This report presents a fatigue evaluation procedure for existing steel highway bridges and a compatible fatigue design procedure for new steel highway bridges. Both procedures realistically characterize the actual fatigue stress and cyclic conditions in bridges. Both procedures use the same detail categories and corresponding fatigue strength data as the present AASHTO specifications; they also use methods of calculating stress ranges that are similar to present AASHTO methods. The evaluation procedure, which is recommended for inclusion in the AASHTO Manual for Maintenance-Inspection of Bridges, gives accurate estimates of the remaining mean and safe fatigue lives and permits these estimates to be updated in the future to reflect changes in traffic conditions. An alternative procedure is also given for monitoring the accumulated fatigue damage during the life of the bridge; it is intended for use in conjunction with the routine inspections that are normally conducted at two-year intervals. An assessment of remaining fatigue life is needed in bridge management systems to assist in scheduling inspection, maintenance, rehabilitation and replacement. It is also useful in evaluating administrative and legislative policies such as truck-weight and overload policies. The design procedure, which is recommended for inclusion in the AASHTO Standard Specifications for Highway Bridges, is consistent with the evaluation procedure and utilizes many of the same concepts. However, it is presented in a format convenient for design; a calculated design stress range is compared with a permissible stress range corresponding to a desired safe design life. The procedure is much more flexible than present AASHTO fatigue procedures; it permits rigorous analysis methods and designs for any desired design life, initial truck volume, and truck-volume growth rate.

Fatigue evaluation procedures for steel bridges

A fracture-critical steel I-girder bridge was instrumented with strain gauges to estimate the remaining design fatigue life. The two girders on the bridge had extensive fatigue cracking. Continuous, dynamic strain data were collected for nearly 2 months to determine an effective stress range and cycle count according to Palmgren-Miner's rule. A simplified rainflow counting algorithm was developed and used to calculate the amplitude of each fatigue cycle. The effective stress range and cycle count were combined with AASHTO's S (stress range)-N (number of cycles to failure) curves to estimate the remaining design fatigue life of certain bridge details. The data revealed that the estimated design fatigue life was exceeded in the east girder (right lane), whereas some life remained in the west girder (left lane). The distribution of observed cracks in the girders was closely correlated with the calculated fatigue life. A method is presented in this paper to index the effective stress range so that strain meas...

Use of Strain Data to Estimate Remaining Fatigue Life of a Fracture-Critical Bridge

Use of structural health monitoring to extend the service life of the Diefenbaker Bridge

The influence of small stress cycles caused by the dynamic response of a bridge upon the fatigue life of welded components was studied. Various loading waveforms were used to load a cantilever welded tee specimen. The fatigue life was measured and means of transforming the loading waveform to constant amplitude waveform producing the same damage were investigated. The waveforms investigated included actual stress histories measured on an in-service bridge loaded with both a single test vehicle and under normal traffic. The results of the study indicate that the small stress cycles cause considerable fatigue damage and cannot be ignored in the design and evaluation of steel bridges for fatigue. Based on the results of the experimental study and an evaluation of the stress histories of three bridges, a simple means for estimating the damage done by these small cycles was developed using a fatigue factor. The design stress range including the normal AASHTO impact fraction for a single vehicle passage should be multiplied by a fatigue factor of 1.15 to include the fatigue damage done by these minor cycles. The factor of 1.15 is the best estimate for medium span girder bridges. Other type and span bridges may produce different values according to their dynamic behavior. The most accurate means of obtaining this value is through field stress measurements of the actual bridge.

K D Swenson

Papers

The application of cumulative damage fatigue theory to highway bridge fatigue design