Slab bridge deflection criteria

[bridgeengineer2007]

Hello, I'm designing a standard reinforced concrete slab bridge and was wondering if there's any published criteria on calculating short/long term deflections of the slab? It seems that you'd still want to incorporate some sort of camber into the slab in order to provide a net long term positive camber to avoid ponding, but I'm having a difficult time finding good information on how to estimate these deflections.

 

[driftLimiter]

What is the span and what is the usage of this slab? You say bridge, do you mean like London Bridge, or do you mean like culvert, or elevated concrete slab for a building?

 

[bridgeengineer2007]

driftLimiter - I'm talking a small bridge structure on abutments, say 40' wide total by anywhere from 20' to 40' long.

 

[driftLimiter]

Surely the governing design code for such a large structure would provide some guidance on this. Which code are you using, and what provisions of it have you looked into regarding this problem?

 

[bridgeengineer2007]

This would follow AASHTO LRFD

 

[driftLimiter]

Since I'm not a bridge engineer I can't really say much more to help you. But from what was needed to Pass the SE exam, I do know that the AASHTO code is quite extensive and covers almost everything needed within the code.

So thats why I asked a two part question. The second part being now the most important part: ...what provisions of it have you looked into regarding this problem?

 

[centondollar]

For permanent load (camber of relevance), the following would be a typical procedure to evaluate precamber:

1. calculate SLS cracked neutral axis height (allow for creep when evaluating modular ratio)
2. calculate SLS cracked second moment of area I
3. calculate cracked bending stiffness Ec*I (elastic modulus reduced by creep coefficient)
4. compute deflection by multiplying the ratio of uncracked to cracked stiffness (EI/EI_cr) with the computed beam or slab deflection: (EI/EI_cr)*w_FEA

Optionally, you can consider tension stiffening (leading to smaller predicted deflection) according to some code formulas.

EDIT: and of course, you are unlikely to be a one-man shop, so ask your coworkers what the practice is in your country/state.

 

[SlideRuleEra]

1) I'm designing a standard reinforced concrete slab bridge...

2) It seems that you'd still want to incorporate some sort of camber into the slab in order to provide a net long term positive camber to avoid ponding.

3) I'm talking a small bridge structure on abutments, say 40' wide total by anywhere from 20' to 40' long.

I'll address your questions as a former bridge contractor (who had previously earned PE).

1) Assume you are talking about a highway "flat slab" bridge (constant deck thickness).

2) You do want positive camber in the final structure, but ponding is not the reason (horizontal crowning of the roadway, often 2% slope each way from center line, will direct water to the parapets where deck drains or slotted parapet eliminate ponding regardless of longitudinal camber).

Primary reason for camber is quality of ride for vehicles traveling on the bridge. Bridge with negative camber will cause a distinct "bump" as vehicles pass over each cap... you will get constant complaints and there is nothing you can really do about it after construction is complete. (I learned this the easy way... from a very experienced bridge contractor, who made that mistake once.)

I can't tell you how much permanent live load camber to design for. Concerning dead load deflection, "means and methods" of a competent bridge contractor's deck falsework can (and will) be designed to compensate / eliminate that factor.

3) Width is not a big deal. A flat slab bridge simple span longer than 30 feet rapidly becomes uneconomical. Evaluate steel or prestressed beams or deck girders before going with flat slab.

 

[BridgeSmith]

Cast in place slab bridges are rarely done anymore, unless there isn't a prestressed concrete fabricator anywhere in the region. Flat precast/prestressed slabs are generally used for 21' to 30' spans only where the available depth for the superstructure is too limited for a prestressed tri-deck section.

Spans 20' and under are typically done with a precast box culvert or 3-sided box.

Anyway, the relatively small permanent load on a slab bridge generally limits the long-term deformation from creep to a fairly small value.