I am checking masonry wall bracing

[DonTee]

I am checking masonry wall bracing through internal bracing and overturning calculations. Foundation overturning is usually the governing failure. My method has been to keep the resultant load in the kern using the .6D+w service load combo. This turns out to be the worst case for most narrow footings. I also have added in a 1" eccentricity and an additional factor for P delta that might result from compression of the soil when the wall and footing begins to rotate prior to picking up load. I feel I may be too conservative. I know what my wall and footing weight is so the .6 is a pure stability safety factor. However, keeping the resultant in the kern is also a safety factor that prevents zero tension on the uplift side of the footing and limits the potential P delta. Sometimes the resultant P/A+M/S loads are quite small and I have fudged on occasion when the resultant is only a small amount outside the kern and soil loads are small. Would be interested in other approaches.

 

[BAretired]

When P is very small, it becomes impractical to keep the resultant within the kern of the section. It is permissible to consider a smaller footing within the area of the actual footing where the resultant is exactly at the kern. If the resultant intersects the bottom of footing at 'x' from the edge, then a footing of width 3x may be considered. If the resulting soil pressure is satisfactory and the footing has adequate resistance to overturning, it should be okay.

BA

 

[DonTee]

My concern is that the footing has to physically rotate in order for the soil to pick up the load and I am always nervous about the P delta increase as this rotation occurs. I have adopted your suggested method in the past and understand it. Indeed, many times the actual soil pressures are low but the question always remains as to how far to allow the resultant to shift as this shifting means that the footing is rotating. This has become a problem in analyzing masonry walls utilizing internal bracing. Many walls are built on 2' wide foundations and if you consider an 18' tall heavy masonry wall on a narrow footing your p delta can start to increase the overturning quickly. I have attached an excel program that I recently wrote to show me exactly how far the resultant has shifted for various DL factors. The specific example shown is for an 18' 8" tall 12" masonry wall on a 3' x 1.5' footing. The wind load is for 40 mph. The .9 DL is at the edge of the kern. The .6 DL has shifted outside the kern but the soil loads are low. I certified this scenario and feel comfortable with it. The question always remains as to how far you can go (the contractors ALWAYS want to go farther) and if there is definitive guidance.

 

[BAretired]

DonTee,

I think you are correct to be concerned about tall cantilevered walls resting on narrow footings. Wind pressure and soil properties are not known with precision and the consequences of a mistake are potentially catastrophic; it is a situation requiring conservatism and considerable engineering judgment.

BA

 

[DonTee]

I agree and appreciate your input.