Florida State University Libraries

Transcription

1 Florida State University Libraries Electronic Theses, Treatises and Dissertations The Graduate School 2013 Analysis of Tilt-Up Building Design and Industry Standard Practices in Tornado- Prone Regions Desiderio Maldonado Follow this and additional works at the FSU Digital Library. For more information, please contact

2 THE FLORIDA STATE UNIVERSITY COLLEGE OF ENGINEERING ANALYSIS OF TILT-UP BUILDING DESIGN AND INDUSTRY STANDARD PRACTICES IN TORNADO-PRONE REGIONS By DESIDERIO MALDONADO A Thesis submitted to the Department of Civil and Environmental Engineering in partial fulfillment of the requirements for the degree of Master of Science Degree Awarded: Summer, 2013

3 Desiderio Maldonado defended this thesis on April 17, The members of the supervisory committee were: Michelle Rambo-Roddenberry Professor Directing Thesis Sungmoon Jung Committee Member Primus Mtenga Committee Member The Graduate School has verified and approved the above-named committee members, and certifies that the thesis has been approved in accordance with university requirements. ii

4 ACKNOWLEDGEMENTS I would like to thank Dr. Roddenberry for her input and guidance on my thesis and graduate studies. I am very grateful for the assistance along the way and could not have done it without her. Secondly, I d like to thank David Conrad for his insight and supervision throughout the process of my research. His input and knowledge regarding the subject were invaluable and absolutely essential to completing the research for my thesis. I cannot understate how much I appreciate your support and guidance. I would like to thank my parents, for always encouraging me to finish what I start and pushing me to realize my potential. Last, but certainly not least, loving and very patient wife Lauren. I cannot count the long nights I ve spent in front of a computer completing my research and she was nothing but supportive throughout the entire process. Without her strength and support, I m not sure I would have ever finished my research. Thank you, from the bottom of my heart. iii

5 TABLE OF CONTENTS List of Tables... vi List of Figures... viii Abstract... xii 1. INTRODUCTION General Objectives BACKGROUND Tornadoes Building Codes and Wind Zones Tornado vs. Hurricane Wind Speeds Recent Tornado-Related Tilt-Up Construction Studies FEMA Mitigation Assessment Team Report Tilt-up Concrete Association Report Structural Engineers Association of Kansas and Missouri Report TILT-UP BUILDING COMPONENTS Roof Deck Roof Deck Fasteners Puddle Welds Screws Power Driven Pins Roof Steel Framing Roof Framing Connections to Steel Columns Steel Columns Roof Framing Connections to Concrete Panel Concrete Panel Walls...29 iv

6 4. BUILDING COMPONENT DESIGN AND ANALYSIS Introduction Traditional Design and Analysis Methods Wind Loading Analysis ACI 551.2R-10 Tilt-Up Panel Manual Design SPWall Software Analysis STAAD.Pro Software Analysis Roof Deck and Fastener Analysis Tilt-up Building Retrofit Feasibility Reinforced Corner Refuge Area SUMMARY, CONCLUSIONS AND RECOMMENDATIONS Summary Conclusions Recommendations...50 APPENDICES...52 A. WIND ANALYSIS...52 B. STAAD.PRO STRUCTURAL ANALYSIS OUTPUTS...62 C. SPWALL ANALYSIS OUTPUTS...71 D. HAND CALCULATIONS...84 REFERENCES BIOGRAPHICAL SKETCH v

7 LIST OF TABLES Table 1 Recorded Tornadoes from 1991 to Table 2 Relationship between Hurricane Wind Speeds and Tornado Wind Speeds...8 Table 3 Tornado Activity within 25 mile radius of Joplin, Missouri from 1950 to Table 4 EF Rated Tornado Wind Speed Effects on Design Velocity Pressure...17 Table 5 Wind Loading Conditions Summary Using ASCE Table 6 Moment Calculations per ACI 551.2R-10 Tilt-up Panel Design...35 Table 7 Moment Calculations per SPWall Analysis...36 Table 8 Steel Roof Framing Member Stresses Summary Table...39 Table 9 Diaphragm Shear Strength for 22 ga Roof Deck...41 Table 10 Diaphragm Shear Strength for 20 ga Roof Deck...41 Table 11 Diaphragm Shear Strength for 18 ga Roof Deck...42 Table 12 Fastener Uplift Capacities for 22 ga Roof Deck...43 Table 13 Fastener Uplift Capacities for 20 ga Roof Deck...43 Table 14 Fastener Uplift Capacities for 18 ga Roof Deck...44 Table 15 Cost Comparison for Metal Roof Deck Thicknesses...44 Table 16 Cost Comparison for Fastener Installation Methods...45 Table 17 Moment and Shear Capacity for Precast Panels for Corner Refuge Area...47 Table 18 Wind Analysis for Building per ASCE Table 19 Wind Roof Uplift Pressure Comparison: Enclosed vs. Partially Enclosed...53 Table 20 Wind Loading Conditions Summary...53 Table 21 Design Wind Pressure Summary for Enclosed Building at 90 mph...54 Table 22 Design Wind Pressure Summary for Enclosed Building at 130 mph...55 Table 23 Design Wind Pressure Summary for Enclosed Building at 150 mph...56 Table 24 Design Wind Pressure Summary for Enclosed Building at 165 mph...57 Table 25 Design Wind Pressure Summary for Partially Enclosed Building at 90 mph...58 Table 26 Design Wind Pressure Summary for Partially Enclosed Building at 130 mph...59 Table 27 Design Wind Pressure Summary for Partially Enclosed Building at 150 mph...60 Table 28 Design Wind Pressure Summary for Partially Enclosed Building at 165 mph...61 Table 29 Maximum Roof System Anchorage Reactions Summary...65 Table 30 Maximum Column Anchorage Reactions Summary...66 vi

8 Table 31 Maximum Roof Framing Member Stresses Summary...67 Table 32 Roof Deck Fastener Capacity Summary...84 Table 33 Roof Deck Diaphragm Shear Capacity Summary...85 Table 34 Cost Estimate for Different Steel Roof Deck Thickness Table 35 Cost Estimate for Different Roof Deck Fastener Types vii

9 LIST OF FIGURES Figure 1 Tornado Activity in the United States...3 Figure 2 Basic Wind Speed Map for ASCE Figure 3 Tornadic Gust Wind Speed with Mean Recurrence Interval of 100,000 years...5 Figure 4 Tornado Safe Room Design Wind Speed Map...7 Figure 5 Aerial View of Home Depot in Joplin, MO and Center Line of Tornado Path...10 Figure 6 Failed Puddle Weld Locations along Open Web Steel Joists...11 Figure 7 Aerial View of Tornado Path and Nearby Big Box Stores...12 Figure 8 Roof Structure in South End of Walmart in Joplin after Tornado...13 Figure 9 Northwest End of Walmart Store...13 Figure 10 Overhead View of Collapsed Home Depot Store in Joplin...15 Figure 11 Common Types of Diaphragms Used in Tilt-Up Construction through the US...18 Figure 12 Standard Roof Deck Fastener Layout Patterns...19 Figure 13 Typical Puddle Weld at Side Lap for Metal Roof Deck...20 Figure 14 Poor Quality Weld at Joist with Blow through on Right Side...20 Figure 15 Typical Metal Deck-to-Wall Connection...21 Figure 16 Standing Drill for Installation of Metal Roof Deck...22 Figure 17 Hilti X-EDNK22 Power Driven Pin...23 Figure 18 Typical Steel Roof Framing Configuration used in STAAD.Pro Model...24 Figure 19 Typical Open Web Steel Roof Joist...24 Figure 20 Open Web Steel Roof Joist...25 Figure 21 Photo of Steel Joist Girder and Steel Column inside Tilt-Up Building...26 Figure 22 Typical Column and Roof Framing Connection...27 Figure 23 Seat Angle for Steel Joist at Wall Connection...28 Figure 24 Typical Steel Joist Girder to Wall Embedded Plate Connection...28 Figure 25 Typical Tilt-Up Concrete Panel...29 Figure 26 Plan View of Building with Wall Wind Load Convention...31 Figure 27 Orthographic View of Building with Roof Wind Load Convention...31 Figure 28 Tilt-up Wall Panel Configuration for ACI 551.2R-10 Analysis...34 Figure 29 Maximum Moment vs. Wind Speed, using ACI 551.2R Figure 30 Maximum Moment vs. Wind Speed, using SPWall Analysis...37 viii

