... E-mail: [email protected] Zhou and Kareem's critical review of models used in the AS... more ... E-mail: [email protected] Zhou and Kareem's critical review of models used in the ASCE 7-98 standard for mean wind speed, turbulence intensity, and gust speed profiles ''suggests notable inconsistencies'' among the defi-nitions associated with those models. ...
... Fahim Sadek, Bijan Mohraz, Michael A. Riley. ... Comparisons of the results using the methods... more ... Fahim Sadek, Bijan Mohraz, Michael A. Riley. ... Comparisons of the results using the methods proposed in this study and in FEMA 273 with time-history analyses for several single-andmulti-degree-of-freedom structures indicate that the proposed procedures give more accurate ...
The policy of NIST is to use the International System of Units in all publications. In this docum... more The policy of NIST is to use the International System of Units in all publications. In this document, however, units are presented in the system prevalent in the relevant discipline, although in some cases more than one system of units may be presented. Use in Legal Proceedings No part of any report resulting from a NIST investigation into a structural failure or from an investigation under the National Construction Safety Team Act may be used in any suit or action for damages arising out of any matter mentioned in such report (15 U.S.C. 281a, as amended).
This paper investigates the degradation of the mechanical properties of concretes made with three... more This paper investigates the degradation of the mechanical properties of concretes made with three types of aggregate affected by alkali-silica reaction (ASR). Three standard testing methods-ASTM C289, JASS 5N T-603 and ASTM C1260-were used to identify the reactivity of ASR of the three aggregates selected. The test results show that all three aggregates are potentially deleterious. A new acceleration method based on JASS 5N T-603 and ASTM C1260 was proposed for concrete specimens. In the acceleration method, cylindrical concrete specimens were used, additional alkali material was added to the concrete mixture and the specimens were stored under conditions similar to ASTM C1260. The preconditioned concrete specimens were then used for evaluating the mechanical properties of the ASR-affected concrete in terms of strength and stiffness. The test results show that special attention must be paid to the effects of two opposing mechanisms on the strength and stiffness of concrete: hydration reactions and ASR. Hydration reactions enhance the mechanical properties, whereas ASR does the opposite. The changes in length of concrete specimens were also measured, which showed that the basic trends for change in length and mechanical properties may be different. It is better to examine the effect of ASR on both change in length and mechanical properties. The size and reactivity of the aggregate are very important factors for the mechanical properties of ASR-affected concretes. Within the two-month testing period, the reactive fine aggregate might cause ASR expansion and the reactive coarse aggregates might not.
This paper presents a systematic investigation of the influence of various parameters on the ther... more This paper presents a systematic investigation of the influence of various parameters on the thermal performance of composite floor slabs with profiled steel decking exposed to fire effects. The investigation uses a detailed finite-element modeling approach that represents the concrete slab with solid elements and the steel decking with shell elements. After validating the modeling approach against experimental data, a parametric study is conducted to investigate the influence of thermal boundary conditions, thermal properties of concrete, and slab geometry on the temperature distribution within composite slabs. The results show that the fire resistance of composite slabs, according to the thermal insulation criterion, is generally governed by the maximum temperature occurring at the unexposed surface of the slab, rather than the average temperature. The emissivity of steel has a significant influence on the temperature distribution in composite slabs. A new temperature-dependent emissivity is proposed for the steel decking to give a better prediction of temperatures in the slab. The moisture content of the concrete has a significant influence on the temperature distribution, with an increment of 1 % in moisture content leading to an increase in the fire resistance of about 5 minutes. The height of the upper continuous portion of the slab is found to be the key geometrical factor influencing heat transfer through the slab, particularly for the thin portion of the slab. Heat transfer through the thick portion of the slab is also significantly affected by the height of the rib and the width at the top of the rib.
