Tuesday, December 9, 2008

Guard Rails...

Written by Chris Goodell, P.E., D. WRE | WEST Consultants
Copyright © RASModel.com. 2008. All rights reserved.

I get this question occasionally...Do I include guardrails on my bridges and culverts as part of the deck station and elevation points? I understand the question here-guardrails, or fencing can be relatively "porous" when the bridge is overtopping. In fact, it's quite conceivable that the bridge could be overtopping, but all the flow is under the guardrail, in which case you would think, "why not leave out the guardrail all together?". However, I think in most cases you should include the guardrail as part of the upper deck. The reason-debris. In most rivers during flood stage, there will be considerable debris floating around. Tree trunks/branches, garbage, cars...etc. It does not take much to plug up the opening under or through a guardrail. So conventional wisdom is you assume the opening at a guard rail will get plugged up during the flood and model it accordingly. It's also the "conservative" approach-meaning this approach might overestimate the extent of flooding. Here's an example of a culvert crossing, with the guardrail included in the upper chord geometry:

3 comments:

  1. thanks, i got it.

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  2. An overly conservative modeling isn't a preferred way an engineer should practice, even though many do!
    Frank

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    Replies
    1. Frank. Given the uncertainty in hydraulic modeling, many applications should lean to the conservative side, and is a preferred way to practice, in my opinion. Most notably, if life-loss is a concern, I think it is appropriate to construct and run a flood event model acknowledging the uncertainty in the inputs and choosing the conservative side of the possible range of input values.

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