Original Posted By: "rambog(at)erols.com"
Jack Textor and group,I've always wondered the same question, whether to use a solid spar or buildup an I-beam with plywood and solid strips. (My selection for the webmaterial is plywood here, if I had that much solid lumber, I would just aswell use a solid spar.) As I see it (insert disclaimer here), there aresome advantages for each...Solid SparThe advantages of this are that it's easy to manufacturer; just rip, plane,and cut to length a long piece of lumber and that's it. Attaching ribs andfittings require no spacers (an I beam would require a filler between thecaps due to the thinner web section). Although, obtaining a spar gradestick of lumber, particularly Sitka Spruce, comes with a price.Built Up SparThe advantages of this are that it is possibly lighter than a comparativelyequal solid spar. Additionally, it may be easier to obtain the materialslocally as plywood is a ubiquitous product and the solid cap strips could bebuilt up from several shorter pieces Sure, you could scarf a solid spartogether but to me personally, I just wouldn't feel comfortable knowingthere was a big-ol' glue joint holding the spar together. Somehow it seemsmore palatable knowing that there are several smaller glue joints holdingthe spar together as in the case with the built up spar. One otheradvantage is that the built up spar could come from "byproduct" material...you've already got the plywood for the firewall, bulkheads, seats, whateverand if planned correctly, the spar webbing could use up the leftover; thesame applies to the caps (this is a general comment, in the Piet, I believethat the material list is defined well enough that very little is leftover).Design?What the heck, I've got the engineering books here on the shelf... I'm dyingto know how much built up spar would equal a solid spar.One quick wood lesson: lumber is very strong in compression and tension inthe direction of its grain (grain in this case means cellular orientation,not growth rings) and weak perpendicular to it. It would be easier to graba 2x4x8' stud and pull it apart into two 2x2's than it would be to pull itapart into two 4' pieces (not actually, considering surface area but you getmy point). Plywood is a trade off. It's laminations oriented 90 degrees toeach other make a strip of it almost as strong parallel to the strip asperpendicular. A spar requires most of it's strength parallel to thelength, good for solid spar, bad for plywood spar.That's where the capstrips come in. The cantilever forces on a spar causethe top of the spar to go into compression and the bottom of the spar to gointo tension, in the center of the spar, there is neither (yea yea, ideallyonly here, sure there is shear at places like just adjacent to the strut,but overlook this just this one time). If you were to remove all of themiddle, you would be left with two capstrips, which really do all the workanyway. In a built up spar, the plywood web in the middle pretty much justholds the two capstrips in place (again, it also works against the force oftwisting or racking in conjunction with the ribs, but overlook this aswell).The next question is... is there a way to make a 3/4" wide built up spar(because I don't want to redesign all the ribs) equal to a solid 3/4" widespar? Answer: no. Like I said before, a 3/4" built up spar would be like a3/4" solid with the middle removed, additionally, the plywood is weaker.The way you want to look at it is... how can I use less solid lumber andretain the same strength (for weight reasons or to save on material costs)?Answer: make it wider at the top, remove the middle (look at an I beam).For this conversation, lets only look at the solid lumber. A solid sparthat is 3/4" wide by 5" tall has a section modulus of 3.125 in3. A builtup spar using four 3/4"x3/4" strips, two at the top, two at the bottom,would have a section modulus (solid lumber only, no plywood) would have as.m. of 4.1in3. (Yea, I did all the calculations but won't bore you withthem here) By the way, section modulus is just a measure of the crosssectional area's ability to resist a moment. Multiply that by the extremefiber stress and the result is the ultimate moment that can be applied.Ok, Ok, based on the solid lumber alone in this example the built up spar is1/3 stronger using 3/5 the solid wood.So where does this leave you? I hope I haven't preached to you all. It wasreally only so that I could maybe help out a little. But the answer isreally why are you considering the built up spar for the Piet?Comparatively speaking, it doesn't amount to much cost or cost savingsconsidering the total project, you've got plenty of other things to build,and the solid spar is already plenty strong enough as proven by years ofexperience.Humbly,Robert HainesDu Quoin, Illinois________________________________________________________________________________