Several prototype steel moment-frame buildings have been designed for the purpose of assessing th... more Several prototype steel moment-frame buildings have been designed for the purpose of assessing their vulnerability to disproportionate collapse. The buildings were designed for moderate and high seismic regions. This paper summarizes the development of threedimensional finite element models of these prototype buildings, with a focus on the modeling approach used for the connections and the composite floor system. Initial simulation results under a column removal scenario are presented to illustrate the model capabilities. Ongoing assessments of reserve capacity and disproportionate collapse vulnerability using these models are part of a larger study aimed at quantifying and comparing the relative robustness of different structural systems.
To achieve structures that are risk-consistent, structural reliability methods must be used that ... more To achieve structures that are risk-consistent, structural reliability methods must be used that account for uncertainties with respect to the relevant parameters affecting the estimation of wind effects. In this paper, we obtain measures of uncertainties in the estimation of the wind speeds upwind of structures. These uncertainties are due to incomplete knowledge with respect to the relevant extreme climatological and micrometeorological parameters. Advances in wind engineering and improvements in computational capabilities now make it possible to improve upon earlier estimates available in the literature. The work presented in this paper is a phase of a broader NIST project aimed at developing user-friendly software for the estimation of probabilities of failure of low-rise structures subjected to wind loads.
ABSTRACT This paper presents a computational investigation of the robustness of a typical concret... more ABSTRACT This paper presents a computational investigation of the robustness of a typical concrete deck-steel beam composite floor system with simple shear connections in the event that a center column has been removed. The study provides insight into the behavior and failure modes of simple shear connections and composite floor systems comprised of such connections. Analyses of a connection subassemblage indicate that loads are primarily resisted by cable action of the beams after column loss resulting in increasing tensile forces in the beams and connections that could eventually precipitate failure. Simulation results show that the floor deck contributes significantly to the floor system response through: diaphragm action to prevent the exterior columns from being pulled inward and membrane action primarily through the reinforcement mesh and metal deck. The analyses indicate that the capacity of the analyzed floor system under the column removal scenario is significantly less than the load specified by the General Services Administration's current progressive collapse guidelines. This suggests that applying these guidelines, the composite floor system studied would be vulnerable to collapse if a center column is lost.
ABSTRACT Peak values in time histories of wind effects may be obtained by using the entire inform... more ABSTRACT Peak values in time histories of wind effects may be obtained by using the entire information inherent in the time series of the wind effect, a method entailing the estimation of the probability distribution of the peak through the application of the classical Rice procedure extended for non-Gaussian time histories. We present estimates of the sampling errors inherent in this method. These are needed for structural reliability calculations and for decisions on the requisite length of wind tunnel pressure records, especially for database-assisted design. If based on the analysis of 1-h-long records generated by Monte Carlo simulation, typical sampling errors in the estimation of peaks of time histories corresponding to windstorms of I-h duration are about 5%. If based on 30- or 20-min records, they are about 1.5 times or twice as large, respectively. Consideration of the sampling errors in reliability calculations entails an estimated increase in the requisite safety margins with respect to wind loading of roughly 2, 3, and 5% if 1-h, 30-min, or 20-min records are used, respectively.
The progressive collapse resistance of seismically designed steel braced frames is investigated u... more The progressive collapse resistance of seismically designed steel braced frames is investigated using validated computational simulation models. Two types of braced systems are considered: namely, special concentrically braced frames and eccentrically braced frames. The study is conducted on previously designed 10-story prototype buildings by applying the alternate path method. In this methodology, critical columns and adjacent braces, if present, are instantaneously removed from an analysis model and the ability of the model to successfully absorb member loss is investigated. Member removal in this manner is intended to represent a situation where an extreme event or abnormal load destroys the member. The simulation results show that while both systems benefit from placement of the seismically designed frames on the perimeter of the building, the eccentrically braced frame is less vulnerable to progressive collapse than the special concentrically braced frame. Improvement in behavior is due to improved system and member layouts in the former compared to the latter rather than the use of more stringent seismic detailing.
