Spread Footing Overturning and Uplift Checks in RAM Foundation

by Seth Guthrie and Josh Taylor

June 13th, 2007


This article seeks to provide the RAM Structural System user with a greater understanding of the criteria settings within RAM Foundation that affect the uplift and overturning checks performed by the program. There are several design options which are critical to the way the program checks for uplift and overturning problems. A basic knowledge of each of these parameters is necessary in understanding the final design results for each footing. The criteria options discussed in this article exist in the Design tab of the Design Criteria menu (see Figure 1), on the right hand side. The user is referred to Section 4.7 of the RAM Foundation Technical Manual, "Analysis of Spread Footings for Soil Pressure" for supporting information.



Figure 1 - Design Criteria menu highlighting options discussed in this article



When a footing is assigned in Modeler, the top of the footing is located at the same elevation as the bottom of the supported member(s). Thus, the component of the overturning moment resulting from the shear at the base of the supported member acting over the footing thickness is not accounted for.

If the Include Moment Due to Shear in Column box is checked, an additional overturning moment is added to the footing that accounts for the footing thickness. This additional moment is calculated as the product of the shear reaction for the respective Load Combination and the thickness of the footing.

This effect could be substantial for isolated spread footings supporting both a column and a brace (see Figure 2). In this situation a significant amount of shear force will be delivered to the footing from the brace and the footing will likely be relatively deep in order to resist the applied forces.


Figure 2 - Spread footing supporting column and brace



This option is included primarily to accommodate the situation in which a specified allowable bearing capacity is assumed to include the self weight of the footing. If this option is not checked, then the footing self-weight and surcharge are ignored in all Load Combinations that produce net compression on the footing. If the net compression is small, but the overturning moment is large, this can result in an overturning instability and a larger footing may be required. It is recommended that this option be selected. The footing self-weight and applied surcharge can have a significant impact in resisting the overturning forces.


When RAM Foundation optimizes the size of a Spread Footing, it will increase the size of the footing so that the user-specified minimum uplift safety factor is satisfied for each Soil Load Combination. If the Increase Spread Footing Size to Prevent Uplift in Concrete Load Combinations box is checked, then during the optimization process the size of the footing will be made sufficient to ensure that the minimum uplift safety factor is satisfied even for the Concrete (ultimate level) Load Combinations such as (0.9 D - 1.3 W1) or (0.9 D + 1.1 E1).

It should be emphasized that even when this criterion is selected, the overturning safety factor will still be calculated with respect to the Soil Load Combinations only, even though the footing has been sized to resist uplift for the Concrete Load Combinations.

It should also be noted that for Soil Load Combinations, when the Dead Load is factored (0.9 or 0.6), that factor also applies to the self-weight and surcharge forces which are part of the dead load case. Thus, using a Dead Load factor of 0.6 is conservative and does not require any additional overturning safety factor.

Note: Since the weight of the footing is calculated from the specified unit weight of the foundation concrete, is important that the user has entered a realistic value for the concrete unit weight. This value can be verified in the Material Properties page of the View/Update dialog (see Figure 3).


Figure 3 - Footing Self Weight value in View/Update dialog