Ambient Vibration Testing of Low-Rise Buildings with Flexible Diaphragms

Transcription

1 Ambient Vibration Testing of Low-Rise Buildings with Flexible Diaphragms Martin Turek 1 and Carlos E. Ventura 2 (1) Graduate Student (2) Professor Room 2010, Department of Civil Engineering, University of British Columbia 6250 Applied Science Lane, Vancouver, BC, Canada V6T 2E7 meturek@civil.ubc.ca; ventura@civil.ubc.ca ABSTRACT This paper presents a review of test results that are part of an ongoing research project to study the dynamic behaviour of low-rise steel buildings with metal roof deck diaphragms. The project includes a large ambient vibration test program on actual building structures located both in eastern and western regions of Canada, due to a lack of data for these structures to date. Test results include natural periods, mode shapes, and damping ratios. This paper focuses on five buildings tested in western Canada. The results have shown that there is a considerable difference between the periods computed from current design and modelling practice and real structures, and indicates a considerable amount of flexibility in the roof diaphragm. BACKGROUND The low-rise warehouse-type structure is a common building type in Canada. These typically have a steel-deck roof system with simple wall construction, made of concrete, masonry or a steel frame. All three of these construction types occur all across the country, but certain types are more common in different regions. For example, the west coast of Canada has more concrete-walled buildings and the eastern regions have more steel-frame construction. These buildings are typically used for light industrial, commercial and recreational purposes. Some of these buildings are located in regions of Canada with active and moderate seismicity levels. These include the west coast of British Columbia, including the Lower Mainland and Vancouver Island and along the St-Lawrence and Ottawa rivers. As illustrated in Fig. 1a, the seismic force resisting system in these structures typically includes a metal roof deck diaphragm that transfers horizontal loads to walls that can include steel bracing. The roof diaphragm is made of corrugated steel deck sheets that are fastened to each other and to the supporting beams and joists (Fig. 1b). The roof diaphragm in these structures is relatively flexible and in-plane roof deformations due to lateral loads are often comparable to or exceed the deformation of the vertical bracing, as illustrated in Fig. 1c. The National Building Code of Canada (NBCC1995) addresses these issues of seismicity in its design principles. The magnitude of the seismic loads at a given site is dependent on the fundamental vibration period of the structure. These can be computed in the code with the use of empirical equations. Those equations are typically derived from multi-storey buildings with rigid floor and roof diaphragms. This would not then give a good estimation of the period of a short building with a flexible diaphragm. It has been seen [Tremblay] that the computed period of these structures is longer than the code empirical equations, as shown in Figure 1c. If it can be proved

2 Figure 1: Typical low-rise steel building: a) Main structural components; b) Detail of roof deck panels; c) Period and design spectrum; d) Plan view of laterally deformed building. that this is a significant and consistent phenomenon, a better more efficient seismic design may be achieved. In order to better understand the difference, actual physical data must be obtained from real buildings. To date, very limited physical evidence has been available in this regard. Although controlled laboratory tests [Tremblay et al.] have shown good agreement between analytical and experimental tests, data from actual buildings [Ventura et al.] has shown a much shorter period than the analytical result [Medhekar]. To further investigate this phenomenon, more field data must be obtained. ONGOING RESEARCH PROJECT To facilitate the study of the seismic behaviour of these types of structures, a research program was recently initiated to expand the existing database and to develop numerical modelling techniques for the seismic design of low-rise steel buildings. This project involves researchers from four Canadian Universities (the University of Sherbrooke, Sherbrooke, QC, the University of British Columbia, Vancouver, BC, and École Polytechnique of Montreal and McGill University, Montreal, QC) and is carried out in collaboration with representatives of the steel industry. A major portion of the work is dedicated to in-situ ambient vibration testing to measure the periods of vibration and damping ratios of actual building structures located in both eastern and western Canada. This paper describes some of the buildings tested by the group in western Canada. FIELD TESTING PROCEDURE The UBC system measures the ambient vibrations in the building with a series of force-balance type accelerometers. Data is acquired with a 16-channel portable system, with adjustable gains and a 50 Hz low-pass filter on each channel. The exact sensor layout of each test varies depending on the size and shape of the building. A typical layout on a building roof, plan view, is

