shear wall anchored footing on rock

[mudman54]

tall building designs are increasing the use of shear walls which are analyzed for overturning and sliding on rock. where the safety factor of overturning moment divided by stabilizing moment is less than 1.5, additional consideration is required by building codes. rock anchors are an alternative to resist the uplift forces and also to resist the base shear between concrete and rock.
my question is how the structural engineers should apply the lock-off loads from the rock anchors to their analyses. i am sure it is an n * tan(phi) resisting force for each anchor, but feel that the dm7 type friction factors for concrete on sound rock might be too low. with all that pre-stress it would seem that a higher friction factor could be used. does anyone have any design guidance that might apply?

 

[Panars]

In my opinion, the anchor load would increase the sliding resistance by increasing the normal force (the "n" in your equation), but the friction angle between the concrete and rock would remain constant (at least theoretically - as in soils, there tends to be a reduction in phi at higher stress levels.)

 

[jdmm]

I would think that it is either the frictional component and the lockoff OR the capacity of the tie-down anchor or the combined effect. Remember that you would not only rely upon the friction but the tension in the anchor itself. If this does not provide sufficient capacity, and the rock is very strong, try embedding the wall and using the intact strength of the rock.

 

[Panars]

I was thinking of a vertical anchor, perpendicular to the plane of sliding. But if the anchor was inclined, then I agree with jdmm.

 

[mudman54]

thank you panars and jdmm for your reply. the anchors are vertical, but could be inclined to counteract the direction of sliding. we are taking the lock-off load times tan(phi) as a resisting force. i'm still trying to justify a higher phi angle once the rock is clamped together.