Composite Infill Beam Camber in Moment Frame 1

[EngineerEIT]

I'm working on a steel office building where the lateral system consists of moment frames in both directions and I had a question with respect to typical practice for whether to camber the infill beams adjacent to the moment frame beams.

I've read that beams with moment connections should not be cambered, so I am concerned with the difference in elevation for the deck bearing for the infill beams next to the moment frames (ie, high to low to high).

 

[EngineerEIT]

No one has any input on this topic?

Some addl info, infill beam camber = 3/4" and beam spacing is 8'-0". My gut says its not a big deal and they can pull the decking down (2" deck) onto the beam that is on the moment frame.

Any input is greatly appreciated. Thanks.

 

[jdgengineer]

Moment frame beams are typically not cambered due to potential fit-up problems at the column where the beam will be welded to the column. I have not heard of any issues regarding the infill beams framing into the moment frame beam. The finished deflections should be similar as the moment frame beam will be much stiffer than the infill beams.

On a side note -- With only a 3/4" camber I would highly consider upsizing the beam a little bit to remove the camber. It's my understanding most steel fabricators would prefer a little extra tonage in exchange for eliminating the camber. Something to run by your steel guy if he is on the job.

 

[mike20793]

I usually avoid cambering beams unless it is a long span or composite joist system.

 

[Hokie93]

I agree with jdgengineer. What is the typical infill beam span? Providing 3/4" is the minimum camber recommended by AISC. If it were my project, I would definitely investigate upsizing the beams to eliminate the camber. If you have a fabricator on board on the project, discuss with them the relative economy of camber versus additional pounds of steel. If you choose to keep the camber, you could check the deflection of the steel deck spanning 16'-0" (between "high" infill beams) with a couple of 200 lb ironworkers standing on it at mid-span. If that deflection is 3/4" or more, you are good to go.

 

[sandman21]

You say adjacent to the moment frame then talk about beams with moment connections, which one? If you are talking about non-moment frame beams then camber away, if you are talking about the moment frame beams themselves needing a camber I would question your camber number.

 

[EngineerEIT]

Apologies if my initial post was unclear. The beams on the grid lines which are moment connected to the columns are not cambered due to the fit-up issue described above (also they have significantly more stiffness to control drift). I've attached a sketch of a typical bay which should make things more clear.

Infill beam span is roughly 40'. Currently, we do not have a steel guy on board yet. To eliminate camber, I would have to upsize the infill beams by about 10 lb/ft of weight (from my research, 5 lb/ft is a typical break even point between cost to camber and cost of additional weight of steel).

 

[sandman21]

Infill beam cambers with moment frames is perfectly fine.

 

[KootK]

Further to Hokie's comments, here's an article that deals a bit with coaxing the deck into place: Link

infill beam camber = 3/4" and beam spacing is 8'-0". My gut says its not a big deal and they can pull the decking down (2" deck) onto the beam that is on the moment frame.

This is very much my gut feel as well given your parameters.

To eliminate camber, I would have to upsize the infill beams by about 10 lb/ft of weight (from my research, 5 lb/ft is a typical break even point between cost to camber and cost of additional weight of steel).

Neat. I'd not heard of this. Could you direct me to any references? I'd like to check them out for myself.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[sandman21]

AISC Specifying Camber

 

[KootK]

Nice. Thanks Sandman!

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[WillisV]

This is done all the time - camber away.