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Seven-wire prestressing strands are commonly used to pretension concrete bridge members. While the industry standard is 12.7-mm (0.5-in.) or 15.2-mm (0.6-in.) diameter strand, interest in developing greater pretensioning forces has resulted in the proposed use of 17.8-mm (0.7-in.) strands. This paper reports an experimental program aimed at characterizing the bond properties of 17.8-mm seven-wire prestressing strand and contrasting these with 9.5-mm (3/8-in.), 12.7-mm (1/2-in.), and 15.2-mm (−0.6-in) strands. The geometric and material properties of the strands are reported, and the evaluation of bond performance is assessed by standard test methods. Beam-end bond tests of both straight and 90° hooked strands are presented. The results of in situ transfer length determined from full-scale girders are also presented. Finally, finite-element analyses aimed at evaluating the influence of the Hoyer effect and strand spacing are presented. In all the tests, bond performance of 17.8-mm strand was found to be adequate and appropriately represented by existing AASHTO transfer and development length equations. The potential for utilizing the embedment of hooked strands into cast-in-place end diaphragms to increase the strand stress that may be developed near girder ends is proposed. From the in situ determination of transfer length, the mean and fifth percentile transfer lengths (determined with 95% confidence) of 0.7-in. strands were determined to be 30.4db (db is the strand diameter) and 53.4db, respectively, both less than the 60db prescribed by AASHTO. Additionally, the conventional 51-mm (2-in.) minimum center-to-center strand spacing appears to be adequate for 17.8-mm strands in terms of release stresses. No local cracking or other deleterious effects associated with strand spacing were observed. It is shown numerically that the lower Hoyer effect strand dilation and larger circumference result in marginally lower crack-inducing circumferential stresses. Although the current AASHTO determination of transfer and development length appears to overestimate these values for all strand diameters including 17.8-mm strands, the overestimation of transfer length underestimates concrete tensile stresses at prestress transfer in the area affected by the transfer length. This may result in unanticipated cracking. To address this, a two-tier approach is proposed: (1) using a reduced transfer length—40db is proposed—to check tensile stresses at prestress release; and (2) using the longer development length—using a transfer length component of 60db—to determine the load-carrying capacity of a prestressed concrete member. [ABSTRACT FROM AUTHOR]

There has been a growing interest in using large-diameter strands to alleviate congestion by using fewer strands, reduce the total number of girders by increasing girder spacing, increase the span length, and allow shallower girders. While the 2020 AASHTO LRFD Bridge Design Specifications included 17.8-mm (0.7-in.) strands by reference to AASHTO M203, these larger-diameter strands were not used in bridge construction primarily because AASHTO LRFD was silent about the design aspects of members reinforced with 17.8-mm (0.7-in.) strands. The presented multifaceted, multiyear research involved an extensive parametric design case study, nonlinear finite-element analyses, material characterization, component tests, and full-scale girder experiments. This paper focuses on the full-scale girder tests, which were used to examine development length, detailing requirements, and flexural and shear behavior and strength. Experimentally determined development lengths were found to be shorter than those prescribed by the AASHTO LRFD Specification. Flexural and shear strength could be determined using established procedures. The current minimum required amount of confinement reinforcement was found to be sufficient to confine 17.8-mm (0.7-in.) strands. The extension of bottom flange confinement reinforcement was found to be inadequate for cases with partially debonded 17.8-mm (0.7-in.) strands, but extension of bottom flange confinement reinforcement to 1.5d beyond the end of the girder was adequate for cases with no debonded strands. The minimum bottom flange confinement reinforcement required by the AASHTO LRFD Specification must be extended to at least 1.5d beyond the termination of the longest debonded length of 17.8-mm (0.7-in.) strands. [ABSTRACT FROM AUTHOR]

Hybrid coupled walls (HCWs) are comprised of two or more reinforced concrete wall piers connected with steel coupling beams distributed over the height of the structure. Extensive research over the past several decades suggests that such systems are particularly well suited for use in regions of moderate to high seismic risk. This paper reviews the state of the art in seismic modeling, analysis, and design of HCW systems. Design methodologies are presented in both prescriptive and performance-based design formats and a discussion of alterative types of hybrid wall systems is provided. DOI: 10.1061/(ASCE)ST.1943-541X.0000186 CE Database subject headings: Seismic design; Coupling; Reinforced concrete; Shear walls. Author keywords: Seismic; Design; Coupled wall system; Reinforced concrete; Walls; Shear walls; Performance-based design; Analysis; Composite.

