Happy Thursday! Andy from Back of the Envelope here – this is where I share fun-ish SE-related things (plus a dad joke), all in 5 minutes or less.

Today’s email is going to be about “diaphragm design forces for structures with out-of-plane offset irregularity” (and as you can probably guess from the title, it has something to do with overstrength, aka amplified forces).

If you have been practicing for a while, you are probably familiar with the diaphragm design force equation from ASCE 7 in section 12.10.1.1, which shows the Fpx and all that good stuff.

And because you’ve seen it a million times, you may have missed (speaking from experience…) a new sentence that was added to the end of the section in ASCE 7-16:

(blurred out for fun – don’t open your book/pdf yet!)

If you already know what this is, you can skip to the bottom and read the dad joke. If not, read on to find out more. Let’s dive in!

(Estimated reading time = 3 minutes & 15 seconds)

Horizontal Structural Irregularity Type 4: Out-of-Plane Offset Irregularity

The blurred section shown above has much to do with the horizontal structural irregularity type 4. This is when the lateral systems (e.g., shear walls) between levels don’t stack.

Here is a graphical plan view representation from ASCE 7-16 commentary Figure C12.3-1:

Table 12.3-1 says that if your structure has this type of irregularity, it needs to meet the requirements of 12.3.3.3, 12.3.3.4, 12.7.3, Table 12.6-1, and 16.3.4.

Here is a summary of what they are:

1. Section 12.3.3.3: Elements (e.g., beams and columns) supporting discontinuous walls or frames must be designed to resist the seismic load with overstrength.

2. Section 12.3.3.4: Design forces need to be increased by 25% for “connections of diaphragms to vertical elements and to collectors,” and “collectors and their connections, including connections to vertical elements of the seismic force-resisting system.”

3. Section 12.7.3: Analysis using a 3D model is required (unless you only have a flexible diaphragm)

4. Table 12.6-1: Permitted analytical procedures (in most cases, no need to worry about this unless the building is over 160’)

5. Section 16.3.4: Nonlinear stuff – no need to worry about this for most buildings.

Now, since the diaphragm itself is not technically supporting the walls above, section 12.3.3.3 doesn’t apply to its design.

It’s also not specifically mentioned in section 12.3.3.4.

So one of the confusions that has existed forever is whether the diaphragm needs to be designed for overstrength since it’s transferring the lateral load from above.

For example, let’s add some hypothetical dimensions and forces to the plan view:

In this example,

• P1 and P2 are the unfactored lateral loads from shear walls above

• R1 and R2 are the diaphragm reactions (i.e., forces going down to the shear walls below)

• V1, V2, and V3 are the diaphragm shears at three locations

To satisfy Section 12.3.3.4, R1 and R2 must be increased by 25%.

This could mean different things depending on the material.

For example:

• In concrete structures, this would be the dowels.

• For steel structures with composite construction, this would be the welded studs.

• For light-frame wood, this would be the shear clips (e.g., see here and here)

Now you might be wondering -- what about V1, V2, and V3? Do those need to be increased for the design of the diaphragm itself?

And that’s where the blurred section in 12.10.1.1 mentioned earlier comes in.

It reads:

Lots of words, but here is a condensed version that I came up with:

“The transfer forces from [shearwall, moment/braced frame] above shall be increased by the overstrength factor before being added to the diaphragm inertial forces.”

Thanks to this, now it’s pretty clear that P2 shall be increased by overstrength first, then we calculate the diaphragm shears accordingly.

And that’s all! Hope that makes sense. Let me know if you have any questions.

(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 word becomes shorter when you add two letters?

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A: Short

Thanks for reading!

Andy Lin
Back of the Envelope/Structural Engineer HQ