LRFD for Geotech – Note 1

2.0 Common Limit States in Bridge Substructure Design

In the AASHTO-LRFD framework, there are five distinct limit states: (a) strength (or ultimate) limit states, (b) serviceability limit states, (c) extreme event limit states, (d) fatigue limit states and (e) constructability limit states.  For most cases, the routine design of a bridge or a component is generally governed by either the strength or the service limit states.  These common limit states are briefly discussed below (Samtani and Nowatzki, 2006):

  • Strength (or ultimate) limit states are limit states that pertain to structural safety.  These limit states may be reached through either geotechnical or structural failure.  Evaluation of strength limit states is based on inelastic behavior of the structure, which is accomplished by using increased or factored loads (i.e., γ > 1.0) and on modification of soil behavior, which is accomplished by using reduced or factored strengths (i.e., φ < 1.0).  From a geotechnical viewpoint, strength limit states are reached when they involve the partial or total collapse of the structure due to sliding, bearing capacity failure, etc.  For well-designed structures strength limit states have a low probability of occurrence.
  • Serviceability limit states are the limiting conditions affecting the function of the structure under expected service conditions.   Serviceability limit states occur before collapse.  These include conditions that may restrict the intended use of the structure, e.g., excessive total or differential settlements, cracking, local damage, rough rideability, etc.  Evaluation of serviceability limit states is usually performed by using expected service loads (i.e., load factors = 1.0), nominal strengths (i.e., resistance factors = 1.0) and elastic analyses.  Compared to strength limit states, the serviceability limits states have a higher probability of occurrence but, if exceeded, involve less danger of loss of life.