No formula I'm aware of... only adding the components... 4-1/2 + 1/8 (washer) + 3/4" for the bolt... so, a 5-1/2" bolt should just about do it... Just check if threads are excluded from the shear plane if required.
The structural Bolting Handbook from SSTC has suggested bolt lengths of 5 1/2 for no washer and 6 inches for one or two washers for 4 7/16 or 4 1/2 length bolts.
You need to consider your fitup and (field ??) assembly and re-assembly requirements: If the bolt will be used to align or compress the two components, very very often an extra inch or 1-1/2 is essential is aligning the pieces before they are bolted up tight. (The parts may be hand held or lifted up by a crane, so the first bolt is used to fit into the hole so the others can be inserted and then tightened.) On the other hand, close spaces or lifting limits on removing the two parts may require a very specific "maximum length" that the bolt can be, you may be limited by the removal length of the bolt!
Equally, you may find you are quoted an obscene price for custom-length bolts when a standard-size 6 inch (instead of 5-3/4 for example) is available off the shelf.
One other thing: Assume you calculate an ideal length just perfect for your assembly and custom-build that bolt.
When any ten thousand of your future customers loses their bolts (or breaks it off, or strips its threads thorugh stupidity or innocent error) will you have the means and customer support and shipping staff and on-line ordering and packing staff available to get him replacements 24-7-365? Or do you expect him to get such replacement from MSC, Fastenal, or Home Depot or Walmart?
I don't understand "very very often an extra inch or 1-1/2 is essential is aligning the pieces before they are bolted up tight." in the context of an A325 bolt. There would not be enough thread length to be useful unless you use type T, but that is restricted to four diameters long, 3" for a 3/4" bolt.
Michael.
Timing has a lot to do with the outcome of a rain dance.
Really helpful everyone. I thought that there might be a formula for this calcualation. I would assume a formula can be made to derive this. The chart works great. For reference: I was trying to keep threads out of the shear plane. It seems that the thread on a 3/4" A325 is 1.5"
Thank you! Although, looking back on things I wish I got that FE I might actually have a chance at a PE someday. That FE seems like a lot being +10 years out
The guys I worked with, used the grip, plus washer, plus nut, plus 1/16" and rounded up to the next standard length. As long as the end of the torqued bolt reaches the face of the nut, there is full engagement.
Michael.
Timing has a lot to do with the outcome of a rain dance.
"As long as the end of the torqued bolt reaches the face of the nut, there is full engagement". Amen.
It is amazing the number of dumb-a$$ inspectors and ironworkers that believe the "old wives tale" that you MUST show 2-4 threads. Sticking threads out into empty air is silly. It is usually somewhat unavoidable, because of there being standard lengths of bolts [vs. something designed to fit the joint exactly], so you usually have 1-3 exposed threads. But there has never been a Code or structural requirement to have more engagement than 'Full', with the bolt flush with the face of the nut.
