Fahim Sadek

A METHOD OF ESTIMATING THE PARAMETERS OF TUNED MASS DAMPERS FOR SEISMIC APPLICATIONS

The optimum parameters of tuned mass dampers (TMD) that result in considerable reduction in the response of structures to seismic loading are presented. The criterion used to obtain the optimum parameters is to select, for a given mass ratio, the frequency (tuning) and damping ratios that would result in equal and large modal damping in the first two modes of vibration. The parameters are used to compute the response of several single and multi-degree-of-freedom structures with TMDs to different earthquake excitations. The results indicate that the use of the proposed parameters reduces the displacement and acceleration responses significantly. The method can also be used in vibration control of tall buildings using the so-called ‘mega-substructure configuration’, where substructures serve as vibration absorbers for the main structure. It is shown that by selecting the optimum TMD parameters as proposed in this paper, significant reduction in the response of tall buildings can be achieved.

Testing and Analysis of Steel and Concrete Beam-Column Assemblies under a Column Removal Scenario

This paper presents an experimental and computational assessment of the performance of steel and reinforced concrete beam-column assemblies under monotonic vertical displacement of a center column, simulating a column removal scenario. The assemblies represent portions of structural framing systems designed as intermediate moment frames (IMFs) and special moment frames (SMFs) for Seismic Design Categories C and D, respectively. The steel IMF and SMF assemblies were designed in accordance with ANSI/AISC 341-02 by using prequalified moment connections specified in FEMA 350. The concrete IMF and SMF assemblies were designed and detailed in accordance with ACI 318-02 requirements. Each full-scale assembly comprises two beam spans and three columns, and downward displacements of the center column are imposed until failure. The study provides insight into the behavior and failure modes of the assemblies, including the development of catenary action. Both detailed and reduced finite-element models are developed,...

Progressive Collapse Resistance of Steel-Concrete Composite Floors

This paper discusses the progressive collapse resistance of steel-concrete composite floors in which steel beams are attached to columns through shear tabs. This is a common type of system used for the gravity bay portions of steel buildings. The study is conducted using computational simulation models validated through extensive comparisons to disparate test data. The models are used to investigate key parameters influencing the robustness of generic composite floors subjected to the removal of a center column. In particular, the effects of deck thickness, steel reinforcement, and the numbers of bolts in the shear tab connection on the behavior of the system are studied as a function of the loading scheme. The simulation results show that the majority of collapse resistance comes from the steel deck and that, for the system considered, increasing connection strength by adding more bolts might not be that beneficial in increasing overall collapse strength. The dynamic impact factor, which is widely used to account for dynamic effects within a static design framework, is also computed and the DIF value recommended in existing design guidelines is evaluated.

Single‐ and multiple‐tuned liquid column dampers for seismic applications

Design parameters for single- and multiple-tuned liquid column dampers for reducing the response of structures to seismic excitations are presented. A deterministic analysis is carried out using 72 earthquake ground motion records to determine the tuning ratio, tube width to liquid length ratio, and head loss coefficient corresponding to a given mass ratio for single-tuned liquid column dampers. A similar analysis is performed to determine the central tuning ratio, tuning bandwidth, and grouping of dampers for multiple-tuned liquid column dampers. The study indicates that by properly selecting the design parameters, single- and multiple-tuned liquid column dampers can reduce the response of structures to seismic excitation by up to 45 per cent. Design examples using single- and multiple-tuned liquid column dampers in a bridge and a ten-storey building are presented to illustrate how the parameters are selected and to demonstrate the performance of the devices under different ground excitations. The response of several structures with tuned liquid column dampers is compared with that using tuned mass dampers where it is shown that both devices result in comparable reductions in the response.

Performance of Steel Moment Connections under a Column Removal Scenario. I: Experiments

AbstractThis 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 10-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...

Performance of Steel Moment Connections under a Column Removal Scenario. II: Analysis

AbstractThis paper presents a computational investigation of the response of steel beam-column assemblies with moment connections under monotonic loading conditions simulating a column removal scenario. Two beam-column assemblies are analyzed, which incorporate (1) welded unreinforced flange bolted web connections, and (2) reduced beam section connections. Detailed models of the assemblies are developed, which use highly refined solid and shell elements to represent nonlinear material behavior and fracture. Reduced models are also developed, which use a much smaller number of beam and spring elements and are intended for use in future studies to assess the vulnerability of complete structural systems to disproportionate collapse. The two modeling approaches are described, and computational results are compared with the results of the full-scale tests described in the companion paper. Good agreement is observed, demonstrating that both the detailed and reduced models are capable of capturing the predominan...

