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This study numerically investigated the dynamic seismic response of multistory precast concrete frame structures using a novel reinforced-concrete buckling-restrained brace component. The maximum demands on the braces were quantified with the intent to guide future experimental testing and validation according to the collapse performance requirements of FEMA P695. A set of 26 braced frame archetypes were designed using seismic procedures consistent with current U.S. building code requirements, and effective linear-elastic as well as nonlinear numerical models of these frame structures were developed. In addition to monotonic static pushover analyses, the nonlinear archetype models were subjected to a set of 44 scaled ground motion records to quantify the system overstrength factor, maximum interstory drift, design story drift, brace ductility, cumulative brace ductility, and end gap closure. It was found that braces designed for a maximum design story drift of 4% and demonstrating ductility capacities of at least 51 and cumulative ductility capacities of at least 297 are needed for braced frame structures in high seismic regions to satisfy the median collapse performance criteria in FEMA P695. [ABSTRACT FROM AUTHOR]

This paper presents a numerical study that simulates the behavior of squat reinforced concrete (RC) shear walls with high-strength reinforcing steel and high-strength concrete. The finite element models are critically evaluated based on previous experiments of four deep-beam specimens and four squat shear-wall specimens with varied material strengths, base moment-to-shear ratios, and section shapes (rectangular and flanged). Monotonic lateral load analyses provided reasonable predictions of the peak lateral strength for squat walls tested under reversed-cyclic loading. However, reversed-cyclic models were necessary for more accurate predictions of the cyclic lateral load versus drift behavior, including cracking, stiffness degradation, lateral load-resistance mechanism, peak strength and corresponding drift, and energy dissipation. Importantly, the model predictions for specimens using high-strength materials were as good as or better than those using normal-strength materials with the same base moment-to-shear ratio. Thus, the use of higher-strength materials did not negatively impact the ability of the models to predict wall behavior. [ABSTRACT FROM AUTHOR]

Resistance factors (ϕ-factors) are reliability-based factors of safety used in load and resistance factor design (LRFD) of structural systems. The ϕ-factors specified in the consensus standard for the structural design of pultruded fiber-reinforced polymer (FRP) structural members and connections are generally lower than the ϕ-factors used for more conventional materials (e.g., structural steel, reinforced concrete, and wood) for comparable limit states, consequently potentially limiting the application of pultruded FRP as a structural material. This work describes a review of 350 published experimental tests of bolted connections in pultruded FRP and compares these test results with the nominal strengths from equations in the consensus standard. Subsequently, a first-order reliability method analysis—following the methodology proposed by Ellingwood for pultruded FRP composites—was performed to re-examine ϕ-factors for the most common limit states of bolted connections in pultruded FRP. All relevant limit states for bearing-type connections as defined by the consensus standard were considered in the study and the derived ϕ-factors were compared with the ϕ-factors prescribed in the consensus standard with the goal of highlighting the need for additional research. [ABSTRACT FROM AUTHOR]

This paper describes an experimental investigation of a novel reinforced concrete buckling-restrained brace for precast concrete lateral load-resisting frame structures in seismic regions. The proposed brace uses ductile reinforcing bars with unbonded lengths across end gaps and bonded lengths at the middle region for lateral stiffness, strength, energy dissipation, and ductility. The experimental program included four isolated diagonal brace subassemblies subjected to reversed-cyclic pseudostatic lateral loading. Local buckling of the energy-dissipation bars across the end gaps is the most critical failure mode that can limit the ductility of the brace in compression. Up to this failure, the bonded and unbonded regions of the braces performed as designed. The results demonstrated the different deformation and stiffness behaviors of the braces in tension and compression. Subsequent loading to large tension displacements provided evidence of the large energy dissipation that can be expected if premature failure of the brace can be prevented. Simplified numerical models provided good predictions of the measured brace behavior until failure. This research featured the first set of tests for this brace, and the results highlighted adjustments needed to design and modeling to achieve the desired behavior of the brace. Recommendations for future research include improved shear dowel and confinement designs to prevent local buckling of the energy-dissipation bars and improved longitudinal reinforcement designs to prevent yielding of the energy-dissipation bars in the middle bonded region of the braces. [ABSTRACT FROM AUTHOR]

