beam calculations?

[sravet]

I have a 24'x30' workshop built on piers. The floor is 2x8 joists, 16" OC, running across the 24' dimension. They are attached to the frame of the floor with joist hangers. I would like to park cars in this building. My thought for reinforcing the floor was to pour 4 concrete beams under the joists to support them. The beams would run the 30' length of the building, perpendicular to the joists. Two would be along the walls to support the ends of the joists, and 2 more evenly spaced in the middle. There's currently a single layer of plywood flooring, I would add a second layer of 3/4".

This gives about a 7' span of 2x8 between concrete beams.

The question is, what weight will that 2x8 support over that span? A car could easily park with an axle directly over a joist, such that it has to carry the entire weight of the axle.

Is there a reference that shows how these calculations are done? I'm an engineer, but of the electrical variety not construction. I'd like to run through the calculations myself.

thanks,
--steve

[Rich]

Here's a similar post regarding parking cars on wood structures.
http://www.construction-resource.com/forum/ftopic644.html

[sravet]

Thanks Rich. That thread, and a $7000 quote for a concrete floor, are what got me thinking about reinforcing the current floor.

I've been doing some reading. I found wood span tables, but what I saw was for uniform loads. I also found a program called beamcalc:

http://web2.ehost-services.com/eb1111/civsoft/beamcalc/BeamCalc_main.html

If I pour 5 pony walls under the joists, each a foot wide, then I'll have a span of 4.75 feet. (overall span 24 feet). I put that into the above program with a 2200 lb point load right in the center of the span. Also a uniform load of 40 lbs/ft for flooring and misc, a modulus of 1.2Mpsi which is the smallest value I found for pine

This gives a deflection of .16 inches, which is right at l/360

One thing that occurs to me is that this geometry (5 walls evenly spaced) means that the wall will be under the center of the car, with the left side in the middle of a span and the right side in the middle of a span. This program only does a beam with 2 supports and a single point load but in reality it's a beam with 3 supports and 2 point loads. I'd have to get something more expensive than "free" to calculate that.

Anyway, am I on the right track here?

thanks all,

--steve

[sravet]

Well right after the last post I found MITcalc:

http://www.mitcalc.com/index.htm

which does more complex loads and supports. Putting in a 12' 2x8 with supports at each and and the middle, with 2 2200 lb loads in the middle of the spans gives a deflection of .04 in and a bending stress of 1735 psi

--steve