Need Advice, Sagging In large Case Pieces
Can anyone please help me? I designing my first dresser. The size will be at least 60 inches wide, and I would prefer to not have a center leg support, for appearance’ sake.
I have looked at case pieces in furniture stores, and mass-produced pieces I have at home, and have come to the conclusion that the way to prevent sagging must be to eliminate all wood weight in the center of the piece, but that doesn’t leave much support for drawers or face. I saw a sideboard carcass on Christian Becksvoort’s website that could be over 6 feet wide with no center support and solid wood in the middle.
Can anyone please offer some general rules of thumb regarding the design of wide case pieces without center leg support? This is my first real foray into designing and building a piece of larger furniture and I want to do it right. Thanks.
Replies
IMMS,
There's any number of ways you can improve the situation, but they depend on the details of each piece so I can't give any one solution for all. A few ideas:
Strengthen the bottom of the case by using a wide face frame on it.
Strengthen the bottom of the case by making it as a torsion box.
Make sure the back of the case supports the bottom, not just sits in a groove or rabbet.
Make a hidden foot set back from the front of the case.
Use the rigidity of a thick top of the case to help support the center weight.
Not having built cases of that width myself, I can't give you specifics. But, those who have can probably help.
At the abstract level, I would assume the design would need to borrow from bridge builders. That is to say, a design that will transfer the weight to the sides of the case via on-edge horizontal supports hefty enough to do the job.
Ring has given you sound general advise as he can't see what you are after but.. it would sure help if you had a picture of the intended so it could be more specific.
First, something I am building that is similar with similar concerns:
I am currently building a very large sideboard for our church out of solid cherry - frame and panel with 3 sets of double doors across the front. It is 102"Lx40"Hx20"D. I initially designed it with four end legs which would have left a span of some 8 feet. I spent a lot of time thinking about sag. I know that it will be overloaded by someone, some day. Peope squirrel away dish sets for 24, etc. I also am using inset doors, so any sag could be a problem. I finally decided to add 4 more small, set back legs in the middle. I now have it dry assembled and it looks good and is rock solid.
Things I learned that may be applicable to your dresser:
My side board is frame and panel - mortise and tenon. There are four panel sets - the end leg frames and the two center leg frames which act as divders. These four assemblies are tied together with long face stretchers front and back which are mortised into the four end legs. The bottom stretchers are 1.5" thick by 2.5" high. The top stretchers are 1.5x1.5. They tie into the center leg post s with locking lap joints. As a result the entire carcase is a large box beam. I have designed the back (1/2" thick plywood) to be glued and screwed to the back. On top will be a 1.5" top. Now that it is dry fitted and together I can tell that at 60 in, no additional legs would be necessary, perhaps even at 80"L. There are lots of wide dresser and sideboard designs without center posts. Take a look at hardwoodartisans in va http://www.hardwoodartisans.com/new_page.asp?Waterfall%20Chests%20and%20Dressers
You should be alright as long as you center drawer separators are locked into you cross rails.
Jerry
I'm guessing that solid wood 3/4" thick will sag less than plywood 3/4" thick, but I'm not positive. In either case, I'm also guessing a full width bottom would sag less than a frame and panel or web (frame and panel without the panel) bottom.
Making the horizontal dividers between drawers wider and thicker will make them stiffer, helping them carry the weight of the drawers. For example, can you design the rails to be 7/8" thick rather than the typical 3/4" or even 1". The extra thickness might not look inappropriate given the overal dimensions of the chest. You can also make the drawer rails wider, up to 4" or so, to make them stiffer. If you don't have enough stock, you could glue a stiff secondary wood to a 1" wide face for the drawer rails to make up greater width. If the sides are solid, the joinery for the drawer rail to the sides can be multiple tenons to maximize wood and joint strength. If the sides are legs with panels, you can use the joinery detailed by Will Neptune din FWW #130 in an article on tables with drawers.
You might make the top slightly thicker, again for extra stiffness, as it can help support the front of the chest via the vertical dividers. These can be dovetailed into the horizontal dividers.
Finally, you could add a piece under the bottom divider, but it's strength will depend on the narrowest width if given a bottom curve.
Assuming that you've drawn the design, on paper or in software, I think the best design approach is to examine how the between-drawer vertical stiles stack up, and how drawer position affects the distribution of weight. If all of the between-drawer stiles align vertically, then the weight is mostly distributed downward, eventually reaching the base frame. Some of that weight, however, can also be distributed upward by better/stronger joinery (DTs, for example) between the stile and the rail above it. The upward distribution eventually reaches the top of the chest, which, along with the rails between drawer sets, distributes part of the weight to the sides. Attaching the back of each of the rail panels to a strong, fixed back panel further distributes part of the weight.
Also, in a three-drawer array, about half of the weight of the two outside drawers is supported directly by the side panel on either side. The balance of the weight of those drawers, along with almost all of the weight of the center drawer is directed mostly downward, except as modified by the inter-stile joinery.
All things considered, I'd still be sorely tempted to add a fifth foot in the center, half way between the front and the back of the chest, so it's not obvious. Ya just never know when someone is going to store ammo in a couple of those center drawers. ;-)
I built an entertainment center that required a very long span (55") to hold a 170lb big screen TV and didn't want a center support under it. So, I bought some angle iron and built it into the unit, screwing it in place every 6" or so It was hid behind the shelf overhand in the front. After five years, there is no hint of sag.
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