Force-based elements with plastic hinge integration are often used to simulate the nonlinear behavior of reinforced concrete (RC) columns experiencing softening behavior. The response from these models can be highly sensitive to the assumed plastic hinge length, L p . The objective of this paper is to evaluate the accuracy and sensitivity of the global and local behaviors of RC columns to the assumed L p . The evaluation is conducted using unregularized and regularized material stress-strain relationships. Additionally, a regularized confined concrete crushing energy equation is proposed to increase the accuracy of the numerical results and reduce their sensitivity to the assumed L p . [ABSTRACT FROM AUTHOR]

This paper describes the results from eight half-scale experiments of unbonded posttensioned precast concrete coupling beams under reversed-cyclic lateral loading. Each test specimen includes a coupling beam and the adjacent concrete wall pier regions at a floor level. Under lateral loads, the nonlinear displacements of unbonded posttensioned coupling beams are governed by the opening of gaps at the beam-to-wall joints. Steel top and seat angles are used at the beam ends to yield and provide energy dissipation. The test parameters include the beam posttensioning tendon area and initial stress, initial beam concrete axial stress, angle strength, and beam depth. The results demonstrate the lateral stiffness, strength, and ductility of the coupling beams under cyclic loading, with considerable energy dissipation concentrated in the angles. It is shown that the residual displacements of the structure upon unloading are small due to the restoring effect of the posttensioning force. The sustained chord rotation capacities of the test specimens are compared with those from previous tests of monolithic coupling beams. DOI: 10.1061/(ASCE)ST.1943-541X.0000212 CE Database subject headings: Post tensioning; Prestressed concrete; Reinforced concrete: Coupling; Shear walls; Seismic effects; Beams. Author keywords: Concrete, posttensioned; Concrete, precast; Concrete, prestressed; Concrete, reinforced; Coupled walls: Coupling; Tests, seismic; Walls, shear.

This paper presents a seismic response evaluation of unbonded posttensioned precast concrete moment frames that use friction dampers at selected beam ends. The dampers utilize the displacements that occur as a result of gap opening between the beam and column members during an earthquake. The parameters investigated include the number of stories, number and strength of the dampers, and amount of posttensioning. Nonlinear static and dynamic time history analyses of prototype structures show that the dampers provide a considerable amount of energy dissipation to a frame, while the posttensioning force provides a restoring effect resulting in self-centering capability (i.e., ability of the structure to return towards its undisplaced position upon unloading). The seismic design of the structures to achieve target displacement-based performance objectives is critically evaluated based on the analysis results. Comparisons with structures that use mild steel (e.g., Grade 60) reinforcement through the beam-column joints as well as structures that use only posttensioning steel (with no friction dampers) and structures that use combinations of mild steel and posttensioning steel are also provided. DOI: 10.1061/(ASCE)0733-9445 (2008)134:1(132) CE Database subject headings: Concrete, post-tensioned; Concrete, precast; Concrete, prestressed; Concrete, reinforced; Damping; Energy; Frames; Seismic design.

This paper presents an analytical investigation of the nonlinear behavior of precast concrete coupling beams under monotonic lateral loading. In contrast to conventional monolithic concrete coupling beams, coupling of reinforced concrete wall piers in the proposed system is achieved by posttensioning the beams and the walls together at the floor and roof levels. Under lateral loads, the nonlinear deformations of the coupling beams occur primarily due to the opening of gaps at the beam-to-wall interfaces. The new system offers several advantages over monolithic systems, such as simpler detailing for the beams and the wall piers (no need for diagonal reinforcement crossing the beam-to-wall joints), reduced damage to the structure, and an ability to self-center, thus reducing the residual lateral displacements of the building after a large earthquake. Steel top and seat angles are used at the beam ends to yield and provide energy dissipation during the earthquake. The paper investigates the effects of design parameters, such as the amount of posttensioning, beam and wall properties, and top and seat angle properties, on the lateral strength and displacement characteristics of floor-level coupling beam subassemblies. The results are used to determine how the system behavior can be controlled by design. A procedure to estimate the nonlinear lateral load-deformation relationship of the subassemblies is developed. DOI: 10.1061/(ASCE)0733-9445 (2007) 133:11 (1571) CE Database subject headings: Concrete, post-tensioned; Concrete, precast; Concrete, prestressed; Concrete, reinforced; Coupled walls; Coupling; Seismic analysis; Seismic design; Lateral loads.

