Marconi Communications Building #5 Warrendale, Pennsylvania

Transcription

1 Kelly Hellyer November 14, 2003 Structural Technical Assignment #1 Structural Concepts / Structural Existing Conditions Report

2 Table of Contents Description Page(s) Executive Summary 1 Introduction & Design Parameters 3 Structural System Summary 4 Floor Plan 6 Gravity Load Analysis 7 Wind Load Analysis 8 Seismic Load Summary 10 Drifted Snow Load Analysis 11 Lateral System Summary 12 Member Spot-Checking 14 Foundation Summary 17 Appendix Wind Load Calculations 18 Seismic Load Calculations 20 Drifted Snow Load Calculations 22 Lateral System Calculation 23 Braced Frame Detail 25

3 Executive Summary: Marconi Communications Building #5 is a four floor, steel frame building covering a total floor area of close to 113,000 square feet. Approximately half of the building area consists of symmetrical bays measuring, 30 feet by 30 feet. The remaining bays contain a variety of shapes and sizes. A typical frame consists of W-shape columns supporting open-web steel joists and girders, which are covered with three and a half inches of reinforced, normal weight concrete. Concluding a brief analysis of the existing design, it was found that most initial design assumptions appear to be within the realm of those made by the original engineer. After referencing Boca 99 for live loads, it is believed that most of the loads used in design, were specified according to exact usage of space. The actual live loads used in design were considerably higher than the minimum recommended loads specified in BOCA. The referenced floor plans do not show exact locations of specific live loads, therefore a given typical floor live load, 100psf, was used in analysis. The ground snow load appears to have been chosen by the engineer from experience or specified by local codes, because they are specified greater than that required by Boca. All assumptions made for seismic loads matched those made by the engineer. Several differences were discovered between the current analysis and the actual design, the most significant being the spot-checking of the braced frame within the lateral system. The braced frame is used to resist both seismic and wind load along the north-south axis of the building. Please keep in mind that this analysis is based solely on strength, drift affects were not included. However, the assumption is that drift will certainly be the limiting factor. Based on the design loads of the existing braced frame (which is detailed in the appendix, page 25) and the design loads used for the current analysis, (shown on page 23 of the appendix) the design loads were found to be approximately 16% greater than the loads used. In addition, the member sizes used were much larger than the sizes required by this analysis, however, this discrepancy is likely due to the exclusion of drift in the current spot checking. Other slight discrepancies found throughout this analysis include the spot-checking of the joist, girder and column. The joist analyzed with typical floor live load, was found to require a smaller size joist; a 24LH05 could have been used, though a 24LH06 exists. One hypothesis, of the differences, is based on the heavier size allowing an increase in safety factor, since the design load for a 24LH05 is close to the safe load. When considering a girder for spot-checking, the slight difference is easily noted, since the girder loads were designed and then sent to the manufacturers to be sized. In this instance only a Page 1 of 25

4 drawing of the depth, spacing of nodes and load per node is required. The girder produced through this analysis is shown in Figure 10. Through analysis of the girder a higher load was found applied than the existing design claims, which contradicts the hypotheses based on joist design. Furthermore, it could be assumed the existing design included different loads. For instance if a smaller live load and higher dead load were used, the factor of safety could account for the discrepancies found. As for the typical column, analysis proved to require a W10x88, while the existing is a W10x77. Again I believe this to be an inconsistency of load assumptions. A better analysis can be given if a floor plan including specific live and dead loads used is obtained. Another possibility for the inconsistencies found, between existing and spot checked members, could be the desire of the designer to use the same size members for a greater area than the loads would actually require. Existing drawings prove that many of the members are of the same size. A very noteworthy possibility of differences is that perhaps the design is based on ASD, a slightly more conservative specification than, LRFD, which was used for this analysis.. Page 2 of 25

