Envelope #1: Site walls and earthquake

Andy Lin
March 17, 2022

This is the first issue of “Back of the Envelope.” Ok, that’s quite a mouthful. Maybe we'll just call it the Envelope.

Yeah that’s better – Welcome to Envelope #1.

(FYI, sidenote: I am writing this as if I am talking to someone who has at least 'some' knowledge of structural engineering and/or has been practicing for a few years. So if there are details I skipped that make no sense to you, just let me know and I can potentially dive in further in a separate email or post. Or I can record a Tiktok/Instagram video to explain it. Jk?)

Anyways, with that said, let’s dive in.

Today I am going to talk about:

  •  Seismic load on a site (or yard) wall

Seismic Load On Site Walls

If you have a site wall or fence wall on your project that is 6’ or taller, they are supposed to be checked for seismic loads, so the thing remains standing during and after a major earthquake.

As far as I know, for about 15 years or so, the International Building Code (IBC) hasn’t been super clear about what kind of load you should apply. (Although, Uniform Building Code, aka UBC, was a different story for you “old-timers.”)

That all changed with ASCE 7-16 (which we are using in the current code cycle).

Basically, “they” (the elusive code writers) added Section 15.6.8: “Ground-Supported Cantilever Wall or Fences” that specifically says you have to design them (the walls, not the writers) per Section 15.4.

And when you go to 15.4, you basically calculate out the Cs value (aka the “seismic response coefficient” as if it’s a building but using the R value from Table 15.4-2 (appropriately named “Seismic Coefficients for Nonbuilding Structures Not Similar to Building” – or you can call it “Table SCFNSNSTB” for short).

An “R” value of 1.25 is given explicitly for “Ground-supported cantilever walls or fences.”

In other words, it’s now abundantly clear what you have to do to determine the seismic load (instead of trying to apply some forms of educated guess or engineering judgment.)

Super helpful? Yes… but there is a tiny-tiny issue.

R of 1.25 will result in loads that are quite high compared to the ‘good ol’ days’ of UBC.

Without going into too much detail, it’s generally about 2.5x compared to 15 years ago (I have my ‘back of the envelope’ calc to prove it 😉) – which is understandable if you flip through the pictures shown on FEMA E-74 Page 248 of the pdf (This is the document cited in the ASCE commentary, by the way).

Now, the issue comes when you have owners or design-build contractors getting mad at you for costing them money to meet the code 🤷‍♂️, because they are used to seeing walls and foundations that were designed for much lower loads.

I suppose one thing you can convince them by showing them some “wall failure” pictures from FEMA-74. Or, you can ask them to lower the wall to 5’-11.99” then you don’t need to meet the load requirements! (sarcasm—ish?)

Alright, that’s it for now. I want to keep this “short and sweet,” if you know what I mean. Let me know what you think and if this kind of stuff is helpful.

Yours truly,

PS: Let me know what you think of today's email?

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