# Calculating the required depth of a plywood box beam

I need to make a temporary work platform to go between two steel joists 8ft apart where the platform would be around 12in wide. To be on the safe side the platform should be able to support 2 adults, so say 400lb. I was thinking of making a simple box beam from 12mm constructional ply as I have enough available. My question is how deep should the beam be to support the required load?

• Adding to @Ashlar - put the joists on edge, put the plywood on top and then you could easily make a wider platform than the 12" which is at best marginal in width. – Ast Pace Sep 1 '16 at 15:27
• Sagulator is not the solution - it is meant for solid beams, not hollow or box beams. You could use it for checking the capacity of the joists, should you decide to go in that direction. – Ast Pace Sep 1 '16 at 15:28
• My gut says a pair of 2x4s (on edge) and ply well screwed to both sides. – Aloysius Defenestrate Sep 2 '16 at 1:29
• @AstPace One should be able to use the sagulator by adding the standing plywood pieces togetther. (given they are square to the load) – LosManos Sep 2 '16 at 6:31
• Remember to factor in dynamic load, two burly men = 400lb, but add in movement and you could get spikes that greatly exceed that weight. Whatever you end up going with I would err on the side of caution and over-engineer the hell out of it, you won't regret making it too strong. I would personally go with a couple of 2x6s, edge up, screwed and glued to the plywood. If you want to go with plywood only in the solution I'd make up substantial glue-ups of the plywood (basically DIY gluelam) that would stand in place of the 2x6s. – Graphus Sep 2 '16 at 8:03

This is not a sagulator calculation. It is intended for box beams.

Weight load as follows: 8ft span : 12in width : 2x4 Douglas Fir : 40lb psf live load : 20lb psf dead load

Range (dependent of weight distribution) Maximum moment, Mmax: 427-480 (ft:lb) Maximum shear force, Vmax: 120-240 (lb) Nailing pattern: 5in on center

If doubling plywood, glue and nailing should be spaced no closer than 2in from lower layer.

EDIT - HOW I CAME UP WITH VALUES: I was a General B Contractor and I have an app that calculates the formula based on materials(plywood and 2x?) used, what type of live and dead loads required, span length, width and returns the strength values of Mmax and Vmax based on various 2x? and nailing patterns. I based it on his 12mm plywood, 2x? douglas fir, with a live load of 40lb psf and 20lb psf dead load. The range is based on weight distribution whether evenly distributed or in one spot.

• It would be helpful to know how you came up with these numbers. – Katie Kilian Sep 7 '16 at 14:28
• Please see edit. – norcal johnny Sep 8 '16 at 2:06
• I'm not sure what the "if doubling plywood" part means, can you expand? Thanks. – Alan Burlison Sep 11 '16 at 23:29
• @AlanBurlison. If you want to use 2 layers of plywood on the walking surface. Make sure your nailing patter is still 5" on center but since the bottom layer has the same nailing pattern, make sure the top layer of plywoods nailing pattern is nailed in a way that the top layer nails are no closer than 2" to the nailing on the bottom layer of plywood. Or it will weaken the strength of the bottom layer of plywood. – norcal johnny Sep 11 '16 at 23:35
• A quick followup: I built three of these in total, 2 x 2"x4" 8' long joists (well, the metric equivalent) with 12 mm ply on either side, screwed about every 4". Completely rock-solid even with two adults on them - even the architects were happy to stand on them :-) – Alan Burlison May 10 '17 at 22:13