Preloaded Bolts One Year Later-prestress loss 3

[IJR]

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

 

[boo1]

Course of Action
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.

 

[boo1]

Check the washers too

 

[ajh1]

You might check the RCSC web site (

http://www.boltcouncil.org)

for information regarding relaxation of the bolts. A certain amount is expected under any condition. Tests have shown that from a tensile strength standpoint, it doesn't matter whether the bolts are pretensioned or not. From a stiffness / rotation standpoint there is benefit to the pretensioning. Check out eithe the Commentary in the RCSC Spec or Kulak's book on Bolted Joints. I would anticipate that there is some information regarding long-term relaxation in one or the other.

 

[CSEllc]

I would not re-torque the bolts. Are the bolts A325 or A490? (It doesn't matter just curious...)

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.

 

[chichuck]

I'm afraid I don't have much help to offer, but a question:

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.

 

[IJR]

Dear Pals

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

 

[chichuck]

IJR,

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

 

[boo1]

Agreed, but the most common faster tensioning method is a torque wrench.

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 RCSC Specification.htm

Fastener Stress Analysis References, Preload

http://euler9.tripod.com/fasteners/

Spreadsheet A490

http://www.arcwallsys.com/Grade5-ASTM A449 StrucFast.xls

Mil-hdbk-5

http://www.grantadesign.com/cgi-bin/stat-tree.cgi?src=MIL5-H.xml&br=10

 

[boo1]

see link at:

http://www.engineeringatboeing.com/docs/BoltAnalysisGuidelines.html

 

[Kenneth]

An additonal ("aerospace") document that may be of general interest to you is MIL-HDBK-60 (

http://assist.daps.dla.mil/docimages/0000/22/90/70455.PD9).

There is some infomation on preload relaxation in Section 5.5.2.