dining table bench deflection question
I’m in the process of making a trestle style bench for a dining table that I finished in December.
The bench seat is a single piece of cherry 6 feet long, 11 inches wide and just under 1.5 inches thick. I’ve placed trestle table style legs 6 inches in from each end. The legs are connected by a 1.5 thick, by 3.25 inch tall stretcher whose bottom edge is about 3 inches from the floor. The stretcher is through mortised into the legs. In the course of dry fitting everything together I had two people sit on the bench and the deflection was visible. The bench does not seem springing and “looks” heavy. I sized the bench from some pictures in a book on Shaker furniture so I thought I should be ok structurally.
The sagulator says I should have about 3/8 inch deflection for 600 lbs of distributed load which seems like a lot
How much deflection is normally acceptable in a bench for a dinning table? Because I haven’t glued everything together yet I’m wondering if I should add a rib type member to the bottom of the seat to stiffen it.
Any help is greatly appreciated.
Eric
Replies
First, make those people move away from the dining table! They are over the OSHA Type I weight limit. If you have linebackers over for supper, I would add two stretchers under the seat. Actually, a slight sag is normal but yours is about twice what is should be. There's probably no danger of it breaking unless it is knotty. You most likely won't have that much weight on it often. Since you are not assembled, I would add a stringer. This will also add some resistance to racking movement.
I seem to recall from my structures courses in college that deflection of 1/360th of span was acceptable for steel and concrete structures. This is an unnoticeable amount. Your bench is deflecting more, but I doubt that a piece of wood 1 1/2" thick will break unless there are knots or cracks affecting its integrity. If you don't want any deflection, then put another vertical member that goes from bench to floor in the center of the bench - this is far more efficient than more stretchers or rails running side to side.
Paul Downs
>> ... I'm wondering if I should add a rib type member to the bottom of the seat to stiffen it.
That's the easiest fix. A rib the same size as your stretcher, 1.5 x 3.25 inches, will be significantly stiffer than the seat, and the seat and the rib glued (and screwed?) together will be dramatically stiffer. If my quickie calculation is correct, the deflection of the whole assembly would about 1/5 the deflection of the seat alone.
This sounds a bit like the trestle style table and benches I have. Benches made from knotty pine; about 1.5" thick by 10-12" wide and 6' long. I had a similar problem when the in-laws sat on it, but I don't remember the deflection being that great. Deflection was more on the order of 1/2" or so. On my bench, the stretcher is only about 2" below the bench top. I put a small block on top of the stretcher so the top board can transfer load to the stretcher. It works well.
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