Pretensioned Beams Research Articles

Considerations for the design of doubly unsymmetrical precast pretensioned beams used for box girder construction

AbstractMany box girder bridges are constructed with doubly unsymmetrical precast beam units. The aim of this paper is to present special considerations for the optimum arrangement of pretensioned strands in a simply supported precast pretensioned beam with a doubly unsymmetrical section. The goal of the optimum arrangement is to minimize the distortion (lateral sway phenomenon) caused by the doubly unsymmetrical characteristic of the beam section at transfer. The study shows that the distortion of the unsymmetrical section is minimized when the resultant stresses at transfer are constant over all of the top flange and over all of the bottom flange of the precast beam for most of the sections within the beam length. In this case, the neutral axes of these sections will be horizontal with respect to the beam section. This approach is verified with the help of two different finite element models. In the first model, the beam is modelled as one‐dimensional space frame elements; in the second model, the beam is modelled using three‐dimensional solid elements. A practical example of a box girder bridge made from doubly unsymmetrical precast pretensioned beams in the APM (Automated People Mover) elevated bridge project in Saudi Arabia is also presented.

Concrete Roadside Barrier Anchorage Details for 5-in., Cast-in-Place Decks on Prestressed Beams

The Texas Department of Transportation commonly builds bridge spans with adjacently framed, prestressed box beams and slab beams and makes them composite with a 5-in.-thick, concrete, cast-in-place deck (CIPD). The presence of a cast-in-place, concrete overlay allows use of more favorable empirical, live-load distribution equations, which results in more efficient bridge spans. This 5-in.-thick CIPD offers limited depth for anchorage of rail reinforcement. AASHTO Load and Resistance Factor Design Bridge Design Specifications has a limiting hook development length of 6 in. for a No. 4 rail anchorage. With a 5-in. deck thickness and a 6-in. minimum required development length, the rail anchorage reinforcement must be cast in the pretensioned beams to meet specification requirements for rail anchorage strength. This need has made it difficult to ensure adequate development of the rail anchorage reinforcement. The objective of the study reported here, therefore, was to develop barrier anchorage details that would meet the AASHTO Manual for Assessing Safety Hardware (MASH) evaluation criteria, minimize damage to the concrete barrier profile and CIPD, and prevent delamination of the CIPD from the precast beams, which was witnessed in a research project performed at Texas Transportation Institute. The anchorage modifications were evaluated successfully under the Manual for Assessing Safety Hardware Test 3–11 and were considered suitable for roadways with traffic conditions appropriate for Testing Level 3 safety hardware.

New Pedestal for Pre-Tensioned Pre-Stressed Beam Construction

Taking the advantages of groove pedestal and pier pedestal, a new type of pre-tensioned pedestal is constructed. The new pedestal makes full use of the deck (also the bottom formwork of the beam) to bear the tensile force so as to reduce the quantity of the pedestal construction. The new pedestal can adapt to variable geotechnical conditions by properly enlarge the arm of the turnover moment and the self-weight. The new pedestal is optimized through the bearing capacity analysis of the anti-overturning, the anti-slipping, the table anti-compressing, the force transfer plate, and the cross tie beams. The stress distribution of local bearing bracket is calculated by the finite element method so as to be reinforced accordingly. Based on practical application in some project, the new pedestal has the features of technical feasibility, security & reliability, affordability, and convenience in operation.

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.

Ultimate load analysis of pretensioned inverted T-beams with circular web openings

The provision of transverse openings in floor beams to facilitate the passage of utility pipes and service ducts not only results in a more systematic layout of pipes and ducts, it also translates into substantial economic savings in the construction of a multi-storey building. In this paper, ultimate load analysis of statically loaded simply supported pretensioned inverted T-beams with circular web openings is presented. Major findings relevant to ultimate load analysis of pretensioned beams with circular web openings are summarized. An attempt has been made to answer the frequently asked questions related to ultimate load analysis on multiple circular web openings. It has been shown that the analysis method for pretensioned beams with multiple large circular web openings can be further simplified without sacrificing rationality.

Analytical modelling of Kirschner wires in Ilizarov circular external fixator as pretensioned slender beams.

