AISC Compression of Members Based on Gross Area vs Net 3

[GearsNsparks]

Is there literature in the AISC commentary explaining why compressive strength capacities (equation E3-1) are based on gross area rather than net area? I think if I turned some angle into swiss cheese by placing bolt holes in it, the compressive strength should be less than without bolt holes.

For tensile capacities there is a yielding check for the gross section and a rupture check for the net section.

 

[WARose]

For pure compression, it's just not significant. (Especially compared to the other buckling modes that will likely govern.) There is enough safety factor against yielding to prevent it from becoming an issue.

I do start thinking about it (and it's affect on bending strength) once the framing bolts start getting bigger than the standard 3/4". But I've rarely seen it have a big impact.

Engineering judgement should (of course) always be on the lookout.

 

[BridgeSmith]

There are spacing limitations on bolt holes that preclude turning "some angle into swiss cheese", and presuming there are bolts in those holes, the deformation before the bolt resists the compression. The check on the net area in tension is based on the resistance to fracture, which is not a concern in compression, only produces yielding.

 

[MotorCity]

This is a great question that I have often wondered about. The capacity is based on some gross area. Obviously that gross area is reduced at bolt holes. Its not even that the bolt holes can be spaced closed together, but you could have several connections to a column at the same elevation that would essentially turn it into swiss cheese. Most columns are controlled by buckling rather than crushing, but even so, we are kidding ourselves into thinking that we have the full gross section in play throughout the column. That said, our current method seems to be working without accounting for the net section.

 

[JoshPlumSE]

Buckling of members is based on a global behavior of the member over its unbraced length. The maximum deflection occurring usually at the locations furthest from support. The net area reductions usually occur over a very limited distance near the support connections.

Fracture isn't really a consideration for compression. Yielding could be considered on net area. But, if there are bolts in the holes, then you shouldn't have much of a reduction as the holes deform. Though if you have a lot of unfilled holes, then I think net are is something to consider for compression yielding.