Preloaded Bolts One Year Later-prestress loss
Preloaded Bolts One Year Later-prestress loss
(OP)
My very dear friend
A large steel framed building had all bolts preloaded to code required torque value. The building is situated in a relatively hot climate.
A year later tests show that some prestress has been lost in most of the bolts
The owner wants the bolts be pre-loaded again. And that is a killer wish.
Could climate be the reason or is it simply that the bolts went through some creep and relaxed a bit?
Is there a code specified tolerance so that we can convince the owner that some percent loss is acceptable?
My respects
ijr
A large steel framed building had all bolts preloaded to code required torque value. The building is situated in a relatively hot climate.
A year later tests show that some prestress has been lost in most of the bolts
The owner wants the bolts be pre-loaded again. And that is a killer wish.
Could climate be the reason or is it simply that the bolts went through some creep and relaxed a bit?
Is there a code specified tolerance so that we can convince the owner that some percent loss is acceptable?
My respects
ijr





RE: Preloaded Bolts One Year Later-prestress loss
Depends on what the contract documents require. Recommend not re-torquing untill cause is determined.
Consider checking bolts material properties and certs for substandard parts. Review site for as-build condition, re-check engineering calcs.... The cause of torque relaxation is not just hot weather.
RE: Preloaded Bolts One Year Later-prestress loss
RE: Preloaded Bolts One Year Later-prestress loss
RE: Preloaded Bolts One Year Later-prestress loss
I would have a bolt removed at tested, and compare the test results with the certs. (just a check to make sure certs match bolts.....darn paperwok can get mixedf up can't it?!)
If the bolts and the certs match, review the conditions to try to determine a cause. As boo1 stated it is not the hot weather.
Also, you don't mention it but do you need the torque or can 6the bolts be "snug tight". If "snug" botls are acceptable, then you might be OK with the relaxed bolts.
RE: Preloaded Bolts One Year Later-prestress loss
how do you test to see that the tension in the bolts has decreased? I thought that there is no way to measure the tension in an installed bolt.
Also, please, if these are high strength stuctural bolts (A325 or A490) you don't "torque" the bolts. You need to get the proper pretension.
RE: Preloaded Bolts One Year Later-prestress loss
MY PERSONAL CONCLUSION HERE:
My special thanks for all your great responses.All posts have been highly valuabel and the post by ajh1 got me where I wanted to be. The boltcouncil's specs and Kulak/Fisher/Struik guide has it all, though no detail on effect of temperature, much is credited to relaxation and geometry. I downloaded all materials from the boltcouncil's webpage and I recommend them to all.
chichuck, what do you mean by "not torquing" the bolts? You can not get pretension without applying torque to the bolt by torque spanners am I dreaming?
respects
ijr
RE: Preloaded Bolts One Year Later-prestress loss
What I mean is, you're trying for a specified pretension, not a specified torque. Do not expect to look in a table and find a torque value that can be expected to give the required pretension. (There are tables like that in existance, but they are very very old, and no longer recognized by AISC.) There are just too many variables that affect torque that have nothing to do with pretension.
Regards,
chichuck
RE: Preloaded Bolts One Year Later-prestress loss
For additional info on Structural Connections see
Guide to Design Criteria for Bolted and Riveted Joints at:
http://www.boltcouncil.org/guide1.htm
http://www.boltcouncil.org/download%20RCSC%20Specification.htm
Fastener Stress Analysis References, Preload
http://euler9.tripod.com/fasteners/
Spreadsheet A490
http://www.arcwallsys.com/Grade5-ASTM%20A449%20StrucFast.xls
Mil-hdbk-5
http://www.grantadesign.com/cgi-bin/stat-tree.cgi?src=MIL5-H.xml&br=10
RE: Preloaded Bolts One Year Later-prestress loss
http://www.engineeringatboeing.com/docs/BoltAnalysisGuidelines.html
RE: Preloaded Bolts One Year Later-prestress loss