Galvanized steel pipe chimney as support for wooden roof trusses at center of round-ish house

We are building a house this summer. Before starting this project, I wish to ask for advice from others who may have had experience in similar project(s).

I want at the center of the round house, a large galvanized pipe. This pipe will be supported by a 8" concrete pad which will also be home to our rocket stove for cooking and occasional heating. Primary heating will be by radiant flooring powered by photo-voltaic/battery for the pumps and heated by a separate batch rocket heater. The chimney pipe will have brackets welded on to support the inner ends of the trusses and as a chimney for our rocket stove.

I am looking to use this pipe primarily because I already have it. It is a galvanized pipe 13" x 17' used for street signal support. I figure if it is heavy enough to support a 10" x 47' galvanized pipe (which I also have cut into 20' and 27' pieces) horizontally from its top side, it should be strong enough to hold up my roof at its apex.

I am aware of the dangers of welding on galvanized metal.
There should be a clean-out at the bottom of the metal chimney pipe.
Drafting may be an issue. There will be a oven/mass bypass forcing the new-fire smoke and gasses out the metal chimney that is at room temperature to preheat the chimney pipe air prior to closing the bypass. This is also our primary method of cooking even in summer so the bypass will be a welcome feature.
I want to know if there is anything I should be aware of with my plans and materials. Will fumes be given off from the galvanization when it is heated by the fire? Zinc is molten at 840F/449C, but I don't find data about off-gassing at the lower temperatures. Anyone have a clue? I don't think the chimney will be getting hot enough to melt zinc even in summer.
Will the metal chimney get too hot to be able to support the wooden trusses at 10' from the floor?

Hi Anthony, welcome to Permies!  Will you have a living roof?  If not, I can't imagine that the pipe isn't strong enough to hold up a roof and snow.  Even if it gets kind of hot.  I don't know anything about RHM exhaust temps but I'm sure someone will be along shortly to comment.  

Just as a side note, have you heard about reciprocal frame roofs?  If I was doing a round building I'd be incorporating that style of roof.  No center support needed and they look cool to me.

Thank you for the reply Mike.

Mike Jay wrote:Will you have a living roof?

We will have a living roof on a round house. I will look at this closer. The internal radius (center to walls) of the roof is 16 feet, πr^2=3.14·162≈804 ft2 and I expect 125 lb/ft2 with living roof and snow load. 804 * 125 = 100,500 lb. The weight supported by the column is 1/2 of the roof. 50,250 is what I am asking the column to support. What does engineering say the column should support? Engineering Toolbox states that a 12" column 3/8" thick and 14' effective length (Top of my trusses with the trusses being attached at the top and bottom of the truss. Also, 2 feet longer and 6,000 lb less carrying capability than length of pipe I am needing) has a load bearing concentric to center of 282 * 10^3 = 282,000 lb. My 50,250 lb < 282,000 lb so I should be okay to proceed with the steel column. Now, what do I need to support the column? Our soil is clay/sand/gravel and will support 8,000 lb/ft2. 50,250 * 1.25 (25% safety factor) = 62813 lb.. Now 62813 lb. / 8,000 lb./ft2 = 7.85 ft2 concrete footer surface area of undisturbed soil or properly tamped aggregate. I plan on a 20 ft2 footer for the stove/heater/chimney with a steel reinforcements and 8" thick for support of at least 8 ft2 under the chimney pipe.

Mike Jay wrote:Just as a side note, have you heard about reciprocal frame roofs?  If I was doing a round building I'd be incorporating that style of roof.  No center support needed and they look cool to me.

I know about reciprocal roofs. We are looking more for the flatter style roof where we can integrate insulation into the same space as the support structure. The amount of work that would needed to be done to make the reciprocal roof zone 4a friendly would be beyond what we are wanting to do. I do agree, though, that it is a very interesting roof. I wish our climate would allow me to use it.

Please, if I have made any errors, feel free to correct me.

Thank you

It looks like you've done some good math considering the living roof.  282,000 lbs is a lot of weight and I'm a bit surprised that pipe could hold it up.  But I don't deal with larger structures like that so maybe someone with better experience can confirm that those numbers "feel" right.  I'm always a bit nervous when the math works out but I don't have an innate "feel" that the numbers make sense.

