What is the maximum load my tower can support?


 
Let’s assume that you are using ⅛” sticks of 18 psf (type BalsaD2).
Using ModelSmart3D create a very short column like the one below:
 
 

 

The column above is ½” long.  It has a universal hinge at its base. It also has a Y roller at the top so it is free to compress. (I used the “Joints|Size|Small” menu option to make the short column easier to see.) I applied a starting load of -1 lb (one pound in the negative Y direction).

 
Test the column using “Analysis|Analysis Options|Find Breaking Force” menu option set.
 
I get -35.94 lbs when I test it.  This is the maximum force that this column can carry.  
The reason I used a very short column is to make sure that the column would fail in a crushing of the material mode rather than a buckling mode.

 
If I increase the length of the column to 1” long the new max load is still -35.94 lbs. This means the mode of failure is still crushing.  (Reset the original load back to 1 lb. before testing again.)

 
I increased the column length to 2” and tested it again.  This time I got -35.14 lbs.
and when I test it at 2.5” long I get 22.49.  These failures indicates a buckling mode failure.

 At 1.9” I get 35.94 lbs; I didn’t buckle.

 
If you were to build a tower using the 1/8” BalsaD2 balsa wood and you braced the columns at no further apart than 1.9” your tower, 
assuming it had 4 legs, should support about 35.94 times 4 or about 144 pounds total. 
This is an ultimate load there is no safety factor.  
This also assumes your bracing holds, there are no imperfections in the sticks, the columns are initially straight and symetrical, and the cg of the load is placed exactly in the middle atop your tower. 

 
(As a rule of thumb, your bracing should be able to support at least 2% of compressive force in the column placed laterally at the brace point.  In this case,  you should apply a lateral load of about .7 lbs at various bracing locations to test the sensitivity of your tower and bracing to lateral load.  It would also be a good idea to assume the test load is not perfectly placed by placing slight more load on two of your columns.)

 
If your tower is x-braced the diagonal that is in compression may be flagged as failing but may actually just buckle and bow out without breaking and remain in place.  This movement can shift the load to the other diagonal of the x-brace for it to take in tension.  To get the correct maximum ultimate structure load you can use the “Member|Negate” menu option to remove the buckling diagonal from the analysis”.  

 
If you have a slanted leg tower the maximum ultimate load would be reduced as follows. 
(This is automatically considered in the ModelSmart3D analysis engine.)

 
The vertical component of the leg would be necessarily less than the force directed along the axis of the skewed column due to geometrical considerations.  Therefore, you would need to correct the maximum force by the factor :

 

 

For example,  assume the coordinate of the base of your column is (X=0,Y=0,Z=0) and the next joint up along your straight skewed column is (X=.858,Y=5.434,Z=.858) then using the correction factor above:


 
5.343/sqrt(.858^2+5.434^2+.858^2)
= 5.343/5.568 = .96

 
Where deltaX = .858-0 = .858, deltaY = 5.434-0 = 5.434, & deltaZ = .858-0 = .858

 
The max ultimate capacity ( no safety factor) of your tower would be 
(4)(35.94)(.96) = 138 pounds.