Journal of Structural Engineering

The unique features of the hybrid frame, which include minimum structural damage when subjected to earthquake loading and the recentering capability, are the result of using a combination of mild steel reinforcement and unbonded prestressing to establish connections between precast beams and precast columns. Using acceptance criteria defined in terms of interstory frame drift and floor acceleration, this paper presents a multiple-level performance-based seismic evaluation for two five-story precast concrete hybrid frame buildings. The design and analysis of these two buildings, established as the displacement-based and force-based design solutions for a prototype building used in the precast seismic structural system (PRESSS) program, were conducted at 60% scale so that the analysis models could be validated using the PRESSS test data. Despite a difference of 40% in the design base shear, the two buildings satisfied the acceptance criteria when subjected to input motions with intensities less than or equal to that of the design-level earthquake. For input motions, equal to 150% of the design-level earthquake, the building designed using the displacement-based principles did not satisfy the interstory drift limit, whereas the force-based solution provided acceptable performance.

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.

Design methods for the shear resistance of reinforced and prestressed concrete beams are based on empirical evidence. Due to different approaches in their development, different equations are used to calculate the shear strength of reinforced and prestressed concrete. Recent research has provided a simplified approach for the shear design of reinforced concrete that eliminates many of the shortcomings of current design procedures and corresponds well to a wide range of test results. The objective of this research was to investigate the applicability of this new approach to prestressed concrete. The applicability of the shear model was evaluated by a comparison of its results with the experimental strengths of 86 specimens which failed in shear. This investigation indicated that the shear model is applicable to prestressed sections. For design purposes, the shear model was simplified to develop a design expression that unifies the design of reinforced and prestressed concrete sections.

Reversed cyclic loading tests of eight prestressed and two nonprestressed concrete exterior beam-column joint assemblies that failed inside the joint cores were conducted. The test results revealed no significant difference in the maximum story shear force between the prestressed and nonprestressed concrete test units although the horizontal input shear forces of the joint cores at the maximum story shear forces of the prestressed concrete units (calculated based on the column shear force and the forces in the reinforcing and the prestressing steels) were greater than those of the nonprestressed concrete units. On the basis of this result, the writers have concluded that prestressing force does not play a major role on the ultimate shear strength of beam-column joint cores. In addition, the design shear force acting on a beam-column joint core should be determined using the average shear force calculated from the bending moment at the beam-column interfaces, not by the horizontal input shear force. These conclusions led the writers to develop design equations for estimating design shear forces and ultimate shear capacities of prestressed concrete interior and corner joints. The equations were compared with test data on 51 beam-column joint assemblies that failed in shear in the joint cores. The test results agreed well with the estimations.

The use of unbonded posttensioned (UPT) precast concrete walls with horizontal joints as the primary lateral load resisting system in seismic zones has been considered in several previous studies. This paper introduces a design-oriented analytical model that uses simple formulae to estimate the nonlinear lateral load behavior of UPT walls, and compares this simple model with available experimental results. A previously developed UPT wall model based on fiber elements is also compared with experimental results. Each model is formulated to consider several critical limit states in the lateral load behavior of UPT walls. Comparisons showed generally good agreement between analytical and experimental results for three different test walls under monotonic and cyclic loading. The simple model is found to be sufficiently accurate for seismic design of UPT walls, and the fiber model is found to be sufficiently accurate for estimating UPT wall response under earthquake loading.

A conceptual model of the response of slender, posttensioned masonry walls to uniformly distributed, transverse loading is introduced and verified through an experimental program. To validate the model, 12 simply supported 3.54 m (11.6 ft) tall walls with $810\times 100\mathrm{mm}$ $(32\times 4\mathrm{in.})$ cross sections were tested under monotonically increasing transverse (i.e., out-of-plane lateral) loads. Six walls were built using cored clay brick, and the remaining six using hollow concrete block. The walls were posttensioned using threaded steel bars with six walls featuring “unrestrained” tendons (i.e., neither grout nor mechanical devices were used to restrain the bars), while the other six had “restrained” bars (i.e., mechanical devices were used to restrain the bars relative to the masonry), and three magnitudes of effective prestress were investigated. Initial response to transverse loads was linear, but cracking was observed over a broad range of loading. The response of the wall specimens is traced through crack propagation, hinge formation, and development of a plastic section.