This paper presents the seismic design of unbonded posttensioned precast concrete frame structures that use friction dampers for supplemental energy dissipation. 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 a trial frame with given geometry, beam and column member dimensions, and selected damper distribution. The proposed design procedure assumes that the lateral strength requirements for the frame have been obtained from a linear elastic analysis of the structure under equivalent lateral forces. Nonlinear reversed-cyclic analyses of friction-damped precast concrete beam-column subassemblies and multistory frame structures under lateral loads are conducted to critically evaluate the design procedure and to identify areas where improvement may be needed. The analytical results show that friction-damped precast frames can be designed to achieve significant energy dissipation levels while maintaining a large level of self-centering capability due to the posttensioning force. DOI: 10.1061/(ASCE)0733-9445(2007)133:11(1501) CE Database subject headings: Concrete, posttensioned; Concrete, precast; Concrete, prestressed; Concrete, reinforced; Damping; Energy; Frames; Seismic analysis; Seismic design.

Eleven half-scale experiments were conducted to investigate the nonlinear reversed cyclic behavior of coupling beam subassemblages in a new type of hybrid coupled wall system for seismic regions. In this new system, coupling of concrete walls is achieved by posttensioning steel beams to the walls using unbonded posttensioning tendons. Different from conventional hybrid coupled walls, the coupling beams are not embedded into the walls. Each test specimen described in this paper includes a steel coupling beam and the adjacent concrete wall regions at a floor level. Steel top and seat angles are used at the beam-to-wall connections to yield and provide energy dissipation. The test parameters include the angle leg thickness, angle gauge length, beam flange cover plate, posttensioning steel area, and beam depth. The results demonstrate excellent stiffness, strength, and ductility characteristics of properly designed subassemblages under cyclic loading, with considerable energy dissipation concentrated in the angles. The specimens were able to sustain large nonlinear displacements without significant damage in the beams and the wall regions; demonstrating that these structural components would require little repair, if any, after a severe earthquake. The residual deformations upon unloading of the specimens were negligible due to the restoring effect of the posttensioning force. CE Database subject headings: Concrete; Post tensioning; Coupled walls; Coupling; Hybrid methods; Walls; Shear.

This paper uses the experimental results presented in a companion paper to develop a seismic design approach for unbonded posttensioned hybrid coupled wall subassemblages. The test results are also used to verify and refine analytical models for the subassemblages. The proposed design approach includes guidelines for the selection of the coupling beam cross-section and the posttensioning steel area, as well as the design of the top and seat angles, their connections, and the design of the concrete wall contact regions. CE Database subject headings: Concrete; Post tensioning; Coupled walls; Coupling; Hybrid methods; Seismic design; Seismic analysis; Walls; Shear.

This paper describes an analytical investigation of a new type of hybrid coupled wall system for seismic regions. Coupling of concrete walls is achieved by posttensioning steel beams to the walls using unbonded posttensioning tendons. Different from conventional hybrid coupled walls, the coupling beams of the new system are not embedded into the walls. The effect of structural design parameters such as the amount of posttensioning, beam properties, and wall properties on the behavior of multistory coupled walls under lateral loads, including the amount of coupling, energy dissipation, and displacement capacity is investigated. Systems with precast concrete walls as well as monolithic cast-in-place reinforced concrete walls are considered. The behavior of posttensioned coupled wall systems is compared with the behavior of systems with embedded steel coupling beams and systems without coupling. Design tools to estimate the nonlinear lateral load-displacement behavior of the walls are developed by quantifying selected limit states for the walls. The results indicate that posttensioned hybrid coupled walls with initial stiffness similar to walls with embedded steel coupling beams can be designed to provide stable levels of resistance under lateral loads over large nonlinear cyclic deformations. The degree of coupling between the walls can be controlled by changing the amount of posttensioning in the beams, as well as other beam and wall properties. DOI: 10.1061/(ASCE)0733-9445(2004) 130:2(297) CE Database subject headings: Coupled walls; Coupling; Concrete, precast; Concrete, posttensioned; Concrete, reinforced; Seismic analysis; Shear walls; Lateral loads.

