Butt vs Lap Joints?

[hearthstone224]

I asked bernard the question of the difference between butt and lap joints, and he gave me this answer:

Here's a document posted on the national website for Wright Stuff: https://www.soinc.org/sites/default/fil ... iad5.0.pdf. The two events are very different yet construction tips carry over between the events. On page 5, from left to right you have: butt/end joint, butt joint secured with a gusset, lap joint, jap joint secured with a gusset. In general, increased contact between two pieces = stronger joint. I can't explain why a lap joint is stronger (probably some explanation involving wood grains) but the science backs it up.

In my opinion, towers are more complicated to understand. Bridges are simpler: https://www.youtube.com/watch?v=OYtY0BxXFgM. The upper section of the bridge is under compression, and the horizontal lower braces are under tension. What if we don't have the horizontal lower braces? Nothing will stop the bridge from doing the splits. What if we don't have the smaller braces connecting the larger compression members (difficult to see from the site, but clearly visible from the top of the bridge)? The larger compression members might shift differently, allowing tilting and shifting of parts = likely unstable. From what I've learned, a little shifting (i.e. settling) right as you add the bucket or load your first scoop is fine; but shifting later on is not good. Balance is important, and if a left member shifts differently from a right member making one of the two weaker, structural symmetry is broken (at least temporarily). And it's possible in that moment the tower/bridge/whatever breaks.

I hope this makes sense,
bernard

Just thought I would post this up so people could read it.

I have a followup question though, and that is this. Should we brace lap joints on the outside of the tower or the inside? Intuitively, I would think on the outside, right?

[Unome]

I asked bernard the question of the difference between butt and lap joints, and he gave me this answer:

Here's a document posted on the national website for Wright Stuff: https://www.soinc.org/sites/default/fil ... iad5.0.pdf. The two events are very different yet construction tips carry over between the events. On page 5, from left to right you have: butt/end joint, butt joint secured with a gusset, lap joint, jap joint secured with a gusset. In general, increased contact between two pieces = stronger joint. I can't explain why a lap joint is stronger (probably some explanation involving wood grains) but the science backs it up.

In my opinion, towers are more complicated to understand. Bridges are simpler: https://www.youtube.com/watch?v=OYtY0BxXFgM. The upper section of the bridge is under compression, and the horizontal lower braces are under tension. What if we don't have the horizontal lower braces? Nothing will stop the bridge from doing the splits. What if we don't have the smaller braces connecting the larger compression members (difficult to see from the site, but clearly visible from the top of the bridge)? The larger compression members might shift differently, allowing tilting and shifting of parts = likely unstable. From what I've learned, a little shifting (i.e. settling) right as you add the bucket or load your first scoop is fine; but shifting later on is not good. Balance is important, and if a left member shifts differently from a right member making one of the two weaker, structural symmetry is broken (at least temporarily). And it's possible in that moment the tower/bridge/whatever breaks.

I hope this makes sense,
bernard

Just thought I would post this up so people could read it.

I have a followup question though, and that is this. Should we brace lap joints on the outside of the tower or the inside? Intuitively, I would think on the outside, right?

Most towers that I've seen have bracing attached on the outside, however I'm not sure whether this is just because it's the way everyone does it (because it's more common/easier to build) or that it's actually better.

[hearthstone224]

I was just wondering, since I don't know if there are significant benefits in bracing on the inside. I doubt there is, since it kind of does the same job, doesn't it?

[Balsa Man]

I was just wondering, since I don't know if there are significant benefits in bracing on the inside. I doubt there is, since it kind of does the same job, doesn't it?

The only downside I can see is that it would be a little more difficult to do- just easier from the outside, but functionally the same

[dholdgreve]

I was just wondering, since I don't know if there are significant benefits in bracing on the inside. I doubt there is, since it kind of does the same job, doesn't it?

The only downside I can see is that it would be a little more difficult to do- just easier from the outside, but functionally the same

Bracing on the inside would result in conflicts with adjacent side bracing, assuming each side has equally spaced and number of tiers.
Bracing on the outside allows each face an unencumbered column face to brace to.