5 Introduction: As already stated, Marconi Communications Building #5 is a steel frame building of 4 floors and approximately 113,000 square feet of floor area, serving as the worldwide headquarters of Marconi Communication Inc. Building #5 is the fifth among a series of six buildings, built by Marconi, located in. The following pages will give insight to the structural system of Marconi Building #5, as well as provide the basic assumptions needed for the design, and the path to creating required assumptions. Included are the load assumptions, a description of the lateral system, and spot-checking of several members. Design Assumptions: The building codes used for the design of Marconi Building #5 include BOCA 96, the Commonwealth of Pennsylvania, Department of Labor & Industry Fire and Panic Regulations, 1998 edition, the Commonwealth of Pennsylvania, Department of Labor & Industry Building Energy Conservation Act 222, 1980, Allegheny County Plumbing Code, 1998 edition, the Americans with Disabilities Act, and the Commonwealth of Pennsylvania, Universal Accessibility Standards, For simplicity in load comparisons BOCA 99 specifications will be used to determine all the required loads except for drifted snow load, where ASCE 7 will be used, because, Boca 99 does not specify a design procedure for drifted snow load. Steel design throughout this analysis will conform to LRFD, 1998 edition, while concrete design is based on ACI The material grades used throughout the building, include a compressive strength of 3000psi for concrete used in foundations and 4000psi strength concrete for slabs. The reinforcing steel has yield strength, of 60ksi, while the structural shapes are specified as ASTM A572 with yield strength of 50ksi. Structural tubes, found sparsely within the building are required to be ASTM A53 with yield strength of 42ksi, and the structural pipes are ASTM A500 with yield strength of 35ksi Page 3 of 25

6 Frame Description: Each floor plan consists of fourteen symmetrical bays measuring, 30 feet by 30 feet. However, due to the building s irregular shape there are a variety of other sized bays surrounding this core of consistent bays. Some of these unsymmetrical bays are consistent throughout the building, while the bays of the atrium change at each floor level. The eastern wall and ceiling of the atrium are skew to the horizontal and vertical, providing a structural challenge and an architectural wonder. An example floor plan is included on page 5, to give you a better feel for Marconi Building #5. Building #5 was a fast-track project. As a result, the lead-time of steel influenced the primary use of open-web steel joist. Therefore, a typical bay consists of an open web joist girder system along, the north-south axis, and an open web joist system along the east-west axis. The floor joists are spaced at 3 center-to-center and are usually specified as long span series joist, most likely to save depth, but there are some k-series joists as well. Since the joist girders chosen do not match any of the manufacturers specified girders, the engineers have labeled the girders with the exact load they are to carry instead of conforming to a typically manufactured size. For example, 32G10N14.3K, are 32 inch girders, spanning 30, which are not typically made with nodes at 3 intervals. From experience, it is understood that loading diagrams are sent to the manufacturer, where the desired girder truss is erected and shipped on site. Although, this does not seem efficient in the least, at the time of construction manufactured open web joists and girders were the most readily available forms of steel. Since the bays have the same length and width, there was no immediate need to have the girders running along the north-south axis. In fact, it may have been more efficient to design the girders from east to west, since the moment frames run along the east-west axis. The necessary increased beam sizes for the moment frame could also have been useful in carrying the extra gravity load girders require. The floor systems are design of 1.5 floor deck with a 3.5 concrete slab, reinforced with W2.9 x W2.9 wire mesh of 6 grid. The floor deck was required to hold an additional 30psf, to account for the concrete before curing. The column framing is as symmetrical as the floor plan would allow, but due to the irregular shape many of the columns have very little load to carry. The typical column was designed as a W-shape and spliced just above the third floor. However, there are some structural tube columns that span to the fourth floor. The tapered columns that are required in the atrium Page 4 of 25

7 were designed as a single element. Consequently, an inefficient portion of the column design, shipping a 70-foot column to a site is very costly. The majority of the column base plates are large enough compared to the column to consider them a fixed support in design. Very little masonry was used in the design of Marconi Building #5. A masonry wall was used where building #5 was constructed directly next to building #4, and masonry walls are also used where the glass staircases are connected to the rest of the building. In addition, the elevator shaft was constructed of masonry. Page 5 of 25

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9 Live Loads: Design Load Used BOCA 99 Typical Floor Live Load: 100psf 50psf + 20psf (partition) >70psf (if loaded w/ files) Corridor Live Load: 80psf 80psf Lab Live Load: 150psf 60psfmin (based on use) Stair Live Load: 100psf 100psf Mechanical Live Load: 150psf not specified Dead Loads: 1.5 B-LOK Floor Deck w/ 3.5 conc.: 43psf Joists: 16lb/ft*1/3 = 5.33psf Girders: 40lb/ft*1/30 = 1.33psf Mechanical equipment: 5psf Drop Ceiling: 5psf Total Floor Dead Load: 65psf Roof Joists: 20lb/ft*1/6 = 3.33psf Girders: 2psf Ceiling: 5psf Mechanical Equipment: 5psf Stone Ballast Roof: 12psf Insulation: 2psf Total Roof Dead Load: 40psf Snow Loads: Pf = CeIPgCt Pg = 25psf (Fig ) Pf = 25psf EXPOSURE: B (assume partial obstruction from surrounding buildings) Ce = 1.0 Ct = 1.0 (continuous heating) I = 1.0 *** For the original design a ground snow load of 30psf was used Page 7 of 25