This paper discusses the nonlinear load-deformation behavior of coupling beam subassemblages in a new type of hybrid coupled wall system for seismic regions. Coupling of concrete walls is achieved by posttensioning steel beams to the walls using unbonded posttensioning tendons. Different from conventional hybrid coupled wall systems, the coupling beams of the new system are not embedded into the walls. One of the most important advantages of the new system is that it can be used to couple existing walls as a part of a strengthening and retrofit scheme. This paper describes an analytical investigation of the nonlinear behavior of coupled wall subassemblages that include a coupling beam and the adjacent wall regions at a floor level. The effect of structural design parameters such as the amount of posttensioning, beam properties, and wall properties on the behavior of the subassemblages, including the amount of coupling, energy dissipation, and deformation capacity is investigated. A simple design method to estimate the nonlinear load-deformation behavior of the subassemblages is developed. The investigation is based on an analytical model using fiber elements, which is verified by comparing with a finite element model. The results indicate that unbonded posttensioned steel coupling beams with initial stiffness similar to embedded steel beams can be designed to provide stable levels of coupling without experiencing significant damage over large nonlinear cyclic deformations. The amount of coupling between the walls can be controlled by changing the posttensioning force. DOI: 10.1061/(ASCE)0733-9445(2002) 128:10(1290) CE Database keywords: Coupled walls; Coupling; Concrete, prestressed; Seismic design; Walls; Post tensioning.

This paper describes a numerical evaluation of the seismic design of precast concrete buckling-restrained braced frames based on the Federal Emergency Management Agency's Quantification of Building Seismic Performance Factors (FEMA P695) methodology. A set of 32 archetype braced frames covering a range of parameters were designed using a procedure consistent with current U.S. building code requirements. Nonlinear numerical models were developed and verified against existing experimental data. The results show that large axial compression and tension forces develop in both beams and columns, thus requiring these members to be designed with large reinforcement ratios or higher-grade reinforcing bars and beams to be designed like column members. Recommended beam and column effective (cracked) linear-elastic axial and flexural stiffness reduction factors provide reasonable estimates of story drifts and brace deformations under design lateral forces. Nonlinear monotonic static pushover and incremental dynamic analyses of the archetypes support an overstrength factor of 2.5, response modification coefficient of 8, and deflection amplification factor of 8 for the seismic design of this system. [ABSTRACT FROM AUTHOR]

This paper investigates the effect of moment-to-shear ratio and flanges on the behavior of squat reinforced concrete (RC) shear walls with high-strength materials. Pseudo-static, reversed-cyclic lateral load behavior of three squat walls--two rectangular walls with uniform thickness and different moment-to-shear ratios, and one I-shaped wall--are compared. The walls used high-strength reinforcement (fy ≈ 850 MPa [123 ksi]), high-strength concrete (fc' ≈ 100 MPa [14.5 ksi]), and the same uniformly distributed vertical and horizontal web reinforcement ratio, ρsw ≈ 0.83%. It is shown that: 1) squat rectangular walls without boundary regions can develop flexural failure, especially when using highstrength concrete; 2) the addition of flange regions at the wall ends increases shear-critical behaviors, such as increased shear deformations, steeper diagonal cracks, and crushing in the wall web; 3) web regions in flanged walls with high-strength concrete can have greater capacity than current ACI limits for normalized shear stress; and 4) current ACI (318-19 and 349-13) and ASCE/SEI (43-05) nominal lateral strength equations and relevant commentaries for squat walls may need to be updated to include the effects of moment-to-shear ratio, flanges, and high-strength materials. [ABSTRACT FROM AUTHOR]

This paper describes a numerical parametric investigation on the effects of coarse recycled concrete aggregates (RCAs) on the time-dependent sustained service-load deflections of reinforced concrete beams. The varied parameters are the volumetric replacement of coarse aggregate, concrete compressive strength, beam geometry (length, depth, width), rotational restraint at beam ends, top and bottom longitudinal steel areas, ultimate shrinkage strain, ultimate creep coefficient, and ambient relative humidity. In addition, RCA quality is quantified and varied through the water absorption and deleterious material content (for example, wood and asphalt). The beams were analyzed using a time-dependent concrete material model that can capture cracking, shrinkage, and creep behavior. The results show that the deflections of beams with good-quality RCA (with low water absorption and deleterious material content) are not much greater than those for beams with natural aggregate (NA). Importantly, RCA amount and quality have larger effects on the deflections of beams with less cracking (that is, shorter and/or deeper beams) and for beams with increased rotational restraint, for which allowable deflection limits would be less likely to govern design. There are significant differences between the ACI 318 time-dependent deflection factor and the numerical simulation results, with ACI 318 generally providing overly conservative estimates of the time-dependent factor, except for beams with minimal or no cracking. It is also found that RCA has a greater effect on the time-dependent deflections than on the initial deflection. [ABSTRACT FROM AUTHOR]