Charts for Preliminary Selection of NU Girder Sections Based On Kansas Department of Transportation LRFD Design Guidelines for Prestressed Concrete Beams

The Kansas Department of Transportation, which currently uses a series of standard prestressed concrete beam sections referred to as K-girders for prestressed beam bridge projects, is considering a switch to use of NU I-girder sections. The NU I-Girder sections are attractive for their efficiency, but Kansas engineers are not accustomed to their use. The aim of this analytical study was to develop an Excel-based tool that can be used to produce charts that engineers can use for preliminary selection of NU I-girder section sizes and strand numbers. The calculations described herein are in compliance with KDOT Bridge Design Specifications

Load Rating Reinforced Concrete Bridges without Plans: State-of-the-Practice

In response to Federal Highway Administration requirements, several states are in the process of ensuring all bridges within their inventories are load rated. A challenging aspect of this effort is load rating reinforced concrete bridges that have no structural plans when there are thousands of such structures within a state inventory. To inform these efforts, the literature was reviewed to identify existing methodologies and a survey was distributed to engineers at state departments of transportation throughout the United States to understand how practicing engineers approach this problem. The survey responses show there are numerous bridges in the U.S. without plans; over 25000 bridges without plans are located in the 18 states that provided responses. Concrete structures comprise 70% of such bridges. To load rate concrete bridges without plans, most responding states report primarily using engineering judgement, which may include reference to performance under existing traffic, era-specific design traffic loads, assumed material properties and reinforcement quantities, or data collected using load tests or non-destructive evaluation. Several methodologies are described and advantages/limitations of each are discussed

A Review of Research on Shear Strength Decay in Members under Load Reversals

In the design of reinforced concrete earthquake-resisting frame members, it is critical that shear distress be limited in order to ensure acceptable deformation capacity and reduce damage. Accordingly, several ACI Building Code1 provisions for beams and columns of frames categorized as “special moment frames” are aimed at minimizing shear distress. Pertinent requirements include using a capacity design approach to calculate demand, neglecting any contribution of the concrete to nominal shear strength in beams, and limiting hoop spacing to one-fourth of the effective (beams) or overall (columns) member depth. These provisions are largely based on findings from early research aimed at understanding the behavior of frame members subjected to cycles of load reversals. The aim of this paper is to review relevant research on the behavior of frame members under earthquake-type demands, beginning with the first tests of flexural members subjected to fully reversed loads and ending with the 1983 ACI Building Code2, as it was the first ACI Code edition to incorporate several provisions aimed at minimizing shear strength decay. This paper describes the basis for pertinent ACI Building Code provisions (other code or design documents were not included in this review), emphasizes the importance of low shear stress demands, and highlights reinforcement detailing options that have been shown to improve member behavior. This review should therefore be of interest to students and structural engineers, particularly those learning or involved in earthquake-resistant design of reinforced concrete structures

Proposed Revisions to the Strength-Reduction Factor for Axially Loaded Members

Modifications correct anomalies for nonprestressed reinforced concrete members subjected to flexure and axial load

Synthesis of Rating Methodologies for Concrete Bridges without Plans

In response to Federal Highway Administration requirements, many states are confronted with assigning load ratings to large numbers of concrete bridges that do not have plans. The AASHTO Manual for Bridge Evaluation does not specify which methodologies should be used to establish load ratings for concrete bridges without plans, nor does it clearly state how extensive the evaluation of such structures should be. To inform engineers responsible for load rating structures without plans, this report highlights available non-destructive evaluation methods that are able, to varying extents, to locate and determine the size of concrete reinforcing bars. It also provides a survey of current and emergent methodologies for establishing load ratings for concrete bridges without plans. Finally, to characterize the state-of-the-practice, results are reported from a survey distributed to state bridge engineers. There are large differences among states in terms of the specificity of established procedures and overall methodologies employed to assign load ratings to the more than 25,000 bridges without plans located in the 24 states that responded to the survey. Recommendations for approaching this problem in a rational and cost-effective manner are made after considering both published evidence and ease of implementation across large inventories of structures

Diagonally-Reinforced Concrete Coupling Beams with High-Strength Steel Bars

The use of high-strength steel in diagonally reinforced coupling beams was investigated with the aims of minimizing reinforcement congestion and increasing the maximum permissible design shear stress without compromising behavior under large displacement reversals. Five large-scale diagonally reinforced concrete coupling beam specimens with clear span-to-depth ratios of 1.9 were tested under fully reversed cyclic loads. The primary variables were yield stress of the diagonal reinforcement (60 and 120 ksi [420 and 830 MPa]), target beam shear stress (1.0 and 1.5 times the ACI Building Code limit), length of the secondary (non-diagonal) longitudinal reinforcement, and axial restraint. All specimens had the same nominal concrete compressive strength and beam dimensions. Chord rotation capacities exhibited by the specimens with Grade 120 (830) reinforcement were between 5.1 and 5.6%, less than that of the control specimen with Grade 60 (420) diagonal reinforcement (7.1%). Neither development of secondary reinforcement nor increases in design shear stress affected specimen chord rotation capacity. The axially-restrained specimen with Grade 120 (830) diagonal reinforcement showed the same chord rotation capacity as a similar specimen without axial restraint, but 14% larger strength. In specimens with secondary longitudinal reinforcement extended into the wall (such that the embedment length exceeded the calculated development length), the localization of damage evident along the beam-wall interface in tests of specimens with bars terminating near the wall face was not observed. Although damage was more distributed throughout the beam span, deformation capacity was not increased. Among the specimens, it was shown that the initial stiffness, area of the shear force-chord rotation hysteresis cycles, and residual chord rotation at zero shear force changed in inverse proportion to the diagonal bar yield stress. A database of results from tests of diagonally reinforced coupling beams was compiled and used to evaluate the sensitivity of coupling beam chord rotation capacity to a range of variables. Variables included aspect ratio, reinforcement grade, transverse confinement reinforcement (type, spacing, and ratio), shear stress, and length of secondary (non-diagonal) reinforcement (whether terminated near the beam-wall interface or developed into the wall). An equation was proposed for calculating coupling beam chord rotation capacity as a function of beam clear span-to-height ratio and the ratio of hoop spacing to diagonal bar diameter. Chord rotation capacity was not correlated with other variables. Modifications are also proposed to the stiffness and deformation capacity modeling parameters recommended in ASCE 41-17 and ACI 369.1-17 for diagonally reinforced coupling beams to account for reinforcement grade

