Designs

[rgdesh]

When it comes to tower designs, i use autocad to draw up my designs. They are accurate, easy to follow, and can be useful when displaying to judges (it doesn't effect judges' opinion). I've been doing towers and bridge building for about 4 years and my best advice for beginners is to do research on frame designs, balsa wood vs basswood, super glue vs wood glue, and reducing weight through oven-heating. This invaluable research will have a big impact on final bridge efficiency. Designs are also crucial. Research designs, draw them up on autocad, build them, and test to see which design has best efficiency. I've been using this method for bridges and it has help me build some successful bridges. Have fun building these towers.

[FunnyFroggy]

I'm kinda new at this, but I have a quick question.

Can you turn the 5x5 loading block, so that it would just fit on a 7cm x 7cm top? Or is that not allowed?
If so, is that better than building a smaller top that would fit the loading block normally?

[lllazar]

You cant have a 7cm x 7cm square top...because that would NOT fit inside an 8cm diameter circle.

And when you say reduction of weight through oven heating, can you please explain the process?

[AlphaTauri]

I've never personally tried this before, but I've heard about it. Basically you build your bridge/tower and then put it in the oven to draw out any water or other moisture left in it, hopefully decreasing the weight by a measurable amount, and therefore increasing your efficiency. For a more detailed description, try here, Week 10 of 2008-2009.

[quizbowl]

I've never personally tried this before, but I've heard about it. Basically you build your bridge/tower and then put it in the oven to draw out any water or other moisture left in it, hopefully decreasing the weight by a measurable amount, and therefore increasing your efficiency. For a more detailed description, try here, Week 10 of 2008-2009.

That sounds like a great way to reduce weight, but it brings up two questions:
!) Is the integrity of the bridge/tower kept?
2) Will the strength of the bridge/tower decrease with the heating process?

[SLM]

I've never personally tried this before, but I've heard about it. Basically you build your bridge/tower and then put it in the oven to draw out any water or other moisture left in it, hopefully decreasing the weight by a measurable amount, and therefore increasing your efficiency. For a more detailed description, try here, Week 10 of 2008-2009.

That sounds like a great way to reduce weight, but it brings up two questions:
!) Is the integrity of the bridge/tower kept?
2) Will the strength of the bridge/tower decrease with the heating process?

From experience, I can tell you that baking a balsa wood structure in the oven at about 200 degrees Fahrenheit for about 20 minutes reduces the weight about 6% without adversely affecting the structure's integrity or strength. However, the structure gains most of the lost weight back in an hour or so, if it is not kept in a very dry and air-tight container.

[Balsa Man]

I've never personally tried this before, but I've heard about it. Basically you build your bridge/tower and then put it in the oven to draw out any water or other moisture left in it, hopefully decreasing the weight by a measurable amount, and therefore increasing your efficiency. For a more detailed description, try here, Week 10 of 2008-2009.

That sounds like a great way to reduce weight, but it brings up two questions:
!) Is the integrity of the bridge/tower kept?
2) Will the strength of the bridge/tower decrease with the heating process?

From experience, I can tell you that baking a balsa wood structure in the oven at about 200 degrees Fahrenheit for about 20 minutes reduces the weight about 6% without adversely affecting the structure's integrity or strength. However, the structure gains most of the lost weight back in an hour or so, if it is not kept in a very dry and air-tight container.

Pretty similar experience here. That 6% number is about what I've seen when the reletive humidity is in the, oh, 30-50% range. If its really humid- like 90-95%, maybe 8%. As to how fast it equillibrates - its not linear over time. Upon exposure to the air, our testing has shown about half the gain lost in the first 10 minutes. I've never been able to measure gain past about 30-35 minutes. Bigger cross section sticks, and higher density slows things down a little- small cross section/low density regains more quickly. Being able to seal the cooked structure against ambient humidity is tricky (to do it effectively, and for any length of time.)

So, whether you can pick up any gain depends on how the event is run. If you can go from a well-sealed container to the official weigh-in less than 10 minutes, maybe you can realize a 2-3% gain. But if you're standing in the check-in line with it in a sealed bag, you may well get asked to hang loose. If its run like its supposed to per the rules, it comes out of containment for check it meets specs, and gets weighed just before testing, and you gain nothing.

[phillies413]

Based on your past experiences, do you all think it is more efficient to have a rectangular base or a square base?

[lllazar]

Well, if you want a rectangular base, im assuming youd do something like 20 cm x W cm, w/e W may be? The fact is, with a base height of 15, the legs would be under a lot of stress due to the angle at which they are from vertical - a square base in this sense would be more practical.

