IES VisualFoundation User's Guide
Stability
Before the internal behavior of a footing can be analyzed, an engineer must make sure the footing will not lift off the earth, will not tip over, and will not slide on the soil. This is done by providing a minimum factor of safety against uplift, overturning and sliding. VisualFoundation allows you to set these minimum factors in the Project Settings (visible when nothing is selected) under the Stability Calculations heading. You select whether stability is checked with service or strength-level loads. A safety factor is allowed for both uplift and sliding.
When piles are present, VisualFoundation assumes the footing has a sufficient factor of safety for all stability conditions. By anchoring a footing with piles, stability requirements are satisfied by ensuring pile reactions are less than each pile's capacity, both in axial force and lateral/shear force directions.
A common cause for an unstable foundation in VisualFoundation is a misinterpretation of the axial-load sign convention. Remember, a negative z-force is downward and a positive z-force is upward.
Uplift forces (Fuplift) must be less than those forces resisting uplift (Fweight). Forces which resist uplift may be assumed to be the footing's dead weight along with the weight of anything lying directly above the footing which can be counted on when the uplift loads are present. When a footing is also supported by piles, the Fweight component may also include the pull-out capacity of the piles. For stability it is common to define a factor of safety (F.S.) as the ratio of strength to demand. For uplift this would be:
F.S. uplift = Fweight / Fuplift
Typical values for F.S. uplift range from 1.5 to 2 and allow for a safety factor guaranteeing a structure that will not lift off to the moon.
The second type of stability that must be assured is overturning. The lateral forces that a structure must resist resulting from wind and seismic loads tend to overturn the structure which in turn tends to overturn footings. Uneven pressures and forces acting on the top surface of the footing can also contribute to overturning. Making an overturning stability check involves calculating the overturning moment (Moverturning), and the resisting moment (Mresisting) of forces acting on the footing and guaranteeing Mresisting to be larger than Moverturning. Again a safety factor is used for this:
F.S. overturning = Mresisting/ Moverturning
Typical values of F.S. overturning are in the range of 1.5 to 2.0. Because moments are calculated in this check, the location where the check is made is important. The critical locations used are the tipping points and these are taken as the corners of the footing. The axes which moments are summed are the axes parallel to the edges of the footing at the tipping points.
VisualFoundation allows the effects of seismic overturning to be reduced as allowed by ASCE 7-10 section 12.13.4. This reduction is taken by checking the box next to Reduce Seismic Overturning, located in the Modify Project Information pane under the Safety Factors heading. When the box is checked, overturning moments generated by seismic loads are reduced by 25% before they are combined with moments from other load sources. These reduced loads are only used when calculating the overturning factor of safety. Full seismic loads are used for all other analysis and design purposes. Since the full seismic loads are needed, the model must have an overturning factor of safety greater than 1.0 when unreduced seismic forces are applied.
The 25% reduction assumes the structure was analyzed using the Equivalent Lateral Force procedure (ASCE 7-10 section 12.8). ASCE allows a 10% reduction for structures analyzed using the Modal Analysis procedure; however, this lesser reduction is not supported by VisualFoundation. ASCE does not allow this reduction for inverted pendulum or cantilevered column type structures.
The third type of stability that must be assured is sliding Horizontal forces are applied to piers, walls, or grade beams.
VisualFoundation will check the resultant horizontal sliding forces for each load combination. You specify parameters in the project properties:
With the above parameters defined, you may apply lateral forces to columns, walls and beams that are assumed to act at the vertical centerline (average) of the mat foundation.
The resultant forces are compared to the sliding resistance due to the friction resistance plus the passive lateral resistance. The friction resistance is calculated as the net downward reaction force for the load combination multiplied by the coefficient of sliding friction. The total passive lateral resistance is calculated by the passive pressure you entered multiplied by the total projected area of the footing in the horizontal direction under consideration (total width * average footing thickness).
Pile supports are assumed to have an infinite sliding resistance, unless you override that in the Pile properties.