The in-plane seismic response of posttensioned concrete masonry walls with openings is investigated by means of shake table testing. A test program was initiated to verify the seismic performance of a wall system for use in residential construction. Two single-story in-plane wall tests were conducted initially to study the effect of door and window openings. This led to the construction and testing of a simple square structure which investigated the influence of multiple door and window openings and wall corners. All walls were posttensioned using unbonded high-tensile steel bars which allowed the wall panels to rock at the wall-foundation interface. Wall damage consisted of masonry crushing in the lower wall corners and vertical cracking under the openings and in the lintels. Residual displacements were minimal, due to the restoring forces provided by the posttensioning tendons. A complete description of the test setup and results is provided, including discussion on wall structural response in terms of flexural strength, displacement capacity, deformed shape, and damage patterns.

This paper presents elements of seismic design for jointed precast cantilever wall units designed to rock about their foundation. Gravity loading and prestressed unbonded tendons provide the restoring force in these walls. Lateral displacements eventually result in a separation gap forming only at the wall-foundation beam connection. The gap reduces the wall stiffness and results in nonlinear response. Design of these walls is made with the explicit objective of ensuring a self-centering response. That is, the wall returns to its pre-earthquake position upon unloading from a large displacement excursion. The integrity of the walls is maintained because no plastic hinges form and there are no residual postearthquake displacements. Energy dissipators, in the way of longitudinal mild steel reinforcement crossing the joint between the walls and the foundation, are incorporated into these walls to add significant energy dissipation capacity while preserving the self-centering response. This paper also describes the results of an experimental program performed on three half-scale precast concrete jointed walls. The wall units were tested under quasi-static reversed cyclic loading conditions. These units were designed for various performance objectives. The test units underwent a prescribed quasi-static reversed cyclic testing regime and attained drift ratios in excess of 3% while preserving their lateral-force capacity and ability to self-center. Two units incorporated energy dissipators. These displayed a characteristic “flag-shape” hysteretic response and had equivalent viscous damping ratios of 14%. The performance objectives selected in the design of the walls were closely met in the experimental program.

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.

This paper presents the results of an experimental investigation conducted in Woomera, South Australia in May 2004 on the blast-resistance of concrete panels made of ultrahigh-strength concrete (UHSC) material. A special concrete supporting frame was designed for testing concrete panel targets against blast loading. Four $2\mathrm{m}\times 1\mathrm{m}$ panels with various thicknesses and reinforcement details were tested under a $6\mathrm{t}$ TNT equivalent explosion at standoff distances of 30 and $40\mathrm{m}$ . Data collected from each specimen included blast pressures and deflections of panels. The test data were analyzed to assess the performance of UHSC and normal-strength concrete (NSC) panels. The test results and observations showed that the 100-mm-thick UHSC panels performed extremely well surviving the blast with minor cracks. The 75-mm-thick UHSC panel suffered moderate damage while the 100-mm-thick NSC panel was breached. Test results from this experimental program were used to validate a finite-element computer code that was developed by the writers to analyze concrete structures subjected to blast and impact loading.

