Camber in Steel Beams 2

[whyun]

I would like to start a thread about cambering of steel members. Following are my general guidelines in cambering but if anyone has a different opinion or have other considerations, please feel free to contribute.

1. typically use 75% of precomposite dead load on a floor beam to calculate camber.
2. minimum camber is 3/4" and in 1/4" increment. ignore any calculated camber that is less than or equal to 1/2".
3. no camber on beams at sloped roof if slope exceeds 1/2" per foot.

Some engineers choose not to provide any camber on floor beams. Instead, they provide for an additional 1/2" of leveling concrete (which is poured together with the deck slab) in the dead load. Is there see any benefit in doing this?

Thanks for y'all's input.

 

[flamby]

For steel bridges, I adopt (100% DL + 75% LL) to compute the camber. The idea is to keep it straight under most likely loading. These factors are recommended by our steel bridge code.

The camber is just for the purposes of 'finish and appearance' under a percieved load and is without any structural value (unless you are a fan of FE-NL analysis). The camber may be important for larger structures like bridges but can be more easily managed by leveling concrete if spans are smaller. I would also resort to leveling layer of concrete, wherever I could manage because it is so cheap as compared to cambering exercise which no fabricator loves to do. Cambering requires bending, reaming of holes to correct tolerances and so on at the cost of large man-and-machine power.

 

[JAE]

AISC has some cambering guidelines in the manual (check the index ... I'm away from my desk so cannot cite the page number. These depend a bit on the depth of the member.

 

[whyun]

Thanks flame and JAE.

My question was intended for buildings so bridge criteria is not the one to follow. For building framing member camber, we normally don't use full dead load because they are overestimated to begin with. General practice here is to use 75% of the dead load. Live load is not included in camber calcs because we dont want a permanent upwards camber when LL is removed in a building. I do follow the logic of including 75% LL in bridge design, though.

JAE, I found on the AISC ASD Steel Manual page 1-147 about camber of rolled sections. Is that the page you had in mind?

Any other relevant discussion on cambering would be appreciated.

Let's not restrict our discussion to steel... How about cambering of Glued-Lam Beams?

Good day, all.

 

[JAE]

whyun - yes, that is the section I was thinking of.

We got into a camber issue recently with two large W21 beams spanning over a roof where a large condenser unit was positioned on top of the steel. The manufacturer's of the unit showed up and tried to set the double condenser on the beams. They of course flexed downward upon the application of the load and the light gage metal on the sides buckled a bit due to the deflection.

We had specified camber in the beams so that with the operating dead weight of the unit the beams would be close to flat. We knew that the variables in getting this beam perfectly flat were the actual weight of the unit, the true E of the steel, and the potential for some degree of fixity in the simple bolted connections at either end.

The unit manufacturer complained that our beams didn't meet their spec. We had used some fairly long spans (don't remember the actual distance) and claimed that the deflection didn't meet what they required in their cut-sheets.

We pointed out to them that they asked in their sheets for the beam to not deflect more than 1/2" or L/360 under load. So we interpreted it to mean the FINAL position of the steel needed to meet that criteria. Instead, they meant that the total deflection of the beam with the addition of their unit would not exceed 1/2".

For our span - this would have required a very, very large section....way more than the owner would have wanted to buy. In any case, they removed the unit, cambered the beam in the field a bit more (with hand torch) and re-set, somehow allowing for our flexibility.

sorry for the long epic story here....just a warning to be sure that non-structural equipment manufacturer's are asked to clearly state their understanding of their own requirements......net deflection or gross deflection.

 

[dougantholz]

A couple of rules we work by,
1. Don't camber any beam less than a W14x22 - just increast the size.
2. Don't camber a beam less than 20 feet long. We have been told multiple times that it is near impossible to camber a short beam because the rigs don't work for that short of a beam.
-Doug