Expert: Roger Zona - 1/30/2012

Question
QUESTION: Hi Roger,
I have a 24x40 double wide modular building(classroom) built in 1992 to BOCA standards. It has a 50lb live - 50lb dead load rating. The floor structure is steel I-beam perimeter frame with 11'10" I-beam cross members set on 4ft centers. I had already added 30% more supports to the frame perimeter in expectation of this use. On top of this steel frame are 2x6 select structural spruce/fir joists 16" O/C with 3/4" OSB flooring. The location is dry - properly drained away from the building and ventilated entirely around the perimeter
I would like to occasionally use part of the building for servicing small automobiles or small tractor. These would weigh 3000lbs gross or 1000lbs maximum per tire. If possible - even a small truck. (1500lbs per tire max) I would expect to add a new plywood floor of at least 3/4" over the area that I want to use for this purpose.
The question I have is if in your opinion the 2x6 floor joists can handle the point loading of the tires for either load range. If not - would adding a thicker plywood overlay make it work? I have 1 5/8" pine plywood available to me.
Thank you,
Ron

ANSWER: Hello Ron,

The building you described sounds very substantial and well built.  It was built knowing that it would be transported and subject to a lot of stress.  

As for your loading capacities:  a small car (6 ft wide x 16 feet long) would have curb weight of less than 3500 lbs but the area would support 4800 lbs.  If you are concerned about additional weight, you must change the joists, not the sub-flooring.  3/4" plywood is very strong.  You should be concerned that all joints are over a joist.  You could add 2"x 6" joists between the existing joists or replace the joists with 2"x 8" or 10" joists.

I hope this has been helpful.

Roger
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---------- FOLLOW-UP ----------

QUESTION: Hi Roger,
Thank you for the quick and thought full reply. The entry to the building is on the gable end of one of the halfs. This places the tires rolling along the length of the 2x6's. I figured that 2 of the 2x6's could handle the load if the tires were rolling between them but didn't know if one could handle the load by it's self. I haven't found any load charts to give me that data for such a short span and high single point load. I don't trust the OSB floor as I have seen OSB get soft with age. I had figured on laying the plywood(4ftx10ft sheets) perpendicular to the joists as this would place the 10ft long seam over top of the steel I-beams and no joints out over the span area of the joists. I just want to get this usable this winter if possible as it's cold here(Michigan) and we need to get a car running. I realize 2x8 or 2x12 joists would have been greatly preferred but I would have to notch them to fit in place now which lessens their strength and loose the insulation and vapor barrier that is in place. Are the existing 2x6 joists sufficient - even just barely - as-is? If not, I can accept that. I would be willing to add more I-beams next summer to reduce the 2x6 joist's span from 4ft to only 2ft if they are not sufficient as is.
Thank you for your insight.
Sincerely,
Ron

Answer
Hi Ron,

Having living in Michigan nearly half of my life, I know what you mean about it being cold!  

You can use the garage as-is as long as you don't exceed a curb weight of 4800 lbs - then next summer add the 2"x 6" joists to increase the load bearing capacity.  You are correct about OSB material.  Plywood is better in this instance.  A tongue and groove edge plywood would be even better.  Secure it to the joists with wood screws rather than nails.  The motion of a car over and over the deck would cause nails to loosen and you could get a puncture.

I hope you have a mild winter and good luck with your project.

Roger
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