J E Breen

EXPERIMENTAL EVALUATION OF STRUT-AND-TIE MODEL APPLIED TO DEEP BEAM WITH OPENING

Where geometric discontinuities exist in structural concrete members, current code documents provide little direction for design. The design of these unusual members can be better understood by using the strut-and-tie model. Combinations of two distinctly different strut-and-tie models were used to design four deep beams each with a geometric discontinuity in the form of a large opening. This paper describes the performance of four physical models that were constructed based on these varied designs. The deep beams were simply supported and tested using a point load. Each of the four beams resisted considerably more than the factored design load. This successful test series reveals the power, versatility, reliability, and predictability of the strut-and-tie modeling technique.

Use of Alternative Materials to Reduce Shrinkage Cracking in Bridge Decks

According to a survey conducted in 1996, respondents in several state departments of transportation indicated that more than 100,000 bridge decks in the U.S. have suffered from early age transverse cracking, a crack pattern that typically arises due to drying shrinkage. Concrete material properties are treated as a means through which to improve the resistance restrained drying shrinkage cracking. Various test methods are discussed as they relate to determining the resistance of a material to shrinkage cracking. Materials-based methods of controlling drying shrinkage are presented. The materials discussed include fibers, shrinkage-compensating concrete, shrinkage-reducing admixtures, and extensible concrete. It was determined in small laboratory specimens, and confirmed in large-scale bridge deck specimens, that several of the alternative mixtures adequately reduced restrained drying-shrinkage cracking.

Behavior and Capacity of Headed Reinforcement

The Texas Department of Transportation (TxDOT) funded studies of the feasibility of headed reinforcement in bridge structures. These research projects focused on two basic anchorage conditions for headed reinforcement: anchorage at compression-compression-tension nodes (CCT); and anchorage in lap splices. This article reports on a study in which the authors combined these techniques to develop design recommendations for headed reinforcement. The authors first summarize important aspects of headed bar anchorage, and then present a model for determining the anchorage capacity of headed reinforcement. This model includes two components that contribute to the total bar stress, head bearing, and bond, each calculated separately. The proposed model is also compared to data from previous headed bar studies. The authors conclude by recommending strut-and-tie modeling for the calculation of anchorage length.

Stresses in External Tendons at Ultimate

Post-tensioned segmental concrete bridges are often used for long, multispan viaducts or medium-span valley and river crossings. This article presents the research that led to the development of the equation for predicting stresses in unbonded tendons at ultimate; this equation is currently used in the AASHTO LRFD Specifications and AASHTO Guide Specifications for the Design and Construction of Segmental Concrete Bridges. The research, performed by the late Robert J. G. MacGregor, involved the construction and testing of a 1/4 scale model of a three-span, continuous, precast segmental-concrete box girder bridge, erected using span-by-span techniques and post-tensioned with external tendons. The authors discuss the results of the tests to ultimate that were dominated by flexural behavior. Predicted tendon stress increases are then compared to a large data base from other tests of beams and slabs with unbonded tendons. The authors conclude that the equation proposed by MacGregor predicts the tendon stress increases in unbonded tendons conservatively and relatively well, compared to expressions that ignore tendon length.

Grouts for bonded post-tensioning in corrosive environments

Problems with corrosion of prestressing steel in post-tensioned structures due to inadequate grouting have become more apparent in recent years. To improve grouting practices, a series of fresh property tests, accelerated corrosion tests, simulated field tests, and long-term corrosion tests were used in this study to develop optimum grouts for post-tensioning. Grouts possessing adequate workability and bleed resistance were tested in an accelerated corrosion test to evaluate relative corrosion protection properties. This paper focuses on the accelerated corrosion test results. The most promising of these grouts were then tested for placeability under simulated field conditions. A fly ash grout and a thixotropic grout are recommended from this testing program. Fluidity, bleed, and corrosion resistance data was also acquired for a number of different combinations of pozzolans and admixtures.

THE BEHAVIOR OF MULTIPLE LAP SPLICES IN WIDE SECTIONS

In most reinforced concrete catilever retaining walls, construction procedures require some type of splicing of the reinforcing steel. Economics often dictate the use of a lap splice. Since a retaining wall has no redundancy, understanding of splice behavior becomes critical to the design of the structure. In order to investigate the behavior of lap splices of the reinforcing steel in such a structure, twenty-five specimens were tested. The specimens were tested with the splice regions subjected to a constant moment along the length of the splice. This loading produced a stress condition as severe as that in the prototype. The main variables in the test program were the splice length and bar diameter, the ratio of the clear cover to the clear spacing of the splices, the edge splice condition, the amount of transverse reinforcement in the splice region, and the casting position. Cracking patterns, steel strain distributions, and failure modes of the specimens were studied to obtain a basic understanding of the behavior of lap splices in wide sections. /FHWA/

Lap splices anchored by headed bars

This research aimed to experimentally study the anchorage behavior of headed reinforcement in lap splices. Observations of cracking behavior, strain measurements of reinforcement, and strength are reported. The behavior of unconfined laps is compared to confined laps, and the behavior of nonheaded and headed bar laps are also compared. Bar stresses are compared with a proposed model for bearing capacity at the head. Test results suggest that noncontact lap splices should be modeled using a truss mechanism with diagonal compression struts between opposing bars. Under such a model, the capacity of the lap is determined by an anchorage length defined by the intersection points of the diagonal struts. Using this model, the anchorage behavior of headed bars is similar to previously reported results from CCT node tests in which anchorage consisted of bond and head bearing components. Results show that headed reinforcement can markedly reduce required lap length of spliced reinforcement.

CCT Nodes Anchored by Headed Bars - Part 1: Behavior of Nodes

The paper examines the anchorage behavior of headed reinforcement in compression-compression-tension (CCT) nodes. Measurements of strains in ties and confining reinforcement, head slip, node and strut capacities are reported, as are observations of cracking behavior. The behavior of confined and unconfined nodes is compared. Strain measurements along ties and the confining steel around the node are obtained to determine the stress state of the node. The strain data show node failure to be related primarily to the anchorage of the tie bar. The anchorage of headed reinforcement is also studied. Headed reinforcement anchorage is shown to consist of bond and head-bearing components. These two components do not peak simultaneously, as bond components generally peak and begin to decline before head bearing components peak.

EVALUATION OF STRUT-AND-TIE MODELING APPLIED TO DAPPED BEAM WITH OPENING

Strut-and-tie modeling is a useful tool for designing irregular concrete members. The ACI 318-02 Building Code contains provisions pertaining to design using strut-and-tie models. This paper presents experimental results from tests conducted on small-scale, simply-supported dapped beams with openings. The design of each test specimen was developed by independent student teams using the ACI provisions for strut-and-tie models. Each of the 4 specimens resisted loads greater than the factored design load and exhibited little distress at service load levels. The successful test series illustrates the applicability and conservative nature of strut-and-tie modeling for design.

POST-TENSIONED ANCHORAGE ZONES WITH SINGLE STRAIGHT CONCENTRIC ANCHORAGES

The authors of this paper present the results of a combination experimental and analytical study that explored the use of the strut-and-tie model for analyzing post-tensioned anchorage zones. The portion of the study described in this paper focuses on concentric anchorage zones. The paper describes 17 specimens used to develop code provisions adopted by the American Association of State Highway and Transportation Officials Load and Resistance Factor Design (AASHTO LRFD) Bridge Specification. The strut-and-tie model successfully and conservatively estimated the capacity of the specimens. An example illustrates how to analyze an anchorage zone using the strut-and-tie model.