Hopefully more people chime in on the RMH part of this.  Good luck with it!

Mike Jay wrote:282,000 lbs is a lot of weight and I'm a bit surprised that pipe could hold it up.

I know it looks like the numbers that I found aren't right, but I looked at it more than once. Plus, my requirements are 1/5 of what the column is suppose to support. I continue to check numbers and strategies. I have thought of having a column within a column, but that's getting overly complicated. Especially when the numbers add up to a single column supporting the roof. I have a couple months before starting my project. Possibly someone will have other data to show me that my numbers are wrong or support an alternative method of support.

Thank you

Your chimney is vastly oversized.
Rocket chimneys are usually sized to match the system.
8" is large for a batch box.
13" is humongous!
I would use this pipe as the outside form for a concrete cylinder.The inside form would be 8" diameter duct, the mix would be perlite and RapidCast Cement.

Consider making your central rocket mass heater your primary heater,since it's likely to be at least  an 8"system.
Then your secondary rocket can be placed on an outside wall,or better still a porch. This will let you use it during hot weather without radiating heat into your living space

The center column will actually support 1/3 of the total load. Divide the roof into triangles, and the load is larger toward the outer edge of each triangle. This calculator will give an idea: Beam Bending Calculator
Select the beam with the triangular "shed roof" load, and look at the "shear" chart. Shear is the vertical stress in the beam, and it will equal the support load at each end. -20 at one end vs. 40 at the other end (take the absolute value for our purposes) gives twice the load for the wall end of each beam as at the column end.

Aside from draft considerations for the RMH, building code forbids a chimney from carrying external structural load unless designed for it (I expect it would take a professional engineer's stamp to satisfy any building inspector if you have to deal with that.) The 2 1/2" of insulation space around an 8" chimney would be adequate FOR RMH USE, I'm sure, but you might not get any authority to put their liability on the line for it.

What is your plan for getting the exhaust into the chimney? A hole in the side will drastically reduce the structural capacity, and I would not risk that without a professional's say-so. I suspect the only way to do it safely is to burrow under the column in the footing and come up in the base.

There is another calculator at engineeringtoolbox that gives support loads directly. The center of mass of a triangular wedge is 1/3 of the way from outer to inner edge, so you can plug 3.33 into the load box for a 10 unit beam. This will give the relative loads, or you can use all the actual dimensions and loads if you want.

William Bronson wrote: Your chimney is vastly oversized.
Rocket chimneys are usually sized to match the system.
8" is large for a batch box.
13" is humongous!
I would use this pipe as the outside form for a concrete cylinder.The inside form would be 8" diameter duct, the mix would be perlite and RapidCast Cement.

You are right! I had only been thinking of getting support from an object that I had laying around. Having proper draft from the pipe was not something I had not thought through. I also need a way of ventilating the home. Getting stale air out along with odors from cooking. I could install an smaller pipe inside the 13" pipe (or even use a smaller pipe, I have 10 1/2" and its load capability is 223,000 lb or 8 3/4" at a load holding 149,000 lb) and use the space between the two pipes for ventilation exhaust...making sure to have the openings far enough apart as to not have ventilation reverse and suck chimney exhaust back inside the house. One hole in the roof serve two purposes.

William Bronson wrote:
Consider making your central rocket mass heater your primary heater,since it's likely to be at least  an 8"system.
Then your secondary rocket can be placed on an outside wall,or better still a porch. This will let you use it during hot weather without radiating heat into your living space

I appreciate the suggestion of having the main heater be the one central to the home, but we desire no cool spots in the home.  We will be able to make sure all of the floor will be heated with radiant tubing and supplied with warm water from our boiler system in our outbuilding holding our large boiler and water storage tanks. The central heater would be for cooking and heat backup only. The central boiler will be for home, greenhouse and water heating. I don't want to have multiple fires over multiple days. I prefer to fire for a day and have heat for multiple purposes for multiple days by storing heat in 1,2 or maybe even 3 5,000 tanks of water. Will it work? How many tanks would I need? I own 3 5,000 gallon poly tanks already. The maximum temperature is 140F/60C. Using water heat, I can also ensure against becoming overheated as I currently get with my fire at camp and remember as a child growing up having to open doors and windows to cool the house because too much heat was coming from the heater. Opening doors is not an efficient way to heat a house.

