Forensic Analysis of Concrete Column

[DEJ123]

Here’s one for the structural forensic concrete experts. I remember seeing some creative construction that I wonder about. I don’t know specifics, so this will be a little bit hypothetical.

It was a column designed as a 30” x 42” concrete column bearing on a thick spread footing. There were 14 vertical #9 or #10 bars (four corner bars, two more spaced evenly along each 30”face and three more, spaced evenly along each 42” face). There were matching dowels embedded in the footing. Ties were #3 or #4 rectangular ties.

That isn’t the way it was constructed though. The dowels were located about 8” off. Most, or maybe all of the dowels were heated with a torch and bent in a dog-leg shape (two 90-degree bends) in order to locate them in the correct position which, of course, made them useless for transmitting any load into the footing. Several of the dowels were cut off because they fell outside the formwork. The column vertical bars and horizontal bars were tied in place, the forms were put up and the concrete was poured. So, you have a column with few or maybe no effective dowels into the footing, but with pieces of dowel tied to the bottom of most of the column vertical bars.

It’s in an area of very low seismic activity and is now several years old.

The questions are: What fraction of the originally intended design strength is this column good for? What is the likely failure mode? A sudden complete loss of strength or something less dramatic?

TIA,
DE

 

[TXStructural]

It depends.

What was the purpose of the column, strictly gravity or uplift due to lateral? Was there a moment expected at the base which was to be resisted by the footing?

How many dowels were compromised? You are certain that there was no attempt made to adhere new dowels into the footing and that all dowels were compromised? I'm guessing that at least 1/2 are in the footprint of the column, but if they were bent 90 degrees at the top of footing, just ignore them - the connection from footing to column will not act in tension (and doubtfully in shear), but the compressive strength of the column could be minimally compromised.

14 #9 bars is minimum reinforcement (1.1%),so it could be a gravity column with little moment and no uplift. Maybe.

If the workmanship is really that bad, I would not trust any structure in the building, since this shows that flexural members are likely improperly built.

As far as being "several years old", since few buildings ever see design maximum loads, I would not expect a failure until that one day in 50 years when demand exceeds capacity. On that day, if the column doesn't do its part for the structure, results could be anywhere from nothing to catastrophe.

The good news is that conventionally-reinforced, cast-in-place concrete is economical even when it is over-built. Many design methods result in significant redundancy. The additional strength gained by concrete as it ages, and the conservative nature of certain calcs and designs help in mitigating problems. When a lower-bound value is used to determine how resisting strength is determined, anything above that value means more strength.

As more designs rely on FEA and complex numerical methods, these redundancies are reduced, and we are seeing more serviceability issues. A faster computer or sharper pencil doesn't make a bad design better or a poor job of construction stronger. We have all been faced with putting a sharper pencil to a project to figure out if the subject of an RFI is acceptable, but this one sounds like they left the engineer out of the solution.

 

[STATICPH]

Hi,

I'm not a structural forensic concrete experts but like to get to that level in the next 300 yr.... so here's my egg.

It all depend on the initial design of the specific column. Long/short/capable of resisting moments or whatever.

If it is a short column subjected to axial load only only, I don’t think it will ever fail unless it is subjected to some sort of large accidental horizontal load.

On the other hand, if it is anything else, and will be subjected to some kind of moment, there will be no full mechanical connection between the column and the footing. That connection should fail in torsion or tension.

Secondly, heating up steel - specialist literature required! You could change the composition of the steel and end up with a lower grade or even brittle metal. But the bars are already compromised anyway, so I doubt if this will be the initial reason for failure in this specific column. However, if all the bars was used but bend – it still bad construction .

Final say – no mechanical connection was achieved.


tx

 

[JAE]

I agree with STATICPH in that I'm also not a forensic expert....does anyone really claim to be?

As far as the dowel concern - I would suggest that the key here is finding out how the column was designed - based on a fixed bottom or not.

For spread footings - I would think many engineers would call it pinned. However, ACI suggests you can consider far ends of columns fixed when designing - again - depends on the original engineer's assumptions in coming up with the columns design.

If the original design considered it pinned, then the dowels don't really serve a huge purpose anyway as long as there is no net uplift, significant shear, etc.

