Question for the Mechanical/Structural Engineers: Shelf Load Limit

Well first of all I wouldn't use plywood outside, even if it was pressure treated and sealed, on a horizontal plane.....This entire design hinges on the strength of the wood....that's a variable...My rule of thumb for working with wood outside is to over engineer every facet of the design....start out with a good solid design and multiply everything by about three....then check it carefully every six months for any weaknesses emerging, and deal with it.....

Is marine plywood acceptable for outside use?

The 'picnic tables' would only be brought out for parties, not left to weather in the elements.

My main concern would be someone leaning on it while eating ("Hey, no elbows on the table!"), using it to push off of while standing up, or (worst case scenario) trying to use it to break a fall.

If you have the room for storage, and enough energy to set up and remove them each time, then why not just use plastic fold up tables and folding chairs??? I have friends that do this....but lugging heavy wooden tables and the storage space involved just not a good idea....either make them permanent or use tables designed to be portable...

$250. for the set....

You do have to assume someone is going to try to use the table edge to lift themselves up or break their fall. It will likely be a person with a lot of weight to heft too.

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The thing that jumps out immediately is the distance between the 'side supports'. I don't know the length of the picnic table, but even if it is not that long, there will be a lot of room between the side supports and the middle of the table.

The table surface in the center will be a long lever attempting to lift the upper support/push the lower back support down.

Over simplifying here, let's pretend the lower back support remains perfectly rigid. 200# force down at the edge 10.5" from the pivot translates to a #1600 force up 1.5" on the other side , trying to lift off the upper support. That can likely move for a distance before the side supports really come into play.

A simple solution would be additional gussets like the side supports. These don't have to be knee killers as big as the side supports. Just something to move the pivot point out to a more comfortable distance. There are also a variety of ready made metal shelf supports that wouldn't steal any room that would be missed.

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Edit: re-reading...are you saying the picnic tables in their longest dimension are 24"? Also, you mention added to railing, then mention a slat wall, and further note this will not be permanently attached.....I am unclear about how the table is attached/supported and to what.

The 'picnic tables' are indeed two feet wide by one foot deep, each table is enough for one diner to eat comfortably while looking out over the back yard watching the kids and/or dogs play. Think of it as a modular system, where the 'table space' can be added or removed based on the size of the party and the staggered arrival/departure of the guests. I might even put horizontal slots into the side supports to make carry handles, so each unit would be like a 'lunchroom tray' that becomes its own table space. Things are still rather nebulous⁽¹⁾ and I'm looking for the 'novelty factor' to impress friends and family⁽²⁾.

The slats of the slat-wall are permanently attached to the railing, and the cleat on the back of the shelf/'picnic table' engages the slat to hold it to the wall.

As I said in the footnotes of my OP, this slat wall system is based on the French Cleat ( https://en.wikipedia.org/wiki/French_cleat ), used primarily as an aid to hanging cabinetry on walls, since, in theory, the wall cleat and cabinet function like a single piece of mountain rail once they've engaged each other.

Notes:

1. The deck is still in the 'plans are being submitted to the construction permit department of City Call and awaiting approval' stage, and I've still got a lot of work to do on the slat-wall system in the garage⁽³⁾, so nothing is moving forward with the 'picnic tables' until spring at the absolute earliest, unless you count building shelves for the garage slat-wall system as practice.
2. When you have three households in the same suburban cul-de-sac, side by side, and with small open field next to the one on the end, it's hard to show yourself as a good party destination when all you have to offer is one house on a standard 1-¹/₂ Chicago lots, with neighbors on either side of the chain link fence.
3. Only have one 4'x4' section of slat wall installed so far, the second piece is cut and painted and awaiting assembly/installation (planned for this weekend) then I need to make more shelves/bins/holders to hold and organize the tools I'm moving from the old pegboard wall system I'm replacing⁽⁴⁾. I'll snap a couple of photos of the garage system this weekend, both to provide more information for this thread, and to show off the garage system itself. I was planning on doing that later, once there was more to see, and the style I was going for was more apparent, but , hey, we're all Engineers here, mostly, we've all seen messy WIP projects before.
4. Pegboard is nice, and at first glance seems like the perfect storage system for a garage, but you quickly find the loading limits of 1/4" thick pegboard, and it's not that high.

I feel like I understand the structure you describe better now. I think if the picnic table itself if well screwed and glued, you will probably be able to jump on it without it disassembling immediately.

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I do have concern mainly in two areas.

The picnic table seems pretty easy to inadvertently dismount, say if a small child goes under the table, or if an adult reaches below to retrieve their napkin. It would be nice there was a spring mounted latch of some sort, whereby the tray would lock in position when mounted and require pressing a release to dismount.

The second concern is in line with several other comments concerning the structure onto which this is to be mounted. Am I correct in understanding you intend to add a slat across the balusters below the railing of your deck?

