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Design of Cantilevered Retaining Walls CE A433 – RC Design T. Bart Quimby, P.E., Ph.D. Spring 2009

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Introduction A cantilever retaining wall is a system of cantilever slabs (i.e. beams) that retain soil. A cantilever retaining wall is a system of cantilever slabs (i.e. beams) that retain soil. The key is to draw the appropriate FBDs so that you can determine the internal forces. The key is to draw the appropriate FBDs so that you can determine the internal forces.

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Cantilever Retaining Wall Shear Key Toe Stem Heel

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Forces ACTING ON the Wall Wall Footing Shear Key Soil on Toe Soil on Heel Active Lateral Soil Pressure

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Reactions Passive Lateral Soil Pressure Vertical Reaction Friction ACTUAL FRICTION is not the same as FRICTION CAPACITY!

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Computing Soil Bearing Stress Resolve applied forces into a concentric vertical force and moment on the contact area. Resolve applied forces into a concentric vertical force and moment on the contact area. I x = bL 3 /12 I x = bL 3 /12 A = bL A = bL c = L/2 c = L/2 max = P/A + Mc/I x max = P/A + Mc/I x min = P/A – Mc/I x min = P/A – Mc/I x P M

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Sliding Driving Force Resisting Capacity V slide = Driving Force = Demand V resist = sum(Resisting Forces) = Capacity FS = V resist / V slide Design for FS > 1.5 Friction CAPACITY = N Not Actual Friction Reaction

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Overturning Driving Force Point of Rotation Resisting Forces M OT = Driving Force*arm = Demand M ROT = sum(Resisting Moments) = Capacity FS = M ROT / M OT Design for FS > 2.0

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Draw FBDs Stem Toe Heel

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Stem Diagrams FBD Shear Moment Demand Capacity Demand Capacity Make stem thick enough for shear Select Steel to provide flexural capacity Add T&S Steel

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Toe Shear Moment Design Shear Design Moment V u can be calculated a distance ‘d’ from face of wall since there is a compressive reaction with the wall. M u is computed at the face of the wall. Flexural Steel extends a development length into the heel and should develop within the length of the toe. Add T&S Steel

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Heel Shear Moment Design Shear Design Moment V u must be calculated at the face of wall since there is a tensile reaction with the wall. M u is computed at the face of the wall. Flexural Steel extends a development length into the toe and should develop within the length of the heel. Add T&S Steel

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The Design Process Select the overall dimensions (height, embedment, footing length and position, and estimated footing & wall thicknesses) based on stability (sliding and overturning) and soil strength (max/min bearing pressures) using service level loads. Select the overall dimensions (height, embedment, footing length and position, and estimated footing & wall thicknesses) based on stability (sliding and overturning) and soil strength (max/min bearing pressures) using service level loads. Check slab (wall and footing) thicknesses using shear criteria and factored loads. Adjust thicknesses as necessary, rechecking stability and soil strength of the values change. Check slab (wall and footing) thicknesses using shear criteria and factored loads. Adjust thicknesses as necessary, rechecking stability and soil strength of the values change. Select the flexural steel for the three cantilever slab elements using factored loads. Select the flexural steel for the three cantilever slab elements using factored loads. Select the temperature and shrinkage steel for wall and footing. Select the temperature and shrinkage steel for wall and footing. Draw the resulting wall cross section (to scale!) Draw the resulting wall cross section (to scale!)

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