10 Figure 31 Building Model in STAAD.Pro Analysis...38 Figure 32 Steel Roof Framing Elements and Steel Columns in STAAD.Pro Structural Model...38 Figure 33 Maximum Combined Stresses vs. Wind Speed, using STAAD.Pro Analysis...39 Figure 34 Roof Diaphragm Loading Example...40 Figure 35 Reinforced Corner Refuge Area Structural Model...47 Figure 36 Plate Contour Output from STAAD.Pro for Moment about X-Axis...48 Figure 37 Plate Contour Output from STAAD.Pro for Moment about Y-Axis...48 Figure 38 Controlling Wind Loading Configurations...53 Figure 39 3D Whole Building Model for STAAD.Pro Analysis...62 Figure 40 3D Roof Truss Model for STAAD.Pro Analysis...62 Figure 41 North Wall Wind Loading in STAAD.Pro Analysis...63 Figure 42 South Wall Wind Loading in STAAD.Pro Analysis...63 Figure 43 Plate Contour Max Moments for M x Axis from STAAD.Pro Analysis...64 Figure 44 Plate Contour Max Moments for M y Axis from STAAD.Pro Analysis...64 Figure 45 Maximum Roof System Anchorage Reactions Graph...65 Figure 46 Maximum Column Anchorage Reactions Graph...66 Figure 47 Maximum Roof Framing Member Stresses Graph...67 Figure 48 Plate Contour Max Moments for Mx Axis for Reinforced Corner Model from STAAD.Pro Analysis...68 Figure 49 Plate Contour Max Moments for My Axis for Reinforced Corner Model from STAAD.Pro Analysis...68 Figure 50 8 inch thick Concrete Wall Panel Capacity Calculation...69 Figure inch thick Concrete Roof Capacity Calculation...70 Figure 52 General Condition for Single Wall Panel SPMats Analysis...71 Figure 53 Pinned Connections for Single Wall Panel SPMats Analysis...72 Figure 54 Applied Roof Loads for Single Wall Panel SPMats Analysis...73 Figure 55 Applied Lateral Wind Pressure Applied to Single Wall Panel SPMats Analysis...74 Figure 56 Max Unit Moment for M y Axis for Single Wall Panel SPMats Analysis...75 Figure 57 Max Unit Moment for M y Axis for Single Wall Panel SPMats Analysis...75 Figure 58 Output from SPMats Analysis for 90 mph Enclosed Building...76 Figure 59 Output from SPMats Analysis for 130 mph Enclosed Building...78 ix

11 Figure 60 Output from SPMats Analysis for 150 mph Enclosed Building...80 Figure 61 Output from SPMats Analysis for 165 mph Enclosed Building...82 Figure 62 Metal Deck and Fastener Properties...86 Figure 63 Roof Deck Structural Fastener Capacity Calculations...87 Figure 64 Roof Deck Sidelap Fastener Capacity Calculations...88 Figure 65 Diaphragm Shear Strength Calculations for Enclosed 90 mph Design Wind Speed...89 Figure 66 Diaphragm Shear Strength Calculations for Enclosed 130 mph Design Wind Speed...90 Figure 67 Diaphragm Shear Strength Calculations for Enclosed 150 mph Design Wind Speed...91 Figure 68 Diaphragm Shear Strength Calculations for Enclosed 165 mph Design Wind Speed...92 Figure 69 Diaphragm Shear Strength Calculations for Partially Enclosed 90 mph Design Wind Speed...93 Figure 70 Diaphragm Shear Strength Calculations for Partially Enclosed 130 mph Design Wind Speed...94 Figure 71 Diaphragm Shear Strength Calculations for Partially Enclosed 150 mph Design Wind Speed...95 Figure 72 Diaphragm Shear Strength Calculations for Partially Enclosed 165 mph Design Wind Speed...96 Figure 73 Tilt-Up Panel Design Calculation General Parameters for ACI 551.2R Figure 74 Tilt-Up Panel Design Calculation for Enclosed 90 mph Design Wind using ACI 551.2R Figure 75 Tilt-Up Panel Design Calculation for Enclosed 130 mph Design Wind using ACI 551.2R Figure 76 Tilt-Up Panel Design Calculation for Enclosed 150 mph Design Wind using ACI 551.2R Figure 77 Tilt-Up Panel Design Calculation for Enclosed 165 mph Design Wind using ACI 551.2R Figure 78 Tilt-Up Panel Design Calculation for Partially Enclosed 90 mph Design Wind using ACI 551.2R Figure 79 Tilt-Up Panel Design Calculation for Partially Enclosed 130 mph Design Wind using ACI 551.2R Figure 80 Tilt-Up Panel Design Calculation for Partially Enclosed 150 mph Design Wind using ACI 551.2R x

12 Figure 81 Tilt-Up Panel Design Calculation for Partially Enclosed 165 mph Design Wind using ACI 551.2R xi

13 ABSTRACT Tilt-up buildings are a popular building construction method used across the United States. These structures offer many benefits, but can also present unique design challenges when compared to other building types. Recent tornado outbreaks have caused over $20 billion in total damages and killed hundreds of people. As with most structures, tilt-up buildings tend to be susceptible to tornado events. These events have brought building performance and safety in tornado-prone regions to the forefront of consideration by residents, building owners, code officials, and design professionals. The research for this thesis was performed to study the major components and connections used in typical tilt-up buildings using current standard wind analysis methods and to identify limiting factors in building performance in hopes of improving future building designs. Standard wind design and analysis methods were used for this research, not tornado-specific wind design criteria. The components focused on are generally regarded as current industry standards and follow local building codes and manufacturer recommendations. Existing retrofit options traditionally used in high seismic regions were also studied to see if any available methods were suitable for preventing tilt-up building failure in tornado events. The construction of internal storm shelters was also investigated as a potential additional method of reducing injuries and deaths in tornado-prone regions. While large tornado-proof buildings may be impractical or cost prohibitive, it is important for design professionals to continue to take proactive approaches to region specific hazards in future designs to reduce property damage and casualties. xii

14 CHAPTER 1 INTRODUCTION 1.1 General Tilt-up building construction is a common method that has been used across the United States for over 30 years, especially for big box stores and industrial buildings. It is a popular method of construction because it can be completed quickly, is relatively inexpensive when compared to other construction methods, and works well for buildings with large square footage. Tilt-up construction consists of forming and casting concrete wall panels on site. Once cured, the panels are lifted into position and braced until the roofing system is put into place. In the earlier days of tilt-up construction, wall panels were supported by columns or pilasters. Wall panels were also commonly tied together with horizontal beams. This configuration created continuity along the entire length of wall, and the panels did not behave independently. Careful design considerations had to be given to the connections to accommodate thermal expansion. Starting in the 1970s, tilt-up buildings were designed to resist lateral loads imparted on the wall panels using the self weight of the panels and connections to the structural roof system [24]. The wall panels in these buildings are typically not rigidly connected to each other to allow for some movement from normal thermal expansion differences between the building components. Tilt-up buildings have had a public perception of being safe and hardened structures, due to their generally large square footage and tall, reinforced concrete walls. This is especially true when compared to buildings with masonry walls. While tilt-up buildings perform very well in normal weather conditions, recent tornado outbreaks have brought this construction method into the public eye and given a need for additional considerations by building design professionals. 1.2 Objectives The goal of this research is to improve survivability of tilt-up building occupants by studying the weaknesses in tilt-up buildings that lead to catastrophic failure from tornado events, based on findings published after the Joplin, Missouri tornado outbreak in the Spring of

15 The objectives of the research performed were to: 1. Compare different existing code wind parameters and design wind speeds for high-wind design regions in the United States. 2. Investigate standard practices in the tilt-up construction industry. 3. Identify potential areas that may improve building performance. 4. Investigate potential retrofit options used in seismic areas to see if they are suitable for preventing roof failure from tornado wind forces. 5. Explore a feasible option for improving survivability of direct tornado impacts inside tilt-up buildings using recommendations from FEMA 361 and ICC

16 CHAPTER 2 BACKGROUND 2.1 Tornadoes Tornadoes are one of the most violent and extreme weather events on the planet. The United States has more tornadoes than any other country, averaging over 1,200 tornadoes per year [1]. Figure 1 shows the location of areas with high occurrence of tornadoes across the United States. Certain parts of the year are more favorable than others for tornadoes to form, yet it is very difficult to predict when and where they may form. Their vortex wind behavior and extremely high wind speeds, as well as potential for missile impacts on structures, make them difficult to design buildings to withstand. Figure 1 Tornado Activity in the United States [2] In the spring of 2011, the US experienced one of the deadliest tornado outbreaks in almost a century. From April 25 to April 28, there were a total of 358 confirmed tornadoes, 3

17 resulting in 324 deaths [3]. Less than a month later, from May 21 to May 26, there was another tornado outbreak resulting in 242 confirmed tornadoes and 178 deaths [4]. These two meteorological events caused billions of dollars in damage, spanning across at least 9 states. The death toll and property damage brings to question if there are ways to reduce or prevent catastrophic damage and improve survivability of tornadoes by means already available and used in other parts of the country. 2.2 Building Codes and Wind Zones The United States is an expansive country, with a large variety of potentially challenging building conditions ranging from seismic and snow to extreme wind zones. The high velocity hurricane zones along the coast of Florida are among the highest basic wind speeds in the world and can be challenging to accommodate in building design and construction. The central part of the US is generally the lowest basic wind speed as outlined in ASCE 7-05, which is shown in Figure 2 [5]. 90 (40) Figure 2 Basic Wind Speed Map for ASCE 7-05 [5] 4

