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When looking at allowable load tables from Simpson for their joist hangers, why are there different numbers for different cases, even though the force is in the same direction?

For example, the LUS24 hanger with SD screws and douglas-fir/southern pine says something like this:

        Uplift (160)    Floor (100) Snow (115)  Roof (125)
LUS24   495             870         1000        1085

I can understand how the uplift load would be different from the floor load (the force is acting in the opposite direction: upward), but why list the snow and roof load and why are they different (the force for all three is acting in the same direction: downward)?

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Loads are based on use...not all loads are the same.

Floor loads are considered permanent loads (100%), while snow loads are considered short term loads (periodically loaded). Snow in some areas are rated very short term, less than 7 days (125% stress). Some snow loads are rated short term, less than 1 month (115% stress).

What this means is that you’re allowed to increase the allowable stress by a factor of 15% or 25%, respectively.

Little known fact: when a single joist/beam is calculated, allowable stress values are less than if they are repetitive. That is to say, joists or beams installed repetitive (say 16” o.c.) can use a higher allowable stress value than a single joist/beam.

  • thanks! ive been misreading these tables! (dont worry, i dont actually do this kind of structural work; its just a curiosity). so what does the 160 for uplift mean precisely? – tau Jul 4 '18 at 19:38
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    @tau Same...60% increase in load. It’s considered an impact load. The highest I know of is 100% increase. – Lee Sam Jul 4 '18 at 19:41
  • okay, but in this case its a 60% increase in load that is considered impact (not short term), meaning the weight that each hanger can handle actually goes down (from 870# to only 495#). is that correct? thank you again! – tau Jul 4 '18 at 19:49
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    @tau Ahh, I see. I would imagine it’s related to the design of the hanger. In lieu of a bottom bearing plate to rest on, it has only fasteners (nails) to resist in shear. IMHO, it is noted improperly. It should be noted as just (60)...implying 60% of “working stress value”, not 160%. – Lee Sam Jul 4 '18 at 20:00

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