Dissertations/Theses - Department of Civil Engineering
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Item Rational and economic design of reinforced concret pavements based on finite element analysis(Department of Civil Engineering (CE), 1992-08) Moazzem Hossain; Hoque, Dr. Alamgir M.In this research the behavior of a conventional concrete pavement of uniform thickness and a thickened edge box type pavement with holes for utility services have been investigated under traffic wheel loads using finite element technique. The effects of thickness, width and length of a conventional pavement, thickness of subbase (of CBR 20) and the subgrade CBR on the pavement deflection, tensile stress and subgrade pressure were studied. For the box type pavement the variables included are the end and midslab thickness, the width and length of slab, the thickness of the subbase and the CBR of subbase and subgrade. Eight noded isoparametric brick element has been used for the analysis. The study shows that for a conventional pavement, the maximum values of pavement deflection, tensile stress and subgrade pressure are reduced with an increase in slab thickness. The presence of a subbase further reduces the above values among which the most significant reduction takes place in the subgrade contact pressure value. An increase in pavement width and length of slab also reduces the tensile stress and deflection. When a better quality subbase is used, the deflection and tensile stress are reduced. Similar effects are noticed for a better quality (higher CBR) subgrade. But the most significant benefit is observed for change in CBR from 1 to 4. The pavement deflections of a box-type pavement, particularly with an equivalent subbase are much lower than the corresponding values for a conventional pavement. The subbase also substantially reduces the tensile stress in the pavement. The type of pavement, whether conventional or box-type, is much more important in reducing pavement deflection and stresses than the quality of the subbase expressed in terms of CBR value. Based on the findings of the present study, a new design rationale has been suggested for both box and conventional type of pavements, which will help to make the design of concrete pavements more economic and rational.Item Simplified equilibrium approach to the design of free standing stairs based on finite element analysis(Department of Civil Engineering, 1999-09) Anwar Zahid, Md.; Sohrabuddin Ahmad, Dr.Because of the absence of any specific code, the process of design of a free standing stair is dependent on some approximate analytical methods. These methods fail to recognise the variation of stress resultants across any cross section of the stair slab and require a lot of computational works. Amanat[4] made an extensive study on free standing stairs using the idealisation of Ahmad's[2] thick shell element and proposed a simplified guideline for easy analysis of free standing stairs. His approach also recognises the stress variation across different sections of the free standing stair, based on finite element analysis, and developed seven empirical design equations. If the loadings and the geometric conditions of the free standing stairs are symmetric, then, only two redundants, namely the lateral shear and the bending moment at the mid landing section are present. Hence, the number of empirical equations can be reduced to two for finding the values of these two redundants. The moments and forces at the other critical locations can be calculated by using equilibrium equations. With a view to developing a more rational and realistic approach by reducing the number of equilibrium equations, and also for increasing the range of applicability of Amanat's equations, this research was undertaken. Sensitivity analysis of the different geometric parameters is carried out over an extended range for two cases of loading. It is found that, in the higher range, the linear equations proposed by Amanat fail to represent the moments and forces as seen from the results of finite element analysis. Based on this study, two sets of empirical equations are developed for two cases of loadings. Other necessary design parameters are obtained from' equilibrium equations which reduce the total number of empirical relations, developed iby Amanat, from seven to four. In this research, detailed study is carried out to select the equations which nearly represent the actual situation. Ten prototype stairs are solved by using the proposed equations, Amanat's equations, and finite element analysis. These results establish the acceptability of the proposed equations. The proposed method has made the calculations for forces and moments, required for design of free standing stairs, very easy, and its estimation is found to be in the acceptable limits for commonly used dimensional ranges of stairs. Recommendations regarding the reinforcement layout have also been made.Item Design rationale for circular silos based on finite element analysis(Department of Civil Engineering, 1994-11) Alauddin, Md.; Sohrabuddin Ahmed, Dr.The current practice of silo analysis is based on several assumptions and idealisation. Analysis of silo by Finite Element method and comparison of the results with corresponding values obtained from conventional method reveals that the. conventional method cannot predict all the stress resultants (forces and moments) required for silo design. Again the functions predicted by the conventional method deviate largely from the actual values for a region near the ring beam of a silo. Conventional method can analyse a silo for axisymmetric loading only and cannot evaluate various types