Designs

[bmbw123]

Are you guys making the chimney section of the tower slightly thinner at the top? For example making the portion of the chimney that connects with the base about 5.6 cm and the top of it maybe 4ish cm?

[Littleboy]

Are you guys making the chimney section of the tower slightly thinner at the top? For example making the portion of the chimney that connects with the base about 5.6 cm and the top of it maybe 4ish cm?

I have thought about it but have not yet tried. I normally do 5 so that it is easy to put the block on strait and square. I do have the base at 5.5cm so I have room for error. Having it further out will increase its ability to spread. I have not tried otherwise so do not use this as full guidence.

[bmbw123]

Alright thanks. I'll try both out sometime when i have the time

[lllazar]

How does everyone make certain that their towers are perpendicular and that the chimney isn't leaning - most of mine are fine because my jigs are pretty precise but they sometimes come out oddly off by a few tenths of a degree.

[Balsa Man]

How does everyone make certain that their towers are perpendicular and that the chimney isn't leaning - most of mine are fine because my jigs are pretty precise but they sometimes come out oddly off by a few tenths of a degree.

Plumb bob- string with a little weight w/ a point.
1) get level surface
2) get a way to hang string (fine sewing thread) from center of the top
3) run intersecting strings (fine sewing thread) from bottom ends of legs- that cross at the center
4) set bob string length so weight tip is just above the strings defining center of bottom
5) adjust leg length (gentle sanding) so weight aligns w/ center of the bottom

If the upper corners of your base section are level, chimney will be perpendicular. Before glueing base and chimney together, re-run above procedure w/ chimney on base; adjust till centered, glue together.

[lllazar]

Wow i can't believe i forgot that our physics teacher showed us that in class...can't remember for wat, i think it was during the pendulum section, but thanks Balsaman!

In other news got a tower under 8 grams for the 1st time!! Hopefully my stiffness check was thorough enough....All of our other ones have held 14-15kg, then again they were all over 10, testing tommorow and we have our fingers crossed~

[Balsa Man]

Very cool.Good luck!

[Freyssenet]

At first glance towers with a triangular base seem more efficient than those with a square or rectangular base. However, there are several constraints that make such towers not as efficient as they seem to:

(1) Spanning the 200 x 200 mm opening at the test base - this results in separation between the legs that is significantly larger than those achieved with either square or rectangular base alternatives, and, hence, the length of the braces between legs ends up being just about the same as with the other alternatives.

(2) Loading block dimensions: the dimensions of the loading block will result in an eccentricity between the point of application of the load, at the center of the 50 mm x 500 mm block, and the centroid of the triangle in the tower resisting the applied load. I do not know how to upload a figure to explain this graphically, but it can be explained as follows: Draw the 50 x 50 mm loading block and locate in it the three struts resisting the applied load. Draw a triangle that passes through the centroid of each of the cross section of the three legs.. We can see that the maximum separation between the legs is attained when the legs are located at the edge of the loading block (in practice the maximum separation between the legs in a triangular tower has to be less than 50 mm). Under these circumstances the distance between the point of application of the load and the centroid of the triangle is 50 mm x cos(30) x 2 /3 - 50 mm / 2 = 3.87 mm. The 50 mm cos (30) is the height of the triangle and the 2/3 is the distance from a vertex to the triangle's centroid. While such distance seems small, it is 8.9% of the triangles height. One would wish the hole drilled in the loading block is oversized so that the rod could be moved from the center of the loading block towards the triangle's centroid, but I think the hole will be enough to pass a 1/4 in. rod through it. Anyhow, the most important issue here is that the legs will not be loaded equally to 1/3 of the applied load. In fact one of the legs could be loaded to 42 percent of the applied load!, an increase of 28 percent over the load that one would think is the load being carried by each leg.

Another point that goes against towers with a triangular base is that they are not easy to build using commercially available standard balsa sticks, I tried and made it work, but it was hard work!

[lllazar]

I made a jig for a triangular base, ill post the pic as soon as i get around to it, it's at my partners house, but anyway it was a lot harder to first of all design than the square base jig, and building it was a nightmare. I went through the work though, for the building experience, and the chimney jig wasn't too bad. I built it and it came out to be 9.4 grams, holding 10.7kg. I got better than that with a square base tower, BEFORE even building a jig. I say this because if it isn't obvious, a well designed jig will make construction easier, but more importantly the structure will be more stable and symmetrical.

In other news we tested our latest tower today, 8.9 grams and held all the 15kg, bringing us to an efficiency of 25.28 mil....we're quite pleased with our design and it certainly payed off to select specific stiffnesses. I think that is truly key after nailing your design and constructing a proper jig - and one final note, i can certainly attest to the effectiveness of x bracing in the base and z bracing in the chimney.

[lllazar]

Sorry for the double post, in the hopes of our getting our bridge mass down we decreased the densities of the compression members and the legs did fine but the bridge prematurely broke because the tension bracing was too weak.

I know higher density = more tensile strength, but is there any other correlative measureable property to tensile strength?