The policy of the National Institute of Standards and Technology is to use metric units in all it... more The policy of the National Institute of Standards and Technology is to use metric units in all its published materials. Because this report is intended for the U.S. building construction industry which uses in-pound units, it is more practical and less confusing to use inch-pound units, in some cases, rather than metric units within quoted text. However, in most cases, units are presented in metric and the inch-pound system. Disclaimer Certain commercial entities, equipment, products, or materials are identified in this document in order to describe a procedure or concept adequately or to trace the history of the procedures and practices used. Such identification is not intended to imply recommendation, endorsement, or implication that the entities, products, materials, or equipment are necessarily the best available for the purpose.
Experience 2003-date Research Engineer ARA, Inc. Mountain View, CA Leading and performing researc... more Experience 2003-date Research Engineer ARA, Inc. Mountain View, CA Leading and performing research projects related to the dynamic response of structures to impact and explosive load. • Implementation geo-material models in structural response calculations. • Development connection models for World Trade Center progressive collapse analysis. • Development simulation based design system to rapidly evaluate the vulnerability of vehicle designs to a mine or above ground bomb.
This paper presents an experimental study of two full-scale steel beam-column assemblies, each co... more This paper presents an experimental study of two full-scale steel beam-column assemblies, each comprising three columns and two beams, to (1) define their response characteristics under a column removal scenario, including the capacity of the beams and their connections to carry loads through catenary action, and (2) provide experimental data for validation of beam-to-column connection models for assessing the robustness of structural systems. The assemblies represent portions of the exterior moment-resisting frames of two ten-story steel frame buildings. One test specimen had welded unreinforced flange, bolted web connections, and the other had reduced beam section connections. When subjected to monotonically increasing vertical displacement of the unsupported center column, both specimens exhibited an initial elastic response dominated by flexure. With increased vertical displacement, the connections yielded, and axial tension developed in the beams. The axial tension in the beams increased until the connections failed under combined bending and axial stresses. The test results show that the rotational capacities of both connections under monotonic column displacement are about twice as large as those based on seismic test data.
ABSTRACT: We descn"be the database-assisted design approach for wind loading, and its applic... more ABSTRACT: We descn"be the database-assisted design approach for wind loading, and its application for estimating limit states -including ultimate limit states --for low-rise buildings and taIl buildings experiencing along-wind, across-wind, and torsional dynamic effects. Database-assisted designs account for the temporal and spatial variation of wind effects and can result in far more risk-consistent, safe, and economical structures than those based on conventional stanllard provisions. Structural reliability methods,are currently applied to da- tabase-assisted designs with a view to developing improved,estimates of probabilities of failure under wind loads. INTRODUCnON,of the type used in U.S.
Semi-active control systems combine the features of active and passive control to reduce the resp... more Semi-active control systems combine the features of active and passive control to reduce the response of structures to various dynamic loadings. They include: a) active variable stiffness where the stiffness of the structure is adjusted to establish a non-resonant condition between the structure and excitation, and b) active variable damper where the damping coefficient of the device is varied to achieve the most reduction in the response. This study is concerned with examining the effectiveness of variable dampers for seismic applications. Three algorithms for selecting the damping coefficient of variable dampers are presented and compared. They include: a linear quadratic regulator (LQR) algorithm, a generalized LQR algorithm where a penalty is imposed on the acceleration response, and a displacement-acceleration domain algorithm where the damping coefficient is selected by examining the response on the displacement-acceleration plane and assigning different damping coefficients accordingly. Two single-degree-of-freedom structures subjected to 20 ground excitations are analyzed using the three algorithms. The analyses indicate that unlike passive dampers where for flexible structures, an increase in damping coefficient decreases the displacement but increases the acceleration response, variable dampers can be effective in reducing both the displacement and acceleration responses. The study indicates that the generalized LQR algorithm is more efficient than the other two in reducing the displacement and acceleration responses. The algorithms are used to compute the seismic response of two flexible structures-an isolated bridge modeled as a single-degree-of-freedom system and a base-isolated six-story frame modeled as a multi-degree-of-freedom system. The results indicate that variable dampers reduce the displacement and acceleration responses of the two structures to a significant degree.