Transfixing thin Kirschner wires (K-wires) are the key components of the Ilizarov fixator regarding its axial stiffness, which affects the mechanobiological environment in which bone is healed. Mechanically speaking, K-wires are slender beams that are axially tensioned, then fixed and transversely loaded. The existing solutions to such a problem either do not accommodate any axial loading prior to transverse loading, or do not account for the change in the axial load (reaction) due to transverse loading. Their applicability is also limited vis-à-vis applied loads and beam dimensions. This work seeks to address those problems by providing a mathematical formulation for a pretensioned slender beam that accounts for the change in the beam tension due to lateral loading. Central loading of a pretensioned beam was studied and new polynomial equations have been derived, the roots of which yield the final tension for a (i) long, slender and heavily loaded beam and (ii) relatively thicker beam subjected to a lower load. Results were produced and discussed for the specific application of pretensioned K-wires in circular (ring) external fixators in orthopaedics (such as Ilizarov's), which were checked (validated) via two- and three-dimensional finite-element analyses.

Computing the dynamic response of an axially moving continuum

The aim of this study was to apply an approximate Galerkin finite-element method to solve the initial boundary-value problem of a viscously damped axially moving pre-tensioned beam including arbitrary support excitations. In order to validate the Galerkin finite-element method the results were compared with exact solutions when available and with solutions obtained using the finite-difference and Galerkin methods. The energy dissipation of the system was considered in the form of an equivalent viscous-damping model. It was also shown that for certain values of the parameters, especially at high velocities, the Galerkin method using stationary string eigenfunctions can give a poor prediction of the dynamic response.

Limit states of cracking in beam-and-block floor systems using pretensioned ribs

Beam-and-block floor systems using pretensioned ribs enable very efficient industrialised production. By ensuring that the ribs are produced under a strict quality assurance programme and that they are safely transported, assembled and properly detailed, an equally monolithic structural performance is achieved. However, calculation methods to control cracking in these structural elements must ensure the durability according to the surrounding environment's severity and predefined exposure classes. The design of beam-and-block floor systems is particularly affected by serviceability limit states. This fact is closely linked to the verification conditions for crack control. Therefore, new design models have been developed to take into account the peculiarities and characteristics of these floor systems. The aim is to find practical and effective crack-control methods for composite beam-and-block floor systems with pretensioned ribs or beams. The verification of the limit state of crack widths has so far been disregarded in the design of this type of lightweight slab. In fact, the current procedure is simply to compare the tensile stress in the lower fibre of the beams with the characteristic strength of the rib's concrete. This paper presents a method to assess crack control. The procedure implies the quantification of a limit bending moment depending on the physical and geometric characteristics of the sections. Simplified calculation methods and verification rules are presented, which allow the establishment of tables and design charts for the indirect verification of this limit state through bar size and spacing limits.

Bending Moment–Shear Force Interaction Domains for Prestressed Concrete Beams

The performance of a prestressed concrete beam, subjected to bending moment M together with shear force V, has been the object of many studies and is an important aspect to take into account in the design. Some models, proposed by researchers and international codes, evaluate the shear strength of prestressed beams by modifying the truss model by Morsch, so as to account for the different slope of stress fields in the web due to the prestressing action. More recent approaches add a strut-and-tie model to the traditional truss model. This paper generalizes a model that was previously proposed for box and I-shaped reinforced concrete cross sections of structural elements. The model, that now includes the effect of prestressing tendons, considers variable-depth stress fields applied to the cross section, subdivided into layers, and allows evaluation of normalized m-v design domains depending both on the web and flange reinforcement and on the slope of the prestressing steel tendons. The reliability of the method has been validated by comparing its numerical results to the strength provided by tests on reinforced concrete beams and on thin-webbed prestressed concrete beams, referred to in the literature. Finally, it has been used in the design of a pretensioned bridge beam to evaluate the additional reinforcement necessary in the flanges, as a function of the reinforcement provided to the web.

Analysis of stress distribution in anchorage zones of pretensioned beams

The stress transmission mechanism in pretensioned concrete beams, though very interesting from an economical point of view, is very complex, integrating various phenomenons such as sliding, bond, bursting. For long the complexity of this mechanism has led engineers to provide a massive rectangular anchorage zone at each end of the beam. The necessity of using such a concrete reinforcement is certainly unquestionable in post-tensioned beams. However in pretensioned elements the stresses induced in concrete in the anchorage zone are smaller than in post-tensioned elements. In this article the stress field in the end zone is calculated numerically and from this analysis the possible reduction of the cross-section of the anchorage block is examined.

