Distributing or not to distribute concentrated live load per ASCE 7

[see_gen_notes]

ASCE 7 Chapter 4.4 states that, unless noted otherwise, concentrated loads are distributed over a 2.5' x 2.5' area. When looking at a live roof load, it’s hard to imagine a person actually occupying that entire footprint. In reality, it seems more reasonable that a single rafter would pick up the full point load. In your designs, would you distribute the load over a 2.5' square, or conservatively assume the entire load is applied to one member (rafter, beam, anchor, etc.)?

 

[driftLimiter]

I always just think about a point load on the rafter of 200#. If the rafter cant support that then something feels wrong. I have only ever invoked the 2.5' sq area in a very specialized floor case.

 

[ChorasDen]

While we do not typically consider the composite effects between wood members and the diaphragm for strength based load sharing, with the single exception of SPDWS section 3.1.1.1, I generally consider it a rational approach to use the 2.5ft area for consideration of point loads. The best way to rationalize this is to apply the entire load to a single rafter, and compare the local deflection to the adjacent member. If these members are spaced at 16" o.c., or even 24" o.c., you'll see the that the radius of curvature of the sheathing/diaphragm material overtop the members starts to grow, and as such, you'd start to expect load sharing to alleviate the deflected shape, otherwise, our assumption regarding the performance related to the stiffness of the structural elements would be invalid.

Basically, if you cause one member to deflect, but the adjacent one does not deflect similarly, there must be essentially zero stiffness associated with the members tying them together (diaphragm), otherwise, the load would redistribute.

 

[ChorasDen]

and continuing, 2.5ft square is probably a committee decided number that seems appropriate for a load sharing area.

 

[RenHen]

Typically, I assume that if someone can walk over my work, there's gonna be a 1 foot (as in the body part, not the unit of measure) point of contact where the full load is applied. I don't work on roof framing though, this mostly comes up for me with canopies, roof perimeter coping, walkable glass surfaces & other systems where I don't think I can necessarily rely on assumptions about roof framing and also where I know some of the framing members could be stood on during/prior to installation of the system. I do spread it over 2"x2" for glass FEA to avoid unrealistic stress concentrations, but for anything else I apply it as concentrated. I think this is probably conservative but I'm fine with that and it doesn't usually drive my designs in a major way.