Masonry Chimneys

[globe trekker]

Greetings to all once again!

I am seeking input on masonry chimneys. I have a "new" SFD with construction plans proposing

to construct a 37 ft. tall (from grade) masonry chimney. The plans indicate a 20" deep footing

with 2 - matts of #5 dia. steel reinforcement bars, spaced at 9" on center each way. The SFD

will be located in Seismic Design Category B.

I am looking for input and ya`lls experience moreso than a question or two. 37 ft. tall seems

pretty tall to me to have only a 20" deep footing (i.e. - lots of weight). We are using the

2006 Edition of the IRC.

What say ye experts out there? Thanks!

.

 

[Francis Vineyard]

I'm not an expert but it's okay in accordance with

R1001.2 & R1003.2 Footings and foundations.

Footings for masonry fireplaces and their chimneys shall be constructed of concrete or solid masonry at least 12 inches (305 mm) thick and shall extend at least 6 inches (152 mm) beyond the face of the fireplace or foundation wall on all sides. Footings shall be founded on natural, undisturbed earth or engineered fill below frost depth. In areas not subjected to freezing, footings shall be at least 12 inches (305 mm) below finished grade.

Another perspective using Table R403.1 an equivalent spread footing on 1,500 lb. bearing soil would support a two-story 8" solid or fully grouted masonry foundation wall.

Francis

 

[JBI]

Footing depth is more a function of projection factor than weight supported.

 

[DRP]

I'm guessing your gut is really saying "anchorage", I see nothing that would preclude an unsupported 500' chimney. Anchorage is not required in B, but it would be a good idea... as would vertical reinforcement in the 4 corners hooked under the footing rebar.

 

[Mark K]

It all depends on the soil properties. Without a geotechnical engineer you really have no rational basis to make a decision.

The big concern is what will keep the chimney from falling over in an earthquake.

 

[Builder Bob]

The width of the footing has more importance than the depth of the footing......... the weight of the chimney will be displaced over the width of the foundation ,,,, kinda like placing a sheet of plywood on thin ice to walk on it..... if the material placed is to heavy, it actually makes the situation worse than better. The reason why foundations are deeper than the minimum the code requires is usually for uplift purposes and they are trying to use the dead weight to overcome the uplift...

 

[JBI]

Builder Bob, there is also something we Nawthunuhs call 'frost depth' in some parts of the country. ;-)

 

[Drepani]

Lots of variables: seismic design category, soil class, frost depth, wind load, area and face dimensions of the chimney, location with respect to the roof slope and height. For a chimney of this substantial height, you should be engaging a qualified design professional to work it all out. I stress qualified, because being licensed or registered doesn't automatically make them proficient in masonry chimney design.

 

[steveray]

Great first post and welcome to the Forum!

Lots of variables: seismic design category, soil class, frost depth, wind load, area and face dimensions of the chimney, location with respect to the roof slope and height. For a chimney of this substantial height, you should be engaging a qualified design professional to work it all out. I stress qualified, because being licensed or registered doesn't automatically make them proficient in masonry chimney design.

 

[Builder Bob]

Builder Bob, there is also something we Nawthunuhs call 'frost depth' in some parts of the country. ;-)

We have a frost depth too... i looked it up one day --- I think it is a 1/2 inch where I am at