Seismic design of basement walls: Evaluation the current practice in British Columbia

Transcription

1 T H E U N I V E R S I T Y O F B R I T I S H C O L U M B I A Seismic design of basement walls: Evaluation the current practice in British Columbia Mahdi Taiebat, Ph.D., P.Eng. Department of Civil Engineering, University of British Columbia Collaborators: Elnaz Amirzehni, Liam Finn, Don Anderson, Peter Byrne, Ron Devall, Ernest Naesgaard, Ali Amini [SEABC task force committee on seismic design of basement walls] 2013 UBC-Tongji-CSRN Symposium August 19-21, 2013 August19, /17

2 Estimating the seismic pressures on basement walls current practice Coulomb theory Mononobe-Okabe (M-O) theory (Credit: Ross Boulanger) Exposed basement wall adjacent to 20-m deep excavation for Four Seasons Hotel located along the Market St in San Francisco August19, /17

3 Motivations for this study 1. Recent changes in the National Building Code of Canada (NBCC) - NBCC 1995: Seismic risk of 10% in 50 yrs return period of 475 yrs PGA used in the M-O method was 0.24g for Vancouver - NBCC 2010: Seismic risk of 2% in 50 yrs return period of 2475 yrs PGA used in the M-O method is 0.46g for Vancouver 2010 National Building Code of Canada Seismic Hazard Maps August19, /17

4 Motivations for this study cont d 2. Stiff wall elements attract load - The M-O method was originally developed for rigid retaining walls with sufficient rigid body displacements to mobilize the active wedge in the soil. - In reality, basement walls have variable degree of flexibility and deformation at different depths. base figure from Budhu (2009) 3. Basement walls have historically behaved well both statically and during earthquakes under a wide range of earthquake shaking. - there have been no reports of damage to building basement walls as a result of seismic earth pressures in recent earthquakes. However, despite the absence of compelling damage or failure due to seismic earth pressures, inclusion of seismic earth pressures is required in the design of earth retaining structures and basement walls in the current building code. (Lew et al., 2010) August19, /17

5 Research approach numerical study A typical 4-level basement wall was considered for this study - The wall was designed by the structural engineer for 6 demand levels, i.e. 50%-100% PGA of NBCC2010 for Vancouver (=0.46g) - 7 earthquake motions were selected and matched to the UHS of Vancouver for the mandated demand by NBCC Nonlinear dynamic analysis was conducted on the 6 walls subjected to the 7 ground motions The analyses show that the current engineering practice for design of basement walls based on the M-O method and using the PGA=0.46g is too conservative, and a wall designed using a smaller fraction of PGA will give acceptable performance. August19, /17

6 Design of walls M-O with difference fractions of NBCC2010 PGA (0.46g) Designed moment capacities along the height of walls 50%PGA 60%PGA 70%PGA 80%PGA 90%PGA 100%PGA August19, /17

7 Numerical model building Gap (c) (d) Mahdi Taiebat (UBC) Seismic design of basement walls: Evaluation the current practice in BC August19, /17

8 Ground motion selection Selection using DGML - NBCC2010 UHS for Vancouver - Site class C - Mw= Distance range Spectral matching to the target UHS in period range of sec using SeismoMatch August19, /17

9 Soil properties Geologic approach - Layer 1 (top): 11.7 m of Till V s1 = 200 m/s, φ = 33 o, c = 0 kpa - Layer 2 (bottom): 12.6 m of Stiffer Till V s1 = 400 m/s, φ = 40 o, c = 20 kpa (Robertson, 1992) No specific site representation - Representative for non-deltaic Vancouver lithology and relevant for high-rise construction in Vancouver August19, /17

10 Lateral earth pressure distribution typical results August19, /17

11 Lateral earth force and pressure 50%PGA wall G1 August19, /17

12 Lateral earth pressure distribution Avg. of G1-G7 August19, /17

13 Lateral earth pressure at the instance of max. earth force Avg. of G1-G7 August19, /17

14 Bending moments Avg. of G1-G7 August19, /17

15 Relative displacements Avg. of G1-G7 August19, /17

16 Performance criterion 1.7% August19, /17

17 Drift ratios Avg. of G1-G7 August19, /17

18 Summary and Preliminary Conclusion A number of simulations were conducted where the walls were designed for different percentages of the NBCC2010 mandated PGA. The analyses show that the current engineering practice for design of basement walls based on the M-O method and using the PGA=0.46g is too conservative The wall designed using a 50-60% of PGA will give acceptable performance. This is a promising finding and further work in under way to look more thoroughly into different aspects of this problem such as: - Effects of using more realistic constitutive models - Effect of megathrust earthquakes - Effect of different geometries of the wall - Effects of inertial interactions -. August19, /17

19 Thank you! August19, /17

20 Effects of the normalized shear wave velocity of soil August19, /17

21 Effects of the modulus reduction and Rayleigh damping ratio of soil August19, /17

22 Effects of the friction angle of the soil wall interface element August19, /17

23 August19, /17

24 from Kramer (1996) Common type of retaining structures (Credit: Ross Boulanger) Exposed basement wall adjacent to 20-m deep excavation for Four Seasons Hotel located along the Market St in San Francisco August19, /17

25 Static pressures on retaining walls Active and passive loading - sufficient wall movement! from Budhu (2009) Methods of analysis - Rankin theory [simplest] - Coulomb theory [convenient] - Log spiral method [more accurate] August19, /17

26 Active and passive loading & everything in-between Range of static lateral earth pressure coefficients K P = 3 10 K A = from Budhu (2009) August19, /17

27 Estimation of active thrust on retaining walls Static: Coulomb theory from Kramer (1996) Seismic: Mononobe-Okabe (M-O) theory August19, /17

28 Estimating the seismic pressures on basement walls current practice Coulomb theory M O using a code mandated PGA August19, /17

29 Shear forces Avg. of G1-G7 August19, /17

30 Similar finding of other researcher by using different approaches (after Lew et al., 2010, SEOAC) (after Al Atik and Sitter, 2010, JGGE) August19, /17