AASHTO LRFD Main Design Loads and Combinations – Dead Loads

As shown in construction stages, the main design load combination includes dead and live load. Dead load is usually represented by self-weight of girders, the weight of deck and superimposed dead loads.

Compression and tension stress limits usually govern the design of the prestressed precast girders. Compression stress and tension stress check have been performed using two different load combinations. These load combinations include only dead and live load cases. The dead load stresses need to be cumulatively added to each other following directly the stages of the construction.

The dead load has three main parts: self-weight of the girder, deck weight and superimposed dead load. Superimposed dead load represents non-structural dead loads that remain permanently on the structure such as wearing surface, sidewalk and fences.

In design, maximum compression and tension stresses develop at mid-span of the structure. It is important to compute the force effects at mid-span due to self-weight of the girder.

The wet weight of the deck will act on the girder section. The girder will deflect due to this additional load that will increase the compression and tension stresses at the mid-span. In stress computations, we will not use the composite section properties since the deck does not gain its stiffness immediately after casting concrete.

The superimposed dead loads are typically non-structural dead loads that have been added to the system at later stages to meet the requirements of a highway pass over a bridge. Some of these items are: wearing surface, sidewalk and railings.

AASHTO LRFD Bridge Construction Specifications, 4th Edition

Bridge Design Flow Chart

Start Material Selection Design Basis Construction Stages Preliminary Girder Design Based on Span Length & Width Dead Load Live Load Service Limit Checks Preliminary Tendon Design Prestress Losses Concrete Stress Checks at Construction Concrete Stress Checks at Service Canber and Deflection Strength Flexure Strength Shear Interface Shear Design Deck Design Bridge Modeling