ASDIP Foundation – Strap Footings Design

ASDIP Foundation
About ASDIP Foundation:   Overview | Benefits | Features | Documentation
Modules:   Spread Footings Design | Strap Footings Design | Combined Footings Design

 

A strap footing is one that usually supports two columns, and therefore is a special type of combined footing. 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 connecting it with the adjacent interior footing by a strap beam. Since this beam is subjected to a constant shear and a linearly varying moment, which are the characteristics of a cantilever beam, this system is called strap footing or cantilever footing.

The use of a strap footing may be justifiable under conditions where the distance between columns is large and a large excavation area must be avoided. It is common practice that the bottom surfaces of the exterior footing, the strap beam, and the interior footing be at the same elevation, but the thickness of each element may be different, depending on the strength requirements.

This module 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 for the interior and exterior footings, 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 for the strap beam. The concrete design is based on the Ultimate Strength Design Method of the ACI 318. Load combinations per the ASCE 7. Both columns may be eccentric in the longitudinal direction.

ASDIP Foundation Strap Footings Design Input DataInput Data

The required input data includes the footings, strap 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, to be combined internally. Alternatively, you can model a set of pre-combined loads.

Example

Consider a strap footing to support two columns 18′ apart. subjected to the following loads: P=90 k, Pu=145 k. Per the soils report, the gross allowable soil bearing pressure is 3.6 ksf. Find out the footing dimensions and design the steel reinforcement.

ASDIP Foundation Strap Footings Design Example

ASDIP Foundation Strap Footings Design OutputOutput

This module checks the overall footing stability under the service combined loads and performs the concrete design of the two footings and the strap beam, which includes the one-way shear, punching shear and bending moments under the factored combined loads. In each case the controlling load combination is identified and reported.

For a quick overview of the design results click the “At a Glance” tab. A more detailed step-by-step calculations are available at the “Detailed” 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. Use the Print Preview command to see a preview of the pre-formatted report on-screen, which includes the color graphics generated by the software.

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.

ASDIP Foundation Strap Footings Design OutputASDIP Foundation Strap Footings Design Output

 

 

 

 

 

 

 

 

 

Explore ASDIP Foundation

About ASDIP Foundation:   Overview | Benefits | Features | Documentation
Modules:   Spread Footings Design | Strap Footings Design | Combined Footings Design