HSS truss fillet welds

I just finished a design for an open web joist made out of square HSS
(hollow structural sections). I notice that the diagrams in AWS D1.1
(god bless inter-library loan) detailing fillet weld fit ups always
show a gap between the members to be joined. They don't dimension
the gap but it's visually there.
Is this just their way of being clear in the drawings or should I
really have a 1/16" gap or so? The particular page I'm talking about
is p72 that has figure 3.2 on it "Fillet Welded Prequalified Tubular
Joints Made by SMAW, GMAW, and FCAW". I was thinking maybe the gap
would be the suggested way to avoid internal stresses developed as the
weld cools (?)
Gory details: This HSS is the cold formed ASTM A500 grade B. The joist
uses "stepped" connections --the webs more narrow than the chords,
which enables the use of fillet welds. The both chords are 1/4" wall
thickness, the webs are all 3/16" wall thickness. The web angle is
60. The span is about 32 feet. The webs will be field welded
completely around with 7018 in the horizontal position. They suggest
chamfering the obtuse side of the web end to enable suffient throat.
Reply to
wahzoo
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I suspect that the gap is for penetration. My Canadian code has an appendix that shows a gap of t/2 plus zero to minus 2 mm for a Tee connection. For a 6mm wall ( 1/4 inch) That works out to one to three millimetre gap. This is the gap at the root after a 45 degree bevel. Land is 2 mm. The table is titled " Partial Joint Penetration Groove Welds in Tubular Structures" On most structural tubing I have done in the smaller sizes the prints called for a natural prep and relied on standard fillet weld geometry. At four inches and above I have had to bevel and place backing bar strips on the two sides of the Tee joint to create a flare bevel joint with backing bar. Internal stresses would not be a big consideration until you get into heavy wall tubing and larger sizes. There is a good article buried somewhere on standard structural practices in the Lincoln website. I will look around in my saved files . Randy
Reply to
Randy Zimmerman
wabelieveote:
Without having the book in front of me, if they didn't detail the gap, it's for clarity. I believe that all of the design details are based off of tight fit.
Sounds like a good idea.
Reply to
Rich Jones
Thanks Randy. AWS offers PJP weld geometries too, but I'm not confident enough in my fitting/welding abilities to excute those. According to my spreadsheet the normal 1/4" leg fillet will be twice stronger than needed anyway.
I was just remembering welding 101 I took a few years back. The first thing they had us do was some play hardfacing with 60?? on a piece of 1/4" plate. At the end of the excercise the internal stresses had built up so much the plate began to curl like a potato chip. Impressive.
Reply to
wahzoo
That plate curled because of massive shinkage forces. It is noteworthy that the plate didn't fracture did it? Structural steel is amazing material and it bends and twists to amazing degrees without failure at the weld joints. Quarter inch wall is just baby stuff. No worries. There have been concerns about the joints when the intersecting tubes come in at a steep angles. Particular attention has to be paid to the weld geometry to assure correct throat thickness. Randy.
Reply to
Randy Zimmerman
Hello Mr. Wahzoo (Gotta include "Mr." With a handle like that)
I don't have my copy of D1.1 w / me. But I know w/ certainty that the code requires as little gap as possible. If the gap exceeds 1/16", it must be added to the legs of the original fillet weld size. Say the weld size on the approved print calls for a 3/16" fillet; if the fit up exceeds 1/16" gap, the weld size becomes 1/4". It keeps going that way until the gap reaches 3/16", @ which point D1.1 suggests using a full pen weld. If I had the code handy I would refer you the relevant para. W / welds this small, base metal this thin, & only 44 KIP base metal (I think that's what A500 Gr B is), internal stresses aren't much of a problem. In any case, gaps are not a good way to control internal stress. Pre heat is. But you're a long way from preheat being required. Although if you're in a cool damp environment @ the start of welding, (for example 35 F & 85% RH or if the iron is damp) it's never a bad idea to put a little heat to it to dry it out. But this is not really a pre heat.
Study the TKY requirements to make sure you have sufficient penetration & throat on the acute side of the tube for it to be prequalified. I don't think 30 degs. is prequalified. I've been retired for a couple years now & it's amazing how much I forget. I could be mistaken. I'm assuming that the web angle is 60 deg, from vertical. If from horizontal, then there is no problem w/ either weld.
Best wishes John
I just thought of something else; I don't remember having seen open web joists w/ anything close to 60 deg web members from vert. Usually ~45 deg. Again I may be mistaken. The AISC Manual of Steel Construction has a great deal of requirements for open web joists. I also assume that you are a licensed engineer or that one is going to stamp your design. As you may or may not know, steel open web joists have been involved in a large # of catastrophic failures over the years. All it takes is for one weld to fail & the whole roof is in the basement.
Reply to
John McGraw
I looked at that page, it's #71 in my 2002 D1.1. In that drawing they are showing the two pieces, not calling for a gap, imo. If they were calling out a gap they would show a dimension, with a +/- tolerance. Other parts of D1.1 that show stepped square HSS connections don't call out a gap either, at least in my quick look thru the book. Being a fillet weld on light material, as Mr. Zimmerman said, you can easily butt the two pieces together and happily weld away. I'm welding a set of stepped, square tube (2" on 4" chords), 41'- 8" long trusses right now, and I'm taking the fit as it comes, some have a little gap, some are tight. We just put a big old 5/16" fillet on all of them and don't worry about it.
regards, JTMcC.
Reply to
JTMcC
As JT says there is a practical side to things. Except for walls over 3/8 inch I haven't seen prints calling for bevel prep on the ends of tubes. These large wall tubes needed full penetration and since the shops I work in do not use open root procedures a backing bar was required along with a gap at the root. I have found Lincoln electric's Welding Innovations very informative. It is actually published through the Lincoln Foundation I believe. There is a good article at :
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The whole series of booklets are available online at:
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Randy
Reply to
Randy Zimmerman
I'm talking 60 degrees from horizontal. And right, there's not even any criteria for acute joints of 30 degrees or less. For joints under 60 but above 30 you have to add a "Z-loss" dimension to the size of the weld to account for the difficulty getting effective throat in these welds.
Thanks, wahzoo
Reply to
wahzoo
What does the HSS refer to ?
hollow steel section ?
Thanks-Jeff Dantzler
Reply to
Jeff Dantzler
Hollow structural shape
John
Reply to
John Noon

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