18 The basis for basic wind speeds is discussed in the ASCE Commentary [5], which includes explanation for the variables and wind speeds as depicted on the wind speed map. It also covers mean recurrence intervals (MRI) and how that correlates to assigning basic wind speed values. Section C of the ASCE 7-05 commentary explains tornadic wind speeds, which are gusts associated with an annual probability of occurrence of 1x10-5 (100,000 year MRI) as shown in Figure 3 [5]. Figure 3 Tornadic Gust Wind Speed with Mean Recurrence Interval of 100,000 years [5] The commentary also goes on to state the following: In recent years, advances have been made in understanding the effects of tornadoes on buildings. This understanding has been gained through extensive documentation of building damage caused by tornadic storms and through analysis of collected data. It is recognized that tornadic wind speeds have a significantly lower probability of occurrence at a point than the probability for basic wind speeds. In addition, it is found that in approximately one-half of the recorded tornadoes, gust speeds are less than the gust speeds associated with basic wind speeds. In intense tornadoes, gust speeds near the ground are in the range of mi/h (67 89 m/s). Sufficient information is available to implement tornado resistant design for above-ground shelters and for buildings that house essential facilities for post-disaster recovery. This information is in the form of tornado risk probabilities, tornadic wind speeds, and associated forces. 5

19 This explanation correlates tornado probability with wind speed. Based on current tornado information available from NOAA, over 62% of tornadoes reported between 1991 and 2010 were rated as EF 0, as shown in Table 1 [6]. A majority of recorded tornadoes fall within the base design wind speed as outlined by ASCE Table 1 Recorded Tornadoes from 1991 to 2010 [6] 2.3 Tornado vs. Hurricane Wind Speeds The highest basic wind speeds as outlined by ASCE 7-05 occur along coastal areas, which are considered hurricane-prone regions. These areas use an annual probability of 0.02, or a 50- year mean recurrence interval (MRI), for a 3 second gust wind speed at 33 ft above ground level in exposure category C [5]. ASCE 7-05 clearly states in Section that tornadoes have not been considered in developing basic wind-speed distributions. As discussed in Chapter 2.1, tornadic wind speeds are associated with a 100,000 year MRI. These wind speeds are used to design community tornado shelters, as outlined in FEMA P-361, which is shown in Figure 4. This document outlines recommendations for designing tornadoresistant and hurricane-resistant structures and takes into account wind speed, as well as windborne debris [2]. This document also outlines and acknowledges that there are three distinct regions of tornadic winds within a tornado. These regions are described below: Near the surface, close to the core or vortex of the tornado. In this region, the winds are complicated and include the peak at-ground wind speeds, but are dominated by the 6

20 tornado s strong rotation. It is in this region that strong upward motions occur that carry debris upward, as well as around the tornado. Near the surface, away from the tornado s vortex. In this region, the flow is a combination of the tornado s rotation, inflow into the tornado, and the background wind. The importance of the rotational winds as compared to the inflow winds decreases with distance from the tornado s vortex. The flow in this region is extremely complicated. The strongest winds are typically concentrated into relatively narrow swaths of strong spiraling inflow rather than a uniform flow into the tornado s vortex circulation. Above the surface, typically above the tops of most buildings. In this region, the flow tends to become nearly circular. FEMA P-361 also explains that tornado wind speeds vary greatly as the distance from the center of the vortex increases. The highest wind speeds in a tornado may actually occur outside the diameter of the vortex and cannot be determined solely from its appearance [2]. Figure 4 Tornado Safe Room Design Wind Speed Map [2] 7

21 Tornadoes and hurricanes both produce extremely high wind speeds. Table 2 outlines the wind speeds associated with hurricane categories and tornado Enhanced Fujita (EF) wind speeds, which were gathered from NOAA.com [1]. Table 2 Relationship between Hurricane Wind Speeds and Tornado Wind Speeds [6] Joplin, Missouri is located within tornado alley and has a very high recurrence interval of tornadoes within a 25 mile radius. In the years from 1950 to 2011, the Joplin area has experienced 116 confirmed tornadoes, totaling 228 deaths and over 2,000 injuries, as shown in Table 3 [7]. Many private residences in the surrounding area have private safe rooms; however, this is not the case with most private businesses. Most businesses have emergency plans in place with a designated refuge area; however, that does not mean the areas are guaranteed to provide protection from tornadoes. Table 3 Tornado Activity within 25 mile radius of Joplin, Missouri from 1950 to 2011 [7] 8

22 2.4 Recent Tornado-Related Tilt-Up Construction Studies The tornado outbreak of Spring 2011 was one of the most destructive tornado outbreaks in the past 50 years. As a result, there were many studies performed following the tornadoes, especially the tornado that struck Joplin, Missouri on May 22, This section discusses the findings of several credible studies and highlights areas related to the research later in this report FEMA Mitigation Assessment Team Report Following severe natural disasters occurring within the United States, the Federal Emergency Management Agency (FEMA) may assemble Mitigation Assessment Teams (MATs) to perform first-hand investigations of the affected areas. The MATs are comprised of a diverse group of qualified professionals from FEMA, local government agencies, and professionals within the private sector with various backgrounds to provide a comprehensive evaluation of the events leading to the disaster, as well as the after effects. According to FEMA s MAT Program Website, The two important components of hazard mitigation are assessing the vulnerability of buildings and increasing building resistance to damage caused by hazard events. The recommendations address improvements in building design and construction, code development and enforcement, and mitigation activities that will lead to greater resistance to hazard events. [8] The week after the Joplin tornado, a FEMA MAT was assembled, and their findings were published as part of FEMA MAT Report P-908: Spring 2011 Tornadoes [8]. Chapter 5 of the report discussed observations on commercial and industrial building performance, including tiltup concrete buildings. The report focused on construction methods, load path, and failure modes. One of the structures investigated was the Home Depot in Joplin. The building was described as being comprised of the following building components: tilt-up concrete wall panels metal roof deck and 5/8 puddle welded fasteners open web steel joists and open web steel joist girders Square tube columns supporting joists and girders Shallow foundations 9

23 The report found that the building was located directly in line with the central path of the tornado, as shown in Figure 5. The EF rating for the tornado at this point is estimated between EF4 and EF5, or an approximate wind speed of 168 to 200 mph. Several of the residents in the area tried to seek refuge inside local businesses due to the perception of safety inside large buildings. HOME DEPOT CENTER LINE OF TORNADO PATH LOCATION Figure 5 Aerial View of Home Depot in Joplin, MO and Center Line of Tornado Path [8] The MAT report concluded that the roof deck to joist fasteners likely failed first, as shown in Figure 6. This failure compromised the rest of the structural elements, leading to the collapse of the building [8]. Joists likely began to fail, allowing the concrete panels to succumb to the wind pressure exerted on the walls. The collapse of the wall panels then lead to the embed plates to be torn away from the concrete wall panels and the racking of the steel columns. It is not believed that wind directly caused the failure of the columns or joist-to-wall panel connections. The MAT also observed that one corner of the building s panels remained intact and partially standing. The metal product racks also appear to have acted as braces, preventing additional collapse of the wall panels in that area. These racks, and many of the products on them, were left still standing and received minimal damage compared to the building 10

24 components. These areas could have possibly been used as areas of refuge as a last resort, however they were not originally designed to protect against wind-borne debris [8]. Figure 6 Failed Puddle Weld Locations along Open Web Steel Joists [8] The FEMA report also investigated the performance of the nearby Walmart store, which was constructed using reinforced concrete masonry unit (CMU) walls and a metal roof deck and open web steel joist roof system. The store was located north of the Home Depot store, just outside the main path of the tornado. Even though the Walmart store did not sustain a direct 11

25 blow from the center of the tornado, the structure suffered roof and wall collapse in the corner of the building nearest to the tornado path. The roof structure for the Walmart was very similar to the Home Depot building: 22 gauge (ga) roof deck with steel open-web roof joists and joist girders, supported by square tube steel columns. The joist girders in the south part of the building remained mostly intact, however, the roof deck, joists, and walls were completely torn away and failed in most of the area, as shown in Figure 7. It was determined the roof deck and open web joists failed first, leading to the collapse of the exterior CMU walls. WALMART LOCATION ACADEMY SPORTS LOCATION HOME DEPOT LOCATION CENTER LINE OF TORNADO PATH Figure 7 Aerial View of Tornado Path and Nearby Big Box Stores [8] The North end of the building fared much better. This side of the building was further from the centerline of the tornado, significantly reducing the wind forces exerted on the roof and walls. Most of the damage sustained to the structure in this area was to the exterior finish of the building. Most of the roof structure and main exterior walls in this area remained intact and still standing after the tornado passed, as shown in Figures 8 and 9. 12

26 Figure 8 Roof Structure in South End of Walmart in Joplin after Tornado [8] Figure 9 Northwest End of Walmart Store [8] 13

