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Mahdi Taiebat (UBC) August19, 2013 0/17Seismic design of basement walls: Evaluation the current practice in BC

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


• Coulomb theory

• Mononobe-Okabe (M-O) theory

Estimating the seismic pressures on basement walls – current practice

Exposed basement wall adjacent to 20-m deep excavation for Four Seasons Hotel located along the Market St in San Francisco

(Credit: Ross Boulanger)


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

http://earthquakescanada.nrcan.gc.ca/hazard-alea/zoning/NBCC2010maps-eng.php


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)


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 NBCC2010

- 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.


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


Numerical model building

Gap

(c)

(d)


Ground motion selection

• Selection using DGML

- NBCC2010 UHS for Vancouver

- Site class C

- Mw=6.5–7.5

- Distance range 10–30

• Spectral matching to the target UHS in period range of 0.05–2.0 sec using SeismoMatch


Soil properties

•Geologic approach

- Layer 1 (top): 11.7 m of Till

Vs1 = 200 m/s, φ = 33o, c = 0 kPa

- Layer 2 (bottom): 12.6 m of Stiffer Till

Vs1 = 400 m/s, φ = 40o, c = 20 kPa

•No specific site representation

- Representative for non-deltaic Vancouver lithology and relevant for high-rise construction in Vancouver

(Robertson, 1992)


Lateral earth pressure distribution – typical results


Lateral earth force and pressure – 50%PGA wall – G1


Lateral earth pressure distribution – Avg. of G1-G7


Lateral earth pressure at the instance of max. earth force – Avg. of G1-G7


Bending moments – Avg. of G1-G7


Relative displacements – Avg. of G1-G7


Performance criterion

1.7%


Drift ratios – Avg. of G1-G7


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

- ….


Thank you!


Effects of the normalized shear wave velocity of soil


Effects of the modulus reduction and Rayleigh damping ratio of soil


Effects of the friction angle of the soil–wall interface element



Common type of retaining structures

fro

m K

ram

er (

19

96

)

Exposed basement wall adjacent to 20-m deep excavation for Four Seasons Hotel located along the Market St in San Francisco

(Credit: Ross Boulanger)


Static pressures on retaining walls

• Active and passive loading

- sufficient wall movement!

• Methods of analysis

- Rankin theory [simplest]

- Coulomb theory [convenient]

- Log spiral method [more accurate]

from Budhu (2009)


Active and passive loading & everything in-between

• Range of static lateral earth pressure coefficients

from Budhu (2009)

KA=0.2–0.4

KP = 3–10


• Static: Coulomb theory

• Seismic: Mononobe-Okabe (M-O) theory

Estimation of active thrust on retaining wallsfrom Kramer (1996)


Estimating the seismic pressures on basement walls – current practice

• Coulomb theory

• M–O using a code mandated PGA


Shear forces – Avg. of G1-G7


Similar finding of other researcher by using different approaches

(after Al Atik and Sitter, 2010, JGGE)

(after Lew et al., 2010, SEOAC)

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Seismic design of basement walls: Evaluation the current ...smartstructures-civil.sites.olt.ubc.ca/files/2014/12/2-Taiebat-UBC... · Mahdi Taiebat (UBC) Seismic design of basement