Pier Caps of Varying Cross Section

[minorchord2000]

My question involves pier caps of varying cross-section. The bridge is a two span structure with a single center pier cap supported by two large concrete drilled shafts. Each span is approximately 132' long. The bridge configuration includes AASHTO girders bearing on the pier cap. The pier cap includes 14'cantilever from the center of each of the two columns and a center span of approx 27 feet. The top of the cap is stepped to accommodate the roadway cross slope but the bottom of the cap is arched, with the narrowest depth at the center of the center span. The center span has a minimum depth of 4'6" and a max depth at the face of the column of 7'-6", and 9'-3" at the center of the column. The radius of the underside of the cap is approx 19'-7".

My question involves the configuration and placement of the bottom bars in the cap. Amongst ourselves here at the design office, we all came to the conclusion that the bottom layer of reinforcing should proceed straight left and right with the proper embedment into the columns as required for development. Of course the bottom layer of reinforcing steel is dependent on the max positive moment in the shallowest section of the cap. The bridge designer detailed these bottom bars to match the profile of the curved soffit instead of provding straight bars. I did some internet searches and could not find anything of value.

What do you other bridge engineers think about this?

 

[Qshake]

I would have done the same with the bottom layer of rebar, ie, follow the arc of the bottom of the cap. I would have made sure the bottom steel was continuous overall. Most beam cages are assembled and then picked to set inside forms.

If I wanted to lessen the amount of steel required for the deeper sections I would have made sure the rebar was sufficiently developed past the max post and neg moment (over the columns).

In my opinion, this would would make assembling the rebar cage much easier than using a series of straight bars and worrying about how the ends of the straight bars would be supported near the middle of the cap beam. Of course, #4 hairpin ties could hold them, I don't like to use them if there is a more conventional means of supporting the bars.



Regards,
Qshake

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[BridgeEI]

Agreed, I just finished several piers that were of a similar style. The reinforcing was detailed to follow the shape of the pier cap, otherwise you may not have enough room to get the necessary development length or you end having to deepen the cap itself to satisfy the flexural/longitudinal reinforcing requirements.