Stream Force on Bridge Piers and Abutments

[jmontoya]

On AASHTO 17th Ed Section 3.18.1.2 the code tells "When the direction of the stream force is other than normal to the exposed suface.....the effects of the directional components of stram flow pressure shall be investigated."
My question is: How do you define expose area, area perpendicular to the flow of the member or area along the member?

 

[msquared48]

The exposed area of the pier is the width of the pier multiplied by the 100 year depth of flow at that location.

However, I would not forget about the instance of logjambs at the piers during high runoff. I've seen it happen and often. That will substantially increase the lateral load to the piers.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[asixth]

Often the piers will be aligned as best they can to the direction of flow.

 

[tai99]

A pier consists of 4 faces - upstream, downstream, and two side faces. The long direction is usually aligned in direction of flow, however, skewed piers happen quite often also. So, other than head-on forces, there is also side pressures. Depending on the usage of the flow channel, you may need to check effects of vessel/log impact, debris and ice loads.

 

[Qshake]

Piers are most often skewed to align best with flow, however there many applications where this is not the case. For example, in a large watershed area that has a small creek but a very large overflow area or valley section. In the overflow area, stream flow may not be aligned with the creek proper and in extreme cases, flow can be somewhat perpendicular to flow. Can you align piers to match overflow? Sure but for long bridges and odd stream meander as well as valley direction it's not always possible.

Regards,
Qshake

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