When is a concrete pier cap considered integral?

Question

When is a concrete pier cap considered integral?

CalebA (Structural)

(OP)

25 Jul 23 15:01

Can someone explain the difference between the pier cap types listed below and how the loads would be transferred for each?

1. Open pier cap
2. Pier cap w/ 1 or 2 rows of dowel bars going into a concrete diaphragm
3. Pier cap w/ stirrups extending from the pier cap into the concrete diaphragm

Here is my understanding of the each type:
1. Loads are not transferred to the piers and go directly to the abutments.
2. Loads get transferred as a moment at the top of the pier cap to the foundation.
3. Loads get transferred as a lateral force at the top of the pier cap to the foundation. (Only type that is truly integral)

BridgeSmith (Structural) · Answer 1 · 2023-07-25 15:55:27

Are you talking about lateral loads longitudinal to the superstructure, such as braking and induced forces due to temperature expansion/contraction, or bending moments in the superstructure?

CalebA (Structural) · Answer 2 · 2023-07-25 15:57:32

RE: When is a concrete pier cap considered integral?

CalebA (Structural)

(OP)

25 Jul 23 15:57

I'm talking about lateral loads.

BridgeSmith (Structural) · Answer 3 · 2023-07-25 18:46:07

For lateral loads, integral or not is irrelevant. Lateral restraint is what matters. Options 2 and 3 are restrained against lateral movement, so they will carry a portion of the lateral loads, based of the bending stiffness of the piers relative to the abutments. Option 1 may be fixed, partially fixed, or free with regard to lateral loading, depending on the type of bearings used.

BridgeNerdGuy (Structural) · Answer 4 · 2023-07-25 19:17:34

Horizontal shear is transferred from the superstructure to the sub and from the pier cap to the pier column by any element that crosses the boundaries between elements or has friction acting at the boundary.

1. I have never heard the term "Open Pier Cap" in my market but no practical system (bearings) is truly 100% free and allows all longitudinal loads to bypass the piers and go directly to the abutments.
2 & 3. Shear can be transferred through a row of dowels, rows of dowels, or the stirrups. Usually there is some protruding bars form the column to cap and cap to diaphragm.
Interface shear between pours at the boundary will also transfer load.

Assuming you mean longitudinal loads, the loads are going to be distributed proportional to the stiffness of the individual supports. Think of the bridge abutments and piers as a system of springs acting in parallel. The stiffness of each element and a total longitudinal stiffness of the bridge can be computed and the loads will go to each abutment and pier proportionally (Ki/Ktot). You seem to be thinking about continuity for moment but that's not relevant for transfer of horizontal shear.

BridgeSmith (Structural) · Answer 5 · 2023-07-25 20:05:18

Quote (BridgeNerdGuy)
...no practical system (bearings) is truly 100% free and allows all longitudinal loads to bypass the piers and go directly to the abutments.

They're not commonly used in new bridges anymore, but rocker bearings come close enough to zero lateral force transfer in the longitudinal direction to be considered free.

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When is a concrete pier cap considered integral?