3 shown in Figure 2. All of the sensors are measuring horizontal vibrations. The four corners are measured in both directions to capture both translational modes and the torsional mode; a cross of sensors in the middle of the building is intended to capture any higher order modes and to identify the degree of flexibility of the diaphragm. At least one set of sensors (x and y) will be placed at the base of the building to establish the baseline ground vibrations. Figure 2: Typical Sensor Layout on Roof The recorded vibration signals are then analyzed using the Artemis Extractor software package, determining the natural frequencies, modes shapes and daming of the building. For all buildings the Frequency Domain Decomposition method is used, and where applicable the time domain SSI method is used to validate the results. TESTING PHASE I: ALBERTA The first testing phase was based on part of a study-term at the University of Calgary, and several tests in Calgary were performed. Market Mall, Calgary, AB Market Mall is a typical commercial mall with a steel structural frame and masonry infill walls. The mall also has a smaller administrative building south of the main structure. The smaller building was tested completely, while only part of the main structure was tested. The main structure is composed of a single storey section, with two-storey sections housing department stores at each end. The roof structure consists of open web steel joists, a 38mm steel deck and a built-up roof. Along the west side of the single storey section is a large grocery store. Due to the large size of the mall, only a portion of the roof near this store was tested, approximately 50 by 60m in area. The test was performed in cold conditions, with the temperature at -8ºC. The Market Mall South Building is an L-shaped single storey steel braced frame building with masonry infill walls, shown in Figure 3. The roof system is a series of steel W sections and openweb joists, with a standard 38 mm steel deck. The building is 45 m by 37 m by 8 m high.

4 Figure 3: Market Mall South Building, Calgary AB From the analysis of the partial test on the main structure, three modes below 10 Hz were identified, shown in Table 1. Although these modes show the general mode shape behaviour of the structure (Description), it is not known if they were global or local modes. Table 1: Market Mall (Main) Dynamic Characteristics Mode Frequency [Hz] Period [sec] Description st N/S st E/W nd N/S From the analysis of the south building, three modes below 10 Hz were identified. Table 1 gives the modal frequencies and a general description of the mode shape. Although the mode shapes are separated in the table, there was significant torsional coupling in the two translational modes. Table 1: Market Mall South Building Dynamic Characteristics Mode Frequency [Hz] Period [sec] Description st N/S st E/W st Torsion Firehall No. 4, Calgary AB Calgary Fire Station No. 4 is a masonry structure with a steel-deck roof supported on typical steel joists. The building has two sections: a 6.3m high garage for the emergency vehicles, and an adjacent 2-storey section for offices and living quarters. The lower storey is 3.6m high, and the floor of the 2nd stoey is a concrete slab poured on a corrugated steel deck. The top of the 2- storey section is a steel-deck roof supported on steel joists. The roof framing consists of 1200mm open web steel joists with three different spacings in the main section ranging between 1265mm and 2094mm. The roof consists of a typical 38mm roof deck.

