Precast/Prestressed Concrete Structures under Natural and Human-Made Hazards: Special Issue

The first two papers of the special issue describe recent developments in seismic precast concrete frame building structures. In the opening paper entitled “Performance-Based Seismic Evaluation of Two Five-Story Precast Concrete Hybrid Frame Buildings,” Rahman and Sritharan provide a comparative analytical study on the seismic behavior of two precast concrete moment frame buildings designed using displacement-based and force-based approaches. The buildings utilize “hybrid” precast beam-column joints with combinations of mild steel reinforcement and unbonded posttensioning steel for lateral resistance. The unbonded posttensioning steel provides self-centering to the structure, whereas the mild steel reinforcement is designed to yield and provide energy dissipation. In a related contribution, Morgen and Kurama investigate the “Seismic Design of Friction-Damped Precast Concrete Frame Structures.” Similar to the hybrid system in the previous paper, the frame concept analyzed by Morgen and Kurama uses unbonded posttensioning strands for self-centering; however, the energy dissipation of the structure is provided by friction dampers placed at selected beam ends. A procedure is described to determine the friction damper slip forces and posttensioning steel areas needed to satisfy prescribed design lateral strength and energy dissipation requirements for the frame.

Reliable shear design procedures are essential to prevent shear failure of structural members under extreme loading events. Current design methods for the shear resistance of concrete beams are based on empirical approaches, and different equations are used to determine the shear strength of prestressed and non-prestressed reinforced concrete. In their paper “Shear Design of Prestressed Concrete: A Unified Approach,” Wolf and Frosch describe a shear design approach that eliminates many of the shortcomings of current procedures and unifies the design of non-prestressed and prestressed reinforced concrete sections. The applicability of the shear model is evaluated by comparing predicted shear strengths with experimentally measured values from 86 specimens. In a related paper, Hamahara et al. investigate the “Design for Shear of Prestressed Concrete Beam-Column Joint Cores.” Reversed cyclic test results from eight prestressed and two non-prestressed reinforced concrete beam-column joint specimens are described and used to develop design equations for estimating the shear strength of prestressed concrete interior and corner joints. The equations are further validated using test data from 51 additional beam-column joint assemblies.

Papers presented in this section focus on precast prestressed concrete and prestressed masonry wall structures under earthquakes, blast, and progressive collapse. In the first paper, Perez et al. investigate the “Analytical and Experimental Lateral Load Behavior of Unbonded Posttensioned Precast Concrete Walls.” These walls are constructed by joining rectangular precast concrete wall panels across horizontal joints using unbonded posttensioning steel placed in ducts running vertically. The posttensioning steel provides lateral strength and stiffness to the structure as well as self-centering capability. The paper introduces a design-oriented analytical model to estimate the nonlinear lateral load behavior of the walls and validates the model against available experimental results and a more detailed analytical model using fiber elements.

Posttensioning can also be an effective method to increase the lateral strength and stiffness of masonry wall structures. In a paper entitled “Behavior of Slender, Posttensioned Masonry Walls under Transverse Loading,” Bean et al. conduct an experimental and analytical investigation of twelve masonry wall specimens under out-of-plane lateral loads. The parameters investigated include walls built with cored clay brick and hollow concrete block, unrestrained and restrained posttensioning tendons, and amount of posttensioning force. In a related paper, Wight et al. describe “Shake Table Testing of Posttensioned Concrete Masonry Walls with Openings.” This study focuses on the in-plane dynamic response of two single-story walls in residential construction with door and window openings. The walls are posttensioned using unbonded high-strength bars to reduce damage and minimize residual displacements.

The greatest setback for the use of unbonded posttensioned precast concrete structures in seismic regions is that, as a result of small energy dissipation, their lateral displacement demands during a severe earthquake may be larger than acceptable. In a paper entitled “Seismic Performance of Self-Centering Structural Walls Incorporating Energy Dissipators,” Restrepo and Rahman investigate the use of longitudinal mild steel reinforcement crossing the joint between the wall and the foundation to provide energy dissipation. The paper focuses on the quasi-static reversed cyclic lateral load tests of three wall specimens as well as the design of these walls to ensure that the desired behavior is achieved.

Energy dissipation can also be achieved by using coupling beams in between two or more wall piers. As an added benefit, coupling beams also result in a significant increase in the lateral stiffness and strength of the structure. In a paper entitled “Nonlinear Behavior of Precast Concrete Coupling Beams under Lateral Loads,” Weldon and Kurama present an analytical investigation on the nonlinear behavior of precast concrete coupling beams. Different from conventional monolithic concrete coupling beams, coupling of reinforced concrete wall piers in the new system is achieved by posttensioning the beams and the walls together at the floor and roof levels. Steel top and seat angles are used at the beam ends to yield and provide energy dissipation during an earthquake, with no need to use mild steel reinforcement crossing the beam-to-wall joints, resulting in more favorable construction details.

