Browsing by Author "Weerasekera, IRA"

Now showing 1 - 16 of 16

item: Conference-Abstract
Analysis of bridge decks for thermal loading for the Sri Lankan industry
(2006) Weerasekera, IRA; Premawardena, VAS; Prasanna, KPN; Samarasinghe, CM
Concrete bridge structures, which are exposed to the sun are subjected to complex thermal stresses, which may vary continuously with time. The magnitude of these stresses depends mainly upon the temperature variations that occur within the cross section of the bridge deck. The nonlinear temperature variation created over the cross section of a simply supported bridge produces longitudinal self-equilibrating stresses (eigen stresses). These can be determined by restraining the simply supported beam and then releasing the restrained structure by reversing and applying the stress resultants. Using the Principle of superposition gives the desired stresses. The resulting internal forces are unaffected but there is a re-distribution of stresses in different components of a cross-section under thermal loading. In a continuous bridge, additional continuity stresses also develop. In this situation internal forces as well as external reactions change. Overall results can be obtained using superposition of self-equilibrating stresses and continuity stresses. The procedure described can be used for any practical situation as long as there is symmetry of the cross section about a vertical axis. A section containing several layers made out of different materials can also be handled in a similar way. In the situation of a complex bridge deck consisting of a grid structure the above results can be combined with a normal grillage analysis to enhance the comprehensive nature of the approach.
item: SRC-Report
Development of a rational procedure for shear design in structural concrete
Weerasekera, IRA
item: SRC-Report
Development of a rational procedure for shear design in structural concrete
Weerasekera, IRA
Over the last century, the design for shear in reinforced concrete beams has been uncertain. The reason for this uncertainty is the lack of clear understanding of the true nature of load carrying mechanism. Researchers around the world have developed several shear design procedure as a remedy to this uncertainty. Most of them are empirical and some are rational which use various mathematical representations of load carrying mechanism. Almost all of these mathematical models are complicated and accuracy of is rather low compared to empirical methods. With the advancement of concrete technology and introduction of new methods, the improvement of accuracy of shear design methods is rather unmatchable when compared with flexural design. However, some of the new shear design methods were adopted by codes of practice and the design formulae were modified by introducing various empirical factors based on test. So the shear design has now become very much empirical. Some of the developed rational methods give good representation of the load carrying mechanism of beams. The method proposed here with the objective of developing a rational yet simple method, also assumes the load carrying mechanism of a beam as a truss. The tension carrying member represents the stirrups and the concrete portion is represented by compression member. For this proposed method, the compression carrying concrete member is idealized as a cylindrical element applied with axial compression. The behavior of this idealized cylinder is modeled by using mathematical techniques like Isotropic and Anisotropic analysis. This developed method is compared with several key codes of practice using available test data for accuracy. It is found that the proposed method is giving sound representation of the test results compared with most other codes of practice. According to the comparison, it is clear that this method is only second to Japanese code of practice.
item: SRC-Report
Development of a Rational Procedure for Shear Design in Structural Concrete
(2016-08-15) Weerasekera, IRA
Over the last century, the design for shear in reinforced concrete beams has been uncertain. The reason for this uncertainty is the lack of clear understanding of the true nature of load carrying mechanism. Researchers around the world have developed several shear design procedure as a remedy to this uncertainty. Most of them are empirical and some are rational which use various mathematical representations of load carrying mechanism. Almost all of these mathematical models are complicated and accuracy of is rather low compared to empirical methods. With the advancement of concrete technology and introduction of new methods, the improvement of accuracy of shear design methods is rather unmatchable when compared with flexural design. However, some of the new shear design methods were adopted by codes of practice and the design formulae were modified by introducing various empirical factors based on test. So the shear design has now become very much empirical. Some of the developed rational methods give good representation of the load carrying mechanism of beams. The method proposed here with the objective of developing a rational yet simple method, also assumes the load carrying mechanism of a beam as a truss. The tension carrying member represents the stirrups and the concrete portion is represented by compression member. For this proposed=method, the compression carrying concrete member is idealized as a cylindrical element applied with axial compression. The behavior ~of this idealized cylinder is modeled by using mathematical technique'S like Isotropic and Anisotropic analysis. This developed method is ..•compared with several key codes of practice using available test data for accuracy. It is found that the proposed method is giving sound representation of the test results compared with most other codes of practice. According to the comparison, it is clear that this method is only second to Japanese code of practice.
