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NDS Load Duration Factors for combined snow & seismic loads

NDS Load Duration Factors for combined snow & seismic loads

(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.

For Dead/Live Load example, what if the live load is very small compared to the dead load?

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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