Girder Camber
Girder Camber
(OP)
I am investigating some crane runway girders that have been cambered excessively (in my opinion).
Some of these girders are 60' in length and have been cambered 2 inches.
This seems simply idiotic to me.
The crane rail had to be shimmed all over the place in order to keep the top-of-rail elevation within tolerances. Not to mention there are many different spans and the camber is different for all... exacerbating the rail shimming fiasco.
The most stringent guidelines use L/1000 for max deflection with a fully loaded crane with no impact. In the case of the 60' girder, this would be an max deflection of 0.72 inches.
The most I can ever see cambering a runway girder is enough to counteract dead load deflection only; in this case, girder selfweight.
Has anyone seen such cambering?
Is this common in highway bridge design?
I think the design-build for this building was a bridge builder.
Any thoughts?
Some of these girders are 60' in length and have been cambered 2 inches.
This seems simply idiotic to me.
The crane rail had to be shimmed all over the place in order to keep the top-of-rail elevation within tolerances. Not to mention there are many different spans and the camber is different for all... exacerbating the rail shimming fiasco.
The most stringent guidelines use L/1000 for max deflection with a fully loaded crane with no impact. In the case of the 60' girder, this would be an max deflection of 0.72 inches.
The most I can ever see cambering a runway girder is enough to counteract dead load deflection only; in this case, girder selfweight.
Has anyone seen such cambering?
Is this common in highway bridge design?
I think the design-build for this building was a bridge builder.
Any thoughts?






RE: Girder Camber
RE: Girder Camber
Why?
Just curious.
I have noticed this many times just driving under highway bridges.
You cannot imagine how many problems this is giving this crane runway.
I'm basically SOL.
RE: Girder Camber
RE: Girder Camber
If you believe that, you've already had a few too many at your local pub.
Mike McCann
MMC Engineering
RE: Girder Camber
I'll have to look through that design guide when I get in the office.I'm pretty sure I have a copy.
AIST also recommends cambering long span girders, but not as much as these are.
In fact, EVERY SINGLE GIRDER IN THIS PLANT IS CAMBERED....even girders 24 ft long.
I cant say anything other than, this is asinine.
I hope it has some recommendations for how to install a level rail on a cambered girder.
I am certain that even for a 20 meter girder with DL + 50%LL still would not even come close to 2 inches...it cant really, because that would exceed allowable deflection by more than two times.
Mike-
Good stuff. This is making me want to go to my local pub...for a good long while.
RE: Girder Camber
The main reason for the camber is to accommodate any future wearing surface that may be applied. If we design for an 8" deck and later it is redecked with a 9" deck or an overlay is added, then that's just more deflection in the girder. It's just not desirable to have a girder that is sagging or almost flat. If the profile grade is in a sag, you could have problems with water ponding on the bridge which would be even worse if the girder is sagging as well. I'm sure a lot of it is a preference thing.
RE: Girder Camber
I would have guessed about the same camber as Hokie suggested, DL+ .5LL, but I've not spent much time designing crane girders. My thought would be that under an average load, you would be running about level; and under max. load you would not have too much deflection. The camber and deflection would both be within the L/1000 range, one + and one -. Under how much load do you level and shim your rails?
RE: Girder Camber
RE: Girder Camber
RE: Girder Camber
You are correct.
Cambering typical crane girders, IMO, is asinine.
It causes way more problems than solves.
RE: Girder Camber
If you have 2" camber on shorter spans, there's obviously a design boo boo.
And yes, cambering is needed on longspans so that the crane doesn't have to climb as much when going towards a column from midspan.
RE: Girder Camber
Just think of this as an extended wear plate system.