Requirements for Foundations on Liquefiable Sites

Transcription

1 Requirements for Foundations on Liquefiable Sites Robert Bachman, S.E. R. E. Bachman Consulting Structural Engineers Laguna Niguel, CA 1

2 Issue Team and ASCE 7 Contributing Members Evolved and Improved at Each Stage of Development Bob Bachman, Chair CB Crouse, URS Geoff Martin, USC Lori Simpson, Langan Treadwell Rollo & Deep Foundation Institute Gyimah Kasali, Rutherford & Chekene Dom Campi, Rutherford & Chekene Jon Siu, City of Seattle Omar Jaradat, Moffatt & Nichol Steve Harris, Simpson Gumpertz & Heger And TC-3 of the ASCE 7 Seismic Subcommittee Martin Johnson, ABS Consulting Ron La Plante, DSA 2

3 Reasons for New Provisions ASCE 7-10 Section (Foundation Design) does not include provisions pertaining to Liquefaction Chapter 18 of IBC now refers to ASCE 7 for liquefaction - POLA Section 11.8 (Geologic Hazards and Geotechnical Investigation) requires explicit assessment of the following for MCE G rather than DE for SDC D, E and F: Potential for Liquefaction and Soil Strength Loss Assessment of potential consequences, including Total settlement Differential settlement Lateral soil movement due to lateral spreading Reduction in soil bearing and lateral capacity Downdrag on piles Gapping Hole No foundation requirements if liquefaction potential exists at site Huge range on what is being done in practice New provisions (provided in new Section of ASCE 7-16) will fill gapping hole by providing - Specific requirements for design and level of ground improvement needed if shallow foundations are to be used - Specific requirements for deep foundations at liquefiable sites 3

4 Some Recent Experience with Liquefaction Moss Landing 1989 Monterey Bay Aquarium Research Institute Foundations with ties Performed well Moss Landing Marine Lab No foundation ties Collapse due to lateral spreading Kocaeli 1999 Large differential settlements resulted in building collapses Christchurch 2012 Large differential settlements resulted in irreparable tilts Pile foundations and foundation ties improved performance MBARI MLML 4

5 New Requirement Concepts Consider reduced capacities - Soil strength and stiffness at MCE G Must also design for non-liquefied condition for DE loadings Level of shaking considered: MCE G In most of CA, MCE G is comparable to MCE R. Zip City MCE R S MS MCE R pga MCE G pga Los Angeles San Diego San Francisco Sacramento Small levels of liquefaction may be neglected (1/4 of Table ) Ground improvement may be considered 5

6 Performance Criteria for Liquefaction NEHRP Recommended Provisions (FEMA P-1050) One implicit set of goals, two explicit approaches Structures designed for Design Earthquake (Life Safety) Liquefaction assessed for MCE G (Collapse Prevention) 6

7 Performance Criteria for Liquefaction Structural Performance Levels Joe s Joe s Beer! Food! Beer! Food! Beer! Food! Operational Immediate Occupancy Life Safety Collapse Prevention What does this mean for foundation design? Life Safety: Maintain gravity and lateral support with some margin Collapse Prevention: Maintain gravity support 7

8 New Requirement Concepts Shallow Foundations Allowed only in certain cases (considering ground improvement): Estimated lateral spreading less than upper limit Estimated differential settlement less than threshold limit or show adequacy by analysis Foundation ties/post tensioning required to hold building together Mat foundations must meet minimum reinforcing or be designed for differential settlement 8

9 New Requirement Concepts Deep Foundations Allowed in all cases Design for normal vertical loading, plus downdrag Piles not expected to remain elastic under lateral spreading Residual strength requirement show adequacy by analysis Ductility through detailing Foundation ties hold footings together (must consider racking) 9

10 Compliance with New Requirements Start Liquefaction or Soil Strength Loss? Yes No lateral spreading? No bearing loss? < 1/4 of diff. settlement limit? Yes OK No No OK Within lateral spreading limits of Table ? No Use Deep Foundations with proper detailing Yes No Within diff. settlement Limits of Table ? No Analysis shows acceptable performance? Yes Yes Shallow Foundations OK with proper detailing 10

11 Lateral Spreading Limits Table Upper Limit on Lateral Spreading Horizontal Ground Displacement for specially tied together Shallow Foundations or Mats Beyond which Deep Foundations are Required Risk Category I or II: Risk Category III: Risk Category IV: 18 in. 12 in. 4 in. Basis for values is experience of collapse resistance at 0.5m displacement, scaled down for higher Risk Categories. 11

12 Differential Settlement Threshold Table Differential Settlement Threshold, Δ v a Structure Type Risk Category I or II III IV Single-story structures with concrete or masonry wall systems L 0.005L 0.002L Other single-story structures L 0.010L 0.002L Multi-story structures with concrete or masonry wall systems L 0.003L 0.002L Other multi-story structures L 0.006L 0.002L 12

13 Differential Settlement Threshold Building Response to Excavation-Induced Settlement Boscardin and Cording (J. Geotech. Engrg, 1989) g y Multi-story structures with concrete or masonry wall systems L 0.003L 0.002L 13