William, thank you for the input

Glenn Herbert wrote:The center column will actually support 1/3 of the total load. Divide the roof into triangles, and the load is larger toward the outer edge of each triangle. This calculator will give an idea: Beam Bending Calculator
Select the beam with the triangular "shed roof" load, and look at the "shear" chart. Shear is the vertical stress in the beam, and it will equal the support load at each end. -20 at one end vs. 40 at the other end (take the absolute value for our purposes) gives twice the load for the wall end of each beam as at the column end.

AH! I did make an error in judgement. If the building was rectangular, each end of the trusses would have an equal load. Since the building is round and the trusses are supported at a central point and the walls on the other end, the area of the roof is being shared by all truss halves supported by the column is smaller than the load supported by the walls. That's definitely not a scientific explanation, but a Tony explanation. Kids, pay attention in school!. So if I take half of the distance from the central point to the wall as the radius of the roof being supported by the column. 8^2 ft * 3.14 = 200.96 ft2 of roof being supported by the column. 201 / 804 = 25%. Roof load of 100,500 * 25% = 25,120 lb. column load. Much less than the 50,250 I originally calculated. Half, really.

Glenn Herbert wrote:Aside from draft considerations for the RMH, building code forbids a chimney from carrying external structural load unless designed for it (I expect it would take a professional engineer's stamp to satisfy any building inspector if you have to deal with that.) The 2 1/2" of insulation space around an 8" chimney would be adequate FOR RMH USE, I'm sure, but you might not get any authority to put their liability on the line for it.

This is why I love forums. So much information! I believe you are right! I have decided to use a two column approach. One column inside another! The larger outer column supporting the roof structure with a smaller inner column used as the chimney with the space between the two used as a ventilation "shaft" for out-going air only. The outside of the column will be painted black to heat up in the summer time to create convection to pull air through the house for nice fresh air. Also, it can be used in the winter for pulling cooking smells (and cooking smoke) out of the house to keep it smelling clean and fresh. In the moderate temperature times of the year, it can be used to pull out air that was overheated as a result of cooking. On the roof, there will have to be an enclosure device to ensure a one-way flow out of the house and perhaps a fan to ensure a flow when convection is not enough to ensure a positive flow.

As far as how the exhaust will get into the pipes. There will have to be holes, of course. The larger column already has a hole in it near the bottom for the chimney pipe to go through. But the hole is not large enough. I would like to be sure I can get an 8" pipe through the hole to the inner column if need be in the future. I am only really interested in 5" or 6" for now for my cook stove/oven since my main heat source is elsewhere. The hole in the outer column will be supported by either a heavy wall pipe welded inside the hole. I could possibly cut a 2 foot section off of the column and cut that in half along its axis and weld each of those halves on as layers giving extra support to the column at the sight of the hole before cutting the hole. Also, I could cut loose and weld the 2" x 13" x 13" plate and its mounting structure at the top of the column where the 50' horizontal traffic light pipe section was bolted to the vertical column to the bottom where the chimney goes through to the inner column. My favored method would be to weld in a heavy walled 9" pipe in the hole at the bottom. I have the materials already. The inner column used for the chimney is a 1/4" x 8 3/4" x 27' pipe taken from the horizontal traffic signal support. It only holds it's own weight and won't rust out in my lifetime. It can be cleaned through a tee in-line with its connection. The inner pipe can be removed through the top of the outer column via crane or hoist attached to the top of the outer column...cutting the inner column off into manageable sized pieces as it comes out. Or should I think about getting stainless chimney pipe and avoid the heavy lifting? I'll have to think about this since all of the materials I talk about here I already own and we are trying to build with little purchased materials. What are the chances of having to remove the chimney pipe after it is completed?



I appreciate the questions! You all make me think more about how it can be done...SAFELY!