How to Read Deflection Limits Table IBC Table 1604.3

Hi there,

Can anyone help explain how to read Table 1604.3 regarding deflection limits? At the beginning of chapter 16, I see that abbreviations are explained, which I have also included as a screen shot.

Specifically, I am unsure which of the three highlighted columns to use, as each respective column refers to a very limited scope of loading:

  • Column 1: L or Lr refers only to roof live loading
  • Column 2: S or W refers only to snow and wind loads
  • Column 3: D + L refers only dead loads and the roof load.

If I am looking to find the deflection limit of a beam supporting the a floor in the middle of a building, which column do I use?

Thanks in advance!


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@coachdarionziegler would you be able to help with this please?

The question stems from a practice problem which says column 1, which only accounts for roof loads, is correct in this instance. Can you please help explain how this is correct for a floor beam, which would be supporting a lot more than just the roof?

Thanks!

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Hi @elisa.ervin ,

My appologies for the slow response, I was travelling for a site visit most of the week.

If you look at the abbreviations you see that ‘L’ accounts for not just the roof live load but also the floor live load. Since you are looking at deflection limits for a floor you will use the ‘L’ value. Because you are looking at a beam on an intermediate level, you will look at the row labled “Floor Members”.

The second column is concerned with snow and wind load. If you had been looking at vertical construction like exterior walls you would need to confirm you met the deflection limits for wind loads. If you were looking at roof systems you would need to confirm you met the deflection limits for snow loads. However, since we are looking at a floor member, we don’t need to worry about this deflection. That’s why there is no value in that cell.

The last column refers to the ‘creep’ that occurs after the construction has been in place. I’m thinking perhaps there was information in the question which would have steered you away from needing to use this column.

I found this article which also talks about this last column which shows deflection limits for D+L. Truss Deflection Changes You Should Know About | SBC Magazine

Hope this helps! I’m going to keep looking around to see if I can find a video on this as well. If I do I’ll post it here!

-Darion

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If you have a copy of the code commentary at your work I’d definitely take a look at what it has to say on this section too!

Hi Darion,

Thank you so much, this is very helpful! I feel silly for not reading the legend more closely to see that L also accounted for the floor live loads.

Looking at the big picture though, I am drawing the assumption that any loading due to the weight of the building itself (dead loads) is accounted for the initial design of the members and that this table only accounts for the deflection of members AFTER construction is complete. This surely must be true because column 1 only accounts for live loads. The member would collapse if it never accounted for the dead weight of the building at any point in the design. Is this correct?

Thanks in advance!

Elisa

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No worries @elisa.ervin !

I don’t look at this table very often and had to spend some time digging into myself to explain it properly. It’s kind of a confusing one - what actually helped me was looking at the table in the 2009 version of the code. It’s a little simpler looking.

NOTE - THIS IS THE TABLE FROM THE 2009 IBC. Values have been adjusted since.
image

You are exactly correct. The third column (including dead + live load) accounts for the allowable deflection after the building as been constructed. All members should have been designed for handling the dead loads. I think this website does a better job explaining it than I can. Plus it has a nice diagram:
image

Hope this helps!
-Darion

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