How much does grain matter in terms of stretch?

[JamesRAHS]

?I am thinking about cutting the sides of towers from a single flat piece of balsa without individually cutting cross beams. Do you guys know if I will lose significant integrity if I manufacture it this way? It would cut out a lot of the glue and build time spent carefully clamping and lining stuff up. So what do you guys think?

[Unome]

I'm not certain, but my first thought is that the weight would be a far more important concern here.

Edit: I misinterpreted what you were saying. I agree with SPP SciO, if you glue well you can overcome any disadvantages from cutting beams vs. a single sheet, but there's not really a way to get around the grain direction if you're cutting from a single sheet.

[SPP SciO]

?I am thinking about cutting the sides of towers from a single flat piece of balsa without individually cutting cross beams. Do you guys know if I will lose significant integrity if I manufacture it this way? It would cut out a lot of the glue and build time spent carefully clamping and lining stuff up. So what do you guys think?

The balsa will be far stronger in the direction of the grain, with tension and compression. If you cut out the whole side from a single piece of wood, many of the pieces will go against the grain and will be very weak. It would be an aesthetically beautiful but structurally weak tower.

[retired1]

If you are referring to a slab sided tower, we had one team that broke their tower without enough time to fix it, so they planked the frame. Held full weight but weighed a bit more that the others, so placed 4th. It was starting to show signs of stress. It is a very easy item to test.

[jander14indoor]

Grain is critical to the strength of wood.
Wood is strongest in tension and compression along the grain, really poor perpendicular to grain.
That said, a few years back, one bridge did essentially just what you are saying. All structure was sheet stock and it was essentially a monocoque structure. Won nationals. But, wasn't simple slab sides, grain direction was carefully chosen in the various pieces to react correctly to the stresses.
I'd say for the narrow portion you could PROBABLY make a pretty good tower with appropriate buckling resistance that way, not sure how weight efficient it would be because you'd start using pretty thin wood to hit weight. And to do it right, I doubt you'd simplify construction much.
For the lower portion where there starts to be significant tension stress around the structure as the legs splay, pretty tough to manage.

Jeff Anderson
Livonia, MI

[Random Human]

Grain is critical to the strength of wood.
Wood is strongest in tension and compression along the grain, really poor perpendicular to grain.
That said, a few years back, one bridge did essentially just what you are saying. All structure was sheet stock and it was essentially a monocoque structure. Won nationals. But, wasn't simple slab sides, grain direction was carefully chosen in the various pieces to react correctly to the stresses.
I'd say for the narrow portion you could PROBABLY make a pretty good tower with appropriate buckling resistance that way, not sure how weight efficient it would be because you'd start using pretty thin wood to hit weight. And to do it right, I doubt you'd simplify construction much.
For the lower portion where there starts to be significant tension stress around the structure as the legs splay, pretty tough to manage.

Jeff Anderson
Livonia, MI

Grain will affect your tower, but really doesnt matter. Its impossible to get all your leg sticks to a c grain, or all your tension sticks to an a grain. Most of the time, you will just have to work with what you have. Grain doesnt matter a whole lot, but it doest matter a bit.

[Balsa Man]

Thinking outside the box is a good thing, and to be commended. The key to evaluating whether something that comes from that is, or may be, really better is understanding… the underlying science, and from that, how the out of the box new idea …lines up with that.

As jander, SPPSciO, and Unome note, (correctly) strength (both tensile and buckling) of wood is MUCH stronger along the grain than across the grain. In the upper/chimney section, with minimal leg lean-in, a ‘cut side’ would have legs with grain pretty closely aligned with leg axis. But braces would have grain essentially perpendicular to long axis. This means the strength of the braces would be an order of magnitude (or more) lower than brace pieces of the same size cut/installed separately (with grain aligned with brace piece axis)

https://www.fpl.fs.fed.us/documnts/pdf1994/winan94a.pdf

In the lower/base section with leg angles on the order of 24-30 degrees (see table previously posted), a “cut side” would have grain angles (vs axis of the leg segments) that would severely weaken the legs (compared to legs with grain running along the leg axis).
So, the bottom line is no way such an approach could begin to be competitive with a design with legs and braces where grain aligns with long axis of the braces and legs.

[SPP SciO]

A couple of years ago I had a student who wanted to make a jointless bridge - the idea being to start with a big block of balsa and build a bridge through subtraction. Just to gauge viability, I gave him a small block to see if he could make it look like a hollow cube. He used a drill and a square rod covered in sandpaper. It was extremely tedious. The end product (first and only attempt) was much weaker than something that could have been built in a small fraction of the time from sticks.

As far as "unconventional" designs go, does anyone anticipate rolled tube towers? My very basic grasp on engineering principles suggests it would work very well, but I doubt a student could build one as perfectly as necessary - and any suitable wood would be very pricey. Still, people roll balsa tubes so plane fuselages can be light and strong..

[JonB]

A couple of years ago I had a student who wanted to make a jointless bridge - the idea being to start with a big block of balsa and build a bridge through subtraction. Just to gauge viability, I gave him a small block to see if he could make it look like a hollow cube. He used a drill and a square rod covered in sandpaper. It was extremely tedious. The end product (first and only attempt) was much weaker than something that could have been built in a small fraction of the time from sticks.

As far as "unconventional" designs go, does anyone anticipate rolled tube towers? My very basic grasp on engineering principles suggests it would work very well, but I doubt a student could build one as perfectly as necessary - and any suitable wood would be very pricey. Still, people roll balsa tubes so plane fuselages can be light and strong..

I am sure we will see some rolled tubes. They are actually not extremely difficult to build based on our experience with Boomilever years back. They worked fairly well but I do not think the top towers in the nation will be rolled tube (but then again, I have been surprised many, many times).

[dholdgreve]

"Sheeting" the sides of the tower with really thin material is known as diaphragm bracing. In House construction, it is the plywood or OSB on the walls that keeps them from scissoring to one side or the other, it is the sheeting on the roof that keeps the trusses from toppling, it is the plywood on the floor, connected to the joists, connected to the basement walls that keep the soils and water from pushing the basement walls in. This is certainly a tried and true method, but as mentioned, it will be very difficult to get the weight down.

One thing to consider:
When you X brace, you create concentrated "nodes" that bear all of the forces at each point of intersection. The fewer the X braces, the greater the concentration of forces on the columns at that point. Convexly, by diaphragm bracing, the forces are not concentrated, but transformed uniformly and continuously to the columns, with no concentrations (as long as the bead of glue is also continuous). In theory, this should allow a substantial reduction in the size and density of the columns, since they are now continuously supported by the diaphragm. I believe there is also a commonly accepted standard that would allow you to install a round hole in the center third of the horizontal dimension of the diaphragm to save weight. So if your chimney part of the tower was 60 mm wide, this theory would allow you to cut 20 mm diameter holes out of the center of the sheet and still provide continuous support of the columns. You will need to figure out the spacing between the holes, but I'd start at 10 mm and move up or down from there.

To even get close to a trade off in weight, I believe you would need probably drop to 3/32 columns and 1/64" super light diaphragm. The problem with the 1/64" material is that it is so thin that it can very easily have a finger punched through it at the wrong spot, and the lignen (the sap that holds the grain together) is so minimal that the sheet can get torn apart easily. The references I gave you earlier comparing to home construction uses plywood, which has grain running in both directions. The sheets discussed in towers only run in one direction, making it susceptible to splitting lengthwise.