HSS 4x4 column-splitting 2

[WWTEng]

We got the call from the homeowner saying that the exterior column supporting deck was splitting. I thought he was talking about a wood post but its actually HSS supporting 3 decks (multistory). The total load is no more then 25 kips, so the column is not failing because of inadequate capacity.

However, the column is not fabricated very well. It appears that fabricator used two pieces for the bottom story, a 2’ long top piece and then another piece to make up the height. The weld between the two pieces is fairly easy to see (see attached picture). The column also seems to be not galvanized and only painted on the outside. So the inside of the column is not protected against corrosion.

I believe that water has been infiltrating from the top of the column, through the joint between the two tube sections and possibly creeping up from below. I couldn’t see any vent holes in the column and thus this creates perfect conditions for corrosion, resulting in material expanding in volume and this may have resulted in the split in the column.

As to why the split happened where it did (near the base), I think there could be three major reasons 1) that’s where the load is maximum 2) most of the water flows down/creeps up near this location 3) the column at the base is restrained by the base plate and possibly the surrounding concrete slab and thus doesn’t have room to expand.

I have asked for the column to be replaced and I think I have a good handle on the cause of the problem but I do have a curious questions:

Could the forces from corrosion-expansion be that strong to split the column at the joint? Or was there possible an inherent fault in the column fabrication. I also think that the weld between the two pieces of column is not good, this may have made the bottom piece to act like a cantilever resulting in k=2.0.

 

[WWTEng]

Another pic.

 

[BAretired]

Is the building located in an area which sees freezing temperatures? Water gathering at the bottom of the column could freeze and cause substantial lateral force on the HSS walls.

BA

 

[WWTEng]

@ BA: Yes, its in the midwest.

 

[MiketheEngineer]

Then it probably did freeze. Seen it many times.

 

[IceNine]

There's an article in this month's Structural Engineer magazine about water getting into tube steel columns, causing the column sidewalls to bulge. Seems like a very real possibility.

Link

 

[TehMightyEngineer]

Given the poor quality of that weld and the obvious signs of water leaking out of the hole in the weld it seems likely that a significant amount of water has penetrated inside the HSS. Given that your loads are so small I would second the freezing water expansion theory.

Corrosion shouldn't cause that kind of failure as far as I know, any surface corrosion would just expand inward inside the tube. There could have been a preexisting crack but there's no way I know of to check that.

You could avoid replacement of the tube if it's still in good shape by drilling the ends of the crack and welding it shut. Grinding and rewelding that joint would make sense, too. Weep holes will obviously be a useful addition.

Maine EIT, Civil/Structural. Going to take the 1st part of the 16-hour SE test in October, wish me luck!

 

[WWTEng]

TehMightyPirate: I believe that salvaging the existing column is impractical. What is not visible in these pics is that 2 faces of the HSS are bowing out where the crack occurs. I think this goes hand in hand with many others have mentioned about water freezing and expanding.

 

[msquared48]

Could you cast a reinforced concrete plinth around it, fill the tube with grout to the top of the plinth, and drill a weep hole to allow water to escape?

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[WWTEng]

msquared: I thought about that, but I am a little uneasy about the weld between the two pieces (second picture). I think the just welded whatever and then grinded it off.

 

[msquared48]

Yea, you're probably right. Looks like he just used whatever scrap material he had to reduce his cost. Happens a lot with ..., well, I won't go there.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[BAretired]

Why not drill a hole near the bottom and examine the wall thickness and the loss due to rusting. The hole can be left for drainage.

If the column is loafing under the load, why go to all the trouble of replacing it? Just fix up the weld, gouge the crack and re-weld.

BA

 

[Ron]

I think BAretired is correct...this appears to be a "single event" issue rather than exfoliation from corrosion....the break is too clean for that.

As for the weld...it sucks. Based on the staining, there is obviously water on the inside of the HSS.

Agree that if the loads are light, just weld the crack, reweld the poor weld joint, provide drainage and move on.

 

[JAE]

...and perhaps also look near the top of the column and see about keeping water out.

 

[WWTEng]

Ron, JAE: I open to the idea of leaving the column in, but I am not sure how the weld is going to be repaired. I think what they did was take two pieces of HSS and weld them without any prep. I don't think this is a partial pent groove weld or a proper weld of any kind. I could see the two HSS section not exactly plumb with each other. And whatever weld they had, most of it was grinded off. So I think a proper fix would be to reinforce the joint with plates and that will be just ugly.

And thats why I am recommending remove and replace.

 

[MiketheEngineer]

You could visually inspect with a boroscope - you can buy a cheap one at Harbor Freight or have it X-rayed. Now you will know!!.

 

[ztengguy]

How much load is at the top, the splice weld above the two floors? can you sleve it and reweled, if the home owner is ok with that?

 

[charliealphabravo]

I saw the exact same condition last summer on a HSS sign pole during a building evaluation in Alberta. I thought it might have been hit by a vehicle but splitting due to water freezing sounds like a better diagnosis since it also explains the bowing.

 

[hokie66]

WWTEng,
I agree, just take it out and replace it.

 

[connectegr]

Just replace the column. Provide drain holes in the replacement (sufficient to release moisture even during the freezing months). Weep holes can freeze closed and collect a lot of water waiting to thaw. And UT the splice next time. Rust is like cancer and even looking inside the column will not provide any real information about the remaining capacity.

http://www.FerrellEngineering.com