Other Bearing Capacity Considerations
There are many other possible considerations in the evaluation of bearing capacity for shallow foun- dations. Some common items are as follows:
Inclined Loads. In addition to the vertical load acting on the footing, it may also be subjected to a lateral load; hence the resultant of the load will be inclined. One possible method as proposed by Merehof is to reduce the allowable bearing capacity based on the inclination of the load. However, this approach has a drawback in that the geotechnical engineer usually does not know the inclination of the various loads when preparing the foundation report. And if the inclinations were known, then numerous allowable bearing capacities would be needed for the various inclinations of the load. A more commonly used procedure is to treat lateral loads separately and resist the lateral loads by using the soil pressure acting on the sides of the footing (allowable passive pressure) and by using the frictional resistance along the bottom of the footing. This is the approach used in the analysis of retaining wall foundations.
Moments and Eccentric Loads. It is always desirable to design and construct shallow footings so that the vertical load is concentric, i.e., the vertical load is applied at the center of gravity of the foot- ing. For combined footings that carry more than one vertical load, the combined footing should be designed and constructed so that the vertical loads are symmetric. There may be design situations where the footing is subjected to a moment, such as where there is a fixed end connection between the building frame and the footing. This moment can be repre- sented by a load P that is offset a certain distance (known as the eccentricity) from the center of grav- ity of the footing. For other projects, there may be property line constraints and the load must be offset a certain distance (eccentricity) from the center of gravity of the footing. There are several different methods used to evaluate eccentrically loaded footings. Because an eccentrically loaded footing will create a higher bearing pressure under one side as compared to the opposite side, one approach is to evaluate the actual pressure distribution beneath the footing. The usual procedure is to assume a rigid footing (hence linear pressure distribution) and use the section modulus (1/6 B2) in order to calculate the largest and lowest bearing pressure. For a footing having a width B, the largest (q′) and lowest (q′′) bearing pressures are as follows: