Establishing beam seats for prestressed concrete beams

[JHart_11]

How is the camber for a prestressed concrete beam accounted for in the bridge beam seat? Is it the mid-span camber that is used to make the beam seat adjustment? If my final camber is an upward deflection, do I raise the beam seat to make up for the camber or lower it? I am thoroughly confused. Any assistance would be greatly appreciated.

 

[BridgeSmith]

Typically, you would have to lower the seat elevation IF you are adding a topping/leveling layer (variable thickness asphalt) to mitigate the camber to meet the required roadway profile.

We typically wouldn't make any adjustments to the seat elevations, as we would only use prestressed beams with substantial camber on 1) A low speed roadway, where the vertical curvature of the roadway profile resulting from the camber is not a concern, or 2) where the camber fits the desired roadway profile, i.e. the designed roadway profile is crest vertical curve at the bridge.

If the project requires a limited amount of camber, the fabricator can usually limit it through the use of harped strands or delaying the release of the strands for a few days. Another option is to use a larger concrete section. Any of those will add to the cost of the girders.

 

[JHart_11]

Thank you HotRod10 for the reply and the information. I was following the guidance provided in the FHWA Publication No. FHWA-NHI-15-058 Link. In Design Step 5.6.7.1 it calculates final camber and then states, "This camber is used to determine bridge seat elevations." If I was going to include camber in by bridge seat elevations, would I determine my desired top of deck (or wearing surface) and subtract from that the minimum deck (or topping thickness), the beam depth, the bearing height AND the final camber to determine the bridge seat elevations that I would show in my plans? I understand that the camber can be made up in a haunch if my beams have a haunch, but I am dealing with box beams that will only have an asphalt wearing surface.

Thanks again for the previous response. Any additional guidance would be appreciated.

 

[BridgeSmith]

Your case would be similar to what I described in my previous post. The camber will affect the thickness of the asphalt at the abutments. Let's assume a straight grade profile and the camber in the beams to be compensated for is the 0.96" in the example. If the minimum thickness of the asphalt is 2", then the thickness at the abutment will be 2.96". That 2.96" would be used to determine the seat elevation.

If you're still having trouble, draw an elevation view of the superstructure and abutments with the camber in the girders, and it should become obvious.

 

[JHart_11]

Got it! Thanks.