Designing Bridge Piers for Impact, Flood 1

[AggieYank]

A current project involves a footbridge with piers ~15' out of the ground to the bridge itself. It is located so that it won't flood in case of a 100 yr flood.

Some considerations:
-Designing for impact of floating debree, tree, waves, differnet water levels, etc.
-General weathering requirements for the piers.

I haven't found a good resource for minimum impact forces. General weathering requirements would seem to be using epoxy coated rebar (not sure if this is necessary), using a 3" cover, and using a low slump concrete for longer service life. Is there any code specifically addressing this? Thanks in advance for any advice / references.

 

[sqrt]

Your hydraulics engineer should give you a velocity, typcially via USACE software. I've assumed and analyzed boulder impacts, but avoided giving an answer, too many unknowns and I wouldn't want something documented pinning me down to a specific boulder.

Your geotech should give you values for the environment to determine final concrete cover. All those things you mentioned need to be assessed by the bridge engineer.

AASHTO Stand Spec's would work as well as any.

 

[MotorCity]

AggieYank,
I was just involved in the design of pedestrian bridges in a nature preserve. Most of the bridges were located in wetlands/floodplain areas subject to frequent flooding. I found that it really depends on what causes the flood. For example, if the flood is caused by the sudden release of water say from a water tower or dam collapsing, that is much different than a low lying area flooding due to rising levels of precipitation. On this particular site, the flood waters were almost stagnant, or moved very slowly according to the park officials. Therefore, I neglected impact of floating objects. However, I did contact the Army Corps of Engineers and obtained velocity data for the river and designed the bridge accordingly. Another feature I used was a galvanized grating for the deck to reduce uplift forces from the flood waters. Good luck.

 

[cvg]

most dam breaks are considered extreme events and likely exceed the 100-year flood by a large margin. Commonly they are considered to be 1,000-year or larger events. Most bridges are designed to pass the 100-year flood with freeboard below the low chord of the bridge and to be able to tolerate the 500-year flood without collapsing. The 500-year flood may overtop the bridge or cause pressure flow through the bridge opening, but should not push it over, off it's foundation or scour the piers and cause failure. You should consider that during the flood, you will have debris wrapped on each pier and during a major flood will have trees and other large debris slamming into the piers. This force should be considered in the structural design. Think back to Katrina - you had barges and oil well platforms banging into bridges, causing catastrophic failure.

 

[Qshake]

I wish I could point you to a direct source on the matter but I cannot. I would recommend that you investigate the Journal of Bridge Engineering by ASCE as a start. However, I don't recall any articles on that matter, but I don't read each and every one.

As to practice - I can offer my experiences. We apply stream forces per AASHTO and where applicable modify per the hydraulic engineer's recommendations.

I have designed many bridges over sustaining streams and dry creeks that swell considerably during flash conditions. Some in heavily wooded areas and others in plains. I have seen debris stacked against bridge piers on major rivers and have see drift wood so thick that people can walk over it as it sits in the water.

I have found that rarely do such conditions control as other lateral forces were often greater. However, I do modify the area of the pier assuming it is locked in drift when such piers are located near the banks.

I have never used epoxy coated reinforcing in piers unless specifically requested by the client. Some use epoxy in the whole structure while others do not. I am not generally in favor of substructure steel being epoxy coated due to the conditions of the coating following delivery, siiting on the job site and then being slung around by rodbenders. Once the coating is compromised it really defeats the purpose. And getting a contractor to fix that coating is like getting a straight answer from a lawyer.

Regards,
Qshake

Eng-Tips Forums:Real Solutions for Real Problems Really Quick.

 

[IFRs]

I sometimes see a steel angle pointing into the current presumably to protect the concrete from impacts.