27 2.4.2 Tilt-Up Concrete Association Report Tilt-up construction is a popular and common method for constructing buildings that has been used across the US for the past 40 years. The term itself is a description of how the buildings are erected: The concrete wall panels of the buildings are formed and cast in place at the building site, then tilted into place using cranes. The walls are then braced, and the roof structure is attached once all the walls are erected. Since the wall panels are generally not connected to each other, tilt-up buildings require careful consideration of the roofing system and connections between the walls and roof supports to properly withstand vertical and lateral forces. Under normal conditions, the buildings perform very well and offer many advantages, including low cost of construction and short overall construction schedule. Over the past 30 years, the construction method has become the topic of discussion after some notable seismic and extreme high wind events, particularly after the Joplin tornado. The city of Joplin, Missouri was struck but a powerful tornado on May 22, 2011, resulting in 158 deaths [4]. One of the higher profile incidents from the storm was the collapse of the Home Depot store, in which the falling panels killed 7 people seeking shelter inside the store. According to a report prepared by the Tilt-Up Concrete Association [9], the building collapse began with the failure of puddle welds that connect the roof deck to open web roof joists, due to high uplift on the roof deck generated by negative wind pressure. The negative wind pressure was created as a result of the glass doors and windows at the front of the building, changing the building s wind behavior from enclosed to partially enclosed. Figure 10 shows overhead view of the collapsed Home Depot store According to the report, the uplift created from the negative pressure caused the metal roof deck to separate from the open web steel joist roof system. Once the roof deck was removed, the building no longer had the roof diaphragm needed to resist the lateral loads imparted on the building from the winds of the tornado. The bar joists and joist girders began to cripple, leaving heavy concrete wall panels without the needed support to remain standing. The 100-kip wall panels collapsed outward, crushing the people who were attempting to enter the store, seeking shelter. According to the report, the concrete wall panels themselves performed as designed and may have withstood the effects of the tornado had the roof system not failed. 14

28 Figure 10 Overhead View of Collapsed Home Depot Store in Joplin [9] The report also examined the performance of other big box stores near the Joplin Home Depot store. These buildings were included in the study because a comment was made in a newspaper article regarding the Joplin tornado that stated concrete block structures may be safer in a collapse than Tilt-Up wall buildings [9]. The report found that the nearby Academy Sports building was located north of the Home Depot and outside the direct path of the tornado and lost only a portion of its roof. This building had an 18 ga roof deck, compared to the 22 ga roof deck used in the Home Depot store. Beside the wall material itself, the building structure did not vary considerably from a tilt-up building. The proximity of the building to the passing tornado and the heavier roof deck appear to be the primary difference in the performance of the building, compared to the Home Depot store. Since the building only sustained a glancing blow, the wind forces exerted on the Academy Sports building were considerably lower than those exerted on the Home Depot store. The final recommendations of the report highlight the need for stronger roof deck and roof system connections, and increased consideration for collapse factor of safety when designing the roof system. The wall panels rely on the roof system for support, so the task force suggested using similar overall structural performance methodology to collapse prevention resistance used for a Maximum Considered Earthquake (MCE) seismic event [9]. The conclusions also highlight the consideration for storm shelters or safe rooms to protect building 15

29 inhabitants, rather than attempting to design the entire building to withstand tornadoes in tornado-prone regions Structural Engineers Association of Kansas and Missouri Report Another independent study performed on the Home Depot collapse in Joplin, Missouri was prepared by the Structural Engineers Association (SEA) of Kansas and Missouri, entitled Investigations and Recommendations based on May 22, 2011 Joplin, Missouri Tornado [10]. The report made the following observations and conclusions of performance regarding the collapse of the Home Depot building: 1. The load path resisting wind uplift appears to be a more significant concern than wind induced in-plane shear at shear walls, and out of plane bending in walls. 2. Metal deck to joist welds appeared inadequate for the uplift forces (22 gauge deck) that were encountered. A thicker gauge deck would help strengthen this limit state. It was reported that a structure with an 18 gage deck performed significantly better than neighboring 22 gauge roof deck structures. 3. The use of typical joist to joist girder welds appeared to be inadequate for the loads incurred during this event. Connections of joists to joist-girders or beams should be designed for all induced forces, including but not limited to the calculated uplift force, diaphragm chord and collector forces. 4. Roof to wall connections, and joist to wall connections appeared inadequate for uplift. Stronger connections, welds, stiffeners, and anchor bolts are encouraged. Use of long rebar welded to embed plates would likely perform better than short headed studs. A hook added to the top of vertical reinforcing bars in CMU walls would help prevent detachment of bond beam from top of the wall. 5. Engineers should use good judgment when calculating dead loads for use in resisting net uplift. Often the dead load is calculated as heavy, which is conservative for downward load combinations. However the lighter extreme should be considered when calculating net uplift. The actual dead load installed may be significantly less than initially considered. Furthermore dynamic wind effects could temporarily negate the downward contribution of dead load in resisting uplift. In fact, it may not be unreasonable to completely neglect dead load for a roof when calculating net uplift forces. 6. Building Codes should consider requirements for a more robust continuous cross ties across the building diaphragm, so as to preserve walls when the roof diaphragm fails. Wind force levels could be EF-0 or EF-1 and allowable stresses could be ultimate, factor of safety equal to 1.0 and allow for significant damage, but minimize the propensity for collapse of the hard wall system. 7. Attention to detail is appropriate for elements attached to the hard wall structures. If an architectural element dislodges and causes harm, the structural engineer may be accused of negligence. 8. Storm shelters or refuge areas need to be considered for any building type, with consideration of tornado activity. ICC 500 or FEMA 361 can be used for developing a 16

30 storm shelter for a given occupancy, based on occupancy category and may be the basis of design for an area of refuge. Based on these findings and recommendations, several items were highlighted for further consideration in research and analysis for this thesis. The items of particular importance were roof deck and fasteners, improving roof-to-wall connections, and additional consideration for storm shelters or designated refuge areas to be considered for tornado-prone areas based on occupancy. The SEA report also spoke to the difference between current wind design as specified by current building codes and the extreme winds generated in tornadoes. While general estimated tornado wind speeds are a good basis for design, the vortex nature of tornadoes makes it difficult to design tornado-resistant structures. The report also discusses that an overwhelming majority of tornadoes fall below 85 mph, which would be contained with the minimum based wind speed as specified by ASCE 7-05 [10]. It also goes on to point out that velocity pressure increases rapidly as design velocity increases. Using the equation for velocity pressure, (qz), from ASCE 7-05 [5], SEA found that the velocity pressure increases 672% for EF 5 rated tornadoes when compared to the base design velocity for 90 mph, for EF 0 rated tornadoes. The actual velocity pressure can be found in the Table 4. Table 4 EF Rated Tornado Wind Speed Effects on Design Velocity Pressure [10] 17

31 CHAPTER 3 TILT-UP BUILDING COMPONENTS This chapter outlines the major structural components for a typical tilt-up building, from top to bottom. The components discussed include the roof deck, roof deck fasteners, metal roof framing system, roof framing connections, steel columns, and concrete tilt-up wall panels. The results of the analysis for each component will be discussed later in chapter Roof Deck One of the most common roof systems used in large tilt-up buildings is light-gauge metal roof deck with open web steel roof joists and joist girders, with intermediate steel columns. This roofing system is commonly used around the country, including the Midwest, Southeastern and Eastern Continental US, as shown in Figure 11. This metal roof deck diaphragm system was used in the Home Depot building in Joplin as well. Figure 11 Common Types of Diaphragms Used in Tilt-Up Construction through the US [13] 18

32 The roof diaphragm is provided by a thin metal roof deck with a special corrugated shape. The shape of the deck provides a strong, light shape to transfer roof loads to the roof joists, oftentimes without the need for additional steel purlins. The roof diaphragm system provides shear resistance to out-of-plane wind forces applied to the vertical walls of the building, and downward and uplift wind forces applied to the roof deck itself [11]. The roof deck typically comes in 3-ft-wide x 3-ft-long panels. The most common fastener layout used when attaching metal roof deck to roof framing members is 36/4, which consists of fasteners every 12 inches in each direction. 3.2 Roof Deck Fasteners Roof deck fasteners are a critical element to ensuring the roof deck performs adequately as a diaphragm to resist shear forces, as well as uplift forces produced from high winds. There are several roof deck fastener types used for standard tilt-up buildings. The fasteners that will be discussed in this report include puddle welds, screws, and power driven pins. Figure 12 shows the typical fastener pattern options used with metal roof decks. The most common type of fastener pattern used is 36/4, which means there is a fastener in alternating ridges in the roof deck. Since the standard roof deck comes in 36-inch-wide sections, this means there are four fasteners across a typical section. These patterns typically repeat every foot, resulting in a fastener pattern of 12 inch x 12 inch across the entire roof deck. Corner and side areas of the roof deck may also be designed with tighter fastener patterns, such as 36/7. Figure 12 Standard Roof Deck Fastener Layout Patterns [13] 19

