- P.O.Box 30052
Addis Ababa
Ethiopia - +251911773055
Asrat Worku
Addis Ababa University, Civil Engineering, Faculty Member
- Asrat is a civil engineer who combines practice with academia with over 35 years of experience in teaching, research ... moreAsrat is a civil engineer who combines practice with academia with over 35 years of experience in teaching, research and consultancy. His research interest focuses on static and dynamic soil-structure interaction, foundation model development and geotechnical earthquake engineering.edit
Research Interests:
A generalized model is presented for a subgrade idealized as an elastic layer overlying a rigid base. In contrast to previous works no stresses, strains or displacements are neglected a priori. A second-order partial differential equation... more
A generalized model is presented for a subgrade idealized as an elastic layer overlying a rigid base. In contrast to previous works no stresses, strains or displacements are neglected a priori. A second-order partial differential equation is obtained similar to Kerr's ...
Research Interests:
A comparison of seismic ptovisions having relevance to the Equivalent Static Force (ESF) method of analysis according to current major building codys worldwide is presented. The codes compared include the latest two editions of... more
A comparison of seismic ptovisions having relevance to the Equivalent Static Force (ESF) method of analysis according to current major building codys worldwide is presented. The codes compared include the latest two editions of the Uniform Building Code (UBC 94 and UBC 97), the International Building Code (IBC 2000), the European Prestandard (Eurocode 8, 1998), and the Ethiopian Building Code Standard (EBCS 8, 1995). The comparison is mady on the basis of the specified base shear coefficient, the vertical distribution of the base shear, the story shear and overturning moment, the considerations for torsion, P-delta effect, and the limitations on the story drift. Substantial differences are observed among the codes and even within different editions of a code, a good example being the UBC series. It is also shown that the most recent American codes like USC 97 and IBC 2000 exhibited drastic changes in the definition of the base shear coefficient and in other pertinent regulations as ...
Research Interests:
Research Interests:
Research Interests:
The dynamic analysis of multi-degree-of-freedom systems interacting with the foundation soil and acted by a system of periodic loads is considered. Such systems belong to the category of non-classically damped systems. The method of modal... more
The dynamic analysis of multi-degree-of-freedom systems interacting with the foundation soil and acted by a system of periodic loads is considered. Such systems belong to the category of non-classically damped systems. The method of modal superposition is employed for coordinate transformation. The resulting coupled algebraic equations are easily solved analytically without resorting to iterations, which would otherwise be necessary in time-domain analyses for irregular excitations like earthquake ground motions. A practical method of compiling the non-proportional damping matrix is adapted for the intended purpose. The proposed method of solution is convenient to use on spreadsheets, with MATLAB or to integrate it with existing software for routine structural analyses. The application is illustrated in detail on a simplified model of a symmetrically-loaded frame founded on a rigid mat. The method is equally applicable to systems the superstructure of which is discretized using fini...
Research Interests:
Abstract A robust two-parameter subgrade model presented recently by the first author is employed to analyze plates on elastic foundations. A finite difference method (FDM) is employed to solve the...
Research Interests:
Abstract A calibrated continuum foundation model of the order same as those of Pasternak, Filonenko-Borodich, and Vlasov is developed. This is achieved by seeking equality of the surface deformation of a Pasternak-type model with that of... more
Abstract A calibrated continuum foundation model of the order same as those of Pasternak, Filonenko-Borodich, and Vlasov is developed. This is achieved by seeking equality of the surface deformation of a Pasternak-type model with that of a calibrated Kerr-type model presented recently without neglecting any stress, strain, or deformation component in the continuum. In order to alleviate the sensitiveness of the model parameters to the layer thickness, this is eliminated from the corresponding expressions through the introduction of a dimensionless calibration parameter. The superiority of the new model over existing models of similar order has been demonstrated. Closed-form calibrated relationships for the two parameters of the new model are provided together with values of the calibration factor. The use of the model is illustrated on a basic problem of a beam on an elastic foundation. As a Pasternak-type model is more familiar to engineers and much easier to handle in applications than a Kerr-type model, its use in practice is recommended. The new model completes a series of calibrated models at three different levels: Winkler, Pasternak, and Kerr as proposed recently by the author.
