To: seaint(--nospam--at)seaint.org, "Vyacheslav Gordin" <scgordin(--nospam--at)hotmail.com>
Subject: Re: Reinforcement of massive pedestals
Date: Wed, 26 May 1999 12:38:24 -0400
If you look at ACI 318, section 18.104.22.168, it states that a steel ratio
of at least 0.005 times the gross area should be used for pedestals,
which this is. UBC has the same wording. Other code provisions lead
us to the conclusion that this is a firm requirement in zones 3 &4.
This works out to 74 - #11 bars vertically. I think most people would
agree that this is excessive, except for rebar suppliers. :-)
What I would propose, is that you look at a "shell" of concrete that
will take all the design loads. Maybe the outer 6" would do it.
Consider this your structural section and use the 0.005 ratio of steel
in this area. Leave the remainder of the concrete in the middle as
plain concrete. It is just a filler.
We have been using much less than the 0.005 ratio for years on our
massive pedestals, with no structural problems or building official
questions that I know of.
_____________________________ Reply Separator _________________________________
Subject: Reinforcement of massive pedestals
Author: "Vyacheslav Gordin" <scgordin(--nospam--at)hotmail.com> at fdinet
Date: 5/26/99 8:11 AM
I need to design a footing for a large vertical vessel in a highly
seismically-active area. Soils are good, though, and allow the footing pad
to be 20'x20'x36." The loads (including considerable weight and seismic
shear and uplift/pullout) are applied through three baseplates @120 degrees
connected to a 12'x12'x4'-6" pedestal centered on the footing. The pedestal
height is dictated by the technological process.
Within the contact areas I installed rebar cages similarly to short tied
"columns." I also tied the tops of the three "columns" together with rebars
to resist considerable shear.
Unsure about the reinforcement of the rest of the pedestal. Both the ACI
0.0018 and Caltrance's 0.0025 reinforcement ratios appear to be excessive.
Any thoughts about how to reinforce the pedestal away from the areas of
contact (apparenly, minimum reinforcement)?
Steve Gordin, S.E.