Sorry 'bout dropping into a black hole for the last.....few weeks; way too busy at work, but in these economic times that's a good thing I guess.
Skimming over postings, a few quick things-
I will get a picture up of the 3 load-point bridge up within the next couple weeks.
On the safety tower - Jeff, thanks for the.....appreciation of what it allows you to do. Its the closest thing to magic I've found over the years. Clarification, its not so much that it slows failure down so you can see it, but that it stops it right after it ocurrs. You can/should adjust the length of the upper chain (the slack in the chain) so the block can fall.....just enough to allow failure. With a very rigid bridge a 16th or just a tad more is enough; if its more flexible, a little more;maybe up to 1/8th. When we tested Peter's State bridge to failure, we were at about a 16th. One of the upper compression members (top cross pieces) broke - right at the edge of the load block - at the point of highest compression load, where the diagonal compression truss member from below joins the top compression member. 1/8th square stock, one end of the break displaced down - the ends of the break are actually still connected w/ wood fibers. Jeff has it exactly right - in terms of using this tool - where you would go from this point; fixing, and continuing to test the same bridge. In this case, carefully pulling the break back in line, putting "splint" pieces on - and splinting/strengthening the .....corresponding places - the other three places where the top member and compression truss pieces join. With those places strengthened, the next loading will a) more than what caused failure the first time - with that area/member/joint strengthened, and b) will be somewhere else - the next weakest place.
On the question on dimensions on the tower, it really doesn't matter; whatever will fit on the test base you use, and clear the structure you're testing, and is steady/stiff enough to hold a 15kg load without visibly flexing - just look at the photo in the gallery.
Looking forward to hearing how the new rules come out, too!
Fort Collins, CO