Research Interests:
The theory of elasticity of continua is employed to show the background of spring formulas that are introduced at the bases of structures to account for immediate static deformation of soils. This is followed by providing sets of such... more
The theory of elasticity of continua is employed to show the background of spring formulas that are introduced at the bases of structures to account for immediate static deformation of soils. This is followed by providing sets of such valuable formulas for use in practical modeling of structures founded on deformable soils. Additional spring formulas that account for primary and secondary consolidation settlement are derived It is shown that these vertical springs can be joined in series to account for all types of soil deformation immediate, primary consolidation, and secondary consolidation or creep. The application of the springs is illustrated using a simple building frame subjected to gravity loads only. The internal forces in the structural members with and without flexible base elements showed notable differences. The significance of the introduction of flexible-base elements in taller and more rigid structures subjected to lateral loads can be expected to be even larger. The...
Research Interests:
Recent developments in the definition of design ground motions for seismic analysis of structures are presented. A summary of results of empirical and analytical site-effect studies are provided and recent findings from empirical studies... more
Recent developments in the definition of design ground motions for seismic analysis of structures are presented. A summary of results of empirical and analytical site-effect studies are provided and recent findings from empirical studies on instrumental records are compared against similar results from earlier studies. Pertinent changes introduced in recent editions of international codes as a result of these evidences are presented. Comparisons of relevant provisions of EBCS 8: 1995 with those in contemporary American, European and South African codes are made. The paper presents compelling evidences showing that the amplification potential of site-soils can in general be significantly larger at sites of lowamplitude rock-surface acceleration up to 0.1g than at sites of larger accelerations. Noting the practical significance of this fact on the seismic design of structures in low to moderate seismic regions, to which many cities and towns of Ethiopia belong, changes to selected p...
Research Interests:
The dynamic response of a multi-degree-of-freedom (MDF) system with non-proportional damping subjected to harmonic loads is considered. Modal substitution is employed to transform the coupled differential equations of motion from... more
The dynamic response of a multi-degree-of-freedom (MDF) system with non-proportional damping subjected to harmonic loads is considered. Modal substitution is employed to transform the coupled differential equations of motion from geometric to modal coordinates. As might be expected, the modal transformation does not uncouple the differential equations of motion because of the inherent non-proportional damping, but transforms them into a system of coupled algebraic equations of convenient form for solution. The modal coordinates are then easily determined with the help of conventional techniques for solving systems of coupled equations. The method presented gives a closed form solution without the need to resort to iterative procedures, which would otherwise be necessary for other types of more irregular excitations like earthquake ground motions. Besides, it presented a technique of compiling the damping matrix of MDF systems exhibiting different mechanisms of energy dissipation in ...
Research Interests:
Based on an isotropic elastic continuum of thickness H overlying a rigid stratum, a generalized formulation for the classical single-parameter Winkler's subgrade model is presented. In this formulation, all the normal components of... more
Based on an isotropic elastic continuum of thickness H overlying a rigid stratum, a generalized formulation for the classical single-parameter Winkler's subgrade model is presented. In this formulation, all the normal components of the stress tensor are taken into consideration, whereas the shear stresses are intentionally dropped with the purpose of providing a useful perspective, with which Winkler's model and its associated coefficient of subgrade reaction can be viewed. The formulation takes into account the variation of the elasticity modulus with depth. It only demands specifying a relationship between the vertical and horizontal normal stresses. Accordingly, two such different assumptions are made to obtain two new Winkler-type subgrade models with the corresponding closed-form relations for the subgrade modulus. The models give consistently larger stiffness for the Winkler springs as compared to previously proposed similar continuum-based models that ignore the later...