When coupled core wall (CCW) systems are built in regions of high seismicity, the ductility demands on the coupling beams can be of critical concern. Steel coupling beams, whether encased in concrete or not, offer a very high degree of ductility relative to common concrete coupling beams. Hybrid core wall systems, that is CCW systems with steel or steel/concrete composite coupling beams, provide excellent lateral stiffness from the walls and coupling action, while providing excellent energy dissipation and ductility characteristics of steel coupling beams. Previous research pertaining to steel coupling beams has made great strides in furthering the understanding of the behavior of steel coupling beams, and recommendations regarding design methodologies have been established. However, as steel coupling beam ends are embedded in the wall piers, postdamage repair can be costly. This paper presents the results of large-scale cyclic tests of a steel coupling beam designed and detailed based on the writers' previous recommendations and an innovative 'fuse' steel coupling beam which provides an added feature to the steel coupling beam in that postdamage repair/replacement difficulties and expenses are minimized. CE Database subject headings: Coupling; Composite structures; Seismic design; Steel beams.

The architectural and structural advantages offered by coupled core walls, constructed using coupling beams, are presented. The design methodologies for the development of next generation of steel and composite coupling beams are also presented.

Adequate strength of headed studs under cyclic loads is a prerequisite for satisfactory performance of outrigger beam-core wall connections that commonly use multiple headed studs. Previous studies have typically not investigated the effects of reinforcement around studs, and have not accounted for the effects of cracking, damage, and yielding of reinforcement on the strength of studs. To remedy these deficiencies, two 1/4-scale subassemblies that contain a cantilever wall and two outrigger beams with and without floor diaphragms were subjected to cyclic loading. The wall reinforcement around the connection was selected according to the anticipated level of cracking and plastic hinge formation. The design methodology followed in this research resulted in connections that could develop and exceed the design forces despite extensive cracking and yielding of wall reinforcement around the headed studs. The presence of heavily confined wall boundary elements around headed studs increases the capacity. Simple methods to account for the influence of cracks and strengthening effects of boundary elements were developed. The resulting analytical model was able to accurately establish the expected mode of failure and capacity of outrigger beam-wall connections. For the outrigger beam-wall connection detail selected, the outrigger beam was found to transfer the majority of diaphragm forces directly to the core wall with negligible participation by the floor. Therefore, floor slab-wall connections can be based on simple details that resist only gravity loads unless the connections are specifically designed to transfer the diaphragm forces to the wall. DOI: 10.1061/(ASCE)0733-9445(2004) 130:2(262) CE Database subject headings: Connections; Composite structures; Cyclic loads; Seismic design; Studs; Walls; Beams.

A key factor behind successful performance of outrigger beam-core wall connections in hybrid structures is the adequacy of headed studs that are typically used to connect a stud plate, onto which the outrigger beam is connected through a shear tab, to the wall. In an effort to better understand cyclic behavior of stud groups under combined action of gravity shear and cyclic diaphragm forces, the research reported was undertaken. Six 1/3-scale subassemblies that involve a portion of the wall, connection, and outrigger beam were fabricated and tested. The test results suggest that a design method which closely follows available equations for computing stud capacity results in connections that reach the design loads; however, the mode of failure is stud pullout. This mode of failure does not have any ductility and lacks energy dissipation capabilities. Moreover, the available design equations fail to capture the substantial increase in strength due to wall boundary element reinforcement around the studs. The boundary element also alters the mode of failure from stud pullout to stud fracture. A new design technique based on dissipating the input energy through yielding and eventual fracture of the shear tab was developed. The performance of a test specimen designed based on this methodology was found to be satisfactory in terms of strength, ductility, and energy dissipation characteristics. Two simple adjustments to available equations were also identified to account for the effects of confinement, by wall boundary element reinforcement, around headed studs. DOI: 10.1061/(ASCE)0733-9445(2004)130:2(253) CE Database subject headings: Connections; Composite structures; Cyclic loads; Seismic design; Studs; Walls; Beams.

