Towers B/C

[jinhusong]

Hi Raleway,

Thank for the prompt reply.

The base legs are already on a pyramid like jig . The angle is in theory, not a building error. The legs are 1/8X1/8 sticks. For the base, they are about 30 degree off vertical. The brace faces between neighbor legs are not in the same plane anymore.

From another point of view, if the base legs are cut levelly, the intersection is not square anymore, it is a diamond. The sharp edge is less than 90 degree now and the neighbor leg is still 90 degree away. Maybe I should suggest them to make the brace taper at the ends so that it will still fully glue to the leg and no bend.

This is the first time he builds square base. Maybe this is the reason that more and more people build rectangle base?

Yes, parents are advices and power tools only here. You know, now they are in in high school, even the advices are less and less. I wondering around in their classrooms during their team time and feel they did not need me anymore. Well, glad to see they grow up.

Best,

Jinhu

[Balsa Man]

Hi Raleway,

Thank for the prompt reply.

The base legs are already on a pyramid like jig . The angle is in theory, not a building error. The legs are 1/8X1/8 sticks. For the base, they are about 30 degree off vertical. The brace faces between neighbor legs are not in the same plane anymore.

From another point of view, if the base legs are cut levelly, the intersection is not square anymore, it is a diamond. The sharp edge is less than 90 degree now and the neighbor leg is still 90 degree away. Maybe I should suggest them to make the brace taper at the ends so that it will still fully glue to the leg and no bend.

This is the first time he builds square base. Maybe this is the reason that more and more people build rectangle base?

Yes, parents are advices and power tools only here. You know, now they are in in high school, even the advices are less and less. I wondering around in their classrooms during their team time and feel they did not need me anymore. Well, glad to see they grow up.

Best,

Jinhu

Jinhu,
First let me echo Raleway’s welcome and support of parental involvement!

It was as a helping parent that I started my Science-O involvement over 15 years ago. Worked with both my sons through junior high and high school, then moved into coaching entire team for a couple of years, transitioned to coaching the building events, then to just working with the ‘balsa structure’ events (towers, bridges, and booms). For both me and my kids it was a wonderful experience; what can I say.
So, on to the problem you’re dealing with. It takes a little digging, with all the threads the towers forum has developed, but there is a lot of detailed discussion and good advice on the exact set of issues you raise. I strongly recommend you take the time to read through. The information developed here is the best and largest body of good information specific to S.O. towers out there. I agree absolutely with Raleway’s advice that neither cutting away, nor adding filler is a viable approach.

You’ve run into the first reason that triangular towers are…. just not a good idea. With square cross section legs, there is no way to get leg faces parallel, and with non-parallel faces, no way to get bracing securely attached. Beyond this primary problem, there are a number of other reasons (as you’ll see when you read through the forum) why 3 legs are just not a good idea. A well engineered square tower will always beat a triangular one.

As to square vs. rectangular tower form, there are advocates of rectangular, rather than square, but I believe strongly that there are inherent advantages of the square approach, and inherent down-sides to the rectangular form. Again, there is specific discussion of this issue here.
Using a good jig (precise and symmetrical) is really, really important. It is, I believe, much easier to make a precise and symmetrical square jig than it is for a rectangular base There is detailed discussion of how that can be done here.

I’ve discussed the issue you see of different cross section in leg segments at the angle break when cut in a plane parallel to the test base, and described the solution (that cut needs to be made on the angle bisecting the angle between the leg segments).
Once you’ve absorbed the info here, do feel free to come back with more questions; there are a lot of aspects to getting to a high-performing tower.

Hope this helps

[jinhusong]

Hi Raleway,

Thank for the prompt reply.

The base legs are already on a pyramid like jig . The angle is in theory, not a building error. The legs are 1/8X1/8 sticks. For the base, they are about 30 degree off vertical. The brace faces between neighbor legs are not in the same plane anymore.

From another point of view, if the base legs are cut levelly, the intersection is not square anymore, it is a diamond. The sharp edge is less than 90 degree now and the neighbor leg is still 90 degree away. Maybe I should suggest them to make the brace taper at the ends so that it will still fully glue to the leg and no bend.

