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sj wrote:I have a couple of questions...
Does this suggest that the top is swaying/bending and needs to be reinforced with more diagonals?

Right now our tops have horizontals 1/16 x 1/16 every 3 cm and diagonals for every 3 of those so like:
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|_/_|
|/__|
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|_/_|
|/__|

Excuse the ASCII art.
The diagonal goes over the middle 2 braces without connecting (no glue).

Your diagonal bracing pattern is not very effective. Generally speaking, in towers and other similar structures where members mainly carry tension and compression forces, you need to avoid having rectangular patterns without diagonals. Put differently, it is best to model and build the tower using triangular patterns only.

Your chimney consists of four segments each having one diagonal member (connecting a lower end to an opposite upper end of the segment). However, each of the rectangular panels (you have three of them in each segment) is not individually diagonally braced. That is a problem!

When a rectangular panel is not properly braced diagonally, its opposing corners would have an easier time to move relative to each other. That, you want to avoid, or, minimize. That is why each rectangular panel needs to have at least one diagonal member for restraining the movement of the upper corners of the panel relative to its lower corners.

Also, with respect to weight (of the wood only and not the glue), your diagonal pattern is not offering that much of an advantage over the standard Z pattern. By my estimation (assuming a 4-cm wide chimney) you have about 58 cm of diagonal bracing for each side. If the Z pattern were to be used for the same chimney, about 60 cm of diagonal bracing would have been needed.

Balsa Man wrote:

thsom wrote:are ladders/horizontal pieces necessary or recommended for the very bottom edge of the base and very top edge of the chimney? Because I have seen designs without them. so it would look like a tower side for the base: top horizontal piece, any number of x's or z's, and then NO horizontal piece and the bottom. for chimney sided: bottom horizontal piece, all z's or x's and horizontal pieces or not, but no horizontal piece at the very top.

Because they're at an angle, at the bottom of the base, the leg ends are pushing outward from the centerline- at full load `couple kilograms. Friction against the base holds them in position to some extent, but not much with a hard, smooth surface; certainly not enough to stop the outward movement. So, bracing between them needs to be tension bracing. At the top, even with a little bit of lean-in (a degree or two), the legs are essentially vertical; the load block induces neither inward or outward pressure; friction between the load block and leg top ends will likely hold them in place, which suggests they're not truely necessary. However, if you're running diagonals to the next ladder down, the start of bowing at that next ladder would pull on the top, and the friction between leg top and block might not be enough. A light ladder at the top would prevent that.

So are you saying that it is needed at the top but not at the bottom? I didn't quite follow you. Or is it needed at the bottom too?

thsom wrote: So are you saying that it is needed at the top but not at the bottom? I didn't quite follow you. Or is it needed at the bottom too?

I'm saying you DO need something at the bottom to prevent the bottom leg ends from sliding out. Those somethings will be working in tension. It could be 4 strips, right at the bottom, joining each of the legs; it could be X-braces from the ends of a ladder higher on the legs going down to the leg ends. At the top, like I said, optional- probably can do without, but may help

Oh ok thank you!! Sorry, I'm complete new to this its my first year (if you haven't noticed yet lol )

I uploaded 2 pictures yesterday but couldn't find them in the image gallery. I don't know what was happening. I'll try again. We tested our tower yesterday. The tower weight is 8g and the height is 48cm; we stopped loading at 13.7 kg but we are pretty sure it can carry the whole 15 kg. We're very happy with the result. Thanks, Mr. Joeris! We built the jig from the info on your post and we were able to build a good tower from that jig.

jma wrote:I uploaded 2 pictures yesterday but couldn't find them in the image gallery. I don't know what was happening. I'll try again.

Images have to be approved by a moderator before them become public. You might PM one and ask that they approve your uploads.

Balsa Man wrote:

thsom wrote: So are you saying that it is needed at the top but not at the bottom? I didn't quite follow you. Or is it needed at the bottom too?

I'm saying you DO need something at the bottom to prevent the bottom leg ends from sliding out. Those somethings will be working in tension. It could be 4 strips, right at the bottom, joining each of the legs; it could be X-braces from the ends of a ladder higher on the legs going down to the leg ends. At the top, like I said, optional- probably can do without, but may help

Wait i do have one question though balsa man. i have seen many pictures of towers online without the bottom horizontal bracing and they have held all of the mass. How is this possible? ( i am asking so many questions about this because i a trying to reduce my base mass, currently at 2.75-2.8 grams. this will shave roughly .35 grams off of the base without the horizontal bottom bracing, any other ideas for lighter bases?)

thsom wrote:Wait i do have one question though balsa man. i have seen many pictures of towers online without the bottom horizontal bracing and they have held all of the mass. How is this possible? ( i am asking so many questions about this because i a trying to reduce my base mass, currently at 2.75-2.8 grams. this will shave roughly .35 grams off of the base without the horizontal bottom bracing, any other ideas for lighter bases?)

