Concrete Pier Foundation Cracking 5

[acer49]

What can cause this type of cracking? My first guess is lack of adequate shear reinforcement or some eccentric loading based on the anchor bolt/column configuration. The piers were constructed in 2009 and designed by someone else. Piers are 5'-0" in diameter with #4 ties at 12" O.C. vertical reinforcement is (8) #7 rebar equally spaced.

Also, have no concrete pour data, break reports, foundation specifications, or soil data available.

 

[r13]

Another possibility to consider is that the two posts have experienced forces in different magnitude. Depending on the arrangement on top, one post might have seen larger load than the other, that created overload locally. Note that the posts each stands on its own base plate, at times they can act independently, especially under lateral load/movement conditions.

 

[JoelTXCive]

Could this be some sort of anchorage failure?

Maybe the vertical 18 #8's are not fully developed at the top of the pier where the anchors pass their tension to the pier?

I'm not sure what a crack pattern from that type of failure would look like though.

 

[SKJ25POL]

acer49 (Structural)(OP),
Could you please provide us the picture of the superstructure and "Just if you have" the reactions at most critical pier/damaged the most (widest cracks) pier?
Have you measured the width of cracks?
Also, may I ask what part of US are you located (I assume by the unit of dimensions provided you are in US)? Any sever weather condition?
And in one of the pictures I see a grating by the pier!!! Whats that grating for?
I have dealt with these issues in past.

Thanks
Skj

 

[bones206]

Definitely a thermal/restraint issue. Similar to the problems with low railings on guardrails. I’d remove the cross beam and inject the cracks.

 

[r13]

Just for fun, let's throw some numbers to see what the result.

Thermal expansion coefficient of steel = 7.2x10-6 (a quick number grabbed from internet)
Assume T1 = 45F at installation, T2 = 100F

Under the assumption, the linear elongation = 7.2x10-6 x (100-45) = 3.96x10-4 x L

Assume L = 4', the result is 0.019" (slightly larger than 1/64")

I may have made mistake in the assumptions, or calculation. Any comment?

 

[bones206]

I’ll give it a shot. working off my phone so bear with me.

α = 0.0000072
ΔT = 55 F
Say A = 4.43 in[sup]2[/sup] (W6X15)
E = 29,000,000 psi

Full axial restraint —> PL/AE = αΔTL
P = αΔTAE = axial force in cross member
P = (0.0000072)(55)(4.43)(29,000,000)
P = 50,874 lbs

The actual force would be relieved somewhat by the outward bending of the posts.

 

[r13]

For that force to develop (p = 50,874 lbs), both ends must be fully restrained. Interesting for bolted joints behave like this. Also, wasn't the bolt hole on the baseplate oversized to allow some movement (albeit some of the anchor bolts might have already touching hole edge during construction)?

 

[bones206]

My guess is the concrete cracked before the bolted connections slipped. Crack pattern looks like an anchor group shear breakout cone, in two directions simultaneously.

 

[hokie66]

We don't really know the answers to that, retired13. But your query raises another possibility, that the cracks could have occurred during erection of the frames due the base plates being forced onto the anchor bolts. But you would have to wonder why no one asked about the cracking for 10 years.

 

[r13]

hokie66,

My thinking is that the slightly greater than 1/64" movement (1/128" each end) should usually be easily accommodated in the bolted connections, unless the bolts are friction type and fully tightened, or the beam was jammed into position. If neither is the case, there is something else to be looked at.

I agree you stated case is another possibility, but not likely happen to all 6 foundations of the same type as observed by the poster. But who knows? They were installed by the same crew, thus same type of mistakes. The cracks might have been hair line-like at the beginning, but widened with time, then speeded up and getting noticeable large recently. Just wonder since when the cracks started to draw the eyes.

 

[JoelTXCive]

Sounds to me like people want to revisit Joel's anchorage questions.....

 

[acer49]

To answer a couple of questions:
[ul]
[li]I am working on getting more pictures of the steel structure above.[/li]
[li]Haven't been to the site to measure the width of the cracks yet.[/li]
[li]The grating by the pier is for a person to stand on for operating some electrical equipment.[/li]
[li]The site is located in the outer banks of North Carolina.[/li]
[li]These issues seem to go ignored for a while because mostly electrical engineers manage the sites and they don't raise concern until it's fairly obvious.[/li]
[li]The steel was erected in 2009 and not sure when the cracks started developing.[/li]
[li]The anchor bolts are 1.25" in diameter with 2'-0" embedment with a 6" projection for a total length of 2'-6".[/li]
[/ul]

 

[canwesteng]

Another thought - the holes for the anchors in the base plates are undersized, anchor placement is slightly off, and the contractor just jammed the supports onto the anchorage. If the bolt holes are db + 1/16" like for structural steel, you'd expect the anchor groups to possibly be 1/8" off in plan, and you could build some restraint there.

Either way, for a repair, epoxy the cracks and try and release the restrain if possible.

 

[JLNJ]

Maybe the w/c ratio of the original concrete was .70 or something similar and it wanted to shrink like crazy.

 

[fel3]

Retired13…

I suspect the total annual temperature change is more than 55°F. I copied the following from a 3-year-old post I made here:

In the early 1980s I designed a 3 million gallon welded steel water storage tank in Westlake Village, CA, which is between Thousand Oaks and the San Fernando Valley. I was then assigned to do the field inspections. One of my tools was a magnetic thermometer for measuring the temperature of the steel during painting. A few days before the painters arrived, I started measuring steel temperatures around the tank at different times of the day to develop a rough idea of what we would be dealing with (I also marked the ringwall with a crayon so I could see how much the tank expanded from morning to mid-day, but that was for my own amusement).

As I recall, on one 105°F day, the steel temperature in the Sun at about 3 p.m. was around 180°F. On the north side of the tank, the shaded steel was around 100°F.​

Fred

==========
"Is it the only lesson of history that mankind is unteachable?"
--Winston S. Churchill

 

[bones206]

I would also concur with KootK that water freezing in the crack could have expanded the original thermal crack widths. I believe they’ve had periods of sustained freezing weather in OBX in recent years.

I thought about concrete shrinkage as well. Perhaps if they used two anchor template plates that were rigidly connected. But I think the crack pattern would look different if they were shrinkage-induced.

 

[r13]

fel3,

Thanks for the wake up call. Yes, the column tie may have experienced much higher heat then previously assumed. My intention was to draw attention to columns-tie connections - whether or not the connections are rigid/tight enough to develop internal force to force the columns to move. Your example is roughly 3 times of mine (3xΔT), so the elongation would be approximately equal to 3/64" (3/128" each end of the tie), which still is something worth to think about. And, the internal force calculated by bones206 would now be 3 x 51 kips = 153 kips, a magnitude can't be ignored, consider it acts simultaneously with other forces on the columns.


To acer49: I suggest to do a little digging on one of the pier to exam the extend of crack below. It may provide more useful information.

 

[slickdeals]

Is it possible that they were installed slightly off and the contractor used some sort of device to spread the columns apart to fit the bottom beam in?

 

[hokie66]

Time has played an effect with the width of the cracks. Narrow at first, but any debris which falls in will prevent the crack from closing...repeat a lot of times.

 

[Ron247]

Is the short beam near the base acting like a fulcrum? Do the columns both move together, or can one drift different than the other? The columns appear battered to me.

Also, do you know if your hoops are lapped correctly?