Seismic Response of Low-rise Steel Frame Buildings Carlos E. Ventura, P.Eng. Mark Bakhtavar Department of Civil Engineering The University of British Columbia
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
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)
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.
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 1970. 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.
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 37 37 25 Northridge 0.30 0.26 Whittier 0.16 0.22 San Bernardino Hospital 5 MRF 46 55 21 Northridge 0.05 0.06 Pasadena Office bldg. 6 F & URM walls 36 38 25 Northridge 0.10 0.15 San Jose office bldg. 3 MRF 76 28 15 Loma Prieta 0.20 0.17 San Francisco Hospital 4 MRF 36 67 16 Loma Prieta 0.15 0.14 Berkeley Hospital 2 EBF 43 36 8 Loma Prieta 0.12 0.11 Richmond Medical Center 3 PMRF 24 50 13 Loma Prieta 0.11 0.08 Redlands Commercial bldg. 7 PMRF 43 28 28 Landers 0.05 0.05 San Bernardino office bldg. 3 PMRF 40 44 13 Landers 0.11 0.10 LA Fire Command Control Bldg. 2 PBF(Chevron)+ BI 26 57 10 Northridge 0.16 0.19 LA 7-story University Hospital 7 PBF(Chevron)+ BI 77 92 36 Northridge 0.16 0.37
Building periods Building Earthquake measured periods (sec) mode E-W N-S Rot. Burbank Northridge 1 1.41 1.39 0.92 6-story office Bldg.- PMRF 2 0.50 0.48 0.32 3 0.27 0.29. --- Whittier 1 1.22 1.33 0.92 2 0.43 0.44 0.03 3 0.24 0.25. --- San Bernardino Northridge 1 0.49 0.48 0.39 5-story Hospital - MRF 2 0.18 0.18 0.15 3 Pasadena Northridge 1 2.24 1.67 1.22 6-story Office bldg. - SF+W 2 0.55 3 San Jose Loma Prieta 1 0.66 0.72 0.49 3-story office bldg. - MRF 2 0.21 0.24 3 San Francisco Loma Prieta 1 0.61 0.69 0.58 4-story Hospital - MRF 2 0.23 0.24 3 Berkeley Loma Prieta 1 0.35 0.33 0.22 2-story Hospital - EBF 2 Richmond Loma Prieta 1 0.70 0.64 0.5 3-story Medical Center - PMRF 2 0.24 0.21 0.18 3 0.14 0.14 Redlands Landers 1 1.45 1.56 1.06 7-story Commercial bldg. - PMRF 2 0.50 3 0.29 San Bernardino Landers 1 0.55 0.56 0.42 3-story office bldg. - PMRF 2 0.20 3 LA Northridge 1 1.0 2-story FCC bldg. - PBF 2 LA Northridge 1 1.22 1.20 7-story University Hospital - PBF 2 3
Detailed analysis of strong motion records Case study: Burbank 6 storey office building
Burbank 6 story SMRF building
Burbank 6 story bldg. This building was designed in 1976 and constructed in 1977. 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.
Recorded motions at Burbank Building E/W ground N/S ground E/W roof N/S roof
Spectral Accelerations from recorded motions at Burbank Building (5% damping) E/W ground N/S ground E/W roof N/S roof
Burbank 6 story bldg. - Damage
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
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 14.98 5.10 5.96 12.37 22.07 7.11 Overturning Moment (% Total Weight x feet) N-S E-W DIFF 14.96 8.84 5.78 546 807 231 Roof Lateral Displacement Relative to the Base (cm) N-S E-W DIFF 16.30 15.78 13.02 9.63 (0.0038)* 9.68 (0.0039)* 1.54 (0.0006)* * Overall drift index values are shown in brackets
Responses at time of Max. N/S Displacements (t=16.30 sec)
Responses at time of Max. E/W Displacements (t=15.78 sec)
Responses at time of Max. N/S Base Shear (t=14.98 sec)
Responses at time of Max. E/W Base Shear (t=5.10 sec)
Responses at time of Max. N/S Overturning Moment (t=14.96 sec)
Responses at time of Max. E/W Overturning Moment (t=8.84 sec)
Responses at time of Max. N/S & E/W Base Shear & Overturning Moment
Responses at time of Max. N/S & E/W Lateral Displacements & Inter-story Drifts
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
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.
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