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Existing mat slab extension.

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G.P

G.P

Structural
Apr 20, 2022
3
Hello folks,

Does anyone have any suggestions with extending an existing mat slab with different depths?

I have a 4 legged lift setup 30 ft. tall with total operation load of 60 k . Its 2 legs land on existing slab and other 2 land on new slab.
See picture attached. Is using dowel bars to connect the existing to new slab a good idea? I have red few comments on other thread where concern were local differential settlement and Heavy loads causing dowels failure. Does anyone see any concern if I use dowel rebars. Any recommendation on dowel design for this conditions would be helpful if using dowels bars is not issue.

Thanks

 
 https://files.engineering.com/getfile.aspx?folder=6a603447-dafe-4d9f-b2b9-05cbac92a442&file=sketch.jpg
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How long is the new slab? If not too long, why not make the slab thickness the same, so the joint can be better prepared.
 
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Le99 The slab is 14 X 9 ft. Any concern using dowel for connecting the old mat and new mat slab?
 
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You probably needs to make a decision if you intend to transfer shear or shear and moment across the joint. IMO, you can do either but you need to consider that when you do your checks on the existing and new mat foundation.
 
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Make sure the subgrade is well prepared, I don't see a problem with doweling, and that's the reason you should make the new slab the same depth as the existing one - better/longer shear friction surface, plenty of room for placing and grouting the dowels, the stress will be more uniformly distributed rather than concentrated at a small area. Concrete is cheap compared to the benefits it will bring.
 
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Guys,
My concern is differential settlment as the existing mat slab have huge load from the column of 3 story building while new slab is gonna take
just the half load from lift. see the attached image for more clarification.

Capture_my9lox.png
 
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Once you made the connection rigid, it acts as a combined footing with stress concentrated at the interface. There is no potential for differential settlement. If the connection is weak, then it could be a problem when the slabs separate, or the thinner slab deflects (similar to a cantilever).
 
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G.P said:
My concern is differential settlement as the existing mat slab have huge load from the column of 3 story building while new slab is gonna take just the half load from lift.
Click to expand...

Why so? With passage of time, the existing mat has likely completed most settlement and the proposed mat is lightly loaded (60 kips on 9' x 14' slab... 0.5 kips / ft[sup]2[/sup]). If new subgrade is high quality and correctly compacted, settlement of the proposed mat should be minimal.

Have you considered the disadvantages of connecting existing and new foundations?

 
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