 

[DEJ123]

Thanks for the replies.

I don’t think the column will see any uplift because it is in a very low seismic area and also not a high wind area. As for moment and shear, it is one of many columns so all supporting a slab so I think there will be enough redundancy and redistribution of force to prevent failure in that way. So axial forces are very probably the only concern.

It is old enough to have been designed without computers, not to mention any FEA. The column only extended up about 20’, so it’s a short column.

It occurs to me that from a point a few inches above the footing upwards, the column was constructed correctly as designed. Also, bearing of the concrete in the footing is not a concern because the footprint of the column is much less that the area of the top of the footing. So I wonder if the probable failure mode would be a little cracking/crushing around the bottom of the column. This is where I lack experience. Probably most of us have seen a standard 6” diameter x 12” cylinder crushed, but how would it fail if there a little rebar cage inside it. I would really like to see a stress-strain curve for that.

Thanks,
DE

 

[STATICPH]

Hi DEJ123,

If this column bother you, make a few assumptions (to side step the unanswered Q’s) and calculate if it could fail in pressure…..or rather, this is an option out.

This is what I suggest:

Assumptions to be:
It’s a short col, No moments or lateral forces present, it’s an old structure and therefore stood a test in time, we assume zero rebar in the 10mm section of column between the footing and where the columns own rebar start (you could get that scanned I quess), test the concrete strength and use the results for concrete strength (Personally I won’t use the ave strength, I’ll use the lowest value obtained and times that by 0.9 as a safety factor, I don’t know if any code say something about this thou) NOW, we assume that first principles are O’K’ to use.

Calculate the loads that the column are subjected to……

Calc : Presure = Force/area ……. “walla” you have an idea of the column’s capability in clean axial load under first principle conditions (Will the codes allow this? - how els can we figure this one out - fire the contractor).

Any other suggestions guys?

 

[DEJ123]

Thanks STATIPCH,

It’s a short column that will only see gravity load. I’m sure the column reinforcement extends all the way down to the top of the footing.

So the rub is that at the bottom of the column, the rebar isn’t developed like the designer thought it was. I think the confinement steel (ties) is important here but I don’t know how to put number to that.

When I put numbers to it, it looks good. (bearing capacity per ACI318, sec 10.17 vs axial strength vs ACI318, sec 10.3). (0.85*f’c*Ag) / (0.8*(0.85*’f’c(Ag-Ast)+fy*Ast)) gives you from 85% to 100% depending on assumed steel ratio.

Therefore failure mode is the question. Given that fact that all is as designed except at the bottom of the column just above the footing (the reinforcement isn’t developed because it isn’t spliced to any dowels) is failure look like some cracking/crushing at the bottom of the column. If the worst that could happen is that the bottom crushed and the column settled a few inches, that wouldn’t be the worst thing in the world.

Comments?

DE

 

[STATICPH]

Gosh, that is a hard one, failure is failure. Failure of an element is failure of the purpose of the element which might deem it unsafe, localised failure could be contained– so it is hard to comment with limited info.

What is the consequences of further failure?

Is further failure possible?

 

[Teguci]

OK, possible failure mechanisms:
1. Shear failure between the column and the footing. This I think is the only one of concern for the given conditions above. This one can be easily fixed as well.
2. Moment Failure - It sounds like this is a typical pinned column to footing condition. No moment capacity is desired.
3. Compression - buckling failure will not happen within two feet of the footing given a constant column cross section.
4. Uplift is a problem if the column has uplift but it doesn't sound like it does.
5. Torsion failure is in line with the shear failure above.
6. Long term column shrinkage which necessitates the minimum steel area in a column shouldn't be at issue since the affected zone is small.

I'd walk under the column in question.

 

[a2mfk]

Sounds like a job for carbon wrap...

JAE- "I agree with STATICPH in that I'm also not a forensic expert....does anyone really claim to be?"

Try bouncing that question off David Caruso on CSI Miami

But in all reality, forensic engineering is a specialty within structural engineering. I have done a lot of forensic engineering for the last 6 years but would only call myself an expert if an attorney asked me, otherwise its pretentious and misleading. Depends on the subject matter, which for me, certainly is not giant concrete columns with messed up rebar Any problems with your house though, that I can help you with...