plywood deck design 4

[kaffy]

Lets say we have a plywood sheet 4' x 8' and I am trying to find out the force this sheet can withstand if the thickness is 18.5mm, 6 plies and material is unsanded douglas fir plywood.

I am not worried about supports, load durations and other factors at this point. I know they are important but our example is fictional so I am gonna assume all are 1.

My first question is

while doing the moment calculations, what is the value of specific bending capacity I need to take?

I think it has to be 930 N.mm/mm because when the force acts, compressive and tension forces are generated in the direction of grain. Hence the angle between them is 0 and as per table in attached file, value is 930N.mm/mm.

My second question is

Except bending, planner shear due to bending and deflection. Do I have to check any other criteria?

Thank You very much
Newbie

 

[le99]

1) Correct, you shall check against the allowable bending stress in direction of the grain. Unlike a compression member, there is no concern about the crushing force/stress (P/A).

2) No comment.

 

[dhengr]

Kaffy:
I think there was some confusion in your earlier thread, particularly in the use of the words force and stress. A force is the load applied, in lbs./sq.ft. or lbs./ft., or other units, for example. Then, the way you span the sht. of plywd., or load it, induces stresses (or a stress field, bending normal stresses, shear or compressive stresses, etc.), and the orientation of this stress field is either parallel or perpendicular to the face grain (the grain of the outer plys). The orientation of the grain in the outer plys, in this lamination, determines its strongest orientation under various loading conditions. The strength of the wood is better/higher when the stresses are parallel to the grain than when the stresses are across or though the orientation of the grain. Thus, the differences in your tabulation, and of course, allowable stresses in tension, compression, shear, etc. are all different too.

Secondly, you add more confusion to the situation by providing some half-assed problem description (in both threads), which many people will reject, out-of-hand, as being too dumb to warrant much wasted time. If the guy doesn’t know that spanning a sht. of plywd. 8’ usually isn’t worth talking about, why should I waste my time? Show and describe the real problem, with some specificity and reasonably accurate proportions. This will take a few more lines in the sketch, some good proportions in the sketch, and a few more sentences in the description. And, you will be surprised at how much more, knowledgeable, experienced engineers will intuitively understand your problem, and be able to offer meaningful and constructive advice. If this is the floor of the cab of an elevator, it will probably have four outside framing members (maybe two diff. sizes of channels for long and short directions) and four or five cross framing members, which the plywd. flooring spans over. Now, you have a real problem, except it probably brings up additional problems for you. What do the shear and moment diagrams look like over the length of the plywd.; and, a point load will not likely act over the full width of the sht. (but over some effective width), and its worst position would be near a sht. edge. A uniform loading will act over the full sht. width, but you might want to take a look at some unbalanced (skip) loading too.

The tabulation you show looks about right to me, but what does the asterisk (*) mean? You should always read the footnotes and surrounding verbiage to be sure you interpret that tabulation correctly. Even number of plys is not the common makeup of structural plywd., because the plys should alternate in orientation, with the outer plys in the same direction.

 

[kaffy]

@dhengr

Thank you very much for detailed reply. I will try to elaborate as much as possible in next threads.
In this particular scenario, * means the most commonly available product.