33 3.2.1 Puddle Welds The industry standard is to connect the deck to metal roof joists with puddle welds performed in the field, and either weld or screw the deck along the side laps. Welding the roof deck to the joists requires downward pressure to ensure the deck makes proper contact with the top of the joist. If the deck does not make proper contact with the steel joist, the weld will not create a proper bond and will not perform as designed [14]. Figure 13 shows a typical puddle weld performed at the side lap of two roof deck panels. Figure 13 Typical Puddle Weld at Side Lap for Metal Roof Deck [11] Roof deck is a difficult medium to weld because the metal is very thin and does not leave much of a margin for error. Also, the welding is performed from on top of the roof, which means the welder cannot see the steel joists below. Often times, welders will miss the joist and have to re-burn the weld. Figure 14 shows a poor quality weld performed on the roof deck at the roof joist. You can see where the weld has blown a hole through the roof deck. Figure 14 Poor Quality Weld at Joist with Blow through on Right Side [14] Welding quality and suitability can also be limited by weather conditions. If the weather is too cold or wet, welding may be difficult to complete properly. According to the Steel 20

34 Decking Institute (SDI) Manual for Construction with Steel Deck, welding should be performed in accordance with AWS D1.3 and during proper weather conditions. [11]. Special care has to be taken to make and inspect each weld. In a building with over 120,000 square feet of roof area, this can be difficult to accomplish and leaves the roof with potentially compromised diaphragm and uplift capacity. In the article Screw the Deck and Welds, the author states one can actually hear welds failing on a deck in the early morning as the sun heats the deck up and expands, shearing a deficient weld. [14] The metal roof deck is typically attached to the concrete wall panels by a continuous steel angle that is positioned around the interior side of the perimeter of the concrete walls. Figure 15 shows a typical wall-to-roof deck connection, as well as roof insulation and waterproof membrane system in a typical tilt-up building. This angle offers support for the roof deck away from the steel joist members. The angle can either be attached to an embed plate cast into the wall panels, or bolted into the wall after the walls are formed [15]. Figure 15 Typical Metal Deck-to-Wall Connection [13] 21

35 3.2.2 Screws Screws are commonly used for side lap fasteners, but can also be used to connect the roof deck to the steel joists below. This method can be done using a standard drill or a stand-up drill, as shown in Figure 16. Installation can typically be done as quickly as welding, and can be completed by a single worker at each connecting location. Inspection is easier to complete through visual inspection, as most deficiencies would be easier to notice than welding issues [14]. However, applicability for using screws to attach to steel framing may be limited, depending on the thickness of the steel framing being drilled into. This is especially true if steel framing member sizes vary throughout the roof system [11]. Figure 16 Standing Drill for Installation of Metal Roof Deck [11] The use of screws for all connections of the roof deck is becoming more common and even preferred by some designers and building constructors [14]. No special training or certifications are needed to operate the drills used for installing screws correctly. 22

36 3.2.3 Power Driven Pins Another roof deck connection type commonly used in the industry is power driven pins. Hilti makes several types of powder actuated pins that have become industry standards and are commonly referred to as Hilti Pins, as shown in Figure 17. Hilti pins are installed about twice as fast as screws or welding, and they offer excellent strength per fastener connection. The pins can be installed using a stand-up tool, allowing the installer to move more quickly. ` Figure 17 Hilti X-EDNK22 Power Driven Pin [11] Attaching roof deck using Hilti pins requires location and marking the layout of the steel joists prior to making connections. Using the installation tool for Hilti pins may require special training to apply proper pressure to reduce rebound. Most power driven pins are installed using powder actuated tools. These tools use.22 caliber cartridges to drive the pins into the steel framing below and must be carefully adjusted to ensure the pins are installed correctly. Using the tools to install the pins is very loud and requires workers to wear ear protection. Selecting the correct power driven pin type for the roof deck gauge and the steel framing thickness is critical to ensure each connection is installed correctly. The pins can also be expensive, as they cost about 10 times more per pin than screws [11]. The additional cost may be made up for with faster installation times, in certain circumstances. 23

37 3.3 Roof Steel Framing Steel framing is the most common roofing support system used in tilt-up buildings. The roof systems can be selected from catalogs, such as Vulcraft. The design of the steel frame members is handled by the manufacturer, not the engineer. Many of the designs are proprietary, and thus not all component sizes are made available to building design engineers. Generally, the load from the deck is transferred to open web steel joists, which then transfer the load to primary joist girders and concrete wall panels, as shown in Figure 18. Figure 18 Typical Steel Roof Framing Configuration used in STAAD.Pro Model Joist spacing is a factor of the design of the roof deck and the load capacity of the joists. Joists are commonly spaced at 5feet to ensure the roof deck is within allowable spans. This spacing also lends itself well to standard column spacings of 40feet or 50 feet. Figures 19 and 20 show typical open web steel roof joists. Figure 19 Typical Open Web Steel Roof Joist 24

38 The joists connect to joist girders along the top chord of the joist girders, and at columns and concrete wall panels. There is typically a continuous steel angle along the perimeter of the wall panel called a seat angle, to which the top chord of the steel joists and roof deck are attached. Careful detailing of connections at the ends of the roof joists is needed to ensure the loads are transferred properly down to the foundation through the joist girders, wall panels and columns. Figure 20 Open Web Steel Roof Joist 3.4 Roof Framing Connections to Steel Columns In a typical Home Depot building, roof framing transfers the load from the roof to the steel columns in the interior bays of the building system. The columns used in the interior of the buildings are typically spaced 40feet to 50feet apart, and their placement determines the span for the steel joists and joist girders. Detailing connections between steel roof framing and columns should follow recommendations from the steel joist manufacturer. Figure 21 shows a typical joist girder-to-steel column connection inside a tilt-up building. Typically, joists and joist girders are connected with a rigid connection at the top chord and a sliding connection on the bottom chord. This ensures the roof framing members behave as simply supported members and loads are transferred properly to all components. If a rigid 25

39 connection of the bottom chord is desired at the columns by the building designer, the joists and joist girders are no longer considered simply supported and should be designed as a continuous frame by the design engineer [15]. STEEL JOIST GIRDER STEEL COLUMN Figure 21 Photo of Steel Joist Girder and Steel Column inside Tilt-Up Building 3.5 Steel Columns Steel columns are the most common type of interior vertical support used in large tilt-up buildings. They are relatively inexpensive, and help building designers achieve acceptable column spacings to reduce member size requirements and keep overall costs down. Typical column spacing for large buildings, such as big box stores, is typically between 40feet to 50feet in each direction. This spacing also lends itself well to joist spacings of 5feet, which falls within an acceptable span for most roof deck requirements. Steel columns are typically connected to the roof framing systems, as outlined in Section 3.4, and as shown in Figure 22. Steel columns can be round or square, although square tube columns are more popular as they are easier to make welded or bolted connections. Typically columns are connected to the foundation through embedded steel plates in concrete footings. The primary forces resisted by the columns are vertical downward and uplift forces received from the steel roof framing above. The steel columns are typically not designed to offer any moment resistance. 26

40 OPEN WEB STEEL JOIST STEEL JOIST GIRDER STEEL COLUMN Figure 22 Typical Column and Roof Framing Connection 3.6 Roof Framing Connections to Concrete Panel Roof framing transfers the load from the roof to the exterior concrete wall panels. These walls are the primary structural support members in tilt-up buildings and require careful design considerations to ensure the connections provide adequate vertical and lateral resistance to the design loads of the building. One of the most common types of steel roof framing-to-wall connections is embedded steel plates with stud anchors welded to the top chord of the steel joist or joist girder, as shown in Figures 23 and 24 [16]. These embed plates are placed and cast into the concrete wall panels when they are being poured in the field. 27

41 Figure 23 Seat Angle for Steel Joist at Wall Connection [16] STEEL EMBEDDED PLATE CONNECTIONS Figure 24 Typical Steel Joist Girder-to-Wall Embedded Plate Connection 28

42 3.7 Concrete Panel Walls The concrete panels of a tilt-up constructed building are the most critical element in tiltup building design. Concrete is a very versatile material and offers additional benefits, such as natural temperature and sound insulation properties, and it performs better than most other materials for protection against flying debris and missile impacts. Tilt-up panels are comprised of reinforced concrete panels typically between 7inches and 10 inches thick, as shown in Figure 25. These panels can be composed of two concrete layers separated by a layer of insulation. This is typically referred to as a sandwich panels, and is fairly common in tilt-up buildings [16]. Many concrete wall panels, including the ones used in the Home Depot in Joplin, are designed with 7.5-inch-thick concrete wall panels. This wall thickness is commonly chosen to stay within ACI 318 requirements for a single layer of reinforcing [17]. Design moment and deflection limits are determined by ACI 551.2, which will be covered in Section Figure 25 Typical Tilt-Up Concrete Panel 29

