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Question on structural strength of 3/4 plywood, weight bearing cabinet..

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Forum topic by xavierCarte posted 04-02-2017 12:11 AM 2181 views 0 times favorited 8 replies Add to Favorites Watch
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xavierCarte

2 posts in 255 days


04-02-2017 12:11 AM

Topic tags/keywords: project loft bed bedroom plywood teenagers cabinetry

Planning to build a loft bed for my sons. My current design uses a 3 piece, 3/4 plywood cabinet (28.25”W x 35”D x 55”H) to support one end of the loft bed.

The plan was to use only plywood in the construction of the cabinet (no interior hardwood supports). I was initially concerned about the cabinets ability to support the vertical load but the more I think about it.. I’m even more concerned with the amount of lateral support (or lack thereof) the cabinet will provide. Obviously both concerns are critical. Having never attempted a build such as this, I thought it best to post this question to those with greater knowledge than I.

-o- The loft bed is supported at one end by 4×4 posts and a bed frame of 2×6s with mattress support using either a 1/2” plywood span or a series of wooden slats.

-o- The posts are anchored to the bottom bunk structure via 4 – 24” 2×4s (only one pictured)

-o- The cabinet is designed as three main components: base (28.25”w x 35”d x 14.5”h); a front cabinet (28.25”w x 17.5”d x 40.5”h) with 3 vertical supports (I I I) sandwiched between a top/bottom uninterrupted span; a rear cabinet in a ’|—|’ or ” I ” configuration (28.25”w x 17.5”d x 40.5”h).

-o- 1/4” or 3/8” depth dadoes/glued to support the shelving, top/bottom horizontal spans.

-o- Each of the three cabinets will be screwed together (front and rear to each other and both to the base)..

-o- To attach the loft bed to the cabinet I have a prone horizontal 2×8 attached to the inside of the bed frame that will lay flat across the top of the cabinet and screwed into the top of the cabinet at the center vertical supports running from the front to rear cabinets.

Pictures being worth a thousand words..

I think the 4 braces attached to the lower bed structure and the fact that the post-end of the bed sits in a corner should be enough lateral stabilizing support at the post end as long as the cabinet offers the same support at it’s end… agreed?

The original plan had a cabinet that was a 3×3 cube, 41”w (as opposed to the 2×3 cube, 28.25w cabinet pictured). I was much more confident in the 3×3 cube’s lateral support but my wife decided that more floor space would be better and considered the 41”x35” footprint of the 3×3 cube to be too large. The original plan also had 1×3 free standing cabinets that were each screwed together.. so each interior vertical support was actually 2- 3/4” plywood pcs back-to-back. I went with a single vertical plywood supports because I thought it looked odd to have 3/4” vert outside supports while the interior vert supports were 1.5” (2- 3/4”).

Questions:
Looking over the cabinet.. my question would be.. Do you think the cabinet itself—made without hardwood framing support, entirely of 3/4 plywood, would provide enough vertical & lateral support for the loft bed: frame, mattress, teenager. Just to be safe, I’d like the cabinet assembly to support 400-500+lbs, possible?

Obviously, using a hardwood framed cabinet would be structurally better but my wife likes the clean lines and maximized storage space of this design and thus I need to find out if this design is structurally sound for this application.

thanks in advance for any info/advice


8 replies so far

View clin's profile

clin

751 posts in 830 days


#1 posted 04-02-2017 01:33 AM

First off, I have to admit to giving up trying to understand your description and therefore I’m basing my comments on your drawing.

I’d bolt the loft to the walls. That would fully support one end and one side the entire length. Then all you need to do is support the one corner with the cabinets.

Maybe you described what is happening. But I see pairs of openings facing out from the loft and wall. Is there a back to these that is in the center of the cabinet? I think so from your drawing, as it would form the side of the openings near the wall, and presumable similar openings facing under the loft.

All I’m really getting at is if the cabinet openings effectively have full sides and full, structural backs, forming a sort of cross in the middle, all from 3/4” ply, the cabinet cannot rack (try to be a trapezoid). If so, it will be very strong. And if the loft is bolted to the walls, the cabinet only has to support the one corner and roughly 1/4 the total target load or about 125 lbs. That is trivial.

Another thing, are those shelves fixed? They should be. That will go a long way to preventing the sides from bending under load.

Keep in mind the strength you get from a plywood build comes from the corners. Clearly the plywood is most likely to bend along its span, but if it is joined to another piece at a right angle, the two piece support each other in their weakest direction. The corner forming a very strong support. So good corner joints are critical to maximize strength. Glue and screw for example.

This is common in kitchen base cabinets. These often have very heavy slabs of granite as counter tops and are not built any differently because of it. Also, think of all the times someone sits on a kitchen counter. If the cabinet is built at all well, it’s a complete non-issue, even with perhaps 200+ lbs on just one corner.

-- Clin

View Lars Gregersen's profile

Lars Gregersen

4 posts in 255 days


#2 posted 04-02-2017 06:30 AM

The weight is not a problem. I would even use 2×4s for the posts simply because I think it’ll look better. Are you able to screw the posts into the wall? That will give you the lateral support you otherwise need. If you can’t fasten the bed to the walls I would add some diagonal support to the upper corners where the posts meet the bed.

I would defitely use slats for the bed. If you use plywood then drill a lot of holes in order to supply ventilation.