Research Interests:
Research Interests:
Contemporary seismic design codes have become more stringent with respect to the requirements for design forces and deformations in building design. This paper demonstrates that it could be worthwhile to consider the introduction of... more
Contemporary seismic design codes have become more stringent with respect to the requirements for design forces and deformations in building design. This paper demonstrates that it could be worthwhile to consider the introduction of soil-structure-interaction provisions into local design codes. This is partly to be able to offset the costs incurred by the high magnitude of base shear demand in most buildings attributed to site amplifications due to soft soil sites, as per the requirements of current codes, including the recent South African seismic design code. This beneficial effect of site soils is as a result of lengthening of the fundamental period and of the increased effective damping of the overall system due to soil-structure interaction, which in most cases lead to reduced design spectral values. The paper shows that, if pertinent provisions in some international codes are properly adapted, a substantial reduction in the base shear force can be achieved, in many cases resul...
Research Interests:
In this paper, an attempt is made to assess some of the important seismic provisions and speciflcations of the 1995 Ethiopian Building Code Standard (EBCS 8) from, the perspective of the theory of structural dynamics and principles of... more
In this paper, an attempt is made to assess some of the important seismic provisions and speciflcations of the 1995 Ethiopian Building Code Standard (EBCS 8) from, the perspective of the theory of structural dynamics and principles of earthquake engineering. For this purpose, a concise but thorough theoretical background is provided on the vibration of structural systems. Only horizontal earthquake excitation of the systems is treated, as this causes the most important effects on building structures. Many concepts that are unknown to the average structural designer, but specified by the Code for strict adherence are clarified and critically assessed. The design spectra of the Code are compared with early research results that served as the basis for the preparation of design spectra of many international codes. Existing discrepancies are shown and illustrated using an example problem. Important conclusions are drawn and some recommendations made. Comparison of the provisions of the ...
Research Interests:
Basic provisions for design ground motions in seismic design codes of sub-Saharan African countries are critically reviewed. The seismic codes of Ethiopia, Kenya and Uganda are selected to represent the eastern region, Ghana to represent... more
Basic provisions for design ground motions in seismic design codes of sub-Saharan African countries are critically reviewed. The seismic codes of Ethiopia, Kenya and Uganda are selected to represent the eastern region, Ghana to represent the west and South Africa to represent the south. The specific provisions considered are those pertaining to site effect and the recurrence period of the design earthquake. The codes are also compared with one another and with selected current international codes from the US and Europe, with respect to selected provisions. The provisions are further viewed from the perspective of the state of the art and the state of the practice. It has been concluded that these basic provisions in most of the sub-Saharan African codes considered are inadequate in guaranteeing safety of human life and limiting damage to property, suggesting a need for immediate updating, an exception being the South African code.
Noting that contemporary seismic design codes have become more and more demanding in terms of requirements related to design forces and deformations for buildings, this paper attempts to demonstrate that it could be prudent to... more
Noting that contemporary seismic design codes have become more and more demanding in terms of requirements related to design forces and deformations for buildings, this paper attempts to demonstrate that it could be prudent to consider the introduction of soil-structure interaction (SSI) provisions into local codes in order to potentially offset the costs incurred by the high demand for base shear attributed to site amplifications by soft soil sites as per current code requirements. This mostly beneficial effect of site soils is as a result of lengthening of the fundamental period and of the mostly increased effective damping of the overall system due to SSI. After introducing the basic concepts of dynamic SSI, the paper demonstrates that if SSI provisions in some international codes are properly adapted, a substantial reduction in the base shear force could potentially be achieved so that the sizes of structural elements would also be proportionally less. With this, the paper attem...
Research Interests:
Research Interests:
A robust two-parameter subgrade model presented recently by the first author is employed to analyze plates on elastic foundations. A finite-difference method is employed to solve the resulting differential equation of the plate-soil... more
A robust two-parameter subgrade model presented recently by the first author is employed to analyze plates on elastic foundations. A finite-difference method is employed to solve the resulting differential equation of the plate-soil system. The proposed method is verified by means of the finite-element (FE) software, Plaxis. The performance of the subgrade model employed is compared with other existing simplified subgrade models and with Plaxis itself over a wide range of cases. The results show that the proposed model performs better than the models compared and can be easily adjusted to yield results in good agreement with the FE outputs over the majority of the plate area. It is demonstrated that the model is simple to use and capable of predicting the plate response by fully accounting for the shear resistance of the subgrade missing in other analytical models. The model parameters are fully defined and availed to users. The work demonstrates that the model is promising for routine and quick analysis of plates without the need to employ FE techniques to discretize the foundation soil.
