Allowable stress in deformed column

[JRGse]

We have an existing 14 x14 HSS square tube column (about 45' long) that was filled with water that froze. This acton caused the four walls of the HSS section to bulge outward and have a permenant deformation of about 1/8". The ice/water has since been removed an now the owner is asking us to determine the load carrying capacity of this column.
How would you determine the allowable stress of the deformed section?

Thanks

 

[civilperson]

No loss of capacity, in fact there is a small gain if using the new deformed shape.

 

[shin25]

I also agree with civilperson, the axial load will directly follow the deformed route to transmit to the base.

 

[Lion06]

I don't know. It seems like this is a localized condition and not along the entire height of the column (is this true grover1?). That being said, wouldn't this be a point for a localized buckling to occur? If it is permanently deformed, it's obviously reached its yield stress locally.

 

[civilperson]

My assumption was that the deformation was exterior face bulging due to ice swelling the interior volume. The fact of previous yieding does not lower the capacity of the steel, in fact strain hardening usually gives higher capacity of area of steel times yield. The bulged section is stronger than the original shape.

 

[Lion06]

doesn't this localized "buckled" shape create a weak spot for local buckling to occur? Especially if you consider some localized P-delta effects?

 

[JRGse]

StructuralEIT - The bulge covers for about 1/2 the height of the column (+/-22').
civilperson - Your assumption is correct, the bulging occurs on the 4 faces for the height stated above. Strain hardening most likely is not a factor here due to the amount of deformation (this amount of deformation would be in the plactic range)

I agree that the yield stress of the steel would not change, however I'm not sure what other effects this deformation may have on the columns' load capacity.

 

[hokie66]

Agree with civilperson. Nothing to worry about.

 

[msquared48]

This is a high strength steel that has, in effect, been cold worked. I do not think that this is a slam dunk here. Due to the temperature, there could be local, microscopic brittle fracture. The presence of such would affect the capacity.

What load is this member designed to take? At 14 X 14, it is a primary member, and should be checked for microcracks in the vicinity of the bulge.

Did this happen during construction prior to sealing the section, or did it come to the jobsite that way?



Mike McCann
McCann Engineering

 

[corus]

Microcracks are present in all structures, particularly fabricated/welded strauctures and aren't of concern unless you're considereing fatigue loading.

Even though you may have an apparently 'larger' section and a greater load carrying capacity, you will have residual stresses in the material though as yield has been exceeded either in tension or compression. The residual stresses should be considered in combination with the nominal stresses you'd expect to see to determine if localised failure would occur either through buckling or by exceeding the allowable stress (some factor of yield).

corus

 

[renko]

It appears that the column is no longer prismatic. I've tried working out the overall capacity of non-prismatic beams like this before, but it's difficult without a great deal of patience or a software package. Professor Nick Trahair did some work on this.

If the deformation has caused a curvature in the longitudinal axis of the column then there would be an adverse effect on the capacity of the column, with regards to overall buckling. If the column remains 'straight' from top to bottom then the 'bulging' should increase the overall resistance to buckling. By how much? Not sure - I'll see if I can dig up my old notes on beams, but it's been a while.

 

[csd72]

If you look at it from a pure sectional point of view it is okay, but when you look at the local buckling of the faces you will have issues.

Somewhere along the length of the member it changes from the standard shape to the bulged shape, this creates a local buckling tendency that cannot be ignored.

look at any cold formed steel text and it will give you the general ides

 

[JStructsteel]

How much load is in the column? Is it part of a lateral stability system. Seems a 14x14 column for a 22' height is large, perhaps there is adequate reserve capacity to call it good.

 

[swearingen]

I'm with csd72 - longitudinally, the walls of the standard shape and the bulged shape do not line up. Axial loads that don't line up create moments and these moments must come out somewhere. I'm thinking that there may be local problems...



If you "heard" it on the internet, it's guilty until proven innocent. - DCS

 

[msquared48]

corus:

I do not know the utility of the column, but if it is a wind column, then you are dealing with a subjection to a fatigue type situation. It is true that microcracks are present everywhere in steel structures, but what I am concerned are those specifically induced by the bulge that would be larger, and consequently more than the normally accepted number, size and length.

It is also way outside the alowable milled tolerances for tube shapes if you will compare the description in this string to the AISC manual standards for the shape - see page 1-123.

Mike McCann
McCann Engineering

 

[civilperson]

An 1/8" bulge at the center of a 14" side is a 1 degree deformation from the corner. The squareness of sides criteria is +/- 2 degrees, (AISC 1-123). What criteria is exceeded?

 

[Lion06]

I doubt that you would want to compare this to the mill tolerances. This is likely not a straight-line deformation such that you can call it 1 degree. It is likely much more pronounced in teh center of teh 14" dimension, like you would expect a fixed-fixed beam to deflect (the wall is fixed at the two corners of the tube.
Either way, this , IMO, creates a localized region of weakness. I don't know how to account for it, but I am hoping someone does. I don't see how you can neglect it for the reasons I stated early in the post and other ones that others have echoed and different concerns of others.

 

[frv]

The permissible variation from specified largest outside dimension is 1%. The Total deviation is 1/4" (it protruding 1/8" on either side) this represents about a 2% variation.

Even if this weren't the case, I'd still agree with StructuralEIT. I don't think it's that clear-cut that this is not an issue- especially due to localized p-delta effects.

 

[hokie66]

csd72 raised the issue of the transition zone from straight box section to deformed box section. I suppose he is right that there would be local effects. If the plate eccentricities cannot be resisted by transverse bending of the tube walls, it may be necessary to stiffen the transition section. One way may be to fill the tube with concrete.

Is there also a transition section near the base plate? There would have been a very large force on the welds to the base plate, so that area should be looked at.

How did the water get into the column? Let me guess, the architect insisted on putting plumbing inside the column.