Newbie Question about Structure Strength Rules of Thumb
JohnnyNumber6
| Posted in General Discussion on
I’m new to woodworking and building a few things by following various plans. However I’m the type of person who likes to understand the details of ‘why things are done certain ways’. For example when recently building a tall rectangular box out of plywood, the plans indicated that having the sides attach to the bottom of the top piece was stronger than having them attach to the sides of the top piece. In other words it was strong for the top piece to sit on top the sides instead of it being recessed in between the sides. Hopefully that is clear… Anyways I’m wondering if there are any good resources (books, websites, etc.) which provide some very basic rules of thumb to help understand these types of things? Thanks in advance!
Replies
In the case of the top resting on the sides, the weight is transferred directly to the sides, without relying upon joinery for strength. If the ends of the top are objectionable from a visual standpoint, the top could rest in a wide rabbett at the top of the sides, leaving just enough thickness to hide the end grain.
There are lots of basic woodworking books out there, try ebay for some good deals. I just gave away several but don't recall the titles. At some of the prices on ebay, you ca afford to buy several. look for those being liquidated by libraries for thebest pricing.
#6, I haven't come across such an engineering book focused on furniture design. But flyfisher111 makes a good explanation.
My two cents. One point I learned helpful to me, is that any load (weight) has to be transferred from the top to the floor vertically. So as long as there is a straight line through wood to the floor, from a table top through a leg, for example, the load is carried safely. If the legs are splayed, like in many chairs, there are bending and racking forces that have to be resisted, by adding braces and runners. You have to look at each thing you are making and decide the direction of loads that need to be transferred.
It gets complicated because the joinery impacts the strength. If you just screwed or nailed the sides of your box to the outside of the top, you can see that all the weight gets transferred to the screws. The wood has to be strong enough not to crack or separate from the screws, and the sides to bow out. With the top on top of the sides (like flyfisher said) the weight goes right from the top down the sides to the floor.
Peruse the woodworking websites and forums, see what's being recommended out there, and go to Amazon or somethig like it looking for used books. They're often quite cheap and unless you're going to put them on your living room coffee table they serve fine for shop references. I have a library of them, based on a similar curiosity, and they are great to go to. Just don't let your wife get hold of them to prop something up! I had Mike Pekovich's book under a lamp for 6 months after only reading it once. I finally got it back :)
Lots of answers to the various methods used. If you're a subscriber here do a search on 'joinery' or 'rogowski, joinery' to see a lot of stuff. For sheet goods case work Bill Hylton's book Illustrated Cabinetmaking is hard to beat but, everyone has their favorite.
I started to ramble on about various joints and their intended use but,realized that this may cover a lot of stuff you don't really care about. Your public library is a good resource if you don't want to build your own collection.
I find many of these books are initially useful but, then become more of a reference material that sets idle for long periods. Better to let the library store them for you ;-)
Strengthwise its of no practical difference unless you're building a box that will hold scrap metal or something LOL.
Aesthetically, would you want the exposed edges of the top and bottom? Well, that depends on whether you can see/hide them or not.
Cabinet boxes are usually made with tops/bottoms between the sides.
Cabinets with exposed sides I usually do a hefty rabbet and attach the bot/top with screws from the top/bot.
Thanks everyone for the info! I apologize for the slow reply, the kids being home definitely impacts my hobby time. Thanks again!
I found this particularly useful in understanding the considerations involved in table legs https://flairwoodworks.com/2020/01/21/designing-furniture-considerations-around-table-legs-and-leverage/
I have been looking for simple/basic information on load and design considerations for furniture (or architecture for that matter) all in one place and haven’t had much luck. However, the following books are good references and contain some of this type of information:
Understanding Wood: A Craftsman's Guide to Wood Technology by R. Bruce Hoadley
The Complete Manual of Woodworking by Albert Jackson and David Day
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