43 CHAPTER 4 BUILDING COMPONENT DESIGN AND ANALYSIS 4.1 Introduction In this research, a typical tilt-up building was analyzed. The building was similar to the Home Depot in Joplin, Missouri that collapsed when it incurred a direct hit from a powerful tornado. The approach taken was to evaluate the major building components from the wall panels to the structural roof system, as these are the areas of interest discussed in the independent studies reviewed in the available literature. Each major component was be evaluated with wind speeds ranging from the typical basic wind speed specified in the Midwest, to higher velocity coastal wind speeds as outlined in ASCE Note that the typical wind analysis for a standard building was performed for regular wind design, not tornado wind analysis, as may be done with critical structures such as nuclear facilities. Figures 26 and 27 show the typical wind loading convention used for the building, per ASCE The analysis methods included the following: typical hand calculations as outlined in ACI 551.2R-10, a typical tilt-up wall analysis using SPWall for concrete wall design and analysis, and a complete building component analysis using STAAD.Pro finite element model analysis software. The results from each method are summarized and plotted in graphs to gain a better understanding of the limitations of each building system. This information may be used to improve future designs and potentially improve existing buildings. The building that was analyzed for this study had similar dimensions and construction as a typical big box tilt-up building in the Midwest United States, such as the Home Depot in Joplin. The building dimensions selected for this analysis are 450-feet- long x 300-feet- wide, with 30-foot-tall wall panels. The wall panels were similar to those in the Home Depot building: 7.5 thick reinforced concrete panels with 30-feet wide x 30-feet- tall tilt-up wall panels. Concrete with 4,000 psi compressive strength, f c, was used for all calculations, with 60 ksi steel rebar. A single layer of steel reinforcing was used for wall panels, based on ACI 318 code requirements [18]. Column spacing for the building was specified as 50 ft in each direction. This spacing is consistent with the typical maximum span for open web joists and joist girders as used in Home 30

44 Depot stores. The columns selected in this analysis were steel HSS8x8x1/4 sections with 46 ksi allowable stress. Figure 26 Plan View of Building with Wall Wind Load Convention Figure 27 Orthographic View of Building with Roof Wind Load Convention Based upon inspection of the photographs taken of the Home Depot store in Joplin, the steel roof framing members were estimated as being similar to LH Series open web joists with an approximate joist depth of 36 inches. The maximum span for the joists was 25 feet with a typical spacing of 5 feet between joists. The joist girders appear to be approximately 60 inches in depth, with a maximum span of 25 feet. The allowable stress of all roof framing members was 31

45 30 ksi. The roof deck selected for the building analysis was 22 ga type B steel roof deck. The roof deck has a 1.5-inch deck depth, with a minimum yield stress of 33 ksi. 4.2 Traditional Design and Analysis Methods Wind Loading Analysis The first step in determining the loading conditions for comparing wind pressures for corresponding wind speeds is to perform a wind loading analysis. The wind load analysis method used for this research was based on ASCE According to a report prepared by the SEA, the wind pressure calculations have not varied greatly over the past 15 years when using Chapter 6 of ASCE 7 [10]. The wind speeds selected for the structural calculations within this research correspond to minimum code basic wind speed for Joplin, the wind speed for a Category 3 Hurricane, and EF 3 tornado wind speeds. The controlling wind pressures are summarized in Table 5. The direction for each loading case used in the analysis is shown in Figures 26 and 27. Based on the wind analysis performed, it was determined that the wind load in Case 1 and Case 3 controlled for the forces generated on the building being analyzed. These loads were then used in the structural calculations contained later in this report. See Appendix A for all diagrams and methods for calculating wind pressures using ASCE Table 5 Wind Loading Conditions Summary Using ASCE

46 Table 2 (Continued) ACI 551.2R-10 Tilt-Up Panel Manual Design A preliminary analysis for determining the moment capacity of a single tilt-up concrete wall panel was performed using the design methods as outlined in ACI 551.2R-10 [17]. Wind pressures were applied along the full length of the wall, as determined in the wind load analysis. The equivalent vertical wind load, and uplift, was applied at an eccentricity of 4 inches from the center of the wall panels. Figure 28 shows the typical loading configuration and summary when using ACI 331.2R-10. The factored vertical load from the roof at the top of the wall from roof live, dead, and wind loads (P um) divided by the gross area of concrete section (A g ) must be less than the compressive strength of the concrete (f c ). The first step in the design process for a conventional tilt-up concrete wall panel is to check the vertical stress at mid height of the panel using the equation below: P um / A g < 0.06 f c Next, check to confirm the factored moment capacity (φm n ) is greater than or equal to the cracking moment (M cr ) per Section , where: M cr = (f r *I g )/ y t φm n = φ*a se *F y *(d a/2) 33

47 The variables used in this section are modulus of rupture (f r ), gross moment of inertia (I g ), effective area of steel (A se ), yield stress of reinforcement (F y ), tension-controlled section variable (φ), depth to centroid of primary reinforcement (d), and depth of compression block (a). If moment capacity is acceptable, the next step is to check the minimum reinforcement (ρ) required by Section Set ρ to equal the total area of steel (A s ) divided by the width of the concrete member (b) times the height of the concrete member (h), as shown below: ρ = (A s )/(b*h) Table 6 summarizes the moment capacity, maximum moment, and percentage of moment capacity utilized based on varying wind speeds as outlined in the wind analysis. Based on the initial wall panel capacity design, it appears the wall panels are sufficient to withstand up to 150 mph winds with partially enclosed wind conditions and over 165 mph winds in enclosed wind conditions, per ACI 551.2R-10 requirements and recommendations [17]. Figure 28 Tilt-up Wall Panel Configuration for ACI 551.2R-10 Analysis 34

48 Table 6 Moment Calculations per ACI 551.2R-10 Tilt-up Panel Design Figure 29 is a graphical representation of the calculated values from Table 3. The moment values begin to increase more rapidly after wind speeds reach 150 mph. Based on the design criteria for ACI 551.2R-10, it appears the limiting wind speed value for the 7.5-inch-thick concrete panels is 150 mph. See Appendix D for all hand calculations using ACI 551.2R-10 for this report. Figure 29 Maximum Moment vs. Wind Speed, using ACI 551.2R-10 35

49 4.2.3 SPWall Software Analysis SPWall is a finite element structural analysis program produced by Structure Point. The program is able to perform design analysis based on in-plane and out-of-plane loading for concrete walls, and to check reinforcing requirements based on ACI 318. The program can also take into account second-order effects due to cracking [19]. The same loading conditions from the ACI 551.2R-10 manual design calculations as covered in Section were also used with SPWall to generate a thorough analysis of the tilt-up concrete wall panels. Based on the results from the structural analysis using SPWall displayed in Table 7 and Figure 30, the maximum moments generated for all wind speeds analyzed fall within the allowable moment capacity provided by a single layer of reinforcing for the tilt-up wall panel provided. Note the resulting moment capacities and maximum moments are displayed in kipfeet per foot units. The analysis generates moments based on two-way concrete wall behavior, resulting in a more precise derivation of moments applied to the concrete wall, as compared to the manual calculation using ACI 551.2R-10. See Appendix C for SPWall analysis outputs used to create summary tables. Table 7 Moment Calculations per SPWall Analysis 36

50 Figure 30 Maximum Moment vs. Wind Speed, using SPWall Analysis STAAD.Pro Software Analysis The primary structural analysis method used in this report is a full building structural model using STAAD.Pro finite element analysis software. All primary steel and concrete elements outlined in Chapter 3 were analyzed based on the wind loading analysis outlined in Section The main focus of the model was to replicate the transfer of wind load to steel roof deck and roof framing members. The roof deck was simulated by a 22 ga ( inch) steel plate at the roof deck elevation. STAAD.Pro does not have a feature to model complex plate shapes, only plate elements with uniform thickness, so the roof deck had to be analyzed by hand. See Section in this report for roof deck and fastener analysis. The building model as used in STAAD.Pro was 450 ft long x 300 ft wide, as shown in Figure 31. The columns were spaced every 50 feet to provide a uniform bay size, as shown in Figure 32. ASCE 7-05 was used to create the wind loading configuration for this analysis. See Figures 26 and 27 for the orientation of primary wind loading conventions. The loads were applied unfactored, and allowable stress design was used to compare calculated maximum stresses to the design allowable stresses for steel members. The American Institute of Steel Construction (AISC) Steel Construction Manual [20] specifies allowable stress as 2/3 of the steel grade s minimum yield stress. The steel roof frame members used were Grade A36, which have a yield stress of 36 ksi and a calculated allowable stress of 24 ksi. 37

51 Based on the analysis for the controlling wind loading conditions applied to the structural model, the 90-mph wind loading conditions were the only load cases that stayed below or around the maximum allowable stress design value of 22 ksi. A summary of the resulting steel roof framing member stresses from the analysis are shown in Table 8. A review of Figure 33, which plots the results from Table 8, shows how significant a different the enclosure category used to design the structure, can significantly affect the acceptability of the members. See Appendix B for STAAD.Pro model outputs. If the building can stay intact and remain enclosed, the roof framing members as currently designed can operate below yield strength at 155 mph design wind speed, as shown in Figure 33. The partially enclosed building, however, experiences stresses greater than yield for wind speeds over 100 mph. Figure 31 Building Model in STAAD.Pro Analysis Figure 32 Steel Roof Framing Elements and Steel Columns in STAAD.Pro Structural Model 38