This is the first time he builds square base. Maybe this is the reason that more and more people build rectangle base?

Yes, parents are advices and power tools only here. You know, now they are in in high school, even the advices are less and less. I wondering around in their classrooms during their team time and feel they did not need me anymore. Well, glad to see they grow up.

Best,

Jinhu

Jinhu,
First let me echo Raleway’s welcome and support of parental involvement!

It was as a helping parent that I started my Science-O involvement over 15 years ago. Worked with both my sons through junior high and high school, then moved into coaching entire team for a couple of years, transitioned to coaching the building events, then to just working with the ‘balsa structure’ events (towers, bridges, and booms). For both me and my kids it was a wonderful experience; what can I say.
So, on to the problem you’re dealing with. It takes a little digging, with all the threads the towers forum has developed, but there is a lot of detailed discussion and good advice on the exact set of issues you raise. I strongly recommend you take the time to read through. The information developed here is the best and largest body of good information specific to S.O. towers out there. I agree absolutely with Raleway’s advice that neither cutting away, nor adding filler is a viable approach.

You’ve run into the first reason that triangular towers are…. just not a good idea. With square cross section legs, there is no way to get leg faces parallel, and with non-parallel faces, no way to get bracing securely attached. Beyond this primary problem, there are a number of other reasons (as you’ll see when you read through the forum) why 3 legs are just not a good idea. A well engineered square tower will always beat a triangular one.

As to square vs. rectangular tower form, there are advocates of rectangular, rather than square, but I believe strongly that there are inherent advantages of the square approach, and inherent down-sides to the rectangular form. Again, there is specific discussion of this issue here.
Using a good jig (precise and symmetrical) is really, really important. It is, I believe, much easier to make a precise and symmetrical square jig than it is for a rectangular base There is detailed discussion of how that can be done here.

I’ve discussed the issue you see of different cross section in leg segments at the angle break when cut in a plane parallel to the test base, and described the solution (that cut needs to be made on the angle bisecting the angle between the leg segments).
Once you’ve absorbed the info here, do feel free to come back with more questions; there are a lot of aspects to getting to a high-performing tower.

Hope this helps

I will go digging. Here is a drawing to explain my question. You can see there is an angle gap between the leg and bracing. The angle is in real scale, calculated by design parameters (base top 78mm diagonal (outside), bottom 300mm diagonal (inside), height 195mm).



Best,

Jinhu

[Chameleon02]

Hey guys
This question may seem kind of confusing but I will try to explain the best I can;
Is there a way to determine the BS of a piece if it were to be 36", although the piece itself is shorter. The reason being is that last year, in Division C, I cut 60cm pieces out of each 96cm stick for the towers. This left about a 30cm piece from each leg. I constructed dozens of towers last year and am left with around 100 of these sticks. Some of these left over sticks are very powerful as they once were the segment cut out of state, regional, and competition towers. These shortened sticks are perfect for use at the base of towers this year. I have several 30cm sticks this year and their BS are coming around 150-230. Is there a way to calculate what these sticks BS would be at 36" so I can compare them with this years pieces?

[Balsa Man]

Hey guys
This question may seem kind of confusing but I will try to explain the best I can;
Is there a way to determine the BS of a piece if it were to be 36", although the piece itself is shorter. The reason being is that last year, in Division C, I cut 60cm pieces out of each 96cm stick for the towers. This left about a 30cm piece from each leg. I constructed dozens of towers last year and am left with around 100 of these sticks. Some of these left over sticks are very powerful as they once were the segment cut out of state, regional, and competition towers. These shortened sticks are perfect for use at the base of towers this year. I have several 30cm sticks this year and their BS are coming around 150-230. Is there a way to calculate what these sticks BS would be at 36" so I can compare them with this years pieces?

That's not confusing at all. There sure is a way, and its easy. It is using that same inverse square relationship you use to calculate increased BS when a stick BS is measured at 36" (which is 91.6cm), and then is braced into shorter segments.
BS is one over the proportion (of the one stick to the other) squared.
Using the 30cm sticks as an example. Let's say you have one with a measured BS of 200 at 30cm length. 1/(91.6/30)^2 = 0.107263. So a 36"/91.6cm stick with the same BS as that one will have a BS of 0.107263 x 200, which = 21.453.

