Venting holes when welding to sealed HSS
Venting holes when welding to sealed HSS
(OP)
I was asked to comment on a deformation observed on a truss's bottom chord fabricated with an HSS. Indeed, the two side walls of the HSS are bulged out for approximately 3/8" for the bottom chord's entire length. It does not seem to be an overstress problem since this member is most likely under a tension load. The truss is 40' long and the damaged bottom chord was made with a HSS 8x6x1/4. I've noted that a reinforcing plate was added to this sealed hollow steel sections without the use of a venting hole. Would this field welding of a reinforcing plate (1/4" fillet weld, 6' long) be enough to increase the internal pressure in the HSS to cause this bulging of the side walls?






RE: Venting holes when welding to sealed HSS
RE: Venting holes when welding to sealed HSS
RE: Venting holes when welding to sealed HSS
The bottom chord of that truss has some unexplained stresses coming from somewhere.
How close to flat/straight is the bottom chord from end to end?
RE: Venting holes when welding to sealed HSS
Section modulus = 1/12 x 0.25^3 x 1"/ 0.125" = 0.0104 in^3.
Stress of 46,000 psi then equates to a moment of 480 in-lbs.
Momeent = 1/8 wL^2, with L = 8" gives w = 60 lbs/inch load required.
That load is the internal gauge pressure, requires 75 psi absolute pressure. Assuming 15 psi initial pressure, with pressure proportional to absolute temperature, that gives (460 + 60) x 75/15 - 460 = 2,100 degrees F average temperature in that tube. That would amount to the entire tube glowing red- or white-hot.
Check my numbers there, but it doesn't sound like sealed tube is the issue.
If there was a lot of water trapped in there, it'd be a different issue.
Weld distortion might be a culprit.
RE: Venting holes when welding to sealed HSS
The bottom chord is pretty leveled. Actually, the truss was cambered 3/8" in order to achieve a "perfect" levelness under the concrete floor self-weight. I just wanted to touch base with somebody to find out if there is enough heat generated in the welding process of this reinforcing plate that could cause such an internal pressure in the sealed HSS. I doubt it is the case, but my knowledge in welding is very limited.
RE: Venting holes when welding to sealed HSS
RE: Venting holes when welding to sealed HSS
I sure look like web crippling but it is not localized under the diagonal connection. In fact the buckling is uniform for the entire length of the bottom chord making it hard to explain.
RE: Venting holes when welding to sealed HSS
RE: Venting holes when welding to sealed HSS
Nobody knows when this deformation did happen. The only area where the HSS side walls are not buckled is where the reinforcing plate was added and locally at the sway trusses ("diagonal bridging") where their vertical connection plates provided enough stiffness to preventing the deformation.
RE: Venting holes when welding to sealed HSS
8x6x1/4...any type of internal press wheather from the heat generated by the weld or from freezing of any liquid inside the pipe would have to be of such an exact level such that it only affected the sides(8") and have no affect on the top & bot(6")..highly unlikely...without any further info I would go with what Lomarandil suggested that it was in that state before fabrication....
RE: Venting holes when welding to sealed HSS
RE: Venting holes when welding to sealed HSS
1) What's the average spacing between bottom chord panel points?
2) How tall is the truss?
3) Is the truss top chord flat or pitched?
4) What kind of section is used for the top chord and do you see any of the same phenomena there?
5) The first photo suggests that gravity load is not being delivered through the panel points. Is that generally the case?
6) Does the degree of the bowing fluctuate at all between panel points? Or is it utterly constant?
5) Any chance the bowing is greater near the supports and somewhat less near mid-span? I'm talking overall here, not between panel points.
Okay, a lot more information.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Venting holes when welding to sealed HSS
1 , 2 & 3) Bottom chord panel points at 10' c/c, Truss span = 50', Warren Flat Truss Configuration with first diagonal in tension, Truss clear height= 85", 5 panel points at truss top chord equally space at 10' c/c.
4)The top chord is the same as the bottom chord (HSS 6x4x1/4") and it does not shows signs of deformation. A bit strange since it is most likely in compression.
