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Assessing Effectiveness of Building Code Provisions

This study evaluates the effectiveness of seismic building code provisions through incremental dynamic analysis on a four-story special moment frame (SMF) office building in the Los Angeles Basin. Emphasizing the 2003 IBC and 2002 ACI codes, the analysis focuses on collapse potential, lateral drift measures, and inherent uncertainties. Results indicate significant differences in collapse probabilities between the current and older design codes, underscoring the importance of modern regulations in enhancing structural resilience against seismic events.

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Assessing Effectiveness of Building Code Provisions

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  1. Assessing Effectiveness of Building Code Provisions Greg Deierlein & Abbie Liel Stanford University Curt Haselton Chico State University … other contributors (PEER TA I & ATC 63) PEER 2007 Annual Meeting

  2. EDPs: Deformations & Forces PBEE: Collapse (SAFETY) Assessment DV: COLLAPSE Decision Variable DM: Loss of Vertical Carrying Capacity (LVCC) Damage Measure Engineering Demand Parameter EDP: Interstory Drift Ratio Intensity Measure IM: Sa(T1) + Ground Motions

  3. Illustration – 4 Story SMF Building • Office occupancy • Los Angeles Basin • Design Code: 2003 IBC / 2002 ACI / ASCE7-02 • Perimeter Frame System • Maximum considered EQ demands: • Ss = 1.5g; S1 = 0.9g • Sa(2% in 50 yr) = 0.82g • Design V/W of 0.094g • Maximum inelastic design drift of 1.9% (2% limit) 8 inch PT slab Typical Perimeter Frame Members Beams: 32” to 40” deep Columns: 24”x28” to 30”x40” Governing Design Parameters - Beams: minimum strength - Column size: joint strength - Column strength: SCWB - Drift: just meets limit

  4. Nonlinear Analysis & Calibration

  5. Capacity Stats.: Median = 2.2g σLN = 0.36 0.82g is 2% in 50 year motion Incremental Dynamic Analysis – Collapse Mediancol = 2.2g σLN, col = 0.36g 2% in 50 year = 0.82g IDRcol = 7-12%

  6. Incremental Dynamic Analysis Simulation Results: Collapse Modes 27% of collapses 40% of collapses 17% of collapses **Predicted by Static Pushover 12% of collapses 2% of collapses 5% of collapses

  7. Collapse Fragility Curve Median = 2.2g sLN, Total = 0.36 Incremental Dynamic Analysis

  8. Uncertainty – Plastic Rotation Capacity Mean (m) Plastic Rotation Capacity Reduced (m-s) Plastic Rot. Cap.

  9. Correlation of Model Uncertainties Type A: Parameters within one element Type B: Between parameters of different elements

  10. Margin 2.7x P[collapse |Sa = 0.82g] = 5% 5% 0.82g 2% in 50 yrs Collapse Capacity – with Modeling Uncert. Median = 2.2g sLN, RTR = 0.36 σLN, Total = 0.64 w/mod.

  11. Mean Annual Frequency of Collapse Collapse CDF Collapse Performance • Margin: Sa,collapse = 2.7 MCE • 5% Probability of collapse under design MCE = 5% • MAFcol = 1.0 x 10-4 (0.5% in 50 years) 2.7 5% Hazard Curve 2/50

  12. The 2% in 50 year ground motion • Illustration: • Site dominated by single event (M 6.9, R 14 km) -- return period of 200 years (MAF 25% in 50 yr) • Boore-Joyner (BJ) attenuation function • Sa (25/50) -- median of BJ. At T=1 sec., Sa = 0.28g • Sa (2/50) -- +1.5s of BJ. At T=1 sec., Sa = 0.56g. Mean Annual Freq. = (Probability of Sa > Sa*, given EQ) x (MAF of EQ)

  13. Ground motion selection (+e effect) • Consider the Loma Prieta (11022 record): • Close match to characteristic event [M 6.9, R 14, Sa(T=1) = 0.65g] • Epsilon: +1.7 at T=1 sec; -0.3 at T = 0.45 sec • General trend for +epsilon records to peak at the +e periods and drop off elsewhere

  14. Effect of Spectral Shape (e) on collapse capacity +e +e

  15. Space Frame 1967 UBC, Zone 4 Design V/W: 0.068 g Member sizes Col. 20x20 to 24x24 Beam depth 20 to 26 No SCWB, no joint check, non-conforming ties 1967 and 2003 Design Comparisons 2003 Design 1967 Design • Perimeter Frame • 2003 UBC/2002 ACI • Design V/W: 0.094 g • Member sizes • Col. 24x28 to 30x40 • Beam depth 32 to 42 • Fully conforming design

