Column axis orientation

[gmailman1]

Sorry if this sounds a bit like a newbie question. Lets say you are designing a two span bent bridge (not roadway but utility like electrical conduit) that is comprised of two WF longitudinal beams and two WF columns, with transverse beams between the two beams and columns as needed. Lets also say the framing of the beams is a bolted clip to the columns at each end of the beams. It may that my head is fried on this Friday morning, but would you have the webs of the columns parallel with the webs of the longitudinal beams (strong axis bending if the beams had a moment connection) or would you rotate them so that the strong axis is in the direction of lateral wind forces? With the beams pinned at the ends they would be an axial member from dead loads, so strong axis wouldn't be necessary with exception of the potential p-delta moments induced by deflection of the beams. Longitudinal wind would create moments on the columns but the exposed end area is relatively small compared to the lateral exposed area. With lateral wind forces, it would induce moments on the column but it would also benefit from frame action due to two column bents.

 

[JAE]

I'd place the strong axis orientation in a way that would resist the larger force demands and best deal with deflection considerations.

I don't think there is a canned answer here. Whichever serves your structural needs best.

 

[paddingtongreen]

JAE is correct, I just add this:

I would expect that the greater wind load would be lateral so the column webs would be perpendicular to the beam web.

You say the beam connections are pinned, but then you refer to frame action! I assume that there is no frame action, but that the columns are cantilevered from the ground and act independently.

Michael.
"Science adjusts its views based on what's observed. Faith is the denial of observation so that belief can be preserved." ~ Tim Minchin

 

[racookpe1978]

There's a couple of ways to even use the webs and flanges to avoid extra steel.

What is the bent carrying, and what future loads do you expect? Multiple electric trays, a single pipeline, several pipes, or what? Is a walkway required?

 

[gmailman1]

Its carrying three levels of electrical conduit (1.5klf) and no walkway required. I'm currently looking into HSS square sections as columns as well, which may make this topic moot but regardless I'm always learning so if you have input I'm all ears!

Paddington, the beams are pinned to the columns but was thinking the deflections of those beams create an axial force within the beam and therefore pull on the columns. The connection would likely slip enough to alleviate much of this force. Its also likely that force is much smaller than that of the wind loads. I have not gone that far into the calculations.

 

[paddingtongreen]

The beams resist bending and do not sag sufficiently to apply horizontal loads to the columns. Catenaries do apply horizontal forces. Catenaries do not resist bending so they sag significantly.

Michael.
"Science adjusts its views based on what's observed. Faith is the denial of observation so that belief can be preserved." ~ Tim Minchin