# Determining bolt diameter.

1. Jan 24, 2009

### billmccai

1. The problem statement, all variables and given/known data

We were provided with the dimentions and specifications for a highway sign, and given assumptions for weight and wind forces acting on it. Firstly, we had to calculate the reactions at the base. And then draw stress blocks and Mohr's circle for four points on the base of the section.

Now I need to determine a suitable bolt size and orientation for the base plate supporting the sign. I'm a bit unsure of how to do this. I basically just need to make a reasonable assumption (say, four bolts) and test what size diameter bolt I would require in order to be sufficient (using manufacturer specs as a guide to their tensile and shear strength, and a factor of safety of 1.5).

But I'm getting myself a bit muddled. In the normal textbook problems it's simple because the force is provided. But I don't know whether I use the stresses from my stress blocks to determine the force on the bolts? Or just my reactions? We can ignore bending of the base plate, I just need to know where I get my values for my forces from?

2. Relevant equations

tau = V/A
sigma = F/A

3. The attempt at a solution

See above.

2. Jan 24, 2009

### nvn

billmccai: I'm not sure yet what you mean by stress blocks for four points on the base of the section. Show your current attempt and/or data. The forces (or reactions) on the base plate are the forces applied to the bolt pattern. Draw a free-body diagram of the forces applied to the base plate and the forces (reactions) on each bolt. Using statics and the forces (or reactions) applied to the base plate, solve for the forces on each bolt in the bolt pattern.

3. Jan 26, 2009

### Dr.D

The reactions you calculated were, I presume, the net reactions for the entire support. Those reactions have to be transmitted through the bolt pattern. The point of drawing the stress blocks was to give you a means to get loads at the points where you were going to locate the bolts (although I'm not quite sure how how you got those stress blocks if they were distributed over the bolt flange).

Perhaps you can tell us a little more about your process and the details of your problem statement.