5 Figure 4: Garage Section Before Deck Installed; Figure 5: Garage Section After Deck Installed The vibration tests were performed on the firehall in two parts. The first part was performed before the main roof deck was installed, as shown in Figure 4. The second part was performed after the main roof deck was installed as shown in Figure 5. All of the testing was performed during construction, and as a result there was considerable vibrations throughout the building. This also created limitations in the use of common reference sensors and access to different parts of the structure. As a result data was collected on the two sections of the structure separately. Because of the way the data was collected, the analysis was performed using four models. Two models were created for the tests done on the first day (no roof) and two models were created for the tests of the second day (roof on). Shown in Tables 3 and 4 are the results of the data collected from the garage section. It is seen that there is a small frequency increase after the deck was installed. Table 3: Dynamic characteristics from Firehall Garage - Before deck installed Mode No. Frequency [Hz] Period [sec] Damping [%] General Description First E/W Second E/W ~ N/A Third E/W Table 4: Dynamic characteristics from Firehall Garage - After deck installed Mode No. Frequency [Hz] Period [sec] Damping [%] General Description First E/W Second E/W Third E/W Testing Phase II: British Columbia The second testing phase focused on buildings on the Canadian west coast, which has the highest seismic risk in the country. This section describes the results of three steel buildings tested in the Lower Mainland.

6 Finning CAT Regional Depot, Surrey, BC The Finning CAT Regional Depot is a single-storey steel braced frame building composed of three separate sections (Finning is a distributor of CATTERPILLAR heavy construction equipment). Two of the sections (warehouse and repair shop) and 10 in height, positioned slightly offset from one another. The third section is a 5m high office space adjacent to the first two. The building has overall dimensions of approximately 100m by 75m. Figure 6: View of warehouse section interior; Figure 7: View of warehouse section exterior The warehouse is made of a frame of HSS columns with steel W sections for the primary roof structure, and 600mm open-web steel joists placed 1.8m o/c as the secondary structure, with a 38mm steel deck, as shown in Figure 6. The perimeter of the structure has a number of braced bays for lateral resistance. Table 5: Finning CAT Dynamic Characteristics Mode Frequency [Hz] Period [sec] Damping [%] Description st E/W st N/S nd N/S rd N/S st Torsion nd Torsion The ambient vibration testing identified 6 modes below 6 Hz, shown in Table 5. The mode shapes are primarily defined by the movement of the warehouse section. There are three apparent types Figure 8: Finning CAT Vibration Mode 1; Figure 9: Finning CAT Vibration Mode 3

7 of modal behaviour: north-south transverse, east-west transverse and torsion. Each of these is dependent on the overall layout of the structure which contains two distinct sections: the more flexible warehouse section and the stiffer office/shop section. For the east-west mode (Mode 1) the entire structure moves in the same direction at the same frequency, as shown in Figure 8. For the north-south modes (Modes 2, 3 and 4) most of the motion comes from the warehouse section, mode 3 is shown in Figure 9. Although there is motion in the shop section, the majority of the deflections and all of the higher order behaviour comes from the warehouse section. In both torsional modes, the two sections move out-of-phase. Versacold Derwent Facility, Delta, BC Versacold is a supplier of frozen and refrigerated storage, distribution and processing services for the food industry. The Derwent Facility contains two separate freezer compartments, maintained at temperatures between -20 and -30ºC. The main freezer section is 11.35m high and is surrounded by two 5.465m high loading docks on its north and west sides. The building is made up of a steel HSS columnbraced frame system, with a standard roof deck. A view of the roof system from inside the west loading dock is shown in Figure 10. Figure 10: Versacold Roof Framing System The actual test was performed twice. The quality of the data from the first test was affected by the presence of large mechanical systems on the roof that operated at high vibration amplitudes. The frequencies were found but no clear mode shapes were defined. The second test was performed on a colder day when the roof cooling equipment was not running. This test confirmed the frequencies and captured the mode shapes. The analysis identified the 3 modes below 10 Hz, shown in Table 6. Table 6: Versacold Dynamic Characteristics Mode Frequency [Hz] Period [sec] Damping [%] Description st N/S st E/W 3? st Torsion IKEA, Coquitlam, BC The IKEA Store in Coquitlam, BC is a 30,000 sq m steel braced frame structure, built on a single storey concrete parking garage. It is shown in Figure 11. The entire structure is built upon a series of over 150 deep piles. The store is 220 m by 100m. Half of the rectangular structure is a two-level structure with the store showroom, and the other half is the full-height warehouse.

