International Concrete Abstracts Portal

In virtually all areas of structural engineering, including the increasingly well-known area of structural reliability assessment, it is commonly assumed that failures will occur suddenly and instantaneously in given failure modes. This assumption affords a valuable simplification of complex real-world problems. However, many failure modes do not obey this assumption, including most serviceability failure modes, strength failure modes of ductile component and/or redundant systems, and failure modes based on cumulative damage. For these cases, a formulation is required with which the transition from complete survival to complete failure can be modeled as being gradual and continuous, and comprised of partial failure levels. This paper proposes such a formulation along with corresponding methodologies for structural reliability assessment and reliability-based design. Various statistical and entropy-based measures which can be used to help characterize the results of the structural reliability assessment are also suggested. Application of the proposed structural reliability assessment and reliability-based design methodologies is illustrated with an example problem involving deflection failure of a reinforced concrete beam. Some potential applications of the proposed methodologies include probabilistic design and code calibration for failure modes modeled as having gradual and continuous failure transitions.

Studies are limited on the early age performance of high-strength cold weather concretes and their shear strength interaction in cold weather. This paper presents shear transfer strength characteristics between regular high-strength concrete and (i) methyl methacrylate-based polymer concrete and (ii) magnesium phosphate based concrete in subfreezing temperatures. Analytical expressions were developed based on shear transfer hypothesis and verified by experimental results. The experimental study included tests on cylinders and L-shaped push off specimens to determine the early age shear interlock and shear frictional resistance between high-strength regular portland cement concrete and cold weather high-strength concretes as is experienced in rehabilitation of bridge decks and other infrastructure systems. Studies indicated that at early age of 24 hours, shear transfer strength of 1400 psi can be obtained with the use of appropriate material and shear reinforcement. The study also indicated the ACI 318-89 code limits on the shear-friction strength are too conservative even at early ages for high-strength cold weather concretes.

Concrete bridges in the United States constitute about fifty percent of the total number of highway bridges. Recent studies indicate that many of these bridges deteriorate due to age, corrosion of reinforcement, fatigue, cracking and spalling of concrete, and/or human error. Limited funds are available for rehabilitation, strengthening, and replacement. Therefore, there is a need for methods to identify the parts of concrete girder bridges most sensitive to damage using reliability models. This may help lower the costs of checking, inspection, and repair. Load and resistance sensitivity functions for the ultimate flexural capacity limit state of simply supported bridge girders are included. The study indicates that the reliability of bridge girders depends mostly on the strength and location of steel.

This paper presents the evaluation of seismic performance of a special moment-resisting (SMR) frame building and an intermediate moment-resisting (IMR) frame building designed in accordance with the NEHRP provisions and ACI Code 318-83. The annual limit-state probabilities for both SMR and IMR frames are determined by integrating the seismic hazard curve and structural fragility curve. From the comparison between the calculated annual limit-state probability and the specified acceptable risk levels, the seismic performance of a structure can be evaluated. In the NEHRP provision, if reinforced concrete frames are used to resist earthquake forces, the SMR frame is required for buildings belonging to higher seismic performance categories such as Categories D and E. Even though the SMR frame has a higher ductility than the IMR frame, the SMR frame is only designed for 50 percent of the strength required for the IMR frame. As demonstrated in this study, the IMR frame may perform better than the SMR frame in the event of an earthquake. Thus, the concept employed in the NEHRP provisions to protect high-risk and essential buildings needs careful reexamination.

A procedure for rational prediction of deformation in pretensioned members is described. Full-scale load tests on stemmed members spanning 30 to 62 ft (9.2 to 18.9 m) were conducted by the author. They showed good correlation with the proposed predictions. Actual deflections were generally less or close to the computed values. It is suggested that the method may be used for loads not exceeding a certain ratio of the ultimate loads.