item: Conference-Extended-Abstract
Development of a rational procedure for shear design in structural concrete
(2009) Sureshkumar, V; Weerasekera, IRA
Shear design equations for reinforced concrete beams are mostly empirical and are based on test results. But some mathematical models have also been developed over the years. The pursuit for a better model which gives a clearer representation of shear carrying mechanism in beams is still on. This paper explains a method based on Cohesive Cracking as a new approach to this ongoing search. The proposed model is giving promising results when compared with key codes of practice.
item: Thesis-Abstract
Development of prestressed concrete beams using controlled detensioning
Sabesh, A; Weerasekera, IRA
Pretensioned prestressed concrete can be produced in a number of ways. Among them several options can be considered depending on the structure constructed and how prestress is transferred. In the recent times radial and longitudinal cracks have been observed due to high tensile stresses developed in concrete around prestressing steel. In practice to eliminate these harmful conditions modifications are required to ensure serviceability functions of the composite high quality material. Often excessive prestress is reduced by lowering the tensile stress in the prestressing steel or / and the magnitude of the eccentricity towards the end of the member which in vulnerable to this type of effect. In the global context debonding of tendons towards the end of a member, drapping of tendon towards the central portion of the member or controlled detensioning can be applied to achieve the desire outcomes. All these techniques require a sound basis for prestress transfer which is achieved by bond. Some of the practices are prohibitive to developing countries due to the high cost of holding down devices buried in the concrete. Further in third world countries cost of hardware is expensive as opposed to cheep labour encountered in production. This research is aimed at strengtheni~g our understanding of bond mechanism by extending cohesive cracking approach established by experiment and matched by a sound theoretical basis to complement each other. Currently some of the global practices are carried out by relying on.intuition as opposed to theoretical formulations. This study covers a 'comprehensive analysis of bond development for the controlled detensioned process. The experimental verification is not part of the present scope. However the parameters influencing such as strand diameter, initial prestress, concrete strength and cover or half the spacing have been identified as main influences to ascertain bond strength based on available test results exhumed from literature. A parametric study has also been carried out and simplified empirical formulae have been developed to predict normal transfer bond length and the type of bond length required for controlled detensioning. The derived theory is also applicable even to debonded tendons as well as drapped tendons accurately. Further study is required to support these findings by providing experimental evidence
item: SRC-Report
Evaluation of Shear Design Procedures Adopted in the Industry for Reinforced Concrete
(2016-08-15) Weerasekera, IRA
Application of reinforced concrete as a construction material was first found in the middle of the 19th century. Over the last one and half centuries it has became a popular and widely accepted construction material. Its applications span from in small domestic structures to large structures like massive dams, bridges, offshore platforms provide evidence for its potential. Shear design is an important area of the reinforced concrete designing process. This study reviews the shear designing approached for reinforced concrete beams. From the beginning the shear behavior of reinforced concrete beams was mysterious. The first analytical model to explain the shear behavior of a reinforced concrete beam was postulated in 1899 by a Swiss engineer called Ritter and a German engineer called Morsch (1902). They independently introduced the Truss Model to use in shear design. Since then various theories have been put forward to explain the shear behavior of reinforced concrete beams. But, still none of them seems to have resolved the issue by producing results relating theory to experiment to a higher degree of accuracy when compared to flexural design.