of moments which may be of considerable magnitude. Since silo is an elevated structure it may be subjected to a considerable amount. of wind load which is non-axisymmetric in nature. Earthquake loading haS significant effect on silo behaviour which is also nonaxisymmetric. Finite Element approach can analyse a silo for axisymmetric as well as non-axisymmetric loadings easily. It was, therefore, felt that the application of Finite Element will lead to a realistic analysis and to a more rational design procedure for silos. With this objective an extensive investigation was carried out on the behaviour of silos of different types under various loading conditions using the Axisymmetric thick shell Finite. Element program by Alunad. A number of parameters influencing silo behaviour were selected and a detail parametric study has been carried out to reveal the sensitivity of stress resultants with respect to a particular parameter. From this study it became obvious that the effect of restraint provided by the ring beam at the bottom of vertical wall can not be ignored. Actually, the moments developed in a silo are due to the restraint provided by the thickened ring beam at the bottom of vertical wall (top of conical hopper). The behaviour of silos under non-axisymmetric loading, such as wind load, has been studied elaborately. It was observed that wind load produces considerable meridional force and circumferential moment in the vertical wall which must be considered in the design of a silo. A detail study has also been carried out to know the effect of temperature difference between inside and outside of a silo. The conventional equations for the computation of meridional moment and circumferential moment have been modified to take into account the effect of restraint provided by the ring beam. Finally, on the basis of the study a design rationale has been presented. Using this rationale the stress resultants required for silo design can easily be computed.Item Design rationale for free standing stair slabs based on finite element analysis(Department of Civil Engineering, 1993-09) Amanat, Khan Mahmud; Sohrabuddin Ahmed, Dr.For abstract please see full textItem Design rationale for stair slabs based on finite element analysis(Department of Civil Engineering, 1989-08) Ishtiaque Ahmed; Sohrabuddin Ahmed, Dr.The results of a study of stair slabs by using finite element technique have been presented in this thesis. The behavior of both Dog-legged and Open-well type of stair have been analyzed. Both thick and thin shell finite elements have been used for this purpose and the results of the analyses by both of these elements compare well. Sensitivity study of the critical parameters of the stair slab has also been carried out to make the findings more generalized. The stair slabs, usually being supported on walls or beams at landing levels, derive significant rigidity from such supports. This reduces the magnitude of the moments that would have otherwise resulted. The leading codes of practice do not provide proper appreciation to this distinctive feature of stair slabs arising out of its supporting arrangements. The design moments for the stair slabs, under study, have been found to be considerably smaller than those commonly suggested by the Codes of Practice. Based on the findings of the present study, a new design rationale has been suggested, for both Dog-legged and Open-well type of stairs.Item Finite element analysis of temperature effects concrete bridge(Department of Civil Engineering, 1997-09) Saha, Debasish; Azadur Rahman, Dr. M.The present study is an attempt to understand the thermal effects in concrete bridge decks with special attention to Bangladesh. Thermal effects in a bridge deck are consequence of non-linear temperature distribution over the deck cross section. As a result, stresses, movements, and even cracks are initiated, the order, magnitude and significance of which are still not fully understood. Although over the past two decades a number of studies have been carried out, codes provide little or no guidance whatsoever on this matter. The earlier studies demonstrated that thermal effects are comparable in magnitude to dead and live load effects. Since the governing factors influencing temperature distribution vary geographically, the results of any such study become limited to specific geographic and meteorological condition. Therefore, previous results cannot be applied to any other situation. This study aims at investigation of thermal effects in the context of Bangladesh. Particular attention is given on the non-linear temperature variation over the cross section of concrete bridge decks and on the resulting longitudinal eigenstresses. Additional continuity stresses that develop in a continuous bridge are also addressed. An analytical model is developed to find out the influence of various parameters on the temperature induced stresses. A method for two-dimensional finite element transient heat transfer analysis is formulated. The mathematical model incorporates as many as 40 factors or parameters, some are interdependent, affecting temperature distribution which are related to the material, geometrical, meteorological and geographical conditions. On the basis of the model a computer program is developed to predict the temperature and stress distributions over bridge cross sections. Results derived from the analytical model are compared with the experimental and theoretical results obtained by other investigators. Several analyses are carried out in order to establish the critical values of parameters causing extreme thermal effects in concrete bridge decks constructed in