Duration of Loading
Duration of Loading
(OP)
I don't do a lot of wood design, but occasionally need to connect guards or handrails to wood structure. I've been using values based on CSA O86, the Canadian Wood Code and treating the load duration as Normal. Since it is unlikely that any loading for these elements would be sustained. Should I be using short term duration for design of these fasteners?
thanks...
thanks...
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates
-Dik
RE: Duration of Loading
RE: Duration of Loading
RE: Duration of Loading
RE: Duration of Loading
RE: Duration of Loading
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates
-Dik
RE: Duration of Loading
drift - I agree that it's unlikely to see full load for a full 10 years, but given the shape of that function, I'm hesitant to reduce it. At 1.15, you're already down to 2 months.
RE: Duration of Loading
I haven't heard back from Vic... until I do, I'll treat it as normal duration, as I usually do, just to be safe.
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates
-Dik
RE: Duration of Loading
I suppose if you come form the other side of things start at permanent then work your way up it makes a bit more sense.
I think we've talked of this before in the past but does anyone have access to or know any of the research that led NDS to use a duration factor in the first place? Would be interesting to see how this plot was developed.
End of the day I hate doing handrails because they have the trifecta; small components, high life safety risk, perceived as simple/easy/cheap by public.
RE: Duration of Loading
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates
-Dik
RE: Duration of Loading
For some reading, check out the USDA/US Forest Service's Wood Handbook: Wood as an Engineering Material (Chapter 5 has information on Creep, Duration of Load, and Creep-Rupture). Breyer's Design of Wood Structures also has some information in Chapter 4, but mostly defers to the Wood Handbook for detailed explanation.
RE: Duration of Loading
However the same event occurs near the end of the structure's life, we would expect more accumulated creep, yet we still have the same duration factor. Seems counterintuitive to me but interesting indeed.
RE: Duration of Loading
D Cd=0.9
D+L Cd=1.0
D+.75(L+Lr) Cd=1.25
D+.75(L+S) Cd=1.15
D+0.6W Cd=1.6
For each load combination, our design load is our "peak load"...so we're adjusting the max allowable stress based on the cumulative time we expect it to be at that load. So even though we check downward wind loading with full dead load, we only expect that full downward wind load to exist for 10 minutes or less over the life of the structure. So dead load goes along for the ride in that check. But when we're just looking at dead load, it's always going to be there so it gets the 0.9 to reduce the allowable from the 'normalized' allowable stress.
I'm sure the sequence of various load levels does play a role in the actual behavior, but as with all things in wood design, this covers it well enough - especially within the typical lifespan of a structure.
RE: Duration of Loading
RE: Duration of Loading
Lori Koch at AWC said use 1.6 for railings in a webinar a couple years back.
There are a couple other factors to consider (just my opinion here, not necessarily worth a squirt of piss):
Is this in a crowded museum? 1.0
Is this in a restricted facility with almost no foot traffic? 1.6
Is this a rail for tourists on top of the grand canyon preventing a mile drop? 1.0
Is this just to denote where people are supposed to stand in line at the airport, all flat floor with no potential life safety issues? 1.6
Does that make it all clear Dik? If it helps, just think of it as a singularity of death. Feel better now?
RE: Duration of Loading
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So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates
-Dik