Column Dents - To Fix or Not to Fix 7

[KootK]

I'm peer reviewing some designs for column reinforcement where 8x8 columns have been whacked by forklifts about 18" above floor level. See the photo below. Our engineer has followed the lead of a previous firm and recommended angle reinforcement on the four corners of the column. My feeling is that no reinforcing is required. These three arguments inform my opinion:

1) The damage is far away from the mid-height of the column where full section stiffness is needed to prevent buckling.
2) There is no column "kink" to speak of. The column is still very straight. It just has the dents now.
3) While some of the cross section near the damage may be unable to carry compression due to P-Delta effects in the bent walls, I feel that I only need to demonstrate that there's enough undamaged section available to carry the load across the damage using squash capacity (As x Fy). I can get this done if I represent the four corners of the columns as imaginary 1.5" x 1.5" angles composed of undamaged HSS wall section. It doesn't take much. Moreover, there's probably some post buckling capacity to be had in the dented HSS side walls anyhow.

It might ultimately be prudent to fence off or concrete encase the columns to protect them from this kind of damage in the future. For now, however, I'd like to solicit comments regarding my "do nothing" proposal and the logic that I've used to justify it. The fact that reinforcement was specified by another firm in the past makes it apparent that one of us must be wrong in our thinking. Maybe it's me.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[Archie264]

I agree with the "do nothing" strategy and your reasoning seems sound to me. I would paint it so as not to draw attention to it and perhaps put up bollards or a concrete curb to help avoid a reoccurrence of it but beyond that wouldn't do anything. If that amount of deformation could cause significant structural problems then there would be warehouses collapsing all over the world.

 

[KootK]

Thanks Archie. You should see the place. You'd swear they're playing forklift bumper cars in there. It's neat how denting on one side results in bulging on the other face. There's some part of me that doesn't really believe in structural theory until I see it in action.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[TehMightyEngineer]

Sounds like a good approach. I do a lot of industrial work and have seen entire buildings where every other column had a good whack (much more than yours) taken out of each wide flange column. Still standing, no issues many years later.

When you actually run the numbers the dents near the point of support don't impact the columns capacity by much, if any. I wouldn't be very concerned. If desired, encasing the columns in concrete has the double effect of protecting the columns and shielding them from future damage.

Maine EIT, Civil/Structural.

 

[Guest262653]

Although I agree with your approach in terms of structural adequacy of the column, I think that one shouldn't put out of the equation the public's perception of safety. If already two firms are involved in the analysis of this problem, maybe a lot of people are very concerned about it...

Just adding a different perspective, but in the end I would follow your line of reasoning.

 

[KootK]

@TME: can you share any details regarding your dent evaluation method? Similar to mine? It's tempting to try to assess the capacity of the bent up walls too.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[KootK]

@avs: I vacillated on that same point. I was tempted to mimic the original repairs for just that reason until I found out that they chipped the SOG out to take the reinforcement down to the base plate. In the end, I found it ethically objectionable to spend the owners money on the perception of a repair. Thanks for sharing your perspective.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[TehMightyEngineer]

Kootk: Similar to your method, I created a model of the column in STAAD where the column section changed right at the base from the W-section (tube in your case) to a custom calculated section where the dented flanges were removed. The unbraced length of the column was modeled as full height in my case. I assumed a pinned base. I ignored eccentricity from the new centroid of the custom dented section being offset from the column centerline. This seemed reasonably accurate considering I was entirely ignoring the post buckling strength of the buckled flanges which I estimated would negate the eccentricity. I then applied any bracing for the the column to match existing and applied an axial load until the column hit failure in the model. From this I determined that the buckled section did not control the strength of the column.

Now, it's worth noting that this doesn't mean it can't fail. I've seen a large building column rust down to paper thin at the base and fail in buckling (dropped about 6 inches before secondary supporting members caught it and saved a building from coming down). However, it took about a 75% loss of cross-section before it did fail.

Maine EIT, Civil/Structural.

 

[JLNJ]

Along the lines of TME's post, it might be worth the exercise to see what varying the I of the column near the base does to the column buckling capacity. I think you will be surprised how small the effect is with regard to the overall buckling of the shape.

This makes sense in that the middle 60% of the column works much harder to prevent buckling than the end 20%.

But don't tell the forktruck drivers about this!

 

[KootK]

This will be a little off topic but, heck, it's my thread.

