Rigid metal frame 50x100 building considerations

[inmaestro]

I've built several steel buildings of similar size. The cost to have an engineer design footings is well worth it. S/he will consider uplift, wind loads, local soil bearing, etc.
That being said, a trench footing 42 " deep with standard piers (the bldg manufacturer might have 'stock' pier details) can be dug. No forms needed if your soils are sufficiently stiff. Just dig with appropriate size bucket.
Rebar cages at piers and in foundation walls should be engineered for your site.
Perimeter foundation insulation is adequate for the floor. Just be sure to use isolation joints at columns. The floor can then float.

Good luck. Share photos when you build.

 

[5030tinkerer]

The building is a clear span, yes. I recognize that the posts will be tied to each other on the common walls if I used a conventional footing or near grade beam system. The question is how to best tie the posts across the span from each other to each other when using a floating slab. The present thought is that cables are the best way to do this, but you guys have convinced me that I need to engage an engineer. Of course, the other option would be to tie the slab to the footings, thus tying the posts to each other across the span and really everywhere, but then the issue becomes what prevents the slab from heaving if the building isn't heated 24/7.

Merely digging a trench of the appropriate width using the proper width bucket seems fine, but a possible waste of concrete. I had envisioned having, say, a 24" wide by 8" tall footing at 42" deep and then maybe a 6" wide frost wall going up from there to just above grade for the posts to rest on.

If I take, say, a 24" wide bucket and dig a trench 42" down and don't use forms, I am stuck with a 24" wide frost wall footing - likely excessive in my application. Of course, I could use a 12" wide bucket, but that still results in the same issue (albeit to a less extent) and, at that point, I begin to be concerned that the footing wouldn't be wide enough to bear the weight of the structure without sinking.

 

[Duffster]

The building is a clear span, yes. I recognize that the posts will be tied to each other on the common walls if I used a conventional footing or near grade beam system. The question is how to best tie the posts across the span from each other to each other when using a floating slab. The present thought is that cables are the best way to do this, but you guys have convinced me that I need to engage an engineer. Of course, the other option would be to tie the slab to the footings, thus tying the posts to each other across the span and really everywhere, but then the issue becomes what prevents the slab from heaving if the building isn't heated 24/7.

Why would they need to be tied together?

Merely digging a trench of the appropriate width using the proper width bucket seems fine, but a possible waste of concrete. I had envisioned having, say, a 24" wide by 8" tall footing at 42" deep and then maybe a 6" wide frost wall going up from there to just above grade for the posts to rest on.

If I take, say, a 24" wide bucket and dig a trench 42" down and don't use forms, I am stuck with a 24" wide frost wall footing - likely excessive in my application. Of course, I could use a 12" wide bucket, but that still results in the same issue (albeit to a less extent) and, at that point, I begin to be concerned that the footing wouldn't be wide enough to bear the weight of the structure without sinking.

You have to balance the waste of concrete vs the waste of backhoe time and form rent .

 

[Raspy]

5030,

I'm building a 48X60 clear span steel frame house with a 12X60 foot expander. It will have a 700 sq ft porch, a 36 x 28 shop/garage and a two bedroom 2 bath home.

We poured square footings that went about 2 feet below the frost line of 18". Each post has one of these footings and the land is sloped, so, in the front, the footings are abut 5 feet high and in the back, right at ground level.

Then a stem wall section was poured between each footing. The forms for the footings had steel sticking out to tie into the stem walls.

Finally, the slab will be poured with radiant heating throughout that is tied to a grid of bar on about 9" centers.

Here are a couple of pix.

These pix are a bit old now. We have the roof on but no walls yet.

 

[Raspy]

5030,

I'm not a casual as some about just digging a trench for the footing pour. We excavated the area to the level of the bottom of the footings. Then we compacted and watered till we could not get further compaction with a vibrator and a jumping jack. Next the form boxes were set and the rebar installed. After these were poured and stripped we formed the stem walls and poured them. Then we backfilled and compacted around the outside and filled the inside with 1 1/2 inch rock and compacted as we went.

Our elevation is 4900 feet and we are on decomposing granite.

The slab will not be floating but will be tied directly to the footings.

If you are worried about heaving in your slab, you might consider void forming it. This leaves suporting stem walls under the slab with compressable temporary cardboard or insulation to pour on top of between the underneath supporting walls. It's an excellent method for expansive soils.

 

[5030tinkerer]

5030,
If you are worried about heaving in your slab, you might consider void forming it. This leaves suporting stem walls under the slab with compressable temporary cardboard or insulation to pour on top of between the underneath supporting walls. It's an excellent method for expansive soils.

I am not following what you are describing. Could you elaborate? Amazingly cool images, btw. Thanks for sharing.

 

[Raspy]

5030,

I'll try to describe it better.

In void forming there is a perimeter footing and a grid of footings under the slab area. The slab is supported by the grid of footings or stem walls under the slab. These might be set up at about 4' on center, for instance, and are deep enough to prevent them from heaving. Between the stem walls under the lab, where the slab would normally sit on the ground, thick cardboard is laid down first, about 2" thick. Then the slab is poured and rests on the cardboard between the stem walls and on the stem walls directly. No cardboard is placed on top of the stem walls.

Over time the cardboard deteriorates and leaves voids. Then when the ground swells there is a place for it to move into instead of lifting the slab.

This design could also be used with underslab insulation that would eliminate the need for the insulation to support the slab as it would do if there was no other means of support inside of the perimeter.

I deal with insulated slabs all the time and I'm concerned about supporting a slab with insulation. I won't do it on my house with a concrete slab and radiant heating. Even though many say that underslab insulation brings a benefit.


John