Requirements for Foundations on Liquefiable Sites

Robert Bachman, S.E. R. E. Bachman Consulting Structural Engineers Laguna Niguel, CA

February 11, 2015 BSSC Colloquium: Seismic Design Technology for New Buildings 1

Issue Team Members

• 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

February 11, 2015 BSSC Colloquium: Seismic Design Technology for New Buildings 2

Reasons for New Provisions

• ASCE 7-10

– Section 12.13 (Foundation Design) does not include provisions pertaining to Liquefaction

– Section 11.8 (Geologic Hazards and Geotechnical Investigation) requires assessment of the following for MCEG rather than DE:

• Potential for Liquefaction and Soil Strength Loss • Assessment of potential consequences, including

– Total settlement – Differential settlement – Lateral soil movement – 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 12.13.7 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 February 11, 2015 BSSC Colloquium: Seismic Design Technology for New Buildings

Some Recent Experience with Liquefaction

• Moss Landing 1989 – MBARI

• Foundations with ties • Performed well

– MLML • 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 February 11, 2015 BSSC Colloquium: Seismic Design

Technology for New Buildings 4

MBARI

MLML

New Provisions Concepts

• Consider reduced capacities - Soil strength and stiffness at MCEG

– Must also design for non-liquefied condition for DE loadings

• Level of shaking considered: pga @ MCEG – In most of CA, MCEG is comparable

to MCER.

• Small effects may be neglected • Ground improvement may be

considered

February 11, 2015 BSSC Colloquium: Seismic Design Technology for New Buildings 5

Zip City MCER SMS MCER pga MCEG pga

90012 Los Angeles 2.52 1.01 0.9692101 San Diego 1.23 0.49 0.5594104 San Francisco 1.50 0.60 0.6095816 Sacramento 0.84 0.34 0.30

New Provisions 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 required to hold building together – Mat foundations must meet minimum reinforcing or be designed

for differential settlement

February 11, 2015 BSSC Colloquium: Seismic Design Technology for New Buildings 6

New Provisions 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)

February 11, 2015 BSSC Colloquium: Seismic Design Technology for New Buildings

7

Compliance with New Provisions

February 11, 2015 BSSC Colloquium: Seismic Design Technology for New Buildings 8

Liquefaction or Soil Strength

Loss?

Yes

No

No lateral spreading? No bearing loss?

< 1/4 of diff. settlement limit?

Yes

Shallow Foundations OK

with proper detailing

Within lateral spreading

limits?

No

OK

Within diff. settlement

limits?

Yes

No

Start

OK

Analysis shows acceptable

performance?

No

Yes No

Use Deep Foundations with proper detailing

Yes

Lateral Spreading Limits

• Table 12.13-1 – 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: 18 in. • Risk Category III: 12 in. • Risk Category IV: 4 in.

– Basis for values is experience of collapse resistance at 0.5m displacement, scaled down for higher Risk Categories.

February 11, 2015 BSSC Colloquium: Seismic Design Technology for New Buildings 9

Differential Settlement Threshold

February 11, 2015 BSSC Colloquium: Seismic Design Technology for New Buildings 10

Table 12.13-2 Differential Settlement Threshold, Δva

Structure Type Risk Category

I or II III IV Single-story structures with concrete or masonry wall systems. 0.0075L 0.005L 0.002L Other single-story structures. 0.015L 0.010L 0.002L Multi-story structures with concrete or masonry wall systems. 0.005L 0.003L 0.002L Other multi-story structures. 0.010L 0.006L 0.002L

Differential Settlement Threshold

• Building Response to Excavation-Induced Settlement – Boscardin and Cording (J. Geotech. Engrg, 1989)

February 11, 2015 BSSC Colloquium: Seismic Design Technology for New Buildings 11

g y

Multi-story structures with concrete or masonry wall systems. 0.005L 0.003L 0.002L

Differential Settlement Threshold

February 11, 2015 BSSC Colloquium: Seismic Design Technology for New Buildings 12

• 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.)

Table 12.13-2 Differential Settlement Threshold, Δva

Structure Type Risk Category

I or II III IV Single-story structures with concrete or masonry wall systems. 0.0075L 0.005L 0.002L Other single-story structures. 0.015L 0.010L 0.002L Multi-story structures with concrete or masonry wall systems. 0.005L 0.003L 0.002L Other multi-story structures. 0.010L 0.006L 0.002L

Differential Settlement Threshold

February 11, 2015 BSSC Colloquium: Seismic Design Technology for New Buildings 13

• Risk Category IV – Threshold is based on functionality, rather than safety. – ATC-58 project assessed fragility of damage onset for jammed

doors: Median drift = 0.0023. – 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 MCEG, as opposed to DE. – Therefore use 0.002.

