pick best foundation expansion detail (see attached)

[eng003]

What do you think is the best existing foundation expansion detail? (see attached

 

[rowingengineer]

My preference would be "B". I can't see how you could construct C unless you doing underpinning. I would suggest staggered dowel, galvanized.

http://www.nceng.com.au/

"A safe structure will be the one whose weakest link is never overloaded by the greatest force to which the structure is subjected” Petroski 1992

 

[BAretired]

I prefer Detail 'A' if the bottom steel can be developed into the existing concrete using epoxy. I am not sure why you call it an 'expansion detail' as it allows for no expansion.


BA

 

[JAE]

A

 

[hokie66]

A combination of A and B. I see no reason to go lower than the original footing, so A. However, I would install the dowels mid-depth as in B. Not a fan of trying to develop reinforcement near the bottom of a footing...just don't think it works. Just consider the dowels as transferring incidental shear.

 

[slickdeals]

I would pick Detail A as well, but want to question how you size/design the new footing.

I don't think the combined footing (new+existing) will take the total load....The existing footing will only pick up a share of the new load (if any, due to them being tied in).

 

[eng003]

Thanks all for your thoughts

BARetired - I guess I meant I am expanding the width of the foundation Do you think adding expansion capability is needed here? how would it be achieved?

hokie6 - you said "just consider the dowels as transferring incidental shear" why do say incidental? development of shear transfer between two footings is one of my main concerns. The new masonry stemwall may be sitting entirely on top of the existing footing, although I could use a larger width block to provide a little bit of direct bearing on the new footing. Maybe I should detail 1/4" amplitude intentional roughening of the side of the existing footing?

slickdeals - why do you say they cannot act as a composite footing to take all applied loads together?

A is of course the easiest and what I would like to go with if I can develop load transfer between the two footings.

B was an attempt to utilize the concrete shear capacity in the shear transfer

C complete underpinning. any experience how this would be easily constructed? how do you shore the structure?

 

[JAE]

With C you could do this in short segments (every other 3 feet) and then come back and do the remaining segments. Assuming the sustained load is manageable with the missing segments.

You would still run the risk of some settlements as this progresses.

 

[ron9876]

If you use A be sure that the embedment of the dowels is capable of resisting the moment across the width of the composite footing.

 

[RFreund]

So you guys would not underpin this.

Is this a suspended slab bearing on the masonry or slab on grade?

Would you guys underpin if this were a masonry bearing wall or a column location?

Sorry don't mean to hijack the thread.

EIT

 

[rowingengineer]

generally I like to avoid underpinning. in general I like to keep the system of footings similar, ie if the existing is strip footing i would review this option first for appropriateness. I do live in an area with large amounts of clay, hence under pinning can cause the nature of the footing behavior to change.

http://www.nceng.com.au/

"A safe structure will be the one whose weakest link is never overloaded by the greatest force to which the structure is subjected” Petroski 1992

 

[hokie66]

eng003,

My comment about "incidental shear" assumed relatively light loading, which I inferred from your section showing stud walls above. And yes, I would roughen the edge of the existing footing, but I wouldn't depend on the dowels to transfer moment.

If heavy dead loading is involved, I would reconsider and possibly underpin, depending on the soil conditions. What are the conditions? For all we know, it could be mud or rock.

 

[eng003]

This is a residential structure with sandy on-site soils. Yes, the slabs are on grade. The existing stud wall is 2x4 supporting roof only (25 ft span). Existing footing is likely 10x20. New 2x6 studwall will support 2nd floor (23 total span) and new roof section. Regarding the several comments about developing the epoxied bars for moment capacity...I don't think there is a lot of moment involved in a footing like this...this is why you see many engineers exclude transverse reinforcement in these light duty continuous foundations...it seams transfer of shear is really the issue?

 

[hokie66]

In my opinion, yes. Light loading, simple solution.

 

[RHTPE]

Why bother with less than 2 courses of CMU? Pour the entire thing with concrete up to the underside of the slab on grade. Install dowels at mid-height of the existing footing. Adhere 1/2" polystyrene to the top of the existing footing that will be over-poured with concrete.


Ralph
Structures Consulting
Northeast USA