... E-mail: [email protected] Zhou and Kareem's critical review of models used in the AS... more ... E-mail: [email protected] Zhou and Kareem's critical review of models used in the ASCE 7-98 standard for mean wind speed, turbulence intensity, and gust speed profiles ''suggests notable inconsistencies'' among the defi-nitions associated with those models. ...
... Fahim Sadek, Bijan Mohraz, Michael A. Riley. ... Comparisons of the results using the methods... more ... Fahim Sadek, Bijan Mohraz, Michael A. Riley. ... Comparisons of the results using the methods proposed in this study and in FEMA 273 with time-history analyses for several single-andmulti-degree-of-freedom structures indicate that the proposed procedures give more accurate ...
The policy of NIST is to use the International System of Units in all publications. In this docum... more The policy of NIST is to use the International System of Units in all publications. In this document, however, units are presented in the system prevalent in the relevant discipline, although in some cases more than one system of units may be presented. Use in Legal Proceedings No part of any report resulting from a NIST investigation into a structural failure or from an investigation under the National Construction Safety Team Act may be used in any suit or action for damages arising out of any matter mentioned in such report (15 U.S.C. 281a, as amended).
This paper investigates the degradation of the mechanical properties of concretes made with three... more This paper investigates the degradation of the mechanical properties of concretes made with three types of aggregate affected by alkali-silica reaction (ASR). Three standard testing methods-ASTM C289, JASS 5N T-603 and ASTM C1260-were used to identify the reactivity of ASR of the three aggregates selected. The test results show that all three aggregates are potentially deleterious. A new acceleration method based on JASS 5N T-603 and ASTM C1260 was proposed for concrete specimens. In the acceleration method, cylindrical concrete specimens were used, additional alkali material was added to the concrete mixture and the specimens were stored under conditions similar to ASTM C1260. The preconditioned concrete specimens were then used for evaluating the mechanical properties of the ASR-affected concrete in terms of strength and stiffness. The test results show that special attention must be paid to the effects of two opposing mechanisms on the strength and stiffness of concrete: hydration reactions and ASR. Hydration reactions enhance the mechanical properties, whereas ASR does the opposite. The changes in length of concrete specimens were also measured, which showed that the basic trends for change in length and mechanical properties may be different. It is better to examine the effect of ASR on both change in length and mechanical properties. The size and reactivity of the aggregate are very important factors for the mechanical properties of ASR-affected concretes. Within the two-month testing period, the reactive fine aggregate might cause ASR expansion and the reactive coarse aggregates might not.
This paper presents a systematic investigation of the influence of various parameters on the ther... more This paper presents a systematic investigation of the influence of various parameters on the thermal performance of composite floor slabs with profiled steel decking exposed to fire effects. The investigation uses a detailed finite-element modeling approach that represents the concrete slab with solid elements and the steel decking with shell elements. After validating the modeling approach against experimental data, a parametric study is conducted to investigate the influence of thermal boundary conditions, thermal properties of concrete, and slab geometry on the temperature distribution within composite slabs. The results show that the fire resistance of composite slabs, according to the thermal insulation criterion, is generally governed by the maximum temperature occurring at the unexposed surface of the slab, rather than the average temperature. The emissivity of steel has a significant influence on the temperature distribution in composite slabs. A new temperature-dependent emissivity is proposed for the steel decking to give a better prediction of temperatures in the slab. The moisture content of the concrete has a significant influence on the temperature distribution, with an increment of 1 % in moisture content leading to an increase in the fire resistance of about 5 minutes. The height of the upper continuous portion of the slab is found to be the key geometrical factor influencing heat transfer through the slab, particularly for the thin portion of the slab. Heat transfer through the thick portion of the slab is also significantly affected by the height of the rib and the width at the top of the rib.
