Designs

[lllazar]

What density 1/16 bracing do you guys generally use? Also Chevron-V bracing seems to work really well, it's lighter than x bracing by a significant amount and though i haven't tested by latest tower (which has a base with chevron v bracing) it seems pretty sturdy.

[soccerkid812]

what exactly is a chevron V brace?

[Jo_squirrel]

What density 1/16 bracing do you guys generally use? Also Chevron-V bracing seems to work really well, it's lighter than x bracing by a significant amount and though i haven't tested by latest tower (which has a base with chevron v bracing) it seems pretty sturdy.

Is Chevron-V bracing the bracing that looks like a sideways V?

[lllazar]

No it isn't, it actually looks like a v opening downwards - so an upside down v. Here's a pic of it:

[deezee]

yes, but the opening will be 20x20 square.

[lllazar]

Regarding bracing for tension, what effect does the cross section have? For example, if a 1/16 x 1/16 piece of balsa had a mass of .2 grams, and a 1/16 x 5/32 piece of balsa had a mass of .2 grams as well, which would be stronger? I know we discussed that density is the primary factor affecting tensile strength, but i wonder if that was only assuming same cross section.

I ask this because i got a really light sheet of 1/16 and don't want to completely waste it, so maybe upping the cross section of the bracing at no added weight would be ok?

[SLM]

Regarding bracing for tension, what effect does the cross section have? For example, if a 1/16 x 1/16 piece of balsa had a mass of .2 grams, and a 1/16 x 5/32 piece of balsa had a mass of .2 grams as well, which would be stronger? I know we discussed that density is the primary factor affecting tensile strength, but i wonder if that was only assuming same cross section.

I ask this because i got a really light sheet of 1/16 and don't want to completely waste it, so maybe upping the cross section of the bracing at no added weight would be ok?

In engineering, tensile strength has a unit of force per cross-sectional area (Strength = Force/Area). That is,

Tensile Strength = Maximum Tensile Load the Member can Carry DIVIDED BY Cross-Sectional Area of the Member.

Let's say tensile strength of medium density balsa is 13 MPa. Since Pa (Pascal) is Newton per unit area (squared meter), then that means a tensile member that has a cross-sectional area of 1 squared meter can carry a force of 13,000,000 Newton.

According to the above equation, a 1/16 x 1/16 (0.00159 m x 0.00159 m) member, can carry a tensile force of

F = Strength x Area = (13,000,000)(0.00159)(0.00159) = 32.76 Newton.

Similarly, a 1/16 x 5/32 (0.00159 m x 0.00397 m) member can carry a tensile force of

F = (13,000,000)(0.00159)(0.00397) = 82 Netwon.

Therefore, if both members have the same density (tensile strength), then the one that has a larger cross-section can carry more force.

But, in your case the members do not have the same density since they both weigh the same but one has more volume than the other. The smaller size member (1/16 x 1/16) is denser than the other one. More specifically, it is 2.5 times denser. From this ratio I can estimate the ratio of the tensile strengths of the two members. It is 4.6. That is, the tensile strength of the 1/16 x 1/16 member is 4.6 times bigger that that of the other member. This means that the smaller size member can carry 1.84 times more tensile force than the larger one. Hence, the 1/16 x 1/16 is a better choice for use as a tensile member.

[lllazar]

Hmm all right, i guess i'll just have to order denser 1/16 next time.

[jpmacguy]

If I have more than four members touching the testing surface on the base of a four sided tower, would each of the members have to bear less weight?

[SLM]

If I have more than four members touching the testing surface on the base of a four sided tower, would each of the members have to bear less weight?

It depends on the geometry of the tower. But, I suspect that for a typical rectangular tower, the bulk of the load goes through the four legs at the base. Adding more inclined members touching the test plate may not necessarily cause a significant redistribution of the loads to these members. To know for sure, you need to analyze the tower.