Integral concrete pier caps 2

[broekie]

Does anyone know if there are design examples and/or other information somewhere out there for a curved steel girder bridge with an integral post-tensioned concrete pier cap?

I'm working on a bridge where we will most likely be utilizing that kind of system. I've done PT pier caps before, just never one that is integral with the beams. Besides the staged construction and the temporary supports necessary for the beams, I'd just like to get my hands on any literature out there that talks about any other design considerations that I need to consider. Thanks.

 

[Zambo]

Sounds interesting and a bit complicated, so will wait for responses from the learned forum members with interest. I don't quite get how the pier cap will shorten during post-tensioning if it is integral with the beams, but I guess there is some particular design consideration requiring this form of construction.

 

[tumbleleaves]

For those of use with no experience or recollection of an integral pier cap with steel beams and/or post-tensioned pier caps: Why are they built this way? Why integral? Why post-tensioned?

 

[yakpol]

Broekie,

Once upon a time we designed this type of caps. The girders were continuous through the cap with the holes in the web for PT tendons. The drawings were very simple but the contractor complained on limited accessiblility and safety issues while placing reinforcement. On the other occasion we did not bother with post-tensioning and it worked just fine (see attached picture).

 

[graybeach]

One reason to build them this way is to increase vertical clearance below the pier. For example if you could make the pier radial ith an integral pier cap instead of skewed with a conventional pier.

 

[yakpol]

I'll take it back, the straddle bent on the back is post-tensioned.

Yakpol

 

[broekie]

Graybeach is exactly correct. Based on the geometry of the interchange, we don't have vertical clearance to do a conventional pier cap. The road underneath can't be lowered enough to make it work.

We also only have room for a single column, so the cap cantilevers out 20' or so from the face of the column. Because of the large negative moment, the cap is going to need to be post-tensioned. Conventional reinforcing will not work.

 

[crossframe]

Not curved, but it looks like a tangent bridge was recently constructed the same way for the same vertical clearance issues. See current issue of Modern Steel Consstruction magazine. If the pier cap can be radial, the curvature effects may be minimized.

http://digital.ipcprintservices.com/publication/?i=52634

 

[tumbleleaves]

Nice picture Yakpol: conventional, P/T, and even an outrigger.

 

[tumbleleaves]

Okay, one more post for us less experienced folks regarding yakpol's nice pic:

1) How many kips P/T?...P/T force designed to "clamp" beam web? Or like broekie's, control appears to be bending?
2) Where exactly was the strand placed?...at the centroid?
3) What does the detail look like at the anchorage, up against the exterior beam's web?

Interesting post and thread.

 

[broekie]

Tumbleleaves:

1. I'm looking at having to provide approximately 3800 to 4000 kips of P/T force in the final condition. The control is bascially bending. I have a single column hammerhead pier cap. Top fiber tension stress at the face of the column is what is controlling (at least so far in my very preliminary calcs). The P/T, in my mind, will also help clamp the beams together since the concrete will want to crack at each beam.

2. I'm providing 6 ducts, each with approximately 20 strands. 2 ducts are straight along the length of the cap about 9" from the top of cap. The other 4 ducts are parabolic and are located at the centroid of the cap at the fascia and will be as high as I can get them at the face of column. Basically the centroid of the strands matches the dead load moment diagram.

3. Not sure what that is going to look like yet. I'm going to have to provide some bigger holes in the fascia girder web to account for the P/T duct getting wider at that point, plus the spiral reinforcing for the anchorage local zone.

 

[yakpol]

Broekie,
Just a couple of notes:

2. "2 ducts are straight ... 9" from the top of cap". You will need more than 9" from centerline of anchor plate to the face of concrete for 20 strand tendon. Review manufacturer catalogue and recomendations for minimum edge distance.

3. I would keep tendon trumpet and spiral away from the girder. Distance from the girder web to the end of capbeam (including anchor blockout)shall be under 3 feet. The piers you see on the picture are not post-tensioned and have just little of concrete outside of webs.

 

[yakpol]

Forgot to mention about "clamping the girders". For construction you will need to provide steel cross-frames at the pier which later will be embedded in concrete. It will do the "clamping".

Yakpol

 

[broekie]

Yakpol,

You're right. There are a lot of things that we need to look at in more detail - clear distances between tendons and edge of concrete being one of them. We're only at about 10% design phase at this point. But we'll have to probably sweep that tendon from 18" (VSL requirement) to 9".

Interaction between web girder and P/T will also be a design consideration.