Database-assisted design for wind: basic concepts and software development

Standard provisions for wind loads on buildings have traditionally been based on summary tables and/or plots suitable for slide-rule calculations. The accuracy in the definition of wind loads inherent in such tables and plots is far lower than that inherent in current methods for stress computation. Advances in computational power now make it possible to reduce this discrepancy and achieve structural designs for wind that are significantly safer and more economical than current designs. This is true both for routine, low-rise structures and for flexible structures experiencing significant dynamic effects. In this paper, we present the concept of database-assisted design (DAD) along with a discussion of the application software Wind Load Design Environment, a user-friendly tool for designers and code writers that employs the DAD approach. The DAD approach entails the use of large databases of aerodynamic pressures, the optional use of databases of directional extreme wind speeds, and the use of structural information needed for the description of linear or nonlinear structural behavior. We present progress achieved to date, describe current efforts and future needs, and discuss the implications of DAD for reliability-based design and performance-based standards development.

Distribution of earthquake input energy in structures

In developing an energy-based design approach and assessing the damage potential of structures, one must know the distribution of earthquake input energy among energy components: kinetic, elastic strain, hysteretic, and damping. This report examines the influences of the ground motion characteristics: intensity, frequency content, and duration of strong motion and the structural properties: ductility, damping, and hysteretic behavior on the distribution of input energy for a oneand a five-story building using 20 accelerograms, ten with short and ten with long duration of strong motion. Results indicate that for certain damping ratios, ductility has a significant influence on input energy and its distribution among energy components in a structure. For a given ductility ratio, small damping ratio (less than 5%) has a minor effect on input energy, but a major influence on the energy distribution. Damping ratios larger than 5% have a significant influence on the input energy and its distribution. Three energy ratios that relate to hysteretic energy were computed: the maximum ratio of hysteretic to input energy (Eh/Eir)m, the ratio of the maximum hysteretic energy to the maximum input energy Ehm/Eirm, and the equivalent number of yield excursions Neq=Ehm/(Fy.up) where Fy is the yield strength, and up is the plastic deformation. It is found that (Eh/Eir)m generally reflects the energy demand for the largest yield excursion, and Ehm/Eirm and Neq reflect the energy demand for the entire duration of accelerogram. The study shows that (Eh/Eir)m is independent of the duration of strong motion and period of structure; however, Ehm/Eirm is independent of both only for periods less than 1 s. Results indicate that as the duration becomes longer the equivalent number of yield excursions Neq increases indicating more structural damage. The influence of ground motion characteristics and structural properties on the distribution of energy parameters for a five-story building with fixed-base, base-isolation, supplemental damping, and semi-active control are examined using the 20 accelerograms. The results show that: 1) the distribution of energy through the height of the building is mostly independent of the frequency content and the duration of strong motion, 2) baseisolation, supplemental damping, and semi-active control reduce the damage potential by reducing the input and hysteretic energy demands and have significant influences on the distribution of energy through the height of the building.

Earthquake Ground Motion and Response Spectra

This chapter surveys the state-of-the-art work in strong motion seismology and ground motion characterization. Methods of ground motion recording and correction are first presented, followed by a discussion of ground motion characteristics including peak ground motion, duration of strong motion, and frequency content. Factors that influence earthquake ground motion such as source distance, site geology, earthquake magnitude, source characteristics, and directivity are examined. The chapter presents probabilistic methods for evaluating seismic risk at a site and development of seismic maps used in codes and provisions. Earthquake response spectra and factors that influence their characteristics such as soil condition, magnitude, distance, and source characteristics are also presented and discussed. Earthquake design spectra proposed by several investigators and those recommended by various codes and provisions through the years to compute seismic base shears are described. The latter part of the chapter discusses inelastic earthquake spectra and response modification factors used in seismic codes to reduce the elastic design forces and account for energy absorbing capacity of structures due to inelastic action. Earthquake energy content and energy spectra are also briefly introduced. Finally, the chapter presents a brief discussion of artificially generated ground motion.

Modeling and Analysis of Single-Plate Shear Connections under Column Loss

AbstractThis paper presents a computational assessment of the behavior of single-plate shear (shear tab) connections in gravity frames under column-loss scenarios. Two-span beam assemblies without floor slabs are considered under push-down loading of the unsupported center column. Both detailed and reduced modeling approaches are used in the computational assessment, and comparisons with experimental data are presented to establish confidence in the models. The models are used to investigate the influence of factors such as end support conditions, span length, connection strength, and postultimate connection behavior on the collapse resistance of gravity framing systems. Rotational capacities of single-plate shear connections under column-loss scenarios are, in some cases, less than half the values based on seismic test data, owing to the axial extension imposed on the connections in addition to rotation.