Well, checking back thru the Image Gallery, all but one have some form of bracing against "bottom spread" (and that one has it (rather strangely) on 2 of 4 legs. As I said, both "strapping" (horizontal strips around/at the bottom), and diagonal from higher on the leg can do the job. Whether its needed also depends on the bracing interval/how many ladders/braced sections. With a single ladder at midpoint (i.e., a bracing interval around 8cm), you're going to need it. With 3 ladders (bracing interval around 4cm), that last bottom section might be short/strong/stiff enough to work, but the extra ladders and Xs would mean a heavier base than one w/ a single ladder.

Under 3 grams on the base is a very solid number, IMHO; it's going to be hard to make a big (say 10%) improvement.

Julian wrote:Your safety towers looks very well made LKN. Considering that its function is to hold up the 15kg if a tower breaks though, I don't think you have to worry that much about how to improve the design. If it ain't broke, don't fix it.

My partner and I made a safety tower over the break too. We weren't exactly sure how to connect the legs in a tripod structure at the top, so we built a rectangular prism instead. It's basically a glorified bar stool with a hook and chain. I'll upload that to the gallery soon. Would you mind giving a quick run down on how you connected them at the top? It looked like Balsa Man's original tower had them interlocking somehow...

Two more quick things I noticed. I like how you just marked out the 20x20 square and then drilled a hole down the center. It took us forever to get that square perfectly cut. I guess there's no need to do that unless you're running the tower event right? Lastly, how is your bucket attached to the chain? We found that the plastic cylinder always gets in the way and is prone to slipping around when pouring, so we broke it off and hot glued our carabiner/chain to the metal handle. Have you encountered any issues with that yet?

To connect the top of the tower, I calculated the angles that the wood would have to be cut. It's just some basic geometry, but it does take a good few minutes. For calculating using the legs of the safety tower, give yourself AT LEAST 2inches of space from the edge of your testing base. I gave myself two inches, and the legs ended up on the very edge of the base when the cutting/screwing/gluing was done. The angles then "fit" together and we screwed/glued the structure at the top. There was no interlocking piece at the top of the structure.

For the 20cmx20cm square, I had the hole drilled then drew out the square. From there I knew the center I was working around for the tripod structure. I also super glued down sand paper in the shape of the bottom of the leg. I marked exactly where the centerline from the tripod was, using thread and a weight from the top of the rig on a level surface, then marked where the feet were. From there, it was just trimming down sandpaper till it fit. Label the leg areas, and glue the sandpaper down on the rig. This should help keep the safety tower from slipping for any reason after the tower has failed.

For the chain/carabiner, we haven't had your problem. It must be the bucket or carabiner size, BUT our carabiner we are using has a nice "nook" on the smaller curved part of the carabiner. This smaller rounded part on the carabiner is towards the ground when testing, and fits nicely into the plastic cylinder on the bucket. The larger, big hook part of the carabiner, is facing upwards and connected to the chain. I didn't plan this out when grabbing a carabiner from the hardware store. My recommendation (if you don't want a permanently glued bucket-carabiner setup like you described) is to take the bucket with you to the store when you pick out a carabiner that has a nice nook that fits into the bucket handle to reduce slipping.

The only complaint I have about my rig right now is the J-hook just above the block. When the block is suspended and supported by the safety tower, the block is tilted a good cm or so sideways. I plan on replacing this soon with an eyebolt. Do you have an eyebolt on the top as well? Has it been working out for you?

Balsa Man-thank you very much for your input; I think that your advice will help us get those extra 5 kilos for 15kg.

However I now have another concern... This week we have two competitions. NJ Regionals at NJIT and Conestoga Invitationals. At Conestoga towers is run superbly with a level testing surface and a hopper. However this is not the case at regionals. Last year for instance the so many things were wrong. First the superviser showed up 20 minutes late to sign in and with the wrong team roster so many teams weren't on the list. He came back just as sign up ended with the right sheet and then rather than just taking name and time slot he started checking in everyone's towers until someone told him this was not an impound event. After that we went to the competition and he first check our specs on a table off to the side and then took us the the loading platform which was neither level nor up to spec. It did not have the 20x20 hole in it rather just a small hole in the middle. However the thing that killed us was the fact that the table was not level. It caused a tower that we tested at home to 15 kgs and knew we had overbuilt to fail at 8 kgs. We noticed this before and asked him to correct it but he refused since other teams had already gone. Now i'm not just posting this to rage.

Do you guys think we should build a shorter tower for regionals in case the superviser does something weird and makes another off level testing base?