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RE: Residential Garage (Help!)

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Although the Code definition is important, it's not the only factor to be
considered. I don't think the local load intensity is particularly important
in finding the solution to this problem - it needs a "big picture" approach.

If the joists are aligned parallel with the vehicle, then a front wheel and
a rear wheel will be on the same joist - half the weight of the vehicle. If
the joists are aligned across the garage, both front wheels will cross each
joist as the vehicle enters, so every joist will at some time be loaded with
as much as 60% of the vehicle weight. And the same again as the back wheels
pass over.

If the floor sheathing or deck cannot distribute this load to other joists,
each joist must be capable of carrying half, or 60%, of the vehicle weight
as two spaced point loads. Unless the joists are short, no 2x member can
support the resulting moment. I expect that it would be possible to find a
deck that could transfer point loads imposed near the centre of the span -
so it may be possible to resist the moments using a regular joist
arrangement.

But shear is another matter. If the load is imposed near the end of the
joist - which it will be when the vehicle enters if the joists are parallel
to the parking direction - then it is unlikely that any wood deck could
transfer this load to adjacent joists. The shear will be close to 2000 lb
between the point load and the support (and it may be more because of the
other wheel load on the same joist) - and no 2x member can support 2000 lb
in shear. Bearing may also be tight.

It is possible to solve this problem with 2-ply or 3-ply 2x lumber, or with
single ply manufactured lumber joists - together with adequate decking for
the situation where the wheels are between joists. But this ends up using a
lot of lumber. Alternatively, if you can constrain the vehicle only to use a
certain path you can use beams beneath the wheel tracks, and regular joists
elsewhere.


Peter James




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