Limit State Fatigue refers to appearance of cracking in the material as a result of
repet-itive application of stress below or way below the material’s static strength.
Fracture is the sudden increase of the crack size induced by fatigue. Depend-ing on the location of such failure and the redundancy of the bridge system,
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fatigue and fracture failure may or may not be a cause of system failure or collapse. Bridge members whose fracture may cause system failure or col-lapse are referred to in U.S. bridge engineering practice as fracture critical members.
Existence of material discontinuity at the microscopic scale may be very difficult to eliminate, depending on the process and procedure used to fabricate the material. However, some may more likely induce discontinu-ity than others. For example, the continuous hot-rolling process of steel shapes produces relatively more uniform material than welding as a process of locally treating and fusing different steel materials. As a result, welding more likely introduces discontinuity resulting from inadequate fusion of the base metal and the weld material (electrode). Therefore, weld details have more often experienced fatigue and fracture failure.
Accordingly, the fatigue limit state in the AASHTO specifications is required to be taken as a restriction on stress ranges. The applied load is specified in the specifications as a single design truck with the number of expected stress range cycles based on an estimation of how many times over the life span of 75 years such trucks may cross the bridge. Depending on the type of weld detail (i.e., how it is completed, such as in the field or in the shop) and the number of truck load repetitive applications, the allowable stress range given in the AASHTO specifications is different. The fracture limit state is taken as a set of material toughness requirements of the AASHTO Materials Specifications.
A typical fatigue-prone bridge component detail is the so called cover plate weld to attach it to a steel I-beam bottom flange for increased moment capacity. Such a weld is subjected to significant bending stress. If the weld is also subjected to a large number of stress cycles applied by trucks of large volume, fatigue cracking has been observed many times. Although such cover plate welding has been significantly reduced or eliminated in new bridge design as a result of this observation and subsequent research on the phenomenon and failure mechanism, many existing bridges still have such details and they need to be evaluated repeatedly to estimate the bridge beam’s remaining life.
2.3 Constructability
Constructability refers to the ability to successfully complete the construc-tion of the bridge being designed. This issue is particularly important simply because it is a prerequisite for the bridge to start its design life by entering the stage of operation. Thus, it is discussed before other general design issues. While it is important, it cannot be exhaustively covered in the speci-fications, since there are a variety of construction techniques and construc-tion procedures. In general, the strength limit states discussed in the next
2.4 Safety 21 section on safety are applicable to the constructability check, but,
depend-ing on the situation, with the load factors reduced to be close to the service limit state level.
The constructability issues explicitly mentioned in the AASHTO speci-fications include, but not limited to, deflection, strength of steel and con-crete, and stability during critical stages of construction. For instance, if the designer requires steel beams with a concrete deck to compositely support both the dead load (self-weight of concrete) and the live load (truck load), this requirement needs to be specified for construction.
Loads applied to bridge components during construction may be different from those during service. Sometimes, construction stresses can be larger than those under normal service conditions. Bridges should be designed in a manner such that fabrication and erection can be performed without undue difficulty or distress and with locked-in construction force effects within tolerable limits. When the designer has assumed a particular sequence of construction, that sequence is required to be defined in the contract documents, such as the plans. If the method selected to construct the bridge structure requires certain strengthening and/or temporary bracing or support during erection, this requirement also needs to be indicated in the contract documents including the plans.
The specifications also identify several other issues that need to be addressed in design. They include, but not limited to, avoiding details that require welding in restricted areas and placement of concrete through congested reinforcing. There also should be adequate considerations to climatic and hydraulic conditions that may affect the construction of the bridge.
2.4 Safety
Conventional structural design and evaluation practices use the allowable stress design (ASD) method and/or the load factor design (LFD) method.
The approaches of the load and resistance factor design (LRFD) and the load and resistance factor rating (LRFR), as indicated by the names, have a format of using different factors respectively for the loads and the resistance.
The LRFD and LRFR methods allow individual treatment of each load or resistance. It is thus more flexible and with a higher fidelity to handle their different levels of uncertainty. This format of design checking can cover var-ious limit states for design or evaluation, such as bending and shear failures, excessive deflection, cracking potential, seismic load, and wind load.
More significantly, the AASHTO LRFD and LRFR specifications have been calibrated with respect to the risk of failure involved. This risk is quan-tified as the probability that the real total-load effect exceeds the real resis-tance. Engineers use nominal values of load effects and resistances to satisfy the design or evaluation requirements. This procedure, however, will not
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eliminate the possibility of failure since the involved loads and resistances vary randomly, sometimes very significantly.
Examples of such variables are severe earthquake load, the maximum truck load over the 75-year life span, and maximum flood load over the bridge life. Apparently different loads are associated with different levels of uncertainty to model and predict for design or evaluation. The load and resistance factors presented in Chapter 3 are meant to address these uncer-tainties as the sources of failure risk.
The design and evaluation methods prescribed in the AASHTO LRFD and LRFR specifications are referred to as calibrated because their load and resistance factors are selected along with the associated nominal loads and resistances to maintain the failure risk at an acceptable level. This section will briefly discuss the calibration conducted for the AASHTO LRFD and LRFR specifications. This discussion provides the background information so that the reader will understand the concept and the limitations and cor-rectly apply the specifications in the bridge design and evaluation.
2.4.1 Uncertainty