Diagonally Reinforced Concrete Coupling Beams with Grade 120 (830) High-Strength Steel Bars

Four large-scale coupling beams were tested under fully reversed cyclic loads to investigate the effects on behavior of diagonal bar grade (60 or 120 [420 or 830]), beam shear stress (9.5 or 14√fc′, psi [0.79 or 1.17√fc′, MPa]), and longitudinal bar detailing (either terminated near the beam-wall interface or developed into the walls). Coupling beam chord rotation capacity was 7.1% for the beam with Grade 60 (420) bars and between 5.1 and 5.6% for the beams with Grade 120 (830) bars, a difference likely due to having hoops spaced at 3.4db and 4db for Grade 60 and 120 (420 and 830) bars, where db is the diagonal bar diameter. Effective stiffness, energy dissipation, and residual chord rotations were approximately inversely proportional to bar grade. Developing the secondary longitudinal reinforcement reduced rotation demands at beam ends but did not improve deformation capacity. Beam shear stress did not affect beam chord rotation capacity

Girder–Deck Interface: Partial Debonding, Deck Replacement, and Composite Action

Results are reported from tests of three precast, prestressed concrete girders under fatigue-type cyclic and monotonic loading conducted after deck removal and replacement. Although deck demolition altered the top surface of the girders, the girder–deck interfaces exhibited shear strengths greater than their nominal strength (based on the 2012 AASHTO LRFD Specification) after 2 × 106 cycles of loading to 45 and 30% of their nominal strength for troweled and roughened interfaces, respectively. A partially debonded detail was used for two of the girders to protect the girder top flange, which was wide and thin, during deck demolition. The roofing felt used to debond the girder–deck interface over the flanges reduced the effort required for deck removal by 65%, compared with the typical detail, eliminated chipping hammer–induced damage to the girder flanges, and still resulted in sustained composite action under 2 × 106 cycles of loading. The width of the bonded interface had little effect on girder stiffness and no observed effect on the width of deck effective in bending

Diagonally Reinforced Concrete Coupling Beams: Effects of Axial Restraint

Diagonally reinforced concrete coupling beams are commonly used in mid- and high-rise buildings to connect adjacent structural walls separated by openings. Under lateral loading, these beams can undergo large inelastic deformations but must retain their strength for the system to behave as desired. It is not known how or whether resistance to axial elongation of the beams, provided by the floor diaphragms and stiff structural walls, affects the strength and deformation capacity of coupling beams. The influence of axial restraint on diagonally reinforced concrete coupling beams was investigated by comparing the results of an axially-restrained coupling beam specimen with those of a nominally equivalent control specimen tested without axial restraint but using the same loading protocol. The presence of axial restraint increased the strength of the coupling beam specimen by approximately 30%, decreased the chord rotation capacity by approximately 10%, and resulted in buckling of diagonal reinforcement at smaller chord rotations

Correlations Between Compressive, Flexural, and Tensile Behavior of Self-Consolidating Fiber Reinforced Concrete

Self-consolidating fiber reinforced concrete (SCFRC) is a hybrid concrete that is both self-compacting and fiber reinforced. Use of SCFRC in reinforced concrete members has been shown to result in improved behavior under shear, flexure, and compression relative to conventional reinforced concrete. The aim of this study was to investigate relationships between the compressive and tensile responses of SCFRC as well as relationships between measured compressive and flexural behavior. Such relationships would simplify characterization of the mechanical behavior of SCFRCs based on a relatively limited number of standard tests. A secondary objective was to quantify the effect of introducing different volume fractions of four types of steel fiber to SCCs with compressive strengths of 6 and 10 ksi. Four different hooked-end steel fibers were used in this study at volume fractions between 0.5% and 1.5%. Results showed that the post-peak slope in compression and the post-cracking flexural and tensile strengths all increased as fiber volume fraction increased, whereas properties up to development of cracking (or peak strength in the case of compression) were not affected by use of fibers. Among the parameters investigated, it was shown that the post-peak compressive response was most closely correlated with the post-crack peak strength in flexure and the flexural strength corresponding to a mid-span deflection of 0.04 in. It was also found that the within-batch coefficient of variation of post-crack peak tensile and flexural loads decreased significantly when T50 was at least 1.0 second, from an average of 40% to 13%. Of the fibers investigated, the RC-80/30-BP had the greatest impact on mechanical performance for a given volume fraction and the 3D RC-55/30-BG fiber had the least