item: Thesis-Full-text
Evaluation of shear design procedures adopted in the industry for reinforced concrete
Wickramage, WKHRE; Weerasekera, IRA
Application of reinforced concrete as a construction material was first found in the middle of the 19th century. Over the last one and half centuries it has become a popular and widely accepted construction material. Its applications span from in small domestic structures to large structures like massive dams, bridges, offshore platforms provides evidence for its potential. Shear design is an important area of the reinforced concrete designing process. This study reviews the shear designing approaches for reinforced concrete beams. From the beginning the shear behaviour of reinforced concrete beams was mysterious. The first analytical model to explain the shear behaviour of a reinforced concrete beam was postulated in 1899 by a Swiss engineer called Ritter and a German engineer called Morsch (1902). They independently introduced the Truss Model to use in shear design. Since then various theories have been put forward to explain the shear behaviour of reinforced concrete beams. But, still none of them seems to have resolved the is~ue by producing results relating theory to experiment to a higher degree of accuracy when compared to flexural design. This study identifies reasons for those theories to deviate from the experimental results. Some of them are conventional parameters used in design equations whereas others are new for these design methods. Also it identifies when these parameters ,become critical for deviation of the predicted results from the experiment. Ultimately this study identifies when these theories are justifiable for shear designing of reinforced concrete. Also it evaluates the practices followed in design offices in Sri Lanka for shear design and recommends the best practises to ensure adequate safe guard against a premature failure. Results of this study shows that Canadian Code General method and Australian Code method give most accurate results and can be recommended to use within the limitations specified in the code. Further this study shows that Japanese Code design method can be recommended for conservative shear designing without any restrictions on parameters. But this method is less accurate than the Canadian Code General method and Australian Code method.
item: Conference-Extended-Abstract
Evaluation of shear design procedures for reinforced concrete beams without shear reinforcements
(2009) Wickramage, WKHRE; Weerasekera, IRA
Many structural concrete beams have been constructed without transverse reinforcements. The main objective of this paper is to review design procedures for slender beams. Five major codes namely: ACI, CSA, BS, Australian, Japanese and a procedure based on Shear Friction are considered. More than 500 slender beams without transverse reinforcements which have been exhumed from literature were used for the evaluation. Parameters influencing the accuracy of each and every procedure were studied using a Multinomial Logistic Regression Analysis.
item: Conference-Abstract
Modified hyperbolic shear deformation theory for static flexure analysis of thick isotropic beam
Jasotharan, S; Weerasekera, IRA
A hyperbolic shear deformation theory for thick isotropic beams is developed where the displacements are defined using a meaningful function which is more physical and directly comparable with other higher order theories. Governing variationally consistent equilibrium equations and boundary conditions are derived in terms of the stress resultants and displacements using the principle of virtual work. This theory satisfies shear stress free boundary condition at top and bottom of the beam and doesn’t need shear correction factor. Results obtained for stresses and displacements using the present theory for static flexure of simply supported uniform isotropic beam carrying uniformly distributed load are compared with other beam theories and the exact elasticity solution.
item: Thesis-Abstract
Nonlinear analysis of cable structures
Peiris, HCT; Weerasekera, IRA
Large structures are widely used in the modem construction industry for infra-structure facilities development. Among these, long span structures with cables are becoming increasingly popular. In this category of structures deformations are large and estimations based on small deformation theory in the normal analysis are inadequate. The large deformation analysis results in nonlinear behavior where principle of superposition does not hold. In geometrical nonlinear analysis, the equations of equilibrium are based on the deformed geometry after the load application. The length of a curved deflected line is longer than the initial length and the basic assumptions used in linear analysis may cause inaccuracies when the deformations are very large. It is also essential that bending effects of cables are considered. Here we deal with large deformations, but small strain problems with linear stress strain relationships. Although there are many methods found in literature to analyze cables exhibiting large deformation nonlinear behavior, it is hard to find a universal approach to describe the exact behavior of a cable considering all geometrical nonlinearity issues. The analysis described in this study recognizes all such influences contributing to geometrical non-linearity. The procedure developed is versatile and gives a state-of the- art analytical tool. This work fills a void in the current practice recognizing large deformation issues without any knowledge of small or large strains as opposed to what is required in commercial software. A numerical solution procedure has been evolved to solve the resulting nonlinear nonhomogeneous integral differential equation. The procedure is converging and a computer program has been developed for practical use. The results are compared with those in literature to validate the findings and to ensure the accuracy of the new large deformation nonlinear analysis technique.