Several prototype steel moment-frame buildings have been designed for the purpose of assessing th... more Several prototype steel moment-frame buildings have been designed for the purpose of assessing their vulnerability to disproportionate collapse. The buildings were designed for moderate and high seismic regions. This paper summarizes the development of threedimensional finite element models of these prototype buildings, with a focus on the modeling approach used for the connections and the composite floor system. Initial simulation results under a column removal scenario are presented to illustrate the model capabilities. Ongoing assessments of reserve capacity and disproportionate collapse vulnerability using these models are part of a larger study aimed at quantifying and comparing the relative robustness of different structural systems.
To achieve structures that are risk-consistent, structural reliability methods must be used that ... more To achieve structures that are risk-consistent, structural reliability methods must be used that account for uncertainties with respect to the relevant parameters affecting the estimation of wind effects. In this paper, we obtain measures of uncertainties in the estimation of the wind speeds upwind of structures. These uncertainties are due to incomplete knowledge with respect to the relevant extreme climatological and micrometeorological parameters. Advances in wind engineering and improvements in computational capabilities now make it possible to improve upon earlier estimates available in the literature. The work presented in this paper is a phase of a broader NIST project aimed at developing user-friendly software for the estimation of probabilities of failure of low-rise structures subjected to wind loads.
ABSTRACT This paper presents a computational investigation of the robustness of a typical concret... more ABSTRACT This paper presents a computational investigation of the robustness of a typical concrete deck-steel beam composite floor system with simple shear connections in the event that a center column has been removed. The study provides insight into the behavior and failure modes of simple shear connections and composite floor systems comprised of such connections. Analyses of a connection subassemblage indicate that loads are primarily resisted by cable action of the beams after column loss resulting in increasing tensile forces in the beams and connections that could eventually precipitate failure. Simulation results show that the floor deck contributes significantly to the floor system response through: diaphragm action to prevent the exterior columns from being pulled inward and membrane action primarily through the reinforcement mesh and metal deck. The analyses indicate that the capacity of the analyzed floor system under the column removal scenario is significantly less than the load specified by the General Services Administration's current progressive collapse guidelines. This suggests that applying these guidelines, the composite floor system studied would be vulnerable to collapse if a center column is lost.
ABSTRACT Peak values in time histories of wind effects may be obtained by using the entire inform... more ABSTRACT Peak values in time histories of wind effects may be obtained by using the entire information inherent in the time series of the wind effect, a method entailing the estimation of the probability distribution of the peak through the application of the classical Rice procedure extended for non-Gaussian time histories. We present estimates of the sampling errors inherent in this method. These are needed for structural reliability calculations and for decisions on the requisite length of wind tunnel pressure records, especially for database-assisted design. If based on the analysis of 1-h-long records generated by Monte Carlo simulation, typical sampling errors in the estimation of peaks of time histories corresponding to windstorms of I-h duration are about 5%. If based on 30- or 20-min records, they are about 1.5 times or twice as large, respectively. Consideration of the sampling errors in reliability calculations entails an estimated increase in the requisite safety margins with respect to wind loading of roughly 2, 3, and 5% if 1-h, 30-min, or 20-min records are used, respectively.
The progressive collapse resistance of seismically designed steel braced frames is investigated u... more The progressive collapse resistance of seismically designed steel braced frames is investigated using validated computational simulation models. Two types of braced systems are considered: namely, special concentrically braced frames and eccentrically braced frames. The study is conducted on previously designed 10-story prototype buildings by applying the alternate path method. In this methodology, critical columns and adjacent braces, if present, are instantaneously removed from an analysis model and the ability of the model to successfully absorb member loss is investigated. Member removal in this manner is intended to represent a situation where an extreme event or abnormal load destroys the member. The simulation results show that while both systems benefit from placement of the seismically designed frames on the perimeter of the building, the eccentrically braced frame is less vulnerable to progressive collapse than the special concentrically braced frame. Improvement in behavior is due to improved system and member layouts in the former compared to the latter rather than the use of more stringent seismic detailing.
