Is Ceiling Part of the Roof System? Load Duration Factor (Wood Design)

[cclin]

Did any one have idea if the CD factor (Load Duration Factor) 1.25 for roof has been used in establishing ceiling load table as shown in Table 23-IV-J-3 of 2001 CBC (1997 UBC). It seems to me that this factor is not used. Does that mean when designing the ceiling joists and ceiling beams, this load factor cannot be used? Does that mean the ceiling portion is not to be treated as portion of the roof? Only the ridges and rafters and the roof diaphragms are to be treated as roof? And then the allowable of these can be increased 25%. Can one clear this up? Thanks.

 

[whyun]

Accessible ceilings should be designed as a floor with LDF of 1.0 and live load based on the use (temporary storage, etc).

Inaccessible ceilings should be designed using LDF of 1.0 and live load of 10 psf.

These are based on 1997 UBC. Not using IBC in my area for a while....

 

[whyun]

My bad... you ARE also in California...

 

[cclin]

Did you treat the ceiling as portion of the roof. For the ordinary conditions, say, nothing is to be stored on the attic. It is just a regular residential house with flat ceiling with attic space between the roor framing members (ridge and rafters) and the ceiling joists. Can you increase the load duration factor by 25%? But if the ceiling beam is used to support the king post that supports the ridge or rafters, can you increase?

 

[LdsUtd]

it depends on how the cieling is designed:

if you use ceiling joists with a pony wall to support your ridge/rafters, then your live load will be 20 psf, and use 1.25 for a load factor.

if an attic is involved, use a factor of 1 on the attic live load.

if a pony wall system (or equivalent) is used to support the rafters, AND there is an attic: use a load duration factor of 1.0. this will be slightly conservative, since the Roof live load will have a factor of 1.25 and only the attic live load will have a load duration factor of 1.0.