Load Duration Factor CD for wood 2

[SE2607]

If a roof deck will be used by people, ASCE 7 says I need to use the same live load as the occupancy served (table 4.3-1).
Is the Cd = 1.00 or 1.25?
Thank you,

 

[phamENG]

Over its design life, will the cumulative time spent at or near design load be 168 hours, or 10 years (87600 hours)?

 

[XR250]

Honestly, probably zero hours at full design loads

 

[SE2607]

Honestly, probably zero hours at full design loads

Can't you say that about any loading?

 

[Eng16080]

Is the Cd = 1.00 or 1.25?

Unless I have a really good reason not to, I follow Table 2.3.2 of the NDS which give CD=1.0 for an Occupancy Live Load.

The approach by pham is reasonable but how are you going to know the number of hours of use "at or near design load"? If you were to use CD=1.25 and there were ever an issue, would somebody really agree with that approach? Now your framing is potentially undersized by 25%.

 

[JAE]

We've always used the duration of load factor (DLF) applicable to each load combination used.

So for your D+L check you'd use 1.0
For a D+Lr check you'd use 1.25
etc. for all the different load combinations - using a DLF based on the basic load case with the highest DLF in each particular combo.
The NDS appendix shows examples of load combinations with appropriate DLF factors for each.

Hopefully you're not confusing load factors with duration of load factors.

 

[XR250]

Can't you say that about any loading?

For occupancy loads - probably

 

[XR250]

The approach by pham is reasonable but how are you going to know the number of hours of use "at or near design load"?

I mean, the lawyers could pull security camera footage

 

[SE2607]

We've always used the duration of load factor (DLF) applicable to each load combination used.

So for your D+L check you'd use 1.0
For a D+Lr check you'd use 1.25
etc. for all the different load combinations - using a DLF based on the basic load case with the highest DLF in each particular combo.
The NDS appendix shows examples of load combinations with appropriate DLF factors for each.

Hopefully you're not confusing load factors with duration of load factors.

I guess that is my question. Is it L or Lr?

 

[phamENG]

Use C_D = 1.0.

I only brought up the time thing because that's what the real question is. The table in NDS is mostly a suggestion. A suggestion anyone would be a fool to ignore, of course, but a suggestion nonetheless. The C_D has nothing to do with what kind of load you're talking about, but all about the cumulative time over the life of the structure we can reasonably expect a floor to see the load under consideration. The type of loading only correlates roughly because different types of loading are usually applied for different amounts of time. A deck? it's going to be more than 7 days.

Honestly, probably zero hours at full design loads

For the entire structure all at once, you're probably right. Average live load in a house is only 6psf, after all. But some areas will see it, and that's the point.

 

[SE2607]

Use C_D = 1.0.

A deck? it's going to be more than 7 days.

Yes, a deck. A roof deck. No one is going to be living (sleeping) on it. I can't see it being loaded for 7 days.

 

[JAE]

I guess that is my question. Is it L or Lr?

Well a deck built and put there for the accumulation of people is a deck occupancy for simple live load.
That would be, in my book, a public assembly function with LL=100 psf.
But the deck also is in essence a "roof" which gets roof live load (Lr=20 psf) and possibly very heavy snow loads (depending on where it's located) where the SL > 100 psf.

I would check all applicable load combinations - each combo using a different DLF for the wood's allowable stress (assuming you're using ASD).
You probably would guess that in a low snow load location the LL=100 psf would control.

 

[SE2607]

Well a deck built and put there for the accumulation of people is a deck occupancy for simple live load.
That would be, in my book, a public assembly function with LL=100 psf.

This is a roof on a single family residence.

But the deck also is in essence a "roof" which gets roof live load (Lr=20 psf) and possibly very heavy snow loads (depending on where it's located) where the SL > 100 psf.

No snow in Huntington Beach, CA, 2 blocks from the ocean.

I would check all applicable load combinations - each combo using a different DLF for the wood's allowable stress (assuming you're using ASD).
You probably would guess that in a low snow load location the LL=100 psf would control.

Yes, ASD.

 

[JAE]

I'd still be tempted to go with 100 psf. Think of the crazy parties that might attract shoulder-to-shoulder people.
At the very least 75 psf.

 

[ANE91]

I’ve always done it the way @JAE has outlined. Different C_D for each load combination.

 

[SE2607]

I'd still be tempted to go with 100 psf. Think of the crazy parties that might attract shoulder-to-shoulder people.
At the very least 75 psf.

Ref. ASCE 7 Table 4.3-1:

100 PSF is the equivalent of 180 lb. people standing 1'-4" apart over the entire area. I think that is ridiculous.
75 PSF is not defensible in that it does not appear anywhere in the code except for bowling alleys (!!).

 

[SE2607]

I’ve always done it the way @JAE has outlined. Different C_D for each load combination.

As do I, as far as using a different C_D.

 

[jerseyshore]

This isn't the question, but I always use a minimum of 60 psf for a deck unless I'm trying to make an existing one work.

As for the question, we always use C_D=1.0.

 

[Eng16080]

This isn't the question, but I always use a minimum of 60 psf for a deck unless I'm trying to make an existing one work.

This is an interesting point. Would a roof deck be considered a deck per ASCE and require 150% live load? I want to say no, but I really don't know.

 

[SE2607]

This is an interesting point. Would a roof deck be considered a deck per ASCE and require 150% live load? I want to say no, but I really don't know.

Balconies and decks (1.5 x 40 = 60 PSF) are listed separately from roof decks (40 PSF) in table 4.3-1. Intuitively, this makes sense in that it seems less likely that a roof deck, not as accessible as a balcony or patio deck at the same level as a floor, would be overloaded.