Bridge Pier Strengthening 1

[tmalik3156]

Good day all.

A bridge pier is showing some vertical cracks near the top. The client wants to restrain the cracks from further widening by applying lateral compressive forces using anchors. Below is the schematics of the idea. The tension rods will be stressed and attached to the anchors at both ends.

But the pier ends are curved (in plan) and sloping (in elevation). So designing the shape of the anchor is a challenge. A custom made anchor with a curved inside will be expensive. So we are thinking of a straight anchor made of steel section (channels or W shapes), with some type of timber shim between the curve of the pier and the straight face of the anchor. The whole system will get embedded in a concrete cap, once the stressing is done. Each of the four tension rods (DYWIDAG thread bar) will be subjected to a tension of about 450 kN.

Do you think this will work? Any other clever idea on how to design the system?

 

[BridgeSmith]

Your client needs to let a consulting engineer with experience in bridge rehabilitation evaluate the pier and design a repair, if it's necessary.

 

[tmalik3156]

Your client needs to let a consulting engineer with experience in bridge rehabilitation evaluate the pier and design a repair, if it's necessary.

Thank you. Yes, evaluation of the pier is completed. Out of several rehabilitation options, the client chose this anchor-tension rod method.
The focus is now on the design of the anchor. I am looking for ideas on how to make the anchor work, given the curvature of the pier.

 

[BridgeSmith]

Unless the cracks were right at the ends of the pier, the typical way I've seen to accomplish external post-tensioning like this would be to add anchor plates to the side faces of the pier, anchored with with high strength threaded bars through the pier to a matching anchor plate on the other side.

If that's not an option, I suppose you'd probably need a custom-fabricated steel anchor block that fits around the curve and is angled to match the taper. I can't imagine wood having adequate durability for this, and the compression capacity perpendicular to the grain is fairly low compared to the other options, making the required contact area much larger. A fiberglass reinforced pad (FGP) or a cotton duck reinforced pad (CDP), 1/2" to 1" similar to what is used for girder bearing pads, might make for an effective cushion between the steel anchor block and the concrete. The CDP will have a compression capacity close to that of the concrete (3.0 ksi), while the FGP compression capacity is limited to 1.0ksi, per the AASHTO bridge design spec.

I would not rely on friction to keep the anchor blocks in place. I would definitely add some anchor bolts into the end of the pier to keep the anchor blocks in place. Drilled holes for threaded rod and adhesive anchorage is what we do. Our current preapproved ashesive products are: CIA-GEL 6000-GP, Red Head C6+, Sure Anchor I J51, and HIT-RE 500 V3. Be sure they clean the holes really well and mix the 2 parts of the epoxy thoroughly.

 

[bridgebuster]

Can you place some concrete at the ends of the pier to create a vertical surface for the anchorage?

 

[tmalik3156]

@BridgeSmith, thank you very much for the detailed reply. Is CDP only available as small box shapes (like bridge bearings), or are they also available as longer sheets (say 4' x 1' x 1")? In the latter case, we could place a sheet on the curved surface, and then place a similarly curved steel plate on it to act as an anchor. Shown below.

@bridgebuster, unfortunately additional reinforcement under the bearing is so crowded that we can not insert dowels to create a vertical concrete surface there. Shown below.

 

[BridgeSmith]

@BridgeSmith, thank you very much for the detailed reply. Is CDP only available as small box shapes (like bridge bearings), or are they also available as longer sheets (say 4' x 1' x 1")? In the latter case, we could place a sheet on the curved surface, and then place a similarly curved steel plate on it to act as an anchor. Shown below.

They should be available in any size you need. If I understand the process correctly, both CDP and FGP are typically fabricated in large sheets and cut to size. I believe they're flexible enough to wrap around the end of the pier. You should check around for a fabricator, and inquire with them. We've only designed and specified plain and steel reinforced elastomeric pads.

 

[BridgeSmith]

With vertical and transverse confinement reinforcement, I'm surprised that external confinement is necessary. Is the existing confinement reinforcement inadequate for the loading?