If so, how far apart are the posts? What are the posts and how are these secured?

On lots of decks, all the stuff below the railing (not the posts) is not that well connected nor rigid. You probably wouldn't want anyone trying to put their full weight on the lower rail of most back yard decks. Everything below the top rail is meant to keep babies and beach balls from exiting via non approved routes. Luckily most lower rails are not that conducive to standing upon. .now imagine a 12 level upon which they can sit.

You are probably going to be depending on the slat you add and its connection to the posts. If the distance between posts isn't fairly short, the slats you add will need to be large in cross section. The connections to the support posts will need to be robust. Of course the posts would also need to be dependable.

The big picture here is that you are inviting loads to be applied to existing structures in ways not originally intended. If and what modifications are necessary depend heavily on what is already there.

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You should attach a slot rail and picnic table and test it out. Would be great to see how you test it and how it fairs.

"The picnic table seems pretty easy to inadvertently dismount, say if a small child goes under the table, or if an adult reaches below to retrieve their napkin. It would be nice there was a spring mounted latch of some sort, whereby the tray would lock in position when mounted and require pressing a release to dismount."

Good point, I could add something to do that, thanks for the tip.

"The second concern is in line with several other comments concerning the structure onto which this is to be mounted. Am I correct in understanding you intend to add a slat across the balusters below the railing of your deck?"

Ah, here is where the cleverness comes in. After enough time has passed that the deck work will be officially 'completed,' by the determination of the Building Inspector⁽¹⁾, the baluster style railing will be removed and replaced with a panel-style railing, with the panels securely fastened inside slots in the top and bottom rails. These panels will be 1x6's or 1x8's of a wood that will complement the rest of the deck, and those will be the 'wall' that the slatwall base will be mounted to.

"If so, how far apart are the posts? What are the posts and how are these secured?"

If memory serves from the initial estimate, (I'll know better after I see the final plans approved by the Building Permit department of City Hall), the posts are 4x4s, and are the same posts that are supporting the deck itself, at whatever spacing is required by the Building Permit. The railing material between the posts when the slat-wall system is to be added will be a 2x4 rail, top and bottom, and '1-by' panels secured into slots in the 2x4s. The top rail will of course be shaped into a proper decorative rail. I may be a fool (as most many some of my posts here definately might show), but I'm not an idiot, I'm making sure the foundation is strong and stable before I build onto it.

Notes:

1. On the form to request evaluation for a building permit, there is a whole page about how nothing beyond what the permit describes may be added, and points out, in very scary, lawyery language about the possibility of severe legal and financial penalties. From that page, they insinuate that a planned 10'x16' deck accidentally built out to 10'x16'2" could be required to be torn down completely, or that adding an unspecified embelishment could also result in fines, jail time, and the tearing down of the structure. That's why I'm waiting until Spring to redo the railing, I do NOT need jail time and being forced to pay to have the entire back porch demolished simply because I wanted something different than that tired old "1x1's nailed to the outside of the rails" style of railing.
   

What you describe sounds sufficiently overkill, that you should be able to stand on your picnic table even if the execution of the project was not something to be particularly proud of.

I would make the side support extend to overlap the slat below. Anything less than that and you might as well just shorten it to just the length of the slat it hangs on. Extending it to overlap the next slot will reduce the stress on the fasteners connecting the slat to the vertical boards.

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With that and insuring the table doesn't easily pop up over the bevel (as previously discussed), along with generally doing a decent job of putting it together and weatherproofing, you can feel confident that lack of structural integrity will not cause or exacerbate problems that a reasonable person would anticipate with typical backyard use.

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That said, nothing beats testing.

I agree with Solar Eagle that you should use solid wood as it is much stronger and if sealed properly will last at least 5x longer than plywood.

Given the questionable quality of plywood due to origin the strength for the type of application you are doing cannot be calculated accurately.

The quality of glue and integrity of the plywood inner layers is not consistent nor of high quality in many cases.

Statements like 'solid wood is much stronger than plywood', require qualification to be meaningful/true/useful.

Solid wood in the direction of the grains can be said to generally exhibit higher flexural stength, tensile strength and stiffness. However it does not fair as well across the grain. Solid wood is also more susceptible to splitting.

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While it is true that there is a lot of poorly made plywood, there are also sources for reliably well made plywood.

Your time and money might be better-spent on (winterizing) your back porch/deck/other first...

I too agree with SE's advice (particularly the "plywood outside" part).

You don't show the fixings - ie, where the screws are, or their size - albeit if you mean wood screws, I would use nuts and bolts

You ask if the shelf is strong enough (and being 'professionally' designed it probably is) but to my mind the wall will buckle under the weight, and even collapse if people are to be carried by it.

I believe you could make use of this cantilever analogy

Those look like the right formulas, But I don't know what the variables are supposed to be. That's why I threw it out to the Structural Engineers.