14 Differential Settlement Threshold Table Differential Settlement Threshold, Δ v a Structure Type Risk Category I or II III IV Single-story structures with concrete or masonry wall systems L 0.005L 0.002L Other single-story structures L 0.010L 0.002L Multi-story structures with concrete or masonry wall systems L 0.003L 0.002L Other multi-story structures L 0.006L 0.002L Risk Category III thresholds are generally ~2/3 of Risk Category II thresholds Single-story thresholds are generally 50% higher than multi-story thresholds Structures without stiff walls are permitted twice the differential settlement. (Values are consistent with drifts for high-ductility frames in ASCE-41.) 14

15 Differential Settlement Threshold Risk Category IV Threshold is based on functionality, rather than safety. ATC-58 project assessed fragility of damage onset for jammed doors: Median drift = Median is divided by 1.5 to account for statistical dispersion (90% non-exceedance) Result is multiplied by 1.5 to account for settlement assessment at MCE G, as opposed to DE. Therefore use

16 Settlement Threshold by Analysis Nonlinear analysis (static OK) required for Risk Category II and III Residual strength of members and connections shall not be less than 67% of nominal strength some degradation OK Usually 20% to 40% Linear analysis is sufficient for Risk Category IV Demands on members and connections shall not exceed nominal strengths 16

17 Shallow Foundation Detailing Foundation Ties Section already requires ties between foundations with a force equal to the larger column load multiplied by 0.10S DS. This is also in CBC Chapter 18, with some slightly different words. Where expected lateral spreading exceeds 3 inches, additional requirements apply in the new provisions: FF tttttt = 0.5μμPP uu μμ = Coefficient of friction PP uu = Sum of factored loads along a line Intended to hold the building together when one part moves relative to another Design for worst condition soil under half the building is moving and other half isn t 17

18 Shallow Foundation Detailing Foundation Ties Example: PP uuii = 100kk each Coefficient of friction: μμ = 0.50 (default) PP uu1 PP uu2 PP uu3 PP uu4 FF tttttt μμμμ uu1 μμμμ uu2 μμμμ uu3 μμμμ uu4 FF tttttt = 0.5 μμ = kk = 100k 18

19 Shallow Foundation Detailing Foundation Ties for Shear Shear resistance between adjacent lines Slab-on-grade integral or connected Mild steel ρ = or 100 psi min pre-compression PTI DC10.5 D & A Post-Tensioned Concrete Slab on Expansive Soils Alternate: diagonal system of grade beams 19

20 Shallow Foundation Detailing Mat foundation detailing (or Post Tensioned) Reinforcing each way, top and bottom Detail according to ACI , Section Minimum longitudinal reinforcing only OR design to accommodate expected differential settlements Elastic design Consistent with small permitted differential settlements 20

21 Performance Criteria for Deep Foundations Design for Collapse Prevention at MCE G Nonlinear structural behavior allowed More difficult engineering Plastic hinges form in piles NEHRP 2015/ASCE7-16 Requirements Pile must maintain gravity support Pile flexural strength must not degrade significantly Special detailing is necessary for ductility Pile must remain elastic in shear Double plastic hinge 21

22 Deep Foundation Design Design for vertical loads + lateral deformations from DE inertial loads Explicit design for MCE G caused lateral spreading deformations Reduce capacity due to MCE G caused liquefied condition Vertical downdrag Lateral softer p-y springs, Lateral Resistance reduced by liquefaction Passive pressure and friction on caps, beams, walls Resistance of soil on piles 22

23 Deep Foundation Detailing Design for Lateral Spreading Analysis Nonlinear analysis required, except for very small deformations Ability to carry gravity load shall not be compromised Residual pile lateral strength must remain at least 67% of nominal Pile shear demand must remain less than nominal capacity Detailing Steel piles must meet requirements for highly ductile members Concrete piles must comply with ACI Sections Confinement as for special moment frame columns At least 7 diameters below depth of liquefaction Similar to current OSHPD and DSA requirements 23

24 Deep Foundation Detailing Foundation Ties Basic requirement (0.10S DS ) still applies For lateral spreading, design for engagement of passive pressure on caps and beams FF tttttt =FF pppp FF pppp FF pppp FF pppp FF pppp 24

25 Design for Lateral Resistance Consider pile resistance and cap, beam resistance Use compatible deformations Behavior of soil is nonlinear 25

26 Downdrag loading on piles Downdrag assessed at ultimate level Safety factor applied to net ultimate capacity Downward (negative) skin friction within and above soils subject to liquefaction Downdrag load is considered a seismic load (1.0 load factor) Liquefied soil 26

27 Summary New requirements provide Consistency with current Code performance goals Rational means of foundation type selection given soil behavior Design and detailing requirements Shallow foundations Deep foundations In the 2015 NEHRP Recommended Seismic Provisions for New Buildings (FEMA P-1050) Coming Soon to ASCE 7-16 and Building Codes (IBC 2018 and CBC 2019) near you! 27

28 Questions? 28