Have you considered making the section hollow to reduce the gross concrete
area, and hence the steel? This is sometimes done on large bridge piers. The
concrete and steel saved might not be worth the additional form cost, and
you would need an extra cage, but if you have oversized this column just to
create a footprint for equipment support, it's an option which will greatly
reduce the column weight and might save you a little foundation cost.
From: Lao, Larry [mailto:LaoL(--nospam--at)vmcmail.com]
Sent: Monday, February 05, 2001 2:59 PM
Cc: Kuhn, Bob; Wiegert, Tom; Lao, Larry
Subject: Min. Steel in Concrete Column
We have a 4'x4' concrete column under 110 k axial load (D+L) and 60
k-ft bending, both are at the bottom of the column, the column height is
18', about 4k of shear on top of the column. The 4'x4' was picked for
supporting upper mechanical device.
Obviously, the min. steel requirement in UBC will put about
0.01*48*48=23 in^2 vertical bars into the column, which will provide about
2200k-f bending capacity or 4,000 kips of compression capacity. I remember
have seen some requirement in the code, saying that if a design can provide
100% capacity over the demand, the 1% steel requirement can be ignored,.
Basically, this is to prevent excessive rebars in a non-structural-sized
column. Can anybody back me up on this?