item: Conference-Extended-Abstract
A practical solution to improve transfer bond in pretensioned prestressed concrete beams
(2006) Weerasekera, IRA; Sabesh, A; Jeyasuthan, Y; Premaraj, M
Pretensioned prestressed concrete is a popular and challenging construction technique to produce precast beams. This type of construction is governed by the effectiveness of the bond between prestressing steel and concrete which will determine the proper anchorage of the prestressing steel. The anchorage requirements of such a beam, is twofold. At the service stage transfer bond dictates the design while at the ultimate stage both transfer and flexural bond are important. The subject area of this research focuses on the transfer bond and the length of the beam associated with it known as the transfer length.
item: Thesis-Abstract
Serviceability analysis of continuous box-girder bridges constructed using different techniques
Thilakarathna, HMI; Weerasekera, IRA
There are different construction methods practiced in the bridge industry to erect bridge super structures. Among these, segmental Box-girder is the latest trend in the industry, which facilitates fast, versatile construction. In contrast to monolithic construction, these bridges consist of pre-cast pretensioned elements joined together by post-tensioning to form continuous bridge structures. Composite bridge cross-sections adopted in segmental construction use dissimilar materials such as precast concrete, prestressing and non prestressing steel components. Behaviour of these continuous bridges, are often influenced by time-dependent nature of the materials used. This study focuses on the analysis of time-dependent properties associated with the bridges constructed using different techniques. To analyse the short, medium and long term behaviour of the bridges, a rational analytical approach is chosen so that it can assess the time-dependant effects such as creep and shrinkage of concrete and relaxation of prestressing steel. So the structural analyses can model mathematically all associated changes in geometrical forms, statical conditions and material properties which are important with segmental construction. A computer program available, and which follows the above procedure has been enhanced by introducing a pre-processor to ease complications arising from the large number of analyses encountered with solution of continuous segmental bridge problems. In this research different construction techniques such as span-by-span construction, balanced and progressive. Cantilever methods and incremental launching have been examined. A four span continuous bridge example where each span is fabricated using eight equal segments have been studied under different construction techniques. These involve evaluating the effects of loads arising from force or displacement induced changes which occur during construction stages and perhaps be temporary or permanent depending on the circumstances. Also it has been possible to compare results and identify both advantages and disadvantages of the various methods. The study reveals the effect of serviceability indices such as stresses and deformations which are affected in different forms depending on the method of construction.
item: Conference-Abstract
Serviceability of continuous box-girder concrete bridges made by the progressive cantilever construction procedure
(2006) Thillakarathna, HMI; Weerasekera, IRA
Continuous segmental bridge construction is relatively new to the Sri Lankan bridge industry. Progressive cantilever method of construction is one method employed for continuous box-girder bridges. This technique is competitive for spans between 30 - 50 m. This construction procedure has several advantages. There is no consideration of balancing cantilevers and provides easy access through parts already constructed. Also horizontal curved shapes can be easily accommodated. From a structural sense progressive placement method is advantageous in substructure design where bearings can be placed immediately. The main disadvantage arising from cantilever action is very high bending stresses. This can be minimized by introducing temporary cables or intermediate supports. The other disadvantages are slow progress and that the superstructure undergoes stress reversal. Often other construction methods may be necessary for the first span.
