Calibration of AASHTO LRFD Concrete Bridge Design Specifications for Serviceability (2014)

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7 CONCLUSIONS AND SUGGESTED FUTURE RESEARCH 7.1 Conclusions 7.1.1 General Conclusions Related to the Calibration of Service Limit States • The main problem in calibrating the service limit states is the lack of clear consequences to exceeding the limit state. • The target reliability index used in the calibration of any limit state is a measure of the frequency this limit state is expected to be violated during the period assumed in the calibration. Due to the lack of clear consequences, the target reliability index for each service limit state has to be selected taking into consideration the reliability inherent in past designs. At this time, there are no justifiable reasons to select target reliability indices that are higher or lower than those inherent in past designs. • Basing the target reliability index for each service limit state on the reliability inherent in this limit state in past designs results in different target reliability indices applied to different service limit states. • For the same phenomenon being addressed, more than one limiting criteria may be used as the limit state function. Each limiting criterion results in a different reliability index. • For the same service limit state and same limiting criterion, the target reliability index may vary depending on the environmental conditions. 7.1.2 Conclusions Related to the Live Load Model for Service Limit States • The probability of heavy correlated trucks existing in two adjacent lanes is very low. For the service limit states calibrated in this report for concrete structures, assuming that the live load only exists in a single traffic lane with no multiple presence factors applied is appropriate • The design load for SLS, i.e. notional load, should not try to encompass all WIM records. Some of the extremely heavy vehicles are permit loads and some are illegal overloads • Some jurisdictions may need to make exceptions based on their legal loads and extent of enforcement • Basic HL-93 load model, scaled by calibrated load factors, is appropriate for SLS • Available information on the scale weights of permit loads, how the actual loads compare to the conditions of the permits and the frequency of permit loads on the highways is insufficient to perform calibration that is meaningful and can be generalized to other locations 144

7.1.3 General Conclusions Related to the Specific Limit States Calibrated 7.1.3.1 Cracking of Reinforced Concrete Decks through the Distribution of Reinforcement • Assessment of current practice leads to a recommended target reliability indices of 1.6 for the base case (Class 1 exposure) and 1.0 for situations when there is increased concern of appearance and/or corrosion (Class 2 exposure). These values correspond to a single lane ADTT of 5000 and annual probability. • The current requirements in the specifications produce uniform reliability across the range of girder spacing considered, so there is no need to change the load or resistance factors. • 7.1.3.2 Tension in Prestressed Concrete Beams For sections designed using conventional methods: • Among several limit state functions investigated, decompression produced the most uniform results and is recommended as the basis for the calibration • For a specific girder of known cross-section and specific number and arrangement of prestressing strands, the reliability index varies based on the design maximum concrete tensile stress, the limiting criteria, e.g. decompression, and ADTT • A uniform reliability index can be achieved uniformly across various span lengths using the load factor developed following the proposed calibration procedure and assuming the decompression limit state • The reliability index is not highly sensitive to changes in the ADTT. While the bulk of the work assumed an ADTT of 5000, based on the analysis of selected cases, using ADDT’s as high as 10000 essentially resulted in essentially the same load and resistance factors • The recommended target reliability index for the decompression limit state is 1.0 for bridges designed for no worse than moderate corrosion conditions and 1.2 for bridges designed for severe corrosion conditions • Based on the reliability indices calculated for different design and load scenarios, to achieve the target reliability index, it is recommended that the following be used for designing for Service III limit state: - Live load factor of 1.0. - Maximum concrete tensile stress of 0.0948t cf f ′= and 0.19t cf f ′= for bridges in severe corrosion conditions and for bridges in no worse than moderate corrosion conditions, respectively. - Girders to be designed following conventional design methods and assuming the live loads exist in single lane or multiple lanes, whichever produces higher load effects. The appropriate multiple presence factor applies. • The results of the calibration demonstrated that girders designed using the conventional design methods and the controlling number of loaded traffic lanes produce uniform reliability approximately equal to the target reliability index provided that the load factor is based on a reliability index calculated using the decompression 145

criteria and assuming one lane of traffic on the load side of the reliability index calculation. For sections satisfying the following conditions: • Time-dependent losses are determined using time step method, • Gross sections properties are used for the calculations, and • The calculations of the force in the prestressing steel neglects the effects of the elastic gain. A load factor for live load of 0.8 may be used. 7.1.3.3 Fatigue of Steel Reinforcement in Tension and Concrete in Compression • The proposed load factor for the infinite fatigue life is 2.0; up from 1.5 in the current specifications. • The target reliability index for fatigue of steel reinforcement in tension and concrete in compression is taken as 1.0; the same as it is currently implied in the specifications for structural steel components, steel reinforcement in tension and concrete in compression. • The current limit on stresses on concrete in compression that is meant to control concrete fatigue results in a reliability index close to the target reliability index. No revisions to the stress limit are required. • The current threshold on the constant-amplitude fatigue for steel reinforcement results in a reliability index higher than the target reliability index. Reducing the reliability index can be achieved using either of the following approaches: - Maintain the current equation used to calculate the constant-amplitude threshold and apply a resistance factor of 0.8, or, - Multiply the current equation used to calculate the constant-amplitude threshold by a factor of 0.8 and apply a resistance factor of 1.0 which matches the resistance factor for all other fatigue cases The latter approach is recommended which results in revising the constant fatigue threshold equations to: For straight reinforcing bars and welded-wire reinforcement without a cross weld in the high-stress region: ( ) minTHΔF 19 0.26f= − For welded-wire reinforcement with a cross weld in the high-stress region: ( ) minTHΔF 13 – 0.26f= 7.2 Suggested Future Research • Research on quantifying the consequences of exceeding different service limit states is needed. This quantification needs also to consider the degree of severity the limit states are exceeded to allow a better understanding of the effect of heavier vehicles on the serviceability of different components. 146

• Instrumentation of a prestressed concrete girder bridge on one of the heavily travelled roads to confirm the assumption that prestressed concrete beams may actually crack in service under heavy loads is needed. The bridge should be located in a jurisdiction that uses the HL-93 loading without jurisdiction-specific heavy vehicles for design. • Data is needed on the distribution of actual weights and configurations of permit vehicles and how they relate to the permitted weights and configurations. The data needs to be collected from several jurisdictions to investigate whether the same trends exist at different locales. • Data on the frequency of permit vehicles on different roads needs to be developed. • Research is needed to investigate whether the deck shrinkage restraint by the girders and the relative stiffness of the deck to the supporting members have an effect on deck cracking. 147