Design prestressed bridge beam for long term downward camber

[MIKE_311]

The bridge owner doesn't want any downward camber in the long term (i.e. no tension in the PS beam bottom flange over time).

PCI has a procedure for estimating long term camber from creep and shrinkage using multipliers, but states that these multipliers are not applicable to bridges with composite cast in place decks, as it serves to limits the effect of creep/shrinkage the beam, nor should the designer include additional camber for long term effects when the multipliers are used.

My question is, what is the typical practice for considering long term camber in PS bridge beam with composite decks? if it is considered, what is the typical procedure used to calculate/estimate it?

 

[Settingsun]

I just remembered that Standards Australia has Handbook 50 which is a glossary of building terms. I feel as though the topic of this thread has changed to that so this is not derailing (any more than already derailed).

"camber (structure) - slightly convex curvature of a structural member, usually introduced during fabrication or into formwork prior to concrete placement, to compensate for deflection under load and provide a level surface after such deflection has taken place."

Whether hogging due to pretensioning counts is still a matter of opinion IMO.

 

[centondollar]

steveh49, that definition does not make much sense to me. Concrete is generally not made with curved formwork to cause upward camber: such changes in cross-sectional depth are instead related to efficient usage of the cross-section, indicated by deep sections near supports (to resist shear force in all beam types and the large support moment for continuous beams) and shallower sections near mid-spans (smaller shear forces) of the bridge.

By performing the calculations, you will easily notice that camber (a curved girder shape in the longitudinal direction) does not significantly reduce deflections, and the reason for this is the drastic reduction of flexural stiffness caused by cracking of the section. Precamber in the fabrication phase, for the purpose of limiting deflections, is done for steel girders, but for concrete, the cambering effect (applicable for an uncracked beam) is achieved by prestressing.

 

[rapt]

Centonthedollar

Precamber is often achieved in insitu concrete by raising the formwork at midspan.

It has the same effect no matter what the level of cracking in a concrete member, it reduces the visual deflection by the amount of the precamber.

Camber only reduces visual deflection, the actual amount of deflection (movement) is still the same (unless you camber it so much it becomes an arch.

 

[Settingsun]

Agree with Rapt. The definition says compensate for deflection, not reduce deflection.