Camber for a beam

[perland]

Hello,

Let's say I have a beam that is 20' long with a DL deflection of 1" and LL deflection of 1/2". I will specify a camber of 1.5" on my plans. My question is how do I check the LL or TL deflection for a scenario where LL limit is l/360 and TL is l/240. Do I just make sure that the LL deflection is within l/360 limit?

Thank You,

Victor.

 

[rapt]

Firstly, I would not normally pre-camber for the full expected deflection. If there is no live load applied, the deflection will be 1/2" upwards!

Is it concrete or steel? Concrete deflection estimates are likely to be more inaccurate, especially if you are using code approximations or some FEM software to calculate them, and over compensating can also be a problem.

Is the deflection limit for visual deflection or damage to attached components.

Pre-camber will only improve visual deflection. It will have no effect on damage to attached components as it only affects the final deflected position, not the amount of movement that is experienced after a certain amount of loading is applied.

 

[perland]

Thank you. The 1.5" camber is my DLx1.5 and not DL+LL. it is a steel roof beam and I am concerned for water ponding only.

 

[Hokie93]

AISC does not recommend cambering beams less than 24'-0" long (it is a practical limitation based on the cambering jig used by most fabricators). Additionally, sloping structural steel, possibly in combination with tapered insulation, would typically provide positive roof drainage, not cambering.

An excellent article on beam cambering can be found in the July 2006 issue of Modern Steel Construction magazine (

http://www.modernsteel.com).

 

[manstrom]

Probably the same article

http://msc.aisc.org/globalassets/modern-steel/steelwise/30755_steelwise_camber.pdf

When I am working on a problem, I never think about beauty but when I have finished, if the solution is not beautiful, I know it is wrong.

-R. Buckminster Fuller

 

[perland]

Thank you for all the responses. There is one item that is still not clear to me. Lest say the the beam can be cambered per AISC, the roof slope is perpendicular to it and I provide a camber equal to 80% of the DL deflection. As far as LL deflection check, all I need to do is to make sure that it is within the l/360 limit correct?

 

[dcarr82775]

Remember that cambering is an inexact science and you will get beams with very different camber then your specified number. If your roof framing system is such that you think you need to camber it, I would look at the ponding checks in AISC manual as well.

 

[hokie66]

I concur with others that cambering a steel beam as short as 20' is unnecessary and impractical.