item: Thesis-Full-text
Study of deep beams using finite element approach
Jasotharan, S; Weerasekera, IRA
Beams are common structural elements in most structures and generally they are analysed using classical beam theories to evaluate the stress and strain characteristics of the beam. But in the case of deep beams, higher order shear deformation beam theories predicts more accurate results than classical beam theories due its more realistic assumption regarding the shear characteristics of the beam. In this study a hyperbolic shear deformation theory for thick isotropic beams is developed where the displacements are defined using a meaningful function which is more physical and directly comparable with other higher order theories. Governing variationally consistent equilibrium equations and boundary conditions are derived in terms of the stress resultants and displacements using the principle of virtual work. This theory satisfies shear stress free boundary condition at top and bottom of the beam and doesn’t need shear correction factor. .A displacement based finite element model of this theory is formulated using the variational principle. Displacements are approximated using the homogeneous solutions of the governing differential equations that describe the deformations of the cross-section according to the high order theory, which includes cubic variation of the axial displacements over the cross-section of the beam. Also, this model gives the exact stiffness coefficients for the high order isotropic beam element. The model has six degrees of freedom at the two ends, one transverse displacement and two rotations, and the end forces are a shear force and two components of end moments. Several numerical examples are discussed to validate the proposed shear deformation beam theory and finite element model of the beam theory. Results obtained for displacements using the present beam theory and the finite element model are compared with results obtained using other beam theories, 2D elastic theory and 2D and 3D finite element models. Solutions obtained using the proposed beam theory and finite element model are in close agreement with the solutions obtained using 2D elastic theory and 2D and 3D finite element models of ‘ABAQUS’.
item: Conference-Abstract
Testing of pre-stressed concrete beams produced using controlled detensioning
(Department of Civil Engineering, University of Moratuwa, 2021-11) Siriwardena, YMC; Weerasekera, IRA; Hettiarachchi, P
Pre-tensioned pre-stressed concrete has been developed to overcome concrete weakness in tension. Generally, there are number of ways for producing pre-tensioned, prestressed concrete beams in practice. Among these, several options can be considered depending on how the structure is constructed, materials used, statical conditions are applied and how prestress is transferred. In the conventional method of pre-stress, radial and longitudinal cracks have been observed due to high tensile stress developed in concrete around the pre-stressing steel. Further, excessive stresses are generated at the two ends when applying this method. Often excessive prestress is reduced by lowering tension stress in the prestressing steel and magnitude of the eccentricity towards the end of the member which is vulnerable to this type of effect. To overcome these problems, control detensioning method has been introduced. Debonding of the tendons towards the end of the member, draping of tendon towards the central portion of the member or controlled detensioning approach are applied to achieve the desired bond characteristics by controlling the bond development systematically. The purpose of this study is to identify conventional methods of prestressing and their limitations together with theoretical concepts of controlled detensioning method. An experimental programme has been carried out at a casting yard of ICC (International Construction Consortium, Piliyandala) by modifying the casting bed. The method of controlled detensioning is a stage by stage releasing technique of prestressing and casting thereby varying prestressing force without any accessories buried in concrete other than simple shear keys at locations of changing prestressing force. The final form of longitudinal prestress varies at the end from concave up, towards the central portion of the transfer zone which is the straight followed by concave down towards the end of the transfer length. In these regions in the conventional beam where we find concave up stress profile which has radial cracking along almost entire transfer length up to the surface, however it does not happen in the controlled detensioning method. In fact, most of the prestressing is transferred, in the straight portion where cracking occurs partially but not to the surface. The experimental programme is partly successful as out of the four beams tested it was not possible to compare the theoretical findings entirely but gives a reflection of the advantages. Hence, there is a major improvement in the bond. This type of issue is beneficial when detensioning, is used to prevent deicing salts used in major roads in winter climates and in bridges, in coastal areas where salty environment is present. These harmful effects can be avoided when cracks to surface can be prevented. Otherwise, it may lead to ingress of saline moisture particles, which are corrosive.