Recommended composite action (25% vs 50%) 3

[WWTEng]

I am a little unclear about the history of the recommended composite action for composite steel beams. I have heard that 25% is the “new” recommended minimum and that it used to be 50%. I haven’t been around long enough to know the history so if someone can take a few moments and bring me up to speed that would be very helpful.

Also,

1) What are some of the ways to increase the composite action in a beam (silly question I am sure)?
2) Can you use a continuous or cantilevered beam as a composite. I think the anwer is no but I could be wrong.

Thanks.

 

[WillisV]

Assuming we are takling about IBC/AISC there is currently no code minimum.

AISC Tables for composite beams go down to 25% and it is therefore the De facto minimum that many people use as well as software vendors.

The AISC Commentary cautions against using less than 50% due to premature departure from elastic behavior and increased ductility demands on the studs, but this is only Commentary.

I have found around 40% to be the sweet spot for economy, which generally works out to be a stud a foot.

1) Read AISC Chapter I. 2) Read the Commentary to AISC Chapter I.

 

[msquared48]

I agree with Willis - a stud a foot usually works. Like 99% of the time.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[SethGuthrie]

It is allowed to design a cantilevered beam as a composite section, but the composite action only helps with positive bending. The bare beam still has to take the full negative bending demand (over the support), so it becomes impractical for cantilevers that are long.

 

[WillisV]

@Seth - unless you provide rebar in the slab, in which case you can determine a "composite" capacity in negative bending, which is covered in the Commentary, which is why I told him to read it

 

[dik]

It used to be that 50% was minimum for strength and that 25% could be used for deflections...

Dik

 

[abusementpark]

I have found around 40% to be the sweet spot for economy, which generally works out to be a stud a foot.

I know an old-timer who doesn't space shear studs further than 12 inches on center. I wonder if this is what is he going for with that rule.

 

[WWTEng]

WillisV, thank you.

 

[WWTEng]

I guess I have a few more questions:

1) what is the ideal % action that one should try to achieve?
2) Can you get 100% composite action?
3) For girders, supporting 2-3 beams, could you use two rows of studs?

 

[JoshPlumSE]

WWTEng -

The ideal percentage differs from region to region and from beam to beam. You have to add more studs to get a higher % composite. So, it's a matter of how much steel costs per pound compared to the installed cost of adding studs.

To get to 100% composite action all you need to do is add more studs (usually). And, girders can certainly have multiple studs per deck flute.