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RE: Backing bar in RBS steel moment connection

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For years many welders have used backing bars and (although not specifically required) have often times removed the backing just to pass the NDT easily. Back about 30 years ago, welders would use brass or ceramic backing plates to make it easier to remove them, arc gouge the trash at the bottom and weld reinforcing welds just to pass the NDT. Ceramic and brass will not fuse to the weld.

Today, it is no different. We engineers in our arrogance and ignorance (me included) have assumed that we have to lead a welder by the hand to get a good performing weld. OK some welders need a spud wrench upside the head, but most of them know how to get it done.

Consider the ever present fusion stud weld. It uses a ceramic ferule to contain the molten fusion steel weld. It is as good of a weld as it gets once the process is set up.

Many iron workers today use a ceramic back up bar. It is clamped or taped in place. Some really sharp welders have had chamfers placed on the back up bar. A well performed weld will then look like it has had a reinforcing fillet weld on the back side, but it just the weld filler of the root bead conforming to the shape of the ceramic back up bar.

Believe me, a good welder will do just about anything to avoid an overhead weld and the resulting sparks down his shirt. With a little practice, I can make any full pen weld look like it had a steel backing bar removed, back gouged, and a reinforcing fillet weld. This weld can be shown in a symbol by showing the fillet on the same side of the line as the full pen symbol, and we can eliminate all of the problems with the backing bar removal. This weld can be performed entirely from the down position just because a good welder will want to get it done quickly and easily.

Check this out:
www.structuremag.org/archives/2006/February-2006/C-BB-Weld-Backing-Feb-06.pdf

Regards,
Harold Sprague





From: bart(--nospam--at)nbse.com
Reply-To: <seaint(--nospam--at)seaint.org>
To: seaint(--nospam--at)seaint.org
Subject: RE: Backing bar in RBS steel moment connection
Date: Fri, 18 May 2007 14:41:24 -0700

For one thing, the bottom flange backing bar is located at the most
extreme location as fas as stress, at the top flange it is not. AISC 358
mandates this as well.  There is a rather extensive commentary in AISC
341 which may have more information.  Both are free at the AISC web site,
last time I checked. Additionally they found many problems stemming from
the Northridge earthquake (parking structures being another one,
which may be one reason they may have mandated I=1.25 for large
structures, different subject)   But there has been an inordinate amount
of research done in this area.  There is a Structural Engineer from the
Bay area who spoke for AISC at the steel frame seminars that had been
going around the country (whose name I don't recall) who was very bright,
and can speak to this issue at great length. (He was also very adept at
answering a lot of design questions)  There are many engineers as well,
who are on the SAC committee, for FEMA 350 etc, who can speak at great
length about this issue as well. I know recently AISC printed the
prequalified RBS connection, someone may know of a link to that diagram.

  ----- Original Message -----
  From: "Michel Blangy"
  To: seaint(--nospam--at)seaint.org
  Subject: RE: Backing bar in RBS steel moment connection
  Date: Fri, 18 May 2007 12:00:04 -0700

As usual excuse my ignorance, but why only the bot. flange? MB

  -----Original Message-----
  From: bart(--nospam--at)nbse.com [mailto:bart(--nospam--at)nbse.com]
  Sent: Friday, May 18, 2007 11:47 AM
  To: seaint(--nospam--at)seaint.org
  Subject: RE: Backing bar in RBS steel moment connection

  At the beam bottom flange, the backing shall be removed. At the top
  flange, removing the backing is optional. (continuous attachment to
  the column with a fillet weld is required) My opinion is that leaving
  backing bars in place is a poor idea, however, it is often cost
  prohibitive to remove them, backgouge and backweld with a reinforcing
  fillet.

    ----- Original Message -----
    From: "Gautam Manandhar"
    To: seaint(--nospam--at)seaint.org
    Subject: RE: Backing bar in RBS steel moment connection
    Date: Mon, 7 May 2007 14:55:31 -0700

List members:

In a RBS steel moment connection, the expected plastic hinge is at the
reduced beam section.  How important is it to remove the backing bar at
the beam-column connection?  I am aware that in the pre-Northridge,
fracture initiated at the bottom CJP joint.   Is anyone aware of tests
done with the backing bar in place.

Gautam



Bart Needham, SE
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Bart Needham, SE
Principal, nbse associates, inc.
Office 206-780-6822
Office 805-452-8152
Fax    206-780-6683
Fax    208-693-3667
Mobile 206-300-2346

Office locations:
629 State Street #230
Santa Barbara, CA  93101

205 Fairview Lane
Suite 100
Paso Robles, CA  93446

365 Ericksen Ave. NE
Suite 328
Bainbridge Island, WA  98110

Mail and Deliveries:
321 High School Rd. NE
Suite D-3 PMB 216
Bainbridge Island, WA  98110

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