9.2 Capacity of Foundations
The ratio of the effective horizontal dimension, at the foundation level of the seismic-resisting system, to the building height (base/height) exceeds 1.4 Av.
For conforming buildings, the evaluator may consider this condition as mitigated, and no calculations are necessary. The deficiency is the concentration of seismic inertial response into narrow elements by the seismic-resisting system, which may overcome the ability of the foundation elements, either structure or soil, to provide adequate resistance. For shallow foundations, evaluate the shear and moment capacity of the foundation elements for adequacy to resist calculated seismic forces. Evaluate the vertical bearing pressure of the soil under seismic loading conditions due to the total gravity and overturning loads and compare to two times the allowable static-bearing pressure. For deep foundations, evaluate the ultimate vertical capacity of the pile or pier under seismic loads. Compare the foundation capacity to the gravity loads plus the overturning loads.
Foundation ties adequate for seismic forces exist where footings, piles and piers are not restrained by beams, slabs, or competent soils or rock.
For conforming buildings, the evaluator may consider this condition as mitigated, and no calculations are necessary. The deficiency is the possibility of significant differential lateral deformations of the foundations. Evaluate the lateral restraint to seismic forces provided by the foundation materials or the structural ties. For shallow foundations, evaluate the horizontal capacity of the foundation soils under seismic loading conditions (the lateral resistance of the footings due to passive resistance on affected sides of the footings plus the friction on the base of the footings) and compare to the base shear of the building. In the evaluation of base friction, consideration shall be given to the effect of the vertical component of ground motion.
The pile caps are capable of transferring overturning and lateral forces between the structure and individual piles in the pile cap.
The deficiency is insufficient capacity of the pile cap to transfer seismic forces from the superstructure to the individual piles. Check the moment and shear capacity to transfer uplift and lateral forces from the point of application on the pile cap to each pile. Conforming buildings which fail this check shall be placed in SPC 4.
Piles and piers are capable of transferring the lateral forces between the structure and the soil.
The deficiencies include inadequate flexural strength and ductility of piles or piers at the connection to the cap and the upper portion of the pile. Compare the maximum lateral resistance of soil against piles or piers and caps against the demand. For concrete piles, check for a minimal amount of longitudinal reinforcement in the upper portion of piles or piers and for hoops or ties immediately beneath the caps. Also check for confining transverse reinforcement wherever bending moments might be high, including changes in soil stiffness. Conforming buildings which fail this check shall be placed in SPC 4.
Pole foundations have adequate embedment.
For conforming buildings, the evaluator may consider this condition as mitigated, and no calculations are necessary. The deficiency is inadequate strength of the pole foundation. Check lateral force resistance of embedded poles using conventional procedures, comparing with conventional allowable pressures times 1.5.
The grade difference from one side of the building to another does not exceed one-half story.
For conforming buildings, the evaluator may consider this condition as mitigated, and no calculations are necessary. If this statement is false, include the horizontal force due to the grade difference, appropriately modified for seismic motions, with the seismic inertial force when checking sliding stability and the lateral-force-resisting system below grade.