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4.5 EXTREME EVENT LIMIT STATE Extreme Event Limit State failure modes are related to the strength and stability of the foundation under unique loading events that have a return period greater than the bridge design life. These events include check flood (year
event for scour,
earthquakes, and/or major vessel or vehicle collision. The design concern is survival of the bridge and protection of life safety with the expectation that some damage to the structure will occur without resulting in collapse. The recommended load and resistance factor for extreme events is 0.80 for geotechnical lateral resistance (Brown et al. 2010). See Chapter 8 fora detailed discussion of the Extreme Event Limit State.
4.6 CONSIDERATIONS FOR LIMIT EQUILIBRIUM APPLICATIONS For evaluation of global stability of retaining structures
and slope stabilization, limit equilibrium applications are needed. Resistance factors and additional load factors in current standards are typically not applied to loads and resistances in limit equilibrium analyses. Using software
to assess global stability, the analyses are performed at the Strength I Load combinations and
targeting a load factor of 1.0 and a resistance factor of 0.75. If supporting
a structure foundation, the resistance factor of 0.75 is used in combination with applying Strength Limit load factors to foundation loads to be consistent with current Strength Limit load groups and load factors. A resistance factor of 0.65 should be used when the geotechnical parameters and subsurface stratigraphy
are highly variable, or are based on limited information. The analysis of deep foundation elements for slope stabilization is further discussed in Chapter 10.
