footing underpinning

[xxpegasus11]

It is proposed to underpin an existing footing in order to add a new floor over an existing single storey by adding a mass concrete footing approx 600mm wide by 600mm high. The soil bearing capacity is well adequate for the proposed loads and size of footing. The contractor proposes to do this in alternate sections of about 4ft long, dry packing each section a day after pouring. Reading through some posts i see theres some debate as to whether each section needs to be tied to adjacent sections through dowels of some sort. Would be grateful for any input on this from anyone whos had experience. Are dowels necessary? If so, what forces would they be designed for? Any other factors from experience/good practice that one should be aware?

 

[PEinc]

Dowels are rarely necessary and are usually avoided. The load being underpinned is usually a continuous load per unit length of wall. Therefore, there is no shear force between underpinning sections. There are not many references on underpinning but it is written that the vertical surface between abutting underpinning piers is rough enough to transfer shear without dowels or shear keys. Refere to Winterkorn and Fang's Foundation Engineering Handbook for good information on underpinning.

 

[jike]

With a section only 24" deep, it would probably be difficult to install any dowels between sections.

For deeper sections, I usually have them install #6's by pounding them into the sidewall of the excavation. When the adjacent section is excavated, those dowels are exposed and then cast into that section.

 

[xxpegasus11]

Thank you both for replying.
The argument was that loaded sections undergo some settlement before the adjacent sections carry load, hence shear developing when the adjacent section is also loaded and that a connection between them was needed for differential settlement reasons. As you both suggest, the shear between the faces would be sufficient, no bending occurs anyway, so i couldnt really see how dowels would help other than "looking better all tied together".
On the other hand, if i extend the concept to a series of strips carrying more substantial differences in loads due to say some loaded with imposed loads and some not, would one still rely on the shear interface to even out differential settlements between unconnected strips?

 

[PEinc]

Look at it any way you want; BUT dowels are not usually used by those who specialize in the design and or construction of underpinning. Especially in deeper underpinning piers. Contrary to what jike said, dowels would be easier to install in shallow piers where a worker does not have to climb in and out of the pits.

 

[pba]

I've not used dowels in the past.

If you do use dowels I would not recommend "pounding them into the side walls" This would leave you with bars protruding into the next work space where they present a danger during the excavation. If you do use dowels, drill them once the excavation has been dug/

 

[PEinc]

Design and construction of underpinning requires experienced engineers and contractors. Or, you could have a big problem. Many engineers and contractors think they know how to underpin buildings. Most do not. See the following link about an apartment building that was being underpinned.

http://abclocal.go.com/wpvi/story?section=local&id=3438587

 

[DRC1]

I have used dowels in tied back underpinning pits to transfer tie back loads without wales.However these are for much deeper pits. If you want to transfer shear reliably, I would use 2x6's to form a vertical key in the concrete. Unless you have very high lateral loads, that should work.