Make sense?

[Chameleon02]

Hey guys
This question may seem kind of confusing but I will try to explain the best I can;
Is there a way to determine the BS of a piece if it were to be 36", although the piece itself is shorter. The reason being is that last year, in Division C, I cut 60cm pieces out of each 96cm stick for the towers. This left about a 30cm piece from each leg. I constructed dozens of towers last year and am left with around 100 of these sticks. Some of these left over sticks are very powerful as they once were the segment cut out of state, regional, and competition towers. These shortened sticks are perfect for use at the base of towers this year. I have several 30cm sticks this year and their BS are coming around 150-230. Is there a way to calculate what these sticks BS would be at 36" so I can compare them with this years pieces?

That's not confusing at all. There sure is a way, and its easy. It is using that same inverse square relationship you use to calculate increased BS when a stick BS is measured at 36" (which is 91.6cm), and then is braced into shorter segments.
BS is one over the proportion (of the one stick to the other) squared.
Using the 30cm sticks as an example. Let's say you have one with a measured BS of 200 at 30cm length. 1/(91.6/30)^2 = 0.107263. So a 36"/91.6cm stick with the same BS as that one will have a BS of 0.107263 x 200, which = 21.453.

Make sense?

Oh... Wow. That is really helpful. I think that I will formulate a table this year of all my last year pieces so I will have a clear "lineup" of which ones are the best. Thanks...

[Raleway]

Hi,

I am monitoring my son do a test tower with square base. I can see the leg faces are not in the same plane anymore. So, if you fully clue the brace X to the leg face, it will bend inward.

Any advice about how to handle that? Like gap filler? File the leg a little where you stick the X brace?

For now, I just told him to pretend it is not there, just pre-tension the brace.

Maybe we should go back to rectangle base. That one, the angle change is ignorable.

Thanks,

Jinhu

In response to your picture... it seems that you are not using square wood then (1/8" X 1/8")? Using square wood would be ok for either a square or rectangular base, with no "angle gap" as you stated. I am a little confused now at your query where the bracing bends inward as with square wood on either a rectangular or square base has the bracing fitting flat on each face. Clarify a little more maybe?

Raleway

[Balsa Man]

Hi,

I am monitoring my son do a test tower with square base. I can see the leg faces are not in the same plane anymore. So, if you fully clue the brace X to the leg face, it will bend inward.

Any advice about how to handle that? Like gap filler? File the leg a little where you stick the X brace?

For now, I just told him to pretend it is not there, just pre-tension the brace.

Maybe we should go back to rectangle base. That one, the angle change is ignorable.

Thanks,

Jinhu

In response to your picture... it seems that you are not using square wood then (1/8" X 1/8")? Using square wood would be ok for either a square or rectangular base, with no "angle gap" as you stated. I am a little confused now at your query where the bracing bends inward as with square wood on either a rectangular or square base has the bracing fitting flat on each face. Clarify a little more maybe?

Raleway

I misunderstood your first post, thinking you were working with a 3-leg tower configuration- sorry ‘bout that. Comments on getting the angle right for the critical joint where upper and lower leg segments meet still apply.

Using a 4-leg, square configuration, as Raleway says, if you have the legs really symmetrically lined up, the faces of any adjacent leg pair will be parallel, in the same plane, and bracing pieces will go on outer leg faces straight, in full contact with the leg faces; no gaps.

While the elongated rhomboid cross section you show in your drawing is correct if you’re looking down perpendicularly onto the cut end of an angled leg, the four faces of that leg stick remain at 90 degrees to each other. See my recent post in the “Base” thread for photos of a tower on a jig – this is a tower from last year, but legs are at an angle. The fact that outer leg faces where the braces go, for any pair of adjacent legs are/will be in the same plane is….supported by the fact that in building this the way we confirmed that the ‘rotational’ alignment of the legs was correct was to tape leg pieces onto the jig and lay a flat plate up against each tower side. That plate would come into full/flat contact with the legs. We actually did this with the little, white plastic angle iron pieces (leg holding strips) in between the leg edges and edges of the tower jig plates. Then, we put some glue spots along the angle iron/jig plate edge joint, and put a second plate against the adjacent tower face, forcing/holding the angle iron in correct “rotational” alignment.

