What is the strongest balsa bridge design? (1 Viewer)

[Hello_World2]

Hi, I have been coming up with a couple of different truss designs like the K-truss and Baltimore truss but I actually don't know which one is the strongest or how I can make my design more efficient? Help would be great! and also can anyone recommend any good bridge testing software?




Details:

- 450 mm length of 2mm x 75mm balsa and four 915mm lengths of 6.5mm x 6.5mm balsa.

- No hot glue allowed, just nails and wood glue



Method of Testing.

The model will be tested by applying a concentrated load at the midpoint

of the carriageway (in your design leave a space of 15 mm square at the

midpoint of the truss.) A saddle will be suspended from the carriageway of

the model, and a bucket, which is attached to the saddle, will be gradually

filled with weights. After failure occurs the load is calculated.

 

[wizzkids]

Have a look at the Second Moment of Area of a beam, "I". The Second Moment of Area is a measure of the stiffness of a beam. Maximum stress in a beam is inversely proportional to "I". If you make "I" as large as possible, the stress will be minimised, and it is stress that causes failure. The parameters that go into the calculation of Second Moment of Area are the width "b" and the height "h" of the structure. "I" is proportional to width "b" but proportional to "h" raised to the power 3.
OK, so what does this tell us?
It tells us that you need to make your structure as tall as possible, and narrow in width, i.e. maximise "h" and minimise "b".
This will give you the most efficient structure.
You should also consider that compression stress will occur in the top plate, and tensile stress in the bottom plate. It is tensile stress that will ultimately be the cause of failure of your structure, so put more material where the tensile stress is greatest.
Good luck with your model! I hope these guidelines help.

 

[fuzi]

Have a look at the Second Moment of Area of a beam, "I". The Second Moment of Area is a measure of the stiffness of a beam. Maximum stress in a beam is inversely proportional to "I". If you make "I" as large as possible, the stress will be minimised, and it is stress that causes failure. The parameters that go into the calculation of Second Moment of Area are the width "b" and the height "h" of the structure. "I" is proportional to width "b" but proportional to "h" raised to the power 3.
OK, so what does this tell us?
It tells us that you need to make your structure as tall as possible, and narrow in width, i.e. maximise "h" and minimise "b".
This will give you the most efficient structure.
You should also consider that compression stress will occur in the top plate, and tensile stress in the bottom plate. It is tensile stress that will ultimately be the cause of failure of your structure, so put more material when the tensile stress is greatest.
Good luck with your model! I hope these guidelines help.

Can you apply second moment of the area to the structure as a whole considering it isn't a single rigid beam with a uniform cross section? Additionally, the entire purpose of a truss is to distribute loads throughout the structure, so it is entirely possible for members along the top chord to be in tension and members in the bottom chord to be compression. T only real way to know is with truss analysis like method of sections or method of joints to know which members are experiencing which force under the given loading conditions.

Hope that helps! And quite curious about the second moment of the area thing

 

[wizzkids]

@fuzi I invite you to do the calculations, use either the method of joints or the method of sections, and I think you will find that my statement is correct. The top members will be in compression and the bottom members will be in tension. If this was not the case, then the whole structure would not be in static equilibrium. Yes, it is a simplification to apply the second moment of area to the whole structure, but it leads to correct general conclusions.