FATIGUE is the process of cumulative damage caused by repeated loadings. Fatigue damage of structures subjected to elastic stress fluctuations occurs at places of stress raisers, where the localized stress (due to the stress concentration effect) exceeds the yield stress of the mate rial. After a certain number of load cycles, the accumulated plastic damage will cause the initiation and subsequent propagation of a crack or cracks. In general, the more severe the stress concentration, the shorter the time required to initiate a fatigue crack.
Fatigue damage was recognized in Europe early in the 19th century. However, implementation of fatigue design criteria in structural engineering did not start until the late 1950's.
The fatigue life of a structure is defined as the number of cycles required to initiate and propagate a fatigue crack to a critical size, which could result in the fractural failure of the structure.
A fatigue-related failure attracted attention of engineers in the mid-1960's, when the failure of relatively young welded crane runway girders occurred significantly more often than the failure of old riveted girders. These failures were mostly represented in the form of cracking of fillet-welded top flange to web, stiffener to web and flange connections. The crack usually started in the welded joint and then propagated to the web or flange. Other cases included the failure of girder to column rigid connections, which restrained the girder support from free rotation, and stitch or plug welded connections of the runway components (eg, walkway plates, girder cap channels, etc).
A significant amount of research was performed and many technical articles have been published regarding premature fatigue failures of welded crane runway girders, some of which were only 2 to 15 years old. The com-mon conclusions and recommendations made in those articles were:
- Fillet welded top flange to web connections per form poorly in the presence of loadings resulting from crane operations. These connections can be loaded in vertical and horizontal shear from crane wheel vertical loads, and in torsion from the crane rail eccentricity over the centerline of the web and crane transverse horizontal loads. In addition, high residual stresses in the vicinity of the top flange web stiffener welded intersection help to create conditions for fatigue crack initiation.
- New crane girders be designed with complete penetration welded top flange to web connections and provide a deep cope (3 to 4 in.) for the stiffener to minimize interference between welds.
- Girder to column connections be revised to provide a free girder support rotation in the vertical plane.
- Replace stitch fillet and plug welds with continuous welds or bolted connections.
On the other hand, n limited amount of research information exists on the fatigue of riveted construction. The majority of research, which has been performed in Europe and North America, has concentrated on riveted bridge members (beams, posts, truss diagonals, etc) and riveted connections. Riveted crane girders comprise a high percentage of the total crane runways, especially in the steel industry. However, no information is available about fatigue-related research performed on riveted crane runway girders. This, most likely, can be explained by a significantly less amount of fatigue failures of the old riveted crane girders vs the failures of relatively young welded girders.
Most fatigue failures of riveted crane girders occur in the girder to column connection region, where the typi cal fatigue-sensitive details are located. These fatigue sensitive details originate from discrepancies between analytical models and design practices. Crane girders were originally analyzed as simple span beams. However, design practices at the time the runways were constructed featured restraining girder support rotations, such as:
- A web splice over the full height of the girder web between adjacent girders at supports.
- A common vertical diaphragm over the full height of the girder web between adjacent girders and the column.
- Knee braces between crane girders and columns.
Modification of the riveted girder support details and a proper repair of detected cracks usually helps to prevent reoccurrence of cracking.
In general, most of the fatigue related crane runway research was concentrated on developing recommendations for repair of existing failures and the proper design of new crane runway girders. The words, proper design, of crane runway girders in terms of fracture mechanics mean the design of girders (or any related structures) to prevent brittle fracture due to cracking. This includes providing an appropriate stress level at the locations where crack growth can occur and elimination (as much as possible) of those details that act as stress raisers.
Another subject of fatigue-relate d design of crane runway girders is an evaluation of the expected fatigue life of crane runway girders. This subject is especially important when the expected remaining fatigue life of the existing runway girders is the main concern in crane runway upgrade projects.
By the nature of a crane operation, most runway girders are subjected to variable amplitude loadings. However, the American and Canadian structural codes do not specify this effect, making design engineers use the maximum stresses in the fatigue analyses, which is an overly conservative approach.