A wide range of experimental data and associated analytical results are used to examine the applicability of two widely used design codes (i.e., ACI and AISC) for calculating the capacity of concrete-filled steel tubular columns (CFTs). The database consists of the measured results from past studies as well as from three specimens tested by the writers. The test specimens include short and slender CFTs made with normal- and high-strength steel tubes filled with normal- and high-strength concrete. To gauge the success of the code-based methods, the capacities are also computed based on fiber analysis techniques, along with a member level iteration algorithm for analyzing members with significant length. In addition to inherent differences between ACI and AISC, the reported study points to potentially large differences between the capacities as computed from these two methods. However, the capacities from ACI and detailed analytical results are generally closer. Neither design method appears to be appropriate for CFTs made with high-strength steel tubes. A simple method based on full plasticity of the steel tube tends to produce significantly improved results for such cases.

The cyclic response of steel coupling beams embedded into reinforced concrete boundary elements was studied. Three half-scale subassemblies representing a portion of a prototype structure were designed, constructed, and tested. The main test variables were the amount of vertical tension/compression stresses in the boundary element, and presence of auxiliary bars attached to the beam flanges for a smoother transfer of bearing stresses. The steel coupling beams exhibited very stable hysteresis characteristics, and could develop the theoretical plastic moment when the boundary and, hence, the connection region were subjected to normal compressive stresses. Tensile stresses in the boundary element reduced the stiffness, and smaller moments could be developed. A significant amount of dissipated energy could be accounted by that energy dissipated in 'plastic hinges' formed in the exposed portion of the coupling beam. The stiffness was found to be different depending on whether the boundary element was under compressive or tensile normal stresses. The initial stiffness of the coupling beams in all the specimens was smaller than the value computed assuming full fixity at the face of the wall. The 'effective fixed point' of the coupling beam was found to be inside the wall at approximately one-third of the embedment length from the face of the wall. Analysis and design of composite coupled walls should account for the actual location at which the coupling beams are fixed in the walls.

Many research initiatives over the last two decades have had the objective of increasing the available spans of prestressed concrete bridge girders and have resulted in optimized cross-sectional shapes. To achieve these long spans, greater levels of prestress force are required. In addition, sections have been redesigned to increase the eccentricity of the prestress, which involves flattening and widening of the bottom bulb. The greater pretension forces andmore slender bulbs have a number of effects at the girder end that affect both the serviceability and the ultimate behavior of the girder. This article identifies some behaviors that are potentially exacerbated when larger prestress forces are introduced and proposes a strut-and-tie modeling approach to better understand and mitigate these effects through improved girder end-region detailing. The utility of the proposed strut-and-tie model is demonstrated in two instances: (1) an investigation of the effects of strand debonding and (2) an investigation of the potential impacts of the larger prestress forces resulting from increasing girder span [specifically by adopting larger-diameter strands of 17.8 mm (0.7 in.)]. In the first instance, the strut-and-tie approach was used to establish guidance for preferred strand and strand debonding patterns. In the second instance, the impact of providing greater prestress forces through the use of larger strands was shown to have little effect on the development of transverse stresses at girder ends. The strut-and-tie approach was also demonstrated to be able to be rapidly applied over a wide range of parameters, allowing trends related to girder geometry, for instance, to be established. Thus, the utility of the strut-and-tie method in general, and the proposed model in particular, is demonstrated to provide a powerful tool for the rational analysis of the complex stress state occurring at the end regions of prestressed concrete bridge girders. [ABSTRACT FROM AUTHOR]

A viable alternative to reinforced concrete coupling beams with heavily reinforced diagonal struts is steel or steel-concrete composite beams. A limited number of previous studies have examined various issues related to design of steel coupling beams. In this and a companion paper, cyclic response of steel-concrete composite coupling beams is addressed. The research results show that nominally reinforced encasement around steel coupling beams contributes to the strength and stiffness, and web stiffener plates can essentially be eliminated as the encasement provides adequate lateral stability for the shear-yielding web coupling beams. Although current design practice is conservative in terms of design and detailing of the beam, the overall behavior of composite coupling beams is not necessarily conservative. The design needs to consider the additional stiffness and strength in order to ensure proper calculation of design forces in the walls and coupling beams, and to dissipate the input energy through formation of plastic hinges in the beam and not in the connection region. Revision of current design guidelines is recommended.

