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, tip over, or slide on the soil. This is done by providing a minimum factor of safety against uplift, overturning, and sliding. VisualFoundation allows these minimum factors to be set in the Project Settings (visible in the tab when nothing is selected) under the Stability Calculations section. Select whether stability is checked with the service-level or the strength-level loads and specify the factor of safety (F.S.) for overturning, uplift, and sliding. When piles are present, VisualFoundation assumes the footing has a sufficient factor of safety for all stability conditions (note: the infinite pile resistance for sliding can be disabled in the tab and a value for the pile lateral resistance can be specified). By anchoring a footing with piles, stability requirements are satisfied by ensuring pile reactions are less than each pile's capacity.
Uplift forces (Fuplift) must be less than the forces resisting uplift (Fweight). In VisualFoundation, the forces that resist uplift are the self-weight of the footing and its components (i.e. the slab, grade beams, walls, etc). When a footing is supported by piles, the Fweight is assumed to be infinite, and the uplift stability is not checked. It may be appropriate to check the uplift capacity including the pull-out capacity for the piles outside of the program. The factor of safety (F.S.) for uplift stability is defined by the following equation:
F.S. uplift = Fweight / Fuplift
The lateral forces that a structure resists from wind, seismic loads, etc. tend to overturn the structure which in turn tends to overturn footings. Uneven pressures and forces acting on the top of the footing can also contribute to overturning. Checking the overturning stability involves calculating the overturning moment (Moverturning) and the resisting moment (Mresisting) of the forces acting on the footing. The critical locations used to calculate Moverturning and Mresisting are the tipping points which 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. The factor of safety (F.S.) for overturning stability is defined by the following equation:
F.S. overturning = Mresisting / Moverturning
VisualFoundation allows the effects of seismic overturning to be reduced as allowed by ASCE 7-16 section 12.13.4. This reduction is accounted for by setting the Reduce Seismic Ovrtrn.? parameter to Yes, in the Project Settings (visible in the tab when nothing is selected) under the Stability Calculations section. Enabling this parameters reduces the overturning moments generated by seismic loads by 25% before they are combined with the overturning moments from other load sources. These reduced loads are only used when calculating the overturning factor of safety and full seismic loads are used for all other analysis and design purposes. The 25% reduction assumes the structure was analyzed using the Equivalent Lateral Force procedure (ASCE 7-16 section 12.8). While ASCE allows a 10% reduction for structures analyzed using the Modal Analysis procedure, this lesser reduction is currently not supported by VisualFoundation. Note: ASCE does not allow this reduction for inverted pendulum or cantilevered column type structures.
The lateral forces that a structure resists from wind, seismic loads, etc. can cause the structure's foundation to slide. Checking the sliding stability involves calculating the sliding forces (Fsliding) and the sliding resistance (Fresisting). In VisualFoundation, sliding forces can be applied to piers, walls, and grade beams. The sliding resistance consists of the friction resistance, the passive lateral resistance, the pile lateral resistance, and the specified external sliding resistance. The friction resistance is calculated as the net downward reaction force for the load combination multiplied by the specified coefficient of sliding friction. The total passive lateral resistance is calculated by the specified passive pressure multiplied by the total projected area of the footing in the horizontal direction under consideration. When piles are included in the model, an infinite pile resistance can be used (to disregard the sliding stability check) or the pile lateral resistance can be manually entered. The factor of safety (F.S.) for sliding stability is defined by the following equation:
F.S. sliding = Fresting / Fsliding