Expert: Dan Griffin - 4/14/2011

Question
We requested consultation from OSHA and were required to place load limit signs on two existing Mezzanines where we store goods.

1.) Warehouse Mezzanine: Has 2 foot on center floor joists, with ¾ inch plywood, 2x8’s are used. The Mezzanine is 10 x 12

2.) Service Mezzanine: Has 16 inch on center floor joists, with ½ inch plywood, 2x8’s are used. Mezzanine is 20 x 38

I am not sure how to calculate these limits and have had no luck finding the equations online. Would love some help.

Sincerely,

Paul

Answer
Paul, I'm sorry.  I wrote a lengthy answer to your dilemma, no idea where it disappeared in cyberland.  Let's try again.

Any questions about load bearing capacity, joists, floors, etc are very subject to a number of things:
1.  Lumber species.  Douglas fir and southern yellow pine are the two strongest structural dimanesion lumbers.  There should be some lumber stamps on every joist in your structures.
2.  Lumber grade.  Based on growth rings, number and type of knots, etc there are different grades within species.  Again, this should be indicated in the stamps on the joists.
3.  Spacing.  16, 24, etc.  YOu've indicated these.
4.  Deflection limits.  How far are you willing to let the joists sag?  Code typically requires L/360 as a minimum which is about 5/8" on a 20' span.
5.  Live load.  This is what OSHA wants you to post.  This indicates loads imposed on the floor system like humans, file cabinets, pianos, hot tubs, whatever.  Code requires house floors to have a minimum of 40#/SF.  Paper and books are among the worst and easily go to 100#/SF or more
6.  Dead loads.  The weight of the material in the system.  2x8's, plywood, nails, etc.  A normal allowance is 10#/SF unless there is something unusual about the framing.  If there is a ceiling fastened to the joists, it must be added to the loads.

Any floor system is also subject to proper framing methods which includes perimeter fastening, bridging, supporting beams, columns, location and size of fasteners if attached to a wall structure.  An OSHA or other inspector may well require an engineer's stamp to verify conditions and capacities.

You've not told me how these mezzanines are carried/supported or which way they span.

There are many span tables available, I like to work with this one:
http://www.awc.org/calculators/span/calc/timbercalcstyle.asp

Let's look at your warehouse mezzanine.  YOu don't indicate what the span is.
Let's assume a few things:
Southern pine
#2 grade lumber (very common)
Floor joists
24" centers
40# loading
L/360 deflection

The calculator allows you to span 11-0
If you use 50# loading you can span 10-0

If you vary the input, you will get a good indication of the load capacity.  YOu do need to use the actual data that applies to your structure.

When I do the same thing for your service mezzanine, I don't like the results - I hope there is more support somewhere.  Again, I need to assume you are trying to span 20'
Southern pine,#2, floor, 16"o.c., 40@, L/360

The calculator will only allow 12'-10".  YOu are way undersized.  

You can input the information
Southern pine, Dense select, ceiling joists, 20#, L/360
The calculator will only allow 15' span

YOu can adjust the variables to match your data and arrive at a loading limit.  I suspect your inspector didn't like something he saw, though loading signage is a standard requirement.

I hope this gives you the information you need.