Happy Thursday! This is Back of the Envelope, where I share bite-sized SE stuff I learned (or re-learned…) recently.

Today, I am going to share something slightly embarrassing. I had forgotten about a key structural consideration during plan check for one of the projects I am working on:

…structural separation.

Let me tell you how I missed it (and how we fixed it), so you can avoid the same slipup.

(Estimated reading time = 1 minute & 42 seconds )

Background

This is a four-story wood over concrete podium project that had undergone several redesigns by the time I inherited it.

Changes were still continuously being made by the owner and the architect, but we managed to complete the structural design and submit the project to plan check.

The project came back with several standard comments – one of which mentioned that we need to check for building separation.

My initial gut reaction was…

What building separation?

This building is more or less isolated, and the building next door is not even close to it.

Then it hit me…

The checker is talking about the property line.

In fact, ASCE 7-16 12.12.3 states that a structure shall be set back from the property line by the maximum inelastic response displacement.

Oops. I knew that!

…but had forgotten because, somehow, I haven’t encountered this situation in recent projects.

Anyhow, after my colleague did some numbers crunching, we determined that we needed 6”, but the architect only allowed 4” (which may or may not have been based on a prior design iteration).

Luckily, by specifying OSB instead of plywood for shearwalls, we were able to make the displacements work 😅 (OSB is much stiffer – that could be another future Back of the Envelope discussion: OSB vs Plywood).

Key Takeaways

This is what I learned:

1. Don’t take memories for granted… especially as I get older ha

2. Have a QAQC checklist to make sure key considerations are not missed

3. Meeting the allowable story drift per Table 12.12-1 alone may not be sufficient, especially when the edge of the building is close to a property line or another building

4. Drift/displacement calc shall be periodically monitored as the project progresses/changes to make sure it still meets the design requirements

That’s all – hope it’s helpful!

Dad Joke

(A colleague once suggested I include a dad joke in the Back of the Envelope because, first, I love those. Second, if you didn’t learn anything from these emails, at least you’ll get a quick laugh!)

Q: What kind of music should you listen to while fishing?

.

.

.

A: Something catchy!

Thanks for reading!

Andy Lin
Back of the Envelope/Structural Engineer HQ