52 Table 8 Steel Roof Framing Member Stresses Summary Figure 33 Maximum Combined Stresses vs. Wind Speed, using STAAD.Pro Analysis 39

53 4.2.5 Roof Deck and Fastener Analysis The roof deck diaphragm in the structural model was checked for shear strength and uplift capacity from the fasteners. The shear loads were resolved from the lateral wind loads from the wind analysis, as shown in Table 5. The uplift forces were calculated using the Steel Deck Institute Diaphragm Design Manual [20]. See Appendix D for hand calculations and summary tables for roof diaphragm capacities. The calculations for shear capacity were provided for roof deck thicknesses varying from 22 ga to 18 ga. The diaphragm design parameters used in all cases were 5/8 inch puddle welds, a 5 ft deck span, three side lap fasteners per span, and a 36/4 fastener pattern. The shear capacity strengths were taken from the New Millennium Building Systems Deck Design Guide [21]. Based on the above criteria, 22 ga roof deck offers 290 plf shear capacity, 20 ga offers 349 plf, and 18 ga offers 456 plf. The maximum applied shear force for all wind conditions was less than the shear strength capacity provided for all roof deck thicknesses analyzed, as shown in Tables 9 through 11. Figure 34 Roof Diaphragm Loading Example 40

54 Table 9 Diaphragm Shear Strength for 22 ga Roof Deck Table 10 Diaphragm Shear Strength for 20 ga Roof Deck 41

55 Table 11 Diaphragm Shear Strength for 18 ga Roof Deck According to the New Millennium Building Systems Deck Design Guide [21], all roof decks provided by New Millennium provide a minimum of 30 psf uplift capacity. Based on a 5foot deck span and 36/4 fastener pattern, an uplift wind pressure of 30 psf would result in 150 lbs of uplift resistance required per fastener. A 36/3 fastener pattern would result in 225 lbs of uplift resistance required. Next, the roof deck fasteners were checked for uplift capacity. A comparison for each type of typical fastener used was performed. The highest resulting roof pressure was used for each for each wind speed from the original wind loading analysis performed. The results of this analysis are shown in Tables 12 through 14. Based on these results, all fasteners tested meet the minimum capacity required to resist wind uplift based on the roof deck thicknesses provided in each case. The important factor to recall is these results assume all fasteners are installed correctly. If several fasteners in a single row are defective, incorrectly installed, or failed over years of expansion and contraction on the roof deck, the uplift forces will be significantly higher on the next available fastener. Once fasteners begin to fail in this manner, the roof deck may begin to unzip, which creates openings in the roof deck and intensify negative wind pressures. 42

56 A basic cost comparison was prepared for metal deck material between 18 ga and 22 ga using AMS Roof Lifecycle Cost Comparison [22] and findings from FSIndustries.com [23]. The cost difference between 22 ga and 20 ga roof deck is around 21%, while going from 22 ga to 18 ga is 65% higher cost. The summary of the roof deck cost comparison is presented in Table 15. Table 12 Fastener Uplift Capacities for 22 ga Roof Deck Table 13 Fastener Uplift Capacities for 20 ga Roof Deck 43

57 Table 14 Fastener Uplift Capacities for 18 ga Roof Deck Table 15 - Cost Comparison for Metal Roof Deck Thicknesses A basic cost comparison for deck fasteners was prepared using the findings from Field Experience with Steel Deck Installation prepared by Gerry Weiler [10]. When compared to the overall cost of the roof system, the cost difference between each roof fastener method is negligible. The results of the roof fastener cost comparison are presented in Table

58 Table 16 - Cost Comparison for Fastener Installation Methods 4.3 Tilt-Up Building Retrofit Feasibility Most building systems have redundancy that will allow a component of the structure to yield without leading to catastrophic failure of the entire structure. Tornado events and earthquakes have exposed that this lack of redundancy in building systems. In the late 1980s, there were several significant seismic events in California resulted in the catastrophic failure of many tilt-up buildings. Retrofitting existing structures with strengthened connections was one method of ensuring that existing buildings complied with new seismic requirements of local building codes [24]. For tornadoes in this research, tilt-up building retrofit solutions that were considered, including reinforcing roof-to-wall anchorage, adding steel ties across girders, using steel rods as cross bracing and adding steel braced frames in intermediate bays [25]. Ultimately, however, the wind loads applied to the walls of the structure proved to be too great for the considered retrofits to prevent failure of the roof structure members. Cross bracing appears to be 45

59 one of the most promising options, but restricts access between columns which is not practical in most big box stores. 4.4 Reinforced Corner Refuge Area Designing a fully tornado-resistant structure is cost prohibitive and not always practical for existing buildings. The typical recommendation for tornado-prone areas is to build a FEMA 361 compliant safe room, or ICC 500 compliant storm shelter. These structures have specific guidelines that must be met, including wind speeds, occupancy density, missile impact testing, and reinforced doors. Building a FEMA 361 or ICC 500 compliant structure is the only way to guarantee that a structure will be able to withstand tornado events [26]. An alternative to building a stand-alone safe room would be to reinforce a corner to create a hardened protected structure in which to seek refuge during tornado events. This room could function as a break room or meeting room, and must remain free of obstructions that would otherwise render the space unavailable during an emergency. This alternative would be suitable for some cases, but may require additional reinforcement or connections to the foundation. For the tilt-up building in this research,, a corner of the existing building s wall panels was utilized, in conjunction with new precast concrete wall panels to form a reinforced corner structure. A STAAD.Pro structural analysis was performed for a 30-foot x 30-foot corner structure using an 8-inch-thick reinforced concrete wall and 12-inch-thick precast concrete roof panels, as shown in Figure 35. This configuration results in approximately 900 sq ft of interior space in the room. ICC 500 occupancy density requires 5 square feet per person and 10 square feet per wheel chair. According to ICC 500 Section , each storm shelter shall be sized to accommodate a minimum of one wheel chair space for every 200 shelter occupants or portion thereof [26]. Using this requirement, 900 sq ft can accommodate 170 standing occupants and five wheel chair occupants. The reinforcing for the walls was #6 bars at 12-inch spacing each way. The reinforcing for the roof panels was #7 bars at 12-inch spacing each way, top and bottom. All connections were modeled as pinned to produce conservative moment values. Wind load pressures for 250- mph design wind were placed on the structure, in accordance with FEMA 361 guidance [2], which ensures the structure can withstand a tornado. The results of the analysis are summarized 46

60 in Table 17 and Figures 36 and 37. All concrete panels met ACI reinforcement requirements for moment and shear. A rough cost, assuming a cost of constructed concrete of $650 per cubic yard, is to approximately $70,000 for this structure to be constructed within an existing building. Figure 35 Reinforced Corner Refuge Area Structural Model Table 17 - Moment and Shear Capacity for Precast Panels for Corner Refuge Area 47

61 Figure 36 Plate Contour Output from STAAD.Pro for Moment about X-Axis Figure 37 Plate Contour Output from STAAD.Pro for Moment about Y-Axis 48

62 CHAPTER 5 SUMMARY, CONCLUSIONS AND RECOMMENDATIONS 5.1 Summary The objective of this research was to investigate potential weaknesses in conventional tilt-up constructed buildings. Industry standard practices were studied, and conventional methods of structural analysis were performed to determine performance limitations in an existing building. Further consideration was given to propose a feasible cost-effective method for accommodating vulnerabilities of tilt-up buildings, to prevent future fatalities associated with tornadoes. Existing guidance from FEMA 361 and ICC 500 were used. The goal of this research is to help designers recognize limitations in building systems in tornado-prone regions and consider incorporating refuge areas into building designs. 5.2 Conclusions The results of the studies of standard construction methods and technologies yielded some surprising results. The following is a summary of notable conclusions reached during the course of this research: 1. Changing enclosure category from enclosed to partially enclosed results in a 40% increase in wind pressure exerted on structures. 2. Welded roof fasteners theoretically have higher capacity than other fastener types, but questionable reliability. 3. Difficulty verifying successful welds reduces their effectiveness and building performance. 4. Screws and power driven pin roof fasteners are faster to install than traditional puddle welds, are easier to install, and have higher reliability than puddle welds. 5. Steel roof framing systems are designed very efficiently, with very little excess capacity. 6. Existing concrete panel walls have considerable excess shear and moment capacity. 7. Wall performance and capacity is limited by the strength and design of the roof framing system. 49