The potential vulnerability of engineering structures to extreme loading actions, due to which a progressive failure of the entire structure or of a major part of it may develop, emphasizes the need to develop comprehensive analytical procedures for the assessment of the potential progressive collapse of structural systems. This paper aims at presenting a method for calculation of the stability of large-prefabricated panel buildings with interior load bearing walls. Assume that the structural system behaves as a plastic system that is subjected to a severe disturbance, which causes a local failure that may develop into a progressive collapse. It is the objective of this paper to assess the damaged structural system in order to determine whether a new stable structural system will form that is able to maintain static equilibrium and carry the loads or whether stability cannot be attained and a process of progressive collapse will follow. That important observation of whether a progressive collapse will follow or not can be done by static analysis of the damaged structure, whereas the progressive collapse process requires dynamic analysis and is beyond the scope of this paper. The basics of the method were originally developed by Strugatsky (1936–2000), but gained limited exposure and were published in sources of limited access [Strugatsky (1980) in Investigation of Bearing Concrete and Reinforced Concrete Structures of Multistory Precast Buildings, MINITEP, Moscow, 74–111; Strugatsky and Eisman (1989) in Design Guide of Residential Buildings, Construction of Residential Building, Appendix 2, SNIP, Moscow, 232–268; Shapiro et al. (2000), Report to Administration of Moscow, Moscow] and are therefore almost unknown. Due to its engineering significance and relevance, it was adopted as the grounds of an extended methodology that was developed recently and is presented in this paper. For the sake of completeness, this paper provides a concise presentation of the main principles of the method that refers to an initial local failure in the exterior envelope of a structure with cross interior load-bearing walls, and further extends the model. The extended model also addresses local failures that develop within the interior part of the structure and generalizes the approach to consider a larger family of buildings that include transverse or longitudinal interior bearing walls. As panel buildings seem to be a basic structural system, the approach developed herein may establish the foundations to an approach that may be later extended to systems that include other structural components such as frames, infill walls, etc.

Floor accelerations are needed to evaluate in-plane diaphragm forces in earthquake resistant design of buildings, and for the design of their connections. Recorded floor accelerations in buildings during some past earthquakes have shown acceleration magnifications that are not properly considered by current building codes. Earthquake damage in some precast buildings seems to point out significant deficiencies in the design of precast diaphragms. Two deficiencies stand out: the magnitude of the design forces and the lack of suitable design methods. This paper describes an experimental validation of a previously proposed analytical-based procedure for the evaluation of earthquake-induced floor horizontal accelerations in regular buildings built with rigid diaphragms. Horizontal floor accelerations recorded from shake table tests of four small-scale structures representing frame and frame-wall multistory reinforced concrete buildings were collected. These buildings were tested on the University of Illinois Urbana-Champaign shaking table during the 1979–1989 period. The horizontal floor accelerations obtained from the tests are compared in this paper with results from four predictive approaches.

Current precast concrete diaphragm design may not necessarily protect the diaphragm from a nonductile shear failure in an overload situation. As evidence exists that diaphragms can attract large inertial loads during strong ground motion, emerging design methodologies are aiming to develop the diaphragm flexural strength. However, the performance target desirable or achievable in design may depend on a number of other factors. An analytical study examines the performance of precast diaphragms with different shear strength relative to design (flexural) strength, termed here “shear reinforcement overstrength.” The objective of the study is to determine the required shear reinforcement overstrength in the precast diaphragm to produce certain performance targets. Appropriate shear reinforcement overstrength design factors are proposed in terms of a number of key parameters related to diaphragm geometry and the properties of the diaphragm reinforcing details.

This paper discusses the design and modeling of a welded chord connection commonly used in pretopped precast double tee panels. In conventional design, the chord is intended to achieve its tension strength based on the axial capacity of the anchorage bars. An experimental study has shown that premature failure of the weld and flexure of the bar may occur, significantly reducing the tension capacity of the connection. A detailed finite-element model is developed and verified with experimental data and used for a parametric investigation. The results indicate that a ductile chord connection, with predictable capacity, can be achieved through concentric placement of the welds relative to the anchorage bar. Variations in faceplate thickness and weld throat are found to alter the load path but do not prevent premature failure. Mechanical debonding of the anchorage bars is found to significantly enhance the deformability of the connection. These findings are incorporated in a force and displacement based design procedure for the chord connection; an example is included for illustration.

The seismic performance of segmental precast unbonded posttensioned bridge columns with hollow cross sections is investigated in this paper. Bonded longitudinal mild steel reinforcement crossing the column segment joints is provided to enhance the hysteretic energy dissipation of the columns, also referred to as energy dissipation (ED) bars in this paper. The ED bars are detailed to provide energy dissipation without premature fracture at the critical column segment joint. For design purposes, a simplified analytical model to predict the lateral force-displacement response (pushover curve) is proposed. Moreover, a detailed three-dimensional (3D) finite-element (FE) model is developed and utilized to predict the behavior of segmental columns under cyclic loading. Parametric studies using the simplified analytical model and the 3D FE model are conducted to investigate the seismic behavior of this type of column construction. At the end of this paper, the comparisons of the results of the response-history analyses between the segmental columns with flag-shape hysteretic behavior and their comparable conventional counterparts are given.