10 Wind Loads: The wind load analysis of Marconi Building #5 is based on the design procedure outlined in Boca 99, using a basic wind velocity pressure of 70mph. For design simplicity, the loads are based on the main building and neglect loads applicable to the mechanical room penthouse. For ease of design, in the diagrams of building wind pressures (located below), the actual pressures are approximated, requiring only two separate design pressures to be taken into account. Based on previous experience, it is understood this is often the procedure taken in professional design. Consequently, the design wind pressures for both axes are approximately the same and therefore will be consider equal. The actual wind pressures at each design interval can be seen along with calculations, on pages of the appendix. In addition, the calculations of components and cladding wind loads for the roof joists and the steel wall studs are included on page 17. Please keep in mind that the total shear loads shown on the building shear diagrams are based on the total loads for that direction of the building, not necessarily what each element will be designed to resist. Figure 1 North-South Wind Pressure Page 8 of 25

11 Figure 2 East-West Wind Pressure Figure 3 Figure 4 North-South Total Shear Wind Loads East-West Total Shear Wind Loads Page 9 of 25

12 Seismic Loading: Exposure Group I (Occupancy Group B Buisness) Av = 0.04 (Table (1)) Aa = 0.04 (Table (2)) Performance Category: A (Table ) Structural System to Resist Lateral: Moment Resisting Frame R = 4.5 (Table ) Cd = 4.0 (Table ) Soil Profile (S1) : S =1.0 Lateral Force Procedure: T = 0.65 Cs = V = CsW = 132k Total Weight: 9409k V2 = 17.2k Weight of 2 nd Floor: 2767k V3 = 30.0k Weight of 3 rd Floor: 2472k V4 = 37.0k Weight of 4 th Floor: 2070k Vr = 47.5k Weight of Roof : 2100k Figure 5 Seismic Loading Diagram **Seismic design loads neglect consideration of mechanical room penthouse. Page 10 of 25

13 Drifted Snow Load: There will be a condition in which drifted snow load will occur. A penthouse for mechanical equipment takes up a portion of area above the fourth floor, therefore the roof nearest the 13 obstruction will receive additional snow load. Although, only part of the area beyond the wall has a roof while the rest has a screen to protect the mechanical equipment, the design ofd the snow load allows for worse case scenario, the area near the section with roof. A diagram is included below, of the drifted snow load, and the calculations are attached in the appendix. Figure 6 Drifted snow load diagram Special Loads on Structural Elements: When designing the roof joists, wind uplift loads will need to be taken into account. In addition, the wind loads on steel wall studs will also need to be considered. Although, neither loads should exceed the allowable, since the loads are so small. These loads have been calculated and can be found on page 17 of the appendix. Wind loads will also need to be considered in the design of the metal roof of the atrium and compared to an allowable load given by the manufacturers. In addition, the atrium roof also has a considerable overhang which will need to be structurally designed. Page 11 of 25

14 Lateral System: The lateral system that carries both the wind and seismic loads along the east-west axis consists of two moment resisting frames, within the main building. For the north south axis, the main lateral resistance is a braced frame. The braced frame is placed eccentrically within the building therefore, the moment frames are required to carry the moment produced by the eccentricity. There are no elements within the atrium to resist lateral load. The atrium was designed to frame into the rest of the building and use the other lateral resisting elements to take wind and seismic load. Diagrams of the limiting load case, which happens to 0.9DL + 1.3WL, and the load distribution to each element, are included. Figure 7 Lateral resisting elements within Marconi Communications Building #5 As you can see by the placement of the braced frame eccentric from the centerline of the building, a torsion effect will have to be analyzed. The moment frames will be the only members to resist any torsion since the braced frame runs perpendicular to them. Page 12 of 25

15 Figure 8 Lateral resisting elements along the east-west axis and their distribution of load Figure 9 Lateral resisting elements along the north-south axis and their distribution of load Page 13 of 25

16 Member Spot Checking: Joist: Wd = 65psf * 3 = 195lb/ft Span = 30 Wl = 100psf * 3 = 300lb/ft Required depth = 24 Wu = 714lb/ft **LRFD conversion factor for charts = 1.65Wsji φ = 0.9 Wsji = 714lb/ft/(1.65*0.9) = 480.8lb/ft **For 24 deep long span steel joists, between span, a total safe load is required Total load = 30(480.8) = 14424lbs 24LH05 safe load max of 15100lbs 24LH06 safe load max of 22300lbs **24LH05 will work ***Joist design is based on the parameters provided in The New Columbia Joist Company catalog of 2000 Girder Joist: Pd = 65psf (3 )(15 ) = 3k Pl = 100psf(3 )(15 ) = 4.5k **These are the point loads from one joist, note that the typical girder receives loads from two joists **For girder loading diagrams, non factored loads are to be given Figure 10 Girder Truss Diagram Page 14 of 25

