HSS/TS sections-residual stress?

[SAIL3]

Has anyone encountered this type of damage on a HSS/TS section?.
It is a TS 4x4x3/16" 304ss.
Thumbnail sketch of project:
12'x12'x47' tall moment/braced frame structure..shop assembled and shipped to field complete.
Dead weight=50kips
Design lift load=50x1.5=75kips
It was lifted, rotated in the air and set on truck and shipped several hundred miles, stored for over a year and then shipped several hundred miles to site.Probably lifted and rotated up to six times.
This crack(about 16" long) was discovered after it was erected.
At first glance, I thought it was a weld seam that had opened on the face of the member due to a bad weld.
It turned out to be on the corner along the radius of the section.Trying to figure out the external load that caused this is almost impossible, after the fact.
I suspect, due to how straight the crack is, that there was some residual stress in the member at the corners due to cold-forming and that combined with the unintended external load it caused an over-stress at that location..using Mohr's circle.
The member was also slightly bowed in the same direction as where the crack was located.
I would appreciate any insights of ideas........thanks

 

[larsacious]

My first thought is that HSS/TS members are typically constructed with A500 mild steel. Since they used stainless, it is inherently more brittle because of the higher carbon content. It is possible that the bend corners were already near rupture and multiple moves was the "straw that broke the camel's back."

Perhaps look into what the minimum bend radius should be for 304 steel...

 

[dhengr]

SAIL3:
Off the top of my head I don’t know exactly how the mechanical and physical properties of 304SS compare with A500 or other structural grades of steel. And, I don’t know exactly what rolling does to the grain structure in thin SS, but it will certainly be work hardened and the grain orientation will be in the length direction on the tube, less than ideal for a tight radius bend. Bending like is done on this tube, with the bend line parallel to the grain structure leads to cross grain cracking and is part of what sets the min. bend radius. Then there will likely be some stretch and thinning in the bend arc length. I’ll bet the member was loaded torsionally in some way and combined torsional shear stress and normal stresses were its undoing. Vibration in transit wouldn’t help matters either, and the way it was supported in transit might have caused some of the overloading.. Drill a hole or a couple holes at the tips of the crack and just weld a 3/16" patch over it.

 

[ToadJones]

SAIL-
They actually cold roll SS tubes?

 

[ToadJones]

That does look like stainless steel to me. Do I need my eyes checked?

 

[ToadJones]

...doesn't....

 

[SAIL3]

Thanks for the replies.
Larscious....ss comes in many grades...304 has the lowest hardeness
dhengr....bending accross the the grain could certainly be a contributing cause along with the shipping loads you mentioned.
The fix you suggested is exactly how we repaired it.
Toad Jones....yes it is 304ss..not polished , ofcourse.
I would think that some of these issues would also apply to A500 HSS sections, but I don't recall this ever being addressed.

 

[ToadJones]

Anyway it was stored in cold weather and had water intrusion into the HSS?
Tubes burst rather easily when filled/frozen

 

[JLNJ]

Where was it made? I would be more suspicious of the manufacture than residual stresses.

Stainless cost is so high I can imagine that corners might be cut.

 

[a2mfk]

Curious, this for the canadian mining/oil industry?

 

[SAIL3]

It was faricated in the US for the food industry but where the steel originally came from is anybody's guess. With all the importing of steel nowadays there is no ironclad guarantee that that there is a consistant and qualitative quality control followed in the production of that steel.