As shown in a companion paper, the embedment length of steel-concrete composite coupling beams inside wall piers should be computed by incorporating the contribution of the encasement. In lieu of detailed fiber-based analytical techniques, a simple design-oriented model was developed and verified through testing of additional specimens with more realistic loading and boundary conditions. Moreover, the influence of face-bearing plates and floor slab was examined. The developed design method results in longer embedment length, and significantly enhanced energy-dissipating characteristics, strength, stiffness, and ductility. Additional improvements are possible by using face-bearing plates. The contribution of floor slab toward stiffness of the coupling beam is lost at small deformations and may be ignored. Slab participation toward the strength of the steel-concrete composite coupling beams is different from that anticipated for conventionally reinforced concrete beams because of the amount of equivalent reinforcement provided by the flanges of the steel coupling beam.

The introduction of steel reinforcing bars having yield strengths exceeding 517 MPa (75 ksi) and often approaching 827 MPa (120 ksi) may allow for significant economies to be realized, particularly when used in conjunction with high-strength concrete. One implication of the adoption of higher-strength reinforcing steel is that reinforcing bar stresses (and therefore strains and consequently crack widths) at service load levels are expected to be greater than when conventional bars [having a yield of 414 MPa (60 ksi)] are used. A study of flexural crack widths of beams reinforced with high-strength ASTM A1035 reinforcing steel is presented. Discussion focuses on the behavior at loads corresponding to longitudinal reinforcing bar stresses of 248, 414, and 496 MPa (36, 60, and 72 ksi), representing service load levels (i.e., 0.6 f y) for steel having f y = 414, 690, and 827 MPa (60, 100, and 120 ksi), respectively. Average measured crack widths obtained from a series of flexural beams having reinforcing ratios ranging from 0.007 to 0.023 are found to be below the present AASHTO de facto limits for Class 1 and Class 2 exposure. The demonstrated conservativeness of existing ACI and AASHTO crack control provisions allows present specifications to be extended to the anticipated higher service level stresses associated with the use of high-strength reinforcing steel. [ABSTRACT FROM AUTHOR]

An evaluation of concrete reinforcing bar fatigue provisions of AASHTO LRFD Bridge Design Specifications as they pertain to the use of high-strength reinforcing steel is presented. Two large-scale proof tests and a review of available published data demonstrate that presently accepted values for the fatigue or endurance limit for reinforcing steel are applicable, and likely conservative, when applied to higher-strength bars. A minor revision to the AASHTO specification is proposed to eliminate the fatigue penalty resulting from the use of higher-strength reinforcing steel. Finally, it is shown that fatigue considerations will rarely affect the design of typical reinforced-concrete members having a specified yield strength, fy≤690 MPa. [ABSTRACT FROM AUTHOR]

Hybrid coupled walls (HCWs) are comprised of two or more reinforced concrete wall piers connected with steel coupling beams distributed over the height of the structure. Extensive research over the past several decades suggests that such systems are particularly well suited for use in regions of moderate to high seismic risk. This paper reviews the state of the art in seismic modeling, analysis, and design of HCW systems. Design methodologies are presented in both prescriptive and performance-based design formats and a discussion of alterative types of hybrid wall systems is provided. [ABSTRACT FROM AUTHOR]

A 45-year old, three-span reinforced concrete slab bridge with insufficient capacity was retrofitted with 76.2- and 127-mm wide bonded carbon fiber-reinforced polymer (FRP) plates, 102-mm wide bonded carbon FRP plates with mechanical anchors at the ends, and bonded carbon FRP fabrics. The use of four systems in one bridge provided a unique opportunity to evaluate field installation issues and to examine the long-term performance of each system under identical traffic and environmental conditions. Using controlled truckload tests, the response of the bridge before retrofitting, shortly after retrofitting, and after one year of service was measured. The stiffness of the FRP systems was small in comparison to the stiffness of the bridge deck, and accordingly the measured deflections did not change noticeably after retrofitting. The measured strains suggest participation of the FRP systems, and more importantly, the strength of the retrofitted bridge was increased. A detailed 3D finite-element model of the original and retrofitted bridge was developed and calibrated based on the measured deflections. The model was used to predict more accurately the demands for computing the rating factors. The addition of FRP plates and fabrics led to a 22% increase in the rating factor and corresponding load limits. During a one-year period, traffic loading and environmental exposure did not apparently affect the performance of the FRP systems. The increased capacity and acceptable performance of the FRP systems enabled the engineers to remove the load limits in order to resume normal traffic. Future tests are necessary to monitor the long-term behavior of the FRP systems. [ABSTRACT FROM AUTHOR]

