How much rise of a curved roof is natural pre-camber? 3

[IJR]

Pals

You have a slightly curved roof. Line between supports may or may not be horizontal, but the roof has a rise and it is curved with a hump.

In deflection checks, without intended pre-camber, how should one evaluate deflection value?Is the hump to be considered a pre-camber? If yes, what would be the criterion for the amount of natural pre-camber, given unknown or nonhorizontal line between supports?

A reference to AISC or any other code will be appreciated.

respects
IJR

 

[271828]

Unless you indicate camber on your drawings, calc as if it's flat.

 

[IJR]

271828

You are basically telling me that there is nothing called "natural camber"?

respects
IJR

 

[civilperson]

Turn the member over and you have "natural ponding".

 

[271828]

LOL, no. It's certainly there--just unreliable.

 

[IJR]

Civilperson

Your message is like an entity in the planned European Constitution. Tough for me

respects
IJR

 

[csd72]

IJR,

How can you guarantee that the contractor wont mess up and put it the wrong way around with the camber downward.

It is also possible (although unlikely) for a beam to have no natural camber.

Anyway you said it was a roof beam and camber has no relevance for wind load. A cambered beam will move just as much under wind as a straight beam.

csd

 

[IJR]

csd72

You are right with respect to wind. But the contractor has no chance of messing up the required curvature. It is architectural and will show up immediately during erection.

In my experience no mistake has ever been done with respect to curvature specified in drawings(within tolerances I mean)

My discussion is: Is unintended camber a camber?

respects

IJR

 

[civilperson]

Curvature of the finished product and camber are not the same but related. In order to have a straight, (flat), finished condition a designed camber is constructed that equals dead load deflection. A curved finish product will be built with a curve greater than the camber/deflection in order to end with a curve. Calcs using staight beam formulas will be good enough until the curve is greater than 5% of a circle.

 

[271828]

I find this to be a very interesting topic. I asked some fabricators a few years ago about cambering and the subject came up. They indicated that most beams have natural camber on the order of 1/2"-3/4". That made me feel good about calling out beams 3/4" camber whenever I needed to. I figured that a fair number of those probably didn't need any work to achieve the camber.

On our drawings, we always included a note to put the camber up instead of down. Whether they did it or not is unknown.

 

[IJR]

All pals

Should share this with you. I am a structural engineer who rarely visits fabricators or site. Easy for me not to trust their work. However I made a quiet research and found out that fabricators do more than I can in most or all projects I am involved. Most are self trained guys who talk of their welding machines the way I talk ofe my excel spreadsheets.

Unexperienced erectors? Yes, they are often have little they talk about, and usually just want the job finished.

Bottom line: Guys got passion with their work

regs
IJR