Help identifying this crack pattern in concrete walls 3

[Robbiee]

Hello,
I was asked to inspect and provide opinion on the cause of cracks developed in load bearing end walls of a building. Please see attached. The building superstructure is precast concrete, 23 storey high and rectangular in footprint. These cracks, as seen, start from the wall vents located near the ends of wall panels and propagate diagonally away from the support. The cracks extend through the wall. To me, they appear to be shear cracks caused by the reduced section of the beam part of the wall/frame, but can't confirm if the reduced section is adequate or not given that no design drawings of the individual wall panels are available for review. What do you think, and what would be the action plan to provide an opinion on the cause?

 

[oldestguy]

Possibly settlement of the supports at or just beyond the left side of the photos.

 

[Robbiee]

Thank oldestguy, These cracks are on the two sides of the centre of the wall as seen in the third photo I attached for the wall elevation.

 

[MotorCity]

Even though they are bearing walls, they may be acting as shear walls if they are stiffer than the lateral force resisting system of the building. If that's the case, they are probably diagonal shear cracks which manifest themselves at the weakest point....openings. Since this is a rather tall structure, the walls may be preventing the building from racking, where the forces would be very high in the lower stories. Do these cracks occur at primarily the lower stories or do they also occur at the upper stories? I am envisioning a sort of coupling beam action with large vertical coupling shear forces where the cracks occur.

 

[hokie66]

In what way are the panels load bearing? There appears to be daylight under the panels, so looks like they are facade, not structure. A section through this would be helpful in understanding.

 

[Robbiee]

MotorCity,
These are bearing walls and also shear walls. They are called on the drawings "walls". Actually they are frames. I agree with your thinking, but what puzzled me is the fact that these cracks started from level 3 and up. No cracks on the lower two floors, which contradicts the expected cracks developing in the lower, worst location due to lateral loads.
hokie66,
The floor precast panels bear on these walls. This is confirmed on the the erection and structural drawings.

 

[hokie66]

It's just that your photo and the elevation are different. I can accept the elevation as representing loadbearing precast, but the photo looks like the panels are supported on the slab. Which way do the floors span relative to the cracked panels?

 

[Robbiee]

hokie66,
The floor panels are perpendicular to these walls. Yes, the floor panels sit on the wall below and the wall above sits on the floor panel as seen on the elevation.

 

[Ron]

The cracks appear to have been sealed and painted. How old are they?

Have you checked the structure for drift? That could explain the cracks starting at the 3rd level.

Agree with MotorCity...they have the appearance of diagonal shear cracks; however, you obviously need to find the driver behind that.

Is there any chance that these occurred as a result of erection of the precast panels? Perhaps they were not designed for the construction loads.

You have a significant section reduction at the openings. Any chance that post-construction shrinkage created these? Check the crack widths at bottom,center and top. If the same, likely uniform shrinkage. If wider at bottom, then some bending is occurring.

Good luck! These are often head scratchers!

 

[Robbiee]

Thanks Ron, The building is fairly new, less than 10 years old. They are not sure when the cracks showed up. We are not the designers. We are only involved in checking these cracks and the reason behind them, and if there is any structural significance to them. Why the designer engineer is not involved? Don't know.
We haven't done anything except for a visual inspection and review of the available drawings. No analysis and no checking of the design. Currently, need to say what possible reasons for the cracks are and what needs be done next.

 

[TehMightyEngineer]

Ron might be onto something about the erection cracking; stresses around rectangular openings for heavy panels can get pretty extreme. I've run a few FEA analysis of various precast items and it's not hard to get the tensile stress above 7.5f'_c^0.5. Once one of these cracks opens up, if there's insufficient reinforcement to restrain it, the normal lateral and vertical drift of the structure could open it up further.

I'd say if all your structural analysis check out (both vertical and lateral related) then take a look at the handling stresses. Keep in mind that a big panel like that may be handled at a lower f'_c than the final design strength.

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

http://www.americanconcrete.com

 

[oldestguy]

In the case of differential settlement between the interior wall and the remaining part of the building, a precise elevation check will answer this one way or the other.

 

[dik]

Are the openings reinforced? ie, a acouple of 10M or 15M bars all around?

Dik

 

[KootK]

I agree with your thinking, but what puzzled me is the fact that these cracks started from level 3 and up. No cracks on the lower two floors, which contradicts the expected cracks developing in the lower, worst location due to lateral loads.

Not necessarily so. In a coupled shear wall system, the peak demand in the coupling beams is at the top rather than the bottom. This lends credence to motorcity's hypothesis (nice). And yours as I do in fact believe these to be permutations of flexural shear cracks.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[dhengr]

Robbiee:
Are these precast panels actually “L” shaped beams with the lower leg pointing inward, to pick up the slab loads? Then you have bending, beam shear and torsion all acting at the corners of those vent openings. Likely some pretty high localized stresses in the conc. Why is the one vent in photo #2 different than the vents in photo #1, and what are the vents really for? How are these vent openings shaped through the beam/panel thickness? Do these cracks only happen in the one vert. portion of the bldg. wall, or do they happen at all of the vents shown in the elev? What other lateral load resisting features/system exists in the plane of that exterior wall, other than these precast panels?

 

[Ron]

Check the painting history of the building. You can also take a small core through the crack surface to determine if more than one coating has been placed since the original. I use a 2" diameter diamond coring bit for such. It will not cause a structural issue to core the surface and penetrate 1/2 to 3/4 of an inch. This might give an indication of the crack age. This might be an important key to finding the source of the cracks.

I do not believe it is settlement since the manifestation does not appear below the 3rd floor. While the framing system might vary in the first 3 floors, if settlement were the impetus for the cracking, there would likely be some manifestation of distress at or near the base of the building.

 

[oldestguy]

Ron: Do we know how well the precast panels fit in their positions. Could it be that later (than at job start) the forms were enlarged some for upper panels because the plenty of slop had to be reduced or more rigid joint filler became used.
And after The mighty engineer (below) maybe a change in bar locations at the opening for the higher floors.

 

[Brad805]

It would be interesting to find out if the blockouts were field cut or blocked out in the plant. Getting correct mechanical information tends to be the final thing one gets in precast production and a field cut hole is unlikely to have any extra rebar.

 

[TehMightyEngineer]

I can all but guarantee that those were blocked out in the plant.

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

http://www.americanconcrete.com

 

[KootK]

@TME: question for you as this has me curious now. We don't know that this is a lateral/strength thing yet but, for now, let's assume that it is. I imagine the process unfolding something like this:

1) Panel designer spots holes and reinforces panels accordingly. Maybe even some sexy strut and tie. Go engineering!

2) Cracks form and owner wants to know if everyone will die.

3) Engineer looks at it and says that everything is fine. That fancy STM reinforcing wouldn't even work until post cracking anyhow, right?

4) Owner wants to know why design team thinks that it's okay for the skin of his zillion dollar building to be all cracked to heck.

So my question is this: when one designs and or supplies something like this, do you design it not to crack under service lateral loads so as to prevent aesthetic problems? I'm guessing not but then there's the potential to end up where we may find ourselves here. And, hopes and wishes aside, it's hard to imagine that a stress concentration like these block outs could be dealt with in a serious, load carrying member without initiating cracking.

Please advise...

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.