Bridge Superstructure Tolerance to Total and Differential Foundation Movements (2018)

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NCHRP Project 12-103 12 2 Current AASHTO LRFD Guidance Related Tolerable Support Movement 2.1 Summary of Current AASHTO LRFD Guidance The only quantitative guidance provided by the AASHTO LRFD Bridge Design Specifications related to tolerable support movement is found in commentary C10.5.2.2. This commentary states that: …studies indicate that angular distortions between adjacent foundations greater than 0.008 rad. in simple spans and 0.004 rad. in continuous spans should not be permitted in settlement criteria (Moulton et al. 1985; DiMillio 1982; Barker et al. 1991). It goes on to state that additional angular distortion limits may be appropriate based on cost, rideability, aesthetics, or safety-related issues. The oldest study referenced in the commentary (DiMillio 1982) examined the performance of shallow foundations through the visual inspection of 148 highway bridges supported by spread footings. The results indicated that the bridges examined could tolerate sizeable support movements without performance problems. The author concluded that the bridges examined “easily tolerated differential settlements of 1-3 inches (25-75 mm) without serious distress.” Around the same time, Wahls (1983) was also examining the performance of shallow foundations through NCHRP Project 107 “Shallow Foundation for Highway Structures.” Through this research, Wahls (1983) concluded: Angular distortions of 1/250 of the span length and differential vertical movements of 2 to 4 inches (50 to 100 mm), depending on span length, appear to be acceptable, assuming that approach slabs or other provisions are made to minimize the effects of any differential movements between abutments and approach embankments. Although these two studies no doubt contributed to the understanding of tolerable support movements, the guidance reflected in AASHTO LRFD is primarily based on the work of Moulton et al. (1985). Moulton et al. (1985) aimed to identify levels of tolerable support movement for various bridge types, and pursued this objective through the evaluation of structural damage caused by support movements to 314 bridges throughout the U.S. and Canada. In addition to these field studies, Moulton et al. (1985) also considered the reliability with which settlements may be predicted and employed simple analytical models to further examine observed trends. This study was consistent with the findings of Wahls (1983), which indicated that the level of tolerable support movement is a function of angular distortion (or

NCHRP Project 12-103 13 differential support movement divided by span length). In addition, Moulton et al. (1985) offered the following quantitative recommendation: The results of this study have shown that, depending on type of spans, length and stiffness of spans, and the type of construction material, many highway bridges can tolerate significant magnitudes of total and differential vertical settlement without becoming seriously overstressed, sustaining serious structural damage, or suffering impaired riding quality. In particular, it was found that a longitudinal angular distortion (differential settlement/span length) of 0.004 would most likely be tolerable for continuous bridges of both steel and concrete, while a value of angular distortion of 0.005 would be a more suitable limit for simply supported bridges. Barker et al (1991) conducted further examination of the data presented by Moulton et al. (1985) and concluded that the 0.005 limitation for simply-supported bridges was overly restrictive. As a result, it was recommended this value be revised to 0.008, which remains in the AASHTO LRFD commentary today. Since the angular distortions of 0.004 and 0.008 are the current guidance for continuous span and simple span bridges, respectively, these values will be used throughout this report as a baseline for comparative analyses. In the remainder of the report, these criteria are referred to as the “AASHTO LFRD criteria” even though it is only located within the commentary. 2.2 Discussion of Comparison with AASHTO LRFD Guidance Throughout Sections 5 and 6, the results obtained for the suite of steel and PS concrete multi-girder bridges are compared against the current AASHTO LRFD guidance. For this comparison, the level of tolerable support movement is considered to be a measure of capacity. That is, when the results of the simulation modeling indicate that a bridge has a tolerable support movement greater than the current AASHTO LRFD guidance, then the current guidance is referred to as conservative. Conversely, when the simulation modeling indicates that a specific bridge has a tolerable support movement less than the current AASHTO LRFD guidance, then the current guidance is referred to as unconservative. In addition, while the current AASHTO LRFD guidance was not intended to apply to the case of TD support movement, it is used as a point of comparison for these results as well. In this case, the maximum displacement at the support that undergoes the TD movement is compared with the current guidance. This was done to provide some context for the TD support movement results, and to evaluate