Research Interests:
Research Interests:
Research Interests:
"""Basic provisions for design ground motions in seismic design codes of sub-Saharan African countries are reviewed and compared. The seismic codes of Ethiopia, Kenya and Uganda are selected to represent the most seismic region of the... more
"""Basic provisions for design ground motions in seismic design codes of sub-Saharan African countries are reviewed and compared. The seismic codes of Ethiopia, Kenya and Uganda are selected to represent the most seismic region of the East, Ghana to represent the West and South Africa to represent the South. The specific provisions considered are those pertaining to site effect and the recurrence period of the design earthquake. These provisions are compared among the selected African codes and against selected current international codes from the US and Europe. The local provisions pertaining to the selected criteria are also viewed from the perspective of the state of the art and the global state of the practice. It has been found out that these basic provisions in most of the sub-Saharan African codes considered are either inadequate to guarantee safety to human life and to limit damage to property or flawed suggesting the need for immediate updating, an exception being the South African Code.
Keywords: seismicity, seismic design, seismic hazard, return period, site effect, design spectra
"""
Keywords: seismicity, seismic design, seismic hazard, return period, site effect, design spectra
"""
Contemporary seismic design codes have become more stringent with respect to the requirements for design forces and deformations in building design. This paper demonstrates that it could be worthwhile to consider the introduction of... more
Contemporary seismic design codes have become more stringent with respect to the requirements for design forces and deformations in building design. This paper demonstrates that it could be worthwhile to consider the introduction of soil-structure-interaction provisions into local design codes. This is partly to be able to offset the costs incurred by the high magnitude of base shear demand in most buildings attributed to site amplifications due to soft soil sites, as per the requirements of current codes, including the recent South African seismic design code. This beneficial effect of site soils is as a result of lengthening of the fundamental period and of the increased effective damping of the overall system due to soil-structure interaction, which in most cases lead to reduced design spectral values. The paper shows that, if pertinent provisions in some international codes are properly adapted, a substantial reduction in the base shear force can be achieved, in many cases resulting in structural-work cost saving. With this, the paper attempts to address the legitimate concern of design engineers regarding the potential escalation of construction costs associated with the introduction of stringent
requirements of contemporary seismic design spectra, especially for soft soil sites.
requirements of contemporary seismic design spectra, especially for soft soil sites.
Research Interests:
A set of analytical formulas for the estimation of foundation model parameters is presented by synthesizing mechanical models at three different levels - Winkler, Pasternak and Kerr - with corresponding variants of a generalized continuum... more
A set of analytical formulas for the estimation of foundation model parameters is presented by synthesizing mechanical models at three different levels - Winkler, Pasternak and Kerr - with corresponding variants of a generalized continuum model presented recently. The generalized continuum model was derived using a unified approach on the basis of a subgrade idealized as an elastic layer of finite thickness overlying a rigid base without making prior simplifying assumptions. The spring stiffness in all variants is inversely proportional to the layer thickness rendering the models sensitive to this parameter with the potential to yield excessively large deflections for thick formations. This problem is alleviated by eliminating the layer thickness through the introduction of a calibration factor in form of a ratio of the layer thickness to the foundation width. It has been demonstrated that the calibration factor for each model type can be established from comparative analytical-numerical studies. Values obtained in this manner are suggested for practical use. The proposed calibrated formulas have potential applications in routine analysis of shallow beam-like and plate-like foundations and rigid pavements.
Research Interests:
A calibrated continuum foundation model of the same order as those of Pasternak, Filonenko-Borodich and Vlasov is developed. This is achieved by seeking equality of the surface deformation of a Pasternak-type model with that of a... more
A calibrated continuum foundation model of the same order as those of Pasternak, Filonenko-Borodich and Vlasov is developed. This is achieved by seeking equality of the surface deformation of a Pasternak-type model with that of a calibrated Kerr-type model presented recently without neglecting any stress, strain or deformation component in the continuum. In order to alleviate the sensitiveness of the model parameters to the layer thickness, this is eliminated from the corresponding expressions through the introduction of a dimensionless calibration parameter. The superiority of the new model over existing models of similar order has been demonstrated. Closed-form calibrated relations for the two parameters of the new model are provided together with values of the calibration factor. The use of the model is illustrated on a basic problem of a beam on an elastic foundation. Noting that a Pasternak-type model is more familiar to engineers and much easier to handle in applications than a Kerr-type model, its use in practice is recommended. The new model completes a serious of calibrated models at three different levels – Winkler, Pasternak and Kerr - proposed recently by the Author.
