Low-cycle fatigue of steel piles in integral abutment bridges

Besides the safety aspects, the economy is the single most important factor when bridges are designed. Lowering the life cycle cost of bridges means that less tax-money would be spent, and that should be in the interest of the general public. Today, bridges in Sweden are generally designed with movable joints and bearings. Leaking joints are a major reason to corrosion problems, and it would be preferable if bridges were designed without these. Integral abutment bridges are bridges without any movable joints. The superstructures are rigidly connected to the abutments, which generally are supported by a single row of flexible piles. The largest benefits of integral abutment bridges are the lower construction- and maintenance costs. Movable joints and bearings are used in order to handle the expansion and contraction of the superstructure due to temperature changes. If these components are not used, then additional forces will be transferred to the abutments. Therefore, abutments in integral bridges will be laterally displaced as the temperature changes. The top of the piles will also be displaced and forces as well as moments will be induced in the piles. Pile stresses can locally exceed the yield strength of the pile material and plastic hinges can be developed. The development of plastic hinges in steel piles is allowed in the design of integral bridges in some states in the USA. The Swedish National Road Administration seems to be more hesitant about allowing pile stresses above the yield strength. And there seems to be a concern about whether or not there could be problems with fatigue involving plastic deformations, low-cycle fatigue. The aim of this thesis is to answer if, how and when low-cycle fatigue failures might happen in piles supporting integral abutment bridges. First of all, a literature review has been done in order to get a better understanding of the problem and to gain knowledge about the research areas that are involved in this report. Integral bridges have been studied in general and especially their thermal behaviour. Other areas that have been studied are piles, fatigue, effective bridge temperature, traffic loads, and the Monte Carlo method. In order to simulate pile strains in integral abutment bridges, a temperature simulation model and a traffic simulation model were created. One example bridge, Leduån Bridge, has been used in the calculations throughout the report. It is a single span composite road bridge with a span length of 40 m. A couple of input parameters have been varied in order to find out in which amount they influence the pile fatigue. Some of the varied parameters are the lateral soil stiffness, pile cross-section, the location of the bridge (different climates), and the length of the bridge. The temperature model is based on shade air temperature measurements during 30 years at five locations in Sweden. These temperatures are transformed into Effective Bridge Temperatures (EBT) in order to simulate the lateral displacements of the abutments. The seasonal temperature changes will give an annual strain cycle in the piles, and there will also be daily temperature variations giving smaller strain cycles. Variations in the vertical temperature gradient in the superstructure are also taken into consideration, since these will give rotations of the top of the piles as well as small lateral displacements. The traffic model is based on vehicle gross weights from BWIM measurements performed by the Swedish National Road Administration. Two traffic models have been used. The first one is based on the traffic intensity and gross weights at the road E22, and the other one is based on measurements from National Road 67. The traffic load model has been combined with the temperature model, and Monte Carlo simulations of pile strains have been performed. The simulation results can be presented as pile strain spectra, involving cycles with periods from seconds up to years. A load spectrum during the designed lifetime of the bridge, 120 years, would involve more than 50 million strain cycles. These cycles have to be identified and counted in order to perform cumulative fatigue calculations. A method called the Rain-flow method has been used to identify the cycles, count them and sort them. The results from the calculations in this report indicate that low-cycle fatigue failures are not expected in piles supporting integral abutment bridges, at least up to a bridge length of 100 m. The calculation model is rather conservative and it is possible that even longer bridges can be constructed without problems with low-cycle fatigue. The importance of lowering the lateral soil stiffness can also be studied in the results, this fact has been noted in several other studies as well. It is also noted that some pile cross-sections seem to be more suitable than others for integral abutment bridges.Validerat; 20101217 (root