Turn of the nut, pre-installation verification

[Inspect87]

In pre installation verification testing of bolts per RCSC, my understanding is that you are testing to verify that the bolt assembly will not fail at the prescribed KIPs per table 7.1 and to ensure that the pre tensioning method used will reach the prescribed kips. For instance, with turn of the nut, if the tension does not reach the prescribed kips at the prescribed rotation, i.e. 1/3 turn, does the bolt fail, or is the allowable plus 60% rotation allowed to be used at this point? I have always viewed it as the +60 applies to installation only and not testing. The way I see it, if the assembly does not reach the prescribed tension with a 1/3 turn (for example) then testing has shown that the method does not work with that bolt assembly. I have been told by our lead that in testing, nut rotation does not matter and you are only verifying that the assembly can reach the desired kips and you can turn it as much as you need to get the kips. The way I am reading the code, the latter is incorrect. Can anyone clarify this for me with referenced reasons as to why?

 

[ajh1]

The preinstallation testing is designed to find a method to achieve the required pretension. Table 8.2 in the Specification is a target point for the tension required in the final installation. Preinstallation testing tensions are required to be 5% higher, which might impact your target. Based on research testing, the specified turns should produce the correct tension at a minimum, but there may be instances where it doesn't quite make it. In that case simply increase the turn amount until the testing shows that it works. A couple of areas where this might occur would be: 1) using fully threaded bolts (the table was created based on shank bolt testing), or 2) bolt length is right at the junction between the different table columns and might need the other column value. Note that the table is also only for imperial threaded bolts. If you happen to be using metric bolts, the thread pitch is different which would result in a different turn requirement. As indicated in the RCSC Commentary, there is a broad range of nut rotation where the tension will be met and overtightening is okay as long as the bolt does not break during the installation. The effect on the bolt during the installation process will always be higher than the bolt will see immediately after. (During installation the bolt is subject to both the created tension force as well as a torque force due to the friction of turning the nut. After installation there is no torque force, only the created tension force.)
The other aspect of the testing is to demonstrate to both the inspector AND the erection crew that the installation method when followed will achieve the required pretension in the actual installation. View this as somewhat of a training exercise for the erection crew.

Al Harrold
RCSC Past Chairman