Design of Plywood over Square Hole? 1

[mtm5248]

I am trying to design/analyze a sheet of plywood over a 24" x 24" hole. I would rather not analyze it as just a simply supported sheet beam over a 24" span because I want to be as accurate as possible. If anyone knows how best to approach this, it would be appreciated.

 

[jayrod12]

Why bother? You can't spend a short enough period of time to make it cost effective. What kind of loading are we talking about?

 

[mtm5248]

Looking at 50psf, which makes this whole thing seem stupid, I was just curious if there was a relatively easy way to figure this out.

 

[PEinc]

Go on line and look for Dayton Superior's site for concrete formwork materials and design. The attached file says "Dayton Richmond" but the company is now "Dayton Superior." The attachment shows 3/4" Plyform good for supporting 100 PSF over a 24 inch span using its strong orientation but only a 19 inch span for its weak direction. At only a 50 psf load, your allowable spans should be greater (but not double). The span lengths are controlled by deflection, which may not be your big concern.

www.PeirceEngineering.com

 

[jayrod12]

My first feeling is just put a piece of 3/4" plywood down and be done with it. Supported on all 4 sides (install blocking as required).

 

[rb1957]

"... this whole thing seem stupid" ... yep, couldn't expressed it better myself

another day in paradise, or is paradise one day closer ?

 

[XR250]

Might as well check some membrane action while you are at it

 

[mike20793]

The span rating of plywood is how far it can span under 100 psf load.

 

[XR250]

The span rating of plywood is how far it can span under 100 psf load.

I believe that assumes a 2-span condition min.

 

[mike20793]

XR250, thanks for adding that.

 

[dhengr]

Mtm5248:
We can make a life’s work out of damn near any problem, however dumb. The trick is, and the real valuable knowledge and experience is gained, by learning which problems are important enough to spend your time on. When your loads are generally conservative guesses/assumptions, and your allowable stresses, etc. have a factor of safety of 2 +/-, why would you try to fool yourself with an elaborate, time consuming, analysis method, on a problem where you can’t really even define the support conditions? With the advent of computer analysis, the almost total loss of any common sense or engineering judgement and our overly complex and convoluted codes and stds., we are mostly fooling ourselves into thinking that we have a more perfect and absolute answer. The funny thing is, the structure is smarter than we are; it can/does recognize a poor design (and analysis) and performs or fails accordingly.

 

[Old Eng]

dhengr: Well stated. As an old guy here I will not be around say 30 years from now, but I hate to think how dumbded down the engineers will be to have to use a computer or what ever is there in the future,instead of common sense. In the subject case, let's say the opening is 5" x 5". Some design method is needed to figure the covering??? I'm really surprised at why the question even is posed. Guess my 30 year time period was about 30 years too long.

 

[XR250]

@dhengr: so eloquent and true - thanks

 

[IFRs]

On the other hand, it could be useful to gain experience with an analysis that has no time pressure, no risk of failure and can easily verified by modeling it in your backyard. Learning on simple no-fault examples paves the way to understand the mechanism, limitations, what to look out fors, etc. of the analysis methodology and having that experience makes you a better engineer with another tool in your belt that you have experience with and confidence in. I support this spirit of question as I read it.

 

[SlideRuleEra]

I'm in total agreement with IFRs. The OP is wise to realize that plywood does not just mimic the structural properties of lumber. Plywood has it's own quirks, such as the concepts of rolling shear, "effective section modulus", and moment of inertia that varies with face grain orientation.

Also, the OP did not tell us what thickness plywood is required for the application. So using 3/4" is a "slam-dunk"... what about 1/2" or 3/8"?

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[jayrod12]

If it's a 50psf area, I would never expect to see anything less than 5/8" and when it's a single span 2 ft square patch, then I'd prefer it is 3/4", however if the existing sheathing (assuming there is some) is 5/8" then I'd probably take a closer look at it. Or not look at it at all and just put 5/8" down to match.

 

[racookpe1978]

OK, OK. I understand the need to "ballpark estimate" this problem (1) it is a very light load (lbs/sq foot) and a very small opening. BUT. The physical requirement and th rreal world safety and practical requirements remain unsolved and undicussed with three parts:
(1) Cover the hole, but make sure the cover does not skid out from somebody's foot or when a pallet jack or forklift truck "pushes" the plywood from over the hole
(2) Cover the hole, but without anything sticking up to trip a person walking over the small hole. This isn't a irregular flooring where you walk around stepping carefully on each edge of each plywood over the whole floor. Nor should you require a person to look at the floor to be sure he/she isn't going to trip.
(3) Cover the hole, but with an adequate fire proof/removeable/permanent/temporary cover appropriate for the chemicals and purpoise (indoor/outdoor/rain water ice) problems that will (or will not) occur.

So, I have seen several cases where a small plywood piece over a hole is correct, and have tripped myself over small plywood boards that stick up, and have had to recover/rebuild small plywood covers (as FME covers) where a two-ply nailed construction was needed not for the weight of the plywood nor its strength across the hole, but so the cover would not get "kicked off" of the hole.

Form. Fit. Function.