Long-Term Behavior of CFRP Prestressed Concrete Beams

This paper presents an experimental program designed to examine the long-term behavior of carbon fiber reinforced polymer (CFRP) pretensionad concrete beams under sustained load conditions. The test program consists of three beams prestressed with CFRP rods and one beam prestressed with steel prestressing stands. The pretensioned beams were subjected to a sustained load of 53 kN (12 kips). Deflections and prestressing reinforcement strains at midspan were monitored for a period of 651 days. An analytical model was developed to predict the time-dependent behavior of the pretensioned beams. The experimental results show satisfactory behavior of CFRP pretensioned beams compared to the steel pretensioned beam. The ratio of long-term to instantaneous deflection at midspan was observed to increase with the level of prestress force. The steel pretensioned beam exhibited higher long-term midspan deflections than the companion CFRP pretensioned beams.

Structural and Economic Benefits of Precast/Prestressed Concrete Construction

This article presents advantages in the application of precast/prestressed concrete in structural frames. Using pretensioned concrete components in conjunction with a composite cast-in-place concrete topping creates a lighter structural system than an all cast-in-place structure. Shoring pretensioned beams and slabs before adding the composite topping can reduce member size even further. Extending reinforcing steel from the precast components creates functional joinery systems that can be used instead of, or in conjunction with, a number of other connection methods, including mechanical couplers. A complete system of precast units can be integrated to form a building frame that behaves monolithically, with sufficient strength, stiffness, and durability to resist seismic loadings.

Durability of Aramid Pretensioned Elements in a Marine Environment

This paper presents results from a 3-year exposure study to evaluate the durability of aramid fiber reinforced plastic (AFRP) pretensioned elements used as piles driven in a marine environment. The aramid/epoxy composite ARAPREE was used in this investigation. Ten AFRP pretensioned beams designed to fail by rupture of the pretensioning rods were precracked and then exposed to wet/dry cycles in saltwater to simulate tide change. Exposed beams were periodically removed and tested to failure to determine their service and ultimate response. Changes in response relative to that of unexposed control specimens were used to evaluate durability. The results showed that exposure had a relatively minor effect on the ultimate capacity of the AFRP pretensioned beams. However, there was evidence of bond deterioration in the failure mode of specimens exposed for 2 years or more. More importantly, characteristic bond splitting cracks developed in one beam that had been exposed for 3 years. This suggests that while AFRP may have adequate chemical resistance to concrete's alkalinity, its bond with concrete is vulnerable under wet/dry cycles. Thus, AFRP is not the ideal candidate for replacing steel in pretensioned piles driven in tidal waters.

Cathodic Protection of Prestressed Bridge Members—Full-Scale Testing

The results of a study on the use of cathodic protection on prestressed and post-tensioned concrete bridge members are summarized. Previous laboratory tests to evaluate hydrogen embrittlement of high strength steel embedded in concrete have proven that cathodic protection will generate hydrogen on high-strength steel in concrete if the potential is more negative than the thermodynamic hydrogen evolution potential. The hydrogen generated will enter the steel and cause a loss in ductility that will adversely affect the steel's performance if a notch is present. Full-scale beams were constructed to further study those phenomena. Four pretensioned beams were constructed. In addition, two post-tensioned slabs were constructed to evaluate cathodic protection of anchorages and tendons encased in metal or plastic conduits. Cathodic protection currents were supplied by IR drop-free potential controlled rectifiers. Good potential control at control points was achieved by using externally mounted silver-silver chloride reference electrodes and a conductive gel bridge. However, inconsistent potential control occurred at locations other than at the control points. Later in the study, constant current power supplies were used on two of the beams. Hydrogen entering the steel as the result of corrosion appears to have masked the presence of hydrogen that might have been produced by cathodic protection. The analysis also revealed that there was corrosion of some pretensioned wires at crossings with interior steel reinforcing bars due to interference (stray current) caused by cathodic protection application. Analysis of the post-tensioned slabs indicated little effect of cathodic protection on tendons inside plastic or metal ducts from the application of cathodic protection. Beneficial effects were noted on anchor points where mortar was in contact with the metal.

