Anchor Bolt Proof Load
Anchor Bolt Proof Load
(OP)
We have specified the use of ASTM A449 2.5" diameter galvanized anchor bolts for bearings supporting a box girder. We specified that the bolts shall be tightened to a proof load of 449KN. My question is two fold:
First, the contractor has complained that this load is astronomical and if it could be reduced. Is this value of proof load unreasonable?
Secondly, the contractor needs the proof load in terms of a torque value and insists that there is no means of converting this proof load into a torque value. Is this true?
Thanks in advance.
First, the contractor has complained that this load is astronomical and if it could be reduced. Is this value of proof load unreasonable?
Secondly, the contractor needs the proof load in terms of a torque value and insists that there is no means of converting this proof load into a torque value. Is this true?
Thanks in advance.





RE: Anchor Bolt Proof Load
A proof load is required for a slip-critical connection - the pretensioning assures the presence of a normal force to develop the friction force. If your anchor design doesn't require a slip-critical resistance, perhaps you can specify the anchor bolt be snug + 1/2 turn - then burr the threads to prevent the nut from backing up. If you need lateral resistance, consider a bearing connection if the DOT allows.
Equating torque to bolt tension is possible (for those who - unlike me - haven't forgotten all their calculus and dynamics lessons) but it also is dependent on the friction coefficient. A direct tension indicator (DTI) is probably the best method of determining the bolt tension in the field, but I don't know if they are readily available for a 2.5" bolt. You might contact some of the bolt manufacturers to determine what's available or typical for tensioning a 2.5" dia bolt.
good luck.
RE: Anchor Bolt Proof Load
Anyway, on to the question of relating tension to torque. To do this you have to have a Skidmore-Wilhelm and a torque wrench. You will be able to calibrate the torque wrench for your application.
One caveat about the torque wrench is that most are of the socket type and I don't know if the socket will fit over the bolt to fit around the nut. Especially if you have much bolt sticking up past the nut. Another concern is the SW and whether it will accomodate the large diameter bolt and one without a head.
RE: Anchor Bolt Proof Load
I have never heard of proof loading anchor bolts for bearings. Typically, I design the substructure for uplift and spec the required anchor bolt.
Regards
VOD
RE: Anchor Bolt Proof Load
This would be maybe a bit of a rough way of doing things, but would probably work well enough if nothing else does. You may want to load/unload the same rod a few times to see if creep, relaxation or polishing of threads changes the required number of turns.
Also, good idea would be to make sure all threads are clean and well lubricated with a high pressure lubricant prior to tightening nut. This way you minimize any polishing of thread effect on the # of turns, and reduce the torque required to turn the nut.
The beauty of doing it this way, is you never have to calculate some arbitrary torque value, nor do you have to find and calibrate a torque wrench big enough for 2.5" rod.
Finding a suitable load cell shouldn't be too difficult. Universities and test labs all use load cells, and probably would be willing to rent one out, or provide tech support to do the tests themselves.