One question about the design. In the bottom bunk there are three openings. What should the one closest to the wall be used for and is the clerance in front of it big enough to get things in and out (it is hard to tell from the drawing). Maybe you need to make a (partial) lid for the bunk?

Lars

View LittleShaver's profile

LittleShaver

206 posts in 454 days


#3 posted 04-02-2017 01:34 PM

The upper bunk looks fine. The lower could use a back to prevent racking. I think that then you could loose the ties to the posts. I think the wife will appreciate being able to move the units independently. I know mine would.

-- Sawdust Maker

View MrRon's profile

MrRon

4492 posts in 3078 days


#4 posted 04-02-2017 05:08 PM

The 2 vertical pieces of the cabinet should be one piece. The shelves will end up serving as a ladder; that you don’t want. The lower bunk needs bracing (long way). Eliminate the (4) 2×4’s and attach the 4×4 posts to the wall; you will need a stretcher spanning the posts to intersect with a wall stud. I would reduce the height of the lower bunk so casters can be attached to the bottom; that will make it easier to make up the bunk. The 3 openings in the lower bunk, I assume are for drawers. Without lateral support, you will never get the openings to maintain squareness for drawer operation. Overall, the strength is there. The problem is not enough bracing to prevent racking. I would cover the raw plywood edges with a thin hardwood strip.

View REO's profile

REO

923 posts in 1908 days


#5 posted 04-02-2017 05:31 PM

the end cabinet with perpendicular panels will stabilize in the horizontals. load bearing no problem. for the low bed cap the end of one of the cubbys or inset a panel at the middle of one of the cubbys it will be fine. My concern would be the window proximity to the low bed.

View xavierCarte's profile

xavierCarte

2 posts in 255 days


#6 posted 04-03-2017 12:00 AM

First, let me thank everyone for your suggestions.. much appreciated.

My sons are currently, 12 & 14, and both working their way quickly to 6’ tall. So this is not a bunk for little kids. Proximity to window is not a concern.

The loft will not be mounted to the walls.

I can see the concern over racking on the lower bed. I had thought that the 4- 2×4s, mounted to the lower bed base and bracing the posts, would be enough to prevent this… no?
If it turns out not to be enough, I will add a back board or maybe a full center support running the length from head to foot. The space between is not meant for drawers but bins.

The two top cabinets (front & rear) do indeed form a cross ’+’ support in the center of their assembly.

All shelving/horizontal pcs will be glued (and possibly screwed) into place.

There will be no stairs/ladder. My 12yr old will hv the top bunk and he will simply stand on the bottom bunk and pull himself up and either step down onto the lower bed or simply jump down from the upper bunk to get down.

I’m sure the wife would prefer that the bottom bunk not be attached to the vertical posts. But I think it would provide needed lateral support to have the lower posts attached to the lower bed assembly. Maybe I’m wrong in this concern.. so any info on this would be appreciated.

I would like to avoid using any diagonal bracing.. I really don’t like that look.

The clearance between the cabinet and the lower bed’s bin opening (directly under the loft bed) is a little more than 24”. I wanted the upper bed’s cross rail to sit as close to the center of the cabinet’s center supports as possible and this meant limiting the space between the lower bunk and cabinet. But it’s enough space to sit at that end of the bed and gather what’s needed from cabinet shelving and pull out the bins underneath (if indeed there will be bins at that end.. it might hold shoes/boots or seasonal items.. etc..

I’m using 3/4” Baltic Birch panels in this build.

——your comments have made me think the following might be better…——
In an effort to bring more stability laterally and vertically.. I’m thinking of switching from 3 cabinets connected together.. to two cabinets—by using one single pc of plywood (per side – ‘L’ shaped) to create the outer vert supports for the front and base cabinets. (pink)

again.. let me thank you for your ideas and critiques.. it really is much appreciated.

forgive the coloring.. I was making layout/cut patterns

View patcollins's profile

patcollins

1605 posts in 2699 days


#7 posted 04-03-2017 12:09 AM


I would defitely use slats for the bed. If you use plywood then drill a lot of holes in order to supply ventilation.

- Lars Gregersen

Unless bed wetting is a problem I don’t see a need for ventilation under a mattress. I’ve always found the best base for a mattress is a good ole piece of plywood.

View clin's profile

clin

751 posts in 830 days


#8 posted 04-03-2017 01:52 AM

I was focused on the loft. That lower bed needs a back. It’s not even an option. There’s nothing to prevent racking, even if tied to the posts holding the upper bed. Given it is not attached to the wall, what keeps them from racking? Sure if 4×4 posts, tied to 2X top bed frame, it’s not going to collapse, but it will wiggle a lot.

And from a design perspective, I think a solid panel, between those post,s would look better than those rails that look like ladder rungs.

I’d put on a back on the lower bed and really consider going with a face frame. Or at least a structural toe kick. In general, having those openings right on the floor is awkward. So raising them up a bit and running support under it could help.

Note: I realize the floor is plenty of support, but if the a toe kick where well joined to sides and to the middle vertical panels (those going all the way to the floor, even if behind the toe kick), would help to prevent racking of the front.

You could also consider a spine under the top. Again joined well to the sides and vertical panels of the lower bed. All in an effort to prevent racking.

-- Clin

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