Cathodic Protection of Prestressed Bridge Members—Full-Scale Testing

The results of a study on the use of cathodic protection on prestressed and post-tensioned concrete bridge members are summarized. Previous laboratory tests to evaluate hydrogen embrittlement of high strength steel embedded in concrete have proven that cathodic protection will generate hydrogen on high-strength steel in concrete if the potential is more negative than the thermodynamic hydrogen evolution potential. The hydrogen generated will enter the steel and cause a loss in ductility that will adversely affect the steel's performance if a notch is present. Full-scale beams were constructed to further study those phenomena. Four pretensioned beams were constructed. In addition, two post-tensioned slabs were constructed to evaluate cathodic protection of anchorages and tendons encased in metal or plastic conduits. Cathodic protection currents were supplied by IR drop-free potential controlled rectifiers. Good potential control at control points was achieved by using externally mounted silver-silver chloride reference electrodes and a conductive gel bridge. However, inconsistent potential control occurred at locations other than at the control points. Later in the study, constant current power supplies were used on two of the beams. Hydrogen entering the steel as the result of corrosion appears to have masked the presence of hydrogen that might have been produced by cathodic protection. The analysis also revealed that there was corrosion of some pretensioned wires at crossings with interior steel reinforcing bars due to interference (stray current) caused by cathodic protection application. Analysis of the post-tensioned slabs indicated little effect of cathodic protection on tendons inside plastic or metal ducts from the application of cathodic protection. Beneficial effects were noted on anchor points where mortar was in contact with the metal.

Strengthening of Prestressed Concrete Composite Beams Using External Prestressed Stirrups

Strengthening of older bridges designed for relatively light loading and traffic is necessary in many places around the world. This paper presents experimental results on the use of external prestressed stirrups to improve the shear strength and concrete-to-strand bond strength of pretensioned composite beams. 5 beams were tested under stationary static loads until failure. The basic unstrengthened beam showed a very dramatic shear failure. The strengthened beams with external prestressed stirrups, however, performed well, even with very short strand embedment lengths. External prestressed stirrups considerably increased the shear capacity of the strengthened beams. Also, when prestressed to adequate levels the stirrups allowed the development of the flexural capacity of the beams with strand embedment lengths as short as 100 times the strand diameter.

Static Response of Fiberglass Pretensioned Columns

This paper presents results from an experimental study to determine the static response of fiberglass pretensioned beams. Three sets of underreinforced pretensioned beams 8-ft span with cross sections of 6 in. × 8 in. (0.15 m × 0.2 m), 6 in. × 10 in. (0.15 m × 0.25 m), and 6 in. × 12 in. (0.15 m × 0.3 m) were tested under two-point loading. The beams were pretensioned using three 3/8 in. (9.5 mm) S-2 glass/epoxy strands. For comparison, three identical beams pretensioned to the same effective prestress by two 3/8 in. (9.5 mm) steel strands were also tested. The results from the study indicate that the precracking response of these fiberglass pretensioned beams was near identical to the steel pretensioned beams, but its postcracking response was mor flexible. Failure of some of the fiberglass pretensioned beams was accompanied by slip. Nonlinear finite-element analysis showed good correlation with the test results. Tentative equations for estimating the failure stress and criteria for designing underreinforced beams are proposed.

Static Response of Fiberglass Pretensioned Beams

Static Response of Fiberglass Pretensioned Beams

Flexural behaviour of concrete beams pretensioned with aramid fibre reinforced plastic tendons

The durability of reinforced concrete structures has become an increasing concern as our infrastructure ages. In particular is the deterioration of steel reinforcements when used in concrete structures exposed to severe corrosive environments. This paper investigates the feasibility of using noncorroding aramid fibre reinforced plastic (AFRP) tendons as a substitute for steel strand in pretensioned concrete applications. Three AFRP pretensioned beams, 150 × 300 × 2100 mm, were tested under static and cyclic loading. These results indicate that load capacities of the beams were not affected by the cyclic loading. Ultimate strengths were generally greater than predicted, primarily as a result of the ability of the AFRP rods to develop greater tensile stresses than expected. Key words: prestressed concrete, nonmetallic tendons, aramid fibres, advanced composite materials.

Durability of Fiberglass Pretensioned Beams

The durability of identical steel and S-2 glass/epoxy pretensioned beams subjected to wet/dry cycles simulating tidal effects was expermentally investigated. Eight identical steel and fiberglass pretesioned beams, half of them precracked at the critical midspan section, were placed in two tanks containing 15% saltwater solution. The ultimate capacity of the beams was determined at periodic intervals and compared against that of unexposed control specimens in air. The tests indicate a complete loss in the effectiveness of the fiberglass strands exposed to wet/dry cycles after an average of 6 months for the precracked beams and 15 months for the uncracked beams. There was no comparable loss of capacity in the identical steel pretensioned specimens.