In an effort to assess the constructability and performance of bridges with fiber-reinforced polymer (FRP) composite decks, the short-term and long-term responses of a 207 m, five-span bridge retrofitted with four different FRP panel systems were monitored. The overall aspects of the panel systems, connection details, and construction techniques are presented prior to presentation of the observed and measured responses. Key design parameters (impact factors, girder distribution factors, and level of composite action) for FRP and reinforced concrete decks are evaluated. This paper demonstrates that FRP replacement decks are a viable alternative to reinforced concrete decks and identifies the differences in performances of various FRP deck systems. Two of the FRP panel systems were found to perform considerably better than the other deck systems. Issues that may reduce the service life of FRP deck systems are presented and discussed. [ABSTRACT FROM AUTHOR]

Four different fiber-reinforced polymer (FRP) panel systems were installed in a 207 m, five-span, three-lane bridge in an effort to assess the constructability, performance, and applicability of bridges with fiber-reinforced polymer composite decks. This paper examines whether four common deck systems are able to realize many of the anticipated benefits of using FRP composites in lieu of conventional reinforced concrete bridge decks. Particular installation issues, connection details, and specific construction techniques for each deck system are described, along with a discussion of the shortcomings in terms of handling, performance, and serviceability. Other factors such as key design parameters (e.g., impact factor and thermal characteristics) and unexpected responses are used to further quantify the performance of four FRP representative deck systems under identical traffic and environmental constraints. [ABSTRACT FROM AUTHOR]

Continuous reinforced concrete slab bridges rely on reinforcing steel bars near the top of the deck over the piers to carry negative moment. Transfer of forces in these bars may be jeopardized by deterioration and repair procedures that involve variable depth removal of deteriorated concrete around the bars. Partial or full loss of continuity could overstress the bottom reinforcement. Truckload testing of three bridges with various levels of damage was conducted before, during, and after repair in an attempt to quantify the level of loss of continuity and to examine the effectiveness of repair in terms of increasing the load transfer and enhancing the overall stiffness. Test results show loss of stiffness during repair but increased stiffness after completion of repair. The continuity was found to be lost during repair, and the slab dead load positive moments may be increased by as much as 50%. After repair, the continuity was restored, and the live-load distribution was essentially unaltered. For the test bridges, the redistribution of dead-load moment to the positive-moment zones did not appreciably affect the overall bridge rating factor. The amount of moment redistribution may be controlled through planning of repair steps. [ABSTRACT FROM AUTHOR]

In order to enhance the effectiveness of portland cement overlays placed over existing bridge decks, the deck may be sealed with an acceptable sealer before placing the overlay. The main purpose for such a treatment of the deck is to seal the existing cracks, and prevent penetration of chlorides into the deck if the overlay cracks. The presence of a sealer at the deck-overlay interface is expected to reduce the available bond strength. The reported research was carried out to investigate the performance of overlays placed over sealed bridge decks, examine the level of bond strength, and develop simple yet effective means to restore the bond strength as much as possible. Test results indicate that the sealer reduces the available bond strength by as much as 50%. Up to 85% of the bond strength can be restored if sand is broadcast over the sealer while it is curing or if the dried sealed surface is lightly sanded. These observations were validated through fatigue loading and loading to failure of a one-third-scale subassemblage of a steel stringer bridge. [ABSTRACT FROM AUTHOR]

In 38 of 48 cases (including all six "Florida I-beam" cases), the limitation that I V SB s sb i not be taken as greater than I V SB u sb / SB v sb i controlled the value of I V SB s sb i used to determine shear-bond reinforcement requirements. In summary, the discussers point out that the stress in the shear reinforcement corresponding to shear-bond failure may be lower than I f SB y sb i , the value used in AASHTO Eq. 5.7.3.3-4 to determine I V SB s sb i . [Extracted from the article]