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(OP)
Hello all,

I'm creating a 3D bolt load distribution spreadsheet that accepts loads in 3 directions and distributes them into a 3 dimensional pattern of bolts (so the pattern does not have to be on a single plane). I'm honestly lost as to why the spreadsheet isn't working.

I found this windows program, which is basically exactly what I'm trying to do, but I can't replicate the results (http://www.chestnutpens.co.uk/misc/boltgroup.html). My spreadsheet matches this program all the way up to the calculation of the "r^2" weighting components, but seems to be doing something wrong in the final determination of the loads on each bolt.

My method is as follows (and is a standard 2d method):
1. Input fastener coordinates and applied loads (applied loads assumed to be at origin)
2. Determine ak based on the weighting of the individual bolt diameters
3. Determine centroid of bolt group
4. Determine moments (Mx, My, Mz) created by Px, Py, Pz about centroid
5. Determine linear forces generated by Px, Py, Pz
6. Determine fastener distance components (dx, dy, dz) from centroid
7. Calculate the r^2 components at each plane, and the sum(ak*r^2) at each plane
8. Determine the components of each fastener of the forces at each plane due to rotation
9. Add the linear and rotational components in each direction to determine the total force in each direction.

If you decide to brave my spreadsheet, apologies for anything confusing. But also, the help is much appreciated.

Do you know which one of the steps is causing the problem?

while i have not thoroughly investigated the workbook, some suggestions are:
- create and apply range names as this helps w/ diagnostic work
- use trace arrows for additional diagnostic work
- be sure to read and understand formulas used. i saw a sumproduct function, so be sure of its use.

Step back to condensing your analysis to a 2D problem, does your spreadsheet give the right answer if you do this? If not solve this and you'll probably see where you may have gone wrong in a 3D sense.

For typical 2D bolted plates there are two calculation methods.
1. The first is the Elastic method, and uses simple superposition of the effects from each force component. This would appear to be what you are attempting to duplicate.
2. The second is the Instantaneous Centre of Rotation method. For standard 2D bolting this is the more accurate method, and I assume this would also be the case for 3D.
Here's a website that discusses this (link). About halfway down it notes:
• When compared with actual test results, the Elastic method is found to be very conservative and to have an inconsistent factor of safety. This is because the use of the principle of superposition assumes that the translational and rotational actions are independent when, in fact, they are not. Hence, the Instantaneous Center of Rotation (IC) method was developed.
There's lots of other info you can research for the IC method. It is an iterative method to find the location so it's not easily implemented directly in Excel, though I have seen a VBA based spreadsheet for the 2D case.

The system of coordinates in the spreadsheet is incorrect. Z-axis should be reversed.

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