AASHTO LRFD & AASTHO 17 (BRIDGES)

AASHTO LRFD Design Code

AASHTO LRFD is the general governing design code for bridges and other highway structures in the United States. In AASHTO all structures shall be designed for specified limit states to achieve the objectives of constructibility, safety, and serviceability, with due regard to issues of inspectability, economy, and aesthetics. These design factors also apply to the design of earth retention systems such as vertical pile walls, gravity walls, etc.

LRFD stands for Load Factor Resistance Design. In the LRFD philosophy the design of each structure is performed by examining a set of load and resistance force combinations. As such, loads are multiplied by partial safety factors while resistances are divided by another set of partial safety factors and this in theory produces an ultimate limit state (i.e. just prior to failure). All partial safety factors are supposed to be calibrated against a wide number of cases that in essence produced similar overall safety factors with older allowable stress design methods.

In AASHTO LRFD the load combinations are described as Limit States. Further AASHTO differentiates between Strength and Extreme limit states with the following specific decriptions:

Strength I—Basic load combination relating to the normal vehicular use of the bridge without wind.

Strength II—Load combination relating to the use of the bridge by Owner-specified special design vehicles, evaluation permit vehicles, or both without wind.

Strength III—Load combination relating to the bridge exposed to wind velocity exceeding 55 mph.

Strength IV—Load combination relating to very high dead load to live load force effect ratios.

Strength V—Load combination relating to normal vehicular use of the bridge with wind of 55 mph velocity.

Extreme Event I—Load combination including earthquake. The load factor for live load γEQ, shall be determined on a project-specific basis.

Extreme Event II—Load combination relating to ice load, collision by vessels and vehicles, check floods, and certain hydraulic events with a reduced live load other than that which is part of the vehicular collision load, CT. The cases of check floods shall not be combined with CV, CT, or IC.

Further combinations are also defined for serviceability and fatigue although the latter are not particularly important for earth retaining structures and deep excavations.

Our software DeepEX includes all AASHTO LRFD 2018 bridge design combinations. Review Combinations

 

AASHTO LRFD Load Combinations in DeepEX software

Fig. Assigning an AASHTO LRFD Combination to a Model in DeepEX

DeepEX Software implements several Structural and Geotechnical Design Codes

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