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(OP)
All references are to NDS 2012 & IBC 2012

If we have seismic & snow loads on a building NDS allows for an increase in the capacity of 1.6 & 1.15, respectively as per Table 2.3.2 of NDS. Which factor should be used?

This situation could occur using Equation 16-14 of IBC.

"God grant me the serenity, to accept the things I cannot change, the courage to change the things I can, and the wisdom to know the difference" -Reinhold Niebuhr

### RE: NDS Load Duration Factors for combined snow & seismic loads

1.6: see 2.3.2.2. This is similar to using CD=1 with dead load (CD=0.9) plus live load (CD=1).

### RE: NDS Load Duration Factors for combined snow & seismic loads

(OP)
Understood.

Do we still us the higher load duration factor? This seems unconservative if so.

"God grant me the serenity, to accept the things I cannot change, the courage to change the things I can, and the wisdom to know the difference" -Reinhold Niebuhr

### RE: NDS Load Duration Factors for combined snow & seismic loads

If the live load is very small in D+L, Dead alone should be checked with CD=0.9 and D+L with CD=1.0.
The maximum stress with D+L only occurs during the shorter duration which is why the larger factor may be used.

### RE: NDS Load Duration Factors for combined snow & seismic loads

(OP)
So the shortest load duration will control which load duration factor to use.

okay I think I got it, thank you.

"God grant me the serenity, to accept the things I cannot change, the courage to change the things I can, and the wisdom to know the difference" -Reinhold Niebuhr

### RE: NDS Load Duration Factors for combined snow & seismic loads

Don't forget to figure in icing loads for heated roofs with exterior cantilevers in accordance with ASCE 7-05 section 7.4.5. Effectively doubles the cantilever load.

I may have a residential project coming up where this drives a 150 psf snow load to a 300 psf snow + ice load. Going to make it interesting...again.

Mike McCann, PE, SE (WA)

### RE: NDS Load Duration Factors for combined snow & seismic loads

You need to divide all of the results from the different load combinations by the load duration factor, CD, using the largest factor.

DL=10 PSF
FLL=40 PSF
SL=50 PSF
WL=15 PSF (POSITIVE WIND PRESSURE)

Load combinations from ASCE 7-05 (Allowable Stress Design)
1) DL CD1 = 0.9 [10]/0.9 = 11.1 PSF
2) DL+FLL CD2 = 1.0 [10+40]/1.0 = 50 PSF
3) DL+SL CD3 = 1.15 [10+50]/1.15 = 52.2 PSF
4) DL+0.75(FLL+SL) CD4 = 1.15 [10+0.75(40+50)]/1.15 = 67.4 PSF <---------Controls
5) DL+W CD5 = 1.6 [10+25]/1.6 = 21.9 PSF
6) DL+0.75(FLL+SL+WL) CD6 = 1.6 [10+0.75(40+50+25)]/1.6 = 60.2 PSF

Load Case 4 would govern this design and the CD to use would be 1.15. This applies only to Bending and Shear, as CD does not effect Mod of Elasticity / Deflection.
I just did PSF for illustration, you would actually be determining the line or point loads and applying them to the member to be designed.

The process is similar for LRFD - just make sure you are using the correct load combinations and duration factors. They are different for each method.

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