Building a raised hearth garden fireplace

[angelboy]

I'm planning an outdoor fireplace and wanted to know your thoughts on the foundations.

This is the plan and as you can see it's a raised hearth.
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My plan would be for the buttresses to go down to foundation level and then build the hearth around it on a 150mm slab.
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The back wall will be a single skin and double at the fireplace which will connect to 2 columns. I'm raising the floor level to 150mm but just putting down flags so no floor slab.
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Each column will sit on pads (900x900x600) and then a shallow reinforced ground beam to take the single courses of bricks between the columns which will created a continuos foundation.

Does this sound OK?

 

[noseall]

What size (footprint) is the structure?

Will the roof be open or will it be covered?

I'd think about how I was going to finish those pillars at the top, especially if the roof is going to be open. Perhaps stop the masonry at the timber ring beam (bottom) and opt for a fascia/soffit arrangement perhaps. Filling in with little bits masonry will be a bit of a faff and will be subject to timber movement.

Look at resisting torsion, roof spread and wind lift especially if you intend covering the roof.

The fireplace needs corbelling down once you get above the throat of the chimney or else it will look too top heavy.

If the structure is to remain open then you may be wise to include some kind of hood over the fireplace opening to protect from rain etc and gather any errant smoke.

Think about where you intend terminating the chimney regards overall height.

 

[angelboy]

Sorry, I've not finished modelling the roof or the chimney.

Yes, the roof will be covered with clay Rosemary's and timber fascias and soffits.

I'll be putting collars on the roof trusses within the bottom 3rd of the length of each truss.

The chimney will be corbelled and not just a large block (it was easier to extrude the model in sketch up but it was originally done just to give me an idea of the scale I wanted). I'll ensure that it terminates within the regs.

The pillars will be filled with concrete as the span between the pillars is planned to be steel. I'll be adding some sort of padstone to the top for the steel to sit on and then cladding it with timber.

The overall footprint is a tad under 6m square so it's a decent size. It's further than 2m from any boundary and its well under 4.2m so PD rights and no building regs. It is a grey area, I think, with the fireplace as its not heating a habitable space but I will check.

I'll post my foundation design when I'm at my desktop tomorrow.

 

[angelboy]

The pads are 900x900x600(min) with the ground beam 300x225 (with a 600 depth at the rear wall) and one course of engineering bricks to bring it to ground level.

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The section that I've left out is the space for the brick & flag design but I've done this so you can see how the column follows through direct to the pad.

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I want to know if my logic makes sense here with regards the two plinth courses around the column. The first plinth extends 50mm which is fine. The lower plinth is 75mm out so rather than build it into the column and then have to extend it into the pad - which would then need to be bigger - I thought it would be OK to be added as you see in the picture below. (Left is what I'm proposing, right is what I don't really want to do).

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The columns will be infilled with concrete and I've drawn in some rebar extending into the pad (like gate pillars) but is this an overkill?

Structurally, my logic is that the ground beam will connect the pads together and also take the couple of courses of brick that will raise the floor level (on the 3 open sides). The forward columns will be taking most of the weight from the steels and part of the roof and the rear columns will be sharing the load along the back wall.

The 3 steels required would be 4.7m (with a 150mm bearing at each end) but I'm unsure of the width, depth and KG yet. I've seen plenty of simple beam calculators on the web but I'm unsure how to use them effectively to get the sizes I need.

Does all this make sense or I'm I off anywhere or should I be doing something differently? Any tips, tricks or ideas are most welcome.

 

[angelboy]

Any thoughts? I'll assume that all is good.