Duration of Load Factor for Live Load
Duration of Load Factor for Live Load
(OP)
(location = USA - IBC building code using the NDS wood specification)
So we've got an exterior wood framed deck that is adjacent to and serves a dining room in a commercial building.
Per code - dining room's live load = 100 psf. Per code, the deck "serves" the dining room so it's live load is 100 psf as well.
The design of the wood joists supporting the composite decking of this deck structure would typically use a duration of load factor of 1.0 for "live loads".
However, typical live loads, say in an office or residence, are usually somewhat sustained over time much more than a deck area serving a dining area.
We can imagine multiple uses of the deck over time where the actual live load would be on the order of 30 to 50 psf. However, for a 100 psf load application,
it seems that the duration of that type of load (large crowd of people gathered at an event) would be more like a snow load application - not occurring all that often or
for all that much time.
Would it be appropriate to utilize the actual load duration curve from the NDS commentary and apply perhaps a Cd factor of 1.10 or 1.15 to this 100 psf live load?
And then perhaps look at Cd = 1.0 with a live load of 50 psf as well?
So we've got an exterior wood framed deck that is adjacent to and serves a dining room in a commercial building.
Per code - dining room's live load = 100 psf. Per code, the deck "serves" the dining room so it's live load is 100 psf as well.
The design of the wood joists supporting the composite decking of this deck structure would typically use a duration of load factor of 1.0 for "live loads".
However, typical live loads, say in an office or residence, are usually somewhat sustained over time much more than a deck area serving a dining area.
We can imagine multiple uses of the deck over time where the actual live load would be on the order of 30 to 50 psf. However, for a 100 psf load application,
it seems that the duration of that type of load (large crowd of people gathered at an event) would be more like a snow load application - not occurring all that often or
for all that much time.
Would it be appropriate to utilize the actual load duration curve from the NDS commentary and apply perhaps a Cd factor of 1.10 or 1.15 to this 100 psf live load?
And then perhaps look at Cd = 1.0 with a live load of 50 psf as well?
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RE: Duration of Load Factor for Live Load
Would that approach be of more help, and just as valid, as a duration factor adjustment?
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RE: Duration of Load Factor for Live Load
As a matter for further consideration, the first edition of the AITC, on page 3-8, agrees with Sliderule to a point, but goes further in stating that "...except that no reduction should be made for areas to be occupied for public assembly."
Although the same verbage is not in the 2012 NDS, I would tend to be conservative here and go with 1.0, knowing the damage parties can do to a structure. Maybe it's just my age here talking though...
Mike McCann, PE, SE (WA)
RE: Duration of Load Factor for Live Load
Some dining rooms have very heavy solid wood furniture such as a buffet or hutch filled with that fine china that you are not allowed to use. These items are usually against a wall, so they should not be contributing much to your bending or deflections. These loads do not leave after the party.
RE: Duration of Load Factor for Live Load
RE: Duration of Load Factor for Live Load
Specifically the duration of load for wood for live load per the NDS.
They specifically show:
combinations with Dead only - Cd = 0.9
Combinations with Live load - Cd = 1.0
Combinations with Snow - Cd = 1.15
Combinations with wind/seismic - Cd = 1.6
Corresponding to this curve:
For an exterior deck of about 12 ft. x 20 ft. - it must be designed for LL = 100 psf.
I get that - no problem.
But for the actual wood joist design we just don't see how a Cd = 1.0 (for live load per the commentary in NDS) applies.
Just seems like Cd = 1.15 would be more in line with the "large party of people" duration.
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RE: Duration of Load Factor for Live Load
Mike McCann, PE, SE (WA)
RE: Duration of Load Factor for Live Load
RE: Duration of Load Factor for Live Load
Not splitting hairs.
LLR is based on statistical aspects of the variability of the live load vs area served.
Duration of load factor is NOT based on statistics but on the unique ability of wood to resist sustained loads vs short term loads.
One is a load variability thing and the other is a material property thing.
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RE: Duration of Load Factor for Live Load
RE: Duration of Load Factor for Live Load
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RE: Duration of Load Factor for Live Load
RE: Duration of Load Factor for Live Load
RE: Duration of Load Factor for Live Load
1. Installed fixtures, such as outdoor tables, chairs, umbrellas, etc. Say, 40 PSF with CD = 1.0
2. People. Say 60 PSF with CD = 1.33
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RE: Duration of Load Factor for Live Load
RE: Duration of Load Factor for Live Load
TBL 2.3.2 references occupancy live load for CD=1.0
Commentary expressly addresses understanding that maximum human traffic loads "may be infrequent and of short duration"