Methodology
The AASHTO Load and Resistance Factor Design (LRFD) specifications were developed to provide a consistent and comprehensive bridge design code based on the theory of reliability and upon current statistical knowledge of loads and structural performance.
A load factor is a factor accounting for the variability of loads, the lack of accuracy in analysis, and the probability of simultaneous occurrence of different loads.
A resistance factor is a factor accounting for the variability of material properties, structural dimensions and workmanship, and the uncertainty in the prediction of resistance.
Each component and connection should satisfy the following governing equation for each limit state:
Equation 14:
where:
η = Load Multiplier
γD = Dead Load Factor
D = Dead Load Effect
γL = Live Load Factor
L = Live Load Effect
φ = Resistance Factor
Ru = Factored Load Effect
Rr = Factored Resistance
Rn = Nominal Resistance
The load modifier, h, is further explained in LRFD Art. 1.3.2. It relates to ductility, redundancy, and the operational importance with respect to the limit states.
Substituting the proper values in Eq. 7 for all the different limit states considered in Precast/Prestressed Girder produces the governing equations for each limit state. Depending on the limit state and the girder selected for analysis, the proper equations are setup and from them, the envelopes are computed to produce the various moments, shears, and deflections.