Concrete Cracking at Stairs/Handrails

[KingKongdoor]

I'm working for an Architect and on one of his previous jobs the concrete stairs at the entrance of the building is cracking. I was not the engineer on the job but on the worst crack it appears there was no steel in the stair nosing. For the other flaking type cracks I'm thinking there was to much water in the mix? Can thermal expansion of the handrail post be the cause? The cracks developed this summer. I'm not sure what time of the year the concrete was poured but I know the building is less than 1 year old when the cracks developed.

 

[Agent666]

On the face of it, it certainly seems to appear that there was significant restraint from the handrail posts with the classic concrete breakout due to shear perpendicular to the edge. Hard to believe though it's just due to thermal.

I wonder if the handrail bridges a seismic or movement joint, that might explain the stair wanting to move but the handrail trying to stop it and the concrete giving up at some point?

 

[WARose]

I vote for a thermal issue as well. Had the posts been taller......that might have relieved the situation as far as the bases were concerned.

 

[canwesteng]

I'd think there needs to be a joint that the handrail is bridging, otherwise the thermal expansion of the concrete and steel should have been similar.

 

[Earth314159]

I had a similar thing happen with an exterior steel beam connected into rigid concrete block walls. Thermal contraction of the beam produced cracks that were about the same size. In your case, the lower rail would stiffen up the balustrade and the concrete stairs are also very rigid. Getting rid of the lower bar would help.

It would think that the cracks would more likely occur in the winter time unless the concrete heated up more during the day and the balustrade cooled down quickly in the evening.

It is also not a great detail for rust jacking.

 

[BridgeSmith]

Looks to me like thermal contraction of the railing is the most likely cause. We usually put something soft around the top few inches of the slab where we have something like that. For one with a vertical face nearby, as at the top of the stairs shown in the first picture, I would have specified it to be sleeved to at least the bottom of the top step.

The thermal expansion coefficients aren't significantly different, but the short term temperature differential between the fully exposed steel railing and the steps can be huge, at least where I live. We can get temperature swings of 50 degrees or more in a matter of hours, and the temperature of the steps would change very little, due to the thermal mass of the concrete and the ground they're sitting on, while the railing would reach the ambient air temperature within minutes.

Rod Smith, P.E.

 

[Tomfh]

Thermal is obviously a possibility (not sure how people know for sure?).

In any case the slab and the steel are moving differentially and there is insufficient restraint to hold the steel in place. I’ve seen it a few times with beams lightly fixed to rigid concrete elements. The anchorages simply tear out.

 

[EdStainless]

This is one reason that the bottom rail is usually not that close to the concrete.
Even if there had been 18" of exposed post it would likely have flexed enough to not crack the steps.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[oldestguy]

How about high slump concrete and over working when surfacing(weak)?

 

[hokie66]

I agree about the bottom rail thermal movement. In addition, it looks like these holes were cored, and the one in the middle looks like it was cored twice...sloppy installation. Was the annular space filled with expanding grout?