Bracing

12thomasal
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Re: Bracing

Post by 12thomasal » February 27th, 2011, 11:43 pm

deezee wrote:i didnt have horizontal bracing, and I did better than when I had :) efficiency of 21
could you clarify that NO pieces were glued to two different legs, if so, you are a miracle and should start your own religion

My guess is that you only had 2 bracings, bottomand transition

soccerkid812
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Re: Bracing

Post by soccerkid812 » February 28th, 2011, 3:20 pm

if you are using too small bracing (lets say maybe 1/64x1/64), where would the tower break?
in the middle of the bracing, the joints, or the legs still?

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Re: Bracing

Post by yousef213 » February 28th, 2011, 4:15 pm

soccerkid812 wrote:if you are using too small bracing (lets say maybe 1/64x1/64), where would the tower break?
in the middle of the bracing, the joints, or the legs still?
As you most likely know, towers will "spontanously explode" when 1 member fails under normal circumstances.

if you look in previous pages, the bracings dont support too much load but are essential for the tower as it makes a reduced effective length.

So theoretically,the bracings would break and the legs would collapse shortly afterword

However, I haven't tested this and cannot confirm
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Re: Bracing

Post by SLM » February 28th, 2011, 7:54 pm

soccerkid812 wrote:if you are using too small bracing (lets say maybe 1/64x1/64), where would the tower break?
in the middle of the bracing, the joints, or the legs still?
The smallest size bracing we have used is 1/16 x 1/32. I think anything smaller may not provide adequate contact area for gluing the bracing to the main member(s). I would be interested to know whether you have had success with using 1/64 x 1/64 for bracing.

The main function of the bracing system in a tower is to prevent buckling of the compression members and to keep the main members upright and aligned. If the bracing system is too weak or if it actually fails, then one of two things would happen.

1. A compression member would buckle leading to the failure of the entire tower.
2. A part of the tower losses its upright orientation, or its alignment relative to the rest of the tower causing either twisting or bending of a few members,or force redistribution and concentration in one or more members. This behavior has a cascading effect on the rest of the tower and very quickly leads to the failure of the entire structure.

soccerkid812
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Re: Bracing

Post by soccerkid812 » February 28th, 2011, 7:57 pm

sorry for the confusion, im not actually using 1/64x1/64, i was just trying to make a point
i am planning on using smaller cross bracing tho, so i just wanted to know the effects

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Re: Bracing

Post by Balsa Man » March 1st, 2011, 7:54 am

Haven’t had a chance to be on the board since Christmas (way busy at work).
As I’ve said before, there are many ways to skin a cat (and do bracing). A little review of the way we’ve chosen to do it (dig back to posts last fall discussing this):
We use (horizontal) ladders and X-bracing. Tower legs are 3/32nds. Ladders are also 3/32nds. Xs are 1/64th x 1/16th to 3/32nd. You can see what it looks like by looking at bridge pictures from 2009 – along the top of the bridge, joining the two top members. In a tower, those top members would be the legs.
Here’s why and how it works. SLM is pretty much right-on on the function of the bracing system- prevent buckling of the legs (the intervals you put ladders at divide the legs up into shorter exposed column length-shorter column lengths can take higher load before buckling) – and keeping legs “upright and aligned.” One important function beyond that; together, the bracing system makes the entire structure (tower) rigid.
Let’s visualize. Looking at just one side of the tower, and one “section”- three ladders- one at top, one in the middle, one at the bottom; laying on a flat surface. You may even want to cut a few pieces of wood and try/feel this. Between any two ladders, you have a trapezoid- parallel top and bottom, with the two sides somewhere between vertical and, oh, maybe 20 degrees sloped in on the base section.
If you “pin down” the four corners (i.e., if they can’t move), and push the middle ladder horizontally, what happens? It moves horizontally, and the legs bow. What happens if you join the corners where the ladders meet the legs with Xs? Let’s start with a brace from the lower left to the middle right, and from the upper left to the middle right. With them in place, what happens when you try to push the middle ladder to the right? Those brace pieces come under tension, and they keep the ladder from moving right. They would do exactly the same if they were string, instead of wood; the only way for the ladder to move right would be for the braces to stretch (lengthen); if they don’t stretch, the ladder doesn’t move. Now, let’s try moving the middle ladder to the left. What happens? It moves; the half-Xs bend (the points they’re attached to move closer together, shortening the distance between them). OK, now put the rest of Xs in place; pieces from the lower right corner to the middle left, and from the upper right to the middle left. Now, which ever way (left or right) you push the middle ladder, it can’t move.
If the legs try to bow inward, the ladder (acting in compression) resists that movement. If the legs try to bow out, the Xs (acting in tension) resist that movement. If now add another side (at 90 degrees in a 4-legged tower, at 60 degrees in a 3-legger), this same bracing works in 3 dimensions. Put all four (or 3) sides together, and you have a rigid structure.
The important thing to understand here is that the ladders work in compression, and the Xs in tension. The Xs (if the rules allowed) could be fine thread. The key is that they be tight. If they have any slack in them, then where they join has to move before they tighten up. That movement – buckling starting in the leg, starts the failure process. If the Xs are tight, and where they join can’t move, then that point of the leg stays in place, and everything is cool. In the upper portion of the tower, where the legs are near vertical, it doesn’t take much strength to hold the point where the Xs meet in place. Down in the base, the Xs going to the lower ends of the legs will be under significant tension load (the bottom ends of the legs trying to spread out). That force is on the order of a few kg. High density 1/64th balsa is amazingly strong in tension; pieces 1/16th wide can hold 2-3 kg (try it). 1/16th provides plenty of glue area (joint will be stronger than the wood).
So, for what its worth; it is one way that works.
Len Joeris
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Re: Bracing

Post by A Person » March 6th, 2011, 12:13 pm

Thanks SLM, I've been considering your post and it is very helpful!
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Re: Bracing

Post by soccerkid812 » April 13th, 2011, 3:14 pm

does using 1/16x1/16 or 1/8x1/32 have an difference on cross bracing if using the same length and density?

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Re: Bracing

Post by SLM » April 13th, 2011, 3:33 pm

soccerkid812 wrote:does using 1/16x1/16 or 1/8x1/32 have an difference on cross bracing if using the same length and density?
Yes, it does.

These sections offer different buckling strengths. Everything else being equal, the buckling strength of the rectangular section (1/8 x 1/32) is 1/4 of that of the square section. So if the bracing undergoes a relatively large compressive force, the rectangular member would buckle first.

The rectangular section offers more surface area for a lap joint than does the square section. This might offer an advantage if your are certain the bracing is going to act as a tension member only.

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Re: Bracing

Post by havenguy » April 16th, 2011, 5:49 pm

What size are most people using for bracing anyway?
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