Column footings

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Column footings

Postby JungleBiker » Fri Dec 16, 2011 7:04 am

Just looking at the photo by Dozer here: http://www.coolthaihouse.com/cthpics/di ... play_media
The rebar for supporting the columns is placed on top of a gravel sub-footing but I've read elsewhere that that there should be 3 inches of concrete covering any rebar that is in the soil to prevent corrosion of the steel. So I think there should have been a 3 inch thick concrete sub-footing below the rebar rather than gravel.

Also in the same album Dozer recommends to make the base of the columns 60 cm square up to the soil surface and says Thai builders don't usually do this but just top up with soil instead. He says his way will help prevent any rocking but wouldn't the footings for the walls do that job (prevent rocking) if they're positioned below soil level?

Just wondering.
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Re: Column footings

Postby Roger Ramjet » Fri Dec 16, 2011 7:50 am

JB,
The columns don't actually support the walls, they support the roof and other floors, the beams support the walls. There are various methods used in Thailand, but you are correct. The rebar in the piles should be exposed, then the ground around the footings should have gravel and sand lade to stabalise the soil under the footings and then concrete should be poured on top of that before attaching the rebar to extend the columns.
It depends on the weight the columns will carry, whether it is one or two or three or four stories, which dictates the size of the footings, rebar and depth of the footings.
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Re: Column footings

Postby dozer » Fri Dec 16, 2011 8:17 am

The rebar for supporting the columns is placed on top of a gravel sub-footing but I've read elsewhere that that there should be 3 inches of concrete covering any rebar that is in the soil to prevent corrosion of the steel.
That is correct about having the concrete cover the rebar. I have seen a lip poured on the bottom and then the rebar frame placed on the lip, but the best method imho is to use spacers to lift up the frame and then have the sub-footing encased in one contiguous pour.
another footing shot
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Re: Column footings

Postby MGV12 » Fri Dec 16, 2011 9:13 am

Roger Ramjet wrote:JB,
The columns don't actually support the walls, they support the roof and other floors, the beams support the walls.


What supports your beams then Roger? :)

“Some days I am an optimistic pessimist ... other days I am a pessimistic optimist”
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Re: Column footings

Postby John » Fri Dec 16, 2011 4:18 pm

Roger Ramjet wrote:JB,
The columns don't actually support the walls, they support the roof and other floors, the beams support the walls. There are various methods used in Thailand, but you are correct. The rebar in the piles should be exposed, then the ground around the footings should have gravel and sand lade to stabalise the soil under the footings and then concrete should be poured on top of that before attaching the rebar to extend the columns.
It depends on the weight the columns will carry, whether it is one or two or three or four stories, which dictates the size of the footings, rebar and depth of the footings.



The columns support everything because load is transferred from the beams to the columns. In most cases the beams are sitting on the ground but this is not considered when calculating the column footing size.
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Re: Column footings

Postby thailazer » Fri Dec 16, 2011 4:46 pm

On our footings, the builder put down about 10 cm of gravel. On top of that went concrete blocks to elevate the steel. When the pour occurred, I am pretty sure that the concrete flowed right into that gravel, incorporating that into concrete. (.8 cubic meter in each footing so a lot of pressure on the gravel.) Don't use bricks to elevate the steel as they are quite porous. Even the concrete blocks aren't perfect but better than dirt. For the ground beams, or builder had one of his crew collect large stones which were used to elevate the steel. Seemed like a good solution.
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Re: Column footings

Postby geordie » Fri Dec 16, 2011 4:55 pm

If you take a look at RR,s thread building in nothaburi you will see he got a load of chairs made up from rebar to give the reabar cages support instead of the usual cement blocks cement spacers bits of brick ect ect Which can be a bit hit or miss obviously because the chairs protruded to dirt they were dipped in a rustproofer before use They looked simple enough to make with a couple of small bends
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Re: Column footings

Postby John » Fri Dec 16, 2011 5:23 pm

Image


Image
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Re: Column footings

Postby geordie » Fri Dec 16, 2011 7:53 pm

John wrote:Image


Image



nice but personally i would still want the reabar on a chair so the concrete can get to grips with the steel with a decent cover a sub base is not a real requirement as even it will be sitting on dirt ? Just a bit more work
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Re: Column footings

Postby John » Fri Dec 16, 2011 8:13 pm

geordie wrote:nice but personally i would still want the reabar on a chair so the concrete can get to grips with the steel with a decent cover a sub base is not a real requirement as even it will be sitting on dirt ? Just a bit more work



Of course the pad steel is raised a little before concrete is poured, sorry the picture does not show this.
It’s kind of an industry standard to set a level base for column pads but having said that there are many situations where they don’t. The base not only makes a good foundation for setting the steel it also allows easy and secure marking of centre lines and offsets.
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Re: Column footings

Postby trader » Sat Dec 17, 2011 8:02 am

MGV12 wrote:
Roger Ramjet wrote:JB,
The columns don't actually support the walls, they support the roof and other floors, the beams support the walls.


What supports your beams then Roger? :)


perhaps :) skyhooks :lol: :lol:
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Re: Column footings

Postby dozer » Sat Dec 17, 2011 10:13 am

I would do as the diagram indicates, except eliminate the 'poured sub base' and have the footing framework sitting on spacers to give at least a 5 cm distance from the lower surface. The footing pour should be one contiguous pour, if it is poured at two different junctures it will create a seam between the concrete layers and give less overall strength.
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Re: Column footings

Postby elgato » Sun Dec 18, 2011 10:21 am

dozer wrote:I would do as the diagram indicates, except eliminate the 'poured sub base' and have the footing framework sitting on spacers to give at least a 5 cm distance from the lower surface. The footing pour should be one contiguous pour, if it is poured at two different junctures it will create a seam between the concrete layers and give less overall strength.



I agree. Pouring onto the "sub base" results in the footing sitting on top of the base and is not truly connected to it.
I thought I was wrong once but I was mistaken.
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Re: Column footings

Postby trader » Sun Dec 18, 2011 10:38 am

MGV12 wrote:
What supports your beams then Roger? :)




silence is golden :)
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Re: Column footings

Postby geordie » Sun Dec 18, 2011 10:48 am

I don,t supose seperation from the sub base would be too much of an issue with the weight of a house pushing downwards on it ??
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