Nikitas Bazeos

Static, seismic and stability analyses of a prototype wind turbine steel tower

Selected results of a study concerning the load bearing capacity and the seismic behavior of a prototype steel tower for a 450 kW wind turbine with a horizontal power transmission axle are presented. The main load bearing structure of the steel tower rises to almost 38 m high and consists of thin-wall cylindrical and conical parts, of varying diameters and wall thicknesses, which are linked together by bolted circular rings. The behavior and the load capacity of the structure have been studied with the aid of a refined finite element and other simplified models recommended by appropriate building codes. The structure is analyzed for static and seismic loads representing the effects of gravity, the operational and survival aerodynamic conditions, and possible site-dependent seismic motions. Comparative studies have been performed on the results of the above analyses and some useful conclusions are drawn pertaining to the effectiveness and accuracy of the various models used in this work.

Drift and Ductility Estimates in Regular Steel MRF Subjected to Ordinary Ground Motions: A Design-Oriented Approach

A simple procedure to estimate drift and ductility demands of regular steel frame buildings subjected to ordinary (i.e., without near fault effects) ground motions is described. Given the strength reduction (or behavior) factor, the procedure provides reliable estimates of the maximum roof displacement, the maximum interstorey drift ratio and the maximum rotation ductility along the height of the structure. The strength reduction factor refers to the point of the development of the first plastic hinge in the building and thus, pushover analysis and estimation of the overstrength factor are not required. This important feature enables both the rapid seismic assessment of existing structures and the direct deformation-controlled seismic design of new ones. The derivation of the proposed relations is based on regression analysis of the results of thousands of nonlinear time history analyses of steel frames. A comparison of the proposed method with the procedures adopted in current seismic design codes reveals the efficiency of the former.

Dimensional Response Analysis of Multistory Regular Steel MRF Subjected to Pulselike Earthquake Ground Motions

An alternative and efficient procedure to estimate the maximum inelastic roof displacement and the maximum inelastic interstorey drift ratio along the height of regular multi-storey steel MRF subjected to pulse-like ground motions is proposed. The method and the normalized response quantities emerge from formal dimensional analysis which makes use of the distinct time scale and length scale that characterize the most energetic component of the ground shaking. Such time and length scales emerge naturally from the distinguishable pulses which dominate a wide class of strong earthquake records and can be formally extracted with validated mathematical models published in literature. The proposed method is liberated from the maximum displacement of the elastic single-degree-of-freedom structure since the self similar master curve which results from dimensional analysis involves solely the shear strength and yield roof displacement of the inelastic multi-degree-of-freedom system in association with the duration and acceleration amplitude of the dominant pulse. The estimated inelastic response quantities are in superior agreement with the results from nonlinear time history analysis than any inelastic response estimation published previously.

Behavior Factor for Performance-Based Seismic Design of Plane Steel Moment Resisting Frames

Simplified expressions to estimate the behavior factor of plane steel moment resisting frames are proposed, based on statistical analysis of the results of thousands of nonlinear dynamic analyses. The influence on this factor of specific structural parameters, such as the number of stories, the number of bays, and the capacity design factor of a steel frame, is studied in detail. The proposed factor describes the seismic strength requirements in order to restrict maximum storey ductility to a predefined value. Interrelation studies between maximum storey ductility and the Park-Ang damage index are also provided for the damage-based interpretation of the performance levels under consideration. Realistic design examples serve to demonstrate the ability of the proposed factor to convert conventional force-based design to a direct performance-based seismic design procedure.

Numerical determination of time-averaged wind loads on complex structures in 2D flows

Abstract A numerical methodology for the determination of wind pressures on rigid civil engineering structures under two-dimensional conditions is presented. The unsteady, incompressible, viscous and high Reynolds number wind flow around structures in numerically simulated by combining the direct boundary element method with the discrete vortex method to take care of the potential and viscous flow characteristics, respectively. The cases considered involve geometrically complex (in plan view or cross-section) isolated or multiple structures for which there are usually no provisions in the various building codes and one has to resort to either expensive experiments of complicated numerical methods. The proposed methodology enables one to determine wind loads on those structures simply, rapidly and with satisfactory accuracy.

A computer program for determining wind pressures on 2-D structures

Abstract A computer program in Fortran for the numerical determination of wind loads on rigid civil-engineering structures is described. The program combines the direct boundary element method with the discrete vortex method in order to simulate the potential and viscous-flow characteristics of wind flow around structures under two-dimensional conditions. The flow chart and the subroutines of the program are given and explained in detail. The program employs linear boundary elements, and the simulation proceeds in a stepwise fashion in time. It is easy to use and can easily run on a personal computer. Numerical examples are presented to illustrate its use and demonstrate its simplicity, speed, and satisfactory accuracy. Thus this program becomes a valuable tool for the design engineer who does not have any further need to resort to expensive experiments or complicated numerical methods for the computation of wind loads on structures.

A New Seismic Design Method for Steel Structures

A seismic design methodology for steel building frames which combines the advantages of the well-known force-based and displacement-based seismic design methods in a hybrid force/displacement design scheme is proposed. The method controls structural performance by first transforming user-specified values of the interstorey drift ratio (non-structural damage) and local ductility (structural damage) to a target roof displacement and then calculating the appropriate strength reduction factor for limiting ductility demands associated with the target roof displacement. The main characteristics of the method are: (1) it treats both drift and ductility demands as input variables; (2) it does not use a substitute single degree of freedom system; (3) it makes use of conventional elastic response spectrum analysis and design; (4) it includes the influence of the number of storeys; (5) it recognizes the influence of the type of the lateral load resisting system (moment resisting frame or concentrically braced frame); (6) it recognizes the influence of geometrical (setbacks) or mass irregularities.

Comparison of Two Currently used and One Proposed Seismic Design Methods for Steel Structures

The forced-based, the displacement-based and the hybrid seismic design methods as applied to plane steel frames are briefly presented and critically compared. The forced-based seismic design method forms the basis of almost all the current seismic design codes, while the displacement-based one is a rather new seismic design method already adopted by some seismic design codes. The hybrid seismic design method is a new method which appropriately combines the best elements of both the force and the displacement-based methods and affects considerable improvements upon these elements. Advantages and disadvantages of these three seismic design methods are presented as derived first on the basis of their description and on the basis of the results of their application to three plane steel frames as compared to those coming out of inelastic time-history analyses involving eight different earthquakes.

Torsional moments on buildings subjected to wind loads

A numerical method is presented for the determination of wind-induced torsional moments on isolated or a group of rigid buildings of arbitrary cross-section. The method combines the boundary element method to account for the potential flow and the discrete vortex method to take care of the viscous flow characteristics of the wind and works iteratively in time. Thus the wind pressure distribution around the cross-section of a building is determined and the resulting torsional moment is easily computed. Numerical examples involving one or two buildings are treated by the proposed method and the results are compared against experimental ones. The method appears to be a valuable tool for the rapid determination of wind-induced torsion on buildings with satisfactory accuracy, especially because there is almost nothing on the subject in wind codes, while experiments are time consuming and expensive.