Happy Thursday! Welcome to Back of the Envelope. This is email #20! My goal is to get to at least #50, then decide if I should continue. For now, let’s talk about some metal studs!

Here is a scenario that happened recently.

A client called up and asked:

Hmm good question.

Let’s dissect that and come up with a solution in 2 minutes and 15 seconds!

What do we know?

1/ First, we know there are gypboard on both sides, so studs are considered fully braced.

2/ Interior non-bearing wall means we must design it for at least 5 psf lateral load.

3/ 3-5/8” x 20ga non-bearing stud means the member designation is 362S125-33. (for more info, see SFIA documents)

4/ The finishes are brittle, so the deflection criteria is L/240.

What are the constraints?

1/ Floor plan is pretty set-in-stone, so changing to 4” or 6” studs is not ideal.

2/ Using a thicker gauge is also not an option since steel transmits sound waves – a heavier gauge actually lowers the STC rating.

In other words, we need to increase the spacing but can’t increase the depth nor the gauge. Interesting challenge – how do we do that?

Oh, by increasing the flange width, you say? Smart. Thanks batman.

Let’s see.

Stud Check

362S125-33 @ 16” non-composite

First, I was curious to see how close we are with 362S125-33 @ 16”.

Pulling up SFIA table (equivalent-ish of SSMA) for “Interior Non-Structural Non-Composite”:

The maximum height is only 15’-5”… not quite close to the intended 16’. Perhaps the architect had specified the proper assembly to allow for composite?

Let’s see if that works.

362S125-33 @ 16”o.c. composite

Pulling up SFIA table for “Interior Non-Structural Composite”:

Alright, that’s better (assuming the correct screw, spacing, etc., are specified).

Now, how close are we using the same stud but at 24”?

362S125-33 @ 24”o.c. composite

Short by 9”. Let’s try 2” flange instead of 1.25”.

362S200-33 @ 24”o.c.

Since the size is not typically used for non-structural applications, it’s not listed on any of the “non-structural” tables. We have to use the Curtain Wall (Single Span) table instead.

Strangely, SFIA doesn’t list allowable height for 5 psf, so we have to open up the SSMA technical guide (maybe we should’ve done that since the beginning…):

That worked for 16’. And the height is based on non-composite so we are all golden.

Shortcut

Well, that was fun – looking up tables without running numbers is nice sometimes.

And what if I tell you, there is even a quicker way?

ClarkDietrich has a suite of free online tools to help you specify their proprietary studs – but part of it also includes a lookup filter for generic studs.

Check it out here: (link)

First you enter all the good stuff that we just talked about:

And the online app automatically gives you the sections that work!

Alrighty that’s all for now – thanks for reading. Let me know if you have any questions and I can perhaps cover them in future emails.

Have a good one!

Andy Lin
Back of the Envelope/Structural Engineer HQ