JLNJ commented on how little the bottom 20% contributes to buckling resistance. On many occasions, I've seen engineers place ground floor columns on much larger below grade piers in order to keep the KL from getting too long on columns where the footings must be deep for one reason or another. They treat the pier/footing assembly as though it were effectively rigid. However, if you look at the column as a single member from top to footing with the pier simply represented as a section with increased I, you find that you've improved KL very little. There's a bunch of other stuff at play out in the real world such as slab on grade restraint and passive soil pressure. Still, though, the logic is often flawed.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[Brad805]

I suspect if nothing is done it could be a problem for them at safety meetings. The angle solution presented seems problematic unless you heat up the one side. I do not see much benefit in jack hammering the slab either. Personally, I am a fan of the concrete bollard.

 

[MikeHalloran]

The downside to 'do nothing' is that when the dented section gets hit AGAIN, it may fail, at a lower impact load than an undented section might have withstood. At least that's how I imagine someone's lawyer might argue it.

I suppose you could present the case to the owner as arguing for complete repair OR addition of buffers/deadmen around the column.




Mike Halloran
Pembroke Pines, FL, USA

 

[TehMightyEngineer]

Agreed KootK. This is one reason I like the direct analysis method is I can model the concrete pier as a simple concrete column and attach the steel column to the concrete pier (column) in the model. Running the model typically the concrete pier provides a lot of support but, as you said, it will still add the to the buckled shape of the column.

However, and I think this is what the "other engineers" are doing; they are assuming a pinned base for the column. Thus, with a pinned base the column and pier do not act as one column and will have their own individual L values. But, you are correct that the piers will shift laterally and this may cause some secondary effects but these are likely negligible.

Maine EIT, Civil/Structural.

 

[KootK]

@TME: I was actually thinking of concrete columns on concrete piers. I neglected to say that however. And, to some degree, it does apply to steel columns as well. The support from the pier would be proportionally more with steel columns I imagine. But then you've also got a difficult to assess stiffness discontinuity at the base plate.

@Mike: we stated that the columns were structurally okay but it would be prudent to protect them somehow. Similar to your recommendation. I was thinking about this issue at lunch. Initially, I worried that recommending a "do nothing" option would give them free license to run into the columns all they like. Then I came to realize that I don't really mind if they keep running into the columns over and over. As long as my three caveats at the top continue to be satisfied, my answer will always be the same. I accept your point regarding lawyers though. I make an active choice to not consider things in terms of future lawsuits. I find it paralysing to think in those terms. No doubt, my first lawsuit will change my tune.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[paddingtongreen]

While we design to a reduced stress for columns, it is really about the column, the actual material is still at original strength so it can take considerable damage before the local load capacity is less than the column capacity. I don't think reinforcing is required for strength.

I do consider that some angles should be placed on the corners supported by some springy flat bars as a guide for the crazed forklift operators.

Michael.
"Science adjusts its views based on what's observed. Faith is the denial of observation so that belief can be preserved." ~ Tim Minchin

 

[EdHenrie]

I tend to look at this situation in the form of: The columns are designed to support a load, and within some damage tolerance will continue to do so. However, you now have introduced some damage, which is some percentage of the total damage it can have before the column fails. Now that it is damaged, does it make it more susceptible to future damage? I'd argue that it probably is. Is it likely that further damage WILL be introduced? Does a bear S*** in the woods? The addition of reinforcement angle is specified, I would suspect, to simply protect the existing structure from further damage, not necessarily to increase the integrity of the column as a load bearing structure, though depending on the specific application could be used as such. The conservative approach in this case is to accept your peers' recommendations and go on with life. You're splitting hairs in an effort to accomplish what? Prove that you can do a fancy engineering calculation and save the company $XXXXX, or should you focus on the protection of the lives and safety of employees and the property of owners/investors? Obviously there's a balance between the two. Worst case scenario: you make a do-nothing recommendation, something bad happens, someone dies. Guess who's going to be scrutinized? If you do the reinforcing, and a forklift smashes into it and it fails and someone dies, there will be no question that you did what needed to be done with regard to the reinforcement. The concrete surround is probably considerably more expensive than the angle reinforcement, but that may be something to investigate. Here's a good option for you: Do the calculations, research and estimate costs for the available options, and have your peers include the data in the report. Give your management something to do.

 

[RPMG]

We evaluate failure modes. Stick to that philosophy, regardless of how someone else analyzed something or original intent. It sounds like the more important issue is that the fork trucks hit something other that columns. I'd recommend putting some gage metal barriers that they can destroy, and not care about.

 

[charliealphabravo]

If you do a search for HDPE column protection you will find many low cost solutions for providing, at the very least, a sacrificial and visual barrier for steel columns.