8 Figure 11: View of IKEA, Coquitlam, BC The store is composed of typical HSS steel framing with a standard roof deck, and feature a special buckling-restrained-bracing (BRB) system [Malmgren]. The analysis of the data found 3 modes below 10 Hz. These are shown in Table 7. The first two translational modes and the first bending mode (2 nd N/S in the Table) are shown in Figure 12. The first bending mode shows the flexibility of the the roof diaphragm between two lines of lateral braces. Table 7: IKEA Dynamic Characteristics Mode Frequency [Hz] Period [sec] Damping [%] Description st E/W st N/S nd N/S Figure 12: IKEA Vibration Modes: 1 st E/W, 1 st N/S, 1 st Bending N/S

9 CONCLUSIONS Based on work done to date, it has been seen that the NBCC-95 design period for low-rise buildings does not compare well with other, more-sophisticated analytical techniques or with measured periods from actual structures. For this reason a major research project has been undertaken to address these issues by developing better modelling techniques, and by collecting data from real structures. The preliminary findings from the project [Paultre et al.] have begun to confirm these issues. From the ambient vibration tests described in this paper, it was seen that the three steel buildings have fundamental periods varying from 0.25 to 0.9 seconds. Since they are similar in height, this confirms the idea that computing period solely based on height is not a good approximation for these types of buildings. It is also seen from the mode shapes of the structures that there is some flexibility in the roof diaphragm. Examples of this are found in the longer steel structures, such as Finning CAT, Surrey, BC (Figure 9) and IKEA, Coquitlam, BC (Figure 14). Further analysis must be performed to determine the exact degree of flexibility. Based on these two observations, regarding period difference and flexibility of the diaphragms, it is apparent that the current design methods are not completely addressing the actual dynamic behaviour of these structures. These tests have begun to confirm the belief that there is a significant difference between the predicted periods and the actual periods. More testing will be performed to expand the database. ACKNOWLEDGMENTS Funding for this project is provided by a Strategic Project Grant from the Natural Sciences and Engineering Research Council of Canada (NSERC). The authors would like to acknowledge the other key researchers who are involved in this project: Patrick Paultre and Jean Proulx, Sherbrooke University; Robert Tremblay, Ecole Polytechnique of Montreal and Colin Rogers, McGill University. The authors would also like to acknowledge the following graduate students who assisted with the field testing: Reiner Herzinger, Malcolm MacKay and Mark Ritchie, from The University of Calgary; Andrew Seeton and Kian Mirza, from The University of British Columbia; Camelia Nedisan and Horea Iepan, from Ecole Polytechnique of Montreal. REFERENCES Malmgren, J.K., Another Way to Brace, Advantage Steel - CISC Magazine, Volume 18, Fall 2003 Medhekar, M.S., Seismic evaluation of steel buildings with concentrically braced frames, Ph.D. Thesis, Dept. of Civil and Environmental Engineering, University of Alberta, Edmonton, AL, 1997 Paultre, P., Proulx, J., Ventura, C., Tremblay, R., Rogers, C., Lamarche, C.P., Turek, M., Experimental Investigation and Dynamic Simulations of Low-Rise Steel Buildings for Efficient Seismic Design, 13 th World Conference on Earthquake Engineering, Vancouver, Canada, 2004, Paper no Tremblay, R., Fundamental Period of vibration of braced frames for seismic design, Earthquake Spectra 2004 (submitted) Tremblay, R., Berair, T., Filiatrault, A., Experimental Behaviour of Low-Rise Steel Buildings with Flexible Roof Diaphragms, 12 th World Conference on Earthquake Engineering, Auckland, NZ, 2000, Paper no Ventura, C., Turek, M., Ambient Vibration Testing of Safeway Canada Store, Boonie Doon Centre, Edmonton, Alberta, UBC EERF Report, EQ 03-08, October 2003