Stability ratio is defined as the ratio of total resisting moment to total overturning moment. Its value is based on service or unfactored loads for all codes except AS 3600. For AS 3600 code, factored load combinations are used. A minimum value of 1.5 is usually recommended for any unfactored load condition. To set these ratios for different conditions such as erection, operation or test, click the Soil Tab under Options Menu.
For all design codes except AS 3600, the stability ratio = Resisting Moment/Overturning moment
For AS 3600 code, Stability ratio = (Resisting Moment * Overturning strength reduction factor)/Overturning Moment
The total resisting moment considers the appropriate load factor in computing the resisting moment due to soil, pedestal and footing. For IS 456 code, the resisting moment due to pedestal, soil and footing weights considers 0.9 times the appropriate load factor.
The overturning and resisting moments for each load combination are reported on the Stability/Sliding dialog accessed from the "Bearing Capacity" dialog.
When the actual stability ratio in one or more directions for each unfactored load combination is less than the allowable stability ratio specified on the Allowables tab of workspace settings, Foundation3D highlights these rows in red.
By default, overturning moment and resisting moment are considered at each edge of the footing, for eg the left edge of the footing and the right edge of the footing. The ratio of the resisting moment to the overturning moment that results in the lowest value considering the two edges is considered the governing edge and the stability ratio is reported based on this governing edge. In the absence of any overturning moment, the stability ratio is listed as 99999.
There may be scenarios in which the stability ratio may be listed as 99999 because there is no overturning moment at the edge of the footing resulting from applied shear or moment. For eg if none of the piers have any applied shears or moments, the overturning moment may be calculated as zero about the edge of the footing. However there will be an overturning moment at the centerline of the footing based on the applied loads and the location of the piers. Under such circumstances, an option to "Calculate Overturning Stability at Foundation Centerline" may be used to calculate overturning moment for regular shaped spread footing, combined footing and mat or raft foundation types. This option is listed under Miscellaneous section of the Soil tab of workspace settings. When this option is selected, the resisting moment is calculated at the edge of the footing but the overturning moment is calculated at the centerline of the footing. These moments are then used to determine the stability ratio.
Note: In general, the stability ratios computed using the above two methods will be different. It is upto the user's engineering judgement as to which method should be used to consider the foundation stability.
Strap Footings:
The layout of tabs in the right pane of this page is discussed in detail in section "Tab Layout of Strap Footings" of topic Strap Footing.Stability ratio calculation in the strap direction considers all 3 components - two footings and the strap beam, acting as one unit. Stability ratio in the transverse direction is based on the assumption that the footings behave as normal spread footings.