Research Interests:
A generalized model is presented for a subgrade idealized as an elastic layer overlying a rigid base. In contrast to previous works no stresses, strains or displacements are neglected a priori. A second-order partial differential equation... more
A generalized model is presented for a subgrade idealized as an elastic layer overlying a rigid base. In contrast to previous works no stresses, strains or displacements are neglected a priori. A second-order partial differential equation is obtained similar to Kerr’s mechanical model. It is demonstrated that several variants of the model can be obtained by specifying two relations: a function for the horizontal-to-vertical stress ratio and another for the variation of the vertical shear stresses with depth. Two such variants are presented. Also, lower-order models are obtained by making just the necessary assumptions. Accordingly, models of the order of two-parameter mechanical models are obtained when the horizontal deformations alone are neglected; Winkler-type models result only when the vertical shear stresses are omitted. Two variants of each of these lower-order models are provided. The proposed models have applications in the analysis of beams and plates on elastic foundations. Such an application together with a calibration work is demonstrated on beams in the companion paper (Part II).
Variants of a new continuum subgrade model presented by the first author (Part I) are employed to analyze beams on elastic foundations. When the higher order model variants are used, the beam’s differential equation becomes of order six.... more
Variants of a new continuum subgrade model presented by the first author (Part I) are employed to analyze beams on elastic foundations. When the higher order model variants are used, the beam’s differential equation becomes of order six. Closed-form solutions of finite and infinite beams are obtained for basic loadings. Deflections are compared with results of finite-element based software and a classical model. Though the comparison shows that the new model is sensitive to the subgrade thickness, a carefully conducted calibration work eliminated this drawback. Numerical examples illustrate that the calibrated variants give results in excellent agreement with the FE results. Also, the influence of the subgrade shear becomes less significant once the calibration is accomplished. This outcome suggests that the classical Winkler model can still be used together with the improved relation for the coefficient of subgrade reaction provided at the end of the paper.
Research Interests:
Three new variants of continuum-based models for an elastic subgrade are proposed. The subgrade is idealized as a homogenous, isotropic elastic layer of thickness H overlying a firm stratum. All components of the stress tensor in the... more
Three new variants of continuum-based models for an elastic subgrade are proposed. The subgrade is idealized as a homogenous, isotropic elastic layer of thickness H overlying a firm stratum. All components of the stress tensor in the subgrade are taken into account. Reasonable assumptions are made regarding the depth-wise variation of the vertical shear stress components and of the horizontal-to-vertical normal stress ratios to simplify mathematical work. The assumptions are based on observation of available analytical results of stress distributions and on knowledge of lateral earth pressure theories. The resulting differential equations are similar in form and order to a high order model developed earlier by Reissner based on a number of simplifying assumptions, but with different coefficients dependant on Poisson ratio. With the help of appropriately selected mechanical models, it has been shown that all of the new model variants consistently give larger effective vertical stiffness and larger shear interaction among the classical Winkler springs for the range of Poisson ratio of practical interest.
Research Interests:
Based on an isotropic elastic continuum of thickness H overlying a rigid stratum, a generalized formulation for the classical single-parameter Winkler's subgrade model is presented. In this formulation, all the normal components of the... more
Based on an isotropic elastic continuum of thickness H overlying a rigid stratum, a generalized formulation for the classical single-parameter Winkler's subgrade model is presented. In this formulation, all the normal components of the stress tensor are taken into consideration, whereas the shear stresses are intentionally dropped with the purpose of providing a useful perspective, with which Winkler's model and its associated coefficient of subgrade reaction can be viewed. The formulation takes into account the variation of the elasticity modulus with depth. It only demands specifying a relationship between the vertical and horizontal normal stresses. Accordingly, two such different assumptions are made to obtain two new Winkler-type subgrade models with the corresponding closed-form relations for the subgrade modulus. The models give consistently larger stiffness for the Winkler springs as compared to previously proposed similar continuum-based models that ignore the lateral stresses. It has also been pointed out that it is only if the shear stress components of the subgrade are taken into consideration that a multi-parameter model evolves regardless of whether the lateral normal stresses are included. Finally, the effective stiffness per unit area of the multiple beds of springs of such a higher order model is exactly the same as the subgrade modulus of the corresponding single-parameter Winkler model presented in this work.
