Seismic analysis to ASCE 7-16

What seismic forces are and where they come from

Unlike wind or live loads, which act on a building from outside, seismic forces originate within the building itself. Laurent explained this using Newton's second law: when the ground accelerates during an earthquake, the mass of the building resists that motion, generating inertial forces throughout the structure. The heavier the building, the larger those forces.

The magnitude of those forces also depends on the fundamental period of the structure, which is the rate at which the building naturally wants to sway back and forth. Laurent demonstrated with a shake-table video showing three buildings of different heights: as the table cycled through different frequencies, each building resonated at a different point. The same earthquake can cause severe damage to buildings of one height while leaving taller or shorter structures largely unaffected. Period is therefore a critical parameter in any seismic analysis, and ASCE 7-16 ties nearly every calculation back to it.

Site class, spectral accelerations, and seismic design category

The USGS compiles acceleration data from seismometers across the country into a response spectrum for each location. ASCE 7-16 condenses that spectrum into three values: Ss (short-period acceleration), S1 (1-second acceleration), and TL (the long-period transition). Laurent walked through the USGS ASCE 7 online hazard tool live, entering coordinates for Page, Arizona and reading off Ss = 0.31, S1 = 0.097, and TL = 6 seconds.

Site class modifies those mapped values to reflect local soil conditions. ASCE 7-16 defines classes A through F based on shear wave velocity. Site class D is the default when no geotechnical report exists, which is typical for small residential work. Laurent noted that confirming a better site class, such as class B or C on hard rock, can reduce seismic loads by up to half in some cases, which may justify the cost of a geotechnical investigation on larger projects.

Seismic design category (SDC) is then assigned from A through F based on the design accelerations and the building's risk category. SDC governs which structural systems are permitted and what detailing is required. Laurent highlighted that ordinary reinforced concrete shear walls are allowed without height limits in SDC B and C but are prohibited entirely in SDC D, E, and F, where special concrete shear walls with additional detailing must be used instead. Confirming the SDC early drives every subsequent system selection decision.

The seismic force-resisting system and its key coefficients

The seismic force-resisting system (SFRS) is whatever keeps the building from tipping sideways during ground shaking: wood shear walls, steel cross bracing, portal frames, concrete shear walls, and masonry walls are common examples. ASCE 7-16 assigns three coefficients to each permitted system type.

The response modification coefficient R captures how much energy a ductile system can absorb through yielding. Laurent explained that nails in a wood shear wall or fibers in a steel moment frame yield progressively as the building sways, dissipating energy and allowing the design forces to be reduced substantially. An R factor of 5 cuts the design force from, say, 100 kips down to 20 kips.

The over-strength factor covers components in the load path that cannot yield, such as concrete anchors and collectors between shear walls. Laurent gave the example of a rim joist collecting forces from a shear wall above and delivering them to a shear wall below: if the shear walls yield at a higher force than nominal, the collector must still remain intact, so its design force is amplified by the over-strength factor to account for that scenario.

The deflection amplification factor Cd works in the opposite direction: because the code allows lower design forces by crediting yielding, the actual deflections under a design earthquake will be larger than an elastic analysis suggests. Cd scales up the calculated elastic deflection to reflect the true inelastic sway, which becomes important for P-delta checks and story drift limits.

Equivalent lateral force procedure and the worked example

ASCE 7-16 offers three analysis methods. For low-rise residential and light commercial work, the equivalent lateral force (ELF) procedure is the standard approach. Laurent explained that ELF converts the dynamic earthquake problem into a set of static forces applied at each floor level, using relatively simple equations that have been calibrated against years of observed building behavior.

The seismic response coefficient Cs is computed from the design accelerations, the period, R, and the importance factor Ie. For a typical one-story house with wood shear walls, the period is very short and the building falls in the flat portion of the response spectrum, so the period has little practical effect on the result. Seismic base shear is then V = Cs times W, where W is the effective seismic weight.

Effective seismic weight includes all dead load, 10 PSF for partitions (unless partitions are accounted for directly), and 25 percent of storage live load. Regular occupancy live loads are excluded because people can move with the building and their lateral contribution is negligible by research. Roof snow load inclusion has a threshold: an attendee during the session flagged that the applicable percentage and threshold may differ between ASCE 7-10, 7-16, and 7-22, and Laurent directed everyone to verify against the code directly rather than rely on a rule of thumb.

Laurent ran through a live demonstration in Calcs.com using a one-story wood-frame house in Page, Arizona with a 25 PSF roof dead load. After entering the USGS hazard values, story height, and seismic weight, the calculator produced the story shear and confirmed load linking to a shear wall calculation so that any change to the seismic weight automatically flows through to the wall design without manual copy-pasting.

What changes in ASCE 7-22

Laurent closed with a summary of changes coming with ASCE 7-22, which will be referenced by IBC 2024. Three differences are relevant for the engineers in the room.

First, seismic design values must now come directly from the USGS web tool, which incorporates site class amplification automatically. The separate Fa and Fv table lookups from 7-16 are no longer part of the standard workflow.

Second, three new intermediate site classes (BC, CD, and DE) have been added between the existing classes. When no geotechnical report is available, the default procedure now requires checking site classes C, CD, and D and using the worst-case result, rather than defaulting directly to class D.

Third, the code has added R, over-strength, and Cd values for cross-laminated timber (CLT) shear walls, filling a gap that previously left CLT designers without formal seismic guidance. Additional new entries cover certain masonry and steel systems as well.