Eccentric Continuous Wall Footings

[JRSE]

I have a two-story masonry wall on a property line and it is to be supported via an eccentric footing. I have read other threads, but could someone post the mechanics (sketch of free body diagragm) of utilizing the slab adjacent to the resist the rotation. Does increase in footing width help or hinder the sitution.

for this situation it is an 8" cmu wall that is two stories. the slab is 5".

to all thanks for the assitance in insights!

 

[BAretired]

Perhaps it would be an idea for you to provide a cross section of the building. What is the elevation of the slab relative to the footing? What is the wall load per unit length? What is the proposed footing size? What is the construction of the next floor above the 5" grade slab?

We may be good, but we are not mind readers.

BA

 

[JRSE]

I have attached the requested section. My original request though is a free body diagram of how the slab on grade is utilized to restrain the eccentric wall footing from rotating. How do eccentric walls perform when subjected to lateral load - wind and seismic. Thanks again!

 

[JRSE]

Currently, I am approaching this by treating designing the footing to span to a combined footing at the glulam/girder locations which are tied via strap footing back to interior column footings. The footing between these locations is resisting the eccentric load by torsional resistance of the grade beam. I wanted to know if this is correct or I should use the slab to resist the eccentric loads. If so, then how? If I treated the eccentric footing using classical methods of eccentric footing analysis it results in a large 7 foot wide footing which I don't agree with. Any insights would be great.

 

[BAretired]

If the wall is on a property line, is it wise to use such a shallow footing? If the neighbor decides to build to the property line with a basement, he will undermine your footing.

You have a 4'-0" square footing centered under columns. This means that the floor and roof loads are not eccentric to the footing. The only eccentric load is the wall itself which is 24'-6" high and weighs less than 1500 pounds per lineal foot.

Using the 5" slab to resist rotation is not a good solution as it unnecessarily restricts future changes to the floor. A narrow strip footing could be used between the square footings to reduce wall eccentricity. The wall could be reinforced to resist footing rotation if it is adequately tied to the building at the next floor.

BA

 

[JRSE]

I have kept the foundation at 18" because this building is built adjacent to exiting building with foundations at the same depth.

Correct, the CMU wall along this line, a total of 20 ft long, is the most eccentric element. The glulalm spacing is 10ft, one beam at each end of the wall and one in the center. The roof glulam beam at this wall location will be supported by this wall. The floor glulalm beams will be supported on a masonry pilaster to assist in moving this reaction farther away from the property line. A pad footing will be provided at the glulam locations. These footing will be part of a combined footing system that is connected to interior columns that are 19 feet away from the exterior face of the masonry wall. The combined footing will redistribute the eccentric loads, as well as, the torisional forces from the eccentric wall (transmitted from the narrow stip footing) between these locations. I find the span short enough to numerically justify that the short span of eccentric wall footing, approximately 6 feet in length (designed to spand the full 10ft of the combined footing), will be sufficiently stiff to transmit the torisonal effect from the eccentricity of the wall to the pad footings. The footing width shall be determined by an assumed uniform bearing pressure underneath. For lateral forces, parallel to the wall, the total "I" shaped wall foundation configuration shall be utilized.

the total gravity load, live and dead is approximatley 2.31 klf. The wall dead load is 90 psf (8 inch fully grouted) * 24.5 feet = 2.205 klf which governs the design. per the calcs, a 24"x18" wide cont footing with 3-#5 T&B and #4@12 ties is sufficient to transmit the eccentric wall loads to the pad footings. This is for gravity alone.

I agree in using the 5" sog to restrain the footing is unacceptable. It will difficult to develop sufficient rigidity within the masonry wall - 8" with #5. Will investigate more. It is a great suggestion. I think the concept to transmit the eccentric forces via torision to the combined footings provides the clearest load path.

I really appreciate all your feedback. Thank you! This is my first eccentric wall foundation. I wish there were more design examples that fully illustrate these concepts - tied foundations and the resultant bearing pressures from utilizing fixed fixed beams to transmit eccentric loadings, foundation pressures by restraining the footing from rotation via a reinforced concrete slab on grade, and foundation bearing pressures via restraining the footing rotation at the wall. Maybe someone can write a book titled "Foundations - from the classroom to office".