• Reqirements for foundations of earthquake-resistant structures are applied automatically according to ACI 18.13


Notation

Ag = gross area of concrete section, in2
As = area of nonprestressed longitudinal tension reinforcement, in2
Ec = modulus of elasticity of concrete, psi
Es = modulus of elasticity of reinforcement, psi
fc' = specified compressive strength of concrete, psi
fy = specified yield strength for nonprestressed reinforcement, psi
Mcr = cracking moment, in.-lb
Mu = factored moment at section, in.-lb


Footings, foundation Mats, and Pile Caps

According to ACI 18.13.2.1, the provisions of ACI 18.13 should apply to structures assigned to Seismic Design Category D,E or F.

According to ACI 18.13.2.2, Longitudinal reinforcement of columns and structural walls resisting earthquake effects should extend into the footing, mat or pile cap, and should be fully developed for tension at the interface.

According to ACI 18.13.2.3, if the column is designed assuming a fixed-end, it should satisfy ACI 18.13.2.2 requirements. Also, if hooks are required, longitudinal reinforcement resisting flexure should have 90-degree hooks near the bottom of the foundation with the free end of the bars oriented toward the center of the column.

According to ACI 18.13.2.4, transverse reinforcement of columns or boundary elements of special structural walls that have an edge within one-half the footing depth from an edge of the footing should be in accordance with ACI 18.7.5.2 , ACI 18.7.5.3 and ACI 18.7.5.4 (These 3 titles are explained in Transverse Reinforcement (CRD)). This transverse reinforcement should extend into the footing, mat or pile cap a length equal to development length, calculated for fy in tension, of the column or boundary elemant longitudinal reinforcement.

According to ACI 18.13.2.5, where uplift force occurs due to earthquake effects in boundary elemants of special structural walls or columns, flexural reinforcement should be provided in the top of the footing, mat, or pile cap to resist actions resulting from the factored load combinations, and should. Minimum area of flexural reinforcement, As,min should be 0.0018Ag.

Grade Beams and Slabs-on-Ground

According to ACI 18.13.3.1, grade beams and beams that are part of a mat foundation that are part of the seismic-force-resisting system should be designed in accordance with Beams of Special Moment Frames (ACI 18.6) for structures assigned to Seismic Design Category D,E or F.

According to ACI 18.13.3.2, for structures assigned to Seismic Design Category D,E or F. slabs-on-ground that resist in-plane earthquake forces from seismic-force-resisting walls or columns should be designed as diaphragms in accordance with ACI 18.12.

Foundation Seismic Ties

According to ACI 18.13.4.1, individual pile caps, piers, or caissons should be interconnected by foundation sismic ties in orthogonal direction for strcutures assigned to Seismic Design Category C,D,E or F.

According to ACI 18.13.4.2, individual spread footings founded on soil with Site Class E or F should be interconnected by foundation seismic ties for structures assigned to Seismic Design Category D,E or F.

According to ACI 18.13.4.3, where required, foundation seismic ties should have a design strength in tension and compression at least equal to 0.1SDS times the greater of the pile cap or column factored dead load plus factored live load unless it is demonstrated that equivalent restraint will be provided by one of provisions given below;

  • Reinforced concrete beams within the slab-on-ground

  • Reinforced concrete slabs-on-ground

  • Confinement by competent rock, hard cohesive soils, or very dense granular soils

  • Other means approved by the building official

Deep Foundations

According to ACI 18.13.5.2, piles, piers, or caissons resisting tension loads should have continuous longitudinal reinforcement over their length to resist design tension forces for strcutures assigned to Seismic Design Category C,D,E or F.

According to ACI 18.13.5.3, for strcutures assigned to Seismic Design Category C,D,E or F, the minimum longitudinal and transverse reinforcement should be extended over the entire unsupported length for the portion of deep foundation member in air or water, or in soil that is not capable of providing adequate lateral restraint to prevent buckling throughout this length.

According to ACI 18.13.5.4, deep foundation hoops, spirals, and ties should be terminated with seismic hooks for strcutures assigned to Seismic Design Category C,D,E or F.

According to ACI 18.13.5.5, for strcutures assigned to Seismic Design Category C,D,E or F or located in Ste Class E or F, transverse reinforcement of deep foundation members should be in accordance with ACI 18.7.5.2 , ACI 18.7.5.3 and ACI Table 18.7.5.4(e) (These 3 titles are explained in Transverse Reinforcement (CRD)) within seven member diameters above and below the interfaces between strata that are hard or stiff and strata that are liquefiable or soft.


Uncased Cast-in-Place Drilled or Augered Concrete Piles or Piers

According to ACI 18.13.5.7.1, for strcutures assigned to Seismic Design Category C,D,E or F, reinforcement should be provided in uncased cast-in-place drilled or augered concrete piles where required by analysis and in accordance wit the requirements in ACI Table 18.13.5.7.1


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