Towers B/C

[Balsa Man]

I know it has been stated that ladders have to be lap joints, and x-es on top of the legs, but what if we make ladders on top as well? Would it decrease the efficiency of the tower itself, or not make a difference. I am not really understanding how to make the cutting jig for the lap joints.

Actually what I've said a number of times is ladders between the legs, and the ends of the ladders are butt-jointed to the legs. The Xs are on outer faces/surfaces, and that's a lap joint.

Looking back, I see in my discussion/posts I hadn't spoken specifically to top ladders. They should be there (regardless of the bracing system you're using). The top side of them should be set just a hair (maybe a millimeter) below the upper ends of the legs (with upper (and lower) leg ends cut/filed/sanded 'to the correct angle', and legs all the same length as closely as you can. That ~1mm clearance of leg tops gives you a way to sand the top ends so load block sits level/parallel to the base, and is in full even contact with all 4 leg tops. The lower ladders see compressive force only if/as legs try to buckle toward ladder ends. The upper ladder does get increasing compressive force because the legs are angled in. It's on the order of 1kg. If it's not braced against, that last upper section will fail. Two reasons you don't want that top ladder set sitting on top of the leg ends. Top ladders are very short (like 4.5cm), so they can be very light (like 0.7, even 0.6gr/36" if you have it). But at that low density (even with stick weight 1.1, 1.2, 1.3 gr/36), they will crush/deform if they're between the load block and the leg ends.

Don't know what to say on understanding of the leg end cutting jig. A picture would be helpful, but can't post to this forum. Re-read the word description, and understand what you're trying to make is something that will guide your cutting tool/sanding strip/file at the angle the ladder end meets the leg; whatever works for you to make that happen..

[Complexity]

I know it has been stated that ladders have to be lap joints, and x-es on top of the legs, but what if we make ladders on top as well? Would it decrease the efficiency of the tower itself, or not make a difference. I am not really understanding how to make the cutting jig for the lap joints.

Actually what I've said a number of times is ladders between the legs, and the ends of the ladders are butt-jointed to the legs. The Xs are on outer faces/surfaces, and that's a lap joint.

Looking back, I see in my discussion/posts I hadn't spoken specifically to top ladders. They should be there (regardless of the bracing system you're using). The top side of them should be set just a hair (maybe a millimeter) below the upper ends of the legs (with upper (and lower) leg ends cut/filed/sanded 'to the correct angle', and legs all the same length as closely as you can. That ~1mm clearance of leg tops gives you a way to sand the top ends so load block sits level/parallel to the base, and is in full even contact with all 4 leg tops. The lower ladders see compressive force only if/as legs try to buckle toward ladder ends. The upper ladder does get increasing compressive force because the legs are angled in. It's on the order of 1kg. If it's not braced against, that last upper section will fail. Two reasons you don't want that top ladder set sitting on top of the leg ends. Top ladders are very short (like 4.5cm), so they can be very light (like 0.7, even 0.6gr/36" if you have it). But at that low density (even with stick weight 1.1, 1.2, 1.3 gr/36), they will crush/deform if they're between the load block and the leg ends.

Don't know what to say on understanding of the leg end cutting jig. A picture would be helpful, but can't post to this forum. Re-read the word description, and understand what you're trying to make is something that will guide your cutting tool/sanding strip/file at the angle the ladder end meets the leg; whatever works for you to make that happen..

Correct me, I meant to say changing ladder bracings from butt joints to lap joints. Thank you for the feedback, and I will go over previous posts for the cutting jig. Seems like I used incorrect wording in my previous question. However, you just solved another problem of mine- I was setting pieces on top of legs for the loading block to rest on. I have experienced the deform, as well as more unevenness.

[USCEngr]

[quote="JZhang1"]
"This elevated bridge apparently got first place Div. B at the national tournament in 2010 according to the best of nats page. HOW DOES IT WORK, and do you think the same principles could be used for building high efficiency towers?"

Nice design but a tower with similar design would be much too heavy to score well.

[0ddrenaline]

When using X braces, should I glue the crossing pieces at their intersection? Does it make a significant difference?

[ashmmohan]

When using X braces, should I glue the crossing pieces at their intersection? Does it make a significant difference?

Nah, don't think it makes a big difference. In fact, I would leave it not glued because due to the downward force, they may want to move a little bit while loading.

[DoctaDave]

When using X braces, should I glue the crossing pieces at their intersection? Does it make a significant difference?

Nah, don't think it makes a big difference. In fact, I would leave it not glued because due to the downward force, they may want to move a little bit while loading.

Actually, I believe that there might be some benefit to gluing them together. You may have noticed that if you use very thin bracing that spans the wide base, the braces may begin to bow and buckle because they are not stiff enough to resist the bending force of the main members, so if this happens you would have to use thicker or denser wood for the bracing. But, gluing the two pieces together effectively "braces" your bracings and perhaps can help increase the stiffness of your bracings, making them more suitable to resist compression.

I'm not completely sure if this effect is really significant though, but it wouldn't hurt to try. Perhaps someone who is more of an expert in this event can chime in and give their opinion as well.

[JojoCho]

When using X braces, should I glue the crossing pieces at their intersection? Does it make a significant difference?

Nah, don't think it makes a big difference. In fact, I would leave it not glued because due to the downward force, they may want to move a little bit while loading.