If you are actually seeing, with legs on the tower jig, the gapping/mis-alignment you describe, you have alignment/symmetry problems with the jig. It doesn’t take much rotational mis-alignment (or jig plates being not identical, or not arranged at true 90 degree angle to each other to see the leg face mis-alignment and gapping you’re talking about.

Hope this helps

[dholdgreve]

I misunderstood your first post, thinking you were working with a 3-leg tower configuration- sorry ‘bout that. Comments on getting the angle right for the critical joint where upper and lower leg segments meet still apply.

Using a 4-leg, square configuration, as Raleway says, if you have the legs really symmetrically lined up, the faces of any adjacent leg pair will be parallel, in the same plane, and bracing pieces will go on outer leg faces straight, in full contact with the leg faces; no gaps.

While the elongated rhomboid cross section you show in your drawing is correct if you’re looking down perpendicularly onto the cut end of an angled leg, the four faces of that leg stick remain at 90 degrees to each other. See my recent post in the “Base” thread for photos of a tower on a jig – this is a tower from last year, but legs are at an angle. The fact that outer leg faces where the braces go, for any pair of adjacent legs are/will be in the same plane is….supported by the fact that in building this the way we confirmed that the ‘rotational’ alignment of the legs was correct was to tape leg pieces onto the jig and lay a flat plate up against each tower side. That plate would come into full/flat contact with the legs. We actually did this with the little, white plastic angle iron pieces (leg holding strips) in between the leg edges and edges of the tower jig plates. Then, we put some glue spots along the angle iron/jig plate edge joint, and put a second plate against the adjacent tower face, forcing/holding the angle iron in correct “rotational” alignment.

If you are actually seeing, with legs on the tower jig, the gapping/mis-alignment you describe, you have alignment/symmetry problems with the jig. It doesn’t take much rotational mis-alignment (or jig plates being not identical, or not arranged at true 90 degree angle to each other to see the leg face mis-alignment and gapping you’re talking about.

Hope this helps

I'm not sure sure of that, Len... I believe the issue is that, as the lower section legs "lean in" toward the "waist" of the tower, if you were to take a horizontal cut through that plane at anywhere in the lower section, you would indeed arrive at the drawing shown, due to the lean. The radial dimension from the center of tower of the columns would indeed be greater than the opposite dimension of the column due to the lean, creating the diamond appearance shown.

Frankly, I just now realized this myself, but it has proven to be a problem in our early testing with braces and tension bands breaking adjacently to the column, proving the issue.

[Balsa Man]

I misunderstood your first post, thinking you were working with a 3-leg tower configuration- sorry ‘bout that. Comments on getting the angle right for the critical joint where upper and lower leg segments meet still apply.

Using a 4-leg, square configuration, as Raleway says, if you have the legs really symmetrically lined up, the faces of any adjacent leg pair will be parallel, in the same plane, and bracing pieces will go on outer leg faces straight, in full contact with the leg faces; no gaps.

While the elongated rhomboid cross section you show in your drawing is correct if you’re looking down perpendicularly onto the cut end of an angled leg, the four faces of that leg stick remain at 90 degrees to each other. See my recent post in the “Base” thread for photos of a tower on a jig – this is a tower from last year, but legs are at an angle. The fact that outer leg faces where the braces go, for any pair of adjacent legs are/will be in the same plane is….supported by the fact that in building this the way we confirmed that the ‘rotational’ alignment of the legs was correct was to tape leg pieces onto the jig and lay a flat plate up against each tower side. That plate would come into full/flat contact with the legs. We actually did this with the little, white plastic angle iron pieces (leg holding strips) in between the leg edges and edges of the tower jig plates. Then, we put some glue spots along the angle iron/jig plate edge joint, and put a second plate against the adjacent tower face, forcing/holding the angle iron in correct “rotational” alignment.