Settlement Threshold by Analysis

February 11, 2015 BSSC Colloquium: Seismic Design Technology for New Buildings 14

• 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 – Why 67%?

• Linear analysis is sufficient for Risk Category IV – Demands on members and connections shall not exceed

nominal strengths

Usually 20% to 40%

Shallow Foundation Detailing

• Foundation Ties – Section 12.13.5.2 already requires ties between foundations with

a force equal to the larger column load multiplied by 0.10SDS. • 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:

• 𝐹𝐹𝑡𝑡𝑡𝑡𝑡𝑡 = 0.5𝜇𝜇𝑃𝑃𝑢𝑢 • 𝜇𝜇 = Coefficient of friction • 𝑃𝑃𝑢𝑢 = 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

February 11, 2015 BSSC Colloquium: Seismic Design Technology for New Buildings 15

Shallow Foundation Detailing

• Foundation Ties – Example: 𝑃𝑃𝑢𝑢𝑡𝑡 = 100𝑘𝑘 each – Coefficient of friction: 𝜇𝜇 = 0.50 (default)

February 11, 2015 BSSC Colloquium: Seismic Design Technology for New Buildings 16

𝐹𝐹𝑡𝑡𝑡𝑡𝑡𝑡 = 0.5 𝜇𝜇 =0.5 4 100𝑘𝑘 = 100k

𝑃𝑃𝑢𝑢4 𝑃𝑃𝑢𝑢3 𝑃𝑃𝑢𝑢2 𝑃𝑃𝑢𝑢1 𝐹𝐹𝑡𝑡𝑡𝑡𝑡𝑡

𝜇𝜇𝑃𝑃𝑢𝑢3 𝜇𝜇𝑃𝑃𝑢𝑢4 𝜇𝜇𝑃𝑃𝑢𝑢1 𝜇𝜇𝑃𝑃𝑢𝑢2

Shallow Foundation Detailing • Foundation Ties for Shear

– Shear resistance between adjacent lines

• Slab-on-grade – integral or connected – Mild steel ρ = 0.0025

• Alternate: diagonal system of grade beams

February 11, 2015 BSSC Colloquium: Seismic Design Technology for New Buildings 17

Shallow Foundation Detailing

• Mat foundation detailing – Reinforcing each way, top and bottom – Detail according to ACI 318-14, Section 18.6.3.1

• Minimum longitudinal reinforcing only – OR design to accommodate expected differential settlements

• Elastic design • Consistent with small permitted differential settlements

February 11, 2015 BSSC Colloquium: Seismic Design Technology for New Buildings 18

Deep Foundation Design

• Design for vertical loads + lateral deformations from DE inertial loads – Explicit design for MCEG caused lateral

spreading deformations – Reduce capacity due to MCEG 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

February 11, 2015 BSSC Colloquium: Seismic Design Technology for New Buildings 19

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 318-14 Sections 18.7.5.2 -.4

– Confinement as for special moment frame columns – At least 7 diameters below depth of liquefaction – Similar to current OSHPD and DSA requirements

February 11, 2015 BSSC Colloquium: Seismic Design Technology for New Buildings 20

𝐹𝐹𝑡𝑡𝑡𝑡𝑡𝑡=𝐹𝐹𝑝𝑝𝑝𝑝

𝐹𝐹𝑝𝑝𝑝𝑝 𝐹𝐹𝑝𝑝𝑝𝑝 𝐹𝐹𝑝𝑝𝑝𝑝 𝐹𝐹𝑝𝑝𝑝𝑝

Deep Foundation Detailing

• Foundation Ties – Basic requirement (0.10SDS) still applies – For lateral spreading, design for engagement of passive

pressure on caps and beams

February 11, 2015 BSSC Colloquium: Seismic Design Technology for New Buildings 21

Design for Lateral Resistance

February 11, 2015 BSSC Colloquium: Seismic Design Technology for New Buildings 22

• Consider pile resistance and cap, beam resistance – Use compatible deformations – Behavior of soil is nonlinear

Downdrag loading on piles

February 11, 2015 BSSC Colloquium: Seismic Design Technology for New Buildings 23

• 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

Summary

February 11, 2015 BSSC Colloquium: Seismic Design Technology for New Buildings 24

• New provisions provide – Consistency with current Code performance goals – Rational means of foundation type selection given soil behavior – Design and detailing requirements

• Shallow foundations • Deep foundations

• Coming Soon to the 2015 NEHRP Recommended Seismic Provisions for New Buildings, Standard (ASCE 7-16) and Codes (IBC 2018 and CBC 2019) near you!