Minimum Compression Steel in Pier

[3Fan]

We are designing a large volume pier (for aesthetics) that tapers from 6'-9" thick at the bottom to 4'-0" at the top. The pier is about 52'long and about 11' tall. It is a wall type pier supported on driven piling.

The controlling Asmin of 1% steel AASHTO Std. Spec. 8.18.1.2, nets me 317 #10's (assuming the average thickness and length). I have then reduced that by a ratio of fy/f'c (similar to the LRFD code allows in 5.7.4.2) which gets me down to about 278 #10 bars. This puts my spacing at around 4.5" c/c. This just seems crazy to have that much steel in this pier. Can this % of steel be reduced even more somehow? I would use larger bars, but have been told that the cost of rebar larger than #10's becomes very expensive. Of course nothing about this bridge is cheap.

Thanks!

 

[civilperson]

0.5% is allowed for columns and piers that are dimensioned for aesthetic purposes. Use #13 bars for fewest number on tension side.

 

[3Fan]

Thanks for the tip. Could you provide me some documentation where a code states this?

Thanks!

 

[PEinc]

Check the AASHTO Standard Specifications for Highway Bridges. I just checked an older AASHTO manual (I assume the newer manual has a similar statement) which said in Section 8.18.1.2 Reinforcement of Compression Members: The minimum area of longitudinal reinforcement shall not be less than 0.01 times to gross area, Ag, of the section. When the cross section is larger than that required by consideration of loading, a reduced effective area may be used. The reduced effective area shall not be less than that which would require one percent of longitudinal reinforcement to carry the loading. The minimum number of longitudinal reinforcing bars shall be six for bars in a circular arrangement and four for bars in a rectangular arrangement. The minimum size of bars shall be No. 5.