Research Interests:
The dynamic analysis of multi-degree-of-freedom systems interacting with the foundation soil and acted by a system of periodic loads is considered. Such systems belong to the category of non-classically damped systems. The method of modal... more
The dynamic analysis of multi-degree-of-freedom systems interacting with the foundation soil and acted by a system of periodic loads is considered. Such systems belong to the category of non-classically damped systems. The method of modal superposition is employed for coordinate transformation. The resulting coupled algebraic equations are easily solved analytically without resorting to iterations, which would otherwise be necessary in time-domain analyses for irregular excitations like earthquake ground motions. A practical method of compiling the non-proportional damping matrix is adapted for the intended purpose. The proposed method of solution is convenient to use on spreadsheets, with MATLAB or to integrate it with existing software for routine structural analyses. The application is illustrated in detail on a simplified model of a symmetrically-loaded frame founded on a rigid mat. The method is equally applicable to systems the superstructure of which is discretized using finite-element method and the foundation system consisting of a set of isolated rigid footings or piles under isolated rigid pile caps.
Research Interests:
The dynamic response of a multi-degree-of-freedom (MDF) system with non-proportional damping subjected to harmonic loads is considered. Modal substitution is employed to transform the coupled differential equations of motion from... more
The dynamic response of a multi-degree-of-freedom (MDF) system with non-proportional damping subjected to harmonic loads is considered. Modal substitution is employed to transform the coupled differential equations of motion from geometric to modal coordinates. As might be expected, the modal transformation does not uncouple the differential equations of motion because of the inherent non-proportional damping, but transforms them into a system of coupled algebraic equations of convenient form for solution. The modal coordinates are then easily determined with the help of conventional techniques for solving systems of coupled equations. The method presented gives a closed form solution without the need to resort to iterative procedures, which would otherwise be necessary for other types of more irregular excitations like earthquake ground motions. Besides, it presented a technique of compiling the damping matrix of MDF systems exhibiting different mechanisms of energy dissipation in different regions. The application of the method is illustrated on a two-mass system, whose foundation interacts with the supporting soil - a phenomenon resulting totally to a four-degree-of-freedom system.
Research Interests:
"Recent developments in the definition of design ground motions or seismic analysis of structures are presented. A summary of results of empirical and analytical site-effect studies are provided and recent findings from empirical studies... more
"Recent developments in the definition of design ground motions or seismic analysis of structures are presented. A summary of results of empirical and analytical site-effect studies are provided and recent findings from empirical studies on instrumental records are compared against similar results from earlier studies.
Pertinent changes introduced in recent editions of international codes as a result of these evidences are presented. Comparisons of relevant provisions of EBCS 8: 1995 with those in contemporary American, European and South African codes are made. The paper presents compelling evidences showing that the amplification potential of site-soils can in general be significantly larger at sites of low amplitude rock-surface acceleration up to 0.1g than at sites of larger accelerations. Noting the practical significance of this fact on the seismic design of structures in low to moderate seismic regions, to which many cities and towns of Ethiopia belong, changes to selected provisions of the local code are proposed."
Pertinent changes introduced in recent editions of international codes as a result of these evidences are presented. Comparisons of relevant provisions of EBCS 8: 1995 with those in contemporary American, European and South African codes are made. The paper presents compelling evidences showing that the amplification potential of site-soils can in general be significantly larger at sites of low amplitude rock-surface acceleration up to 0.1g than at sites of larger accelerations. Noting the practical significance of this fact on the seismic design of structures in low to moderate seismic regions, to which many cities and towns of Ethiopia belong, changes to selected provisions of the local code are proposed."
