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[SPP SciO]

So we've got an invitational tomorrow and my students finished their tower a few hours ago. It looks great - 1/8" sq legs, 3/32" ladders (tucked nicely between the legs with butt joints, made with a tool described on this forum) and 1/64" sq basswood X bracing. Altogether it's just a bit under 9 grams.

Here's the catch - the very top of the tower is a precise 5.0cm square- to the inner edge of the wood, not the outer. Essentially, to fit the loading block, they'll have to caddy corner it so it won't be on top of any of the 4 legs. Instinct says one of those butt joints at the top or one of the top ladder pieces will give long before the legs buckle (but, often, my instinct is wrong with this stuff).

Is there a "quick fix" that can alleviate this? It wouldn't be much trouble for them to tack on a few pieces in the morning. Is this tower destined to fail? The coach in me thinks, It's just an invitational, and would mark the 2nd time dimensions weren't carefully planned, so maybe this is the wake up call they need. Still, my teacher/human side wants to see them earn a medal. Suggestions welcome!

(Btw, Balsa Man, I know you're about to stress the importance of building on a jig - hopefully that's built before our season is done!)

[Balsa Man]

No, I simply....observe this is the kind of thing that can happen without a jig.... Now they, hopefully, understand. Sorry to hear..... I feel for them. I'm sure they put a good amount of time and effort in- 9gr is a respectable weight.

I can't think of a fix that doesn't involve a fair amount of additional weight.... there may be some clever angle I'm not thinking of. If I had to deal with it, I'd suggest a 'mini truncated pyramid on the top- leg pieces like 1- 1/1/2cm, leaned in enough to get tips under the load block, This mini tower will, to work, require ladder pieces at the top, and tension control strips at the bottom. (because the tops will be being pushed inward, and the bottoms outward with fairly significant force.

One other thing that may cause problems is using 1/64th x 1/64th for Xs. Basswood at that dimension....may to probably has the tensile strength, but a glue area only 1/64th wide may be insufficient. That's why I ....advocate 1/64 x 1/16 or 3/32 for tension loaded strips- to get good glue area.

[Balsa Man]

It may be too late, and you may not have on-line access/be checking here.....

Woke up thinking about how to deal with this; one other idea that would be easy to implement, if you have some 1/16" sheet.

Cut 4 right triangles....oh, 3/8" on the sides. Glue one on each of the leg tops- w/ the right angle aligned with the outsides of the leg- glue to both the leg top, and the top side of the top ladder.. This creates 'corner brackets'. Then cut 4 more right triangles, with a long side of....oh, an inch, inch and a half, and a short side of whatever the measurement, looking down from the top, is from inside corner of leg to the edge (the hypotenuse) of the 'corner bracket'. Glue these on so they're glued to the corner edge of the leg, and the underside of the corner bracket. Again, looking down, the upper edge of the 'bracing triangles' will run from the corner edge of the leg toward the center line of the tower.
This should distribute the load enough to get it onto the legs, and be lighter than the 'mini truncated pyramid' I initially described. Sorry I didn't think of this last night....

[baker]

Sounds like a good fix with min weight added.

[SPP SciO]

Thanks for the replies! I was hardly around the team in the morning, busy with ES responsibilities, but I heard they wound up adding 4 ~1 cm pieces of 1/8" sq (what they had available) at the corners and loaded the block on top of that. The overall score was ~700, which finished 11th place at this particular tournament. It's a shame because all members but one survived, but I think they're on the right track. Regionals in 3 weeks - plenty of time to do it right.

[Balsa Man]

Thanks for the replies! I was hardly around the team in the morning, busy with ES responsibilities, but I heard they wound up adding 4 ~1 cm pieces of 1/8" sq (what they had available) at the corners and loaded the block on top of that. The overall score was ~700, which finished 11th place at this particular tournament. It's a shame because all members but one survived, but I think they're on the right track. Regionals in 3 weeks - plenty of time to do it right.

You're welcome, of course. Given the problem, respectable outcome.
Does sound like you're on the right track. May the next version do much better!