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Re: Garage Floor Design

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I've been called to investigate a residential garage floor to see whether an
additional weight over the raised garage floor was possible.

The problem was that the garage was at the end of a long down-sloping, fairly
steep driveway with a 6" trench drain, 3' away from the garage door and fairly
level beyond the trench to the garage. Originally, the concrete level inside
and just outside the garage where at the same level (big mistake). Then, about
1/2" pavers were placed on top of the driveway slab. This made the situation
worse by letting water flow into the garage floor.

A waterproofing contractor wants to raise the garage floor level by adding a
leveling concrete topping on the existing floor ( The most economical way,
short of reinforcing the floor joist or reworking the approch to the garage).

The garage floor consists of 4" regular concrete reinforced with #5 bars @
16", over 1-1/8" plywood, over 14" TJI/35 "Residential Silent Floor" joists @
12" o.c.,
spaning 20'.

My preliminary calculations indicate that the floor is underdesigned as it is
when considering the minimum 2000# concentrated load distributed over 20"x20"
area along with uniform dead load ( the 50#/sf live load works). I use 1000#
concentrated load per joist for shear and bending, assuming the 2000# will be
shared by two or more joists (Some plan checkers question the use of 1000#
instead of 2000# !!!).

Does anybody know somthing that I am missing here. Could there be a
justification for using less than 1000# on a single joist?

To make the responses short, I am looking for an answer to my question
regarding the use of less than 1000# concentrated load per joist.

Oshin Tosounian, S.E.