ASCE 7 design loads: wind, snow & seismic, explained

Reading the design loads, in plain language

Look up site values free in the ASCE Hazard Tool. Enter an address, pick the ASCE 7 edition and risk category, and it returns the mapped wind, snow, seismic, ice, rain, and flood data. ASCE 7 itself (Minimum Design Loads and Associated Criteria for Buildings and Other Structures) is sold by ASCE; this page explains it without reproducing it.

The general notes are a summary of the engineer’s load assumptions. Risk category sets how conservative everything else is. Wind is described by the basic wind speed V, the exposure category (how rough the terrain is upwind), and the enclosure classification. Snow starts from the ground snow load pg and is adjusted for exposure, heat loss, and importance. Seismic starts from the mapped accelerations Ss and S1, adjusts for soil (site class) to get SDS and SD1, and lands on a Seismic Design Category.

Worked example. At V = 115 mph, the raw velocity pressure is 0.00256 × 115² ≈ 34 psf — that’s pure physics (the kinetic energy of moving air). ASCE 7 then adjusts it for height, exposure, terrain, and building shape. The field lesson is the square: a little more wind speed is a lot more load, which is why screened fences and tarped scaffolds fail in storms.

Why it matters to trades. Temporary works — windscreen fencing, enclosed scaffolds, tilt-up bracing, material staged on decks — see the same wind and snow as the finished building, often before the structure is complete. In higher Seismic Design Categories, electrical, plumbing, and mechanical contractors also install seismic bracing for conduit, pipe, duct, and equipment. ASCE 37 covers design loads during construction; the engineer decides how it applies.

Related: the OSHA ladder requirements chart, the crane power line clearance chart, and the IBC live loads guide. Log wind holds and weather delays with the weather delay log, and keep them with the job’s daily reports in Field PM.