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Re: Residential Garage (Help!)

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James L. Getaz wrote:

. >         List,
. >                 I have heard that the origin of the 20 square inch
. > requirement for a point load in a garage is the load from a jack, of the
. > style that used to be used under a chromed steel bumper. Is this an urban
. > legend? I do not know, but it seems more reasonable me than a tire. Also,
. > this area is used in more Codes than UBC.

People can rationalize anything in attempting an explanation for a
requirement.

Bumper jacks came out after WW2 (1946) when modernized car appearances had
the fender skirts extend below the tops of tires.  The only way to change the
tire was to dangle the tire/axle from the springs, hence the bumper jack.

The 1985 UBC has *all* concentrated loads distributed over a 2'-6" square,
including the 2,000 lb. automobile load.

But, are we really dealing in nit-picking in this discussion?  It seems to me
that the only concern where the area of load distribution comes into play is
in consideration of punch-thru.  For flexure, whether you consider the 2,000
pound load distributed over a length of 20", 4", or 2' isn't going to make a
hill of beans difference in the design of the floor joists.  Even if we
consider load contribution to a joist based on the tributary area loaded, we
are making a theoretical error greater than considering the wheel load as a
point load on the joist in question.

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

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