Composite Steel Floor Beams

[spats]

I'm designing a typical office-type composite beam/composite deck floor. Generally speaking, the span on my girders are approximately 25', and my secondary beams span as much as 35'. I had an experience in the past with composite 35' secondary floor beams in an office, and the occupants complained about floor vibration. Is 35' too much? I'm having trouble getting the AISC Design Guide 11 procedure to produce favorable results but, in all truthfulness, I always have problems with DG11 results, even for shorter bays. What's the experience of others?

 

[SteelPE]

What is your beam spacing and slab thickness?

 

[spats]

Beam spacing is anywhere from 4' to 6'3 on center due to geometry. Slab thickness is 4" total on 1 1/2" composite deck.

 

[bouk715]

spats - I've found similar results when trying to meet DG11 for composite floors for new schools. Whenever my beam span exceeds around 30'-0", DG11 typically controls the design. I find this especially true when design live loads are 60 psf or less (like office or classroom space). I almost end up with the same design as a 100 psf live load floor. Classrooms are a little more forgiving in terms of vibration versus wide-open office space though.

 

[bouk715]

Also, I'm not sure what your bay spacing constraints are but you may be better off extending the girder spans to 30'-0" +/-. You could use 2" or 3" composite deck with more concrete (5"-5.5" total) that will give you a bit more mass to dampen vibrations. I've typically used 10'-0" beam spacings in this case. You end up with deeper beams and girders, but you were probably headed that direction anyway to meet DG11 and overall steel weight is lower due to increase spacings. Just a thought.

 

[structSU10]

As bouk has said, a thicker, heavier slab can really help with vibration problems. In some instances we have used a 7" slab (5" over 2" deck) to help. You have two things that you can do - add mass or add stiffness, and beefing up the deck does both.

 

[jike]

I agree with others in ADDING CONCRETE to control transient vibrations.