Crack in beam

[stmax]

Hi,
Please see the attachment and advise regarding cause of cracks in a beam. This is a 5 meter continuous beam supporting a 2.5 meter high walls enclosure above for a water tank. The cracks appears after filling the tank above with water. On top of beam there is a wall of the tank. Tank's wall and beam will resist compositely (deep beam) the stresses.
Design has been checked and found adequate. I just want to know, are these structural or non-structural cracks and what are remedial measure. Epoxy injection OR strengthening? Is torsion be a problem from water pressure on wall? which I don't think as there are cross walls at the end of span which along with the slab at bottom will distribute the torsion. Even if I consider torsion the composite section of beam and wall above is sufficient.

 

[ishvaaag]

Seeing the photos I wouldn't discount that the effects of the weight combined with shrinkage effects may be producing the crack. I see particularly worrying that corrossion has affected the reinforcement in the beam, what converts the problem in structural if it was not. Further investigation -and, of course, if what existent is to be kept, complete waterproofing of the tank- is of order.

 

[dik]

What is your concrete specification and what reinforcing steel was used? Stress level in the rebar?

Dik

 

[JAE]

Vertical midspan cracks such as this - where they are of uniform width from bottom to top would be indicative of some shrinkage effects which are induced by possibly some restraint at the ends of the beams (monolithic with some fairly rigid walls, etc.). The midspan location occurs due to the combination of the shrinkage tensile stresses plus bottom beam flexural tensile stress from the loading above occurring at midspan (max. moment).

The crack is fairly think looking and I see no structural concern based on your assertion that it checks out in calculations. The concern would be the long term effects of the extra oxygen and water getting into the rebar inside and rusting.

 

[IDS]

I just want to know, are these structural or non-structural cracks

Most likely a combination of both. The concrete in the beam would have more shrinkage than that in the wall, so you would expect any flexural cracks in the beam to be larger. The fact that it occurred after filling of the tank suggests that it is primarily flexural.

What tensile stress in the concrete do you get from the analysis, taking account of shrinkage? If it is more than the tensile strength of the concrete then cracks are to be expected.

What is the crack width? Is it large enough to require remedial work?

If it is, remenber that the crack will continue to grow over time with shrinkage and creep of the concrete. A crystalline crack filler may be better than a rigid epoxy repair.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[JedClampett]

When you say the design has been checked and is adequate, how "adequate" is it? ACI 318 allows a 1.4 factor for liquid loads. But ACI 350 increases this to 1.7 and depending on the reinforcing, it might go even higher (I routinely use the 2.21 (1.7 x 1.3) of the ACI 350-89). Has crack control been checked? Have all distributions of the liquid and other loads been considered? It might not be as simple as using the tributary area above the beam.
As long as the Factor of Safety is greater than 1.0, you're not in danger. But if the working stress is approaching 30 ksi or even a little less, you might get some cracking.

 

[IDS]

But if the working stress is approaching 30 ksi or even a little less, you might get some cracking.

Cracking in concrete is controlled by the stress in the concrete, not the stress in the steel. The stress in the steel after first cracking of the concrete will often be much less than 30 ksi. The crack width is related to the steel stress after cracking, but it is also related to bar diameter, depth of cover, bar spacing, bar type, and concrete shrinkage and creep strains. If the design complies with ultimate load strength requirements then cracks in the concrete should not be a concern for strength. If the crack widths are less than the allowable for the type of structure then there is no problem. If they are greater, then they should be repaired.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[charliealphabravo]

I understand that the design has been checked. Is this check based on as-built drawings? What is your comfort level that the construction matches the plans? This may determine whether you consider trying to map the actual reinforcement size and pattern.

As far as cracking being controlled by the stress in the concrete, this is true for initial crack appearance since the cracking moment is independent of the reinforcement. But this occurs far below design strength levels. Once the crack has appeared it is the steel stress that must control if it is designed properly. This ensures predictable yield failure and not sudden concrete compression failure. The widening of the crack under load is consistent with proper control/performance of the design for steel yielding.

I agree with JAE that this looks like a combination moment/shrinkage effect. I am a little befuddled why the condition is so precisely centered and not a little more distributed, not withstanding the mathematical/theoretical stress maximum at the center. It almost looks like there was a construction joint in the center of the beam.

The oxidation could also be troubling but this might be expected if the crack goes up into the tank. Are there any chlorides or corrosives in the water?

 

[JAE]

charliealphabravo,
I saw that too - it does look a little like there was a form joint at the location of the crack. Typically form joints don't initiate cracking but only leave a rough, irregular surface along the form joint. Might just be a coincidence that the crack and form joint aligned.

 

[TXStructural]

You say that the tank wall above will resist the forces "compositely" as a deep beam. Are you certain the the crack you show is not the result of midspan tensile stresses, where the (laterally-unsupported?) tank wall above is taking all of the compression? That would be my first guess at what we see here.

And what is sticking out of the inside, beam-slab corner? Epoxy injection ports or some other evidence of construction deficiencies?