But don't take my word for it, im sure other more experienced builders will try to answer your question to the best of their abilities.

[Balsa Man]

Well, if you want a rectangular base, im assuming youd do something like 20 cm x W cm, w/e W may be? The fact is, with a base height of 15, the legs would be under a lot of stress due to the angle at which they are from vertical - a square base in this sense would be more practical.

But don't take my word for it, im sure other more experienced builders will try to answer your question to the best of their abilities.

Illazar, you’re absolutely right that the more the legs are leaning, the more force they have to carry. Per earlier post, if legs were vertical, they would carry ¼ (for a 4-legged tower), or 1/3 (for a 3-legged tower) of 15kg. As the angle from vertical increases, the load increases as a function of 1 over the cosine of the angle. The trick is understanding what “the angle” is. You want to minimize that angle to minimize the force the legs need to handle, because the less the force, the lighter the legs can be. What's a little tricky here is visualizing/understanding things in three dimensions.

At the base/bottom, the legs need to span the 20cm square hole in the test base. At the 15 cm (30cm for B), the legs need to fit inside an 8cm circle. For a square base, by putting the legs at the mid-points of the sides of the 20x20 hole, you end up with the legs (on each of the sides), about 16cm apart. At the top, the distance apart is the side of a square that will fit in an 8cm circle- I don’t have drawings with me, but let’s say for discussion purposes, that distance is 6cm. The base portion looks like a truncated pyramid – a square w/16 cm sides at the bottom; a square with 6cm sides at the top. The legs lean in toward each other, and in toward the center of the tower.

OK, now let’s consider a rectangular base. Looking at one side, the legs, at the base, need to be 20+ cm apart (tapering in to end up 6cm apart at the top. Presumably, these sides would be put together 5cm apart - Illazar's "W" (so the legs at the top of the tower will fit under the 5cm square load block. You’d have two flat sides, and two sides where the base is leaned in quite a bit, and the upper portion is leaned in slightly. The lower portion would be…..very much like a short bridge; clear span of 20cm, top spam of 6cm, width of 5cm.

For simplicity, let’s just consider the base portion, now. It would seem, at first thought that the legs in the rectangular configuration would be angled in more; 20 cm apart (as opposed to 16cm) at the bottom, and 6cm at the top. However, let’s look more closely at the square. If you look at the plane defined by two opposite sides, the base ends of the legs are 20+ cm apart, and the top ends fit inside the 8cm circle (our assumed 6cm apart); this is the same angle as in the rectangular case. It is the angle in toward the centerline of the tower. If we look at the base “pyramid” from one side- looking at the…..outline, or silhouette, or shadow, we see legs leaning in from 20+ cm to 6cm, BUT, those legs are in a tilted plane – a plane tilted from the base in toward the center of the tower. The angle of that tilt is the same angle as we see in looking at the outline. Looked at in this view, It’s a compound angle. However, by rotating the tower/base section we’re looking at 90 degrees – rotating around the centerline of the tower, now the outline we’re looking at has a pair of legs at the outside (a pair that are diagonally apart from each other), and the other pair of legs line up with each other- the front leg lined up on top of the back leg. The angle the outside legs lean in toward each other – and, this is the important part; in the plane we’re now looking at, toward the centerline of the tower, is the same as the angle they lean in at in one of our flat sides from the rectangular configuration. So, “the” angle; the angle that determines how much more force the legs see than if they were vertical, is the same in both cases.

So which is “more efficient”? (as in lighter). If you figure a ladder piece joining the tops, and a ladder piece joining the mid-points of the legs, in the rectangular, you have a total of 60cm, and in the square 68cm – a little over 10% in favor of the rectangle. but, the ladder piece in two of the sides is longer (13 cm vs 11cm)- meaning they have to be stiffer (i.e., heavier), while on two of the sides they are a lot shorter (5cm vs 11cm), i.e., a lot lighter. You’ll have to figure out how these trade-offs work.

There is, I believe a big downside to the rectangular configuration (with flat, parallel sides), though; overall tower stability. With the load on top of the tower 50cm above the base, only a tiny amount of bucket sway across (perpendicular to) the plane of the flat sides will create a force that will act to tip the tower over. Likewise, only a tiny amount of distortion out of the plane(s) of the flat sides will do the same. With all sides leaning in some toward the center you gain overall stability against both distortion and bucket sway.

I think a – perhaps the - really important question this year is how do the tradeoffs between a square (4-legged) tower, and triangular (3-legged) tower work out……….