Brandon E. Ross

Experimental and Analytical Evaluations of Confinement Reinforcement in Pretensioned Concrete Beams

The AASHTO LRFD Bridge Design Specifications require that confinement reinforcement be placed around prestressing strands in the bottom bulb of pretensioned concrete beams. Although the AASHTO specifications contain prescriptive requirements for the quantity and placement of confinement reinforcement, the effect of such reinforcement on end region behavior is not well understood. To evaluate the function and effect of confinement reinforcement, 12 tests were conducted on precast pretensioned beams loaded in three-point bending at a shear span-to-depth ratio of 1.0. Variables in the test program included strand size, strand quantity, prestressing force, and the presence or lack of confinement reinforcement. In the tests, the confinement reinforcement neither prevented nor delayed strand slip but improved shear capacity and displacement ductility and prevented splitting failure. Experimental results also indicated transverse strain in the concrete above the bearing pad, which eventually led to splitting failure in test beams without confinement reinforcement. An elastic finite element analysis of the test beam was conducted to model the strain distributions in the test beams before cracking. Results from the analyses matched well with the experimental results. In particular, the magnitude and distribution of the transverse concrete strain measured at the end of the beam agreed well with the experimental results.

Experimental Study of End Region Detailing and Shear Behavior of Concrete I-Girders

AbstractThis paper presents the results of an experimental program conducted to evaluate the effects of end region detailing on the capacity and behavior of pretensioned concrete I-girders. The program was conducted using six Florida I-Beam specimens, 1,372 mm (54 in.) in depth, which were load-tested in three-point bending at a shear span/depth of 2.0. Variables in the program included confinement reinforcement configuration, presence/lack of steel bearing plate, strand shielding pattern, and strand layout. Specimen behavior and capacity were found to be significantly affected by end region detailing, especially placement of fully bonded strands. Experimental results were compared with theoretical models for web-shear, bond-shear, and lateral-splitting capacity. By considering multiple models, calculations were able to accurately identify the failure mode of each test specimen.

Model for Nominal Bond-Shear Capacity of Pretensioned Concrete Girders

The 2010 AASHTO LRFD Bridge Design Specifications includes requirements for proportioning flexural reinforcement at the end of pretensioned girders to carry longitudinal tie forces acting above the support. To prevent bond failure of the longitudinal tie, AASHTO requires that “any lack of full development [of the tie] shall be accounted for.” This paper proposes a model for calculating nominal bond-shear capacity, which is defined as the attendant shear force at bond capacity of the longitudinal tie. The model gives explicit consideration for tie development length. Variables in the model include bearing and girder geometry, longitudinal and transverse reinforcement details, and inclination angle of cracking. Derivation of the model is presented, and the model is compared with a database of experimental results compiled from the published literature. The proposed model can be used for designing girders that are resistant to bond-shear failure, particularly when partial strand debonding is employed. In some circumstances the model may justify exceedance of the AASHTO limits for partial strand debonding.

Design Model for Confinement Reinforcement in Pretensioned Concrete I-Girders

The 2007 edition of AASHTO LRFD Bridge Design Specifications contains prescriptive requirements for the quantity and placement of confinement reinforcement located in the bottom bulb of pretensioned concrete I-girders. The proposed model can be used to design confinement reinforcement as an alternative to AASHTOs prescriptive requirements. The model considers a range of I-girder conditions and variations, yet is intended to be practical enough for use by bridge design engineers. Variables in the design model include flange and bearing geometry, strand size and placement, effective prestress force, concrete and steel material properties, and the effects of steel bearing plates. The model is based on strut-and-tie and shear-friction concepts and considers the lateral-splitting failure mode. Derivation of the model is presented, and the model is compared with experimental results from the published literature.

A modelling approach for evaluating the effects of design variables on bridge condition ratings

Abstract While routine inspections are commonly used to assess the structural integrity, safety and maintenance needs of individual highway bridges, data from these inspections can also be used to study performance of bridges at the inventory level. This paper presents a novel method by which inspection data can be used to evaluate design variables and inform future designs. In particular, inspection data from prestressed concrete bridges in south-eastern United States were used to develop artificial neural networks models for estimating the condition rating of bridge decks and superstructures as a function of skew angle and span length, as well as, bridge age, width and traffic level. Once developed and validated, the neural network models were used for an array of simulations that were designed using a full factorial approach. The objective of the simulations was to identify skew angles and span lengths that correlate with the highest inspection ratings. It was determined that deck ratings are highest for smaller skew angles and shorter span lengths, whereas superstructure ratings are minimally impacted by larger skews and unrelated to span length. The conclusions of this study will be helpful in understanding the implications of bridge design variables on the long-term performance of bridge decks and superstructures. Though the trends and conclusions noted in this study are to be seen within the scope of the data considered, the approach demonstrated in this paper can be applied to address other questions of bridge performance.