17 Column: **A first floor column from a typical bay Axial Loads/Floor: At = 30 (30 ) = 900ft² Pd2 = 900ft²(65psf) = 58.5k Pl2 = 900ft²(100psf) = 90k Pu2 = 1.2(58.5k) + 1.6(90k) = Pu3 = 214.2k Pu4 = 214.2k Pdr = (900ft²)(30psf) = 36k Plr = (900ft²)(25psf) = 22.5k Pur = 1.2(36k) + 1.6(22.5k) = 79.2k Total Axial Load = 721.8K For a W10x88, with Lb = 16 ΦPn = 746k Page 15 of 25

18 Lateral Element: The lateral element chosen for consideration is a braced frame placed along the northsouth axis. The frame is shown below with loads and existing sizes. The other lateral elements within the building consist of two moment resisting frames in the east-west axis of the building. Therefore, the braced frame is required to resist all lateral loads from wind or earthquakes, along the north-south axis. The two moment resisting frames will then need to resist the torsion from the braced frame. The near symmetry of the moment frames within the east-west axis will not limit the design of the braced frame. Figure 11 Braced Frame **On pages xx of the index is the design for the braced frame and a detail of the existing braced frame is included on page xx. Page 16 of 25

19 Foundation System: Weight placed on the perimeter columns is transferred to the earth, first through piers. The piers at the base of the columns then rely on grade beams place just below the frost line. The grade beams in turn span from caissons, which then transfer the weight to the earth. The caissons are cast-in-place, drilled at least 20 deep and have diameters ranging from 30 to 48 inches. The soil is quite obviously not capable of withstanding very much load. No soils information is currently available for analysis, but based on the foundation system and the size of the building, there is confidence in the assumption of a very low bearing capacity. When designing the foundation, special attention will need to be paid to the foundations for the columns in the moment frames and the slanted columns of the atrium, to be sure they are capable of resisting the required moment. Page 17 of 25

20 APPENDIX BUILDING MAIN WINDFORCE-RESISTING SYSTEM NORTH-SOUTH WINDWARD P=Pv*I (KzGhCp) V = 70mph (Figure ) Pv = 12.5psf (Figure (3)) P=12.5psf(0.8KzGh) I = 1.0 Height Kz = Kh Gh P (psf) L/B = 225'/195' = Cpz = LEEWARD P=Pv*I (KhGhCp) V = 70mph (Figure ) Pv = 12.5psf (Figure (3)) P=12.5psf (0.68)(1.39)(-0.47) I = 1.0 L/B = 225'/195' = 1.15 P= -5.55psf Kh = 0.68 Gh = 1.39 Cpz = EAST-WEST WINDWARD P=Pv*I (KzGhCp) V = 70mph (Figure ) Pv = 12.5psf (Figure (3)) P=12.5psf(0.8KzGh) I = 1.0 Height Kz = Kh Gh P (psf) L/B = 195'/225' = Cpz = LEEWARD P=Pv*I (KhGhCp) V = 70mph (Figure ) Pv = 12.5psf (Figure (3)) P=12.5psf (0.68)(1.39)(-0.5) I = 1.0 L/B = 195'/225' = 0.87 P= -5.91psf Kh = 0.68 Gh = 1.39 Cpz = -0.5 Page 18 of 25

21 BUILDING COMPONENTS & CLADDING ROOF 6'c/c ZONE 1 METAL 6'c/c ZONE 4 P = PvIKh [(GCp) - (Gcpi)] V = 70mph (Figure ) Pv = 12.5psf (Figure (3)) P = 10psf [1.2 - (+/-)0.25] I = 1.0 P = 14.5psf ASSUME Kh = 0.68 FOR HEIGHT OF 60' At of a joist = 6'(30') = 180sq ft GCp = 1.2 P = PvIKh [(GCp) - (Gcpi)] V = 70mph (Figure ) Pv = 12.5psf (Figure (3)) P = 15.5psf I = 1.0 ASSUME Kh = 0.68 FOR HEIGHT OF 60' At of a stud = 1.5'(16') = 24sq ft GCp = 1.3 **LOADS BASED ON BOCA '99 **WIND LOADS BASED ON THE MAIN BUILDING, THE MECHANICAL ROOM ON THE ROOF IS NEGLECTED IN THIS ANALYSIS **ASSUME THE ROOF SLOPE IS LESS THAN 10' Page 19 of 25

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