Structual attachment of new foundation?


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Old 10-23-06, 10:11 AM
C
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Structual attachment of new foundation?

Getting ready to pour a new slab on grade foundation for an addition we are building. Should I consider structually attaching (ie running rebar into) the new footing wall and/or slab to the existing foundation? If so, how is that done?
 
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Old 10-23-06, 06:58 PM
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Your structural engineer shold have specified what you will require. If you are winging it, then it will depend upon how the addition is tied to the house. Is there a movement joint in the walls between the new and old or are you tying directly to the structual elements of the framing? What is the roof detail?
 
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Old 10-24-06, 11:03 AM
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Going through the permit process now on a sizable addition now.

Our structural engineer and the Florida Building Code requires connection methods to the existing slab. Will assume the building code in your area will require something similar as we believe the "joining" of slabs for habitation to be standard engineering practice.

He has us embedding #5 rebar, 4 inches into the existing footing cemented by epoxy, extending within three inches of the far end of the new footer.

Good Luck
 
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Old 10-24-06, 11:16 AM
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somehow, dont see my edit....probably fat fingered

The rebar embedded is required every 6'
 
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Old 10-24-06, 11:19 AM
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Structural attachment of new foundation?

You two (CougGuy and hopro) are talking about two different things. One is apparently a "slab on grade foundation" and the other is for a "footing"

CougGuy - according to your other post, you have a footing with a stem wall and them some sort of a possible continuous pour with a "slab on grade". It is difficult to figure out what your engineer (if you have one) has designed for you. Arbitrarily tieing the slab to the stem wall could lead to a lot problems if you do not have great soil and compaction under the slab or if your slab is not designed to span the 25'.

Tscarborough had a good point that your structural engineer should tell you how to build instead of just putting things together.

Hopefully, you will have an inspection to save you if you have a DIY design. It is a shame to waste good free DIY labor on a impractical design.

Dick
 
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Old 10-24-06, 12:36 PM
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Checked with the city: WA code requires a min of two rebar (#4 min), one in the footing and one in the stem wall to attached to the existing foundation. They recommend the rebar protrude at least 3" into the existing foundation (epoxy glued) and about 10" into the new footing/stem. Based on a conversation with the inspector, I will put two rebar in both the footing and the stemwall. There's no requirement to attach the slab to either the new stem wall or to the existing foundation.

Dick -- you said attaching the slab to the the new stem wall or existing foundation could cause problems. Would you recommend not doing that then? The slab is not designed to be spanning.
 
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Old 10-24-06, 01:15 PM
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Structual attachment of new foundation?

Regarding the potential problems -

I referred to the situation where you have soil under the slab that settles due to quality and degree of compaction. If there is settlement (as little as 1/4"), the slab would be unsupported and would have to span between the stem walls if the slab was connected or bearing on the stem wall.

It it is bearing on the stem wall, it could crack. If it is connected with to the stem wall with dowels, the dowels would shear or bend and crack or shear the concrete. This is one of the problems that you can create by connecting to many things together - you create conflicts. Especially common when concerned with settlement and seismic design.

Depending where you are in Washington, your code may require enough steel in the slab. If you have a one story home with no interior wall loads or garage loads, you may be OK.

In many areas, the floating slab is truely floating and is not connected to the stem walls. In that case, the soil can settle slightly and the slab would follow with a very small amount of deflrction, but no extra load or different loads and chance of cracking.

The benefit of a true floating slab is that it has less reinforcement since it is always supported by the soil and not forced to span over a void.

Glad to see you are in contact with your local inspector. He is there to see the actual situation and materials and does not have to rely on the internet.

Dick
 
 

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