Bracing

[_HenryHscioly_]

My first tower I made I used lap joints, but it performed very poorly.
So I've been using end joints for all my parts now, and my tower's have been breaking at the base because the legs end up sliding outwards, the joints fail
Should I use lap joints instead?And my first tower didn't do well because it was my first one?
How about notched joints?

[iYOA]

USE LAP JOINTS no joke. The reason it performed poorly was probably because it was your first one and there may have been some construction issues. anyways, the joints most likely were not the reason for breaking. a good tower should not have a joint failure so there is no point in going through the effort of notched joints. An ideal tower failure is probably when an unbraced length of the primary compression members buckles. A joint failure is usually fixed with improved construction.

[hogger]

My first tower I made I used lap joints, but it performed very poorly.
So I've been using end joints for all my parts now, and my tower's have been breaking at the base because the legs end up sliding outwards, the joints fail
Should I use lap joints instead?And my first tower didn't do well because it was my first one?
How about notched joints?

At the spot where the top meets the legs, I would stay with butt joint (end joint as you put it) but use gussets on both sides of the pieces that have the same parallel plane. In fact, you can keep adding more pieces of gusset next to the first one outward up and down the joining pieces. Make sure you don't violate the 8 cm cylinder diameter rule at the joining spot though. Everywhere else especially for the braces, I would use lap joints. See Double Gusset 2 below.

http://www.garrettsbridges.com/building/bridge-joints/

[SLM]

for the top part of the tower chimney,
is it better to have the bracing sides connected all the same way or where four pieces of bracing connect? (assuming you use one bracing per part of the tower) ...

An interesting question with no simple answer.

In towers, bracings mainly control member buckling and lateral movement of the structure.

With regard to member buckling, it does not matter how the bracings are oriented on the four sides of the chimney. What matters is how the bracing pattern on each side divides the long (vertical) compression members into shorter (un-braced) segments. As long as these segments are short enough (according to the Euler’s buckling equation), then theoretically no buckling will take place.

Lateral movement of the structure, however, could be effected by the orientation of the bracing pattern on each of the 4 sides of the tower. Let’s use an example to see how.

Consider the example tower used in my third post on the previous page. Let’s assume the tower uses bracing pattern P3 on all four sides where the diagonals are slanted with a negative slope (connecting an upper left node to a lower right node), as shown below.


Note: For visual clarity only bracing on two sides are drawn.

As it was shown previously, each side of the tower, if considered in isolation, displaces slightly to the left at the top of the tower, like this:



Here is a visualization exercise. In your mind, move round the tower and examine each side. As you face a side, you should see it being deformed in a manner similar to the image shown above. Can you see what happens to the whole tower when each side behaves the same way (sidesway to the left)? The tower twists, like this:



Now, let’s look at a different bracing orientation for the four sides.



In this case, let's assume the bracings on the opposite sides are oriented in the same direction, but the adjacent sides are mirror image of each other. Can you figure out how the tower is going to deform? Give it a try. Hint: It is not going to be by twisting.

The point is that different bracing patterns or different orientations of the same pattern on the four sides of a tower lead to different deformation patterns for the tower. As to which bracing pattern or orientation is a better choice, there is no simple answer. Bracing orientation A could result in excessive twisting in a member causing it to fail. On the other hand, orientation B could create significant stress at a glued joint causing it to come apart. You need to run experiments to determine what works best for your tower.

[soccerkid812]

although I was referring to the P2 bracing, but I understand the main point

thanks SLM

[Sgt Evans]

What type of bracing r u using???

[sciolyswizzle16]

would you guys suggest using both end joints and lap joints when bracing a tower?
for example, using end joints on the horizontal pieces connecting the top of the tower and the bottom of the tower then using lap joints for the rest of the bracing?

[12thomasal]

slm, If I tilt the chimney section, does that reduce the swaying factor

[SLM]

slm, If I tilt the chimney section, does that reduce the swaying factor

No. The compression force in the chimney increases as the tilt angle increases. The sidesway of the chimney frame is directly proportional to the intensity of the axial force in the compression members: the larger the force the more the frame would displace. So, tilting a chimney frame actually increases the sidesway of the frame.

However, (1) the sidesway is very small to begin with, so it should not be of a major concern for most tower, and (2) the tilt angle is very small (in our case about 4 degrees), so its impact on the behavior of the tower is insignificant.

[SLM]

would you guys suggest using both end joints and lap joints when bracing a tower?
for example, using end joints on the horizontal pieces connecting the top of the tower and the bottom of the tower then using lap joints for the rest of the bracing?

It can certainly be done that way. Although, you need to pay extra attention when constructing a butt joint; you need to make sure that either end of the horizontal piece is completely flat and parallel to the member it is being glued to (you want to avoid any gaps between the two pieces).

We've found that most of the time lap joints work very well even for horizontal pieces. They are easy to construct and, in most cases, their construction doesn't require a lot of precision.

[sciolyswizzle16]

thanks SLM, i definitely will consider that

[yousef213]

on the base of the tower, or any part really,, could you skip the horizontal pieces between the x bracings, or would that weaken it significantly?

[jander14indoor]

on the base of the tower, or any part really,, could you skip the horizontal pieces between the x bracings, or would that weaken it significantly?

Probably not, especially the bottom.
Key failure mode on bottom is legs spreading out (especially on a smooth surface, common in contest test setups), increasing bending load and causing premature failure or tip over. Need horizontal members around the bottom to constrain this.

Probably not on top either unless the angled pieces are fairly horzontal anyway. Again, you want to prevent the uprights from bowing out (or in) as well as sideways, and an occasional horizontal strip is an efficient way to do that.

Note, on SLM's diagrams above, the uprights without horizontal cross bars tend to bow out, those with tend to tilt.

Jeff Anderson
Livonia, MI

[deezee]

i didnt have horizontal bracing, and I did better than when I had efficiency of 21

[TYG]

i didnt have horizontal bracing, and I did better than when I had efficiency of 21

Was it able to hold all 15kg?