Research Interests:
A comparison of seismic provisions having relevance to the Equivalent Static Force (ESF) method of analysis according to current major building codes worldwide is presented. The codes compared include the latest two editions of the... more
A comparison of seismic provisions having relevance to the Equivalent Static Force (ESF) method of analysis according to current major building codes worldwide is presented. The codes compared include the latest two editions of the Uniform Building Code (UBC 94 and UBC 97), the International Building Code (IBC 2000), the European Prestandard (Eurocode 8, 1998), and the Ethiopian Building Code Standard (EBCS 8, 1995). The comparison is made on the basis of the specified base shear coefficient, the vertical distribution of the base shear, the story shear and overturning moment, the considerations for torsion, P-delta effect, and the limitations on the story drift. Substantial differences are observed among the codes and even within different editions of a code, a good example being the UBC series. It is also shown that the most recent American codes like UBC 97 and IBC 2000 exhibited drastic changes in the definition of the base shear coefficient and in other pertinent regulations as compared to the classical forms familiar to most users.
Research Interests:
In this paper, an attempt is made to assess some of the important seismic provisions and specifications of the 1995 Ethiopian Building Code Standard (EBCS 8) from the perspective of the theory of structural dynamics and principles of... more
In this paper, an attempt is made to assess some of the important seismic provisions and specifications of the 1995 Ethiopian Building Code Standard (EBCS 8) from the perspective of the theory of structural dynamics and principles of earthquake engineering. For this purpose, a concise but thorough and lecture-note-type theoretical background is provided on the vibration of discrete mass systems. Only horizontal earthquake excitation of the systems is treated, as this causes the most important effect on building structures. Many concepts that are unknown to the average structural designer, but specified by the code for strict adherence, are clarified. The use of the design spectra annexed by the code for use in dynamic analysis is well illustrated and compared with early research results using an example problem. Important conclusions are drawn and some recommendations made.
Research Interests:
Research Interests:
A parametric study conducted on the influence of flexible bases on the response of building structures subjected to both gravity and lateral static loads is presented. The most important parameters that are varied include the type of... more
A parametric study conducted on the influence of flexible bases on the response of building structures subjected to both gravity and lateral static loads is presented. The most important parameters that are varied include the type of structural system, the type of soil and the embedment depth of the foundation. Both framed and dual structural systems are studied with height varying from six to twenty one stories. Base springs are established in accordance with relations available in pertinent literature and presented in a companion paper. Analyses of three-dimensional models using ETABS V 8.00 are carried out. The results presented show that all three factors have strong influence on the responses. The differences in internal forces of essential lateral-force resisting structural elements like columns and shear walls between the fixed-base and the flexible-base models are found to be significant. The fixed-base model underestimates both axial forces and moments in some columns of the dual system. The fixed-base model tends to underestimate the shear wall bending moments and axial forces, whereas it consistently overestimates the shear forces. Significant differences in the reaction moments at the foundation level are also noted between the fixed-base and flexible-base models.
Research Interests:
The theory of elasticity of continua is employed to show the background of spring formulas that are introduced at the bases of structures to account for immediate static deformation of soils. This is followed by providing sets of such... more
The theory of elasticity of continua is employed to show the background of spring formulas that are introduced at the bases of structures to account for immediate static deformation of soils. This is followed by providing sets of such valuable formulas for use in practical modeling of structures founded on deformable soils. Additional spring formulas that account for primary and secondary consolidation settlement are derived. It is shown that these vertical springs can be joined in series to account for all types of soil deformation - immediate, primary consolidation, and secondary consolidation or creep. The application of the springs is illustrated using a simple building frame subjected to gravity loads only. The internal forces in the structural members with and without flexible base elements showed notable differences. The significance of the introduction of flexible-base elements in taller and more rigid structures subjected to lateral loads can be expected to be even larger. The influence of the consolidation springs could particularly be more significant if different foundation elements of a structure rest on compressible layers of different properties and varying thickness. A companion paper deals with a parametric study on the influence of elastic base springs, in which the height and type of the structural system, the soil type, and the embedment depth of the foundation are varied.