There is a (good, with reservations) ruby program called "simplebeam.rb". It provides a dialogue box for you to input the size of your piece of lumber or plywood. I have used it and I like using it. There are several things to remember with this program:
You have to go in and change the centimeters to inches inside the text portion of the ruby, that is if you use inches.
The program has a major flaw, after you create one component with the program, if you wish to alter any component or one created by the simplebeam ruby you get 3 error boxes when you right click on any component.
I have used this ruby, and then disabled it when I am done. I have tried to look at the innards of this ruby to find the cause of the errors, but being a ruby newby I could not find the flaw. I hope that someone repairs this ruby, because it is most useful.
Best,