The last two papers of this section focus on the blast resistance and progressive failure of precast concrete panel structures. First, Ngo et al. describe the “Behavior of Ultrahigh-Strength Prestressed Concrete Panels Subjected to Blast Loading.” The paper presents the results of an experimental investigation on four $2\mathrm{m}\times 1\mathrm{m}$ prestressed concrete panels with various thicknesses and reinforcement details under a 6 ton TNT equivalent explosion at stand-off distances of 30 and $40\mathrm{m}$ . The experimental results are used to validate a finite element model for the analysis of concrete panel structures subjected to blast and impact loading. Then, Yagust and Yankelevsky describe an analytical investigation “On Potential Progressive Failure of Large-Panel Buildings.” This paper presents a method to determine the stability of large precast panel buildings with interior load bearing walls. It is assumed that the structure behaves as a plastic system when subjected to a severe disturbance. The damaged structural system is evaluated to determine whether a new stable structural system can form, or if stability cannot be attained and progressive collapse will follow.

Previous earthquakes have shown the importance and potential vulnerability of precast concrete floor diaphragms under severe seismic loading. Three papers in this section of the Journal focus on this issue. First, Rodriguez et al. investigate the “Seismic Design Forces for Rigid Floor Diaphragms in Precast Concrete Building Structures.” The seismic design of precast floor diaphragms requires reliable procedures to estimate the in-plane diaphragm forces that develop during an earthquake. The paper presents the horizontal floor accelerations recorded from shake table tests of four small-scale multistory wall and frame-wall structures tested between 1979 and 1989. The floor accelerations from the tests are then compared with four predictive approaches for regular buildings with rigid floor diaphragms. The second paper in this section of the special issue, entitled “Appropriate Overstrength of Shear Reinforcement in Precast Concrete Diaphragms” authored by Fleischman and Wan, describes an analytical study on the performance of precast diaphragms with different shear strength to flexural strength ratios over a range of design parameters. Appropriate shear reinforcement design overstrength factors are proposed in terms of diaphragm geometry and tension deformation capacity of the reinforcing details. The effects of confining forces and force combinations are also examined. The last paper on precast concrete floor diaphragms investigates the “Design of Precast Diaphragm Chord Connections for In-Plane Tension Demands.” In this paper, Cao and Naito discuss the design and modeling of a welded chord connection commonly used in pre-topped precast double tee floor members. A detailed finite element model is developed and verified with experimental data. Then, a parametric analytical investigation is conducted considering various connection details and the findings are used to develop floor connection design recommendations.

Precast/prestressed concrete members are widely used in bridge structures. Significant advances have been made in recent years in the area of precast concrete bridge piers, bridge girders, and bent caps, which are the focus of four papers in this section of the special issue. The first two papers investigate bridge piers using unbonded posttensioning for lateral strength and self-centering during an earthquake. Ou et al. discuss the “Seismic Performance of Segmental Precast Unbonded Posttensioned Concrete Bridge Columns.” Bonded reinforcing bars crossing the segment joints are used to enhance the energy dissipation of the structure. A simple analytical model and a more detailed three-dimensional finite element model are developed to investigate the optimum amount of energy dissipating bars to achieve maximum energy dissipation while maintaining minimal residual displacement upon unloading. Response comparisons with conventional monolithic concrete bridge piers are also provided. In a related paper, Palermo et al. discuss the “Design, Modeling, and Experimental Response of Seismic Resistant Bridge Piers with Posttensioned Dissipating Connections.” In this contribution, results of quasi-static reversed cyclic tests on $1∕3$ -scale bridge pier specimens incorporating unbonded posttensioning steel with or without supplemental energy dissipaters are presented. The test results are used to validate a simplified analytical procedure and modeling approach for the piers. Experimental comparisons with a benchmark specimen representing conventional ductile monolithic reinforced concrete construction are also provided.

The third paper in this section of the Journal is on the “Effects of Existing Shear Damage on Externally Posttensioned Repair of Bent Caps” by Aravinthan and Suntharavadivel. The effect of existing shear cracks in precast concrete bridge bent caps strengthened by external posttensioning is experimentally investigated using model specimens. It is found that, when shear cracks exist, the shear capacity of a bent cap is not enhanced by external posttensioning. The existing shear cracks need to be repaired prior to the application of posttensioning. In the last paper of the special issue, Baran et al. focus on the “Behavior of Girder-Floor Beam Connections in Prestressed Concrete Pedestrian Bridges Subjected to Lateral Impact Loads.” This paper investigates prestressed concrete through-girder pedestrian bridges under transverse lateral loads caused by collision with over-height vehicles. A series of static tests on full-scale floor-beam-to-prestressed-concrete-girder connection subassembly specimens with different types of steel inserts are presented. Finite element models of the specimens are also developed and calibrated with data obtained from the experiments.