My goal here is basic safety, I just don't want them pulling free of the slat wall mounting if someone leans/falls on them.

Complex engineering formulae are fine but as you say, not much help because you don't know the required parameters/coefficients. Forget about beams and bending, just use simple moments about a fulcrum.

If the shelf is strong enough to carry the weight, then it will depend on the fixings you use, and if strong enough, the wall takes the weight, and if strong enough, fine!, but if not then the wall will buckle or collapse.

#7 warned of this first, then #13 and then #15. And #15 got a GA for it.

You need a simple hinged flap with a fold-away leg to transfer the weight to the floor. This will carry all the weight you need.

As an exercise - how many people could stand/sit/lay on your dining table before it collapses?

Why not build a bench system into the deck? The benches function both as a place to sit and as place to set a drink or plate of food. You could have a folding picnic table that stores off to the side when not in use, but could be set down by the bench when you need an actual table.

Well, this is a narrow deck off the back porch, and a permanent bench would take up too much of the available walking room. The 'picnic tables' are intended to be brought out for the outdoor dining, then put away when the eating's done.

My concern is about the 'picnic tables' giving way from someone leaning on them or falling against them.

Don't think just about the shelf! That load will be transferred to the wall, becoming a point horizontal load away from the wall at the top fastening, and a distributed (but assume a point) load toward the wall at the bottom plus the vertical load. This all puts a moment on the wall trying to tip it over. Unfortunately I'm not familiar with a slat wall. This moment then has to be resisted by the wall posts and their anchors. Your loading assumptions will also be extremely important.

Now that's a ''Good (alternative) Answer'' in kind, even if in this specific case, the actual available space is too narrow to fit the specific example shown. Here's my GA to you for it, in any case.

Sweet looking table, but it won't fit on the deck, the thing's only 6'wide. Chicago lot, remember? I build a nice, spacious deck, I lose the grass space in the back yard.

That site in general looks really neat, lots of inspiration. Thanks for sharing.

OK, but, getting back to basics, just exactly how wide is the total (available/usable) porch space going to be?

The section that would be getting this is planned to be about 6 feet wide by about 25 feet long. It's a decent-sized back yard for a city lot, about 20'x30', which means 'small' to you subarbanites, and the Rural fold would call it a 'postage stamp.' So I don't want to take up too much of it with the deck.

Well, allowing for a 4' wide aisle, there's room for only about 4 small (18" square) tables, with 2 chairs per each, after allowing one space to stay open to allow people to get from the yard, through the porch, and into the house, and back.

So, keep the local Fire Marshall happy, and never exceed a occupancy of 4 adult, slim, couples on it at one time.

By the way, how far do you live from the nearest suitable park, as an alternative venue?...

( Is it too late to cantilever the porch a few more feet out over the yard? )

"By the way, how far do you live from the nearest suitable park, as an alternative venue?..."

The nearest park is only a few blocks away, but A) They don't allow open fires or BBQ grills, B) even if they did, it wound be a big hassle dragging my two grills over to cook there, C) even if I could and did bring the grills over, they don't have a natural gas feed I can use for my second grill, and D) the 'outdoor only' setting of the park makes it hard for me to show off the other big draw for parties at our place: my collection of 'unusual, fascinating, and beautifully detailed' board games (Read, expensive, and with lots of bits that could get destroyed by rain/lost in a gust of wind).

"( Is it too late to cantilever the porch a few more feet out over the yard? )"

I'm already extending two feet over the grass. The house faces North, so the sun should still reach under the back porch that far to keep the grass from dying and turning the underdeck into a barren mudpit for the dog to track into the house.

Given all the above, I sincerely suggest you review all your personal liability insurance coverage(s), without further adieu...

The deck is being built by a contractor, who is using plans submitted to and approved by the Cit Hall department of Buildings. Since all I specified was the size and shape of the deck, not how it is to be supported (support will be determined by the contractor, following the Chicago Building codes), I should not be liable for any porch collapse, as long as I do not overload the deck with people, and I find it highly unlikely that we will have that many people at the house at any one time, much less that many people all wanting to stand out on the deck at the same moment.

The shelf/table units would not be added until the Spring at the earliest, and I will be using that time to test the system out in the garage. If I find any problems I can't overcome, I will obviously abandon the plan for modular deck dining.

An update on the deck situation: The new deck has been constructed, and the wider spacing of the stiles on this deck as opposed to the 'really narrow' spacing on the old one makes it look a lot more appealing. Also, these stiles were mounted to the INSIDE of the deck rails, instead of the outside, as the old ones were, so the plans to replace them after City inspection, and the plans to add the modular tables, are being but on 'indefinite hiatus.' (I won't say 'scrapped,' because it may be revisited in the future, and I'm keeping all my notes in case I find a similar project in the future that will benefit from them.

Thank you all for your consideration and advice.