AASHTO Water Loads

[sforesman]

My question is in reference to AASHTO LRFD Bridge Design Specifications 3.7.3 - Stream Pressure.

We are looking to underpin an existing wall type pier within a river and need to calculate the water loads. We know that the velocity, V = 17 ft/s and that the CD for the pier is 0.8 from Table 3.7.3.1-1 and therefore our longitudinal pressure p = 231 psf. What area is this pressure applied to?

My confusion lies with CD, the drag coefficient, which makes me want to apply this force longitudinally along both sides of this pier i.e. (2 sides)*(14ft high)*(50ft long)*(231psf) = 323 kips, sounds high.

However only applying the pressure to the end of the pier i.e. (3ft wide)*(14ft high)*(231 psf) = 10 kips, sounds low.

Any insight with references would be great, thanks.

 

[wiktor]

Your calculations appear to be right, and the resulting forces, although small, are reasonable.
These forces should be re-calculated using “debris raft” provision (commentary section) using engineering judgment when increasing stream velocity and sizing the raft.
Compare these forces to the ice loads (3.9) to have some form of checking the magnitude of your forces.