moment connection bolt pretensioning

By CHGuilford (****)

Date 09-14-2007 16:45

I do not know where it is in writing, I thought bolts were tensioned after welding.

By swnorris (****)

Date 09-14-2007 18:39

I'm not sure that it is in writing, unless it's a requirement in the contract documents. A seismic rating of greater than an R3 may require something like this, but for a non-seismic bearing condition, snug tight is all that's really required.

There is a requirement that exists with bolts in slip critical connections when the bolts share the load with welds. When this occurs, it's adviseable to fully tension bolts before the weld is made, because if the weld is made first, angular distortion from the heat of the weld might prevent the faying action required for the development of the slip crtical forces. This is the only instance that I'm aware of that requires tensioning of the bolts first.

It seems to me in your situation, that if bolts are tensioned first, shrinkage forces of the weld would induce additional stresses on the bolts.

By jwright650 (*****)

Date 09-14-2007 19:03

That was my thought too, but without being able to produce it in black and white I just didn't say anything. The heat that soaks into the beam web from welding that connection would also concern me a bit when tensioning HS bolts...the bolts are heat treated to give them their strength and I'm not sure that you wouldn't be altering the heat treat of those bolts, plus the distortion that Scott mentioned surely could stretch the bolts further since they are already tensioned.

I would be interested in seeing it in writing somewhere, if someone has seen it please post up the source.

By 803056

Date 09-14-2007 21:44

Again, my two cents.

The welded joint in the moment connections shrinks as the weld solidifies and the joint contracts as it cools to ambient temperature. I typically allow for 3/32 inch per welded joint when the flanges are ¾ inch or thicker. If the bolts are snug tight, the joint has freedom to accommodate the contraction. If the full 3/32 inch of contraction occurs, the bolts will go into bearing against the side of the hole. Some building I've worked on used short slotted holes to keep the bolts from fetching up on the side of the bolt hole.

If the bolts are fully pretensioned before welding, they will slip into bearing all the same. The residual stress from welding is on the same order of magnitude as the yield strength of the weaker component, i.e., the filler metal or the structural member. The cross sectional area of the member times the residual stress is going to over-power the clamping force of the bolts in all but the thinnest of members. However, there is the chance the clamping force of the pretensioned bolts with offer sufficient rigidity to cause cracking in the first few passes of weld if there is any interruption in the welding operation such that the joint cools to ambient temperature.

Also, the building must be plumbed before welding or pretensioning of the H.S. bolts. As an inspector, a common mistake I see is when the "newbie" ironworker tightens the bolts, welds the moments and tries to plumb the build with the decking welded in place.

"Hello dear, book another week in the Caribbean this winter. This job is a money maker!"

Best regards - Al

By natecf

Date 09-15-2007 03:13

A.I.S.C publishes a book called " steel construction manual". you may already have this, but it would most likely be there. I have the previous version called "allowable stress design" I know there is info on splices with bolts used in conjuntion with welds. I can do a little research on monday and see if there is anything you can use

http://www.aisc.org/

By trapdoor (**)

Date 09-15-2007 03:44 Edited 09-15-2007 03:46

Have the inspector show you where it indicates to do so either in the contract documents or the referenced codes. If he or she is unable to do so and they still insist that the bolts be pretensioned prior to welding the web then i would ask the engineer, since everybody is unsure what to do now. That is exactly what i would do in your situation.

That being said, from my experiance with moment connections the bolts are usually tensioned after welding the flanges and when the web is CJP welded to the column then the bolts (two) have been for erection purposes only. Also the moment connection details that i have seen in heavy construction have been very clear about such things.

By JA

Date 09-15-2007 12:10

if it makes a difference , the connection is a (jay allen) slotted web steel moment connection.........

By JA

Date 09-15-2007 23:31

well , thats what i did , and in the structural notes it states , all bolts shall be fully pretensioned before welding.................

By trapdoor (**)

Date 09-17-2007 04:04

There you go, J allen wants his bolts pre tensioned proir to welding.

By swnorris (****)

Date 09-18-2007 13:17 Edited 09-18-2007 13:20

I respectfully disagree with Mr. Allen. If the bolts are tightened first and the welds made subsequently, the weld shrinkage would cause an indeterminate but significant amount of pre-load in both the bolts and welds.

By BF

Date 09-18-2007 18:08 Edited 09-18-2007 18:24

While performing inspedctions in my younger days in So Cal, I always required the bolts be tensioned first before welding. It was referenced in the AISC Manual and the AWS D1.1. In the 2004 D1.1 section 2.6.7, Welds with Rivets or Bolts is explained/referenced.
Hope this sheds some new light on the subject.
Also in my old 8th edition of the AISC Steel Construction Manual, again Rivets and Bolts in Combination with Welds, it says high strength bolts used in frictin-type connections it is advised that welds be made after the bolts are tightened due to the possible interference with the development of the high contact pressure between the faying surfaces (top of page 5-145 if you have the red 8th edition).

Carpe Diem
BF

By swnorris (****)

Date 09-18-2007 18:40 Edited 09-18-2007 19:24

BF,

I agree with the slip critical. I mentioned and explained it in my initial post on this thread, but from the way the original poster explained it, it doesn't sound to me like his is a slip condition, because, how would slip be a factor if the beam is welded?

By JA

Date 09-19-2007 11:05

i agree with swnorris............

By blanier (*)

Date 10-17-2007 15:26

Structural engineers are the resource this question should be addressed to but considering the (welded) flanges as individual components of a connection and the (bolted) flange as the third, then why would only two of the three be connected in a manner to be rigid?? The web will act independently of flanges and the load path for all should be able to transfer load equally which requires no movement between the connnected parts. That is the purpose of the Slip-Critical tightening method.

By swnorris (****)

Date 10-17-2007 17:31 Edited 10-17-2007 17:44

Section J1.9 of the AISC Steel Construction Manual states that a connection with bolts designed as slip critical high strength bolts are permitted to share loads with welds.... but it is advisable to fully tension the bolts before the weld is made. As I've previously posted, if the weld is placed first, angular distortion from the heat of the weld might prevent the faying action required for development of the slip critical force. When the bolts are fully tensioned before the weld is made, the slip critical bolts and the weld may be assumed to share the load on a common shear plane. So, if welding is done first, faying action cannot be assured and slipping of bolts may occur followed by premature cracking of welds. Then, bolts must be conservatively considered in bearing action, not sharing loads, and welds designed to support the whole load, due to their inherent stiffness.
Also, as I have previously posted, there are connections involving bolts and welds in combination but with more than one connection plane, where welding should be done first. An example of this would be the moment connection of a wide flange beam to a column, where the flanges are complete penetration welded to the column face in the field, and the beam web is connected by means of a shear plate or angle (having short horizontal slots) that is shop welded to the column and field bolted to the beam web. In this case, the bolts are placed first and snug tightened, and basically serve as erection bolts. The welds are made subsequently and allowed to shrink. Finally, the bolts are tightened as required. If the bolts were tightened first and the welds made subsequently, the weld shrinkage would cause an indeterminate but significant amount of pre-load in both the bolts and welds. I'm not an engineer by any stretch of the imagination. This is just how I've understood it to work.