If you are actually seeing, with legs on the tower jig, the gapping/mis-alignment you describe, you have alignment/symmetry problems with the jig. It doesn’t take much rotational mis-alignment (or jig plates being not identical, or not arranged at true 90 degree angle to each other to see the leg face mis-alignment and gapping you’re talking about.

Hope this helps

I'm not sure sure of that, Len... I believe the issue is that, as the lower section legs "lean in" toward the "waist" of the tower, if you were to take a horizontal cut through that plane at anywhere in the lower section, you would indeed arrive at the drawing shown, due to the lean. The radial dimension from the center of tower of the columns would indeed be greater than the opposite dimension of the column due to the lean, creating the diamond appearance shown.

Frankly, I just now realized this myself, but it has proven to be a problem in our early testing with braces and tension bands breaking adjacently to the column, proving the issue.

Well, Dan, after putting some pieces together and playing with them on a jig, I have to say, you and Jinhu are right and I was just plain wrong. This is a real issue. What can I say, except …always learning. I’ve always tried very hard to be sure before I say… something is right, ‘this is the way this/this works’, or something is wrong, ‘no, that’s not how that works. Blew it big time on this one.

I think because of the lesser angle to the legs in last year’s towers, the effect was less- enough to not see it in putting plate up against a side, combined with what we knew to be less than perfect rotational alignment of the angle iron we were using for leg holding.

So, to check things out, I put together a (1/8”) a leg segment, then glued 3 pieces onto one face of it- 1 perpendicular to the long axis, one at about 10-12 degree angle to the long axis, and one at about 24-25 degrees. Then glued ‘corresponding’ pieces on adjacent face of the leg stick – parallel to that face, so looking down the line of the leg piece, the glued on pieces were perpendicular to/at 90 degrees to each other. Then, with leg pieces taped onto two opposite jig plates, took the leg piece with the pieces glued onto it, aligned it parallel with the edge of the jig plate in between them, and moved it in toward the jig plate edge. Before it got into contact, the pieces glued onto it perpendicular to it contacted the adjacent legs.

Continued pushing it in toward the jig plate edge (with the perpendicular pieces bending), and next the pieces at 10-12 degree angle came into contact, kept pushing until the pieces at 24-25 degrees came into contact (which was about when the leg segment was fully up against the jig plate edge. So, while I recognized early-on the issue of square deforming into elongated rhomboid as cut angle got away from a 90 degree/perpendicular cut (and need to cut both lower and upper leg ends at the angle bisecting the angle between upper and lower leg segments), I missed/blew off how the 3-d geometry of leg faces was working- that adjacent faces weren’t in the same plane, and how the extent to which this was true varied with the angle of bracing pieces connecting two adjacent legs.

So, recognizing and understanding what’s going on, what does that mean, and how can it be dealt with?

Problem is most severe in a C tower base, essentially non-existent in C chimney, and almost non-existent in a B chimney.

You definitely want brace pieces to be straight, not put on with significant bowing in them. (as discussed before, even with 1/32” thick X braces, there is very slight bowing- half of 1/32”-1/64”; no way around that, but you don’t want any more than you have to.

As previously discussed, filing/sanding of leg faces where braces go definitely compromises leg strength, by reducing cross section (hence buckling strength) in the filed down/sanded down zone. However, the presence of the glued on brace pieces increases cross section in that area. Last year (thinking we were dealing with just rotational mis-alignment, we did do some careful filing along the contact lines, and legs worked – had the calculated buckling strength; held under full load. The other thing we did was careful trimming of X brace piece ends so they contacted each other, and put a tiny spot of glue on at that contact. That may help the tear-away problem you’re encountering.

The greater the angle (from horizontal) of brace pieces, the less the problem, which says wider bracing intervals may turn out to be ultimately more efficient (if calculations show tower weights to be close between two bracing intervals. Also, in situations where the angle/gap is small, using medium/thick CA as gap filler is viable- just a bit of extra glue weight.