Roof live load wood duration factor 2

[ResidentialStructGuy]

What duration factor would you use for wood beam supporting a roof live load and roof dead load? Based on the definition of a roof live load in ASCE 7-16 chapter 4 section 4.1, it seems to me that it should be 1.25 (construction live load). This is for a residential structure with a sloped roof. Under ASCE 7-16 table 4.3-1 "Roofs" it states that the live load for ordinary flat, pitched, and curved roofs to use 20psf. It also repeats the 20psf for roof areas not intended for occupancy.

 

[ANE91]

Yes. Table 2.3.2 and Appendix B of NDS.

 

[ResidentialStructGuy]

Do you happen to know if there is a code reference explicitly stating that the 20psf roof live load is a construction live load? The reason I am asking this is a plan reviewer is requesting I use the occupancy live load and it causes a beam to fail... Usually I would just upsize the beam to appease the reviewer however in this case the beam is already 6x12 LVL and increasing the size feels like overkill. Thanks in advance.

 

[JAE]

Roof Live loads are defined in ASCE 7 under the generic "LIVE LOADS" chapter and RLL are defined as:
"A load on a roof produced (1) during maintenance by workers, equipment, and materials and (2) during the life of the structure by movable objects, such as planters or other similar small decorative appurtenances that are not occupancy related."

It doesn't mention construction loading at all and personally I've never presumed that RLL in the code was a temporary construction load. Yes, we've typically used 20 psf for temporary construction live load as a value - but technically a RLL is simply a live load.

Under part (2) above the description is not that of a 1.25 DLF I'd say.

 

[jhnblgr]

State building codes for roofs seems to be 20 psf. I would be concerned with a roof that can’t support that.

 

[ANE91]

 

[OldDawgNewTricks]

Breyer's textbook says roof live load has a duration factor of 1.25 (see screenshot below). I would think that should be good enough for the plan reviewer.

 

[SE2607]

Calling it roof live load instead of construction load will solve your problem. This is one of the (few) exceptions to the "if you call a tail a leg..." debate technique.

 

[XR250]

Do you happen to know if there is a code reference explicitly stating that the 20psf roof live load is a construction live load? The reason I am asking this is a plan reviewer is requesting I use the occupancy live load and it causes a beam to fail... Usually I would just upsize the beam to appease the reviewer however in this case the beam is already 6x12 LVL and increasing the size feels like overkill. Thanks in advance.

Failing in bending or deflection?

 

[RWW0002]

NDS Commentary for Load Duration Factor appears pretty clear.
C2.3.2 Seven Day Loads.
Where the minimum roof uniform load specified by the applicable specified by the applicable building code exceeds the design snow load.... .. it is conventional practice to consider this load a construction type load for which a 7 day or 1.25 load duration factor is applicable....

 

[ResidentialStructGuy]

Failing in bending or deflection?

Failing in shear, the beam is slope cut at the exterior wall to match existing 2x6 rafters.

NDS Commentary for Load Duration Factor appears pretty clear.
C2.3.2 Seven Day Loads.
Where the minimum roof uniform load specified by the applicable specified by the applicable building code exceeds the design snow load.... .. it is conventional practice to consider this load a construction type load for which a 7 day or 1.25 load duration factor is applicable....

Thanks! This is exactly what I was looking for.

 

[Respawn]

Failing in shear, the beam is slope cut at the exterior wall to match existing 2x6 rafters.


Thanks! This is exactly what I was looking for.

What exactly is this roof beam supporting? It sounds like it is running the same direction as the rafter? Or is it a ceiling beam. In that case I understand the slope cutting the end of the beam. You might want to entertain a wider LVL or bump to PSL. Hopefully that helps your shear issue due to the slope cut.

 

[jhnblgr]

Failing in shear, the beam is slope cut at the exterior wall to match existing 2x6 rafters.


Thanks! This is exactly what I was looking for.

Did you check bearing strength of what’s below? Seems likely the top plate be over loaded also