Cracked HSS Post 1

[Eng_Struct]

Hi Group,

Point 1: I have a 10" x 10" x 5/8" HSS stub post for an equipment platform that has a crack running in the vertical direction along one of the corners. The post also has some bulging on the sides. The post is adequately designed for the loads. However, I am unclear what may have caused the crack. Even if the post was underdesigned, in my mind, none of the failure mechanisms (pure compression or flexural) will result in a vertical crack. Any thoughts if you think a load design failure can result in a vertical crack?

Point 2: The crack appears to be more similar to some of the other discussions on the forum related to water freezing inside the HSS. For the crack and bulging to occur, I would think that the post will need to be filled with water (note that the post is only 2' high). There are both top and bottom plates welded to the post and for the life of me I cannot figure out how could the water get inside. Any similar experiences or examples where water can still get inside the post and cause cracking?

Point 3: The post will need to be repaired. However, I want to analyze the current condition with crack and determine if there is still sufficient capacity. I am thinking of taking out a 1"x5/8" section at the cracked location and analyzing the remaining section as an "open" shape for combined bending and axial load. Any other ideas on analyzing HSS sections with vertical cracks for bending and axial.

Point 4: This ties back to point 3, for the bulging of the side walls, I am thinking I can check the effect of the walls locally considering the moment in the side wall plates caused by the eccentricity of the axial load. Any other thoughts?

Thanks

 

[HTURKAK]

I am not sure if the post is adequately designed ..

You did not mention for the loads. The crack and ' bulging ' implies dynamic impact loads.

Can you post some descriptive pictures and info. regarding the subject equipment and loads ?







Don't underestimate a nail. A nail saves a horseshoe, a horseshoe saves a horse, a horse saves a commander, a commander saves an army, an army saves a whole country.. GENGHIS KHAN

 

[sbisteel]

Is it galvanized? Might be hydrogen assisted cracking.

Best way to see if water is the culprit is to drill a 1/4" weep hole near the base plate, water will come out if that is the problem. I've seen this issue in the past, but the cracks were enormous and there wasn't much question as to why they were there.

 

[dik]

Any chance for water penetration and freezing?

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[Eng-Tareq]

I agree with dik, water might have penetrated through, and with the temperature fluctuations the failure mechanism took place. I have seen multiple hollow sections which are supposedly welded to not allow water ingress but they seem to still draw moisture (maybe due to condensation). Anyways, it is a good idea to provide a weep hole at the bottom to allow egress of any moisture buildup.

Regarding point 3, I think your method works but I would be more conservative and just analyze it like an open section with 3 sides (C shaped) and totally ignore the 4th side which is cracked. I agree with your approach for point 4, not sure if I have a better way of idealizing that.

 

[dvd]

Some discussion on Charpy v-notch toughness and cold forming at corners of HSS

 

[Once20036]

I`m with sbisteel - is this galvanized? I`ve posted some links in the past about the phenomena that leads to these cracks.

dvd - I didn't see any links in that link about cracking of HSS corners. How does it relate to the question at hand?

 

[dik]

dvd... thanks for the link, anyway,,,

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[dvd]

Once20036 - ?

 

[Once20036]

Sure - the toughness is different in the corner.
How does that lead to the cracking described in the original post?

 

[Eng_Struct]

Thanks, Everyone!


I have calculated the capacity of the cracked section using open section properties however, I am not sure how to account for bulging and if bulging will reduce the capacity of the post significantly.


Can creating a FEM with plate elements help? I can apply all axial loads and moments including moments due to eccentricity caused by bulging. The post is only 3ft tall, I am thinking Euler buckling may not govern and I can look at the stresses to ensure they are below yield.

 

[dvd]

A picture or sketch and discussion of the loading would go a long way to getting better input. This is an equipment platform - has it been subject to vibrations, load reversals, shock loads, etc.?

 

[Eng-Tareq]

FEM with plate elements is a good idea and try to model the bulging of the HSS as accurately as possible and compare the von Misses stress to the phi*Fy value of your steel.

 

[SWComposites]

You also need to check local buckling of the open section sides, not just column Euler buckling. Best done with a shell element FEM with bulging displacements modelled.