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RE: Residential Garage (Help!)

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The 1991 UBC still had concentrated loads, including automobile loads,
distributed over a 2.5-foot square.  The 1993 Accumulative Supplement to the
UBC changed the requirement to a concentrated load in passenger car garages
being distributed over 20 square inches.

The change was proposed by SEAOC [No. 120, Building Standards, Part III,
November-December 1991, pg 86], with the reason given as:

"This revision will make the Building Code concentrated loading distribution
consistent with ANSI standards for concentrated loads for passenger cars.
The lack of a definite surface over which the 2,000-pound load is to be
distributed has created design and plan check problems for many years.  This
change will produce a uniform code application for this design criteria."

Analysis [by ICBO staff]:  None

It should be noted that ANSI/ASCE 7-88, as well as ASCE 7-93, requires for
garages for trucks and buses the concentrated load be, "...maximum *axle*
load on an area of 20 in^2." [Emphasis added.]

A. Roger Turk, P.E.(Structural)
Tucson, Arizona

Jim Lutz wrote:

You're right! I should have looked this up. I was thinking about the 2.5 ft
square provision for application of concentrated floor loads in UBC
1607.3.2. I gotta get out of the habit of pulling the trigger before the gun
is out of the holster!

-----Original Message-----
From: J. K. [mailto:structure_r_us(--nospam--at)]
Sent: Wednesday, March 27, 2002 7:54 PM
To: seaint(--nospam--at)
Subject: RE: Residential Garage (Help!)

--- "Lutz, James" <JLUTZ(--nospam--at)> wrote:
> I think the code reads 20 inches square, or 20 x 20.
> That gives you an
> average bearing pressure on the loaded area of
> 2000lbs/4000 = 50 psi,
> which is a typical high tire pressure for light
> vehicles.

!!!!!Watch out!!!!!
UBC contact area is 20 in^2, not 20"x20". Pressure
becomes 2000 lb/20 in^2 = 100 psi. It can not be 50
psi - you are not designing a roof rafer or a ceiling

It could be the shear that controls the design.

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