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RE: Garage Floor Design

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Although the 94 edition of the UBC was the first to specifically require the
20 square inch contact area for private garages, I believe the requirement
actually dates all the way back to ANSI A58.1.  I am also familiar with the
prescriptive use of 2x12's +AEA- 12+ACI- o.c. spanning 12'-0+ACI- max.  Based on some
full scale testing here in Anchorage a year of two back, the Building Safety
Department is still allowing the prescriptive use of 2x12's +AEA- 12+ACI- o.c. / 12'
long.  My experience has been that proper detaling and attention to
waterproofing and ventilating the crawl space under the garage framing has a
lot to do with long term performance.  In addition, I have seen some cases
where the driveway has settled in front of the garage slab leaving a 2+ACI- - 3+ACI-
ledge to drive over.  Most homeowners don't take the time to feather the
driveway or ramp up to the garage slab and when the joists are run parralel
to the garage door opening, some distress to the framing can occur due to
the vehicles impacting the slab.  Seems like the way to get over the ledge
is to build up a little more speed.

Regards...Henry David Kim

+AD4- -----Original Message-----
+AD4- From: Randy Vogelgesang
+AD4- Sent: Saturday, November 21, 1998 11:37 AM
+AD4- To:
+AD4- Subject: Re: Garage Floor Design
+AD4- Greg,
+AD4- Both Henry +ACY- Ernie are right, the code was revised a few years back to the
+AD4- 20 sq in area.
+AD4- I also do a lot of residences with raised garage floors up here in the
+AD4- mountains.  For years the standard in this area was to use 2x12's at 12+ACI-oc
+AD4- spanning 12' (+-/-) with 3/4+ACI- ply +ACY- 3.5+ACI- of concrete (not just my
+AD4- office but
+AD4- most of the
+AD4- raised garages built up here).  The code change to the 20 sq in and the
+AD4- lowering of the timber stresses a few years back made the 2x12's not calc.
+AD4- These 2x12 floors have performed well and I did not see the
+AD4- reason to change
+AD4- from the 2.5 ft sq to the 20 sq in .  I also use to use an area of 2.5' x
+AD4- 2.5' (three joists), but that was in the days when the you were allowed to
+AD4- use a little judgement.  :)
+AD4- After the code changed, I had to fight like hell to get plancheck
+AD4- to accept
+AD4- using just two joists +AEA- 12+ACI-oc to support the point load.  They must be
+AD4- trying to fix a problem that doesn't exist.  I have been thinking
+AD4- of trying
+AD4- to use the Caltrans S/4 criteria along with
+AD4- the APA report +ACM- 139 that I mentioned in a previous post so I can get back
+AD4- to using reasonable size lumber.
+AD4- By the way I have seen 25 year old garages with 2x10's +AEA- 16+ACI-oc
+AD4- spaning 12',
+AD4- 5/8+ACI- ply with no concrete , and the Surburban parked inside+ACE-
+AD4- Randy Vogelgesang S.E.
+AD4- S. Lake Tahoe
+AD4- -----Original Message-----
+AD4- From:
+AD4- To:
+AD4- Date: Friday, November 20, 1998 8:25 PM
+AD4- Subject: Re: Garage Floor Design
+AD4- +AD4-Maybe there is something that I've been missing but getting away
+AD4- with. I've
+AD4- +AD4-designed several hillside homes with elevated garage floor
+AD4- systems here in
+AD4- Los
+AD4- +AD4-Angeles. The construction has been very similar to the one you've
+AD4- described.
+AD4- +AD4-If I'm not mistaken, Section 1607.3.3 +ACI-Concentrated Loads+ACI- in the UBC,
+AD4- states
+AD4- +AD4AIg-...placed upon any space 2 1/2 feet square,+ACI-. This does not state 2 1/2
+AD4- +AD4-square feet. I've always interpreted this to mean 2 1/2 feet x 2 1/2 feet
+AD4- +AD4-which would distribute the 2000 pound point load over three
+AD4- joists and not
+AD4- +AD4-two. This reduces the load to 667 pounds per joist. At this point, a
+AD4- uniform
+AD4- +AD4-live load of 667 pounds/2.5 feet (266 pounds/foot) would be
+AD4- applied to that
+AD4- +AD4-area of the joist. Using this methodology, the joists might calc-out.
+AD4- +AD4-
+AD4- +AD4-I would be interested in other engineers' takes on the subject.
+AD4- I've never
+AD4- had
+AD4- +AD4-a problem getting a plan checker to buy into this, and I've never had a
+AD4- +AD4-performance problem with any of the floors I've designed using this.
+AD4- +AD4-
+AD4- +AD4-Greg Riley PE
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