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RE: Bolt Tension

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See below for reply.

Harold Sprague
Krawinkler, Luth & Assoc.
4412 W. Eisenhower Blvd.
Loveland, CO 80537
Voice: 970 667-2426
Fax: 970 667-2493
Email: hsprague(--nospam--at)klaalov.com



-----Original Message-----
From:	Paul McEntee [SMTP:pmcentee(--nospam--at)home.com]
Sent:	Wednesday, April 29, 1998 11:30 PM
To:	seaoc(--nospam--at)seaoc.org
Subject:	RE: Bolt Tension

While were on te subject of bolt tensioning...

We were having this debate over whether or not to tension high strength threaded rods in a fixed base connection for a moment frame.  Some argue that you must tension the bolts, otherwise you'll get large rotations of the base plate before it starts taking moment.  
It is true that your rotations will be quite large if you do not preload the bolts.

Others were of the opinion that pre-tensioning the rods loads the concrete, causes it to creep, and the bolt tension will relax anyway.  
You will get some creep, but this can be calculated.  Generally the relaxation is not that great if you have a long enough anchor bolt and grease the shaft to prevent bond and create a length of rod that will stretch.

I'm kind of with the creepy group myself (couldn't resist).  The issue of it being difficult and expensive to tension the rods came up.  
In certain nonbuilding structures applications, preloading anchor bolts is common.

In one particular case, our detail calls for 1.25" diameter A449 Rods tensioned to 48 kips, and we are now in the process of getting the inspection company to take samples of the rods into their lab (rods are too big and too strong to use a skidmore on site) to measure the torque required to get that tension using the contractors wrench. 
1.25" rods are too big for most tensioning methods; A449 rods should be manufactured from A193-B7 stock (otherwise quality is highly suspect); torque is a very, very poor predictor of tension loads (use a hydraulic tensioner like Biach 908-276-3110)

I don't sympathize with the difficulties this causes the contractor too much if the pre-tensioning of the bolts truly increases the fixity of the connection, but if the tension is going to relax in a few months anyway, I'd hate to have the owner paying extra for this.  Not to mention the fact that there is a risk of breaking the bolts while doing this.  
If you break an A449 bolt at 48 kips of tension, you have a much more serious problem.

Appreciate any fresh (ie, not from my coworkers) opinions on this.

Paul

A self proclaimed lurker

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