Differential Deflection - Parallel beams

[Paisace]

I was wondering if anybody happens to know the limits (if any) to differential deflection between two or more parallel beams. I have a case where the differential deflection between parallel-adjacent beams is as much as 1/4". The beams have a span of 24 ft. Due to the architecture I can not get the beam closer of deeper.

 

[BAretired]

Doesn't sound like a concern to me. What is the spacing of the parallel beams?

BA

 

[Paisace]

The beams are space 10'-0" On Center for the most part. There are few places where they are 8 ft apart

 

[Lion06]

1/4" differential deflection over a minimum of 8' would not concern me. That would be a differential L/768 if the adjacent span is also 8' and has the same 1/4" differential deflection. Not that you would really measure floor deflection in that manner, I was just trying to put an approach to something that's not commonly checked.

 

[BAretired]

Unless super flat floors are required for a special purpose area, I would not be concerned and if that is the case, you should probably be controlling deflection to a more rigid than normal standard.

One common specification for flatness is 1/4" maximum deviation in any ten foot section. A ten foot straightedge is used anywhere on the floor to check whether or not the specification is met. I don't think it is often checked and I suspect that not many floors meet the spec.

With uneven loading, differences of that sort could be expected even if the floor is flat when unloaded.

BA

 

[Ron]

Paisace...good for you for checking this!!
It rarely gets checked either for floors or walls, but can be quite significant, particularly if covered by rigid materials....tile, stucco, etc.

1/4" in 10 feet is a reasonable tolerance so it would appear that you are ok on that count.

 

[dhengr]

Paisace:
Are all the beam bearings at their exact bearing elevations? The .25" Δ doesn’t sound too bad when you consider potential mill and fab. tolerances in beam depth, and camber, etc. Are they all the same size beam, and loaded the same in terms of deflection? Did they all match before being installed?

 

[johnbridge231]

I would use a limit of L/500

Analysis and Design of arbitrary cross sections
Reinforcement design to all major codes
Moment Curvature analysis

http://www.engissol.com/cross-section-analysis-design.html

 

[TDIengineer]

Just curious.. what load combo generated that differential deflection? If it included live load then it is definitely ok, as loading the structure to its design live load is rare.