Strength Based Foundation Design

Martin Johnson

Current Design Practice

1. Above-ground structure designed using USD (or ASD) Load Combinations

2. Foundations are proportioned using ASD Load Combinations

3. Foundation reactions are recalculated into USD Load Combinations

4. Steel and Concrete Foundation components are designed

Design Basis • Traditional engineering practice

– Foundation design is based on allowable stresses, with allowable foundation load capacities for dead plus live loads, to limit static settlements.

– Provides factor of safety against exceeding ultimate capacities. – Allowable soil stresses for dead plus live loads are increased

arbitrarily by one-third for load combinations that include wind or seismic forces.

– Conservative and not based on explicit consideration of the expected strength and dynamic properties of the site soils.

• Strength-based design of foundations facilitates more direct satisfaction of the design basis.

New Definitions

• FOUNDATION GEOTECHNICAL CAPACITY: – The capacity of a foundation based upon the

supporting soil, rock or controlled low-strength material.

• FOUNDATION STRUCTURAL CAPACITY: – The design strength of foundations or internal

foundation components • determined in accordance with adopted material

standards (typically either ACI or AISC standards).

Factored Load Combinations

• Load Combinations are identical as those used to design the supported structure. – Foundation dead loads need to be included.

– 25% (for ELF analysis) and 10% (for modal analysis) Reduction in Foundation Overturning Provisions … (Unless inverted-pendulum or cantilevered-column type structure) …are still permitted.

Geotechnical Resistance Factors, ø

Direction and Type of Resistance Resistance

Factors

Vertical Resistance Compression (Bearing) Strength 0.45 Pile Friction (either upward or downwards) 0.45 Horizontal Resistance Lateral Bearing Pressure 0.5 Sliding (by either Friction or Cohesion) 0.85

Based on values from AASHTO LRFD Bridge Design Specifications.

Nominal Strength

• The Nominal Strength is characteristically defined as the strength that is determined using conservative estimates of material properties.

• Example: Where the Nominal Strength of the soil beneath a foundation is determined based upon the Specified Minimum Compaction of the soil beneath the foundation.

Nominal Strength, Qns

• The nominal foundation geotechnical capacity, Qns, determined using any of the following methods: – presumptive load-bearing values, – by a registered design professional based on

geotechnical site investigations that include field and laboratory testing to determine soil classification and as-required active, passive and at-rest soil strength parameters, or

– by in-situ testing of prototype foundations.

Presumptive Load Bearing Values

• Found in IBC Table 1806.2. • permitted to be multiplied by 3.0 when used

with the strength design load combinations. • No additional increases to the resulting values

of vertical foundation pressure or lateral bearing pressure are permitted for load combinations that include wind or earthquake loads (i.e., no additional 1/3 stress increase).

Acceptance Criteria

• For linear seismic analysis procedures, factored loads, including reductions where permitted, shall not exceed foundation design strengths, = ø Qns.

• Example, for: ø = 0.45, ø Qns = 0.45 x 3.0 x ASD = 1.35 x ASD ≈ equal to the typical ratio of USD/ASD load combinations

• Thus, near parity between ASD and USD Methods.

ASCE 7-16 Proposal

• Nearly identical, except: – Occasional slight differences in wording. – ø = 0.8 is permitted when nominal

foundation capacity is determined by AHJ-approved testing of full-scale prototypes.

Example: ASD vs. USD Design

• Forces at base of footing • D = 100 kip, L = 50 kip, E = +/-10 kip vert., 30 kip horiz.

• ASCE Load Combinations

ASD Load Combinations D + L = 150 k (vert)

D + 0.75 (L + 0.7 E) = 145.5 k (vert) = 16.9 k (horiz)

D – 0.7 E = = 107 k (vert) = 21 k (horiz)

USD Load Combinations 1.2 D + 1.6 L = 200 k (vert) 1.2 D + L + 1.0 E = 180 k (vert) = 30 k (horiz) 0.9 D – 1.0 E = = 80 k (vert) = 30 k (horiz)

• Soil = sandy gravel

Example: ASD vs. USD Design

ASD Design Allowable vertical pressure = 3.0 ksf Lateral sliding friction coeff. = 0.35 Sliding, D/C = 21 k / (0.35 x 107 k) = 0.56 Try 7.5’ x 7.5’ ftg., A = 56.25 sf Max vert. pressure = 150 k / 56.25 sf = 2.67 ksf < 3, ok D/C = 0.89

USD Design Resistance factor, Ø Compressive bearing, Ø = 0.45 Lateral Sliding, Ø = 0.85 Nominal Strength, Qns = 9.0 ksf Ø Qns = 0.45 x 9 = 4.05 ksf Sliding, D/C = 30 k / (0.85 x 80k) = 0.44 Try 7.5’ x 7.5’ ftg Max vert. pressure = 200 k / 56.25 sf = 3.56 ksf < 4.05, ok D/C = 0.88