Characterization of Bond-Loss Failures in Pretensioned Concrete Girders

AbstractFailures of strand–concrete bonds have been widely observed in load tests of precast-pretensioned concrete I girders. Through a review of 22 test programs, 15 terminologies were identified to describe failures associated with bond loss. In many cases, previous researchers used different terms to describe the same failure behavior. In response to the wide-ranging and sometimes inconsistent terminologies used in the literature, this technical note makes two contributions. First, the different types of failure that involve strand–concrete bond loss are characterized, and a consistent labeling scheme is proposed. The 15 labels given in the referenced test programs are condensed into four primary behaviors. Second, a flowchart is presented for assisting future researchers in characterizing and labeling bond-loss failures. Decision points in the flowchart are based on a synthesis of the reviewed test programs, which included 120 unique load tests having some type of bond-loss failure.

Evaluation of the AASHTO LRFD strand debonding limitations in the context of bond-loss failure

Abstract Debonding of select strands is an effective means of controlling stresses and cracking at the ends of pretensioned concrete girders, but can also have adverse effect on shear capacity due to loss of strand-concrete bond. To ensure sufficient shear capacity, the current AASHTO LRFD Bridge Design Specifications recommend that strand debonding be limited to no more that 25% of strands. This paper analytically evaluates the conservatism of the 25% debonding limitation with respect to shear failures involving loss of strand-concrete bond (i.e. bond-loss failure). This is accomplished by calculating the bond-loss capacity of six in-service bridge girders from different states and for varying levels of strand debonding. Calculations are based on a model previously published by the authors. It is determined that limiting strand debonding to 25% is conservative for all girders; however, the degree of conservatism is inconsistent. The demonstrated methodology can be used to balance the competing criteria of preventing bond loss failure and controlling end region cracking.

Compressive Strength of Dry-Stacked Concrete Masonry Unit Assemblies

AbstractDry-stacked masonry construction consists of individual units stacked directly without mortar at the bed and head joints. Although dry-stacked construction offers many benefits including speed of construction and minimal need for skilled labor, its use has been limited by lack of technical information. This paper presents the results of an experimental program investigating the compressive strength of dry-stacked assemblies built from nonproprietary standard concrete masonry units. The program included 124 tests of dry-stacked prisms; variables in the program included compressive strength of the units and treatment of the interface. Roughness of the interface was found to have significant effect on the load–displacement behavior and ultimate capacity of dry-stacked assemblies. Based on the experimental results, a unit strength method is presented for qualifying dry-stacked systems.

Detailing Steel Roof Decks to Control Damage from Wind-Borne Debris Impact

AbstractStructurally hardened safe rooms are a proven strategy for enhancing life safety during tornadoes and other severe wind events. Performance criteria for safe rooms include resistance to both wind pressure and impact from wind-borne debris. This paper presents the results of a test program evaluating debris impact resistance of light-gauge steel decking for use as roof cladding on safe rooms. Particular attention is given to the modes of damage caused by debris impact and strategies for controlling and mitigating such damage. Recommendations are provided for detailing lap joints and bearing connections and for selecting appropriate support spacing. This information will aid design engineers in detailing light-gauge steel roof decks for use in tornado safe rooms. Recommendations provided in this paper are also applicable to other scenarios where steel roof decks are at risk of wind-borne debris impact.

Effect of Bearing Pad Arrangement on Capacity of Slab Panel Bridge Members

Slab panel bridges (elsewhere referred to as flat-slab bridges) are constructed with multiple precast pretensioned slab panels that are placed side by side and act compositely with a cast-in-place concrete topping. Design standards from the Florida Department of Transportation specify that individual slab panels be supported by a tripod system of bearing pads. A single bearing pad is placed at one end, and two bearing pads at the opposite end. This paper presents experimental research on the effect that this bearing pad arrangement has on the behavior and capacity of slab panel members. Four full-scale tests on slab panels were conducted, with variables that included the bearing pad arrangement (single or double bearing), and the position of the load point [shear span-to-depth (a/d) ratio of 2.0 and 3.3]. For slab panels loaded at a/d of 3.3, test results indicated that the behavior and capacity were independent of the bearing pad arrangement. For the panel loaded at a/d of 2.0 with a single bearing pad, the failure mode was bond shear. The panel loaded at a/d of 2.0 with two bearing pads failed in flexure. Experimental results were compared with theoretical capacities calculated with American Concrete Institute and AASHTO codes. Design recommendations were made on the basis of the experimental results and on the comparison with theoretical capacities.