Strip Footing On Boundary 2

[ETET001]

Hi Guys, I am just wondering how you guys deal with strip footing on boundary. The project I am working on now is a 3 storey building. Allowable bearing capacity is 100kPa for 100mm into silty clay and 250kPa for 600mm into silty clay.
As the external walls are adjoining to the boundary so there will be eccentricity for load apply on the strip footing and as a result, the end reaction will be over 250kPa due to this eccentricity. I am wondering how you guys deal with this? Thank you.

 

[JAE]

If you can't avoid the eccentricity then some type of internal stabilizing strap footings or underground grade beams could be used to take the edge rotational twist away from the edge footing.

Sometimes we've used a series of cantilevered underground beams, launched off of spread footings placed just inside the exterior wall. The grade beams extend back some distance to a sort of counterweight footing to hold the back-span down.

This is fairly expensive but a solid solution.

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[ETET001]

Thanks for your reply JAE. How do I know how many internal strip footings are enough using simplified method? As I have slab on ground sitting on the strip footings as well. Can I take that into consideration and how do you normally comp it if so? Thanks.

 

[civeng80]

What I have done in the past is to bring the wall in 75mm or 100mm so that it is concentric or close to concentric with the footing, losing a bit of land but with confidence and economy in the concentric strip footing.

Sometimes it is hard to make the strap beam to work because there is nothing to strap too. Tying to grade beams is also pretty futile in most cases.

 

[ETET001]

Thanks for your reply civeng80. Unluckily I can't move the external walls. They sit just next to the boundary with no gap.

 

[JAE]

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[DETstru]

Use the slab on grade to balance the rotation.

You have your footing, a rigidly attached concrete stem wall, and a slab attached to the top of the stem wall. The slab restrains the rotation via horizontal reaction by friction against the subgrade.
All you have to do is make sure to dowel your slab to the concrete stem wall (instead of the typical isolation detail) and to make sure you have enough slab to generate the friction required for the reaction. Much cheaper than grade beams.

You can prove this works if you build a little RISA model with/without the slab. Like this:
(model with slab on the left, model without slab on the right. note how the one on the left deflects vertically and the one on the right rotates significantly beyond what would be acceptable/possible)

 

[hokie66]

JAE's model is the way to address this if the eccentricity is significant.

 

[ETET001]

Thanks guys. Very helpful information.

 

[civeng80]

That is a good detail which may work. But really depends on the next door footing details.

 

[kmart30]

transfer bars!

 

[civeng80]

You mean like the attached detail ?
It can work but I have seen cases where the footing still rotated.

 

[civeng80]

DETstru

The walls are pinned, difficult to get a rigid connection between walls and footing.