  16. Comparison of 1967 vs. 2003 Designs Column Hinge Backbone Parameters Qp,cap : 1967 = 0.02 rad (COV 50%) 2003 = 0.06 rad Kc/Ke: 1967 = -0.22 (COV 60%) 2003 = -0.08 Static Pushover Response Wu : 1967 = 2.4 2003 = 2.7 Du: 1967 = 1.5% roof drift ratio 2003 = 5.0% FEMA 356 Qp limits: 1967 = 0.006 rad 2003 = 0.015 rad

  17. Incremental Dynamic Analysis, Controlling Component, 1967 Analysis Model 2.5 2 1.5 Sa (T=1.0s) [g] 1 0.5 0 0 0.05 0.1 0.15 Maximum Interstory Drift Ratio Incremental Dynamic Analysis – Sidesway Collapse Median Sa = 2.2g Median Sa = 1.0 g = col 0 g IDRcol = 7-12% IDRcol = 3-6% 1967 Design Strength: Median Sa = 1.0g, COV = 30% Deformation: IDRmax = 3 to 6% 2003 Design Strength: Median Sa = 2.2g, COV = 36% Deformation: IDRmax = 7 to 12%

  18. 50% 2.7 1.0 4% Simulated (sidesway) collapse fragility: 4-story building • FACTORS CONSIDERED • Beams & Cols: flexure-shear • B-C Joints: shear/bond • Modeling Uncertainty • Spectral Shape (e) • Margins (mcollapse/MCE) • 2003: 2.7 • 1967: 1.0 • P[C/MCE] • 2003: 4% • 1967: 50%

  19. 1967 Sidesway and Vertical Collapse (4-story) Total Collapse Probability Sidesway Collapse Probability at IMi Probability of LVCC (given drift ratio) Probability of No SS Collapse at IMi = + X • Per Elwood/Moehle & Aslani/Miranda: • Column Shear Failure: • Column IDR = 0.024 (mean) • Column Axial Failure: • Column IDR = 0.056 (mean) Shear failure reduces median capacity by about 15% Recall – Sidesway collapse occurs at peak drift ratios of 0.03 to 0.06.

  20. RC Building Archetype Study • Archetype Design Space & Parameters • heights & configurations • seismic design shears • capacity design/detailing • Archetype Analysis Model • 3-Bay Multistory • Interior/Exterior Joints • Deterioration, P-D • Archetype Index Buildings • Heights: 1, 4, 8, 12, 20 • Space & Perimeter Space Frame (Atrib/Atotal = 1.0) Perimeter Frame (Atrib/Atotal = 0.16)

  21. Effects of Codes (’67 vs ’03) and Building Heights 1967: 8 – 12 – 4 stories 2003: 12 – 8 – 4 stories Normalized Sidesway Collapse Fragilities

  22. 1967 Sidesway and Vertical Collapse: 8-story Total Collapse Probability Sidesway Collapse Probability at IMi Probability of LVCC (given drift ratio) Probability of No SS Collapse at IMi = + X • From Elwood/Moehle & Aslani/Miranda: • Column Shear Failure: • Column IDR = 0.022 (avg.) • = 0.014 (1st-story) • Column Axial Failure: • Column IDR = 0.050 (avg) • = 0.025 (1st-story) AXIAL collapse reduces median by ~ 40% Sidesway collapse occurs at peak (median) drift ratio of 0.038.

  23. SUMMARY – Key Collapse Results Simulated Sidesway Collapse Statistics 10 to 30x 5 to 12x • Including Shear-to-Axial Column Failure for 1967 Designs: • 4-story building: little change • 8-story building: significant change (column IDR = 0.025) • MAF,collapse = 190 x 10-4 c/yr (35x rate of 2003 design)

  24. Comments on Collapse Assessment • Accuracy of Assessment Procedure • stiffness/strength degrading models • characterization of ground hazard (spectral shape effect) • modeling uncertainties .. • Comparison of 1960-70’s versus modern frames • “regular” frames have 10 to 30x collapse risk • what about irregular frames? • validation & corroboration of results • appropriate level of safety? • Interpretations and Implications • communicating risks in consistent & meaningful ways • providing tools and engineering solutions (new buildings & retrofit) • action/implementation strategies

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