precast beam cracking pattern at bearing connection 7

[jrfroe]

A client of ours has a 2-story manufacturing facility that is primarily precast concrete construction. We noticed an unusual cracking pattern in one of their precast beams that is exposed in a stairwell. Analysis of the beam and design of a fix are out of scope at this time, but we want to do our best to understand what is happening before asking for a change order. Therefore, I'd like to see if anyone else has seen this type of cracking and to provide any thoughts concerning what may be occurring.

The attached photos show the precast beam at its bearing on a precast column. There is also a steel column that appears to have been added at some point adjacent to the precast column. We are unsure why the column was added, but it doesn't pass the eye test for being appropriately sized if it was intended to support the beam. It also appears to have experienced local flange buckling near mid-height.

The diagonal cracking appears to run the opposite direction you would expect from a shear failure which makes me think that the presence of the steel column is causing the beam bearing connection to experience tension. In thinking about a fix, I'm leaning towards just removing the column, since it seems to be doing more harm that good, and doing whatever is needed to repair the precast beam. However I'm hesitant to take something out when I don't fully understand its purpose.

Does anyone have any thoughts?

 

[rapt]

Last thing I would be doing is remove the steel column. Looks like it has been put in to save from collapse a very badly detailed precast beam in the area of the connection.

Any tension is probably because the beam is not free to move from its seat. And there is probably no reinforcement developing into the connection above the seat to take the tension.

I would prop the beam and either replace the steel column with one that is adequate or strengthen the existing one if it needs it.

 

[MIStructE_IRE]

I certainly wouldn’t be removing that steel column. It was put there for a reason. There are countless possible reasons, so until you’re certain, leave it in.

Any closeup photos of the “buckle”? Perhaps its impact damage?

 

[jrfroe]

Thanks... If its not wise to take out the column, I would for sure want to replace it since it seems like it is also twisted slightly. I'm pretty sure the "buckle" isn't impact damage since its in a stairwell underneath a flight of stairs at a location that isn't accessible to anything that could impact it. Only way it could be impact damage is if it was damaged prior to being installed. I don't have a close-up view but here is a slightly different angle.

 

[Settingsun]

Are you able to speculate on how the steel column was brought into position? Could have been dinged on the way in through tight spaces inside a finished building.

As for size of steel column, does it look small in comparison to the concrete corbel depth? Hard to get perspective on the column length but a column loaded axially is much better off than a corbel. Maybe the load isn't all that big.

Crack could be hard to diagnose. Direct tension as Rapt said, initiated at the reentrant corner; or cracks 2, 3 and 4 meeting up.

 

[rapt]

steve49,

I only mentioned the tension because it was suggested it might be from the column.

As is required for several effects, and I would guess it is either not there or is not developing properly. Av and Ah might not be there or not in the correct position either.

The step in the bottom of the beam may have been an afterthought when they realized they could not have perpendicular corbels at the same level and not detailed accordingly. The perpendicular beam has no step!

 

[Settingsun]

I was wondering why bother with such a small step when there doesn't appear to be any headroom issue. Most Australian Main Roads departments have outlawed stepped beams because of the hassle dealing with cracks. Guess it's a problem in buildings too.

 

[BAretired]

With sudden change in temperature, the steel column tends to shrink faster than the surrounding concrete. Cracks which formed prior to the installation of the steel column would tend to re-open with a sudden blast of cold air. Insulation might help, but removal of the column is definitely not advised.

BA

 

[Brad805]

I think rapt is on to something when you look at the corbel elevations. The corbel elevation of the beam in question makes no sense. It could be a case that the corbel elevation was an error that was not discovered until the beam was on site. That beam could have been cut on site and the bearing added. That region does not look like it came out of the precast plant. You will need to track down drawings or get someone on site to figure out what you have for rebar. After that, you can look into repair options. Regardless, column must stay and this must be looked into to figure out:
1. Is the steel column adequate and what does it sit on?
2. What do you have for rebar in in the beam region above the corbel. It may be feasible to fix.

 

[rapt]

Brad805,

Especially when you look at the perpendicular beam which appears to have depth added equivalent to the difference of the corbel heights with a cold joint at the lever of the upper corbel!

 

[dik]

Do you have any information about the reinforcing in the beam? From shop drawings and not structural drawing?


Too localised and in wrong spot; don't think it's a buckle.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[BAretired]

I agree, dik. It can't be a buckle. Looks like it was caused by a local impact.

BA

 

[jrfroe]

With sudden change in temperature, the steel column tends to shrink faster than the surrounding concrete.

There may be something to this. The stairwell this is in does get cold in the winter. I don't think they have heating in the stairwell and the wall in the picture is an exterior wall.

I'm not yet convinced its not a buckle knowing the location is protected by the nearby stairway. If its due to impact it would have needed to have occurred prior to installation. I plan to make another visit and look at it up-close to verify.

Also, length of the column is 17'-4". I'm not sure what the section is, but its 8" deep and 6" wide.

 

[Tmoose]

How old is the building ?

I'd be taking a careful look at the equivalent detail on both ends of all similar precast beams.
Can you get a good look at the top surface of the cracked beam? And maybe of the back side?
Is that steel angle or plates at the bearing point of the corbel/seat? Some pretty raggedy looking details showing thru the paint. I'd be removing the paint from all sides of the end of the beam from the very end to 12" beyond the visible cracks to get a better look.

I'd be interested in the precast beams' design reinforcement, the as-build inspections and any pictures, and the design loads used to select the beam.

X 4 for local impact causing the column dent. If the column is going to stay I think a little work with a hydraulic jack and some heavy strong back plates would cold-flatten out that kink nicely.

https://www.enerpac.com/en-us/c-clamp-presses/c-clamp-press/A210

 

[phamENG]

I would be shocked if that turns out to be local buckling. For one thing, flange local buckling is a flexural effect - a slender flange in compression due to bending stresses buckles locally. It's not a compression limit state. In flexure, the compressive stress in the flange varies along the length with the internal moment at that location, so only a small section will have sufficient stress to cause buckling (hence the local). In a compression member, the compressive stresses tend to be fairly uniform (neglecting self weight of the column, of course) and so you end up with a global buckling failure rather than a local one. Based on your dimensions (W8 with a 6" flange), it's probably a W8x24 or W8x28. Both of them are compact in flexure and flange local buckling is not an applicable limit state for that section anyway.

I realize that it could only have happened prior to or during installation - I'm not sure why you seem to be using that as a means of dismissing the idea. Steel columns and beams get dinged up like that all the time. Installing it in a tight location increases the likelihood that it'll happen. And once it's in there, the erector isn't about to volunteer the fact that they dented the beam and have to fish it back out and replace it.