ASDIP Foundation – Combined Footings Design
About ASDIP Foundation: Overview | Benefits | Features | Documentation
Modules: Spread Footings Design | Strap Footings Design | Combined Footings Design
A combined footing is one that usually supports two columns. If a property line exists at or near the edge of an exterior column, an isolated footing would be placed eccentrically under this column and it would tend to tilt. Overturning of the exterior footing is prevented by supporting the two columns on a common footing.
The use of a strap footing may be justifiable under conditions where the distance between columns is short and the stability of the exterior footing would be compromised. It is common practice to size the combined footing so that the resulting soil bearing pressure is uniform. To accomplish this, the footing shape is sometimes trapezoidal or rectangular, depending on the loads. A combined footing is usually analyzed as a beam in the longitudinal direction and as a footing in the transverse direction.
The program computes the soil bearing pressures induced by a cantilever footing under the action of vertical loads and bending moments, per the latest ACI design criteria. It designs the reinforcing steel, and checks the one-way and two-way shear stresses. In addition, the program generates the shear force and bending moment diagrams in order to design the reinforcement in the longitudinal direction. The concrete design is based on the Ultimate Strength Design Method of the ACI 318. Load combinations per the ASCE 7.
Input Data
The required input data includes the footing and columns dimensions, the materials’ properties, the allowable soil bearing pressure, and the acting service and factored loads. In addition, the program accepts a number of load cases, such as dead, live, snow, wind and seismic.
Example
Consider a combined footing to support two columns 15′ apart. subjected to the following loads: Pext=30 k, Pint=200 k. The gross allowable soil pressure is 3.0 ksf. Find out the footing dimensions and design the steel reinforcement.
Output
The program checks the footing stability for the service combined loads and performs the concrete design, which includes the one-way shear, punching shear and bending for the factored combined loads. In each case the controlling load combination is identified and reported.
A detailed step-by-step report is available to the touch of a tab, which is updated with every new change. In addition, ASDIP Foundation uses a pre-formated colorful text-with-values output for easier identification of the problem areas.
ASDIP Foundation generates a graphical view of the designed footing and the resulting pressures and forces, as shown. The program also generates the moment and the shear diagrams for the controlling combination, as well as a view of the construction section and elevation with the reinforcement information.
Explore ASDIP Foundation
About ASDIP Foundation: Overview | Benefits | Features | Documentation
Modules: Spread Footings Design | Strap Footings Design | Combined Footings Design