[SPP SciO]

My students (who are slowly and surely getting a feel for this event, especially after an invitational) have struggled with butt/lap joints on their tower. They have a typical "ladders and x's" design, and tried their best to fit all the ladders snugly between legs with butt joints. This was a time intensive process, and I don't suspect it's worth it, because there are places where you can see the members aren't flush, and there's a hair width gap filled with glue.

That also had the side effect of creating a flat surface for the X's. Since each of the 4 corner points on the X are in the same plane, the top piece (the 2nd member of the X set in place) was slightly displaced and thus pre-tensioned.

I'm advising them to stick with lap joints for the ladders - They can cut the members a hair too long and easily sand them down flush, and one X member can connect leg to leg, while the other crossing member can sit comfortably above, connecting ladder to ladder. Is this reasonable? I think they'll find it much easier, and we're just trying to break 1,000 points - certainly not an upper echelon tower.

[Balsa Man]

My students (who are slowly and surely getting a feel for this event, especially after an invitational) have struggled with butt/lap joints on their tower. They have a typical "ladders and x's" design, and tried their best to fit all the ladders snugly between legs with butt joints. This was a time intensive process, and I don't suspect it's worth it, because there are places where you can see the members aren't flush, and there's a hair width gap filled with glue.

That also had the side effect of creating a flat surface for the X's. Since each of the 4 corner points on the X are in the same plane, the top piece (the 2nd member of the X set in place) was slightly displaced and thus pre-tensioned.

I'm advising them to stick with lap joints for the ladders - They can cut the members a hair too long and easily sand them down flush, and one X member can connect leg to leg, while the other crossing member can sit comfortably above, connecting ladder to ladder. Is this reasonable? I think they'll find it much easier, and we're just trying to break 1,000 points - certainly not an upper echelon tower.

Given the goal, you should probably be able to get away with this. That said, couple other comments:
If you put together an end cutting jig, getting good end fit (ladder/leg joints) is....fairly easy/quick.
Ladders in between the legs get axial loading (straight onto the ends); ladders on the face of the legs get non-axial loading, which, with any force starts to induce bowing, so the in-place buckling strength is lower that it would be if the ladder was between the legs, which means ladders have to be heavier/stronger.
Having the Xs in the same plane makes it easier to have them pre-tensioned, which is important to them working....properly.

[Random Human]

I asked bernard the question of the difference between butt and lap joints, and he gave me this answer:

Here's a document posted on the national website for Wright Stuff: https://www.soinc.org/sites/default/fil ... iad5.0.pdf. The two events are very different yet construction tips carry over between the events. On page 5, from left to right you have: butt/end joint, butt joint secured with a gusset, lap joint, jap joint secured with a gusset. In general, increased contact between two pieces = stronger joint. I can't explain why a lap joint is stronger (probably some explanation involving wood grains) but the science backs it up.

In my opinion, towers are more complicated to understand. Bridges are simpler: https://www.youtube.com/watch?v=OYtY0BxXFgM. The upper section of the bridge is under compression, and the horizontal lower braces are under tension. What if we don't have the horizontal lower braces? Nothing will stop the bridge from doing the splits. What if we don't have the smaller braces connecting the larger compression members (difficult to see from the site, but clearly visible from the top of the bridge)? The larger compression members might shift differently, allowing tilting and shifting of parts = likely unstable. From what I've learned, a little shifting (i.e. settling) right as you add the bucket or load your first scoop is fine; but shifting later on is not good. Balance is important, and if a left member shifts differently from a right member making one of the two weaker, structural symmetry is broken (at least temporarily). And it's possible in that moment the tower/bridge/whatever breaks.

I hope this makes sense,
bernard

Just thought I would post this up so people could read it.

I have a followup question though, and that is this. Should we brace lap joints on the outside of the tower or the inside? Intuitively, I would think on the outside, right?

Most towers that I've seen have bracing attached on the outside, however I'm not sure whether this is just because it's the way everyone does it (because it's more common/easier to build) or that it's actually better.

You get more points of contact having the braces atatched from the outside, at the cost of a small bit of wood.