Do you have a link to the Z bar? That would be very helpful.

It is possible that the rating is the same as it wold be based on the type of material and size. However, it's impossible to tell without seeing the specs.

 

Lumbering, the site is
http://www.zbarhanger.com/
The product is: Large Zbar Hanger ,5 CZ12 1 41.50 sizes: = 72inch pair

 

Maybe, I'm just looking at the wrong product, but from what you describe in your original post the 16" bar is only rated at 30lbs

http://www.zbarhanger.com/16lazbhahaha.html

However, the 48" bar is rated at 300lbs, as you describe.
http://www.zbarhanger.com/48lazbhahaha.html

This seems to make perfect sense, and you'd be surprised at how much some things can support. I have about 150lbs of lumber resting on three cantilever shelves with only 3 #8 screws per shelf. This may not seem like a lot, but each #8 screw could support the weight of the wood by itself.

However, if you're worried, you could double up the cleats, but I'd contact the company first. If it's anything like most things that support weight on a wall, it's likely that the actual weight it can support is 2-3 times higher. Going back to the #8 screw, I calculated it's 'safe' support at ~95lbs. However, this is at 40% of it's max shear weight, and I doubt that this company would just give the weight the bar would hold before tearing off the wall.

 

First keep in mind the zbar weight rating isn't dependent on length, the thickness and height determine its strength. Looking at what you've got there I'd run 2 sets of cleats though, one at the top and another midway or so, as close to the bottom as you can.
In case you weren't already planning on it you'll probably need to scribe to the wall and it looks like you have enough meat around the edges to do so

 

I get that it's supposed to float, but most of the underside will never be seen. If it were me, I would bring along some lengths of 6" 3/4 ply painted to the wall color. Hang the entire unit on the Z-bar, and then measure supports from the bottom edge of the unit to the floor and then install them on studs to transfer much of the load to the floor.

 

I don't see several hundred pounds across the Z bar as an issue. It's going to be more or less spread evenly across the bar. I too would be more comfortable with steel than aluminum for these.

This design and discussion seems very familiar to me. OP, did you start a thread don this a while back? Regardless, I know my concern in that other thread, and for this, is the "floating" nightstands. But if I understand what I'm seeing in the photos, the nightstand has a back extending up about 2 feet.

I agree with the idea of strengthening that back. It needs to be an integral part of the nightstand and resist the loads on the nightstand trying to pry the nightstand from the wall. I would run ribs from top to bottom rather than double up the sheet material. I would make these full depth (same as the frame). Glue and screw from the back side.

As this is, it will hang from a cleat near the top. But the weight and load on the nightstand will be pulling at this back, at your tenon location, as you obviously know. This will be trying to pull the back out and I think that back may start bowing. The added ribs would help resist this. 2 or 3 full depth ribs would add a lot more stiffness than doubling with a layer of 1/2" ply.

You could add another cleat near the tenons to help hold the back to the wall and resist the force trying to bow it. If it were me, I'd screw the thing to the wall with screws inside the night stand. located as high as possible. This would pull the night stand tight to the wall.

Your total weight really isn't much of an issue. In many ways, what you have is just a bank of wall cabinets. And it's going to hold pillows not heavy plates. The night stands are the issue becasue they stick out 2 feet and will want to sag under their own weight and of course, someone will put their weight on the front edge of these someday for some reason.

I like what you have and I think in the end it's going to work out well.

 

The Z bar weight limit will depend on if you hit studs or not. A 16" piece with pre drilled holes will likely miss all studs.

If you want to use them, ignore the predrilled holes, and drill where the studs are and hit everyone. Buy the longer lengths and run them the full length. if you do that, weight will never be an issue.

Kind of a waste of money though, as you can acomplish an even stronger system with a hardwood french cleat.

And by the way, "torque and strains of human interaction" is an awesome quote and will forever be saved in my repertoire.

 

Lumbering,
Thank you. I must have seen the specs on a different site. The Z bar I received seems pretty flimsy, It is however rated for 300 pounds. If I use two Z Bars each holding up 1/2 of the 260 pounds (130 Each, it would seem strong enough. Still, I'm one of those people who always over builds. Perhaps it's not necessary. Thank you for your input.

 

Grany A, Thank you for your input. I considered using two rows but I am not confident about getting them installed exactly where the can share the load equally. Also there may be some scribing but the owner said the builders used a laser to line the the studs when building the wall. I checked the wall with a straight edge and surprisingly it's pretty darn flat.

 

Chef, Good idea. I can do that, and will under the headboard but it would show under the nightstands

 

Clin, The "backer boards" on the nightstands are 29" tall Made of 1/2" Baltic Birch. My thinking on reinforcing it with another layer of BB is to make it thicker (more layers). I like your idea about using ribs though. Perhaps I will do ribs glued to the two sides of the frame and plywood between them. You have given me great ideas, and more to think about. Thank you

 

Cw, I have a feeling you are right about hardwood cleats. instead.