Cast Stone Top Anchor Freeze/Thaw

[bookowski]

This is for unreinforced history stone & mass masonry walls. I am looking at a building where a stone failed by splitting. There have been two engineering investigations/reports and they differ in their opinion as to the cause. I've been asked to review both.

Leaving aside their opinions here, I am wondering about the susceptibility to freeze thaw created from the pocket that top anchors create for stone/cast stone facades.
W/out sharing the actual project, I'm talking about anchors similar to the ones shown here: https://masonrymagazine.com/Default?pageID=2252, about 1/4 down the page labeled "mass masonry". This is close enough to what I have (multi wythe brick backup and stone facade).

The top anchors are common in this type of construction. In reviewing this failure, and the opinions, it seems to me that this detail is very likely to have a small pool of water in the anchor pocket. These things are a zillion joints, no way that water isn't getting into at least some of them. Is the idea that ice can expand vertically enough to not typically be a problem? It seems that 99% of the time this is working. In my particular case the stone split right at the anchor. The iron anchor itself shows only surface corrosion, it doesn't appear to be enough that corrosion is what split the stone. If it is freeze thaw I don't see any way to opine on whether or not this would happen at other stones. To me the detail seems inherently problematic in this regard. But I must be missing something because in reality they seem to usually work.

 

[ANE91]

My intuition is that compressive force along the axis of the facade is more likely to buckle the panels off of the backing wythe before splitting them. Consider thermal expansion, for example.

Is there mortar between the panels? If so, then consider that the mortar wants to “spread out” under compression, due to the Poisson effect. It’s a traditional masonry problem at that point.

I’d be scouring the forensics congresses for any case history. Unlikely that this phenomenon hasn’t been observed/addressed before. Based on your post, freeze-thaw seems more likely than not, but I share your skepticism re: the detail. You never know; maybe that one panel had an inherent plane of weakness.

 

[bookowski]

@ane - Let me clarify. This particular condition happened at a stone corbel. The diagram below "1" is not the same but gives you the idea. In my case there is no steel beam, it is all brick masonry backup. The top anchors shown here provide the same concept, it's a pocket carved out on top for the iron anchors. Additional stones sit above the stone that failed, similar to what you see in the right of diagram 1. The 2nd image shows similar details. None of these exactly match my condition, but they depict the relevant detail. After speaking to someone that works in this area they clarified that what I am referring to would be "frost wedging" as opposed to freeze thaw (but they were not able to opine on why this doesn't happen more often on this type of detail).

 

[ANE91]

I’d venture the following, non-comprehensive guesses, if you’ll forgive me the pontification:

1. Not all pockets are going to be contain the same volume of water.
2. Freezing is likely non-uniform along the exposed facade.
3. Initial compression provided by the anchors is a function of their installed condition (read:torque) and also variable.
4. Overlying and underlying surfaces may be providing variable confinement against splitting, similar to how sulfur-capped cylinders behave differently than cylinders in direct contact with steel during concrete compression tests.