How to Determine Loads on a Cantilever Concrete Slab

Cantilevered beams and floors extend into open space, without any apparent supports.

Cantilevered objects, such as buildings, beams or concrete slabs feature direct support under one end, while the other end extends past the foundation wall into open space. A cantilevered concrete slab rests on the ground at one end and extends a few feet past the foundation wall on the other end. Cantilevered buildings and slabs give architects the ability to create intriguing building designs. Cantilevered designs also create unique stresses on the suspended slab, which must be carefully engineered.

Things You'll Need

• Calculator

Instructions

1. Measure the length which the slab hangs into the air, extended beyond the foundation wall. For this example, we will use a slab which extends 6 feet past the foundation wall, which is 20 feet long and has a thickness of 9 inches.

2. Calculate the total weight of the cantilevered slab using the concrete's density and volume. Concrete's density ranges between 52 – 103 pounds per cubic foot. We will use 100 pounds per cubic foot for our example. Therefore, the total weight of the cantilevered slab is 100 pounds/cubic foot. * 6 feet * 20 feet * .75 feet = 9,000 pounds.

3. The shear force pressing upward, or bending moment pushing downward (these forces are balanced in any static system) on the concrete slab by the foundation wall is 9,000 pounds, or 450 pounds per linear foot. This force diminishes uniformly as you move farther away from the foundation wall. Therefore, the concrete must be able to withstand a shear force of 450 pounds per liner foot or the cantilevered slab will eventually break.

References

• Efunda: Loading Options
• Free Study: Edexcel National Certificate Mechanical Principles
• Everything About Concrete: Density of Concrete

Resources

• Fides Software: Calculation of Cantilevered Walls
• Innovate US: What Is a Cantilevered Beam