Greetings from Bridging Solutions!

This installment of “A Bridge Moment” is the first in a series that will summarize the significant permanent and transient design loadings for bridges. The AASHTO bridge design specifications provides the required design loadings and recommends groups of loadings that should be combined by the bridge designer for the design of a bridge.

The primary function of a bridge is to carry traffic loads but for most medium and long span bridges, the largest force that must be considered in the design of the bridge is the weight of the bridge itself. The self-weight of a bridge and all of the permanent attachments is what bridge engineers refer to as dead load. On short span bridges the traffic loadings or live loads usually induce higher stresses than the dead load.

The dead load of bridges varies depending on the materials that are used for the bridge components. Concrete bridges are usually heavier than bridges constructed using steel and timber components. The following list summarizes the unit weights or densities bridge engineers use to compute the dead loads of the bridge components.

                        Steel                            490 pounds per cubic foot

                        Concrete                      150 pounds per cubic foot

                        Timber                            50 pounds per cubic foot

                        Compacted earth          120 pounds per cubic foot

                        Stone masonry             170 pounds per cubic foot

                        Asphalt pavement         150 pounds per cubic foot