General framing economy

[CTSeng]

Having a discussion with colleagues about which is the more economical framing layout for 2-way framed bays with unequal spans; secondary framing (purlins/joists) spanning the long direction and primary framing (girders) spanning the shorter or secondary framing (purlins/joists) spanning the short direction and primary framing (girders) spanning the longer.

I know this is very general and in reality is influenced by a multitude of other factors but just in general for average sized varying bays in the 20 to 50 foot range, floors or roofs, and excluding labor costs, is one more economical than the other? Would your answer vary for different materails, steel, concrete or wood?

Any thoughts are appreciated.

 

[cedent]

We span our floor joists the long direction and the girders the short direction. With a composite floor system, this helps us minimize the overall depth of floor system. Architects tend to like that.

Have you tried the parametric bay studies on AISC's website?

 

[structuralaggie]

That is a really general question. One thought I had is OSHA requires bar joists over 40 feet in lenght have bolted bridging in place before slackening of the hoisting lines. This really slows installation, so it would increase labor.

 

[Lion06]

We generally have girders spanning the short direction. I am not sure if this is more a factor of economy or a more consistent way to get beam depths similar to girder depths. In limited floor-to-floor height buildings, that is just as important as steel weight.
Having beams in the short direction and girders in the long direction will have you framing W10 or 12 beams into W24 girders, rather than W16 beams into W18 girders.

 

[JAE]

Yes, I think that typically, girders spanning the short direction and repetitive members spanning in the long direction....first, it gives you the least weight, second, better deflection control, and third, you have fewer members to hoist in and set.

 

[jike]

I generally agree (girders in short direction and floor beams in long direction) when it is important to keep the heights about the same. However, when depth restriction of the girder is not required, you might consider spanning the girder in the long direction and the purlins in the short direction similar to metal buildings. Please double check me on this because it has been a long time since I studued this. Run thru several designs to prove this to yourself.