Bangladesh. Special attention is given to model the seasonal and diurnal variation in boundary conditions. Statistical analysis of extremes is carried out to determine the extremes of meteorological parameters for 30-yr return period. Finally, temperature distributions to be applied in Bangladesh in slabs, beams and webs of box girder of various depths encountered in practice are proposed. Simplified hand calculation method is also presented to calculate the magnitude of thermal stresses due to proposed distributions.Item Improved design rationale for mat foundation based on finite element analysis(Department of Civil Engineering, 1999-07) Alok Sutradhar; Sohrabuddin Ahmed, Dr.For abstract please see full textItem Design rationale for mat foundation based on finite element analysis(Department of Civil Engineering, 1996-09) Monzurul Morshed, A.S.M.; Sohrabuddin Ahmed, Dr.Recently, mat has become quite popular among foundation engineers. Although there are several methods available for the analysis of mat, none of these is accurate enough to picture the actual behavior of mat. In the current study, two popular methods for the analysis of mat, namely Conventional method and ACI Approximate Flexible method, have been compared with the Finite Element solution of mat using Ahmad's thick shell element. A general computer program, capable of solving any shell type structural problem, has been used for this purpose. Various data generation and result interpretation modules have been added to the original program to convert it into a versatile mat analysis software. The software is capable of analyzing mats with any kind of column and shear wall arrangement as frequently encountered in engineering practice. Also, separate computer programs have been written for analyzing mat using Conventional and ACI methods for the purpose of comparison. In performing the design of mat, the USD method of ACI code of practice has been followed in this study. It has been found that Finite Element method gives the most economic solution and its economy increases sharply with the increase of column loads. It has also been observed that frequently minimum requirements of ACI code govern the design of mat and because of this, Conventional method results in only slightly uneconomic design in comparison to Finite Element method despite its inaccuracy and various drawbacks. For heavy column loads, ACI method has been found to give considerable overdesign in comparison to Finite Element method despite its rigorous nature of analysis. A parametric study has been made in order to identifY the parameters which play the most important roles in defining the behavior of mat. It has been found that mat response is not much sensitive to most of its parameters. The most significant role played in this regard has been identified to be that of mat thickness. Finally, the economic prospect of a reshaping scheme for mat has been examined. Relative performance of mat with non-uniform thickness with respect to mat with uniform thickness, which is the most popular practice so far, has been evaluated. Effects of the new geometric parameters associated with mat with non-uniform thickness have also been investigated in order to find a safe guide line for this redimensioning proposal. It has been found that mat with non-uniform thickness offers about 20 to 30% saving of concrete and reinforcement and its design is not much complicated compared to the design of mat with uniform thickness. In the end, scope for future research has been indicated.Item Improved design Rationale for helicoidal stair slabs based on finite element analysis(Department of Civil Engineering, 1998-09) Saiful Amin, A.F.M.; Sohrabuddin Ahmed, Dr.Attractive appearance of the helicoidal stair slab has drawn the attention of the users and architects. For this reason, the structure is now being increasingly used in many buildings. Due to complex geometric configuration of this structure, the conventional methods of analysis are based on different idealisations and assumptions. All these analytical methods fail to utilise its inherent structural efficiency that is derived from the helicoidal shape. With a view to developing a rational design procedure for this structure, finite element approach was introduced to study the actual behaviour of heliocidal stair slabs without any geometric idealisation using eight node curved thick shell element. The results of finite element analysis were compared with those obtained from the traditional helical girder solution. The comparative investigation revealed that the helical girder solution largely over estimates the vertical moment, lateral moment, lateral shear force and thrust with an under estimation of torsion. Apart from these aspects, the sensitive non-linear response of the structure due to the variation of different geometric parameters including central angle was also distinctly observed from an extensive parametric study. The investigation on the effect of variation of mesh size on the vertical displacement for an 800 stair indicated that a coarse mesh (even 2 x 4) is also capable of performing the analysis satisfactorily. Evidently this confirms the efficiency of the developed software and displays the power of the "Curved Shell Elements". The deflection pattern and the load deflection response of the structure were also found to be in good agreement with the available results of model studies. Based on all these findings, a rational design procedure based on finite element analysis has been proposed. A comparative design exercise has illustrated that around 37% overall economy of reinforcement is attainable in the proposed design process.