63 8. Some tilt-up buildings can be retrofitted to contain structures that serve as refuge areas that also comply with FEMA 361 and ICC 500 guidelines. 5.3 Recommendations The tornado outbreak of Spring 2011 serves as a reminder that powerful storms will continue to be a serious threat to many parts of the United States, especially the Midwest region. As technology advances, researchers will continue to improve methods for early detection of tornadoes and sending warnings out to residents in surrounding areas. Detecting tornadic activity is only part of the challenge. Improving building performance and striving to decrease casualties due to tornadoes should be considered by building designers and building owners. Designing and constructing tornado-proof buildings, particularly tilt-up buildings, may never be cost-feasible or even possible in some cases, but there are proactive steps that can be taken for new construction and in existing buildings that can increase survivability within tilt-up buildings. New buildings can be designed with more reliable roof deck fasteners to ensure the roof deck performs as designed. The designer and building owner may also consider using a thicker roof deck to accommodate higher uplift capacity. These considerations can keep the roof deck intact during higher uplift conditions and prevent the roof system from being compromised in extreme wind events. Keeping the roof deck attached to the roof framing is critical for ensuring that the loads applied to the roof are transferred to the foundation through the proper load paths, as designed. Concrete tilt-up wall panels are very strong and are generally good at providing protection from flying debris and missile damage; however, the wall panels require support at the top to keep the walls upright. Other than the wall panels themselves, the roof and column systems used are very similar to CMU buildings. Concrete wall panels can offer protection from airborne missiles. The wall panels can also become destructive entities themselves when they are no longer supported at the top end of the panels. It is not practical to design wall panels with a cantilevered wall at the foundation due to the required height of the walls, so consideration should be given to improve reliability of roof-to-wall structural connections. One potential method of keeping the wall panels upright if the roof system fails is to utilize the heavy steel racks that are already being used for merchandise storage and display. These racks are made of heavy steel members and can act as wall bracing during a tornado event. 50

64 While this method may not be feasible as a retrofit option, it might be considered when designing new buildings in tornado-prone areas. If the building will not have large steel rack frames, additional steel bracing can be added to provide additional support against lateral wind loads applied to the wall panels. Panel-to-panel connections may be required to ensure that loads transfer from the upright panels to the bracing, should this method be considered. In cases where buildings are too large to retrofit to provide tornado wind, the best option to prevent injury or loss of life is to provide a stand-alone storm shelter or utilize the natural properties of the existing tilt-up panels to construct a hardened and protected corner in the building. To ensure that the structure can withstand any tornado, it should comply with FEMA 361 or ICC 500 requirements for wind speed, occupancy, and ventilation. If these types of tornado shelters are considered during the design and construction of a large tilt-up building, the cost to include them in the final building could be minimized. This can be an excellent proactive approach to furthering the safety of the building occupants. 51

65 APPENDIX A WIND ANALYSIS Table 18 Wind Analysis for Building per ASCE

66 Table 19 Wind Roof Uplift Pressure Comparison: Enclosed vs. Partially Enclosed Table 20 Wind Loading Conditions Summary Figure 38 Controlling Wind Loading Configurations 53

67 Table 21 Design Wind Pressure Summary for Enclosed Building at 90 mph 54

68 Table 22 Design Wind Pressure Summary for Enclosed Building at 130 mph 55

69 Table 23 Design Wind Pressure Summary for Enclosed Building at 150 mph 56

70 Table 24 Design Wind Pressure Summary for Enclosed Building at 165 mph 57

71 Table 25 Design Wind Pressure Summary for Partially Enclosed Building at 90 mph 58

72 Table 26 Design Wind Pressure Summary for Partially Enclosed Building at 130 mph 59

73 Table 27 Design Wind Pressure Summary for Partially Enclosed Building at 150 mph 60

74 Table 28 Design Wind Pressure Summary for Partially Enclosed Building at 165 mph 61

75 APPENDIX B STAAD.PRO ANALYSIS OUTPUTS Figure 39 3D Whole Building Model for STAAD.Pro Analysis Figure 40 3D Roof Truss Model for STAAD.Pro Analysis 62

76 Figure 41 North Wall Wind Loading in STAAD.Pro Analysis Figure 42 South Wall Wind Loading in STAAD.Pro Analysis 63

77 Figure 43 Plate Contour Max Moments for M x Axis from STAAD.Pro Analysis Figure 44 Plate Contour Max Moments for M y Axis from STAAD.Pro Analysis 64

78 Table 29 Maximum Roof System Anchorage Reactions Summary Figure 45 Maximum Roof System Anchorage Reactions Graph 65

79 Table 30 Maximum Column Anchorage Reactions Summary Figure 46 Maximum Column Anchorage Reactions Graph 66

80 Table 31 Maximum Roof Framing Member Stresses Summary Figure 47 Maximum Roof Framing Member Stresses Graph 67

81 Figure 48 Plate Contour Max Moments for M x Axis for Reinforced Corner Model from STAAD.Pro Analysis Figure 49 Plate Contour Max Moments for M y Axis for Reinforced Corner Model from STAAD.Pro Analysis 68

82 Figure 50 8 inch thick Concrete Wall Panel Capacity Calculation 69

83 Figure inch thick Concrete Roof Capacity Calculation 70

84 APPENDIX C SPWALL ANALYSIS OUTPUTS Figure 52 General Condition for Single Wall Panel SPMats Analysis 71

85 Figure 53 Pinned Connections for Single Wall Panel SPMats Analysis 72

86 Figure 54 Applied Roof Loads for Single Wall Panel SPMats Analysis 73

87 Figure 55 Applied Lateral Wind Pressure Applied to Single Wall Panel SPMats Analysis 74

88 Figure 56 Max Unit Moment for M x Axis for Single Wall Panel SPMats Analysis Figure 57 Max Unit Moment for M y Axis for Single Wall Panel SPMats Analysis 75

89 Figure 58 Output from SPMats Analysis for 90 mph Enclosed Building 76

90 77

91 Figure 59 Output from SPMats Analysis for 130 mph Enclosed Building 78

92 79

93 Figure 60 Output from SPMats Analysis for 150 mph Enclosed Building 80

94 81

95 Figure 61 Output from SPMats Analysis for 165 mph Enclosed Building 82

96 83

97 APPENDIX D HAND CALCULATIONS Table 32 Roof Deck Fastener Capacity Summary 84

98 Table 33 Roof Deck Diaphragm Shear Capacity Summary 85

99 Figure 62 Metal Deck and Fastener Properties 86

100 Figure 63 Roof Deck Structural Fastener Capacity Calculations 87

101 Figure 64 Roof Deck Sidelap Fastener Capacity Calculations 88

102 Figure 65 Diaphragm Shear Strength Calculations for Enclosed 90 mph Design Wind Speed 89

103 Figure 66 Diaphragm Shear Strength Calculations for Enclosed 130 mph Design Wind Speed 90

104 Figure 67 Diaphragm Shear Strength Calculations for Enclosed 150 mph Design Wind Speed 91

105 Figure 68 Diaphragm Shear Strength Calculations for Enclosed 165 mph Design Wind Speed 92

106 Figure 69 Diaphragm Shear Strength Calculations for Partially Enclosed 90 mph Design Wind Speed 93

107 Figure 70 Diaphragm Shear Strength Calculations for Partially Enclosed 130 mph Design Wind Speed 94

108 Figure 71 Diaphragm Shear Strength Calculations for Partially Enclosed 150 mph Design Wind Speed 95

109 Figure 72 Diaphragm Shear Strength Calculations for Partially Enclosed 165 mph Design Wind Speed 96

110 Figure 73 Tilt-Up Panel Design Calculation General Parameters for ACI 551.2R-10 97

111 Figure 74 Tilt-Up Panel Design Calculation for Enclosed 90 mph Design Wind using ACI 551.2R-10 98

112 Cracking moment of inertia Calculated moment capacity o.f Concrete Panel Check minimum reinforcement reauired by Section 14_8 24 "No Good" otherwise PJ Z Minimum reqcired reinforcement ratio per Section As p :a a bpaner 1 panel Reinforcement tatio provided Check minimum reinforcement reauired by Section 14_3 2 "No Good" otherwise a "Ok" Z 48 Ec lcr Kb, kip 5 1c- Pane/sOffness Ultimate appned moment Mua 1 Mu Z ( ) kip pum P N W U Ultimate calculated moment Check applied moment ャュッセZ\A\ィ c@,_ "Ok" if 4>M 0 > Mu - "Ok" "No Good" otherwise 99

113 Figure 75 Tilt-Up Panel Design Calculation for Enclosed 130 mph Design Wind using ACI 551.2R

114 101

115 Figure 76 Tilt-Up Panel Design Calculation for Enclosed 150 mph Design Wind using ACI 551.2R

116 103

117 Figure 77 Tilt-Up Panel Design Calculation for Enclosed 165 mph Design Wind using ACI 551.2R

118 105

119 Figure 78 Tilt-Up Panel Design Calculation for Partially Enclosed 90 mph Design Wind using ACI 551.2R

120 107

121 Figure 79 Tilt-Up Panel Design Calculation for Partially Enclosed 130 mph Design Wind using ACI 551.2R

122 109

123 Figure 80 Tilt-Up Panel Design Calculation for Partially Enclosed 150 mph Design Wind using ACI 551.2R

124 111

125 Figure 81 Tilt-Up Panel Design Calculation for Partially Enclosed 165 mph Design Wind using ACI 551.2R

126 113

127 Table 34 Cost Estimate for Different Steel Roof Deck Thickness Table 35 Cost Estimate for Different Roof Deck Fastener Types 114