Seismic · Code
ASCE 7-22 quietly made borings insufficient for site class
Nine site classes instead of six, and shear-wave velocity is now the only accepted basis for assigning most of them.
For years a site class could be argued from a boring log. Standard penetration resistance or undrained shear strength would do, and on many projects nobody measured a velocity at all. ASCE 7-22 ended that. The distinction between site classes is now based on shear-wave velocity alone, and the six familiar classes became nine with the addition of BC, CD and DE.
The practical consequence is easy to miss because nothing on the drawing looks different. A geotechnical report still arrives, it still contains a site class, and the structural engineer still designs to it. But if that class was assigned from blow counts, it no longer rests on an accepted basis.
What the code now asks for
Classes C through E are assigned from the average shear-wave velocity over the top 100 ft, and that average is a harmonic mean rather than an arithmetic one — soft near-surface layers weigh far more heavily on the result than their thickness suggests. Site Class B can only be assigned where velocity has been measured on site. So can A.
Where nobody measures, the default applies: a combination class that takes the most critical spectral response across C, CD and D. It is deliberately conservative, because it has to cover the possibility that the ground is worse than anyone checked.
Why this has become a data center problem
Large-footprint, fast-track projects feel this most. Seismic design governs a great deal of structure on a big slab, the schedule leaves no room to discover a problem late, and the permit set needs a defensible number rather than an assumption. Warehousing, manufacturing and healthcare are running into the same requirement.
Measuring it without a borehole
Two surface methods answer the question directly, and they complement each other:
- MASW — multichannel analysis of surface waves. An active source, with Rayleigh-wave dispersion inverted to a velocity profile. Strong resolution through the shallow section.
- ReMi — refraction microtremor. Passive, using ambient traffic and plant noise as the source, so it works on live industrial sites and reaches deeper than an active source alone.
Run together they extend the dispersion curve at both ends and produce a better-constrained profile than either on its own. Neither requires a borehole, coring, or anything left in the ground. Most sites are a single day.
What you should ask for
A shear-wave velocity profile, the computed V̄s30, and the site class stated against ASCE 7-22 — in a report your structural engineer can put in the permit set. If a report gives you a class without a velocity profile behind it, ask how it was derived.
Whether a measured class reduces your design demand is the structural engineer’s call, not ours. What measurement changes is that the decision is made on evidence rather than on the conservative assumption the code applies when nobody looked.