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RE: Parking Garage Retrofit

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For your consideration:

1. Use a light weight concrete for the additional level and columns. You can get a 110 pcf light weight fairly easily and save a significant amount of dead load over the original level considerations. It will also reduce seismic demand.

2. There are many ways to reinforce the existing foundations if required. It depends on what the foundations are currently. Consider helicals or auger cast piles.

3. If the joists require reinforcing, I doubt that the economics will work. Wrapping the columns should not be that big of a deal.

4. The shear walls are the best way to go. There are some details that you should consider to allow vertical movement while restraining lateral forces.

Regards,
Harold Sprague





From: "Mike Zaitz" <mzaitz(--nospam--at)hgbd.com>
Reply-To: <seaint(--nospam--at)seaint.org>
To: <seaint(--nospam--at)seaint.org>
Subject: Parking Garage Retrofit
Date: Tue, 28 Sep 2004 11:31:10 -0400

Morning;

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:

.        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.

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.

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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