Leaning Chimney 4

[zero1238]

I inspected a leaning masonry brick chimney for a client the other day. The house was built in 1930 and the chimney itself is around 25' tall. I noticed some separation nearing the top of the chimney away from the house and when I put my level on it, it appeared as though it was leaning outward as far as 1" over my 48" level. I don't think that this is consistent the whole way up to the top but it is certainly concerning. I've heard of people underpinning chimneys with helical piles but is there a certain "lean" tolerance or some threshold at which point it's clear it has to come down or is something like this repairable?

 

[JAE]

So I see the orange substance between the brick and the white wood siding. Is that some kind of spray foam insulating fille that was previously used and now has turned orange due to sunlight?

If the chimney is leaning out that far, yes, I'd be concerned (if it was my house). It could be that the weight of the brick has settled and/or rotated the chimney foundation outward. Or the house could have moved away from the brick although you said your level shows the lean in the brick.

In any case, if the brick is otherwise competent, one first response might be to install some type of horizontal tie - perhaps a series of steel rods going through the brick and into the house - at the floor and roof levels - to tie into the floor/roof framing. At least this would arrest any further movement outward and away from the wood structure.

Underpinning with helicals is an idea but I'd first want to use a positive tie-in to the diaphragms.

The helical piling could be used to underpin and hopefully stop any further settlements or rotations but the tie-rods are a must in my book.

 

[XR250]

Around here, we just underpin them with helicals.

 

[zero1238]

Yes, it is spray foam, which is another concern because that's highly combustible. I was thinking about underpinning it but as far as the strap, is there a product out there that you've had success with? Also. is there a certain point at which underpinning isn't an option and it definitely needs to come down? I'm just having a tough time making that distinction...

 

[milkshakelake]

What JAE suggested is not straps but rather anchor rods tying into the structure itself, which will be restrained by the diaphragm.

This is not a case where the whole thing needs to come down. I had a chimney situation that "suddenly" collapsed (though the telltale signs were there), city got involved, there was emergency demolition of the remaining parts using boom lift, and it was a huge mess. That one was cracked and degraded, with mortar missing between the joints, and almost completely independent of the structure (i.e. no ties). After the fact, I also observed the foundations to be crumbling. This doesn't look like that situation. I don't think there's a way to "push" it back plumb, but you can minimize further movement.

 

[zero1238]

I'm not sure how you would anchor it into the framing without using some type of strapping? You're saying anchor rods through the masonry into the framing? They would have to be some very long anchor rods...

 

[JAE]

Yes - long anchor rods with threaded ends. Usually some type of exterior "washer" plate is used to lay flat on the exterior brick. The rod is extended through a hole drilled through the brick chimney (yes a long drill bit) and extended into the floor structure. The floor / roof joists may be perpendicular or parallel to the rod so you'd have to verify that and detail a connection of some kind betwween the rod and the wood framing. Usually some type of additional blocking and perhaps a small 3x3 clip angle for the bolt to fasten to.

I would be hesitant to try to crank on the rod nut in an attempt to pull it back level. This is only a method to stabilize the overall chimney and mitigate further movement away from the house.

With the rods installed, further settlement of the foundation would probably cause exterior cracking to increase on the brick face (rods pulling back, settlement bending the chimney) so the whole assembly should be monitored on a regular basis for the next couple of years to ensure things are still stable.

Underpinning with piling would be a second step to help minimize settlements and the resulting cracking. Even with helicals I would recommend monitoring.

 

[phamENG]

I agree this doesn't look too bad. Important to check the mortar joints and make sure the mortar is still sound. As far as "how much" is "too much" I'd say run the numbers. It's a hollow, unreinforced masonry column. Do a P-Delta check on it and see what amount of lean produces a moment that a) destabilizes the foundation or b) exceeds allowable flexural tension perpendicular to the bed joints.

I'll counter milkshakelake - it's usually a combination of straps and bolts. For retrofits, it's usually a steel strap that goes around the outside of the chimney and then gets bolted to a tension tie either fastened to the joists or to blocking between joists that gets nailed to the diaphragm.

These are pretty standard details in seismic country where the vastly different response of a masonry chimney and light frame wood house would have them slamming into one another irreparably at the slightest ground movement. (See IRC R1003.4 Seismic anchorage.)

 

[milkshakelake]

The only issue I see with a strap is how it'll be tightened enough to restrain the chimney. If it's used like a belt around the chimney, it's flexible enough that the corners could pull out. A bunch of bolts around the chimney should mitigate that because at least it'll be in tension between the bolts. If this is the solution paired with what JAE suggested, I imagine any generic 3" or 4" 18 gage or 20 gage Fy=36 ksi strap should work. They can take crazy amounts of tension for how thin they are, like 2 kips or more. I wouldn't go to 2" straps just because of edge distance and tolerance issues.

 

[zero1238]

This is all great info, can't thank everyone enough for the tips. I've got my homework now and will definitely look into these products/analyses. Appreciate it!

 

[phamENG]

I agree, JAE's solution is cleaner and looks better...but it may be tough to set those effectively depending on the size of the chimney. I've seen it used on large chimneys with several wythes of brick to get into...but a lot of single wythe residential chimneys just don't have the 'meat' to get it in and not risk damage to the flue.

Yes, the 'belt' is tacky and prying at the connection is an issue...but one that can be accounted for. They're usually cheaper (a lot less labor) and, if painted the same color as the brick, often go unnoticed by the casual passerby.

 

[Compositepro]

Does not strapping to the structure in the case of settlement ultimately result in the chimney hanging from the structure rather resting on a foundation?

 

[zero1238]

That was my original hesitation with the strapping.

 

[XR250]

I can't imagine strapping one of these to the framing. Let's say you have a 10k chimney with 36" projection from the house with attachment at 20 ft. above the footing. That creates a sustained 1,500 lb force on the roof diaphragm that has to be dealt with. I would not touch that.

 

[JAE]

The photo showed about a 2" lean out from the house.

If the whole chimney weighed 10,000 lbs. and you tied-back at floor and roof, you'd only have about 60 to 100 lbs. of force at each level.

(10k x 1" lean at mid-height) = 10 in-kips OTM

Height of combined tie-backs (floor and roof) = 15 ft. (mid-way between floor and roof and assuming flexible diaphragms equalizing the force on each level)

10 in-kips = P x (15ft x 12"/ft)
P = 56 lbs.

Your resisting lever arm vertically is way higher than the 2" outward lean.

 

[XR250]

@JAE,

I honestly did not look at the original photo. But, yeah, that would work in this case.
Most of the ones I get will not work though.