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Concrete Columns supporting PT Slab needs drop caps retrofit
- To: seaint(--nospam--at)seaint.org
- Subject: Concrete Columns supporting PT Slab needs drop caps retrofit
- From: Michael Hemstad <mhemstad(--nospam--at)mbjeng.com>
- Date: Wed, 31 Aug 2011 08:40:22 -0500
Aaron Erickson and Daryl Richardson's advice sounds pretty
We fixed a problem like this a few years ago -- PT deck
over small columns. In that case the original design was marginal for
shear, and then they lost a number of strands over the column head to
A co-worker designed conical caps to be poured around
the tops of the columns, using the references Aaron cites. The issues we
1. When roughening the bonding surface on the
column, how rough is rough enough? If I had it to do over again, I would
sawcut about 1/2" near bottom of cap and chip out a band at least 4 to 6"
tall as a key. Depending on how badly the deck is evidencing a punch
failure, it might be a good idea to shore before you start taking the column
apart. You might even want to try jack it back up if it has really started
to punch. Then epoxy infect the punch zone.
2. How to get a
continuous ring of reinforcing around the column. If I had it to do over
again, I would use a #3 spiral with a lot of turns. The ironworkers would
have to wind it onto the column like putting a key on a split keyring, but it
can be done. I like this better than an external ring of steel, because at
least around here, we want any steel in a parking ramp to be buried in concrete
due to constant infusion of de-icing salt.
3. The cap had to be poured about 4" below the
bottom of deck, then grouted with non-shrink grout. I think we used a
dimension of 3", and it made the contractor's life difficult getting them
poured. If I remember, we used a pea-gravel mix, maybe 6000
Anyhow, for all that, the repairs worked very well and
actually looked great. The contractor had stainless steel conical forms
made, which were a large part of why they looked so good. (Then Halloween
came and vandals stole half of them for scrap. The contractor was
understandably really, really mad.)
Mike Hemstad, P.E., S.E.
Meyer Borgman Johnson