Seismic Response of Low-rise Steel Frame Buildings

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1 Seismic Response of Low-rise Steel Frame Buildings Carlos E. Ventura, P.Eng. Mark Bakhtavar Department of Civil Engineering The University of British Columbia

2 Outline of presentation Background of project Objectives of study Buildings studied Studies conducted Vibration properties of buildings studied Case study detailed analyses Work in progress Conclusions & Lessons learned

3 Background of project Sponsors: the Steel Structures Education Foundation (SSEF) and the Natural Sciences and Engineering Research Council of Canada (NSERC) Reasons: The behaviour and response of low-rise (< 7 stories) steel structures is not completely understood. Simple design requirements to achieve economical ductile and robust responses are required. Some areas of interest are: dynamic characteristics, deck-to to-framing connection response & force and inelastic displacement relationships. Data sources: strong motion data obtained from instrumented buildings during recent earthquakes (California data)

4 Objectives of study to gain a better understanding of the factors that control the behaviour of low-rise steel frame buildings by studying in detail the seismic response of buildings subjected to similar level of shaking but with different lateral force resisting systems. to determine how much information can be obtained about the performance of a steel frame building from analyses of recorded motions and use this information to predict the ultimate capacity of the building; and to establish reliable approaches to determine the ultimate capacity of low-rise steel frame buildings and help develop design guidelines suitable for implementation in Canadian seismic design practice.

5 Buildings studied The buildings selected for this study are mostly located within a 100 km radius of the epicentre of the 1989 Loma Prieta and 1994 Northridge earthquakes. General characteristics of these building are: Buildings are between 2 to 7 storeys high. The level of structural shaking experienced by each building is over 0.10g. Only one building was built in the 1960s, while the other buildings were built after The lateral force resisting system of seven of the buildings is a moment resisting frame. Two of the braced frame buildings are base-isolated. Building drawings and recorded motions are available.

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7 Buildings studied general description Building Stories Framing Dimensions (m) Quake PGA (g) E-W N-S height E-W N-S Burbank Office bldg. 6 PMRF Northridge Whittier San Bernardino Hospital 5 MRF Northridge Pasadena Office bldg. 6 F & URM walls Northridge San Jose office bldg. 3 MRF Loma Prieta San Francisco Hospital 4 MRF Loma Prieta Berkeley Hospital 2 EBF Loma Prieta Richmond Medical Center 3 PMRF Loma Prieta Redlands Commercial bldg. 7 PMRF Landers San Bernardino office bldg. 3 PMRF Landers LA Fire Command Control Bldg. 2 PBF(Chevron)+ BI Northridge LA 7-story University Hospital 7 PBF(Chevron)+ BI Northridge

8 Building periods Building Earthquake measured periods (sec) mode E-W N-S Rot. Burbank Northridge story office Bldg.- PMRF Whittier San Bernardino Northridge story Hospital - MRF Pasadena Northridge story Office bldg. - SF+W San Jose Loma Prieta story office bldg. - MRF San Francisco Loma Prieta story Hospital - MRF Berkeley Loma Prieta story Hospital - EBF 2 Richmond Loma Prieta story Medical Center - PMRF Redlands Landers story Commercial bldg. - PMRF San Bernardino Landers story office bldg. - PMRF LA Northridge story FCC bldg. - PBF 2 LA Northridge story University Hospital - PBF 2 3

9 Detailed analysis of strong motion records Case study: Burbank 6 storey office building

10 Burbank 6 story SMRF building

11 Burbank 6 story bldg. This building was designed in 1976 and constructed in The vertical load carrying system consists of 3 concrete slab over metal deck supported by steel frames. The lateral load resisting moment frames are located at the perimeter of the building. The foundation system includes concrete caissons approximately 32 feet deep. The largest peak horizontal acceleration recorded at the base was 0.36g and at the roof 0.47g. The peak velocity at the roof was about 48 cm/sec.

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13 Recorded motions at Burbank Building E/W ground N/S ground E/W roof N/S roof

14 Spectral Accelerations from recorded motions at Burbank Building (5% damping) E/W ground N/S ground E/W roof N/S roof

15 Burbank 6 story bldg. - Damage

16 Observations: The 0.22W maximum base shear apparently experienced by the building in the E-W direction significantly exceeds both the 1976 and 1994 UBC strength design base shears of 1.4x0.07W= 0.10W for UBC-76, and 1.4x0.052W =0.07W for UBC-94. visits to the building and interviews conducted revealed no sign of structural damage. Most of the content damage was caused by tearing of a small water pipe at the penthouse which resulted in flooding of the building. The anchorage of a roof mechanical equipment was also damaged

17 Response Summary for Burbank 6-Story 6 Building. Response Parameter Direction Time of Maxima (sec) Peak Value Base Shear (% Total Weight) N-S E-W DIFF Overturning Moment (% Total Weight x feet) N-S E-W DIFF Roof Lateral Displacement Relative to the Base (cm) N-S E-W DIFF (0.0038)* 9.68 (0.0039)* 1.54 (0.0006)* * Overall drift index values are shown in brackets

18 Responses at time of Max. N/S Displacements (t=16.30 sec)

19 Responses at time of Max. E/W Displacements (t=15.78 sec)

20 Responses at time of Max. N/S Base Shear (t=14.98 sec)

21 Responses at time of Max. E/W Base Shear (t=5.10 sec)

22 Responses at time of Max. N/S Overturning Moment (t=14.96 sec)

23 Responses at time of Max. E/W Overturning Moment (t=8.84 sec)

24 Responses at time of Max. N/S & E/W Base Shear & Overturning Moment

25 Responses at time of Max. N/S & E/W Lateral Displacements & Inter-story Drifts

26 Summary: M/V code = 57 ft Ratio of maxima: M/V N/S = 46 ft M/V E/W = 36 ft Ratio at time of maximum V: M/V N/S = 46 ft M/V E/W = 36 ft Ratio at time of maximum M: M/V N/S = 46 ft M/V E/W = 67 ft

27 Work in progress: Computer model of the building to compare measured & calculated motions. Investigate 3-D 3 D nonlinear behaviour of building using amplified ground motions. Evaluate effectiveness of pushover analysis for predicting yield patterns.

28 Conclusions: Measured periods larger than code periods Ratio between measured & code periods varies between.95 to 2.75 value of empirical formulas??? Actual period is sensitive to level of shaking Torsional period is slightly less than lateral periods 3-D D response is significant Displ,, Drift, Shear and Moments can only be adequately captured with time history analysis