An increasing interest in the development of high-performance seismic resisting systems based on posttensioned, jointed ductile connections has been observed in the last decade. The extensive experimental and numerical studies carried out under the PRESSS program developed efficient alternative solutions for seismic resisting frame or wall systems in precast concrete building construction, typically referred to as jointed ductile connections. Low structural damage and self-centering behavior, leading to negligible residual displacements after an earthquake event, were recognized to be the main features of such systems. Recently, the extension and application of similar technology and seismic design methodologies to bridge piers and systems have been proposed in the literature as a viable and promising alternative to traditional cast-in situ or precast construction. However, a broad acceptance of these solutions in the bridge design and construction industry has yet to be observed. Valid justifications can be found in the lack of official guidelines for design and construction detailing as well as in the general apparent complexity of the design procedure and analytical models presented by the scientific community. In this contribution, confirmations of the unique design flexibility, the ease of construction, and the high seismic performance of jointed ductile hybrid systems, combining recentering and dissipation capabilities, are presented. After a presentation of simple design methodologies and modeling aspects herein adopted to fully control the seismic response of these systems, the experimental results of quasistatic cyclic tests on five 1:3 scaled, bridge pier specimens are reported and discussed. Four alternative hybrid configurations are implemented by varying the ratio between the posttensioning steel and the internal mild steel as well as the initial posttensioning load. Lower levels of damage and negligible residual/permanent deformations are observed in the hybrid solutions when compared to the experimental response of the benchmark specimen, representing a typical monolithic (cast-in situ) ductile solution. In addition, the efficiency of the simple analytical procedure adopted for design and modeling is further validated.

External posttensioning can be considered an effective strengthening method for bridge members, which are deteriorating due to extreme loading conditions and progressive structural aging. The effects of existing shear cracks in bent caps strengthened by external posttensioning were experimentally investigated using model specimens. Initial investigation revealed that the shear capacity of a bent cap is not enhanced by external posttensioning only, when shear cracks exist. Moreover, it was found that the repair of existing cracks using epoxy injection substantially increases the shear capacity of the bent cap. The Tenthill Creek Bridge in South East Queensland, Australia provides a case study where the bent caps have been strengthened by external posttensioning. The model tests of the bridge bent caps confirmed the initial findings on the effects of existing shear cracks on the shear capacity. The experimental results emphasize the need for appropriate repair of existing shear cracks prior to strengthening by external posttensioning to enhance the shear capacity of bent caps.

Each year a large number of concrete bridges are subjected to impact by overheight vehicles or vehicles carrying overheight objects. Most bridge owners in the United States are concerned with the increasing trend of bridges impacted by overheight vehicles, thereby increasing the urgency to evaluate the resistance of bridges to lateral impact loads. The present integrated numerical and experimental study investigates the behavior of a critical connection, between girders and floor beams, in a type of bridge that is considered to be particularly vulnerable, prestressed concrete through-girder (PCTG) pedestrian bridges. PCTG pedestrian bridges comprise precast, prestressed concrete girders connected by cast-in-place, reinforced concrete floor beams and a cast-in-place deck. The deck is connected to the floor beams only, and the floor beams are joined to the girders using embedded concrete anchors. The investigation combines three-dimensional finite-element analyses with physical testing to elucidate the load-deformation characteristics of the girder-floor beam connections. A series of six girder-floor beam connection subassemblage specimens were built and tested, three of which were subjected to vertical loading, while the other three specimens had a combination of vertical and horizontal load. Three types of anchors were investigated, including two types of loop inserts and one bolt insert. The study revealed that specimen response depends upon the characteristics of the embedded concrete anchors. Deterioration of specimen load capacity was found to be associated with concrete cracking, formation of a cone breakout surface, yielding of the anchors, and fracture of the inserts. The floor beam-girder subassemblages were able to resist large displacements after attaining peak load, even though postpeak load carrying capacity was undermined in most cases.