Camber for Steel Beams 2

[LPPE]

Is there a rule of thumb for cambering steel beams? AISC doesn't seem to have much on the subject, except that it is up to the discretion of the engineer. I've usually just used the dead load deflection as my camber, but I've heard of also using a portion of the live load. My basic problem is I have a 30' span and can only sneak a W16 in. The DL is .5k/ft and LL is 1.25k/ft. In a similar situation, an engineer used a W16x45 c=3/4". The c=3/4" takes some legitimate LL deflection. Any thoughts?

 

[Rich2001]

Like you I usually use DL only for determining camber. But I have include LL in some cases (chemical plants and refineries) where I need to reduce stresses in piping and equipment attached to the steel.

 

[Taro]

Cambering for live load is generally not recommended, for several reasons:
1. Actual live load is usually much lower than design live load.
2. Deflections of steel beams are usually smaller than calculated due to end restraint of connections.
3. Actual camber may be larger than specified due to standard tolerances.
4. Excess camber can lead to problems placing decks and slabs.
5. Over-reliance on camber produces beams with lower stiffness and can cause vibration problems.

For these reasons, most engineers do not camber for live load. In fact, many engineers only camber for a portion of the dead load, say 60% to 90% depending upon end restraint conditions.

 

[Qshake]

I agree with the above and would like to note that we, in the bridge industry, do not camber plate girders for live loads only for dead load. We rarely camber rolled sections, preferring to make up the difference due to dead load deflections by increasing the thickness of the slab in a limited area which we refer to as a "haunch".

If we did camber for live load, the end product would be rolling hills on an otherwise flat application. Due mainly to load that isn't there.

 

[whymrg]

We use 70-75 % of DL deflection. Deflection does not occur immediately. If 100% of DL deflection is used, headed studs may be too close to the top of slab, or above the top of slab (we had this case on one of the projects and learned from it).