The policy of the National Institute of Standards and Technology is to use metric units in all it... more The policy of the National Institute of Standards and Technology is to use metric units in all its published materials. Because this report is intended for the U.S. building construction industry which uses in-pound units, it is more practical and less confusing to use inch-pound units, in some cases, rather than metric units within quoted text. However, in most cases, units are presented in metric and the inch-pound system. Disclaimer Certain commercial entities, equipment, products, or materials are identified in this document in order to describe a procedure or concept adequately or to trace the history of the procedures and practices used. Such identification is not intended to imply recommendation, endorsement, or implication that the entities, products, materials, or equipment are necessarily the best available for the purpose.
Experience 2003-date Research Engineer ARA, Inc. Mountain View, CA Leading and performing researc... more Experience 2003-date Research Engineer ARA, Inc. Mountain View, CA Leading and performing research projects related to the dynamic response of structures to impact and explosive load. • Implementation geo-material models in structural response calculations. • Development connection models for World Trade Center progressive collapse analysis. • Development simulation based design system to rapidly evaluate the vulnerability of vehicle designs to a mine or above ground bomb.
This paper presents an experimental study of two full-scale steel beam-column assemblies, each co... more This paper presents an experimental study of two full-scale steel beam-column assemblies, each comprising three columns and two beams, to (1) define their response characteristics under a column removal scenario, including the capacity of the beams and their connections to carry loads through catenary action, and (2) provide experimental data for validation of beam-to-column connection models for assessing the robustness of structural systems. The assemblies represent portions of the exterior moment-resisting frames of two ten-story steel frame buildings. One test specimen had welded unreinforced flange, bolted web connections, and the other had reduced beam section connections. When subjected to monotonically increasing vertical displacement of the unsupported center column, both specimens exhibited an initial elastic response dominated by flexure. With increased vertical displacement, the connections yielded, and axial tension developed in the beams. The axial tension in the beams increased until the connections failed under combined bending and axial stresses. The test results show that the rotational capacities of both connections under monotonic column displacement are about twice as large as those based on seismic test data.
ABSTRACT: We descn"be the database-assisted design approach for wind loading, and its applic... more ABSTRACT: We descn"be the database-assisted design approach for wind loading, and its application for estimating limit states -including ultimate limit states --for low-rise buildings and taIl buildings experiencing along-wind, across-wind, and torsional dynamic effects. Database-assisted designs account for the temporal and spatial variation of wind effects and can result in far more risk-consistent, safe, and economical structures than those based on conventional stanllard provisions. Structural reliability methods,are currently applied to da- tabase-assisted designs with a view to developing improved,estimates of probabilities of failure under wind loads. INTRODUCnON,of the type used in U.S.
Semi-active control systems combine the features of active and passive control to reduce the resp... more Semi-active control systems combine the features of active and passive control to reduce the response of structures to various dynamic loadings. They include: a) active variable stiffness where the stiffness of the structure is adjusted to establish a non-resonant condition between the structure and excitation, and b) active variable damper where the damping coefficient of the device is varied to achieve the most reduction in the response. This study is concerned with examining the effectiveness of variable dampers for seismic applications. Three algorithms for selecting the damping coefficient of variable dampers are presented and compared. They include: a linear quadratic regulator (LQR) algorithm, a generalized LQR algorithm where a penalty is imposed on the acceleration response, and a displacement-acceleration domain algorithm where the damping coefficient is selected by examining the response on the displacement-acceleration plane and assigning different damping coefficients accordingly. Two single-degree-of-freedom structures subjected to 20 ground excitations are analyzed using the three algorithms. The analyses indicate that unlike passive dampers where for flexible structures, an increase in damping coefficient decreases the displacement but increases the acceleration response, variable dampers can be effective in reducing both the displacement and acceleration responses. The study indicates that the generalized LQR algorithm is more efficient than the other two in reducing the displacement and acceleration responses. The algorithms are used to compute the seismic response of two flexible structures-an isolated bridge modeled as a single-degree-of-freedom system and a base-isolated six-story frame modeled as a multi-degree-of-freedom system. The results indicate that variable dampers reduce the displacement and acceleration responses of the two structures to a significant degree.
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Papers by F. Sadek