Spring 2019

ASPIRE is a quarterly magazine published by PCI in cooperation with the associations of the National Concrete Bridge Council. The editorial content focuses on the latest technology and key issues in the Concrete Bridge Industry.

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Page 52 of 55

ASPIRE Spring 2019 | 51 O ur ASPIREĀ® team recently received a question about when and why we use the provision for prestressed (pretensioned and post-tensioned concrete) substructure solutions with the load combination for the Service IV limit state, which is one of many limit states in the American Association of State Highway and Transportation Officials' AASHTO LRFD Bridge Design Specifications. 1 This article aims to shed light on that topic. Before we address the Service IV load combination specifically, let us first consider the historical development of bridge design specifications. From the issuance of the first bridge design specifications in the late 1920s until early 1970s, bridges were designed by using allowable stress design (ASD) principles. In a nutshell, ASD is based on the premise that stresses created by loads acting on bridges and their c o m p o n e n t s c re a t e "w o rk i n g" o r allowable stress conditions that must be kept below a certain fraction of actual material strength. A f t e r n e a r l y h a l f a c e n t u r y o f successful use, ASD was set aside, and a new direction was taken based on the more rigorous notion of the factor of safety against failure of the section. This was intended to give a more uniform factor of safety against failure, which was not possible with ASD. For load factor design (LFD), the strength (resistance) of the section was compared to service loads that were individually increased by load factors that were greater than 1.0. Prior to the comparison of strength to demand of the applied loading, the resistance was reduced by a resistance factor that was typically less than 1.0. The magnitudes of the load and resistance factors were based on judgment, considering uncertainty and variability of the loads and section properties. The transition to load and resistance factor design (LRFD) for concrete simply involved the formal calibration of the load and resistance factors used in LFD design by using statistical methods and field data on loads, material properties, and other aspects. Subsequently, variability in material resistances, the accuracy with which structural capacities can be estimated, and the consequences of different modes of failure were all brought into the LRFD design process. The first edition of the AASHTO LRFD specifications was published in 1994. Since 1994, the AASHTO LRFD specifications have been revised or updated as our knowledge about the performance of concrete bridges has evolved. Our decisions while developing new techniques, revising older methods, and calibrating load and resistance factors have been informed by data generated in new experimental programs, the advent of increased computing power, the associated development of advanced analysis techniques, and the field performance of concrete bridges. In the eighth edition of the AASHTO L R F D s p e c i f i c a t i o n s , w h i c h w a s published in 2017, loads and load combinations are covered in Section 3. 1 As stated in that section, we have five strength limit states, two extreme event limit states, and four service limit states, in addition to the two fatigue and fracture limit states. The Service IV limit state relates to controlling tension in prestressed concrete columns, with the ultimate goal of preventing or minimizing the chances of cracking in these substructure elements to improve durability and maintain a higher flexural stiffness. by Dr. Oguzhan Bayrak, University of Texas at Austin AASHTO LRFD Bridge Design Specifications: Service IV Load Combination Load combinations and load factors from Table 3.4.1-1 in the AASHTO LRFD specifications. Figure: AASHTO LRFD Bridge Design Specifications, 8th edition. A A S H T O L R F D

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