Shop expansion

[PeterDE4]

Well, it looks like the boss is going to let me expand my shop. My current building is 14x18, and I'll be adding a 22x18 section. Due to the grade of the yard, the new section will be 8" higher than the existing shop. I've been using propane forges up until now, but I have a coal forge stored away and I want to be able to set up in the new section. The builder is proposing a block chimney with an 8" square flue liner, and there will probably be a round 8" thimble in the side to connect to the hood. Based on another post I saw on this site, that sounds like it may not be large enough. Can anyone point me to a reference so I can make sure that the flue is sized properly?
The current building is on a monolithic foundation/slab, 6" of 4,000 psi concrete that's been in place since 1992. I have a Kinyon style air hammer with a 50# ram that sits on a 1.75" steel plate, 16"x48". The total weight is 1,500#. The base plate is mounted on a double thickness of 2x lumber, so it's about 3" thick. I haven't had any problems with cracking the slab. I don't have any plans to go to a larger or different style hammer. I was planning on doing the same type of foundation/slab, with reinforcing wire and rebar. Do I need to make any special provisions for the hammer, other than letting the concrete get well cured before I start hammering? I'm a little reluctant to pour a special foundation as I may need to move it around to different areas of that shop as my needs change. Any advice would be appreciated.

[David Einhorn]

The general consensus seems to be that you will need a 12" by 12" flue, or you will choke on the smoke. Don't let him talk you into an 8 inch flue liner!

[Mick]

Peter without going into a full engineers dissertation on the design of a reinforced concrete slab your first concern should be the nature of the ground you are placing it on. A slab designed for non reactive stable sandy soil is going to be much lighter design than one for the other end of the scale, highly reactive clay soils. Forget about swampy country, that's a whole other issue. As soil test will indicate the bearing capacity of the ground and is generally a major consideration in the slab design along with the size and types of loads to be applied to it.
Your next concern is proper preparation of the site, making sure of levels and that you are in clean undisturbed soil free from vegetable matter, (think major tree roots and the like.)
Next make sure the reo is placed accurately, and that the concrete they deliver is at the correct water content, and when poured it is vibrated correctly to remove all voids.
Make sure all the correct protocols are followed for curing the concrete and 28 days later your slab should be at full design strength.
Having said that a 6 inch slab is rule of thumb standard for light industrial use over here and there is more slab failure from bad site prep and soil movement and badly poured and cured concrete than there is from excessive loads.
But most important make sure you give yourself a nice low gradient transition ramp between the two floor levels that 8 inch step would be a killer.

[ironstein]

i agree with Mick whole-heartedly. Some thing to add being that i am a union ironworker that deals with reinforced concrete on a daily basis in the greater los angeles area (earthquake country). I would throw some extra rebar in areas that you think may be prone to cracking, or even throughout the whole slab. 5/8 rebar at ten or twelve inches on center would give you a seriously strong slab. I have worked on slabs designed to be used for material handling with heavy forklifts and machinery which were the latter. The final suggestion would be to saw cut the slab in sections to relieve stress, or add felt joints, this greatly decreases cracking, and gives you several independent slabs. if it were me designing my own shop, i would pour an elevated mechanical pad with horizontal and vertical rebar twice as thick as the slab surrounding it, and have a ramp so that i could place my power hammer (if i had one) on an isolated super strong, highly reinforced pad. Depending on your budget, you could go with post tension cables in your slab. These cables are stressed to a few thousand pounds per square inch after the slab is poured and in effect they "hold" the slab together and thus prevent cracking. That was probably more info than you needed but it is what i do when i'm not blacksmithing. if you need any detailed info pm me.