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# Help determining correct bolt size & number of bolts

## Help determining correct bolt size & number of bolts

(OP)
Hello,

I am trying to calculate the Moment and stress acting on the bolts connecting directly to the pan (please see attached images) to determine the bolt size and # of bolts to use. We are currently seeing the bolts fail way more often then they should be.

The method I have been using now is saying both ends of the pan are fixed and assuming the worst case scenario. Using the equation M = (w*L^2)/12 to determine the moment acting on the brackets.

Where w is the distributed load from the pan = 4720LB/29.145ft = 161.91 lb/ft
Where L is the entire length of the pan = 29.145 ft
Where M is the moment acting on the bolts.

From the equation I get that the moment acting on the bolts is 11460.94 lb-ft which is equal to 137531.37 lb-in.

Then using the equation sigma = (32*M)/((pi)*d^3) to determine the stress seen on 1 bolt (then divide stresses by proof strength of material to determine # of bolts required)

Where M is the moment we just calculated in the above equation (137531.37 lb-in)
Where d is the minor diameter of a 3/4" bolt which is 0.6255"
Where sigma is the stress seen on a single bolt.

From the equation I get that sigma is equal to 5,724,267.96 psi which to me does not seem to be reasonable and make much sense.

Is there something that I am missing? Am I going about solving this problem all wrong? Can someone please provide some insight as to how to approach this problem to determine the correct bolt & # of bolts to use?

Please let me know if you require any additional information in order to help me determine the proper solution to this problem.

### RE: Help determining correct bolt size & number of bolts

Firstly, I feel as though your analysis is slightly incorrect. 2 bolts in the orientation shown will never act as a fixed connection. Therefore I expect the tank to be simply supported by those two bolts at each angle.

Therefore your moment on the 4 bolt pattern would be in the range of 25,000 in-lbs. (2500 lbs * 10" assuming even distribution of the ~5000lb tank weight full).

The stress in the bolts then depends on the distance between the bolts at the four bolt pattern.

That's how I would look at this problem, but it doesn't necessarily mean I'm correct. Also, depending on the flexibilty of that angle, you may be seeing quite a bit of prying force on the bolts.

### RE: Help determining correct bolt size & number of bolts

(OP)
For my own understand why would the 2 bolts not be acting as a fixed connection? What would need to be different in order for it to act as a fixed connection?

I understand if we were to look at it as a simply supported beam, the reaction forces on either end of the beam would be ~2500lb. But wouldn't that then mean that the moment on the 2 bolt pattern is 25,000 lb-in? not the 4 bolt pattern?

Or is the reaction force actually coming from the 2 bolt pattern?

The reason why I am asking this is because the failure we are seeing right now is on the 2 Bolts connected directly to the pan, not on the 4 bolts. Could you please provide some more clarification on the method you are talking about?

I have also attached a small sketch of the drawing for the 4 bolt pattern incase it is required to solve this problem.

### RE: Help determining correct bolt size & number of bolts

Then you could be seeing prying action in the 2 bolt pattern. 2 bolts rarely can provide enough fixity to be considered a fixed connection, specifically if the 2 bolts are in a horizontal orientation as shown. Essentially they are free to rotate.

I think you should google bolts in prying. That really could be the cause for the failures you're seeing.

### RE: Help determining correct bolt size & number of bolts

I'd agree - you may have significant prying action going on along the wall of your tank at the 2 bolt connection.

This depends a lot on the stiffness of your pan wall as well.

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