Actually, I believe that there might be some benefit to gluing them together. You may have noticed that if you use very thin bracing that spans the wide base, the braces may begin to bow and buckle because they are not stiff enough to resist the bending force of the main members, so if this happens you would have to use thicker or denser wood for the bracing. But, gluing the two pieces together effectively "braces" your bracings and perhaps can help increase the stiffness of your bracings, making them more suitable to resist compression.

I'm not completely sure if this effect is really significant though, but it wouldn't hurt to try. Perhaps someone who is more of an expert in this event can chime in and give their opinion as well.

I would suggest gluing the centers as well, as the previous replier has said, especially with thin bracings like 1/32 or 1/16 pieces, they like to buckle as weight is being pushed on the tower. For our tower we did the X bracings and glue all the centers and it seems to work well.

[dholdgreve]

Nah, don't think it makes a big difference. In fact, I would leave it not glued because due to the downward force, they may want to move a little bit while loading.

Actually, I believe that there might be some benefit to gluing them together. You may have noticed that if you use very thin bracing that spans the wide base, the braces may begin to bow and buckle because they are not stiff enough to resist the bending force of the main members, so if this happens you would have to use thicker or denser wood for the bracing. But, gluing the two pieces together effectively "braces" your bracings and perhaps can help increase the stiffness of your bracings, making them more suitable to resist compression.

I'm not completely sure if this effect is really significant though, but it wouldn't hurt to try. Perhaps someone who is more of an expert in this event can chime in and give their opinion as well.

I would suggest gluing the centers as well, as the previous replier has said, especially with thin bracings like 1/32 or 1/16 pieces, they like to buckle as weight is being pushed on the tower. For our tower we did the X bracings and glue all the centers and it seems to work well.

When x braces are put in tension, they pull tight and straight. When they are put in compression, they will bow and buckle. Using 1/32" thick X braces provides virtually no compressive resistance, especially on the longer spans. The intent of using x braces is that they work as a pair... as one part of it is put in compression, the opposite half is put in tension, thus reducing the need for the compression side to pick up that load...

You know what they say about opinions, but mine is that gluing the intersection is just adding dead weight... and could even possibly pull the tension member up or down at the center... we do not glue that intersection.

[Balsa Man]

Actually, I believe that there might be some benefit to gluing them together. You may have noticed that if you use very thin bracing that spans the wide base, the braces may begin to bow and buckle because they are not stiff enough to resist the bending force of the main members, so if this happens you would have to use thicker or denser wood for the bracing. But, gluing the two pieces together effectively "braces" your bracings and perhaps can help increase the stiffness of your bracings, making them more suitable to resist compression.

I'm not completely sure if this effect is really significant though, but it wouldn't hurt to try. Perhaps someone who is more of an expert in this event can chime in and give their opinion as well.

I would suggest gluing the centers as well, as the previous replier has said, especially with thin bracings like 1/32 or 1/16 pieces, they like to buckle as weight is being pushed on the tower. For our tower we did the X bracings and glue all the centers and it seems to work well.

When x braces are put in tension, they pull tight and straight. When they are put in compression, they will bow and buckle. Using 1/32" thick X braces provides virtually no compressive resistance, especially on the longer spans. The intent of using x braces is that they work as a pair... as one part of it is put in compression, the opposite half is put in tension, thus reducing the need for the compression side to pick up that load...

You know what they say about opinions, but mine is that gluing the intersection is just adding dead weight... and could even possibly pull the tension member up or down at the center... we do not glue that intersection.

I agree with dholdgreve. But a couple subtlties/caveates...

In a system with ladders and X strips- 1/64th, 1/32nd, the Xs work entirely in tension- if one or the other end of an X strip is trying to pull away (leg trying to bow/pull away at the braced point), then the strip acts as cable/string brace; tensile force; its pulled along its axis, but as long as it holds, prevents any movement of the braced point in the direction its being pulled. Ladder prevents movement/braces against force in the opposite direction. Thin strip Xs provide essentially nothing in compression loading, and, hence, glueing together at the crossing point (while it does, by shortening increase buckling strength) doesn't stiffen/increase buckling strength to a point where it....matters, or contributes any meaningful bracing. Also, it doesn't take very much distortion of the tower to create a problem where X strips cross, if glued at the crossing point- that point wants to move a bit, and that can put a bending force in that at some point will cause failure.

If, instead of X strips (in a ladders and Xs setup), you're using X sticks (1/16th", or >) then they do have some meaningful buckling strength, and glueing the cross point will increase that....a bit. But, sticks are going to be heavier than 'pure tension' strips, and to handle the tension forces they'll see, they'll need to either lap jointed to leg faces, or gussets added if butt-jointed (adding more weight).

In a 'pure Xs' bracing approach, you need sticks, that have both some significant buckling strength, and tensile strength; they need to be able to operate independently, depending on how the tower's trying to distort, and glueing at the crossing point interferes with that..

[USCEngr]

When using X braces, should I glue the crossing pieces at their intersection? Does it make a significant difference?

Nah, don't think it makes a big difference. In fact, I would leave it not glued because due to the downward force, they may want to move a little bit while loading.

A good way of attacking 0ddrenaline's question is to look at what engineers design/build in the real world. Radio towers are the most similar structures to the SciOly towers. Pic below shows that crossing pieces are connected. The pic does show a horizontal piece going through the crossing pieces but a google search would show you other examples with crossing pieces connected, usually with a bolt through the connection. I used this image because you can see the detail clearly whereas most images do not give a close up of diagonal pieces.