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RE: Parking Garage Retrofit

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Mike,

I have heard of external post-tensioning for the retrofit of parking garage
beams and joists.  I'm not sure about the economics of it, but I expect it
could be fairly efficient.  Nick Watry, SE, spoke about this method at a
seminar I attended recently.

Dmitri Wright, PE
Progressive Consultants Inc.



From: "Mike Zaitz" <mzaitz(--nospam--at)hgbd.com>
To: <seaint(--nospam--at)seaint.org>
Subject: Parking Garage Retrofit

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Morning;
=20
We are getting involved in a study for a vertical addition to a 2 story
parking garage.  The original design is precast girders and double tees =
with
cast in place fixed based columns on pile caps with piles. A note on the
drawings indicates that the structure was designed for 1 future level =
for
gravity only.  Any seismic and wind resistance for the future level =
would
require additional bracing or shear walls.  Based on some preliminary
research the following has been discovered:
=20
.        The structure was designed under the 1994 Standard Building =
Code
(1994 SBC) and the expansion will require design per the 2000 Standard
Building Code (2000 SBC aka the IBC 2000).

.        The seismic forces in the cantilevered columns have increased =
82%
with the current configuration and the new seismic loads.  The columns =
as
currently reinforced are inadequate for these new forces and will =
require
retrofitting.

.        The 2000 SBC has changed the method of reducing Live Load for a
passenger car parking garage.  It allows a 20% reduction in Live Load =
for
members supporting 2 or more floors.  This would allow the reduction to =
be
applied to the columns and the footings.  No reduction in Live Load is
permitted for horizontal members.  The 1994 SBC allowed a reduction of =
up to
40% for horizontal members and 60% for vertical members based on the
tributary area of the member.  The calculations for the precast members
needs to be reviewed to determine the final impact of this.  I am =
waiting
for these calculations to be sent to me but there is a distinct =
possibility
that the existing precast members will require retrofitting.  This will =
also
impact the foundations. =20

What I am trying to do now is to determine what methods of retrofitting =
the
project are even in the realm of feasibility (throwing mud on the wall =
and
seeing what sticks as my boss calls it).  I am looking at adding shear =
walls
to take ALL of the lateral forces and then the existing columns will =
only be
required to resist gravity loads (will make the tying of the existing
diaphragm to the shear walls interesting).  Based on this I believe that
wraps on the columns are a feasible approach for increasing the capacity =
of
the columns.  Another idea is to jacket the columns with additional =
concrete
and steel.  A possibility for retrofitting the double tees is to use the
carbon fiber wraps on the stems and then pouring a 2" topping slab.  =
This
will cause numerous problems with the added weight, etc.  No real ideas =
for
reinforcing the girders. =20

With the budget set at about $10,700 per space I do not think there is =
any
chance that retrofitting the garage is economically feasible.  I am =
guessing
that it will be more in the range of $15,000+ per space.  It looks like =
our
primary solution to the parking issue will be to build a new parking =
garage
near the existing.

Any comments or suggestions?





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