Rules of Thumb on Final Camber

[VoyageofDiscovery]

Does anyone have rules of thumb on final soffit profile for composite concrete deck on steel girder bridges as a function of span length, etc?

Shrinkage and other camber losses come to mind for me as I don't want a sag condition to develop over time.

VoD

 

[VoyageofDiscovery]

Correction "Shrinkage and creep and other camber losses"

 

[wiktor]

I have never heard about cambering of the deck, and never come across in any textbook. Typically, the permanent stresses in the deck are so low that creep is of no concern. LL stresses will be acting both ways, so also there is no effect. Shrinkage will put the deck in tension (if any), preventing sagging, and under normal conditions is over within few months.

 

[VoyageofDiscovery]

Hi wiktor,

Sorry, I meant for the camber of the steel girder, not the concrete deck. Rules of thumb for how much positive camber.

VoD

 

[wiktor]

Typically steel shall be cambered for 100% DL, 100% shrinkage, 50-75% SDL, and in some cases 25%-50% LL. The cambering for LL is to avoid rather uncomfortable appearance of sagging span. Check also AASHTO requirements.

 

[bridgebuster]

Check your DOT requirements for vertical curve camber.

Also, if your bridge is being constructed in stages you have to consider the effect in the camber calculation.

 

[msquared48]

I have always used 1.5X the dead load deflection for the camber.

Mike McCann
MMC Engineering

 

[asixth]

I'm thinking back a few years ago now but we had some girders in our office that had a 1.25" precamber for an 80' span (span/720 approx.)

 

[VoyageofDiscovery]

Thanks all!

 

[hokie66]

If your bridge is a short span/trestle type, too much camber is just as bad as not enough. Driving over a prestressed trestle which has creeped a lot is not pleasant.