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I'm building a murphy bed. I've build a box out of plywood that holds the mattress (the box is shaped like a sandbox. See image below). I plan on putting 5 different 25lb springs across the long side of the bed frame, but want to know if the frame can withstand 125 lbs of constant force.

enter image description here

So, what I did was build three different L-shaped test pieces, joined the same way I did the box (rabbit joints + dowels + screws + glue), to test their breaking force. (I'm going to use a similar setup to http://woodgears.ca/joint_strength/).

Here's my question. Does each of those L-shaped test pieces need to withstand 125 pounds of force? Or do they need to withstand only 25 pounds (assuming the L-shaped pieces are the same length as the average distance between springs)?

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A diagram of the planned bed frame would be very helpful here. I have some concerns based on what you described. –  mac Feb 13 at 20:34
    
Good idea, mac. I've included a quick sketchup image and edited the post. (Sorry the "springs" don't look like springs. I'm not too savvy at sketchup). –  dfife Feb 13 at 21:08
    
thanks for the diagram. I still have some questions/concerns. 1) What is the purpose of the springs? 2) When you say "25 pound spring" do you really mean a 25 pound-per-inch spring, or are you saying that each spring will be guaranteed (by some other design feature) to exert 25 pounds of force on the bed frame? –  mac Feb 14 at 17:12

1 Answer 1

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From what I can see the 2 corners are the only thing that need to hold the force. Each would hold half the 125 lbs. Not only that but the corners would have to handle the side strain, so to speak, that would be applied as the bed is turned from vertical to horizontal. Any springs you may have in the middle will still transfer their pulling force to the corners too. Maybe even bow out the side a little. I myself would not use springs in the center, just 2-75 lb. springs in the corner if available. Since the ends are really the area where all the force will be applied anyway (the corner joinery after all is what holds it all together, the bottom serves its own purpose). To contain the force needed to hold it together e.g., keep the springs from ripping the wood apart from the force exerted at the corners, I would incorporate metal to hold the spring in place and maybe even include the pivot in it. It can be installed on the inside the box frame and hole or slot in the wood frame will allow installation of the spring on the metal ring or whatever you decide to protrude through the hole or slot to connect the spring to.

I ran on a few times, I hope I got the idea across.

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Just to clarify--all weight will be transferred to the corners? In other words, each corner will hold 62.5 lbs of force? If so, interesting. Why is that? –  dfife Feb 14 at 3:39
    
I guess the simplest way to put it, is if the corners just met, the rest would fall. So the corners would have to be strong enough to hold the rest of the wood, mattress, etc. I only refer of the corners where the springs are located. The other 2 corners would not have the same stress. My math is bad, I wrote 75 lbs. –  Jack Feb 14 at 12:34
    
Great. Thanks Jack! –  dfife Feb 14 at 15:42

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