5)Most of the gravity loads are transferred to the top chords and they are uniformly distributed. They come from a concrete slab on deck sittings on the top chord. For the bottom chord, there is a little bit of gravity loads transferred from a "deck" that serves as a walking surface within an interstitial mechanical space. This deck is perpendicular to the bottom chord and it is supported by an continuous angle welded under the bottom chord.
6) The bulging is fairly constant with the exception of two following locations where there is no bowing:
- at the reinforcing plate;
- at the four connection plate welded perpendicular to the side of the bottom chord for connecting the sway trusses.
7) The bowing is the same for the full 50' length, therefore it appears as if it does not vary with the axial load present in that bottom chord.
How much water would it take for it to bulge in freezing conditions? If the reinforcing plate was added because of the presence of a damage such as a hole that would have allowed water infiltration within that bottom chord designed to be sealed. The bulging might be explained by the freezing of this water during the first year construction cycle (before it was protected from winter conditions).
Thank your for your comments,
RE: Venting holes when welding to sealed HSS
I'm relatively confident that for ice formation to cause that bulge, the HSS would need to be completely full or nearly completely full with water.
RE: Venting holes when welding to sealed HSS
RE: Venting holes when welding to sealed HSS
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Venting holes when welding to sealed HSS
Even if there's compression in the bottom chord, it's hard to imagine that you'd get such uniform crippling without some evidence of global chord buckling. I'd also expect longitudinal crippling to be wavy along the length of the chord, not uniformly bulged out.
The concept of a chord damaged prior to erection is attractive except for thise locations where there is no bulging. They would have had to have fixed the bulging at those locations if, say, the HSS was crushed in storage or something.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Venting holes when welding to sealed HSS
RE: Venting holes when welding to sealed HSS
The side walls would buckle in this exact pattern - everywhere except where there's extra steel.
RE: Venting holes when welding to sealed HSS
RE: Venting holes when welding to sealed HSS
RE: Venting holes when welding to sealed HSS
@StructMB: Is the section 8x6 or 6x4? 40' long or 50' long?
My initial thought: There's a fixed condition at the bottom chord inducing compression. Hokie93 brings up a good point about the camber. Distortion can occur if not done correctly. However 3/8" over 40 to 50 feet isn't much. If the bottom of the bottom chord is flat, probably not a camber issue. If it were due to the camber I'd expect to see a concave surface on the bottom of the HSS.
You could drill a hole in the chord to see if there is water; that may solve the mystery.
Here's my dark horse theory: If it's not compression in the bottom chord the offset of the gusset plates is creating an eccentric load - what do you think KootK? On a bridge truss we would never do that.
RE: Venting holes when welding to sealed HSS
What do I think? I think that you may be my brother from another mother. My dark horse theory is identical to yours. That's where I was headed with these questions:
I envisioned a scenario where you'd be getting a compression strut in the side walls between successive panel points due to the eccentricity that you've described. The joints are concentric in terms of the member center lines but still require that there be a moment where the gussets meet the chords. With the distance between panel points being 10', I deemed that too far for this effect to produce uniform buckling along an 8" deep HSS face. That's just instinct however.
I figured that, if it were a flat truss with similar top chord panel point spacing and member size, the phenomenon would manifest itself at the top chord as well. That is, apparently, not the case.
I figured that, if gusset eccentricity were the culprit, the effect would be less apparent at locations of low truss shear. Apparently that is not the case either.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Venting holes when welding to sealed HSS
RE: Venting holes when welding to sealed HSS
RE: Venting holes when welding to sealed HSS
RE: Venting holes when welding to sealed HSS
If the truss had been stood up in a vert. position, and standing on a conc. floor slab on its bottom chord, and then loaded from above on the top chord, for some reason, or maybe dropped in the fab. shop...; the vert. load from above would have been transmitted down the diag. web members to their connection gussets, and then on down to the slab. This would cause the bot. chord HSS webs to have a bending moment from the diag. web connection gussets, plus a sizeable compression load on the bot. chord webs. The sum of these loads might have caused the kind of web bending/buckling I think I see in your photos.
RE: Venting holes when welding to sealed HSS
RE: Venting holes when welding to sealed HSS
RE: Venting holes when welding to sealed HSS
It is good to inspect your job (and photograph) frequently for this very reason.