Cracking in Brick Gable

[bvbuf]

I'm looking at a church built in 1922. The exterior walls are 3 wythes of unreinforced brick. Near the top of the front gable is a 36" diameter circular window. There is a perfectly horizontal crack in the grout at the CL of the small window extending the entire width of the gable at that location. There is no other sign of significant cracking in the brick in this wall. My first guess at the cause would be wind loading (very low seismic area). The dead load up there isn't big and the tensile stress (fatigue?) in the mortar from out of plane bending is the cause. Anybody care to offer an opinion?

 

[Teguci]

How about a thermal stress? Either from the window frame or even the roof.

I'm thinking a scenario like - Window frame is metal and heats up quicker than the surrounding brick, there is no relief and little dead load so the frame pushes up and out on the brick. I'd think this would be more of a diagonal crack though.

Or - the roof has a tension tie on the inside of the wall. The wall, due to a long term thermal gradient expands through the day with the roof, but the roof contracts quicker at night putting a horizontal compression on the gable with perpendicular tension. - If the roof isn't tied/restrained near the gable then this one can be thrown out.

Could be wind if the roof has a shallow slope and produced uplift.

 

[bvbuf]

Thanks Tegusi!

The window frame is wood and there is a wood tie on the inside of the wall. See attached photo

 

[Archie264]

Wow, when you said "crack" you weren't kidding! It also looks like there's a faint vertical crack starting at the top of the window and zig-zaging a bit. And it *almost* looks like there was a beam pocket there at some point. I can't see if the collar ties are at each rafter but if not, or if they are not connected well to the rafters then it's possible that that ridge board at the top acted more like a ridge beam and put a point load on the wall above the window. That could have cracked the top of the arch causing the horizontal resultant force to seek the weakest planes, those two almost-at-the-same level mortar joints. (Or, more precisely, the top and bottom mortar joints of the same brick course.) And to be technical it would have occurred the reverse order: the horizontal joints failed causing the top crack to form.

Just my thoughts.

 

[Teguci]

Great photo.

I don't think its thermal because the crack is still open which suggests the load is still acting. Same with out of plane bending - I wouldn't expect to see through the crack - one side should be closed in compression.

Swelling of the window frame? Does the crack close towards the outside edges? Looks like there are some slight cracks stepping up to the ridge.

If it were window swelling I would check for a wet frame. Check if the frame is tight. Looks like the frame is rigid enough for some decent loading. Check if there is anything else holding the gable up.

 

[KootK]

Could the gable wall be settling or shrinking and that upper wedge getting hung up on the roof framing somehow? The window opening may just be serving as a crack initiator.

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.

 

[KootK]

Perhaps the rafters are swelling due to ambient moisture changes and are somehow forcing the roof peak upwards with the brick going along for the ride somehow. Kind of like residential wood truss uplift but for rafters.

1) Do we know anything about the age of the cracks?

2) Is there any reason to believe that the humidity environment in the building would have recently changed?

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.

 

[hokie66]

I would guess that the crack was initiated by a high wind event. But does it matter? Unreinforced gables are notorious weak links.

 

[Archie264]

Yeah, that makes sense. That's at the level that the lateral bracing from the side walls end and those roofing boards don't look capable of transferring much shear to the side walls via diaphragm action.

 

[bookowski]

In the photo it's hard to tell how much on the right is a crack and how much is a shadow. Definitely some of that is shadow.
- Is the crack the same/similar on the outside?
- Did the window go with it or is there a separation? I doubt it's the window that is lifting that peak. Looks like I can see a gap over the window, towards the left.
- Any embedded steel? What is the green flash reflection on the left? Also something sticking out of the wall at the far left, iron?

 

[chicopee]

Did you check the plumbness of the walls?.