Seismic Design Methods for Steel Building Structures

This chapter presents fundamental aspects of seismic structural design and together with the next chapter devoted to seismic structural analysis serve as an introduction and basic reference to all subsequent chapters dealing with specific seismic design methods. After some historical notes on the development of seismic design methods, the basic characteristics of earthquakes pertaining to their generation and propagation, magnitude and intensity, duration, attenuation, frequency content and their distance from the source are briefly described. Basic concepts in seismic design, such as inelastic deformation, energy of dissipation, ductility, damage, behavior factor and overstrength, torsional effects and response combination rules are presented and discussed. Capacity design rules and their importance in design are explained. Performance-based design in its two forms, deterministic and probabilistic is briefly discussed and its importance in design is stressed. Performance levels defined by pairs of seismic hazard and performance objectives are stated. Finally, related to performance-based design subjects, such as seismic intensity measures, fragility curves and incremental dynamic analysis are also briefly presented.

This chapter briefly describes fundamental aspects of the analysis of building structures under seismic loads for reasons of completeness and easy reference. Emphasis is placed on steel building structures. Modeling procedures in the framework of the finite element method are presented and discussed. Linear elastic global analysis methods involving modal superposition and stepwise time integration are discussed. The special case of determining the maximum response for design purposes by combining modal superposition and elastic spectra is also presented. Nonlinear global analysis methods by stepwise time integration are described. Both material and geometric nonlinearities are considered. The special case of determining the maximum response for design purposes with the aid of inelastic spectra is also presented. The static nonlinear (pushover) method of analysis where seismic loads are applied as gradually increasing lateral forces is also briefly discussed. Some special topics, such as hysteretic material modeling under cyclic loading including deterioration and selection and scaling of earthquake records for nonlinear time history analyses are also briefly presented. Finally, special and general purpose computer programs for seismic analysis of steel building structures are presented.

This chapter briefly describes the force-based design (FBD) method as applied and used for the seismic design of steel building structures in the framework of Eurocode 8 (EC8). The FBD method of EC8 uses forces as the main design parameters and performs the design in two steps involving the strength checking of the structure under the design basis earthquake and the displacement checking under the frequent occurred earthquake. Both the simple lateral force method and the modal response spectrum analysis method for determination of the seismic design forces are presented and discussed. The concepts of the behavior factor to account for inelastic effects in the framework of elastic analysis and that of capacity design in order to ensure high levels of ductility before collapse are also presented. The chapter consists of six sections describing, besides some introductory remarks, performance design requirements, seismic action and soil types, design rules for buildings in general and steel buildings in particular, illustration of the theory with characteristic numerical examples involving steel building frames of the moment resisting and the braced types and a number of conclusions at the end.

The direct displacement-based seismic design method as applied to steel framed buildings in accordance with the Model Code DBD12 is presented. Moment resisting, eccentrically braced and concentrically braced frames are mainly considered. The method employs displacements as the main design parameters of the problem and succeeds in effectively controlling damage. It is based on the construction of an equivalent linear single-degree-of-freedom system to the original nonlinear frame and a displacement design spectrum with high amounts of viscous damping. Thus, by assuming a target interstorey drift ratio and determining the design displacement and the equivalent damping of the single-degree-of freedom system, one can obtain from the displacement spectrum the required period and hence stiffness and design base shear necessary for the structure to achieve the assumed deformation. Using the computed design base shear, one can distribute it along the height of the frame and dimension beams and columns in conformity with the capacity design rule. Numerical examples involving steel moment resisting and braced (eccentrically and concentrically) frames are presented for illustration purposes and demonstration of the advantages of the method. New developments of the method pertaining to various improvements and further applications are also briefly discussed.

The hybrid force/displacement (HFD) seismic design method for plane and space, regular and irregular, unbraced and braced steel building frames is presented. The method combines the advantages of the force-based design (FBD) and displacement-based design (DBD) methods. The HFD design method starts by considering both non-structural and structural target deformations in the form of maximum interstorey drift ratios (IDR) and member rotational ductilities μ_θ, respectively. These target deformations are transformed to a target roof displacement. Thus, the behavior (strength reduction) factor q is determined as a function of the target roof displacement ductility. After that, the HFD method proceeds as a FBD approach for strength checking using the pseudo-acceleration design spectrum analysis for seismic force determination. The necessary explicit empirical expressions needed for the determination of q are obtained by regression analysis on response databanks created for the various types of frames considered here. These databanks are obtained with the aid of extensive parametric studies involving many frames under many seismic motions analyzed by nonlinear time history analyses. The HFD method is constructed to be a performance-based seismic design method. Numerical examples are presented to illustrate the application of the method and demonstrate its advantages over the FBD and DBD seismic design methods.

A seismic design method for plane steel moment resisting frames is presented. The method designs moment frames with desired ductility and failure mode. The failure mode is of the global type, which ensures a global ductility supply and energy dissipation capacity of these frames. Use is made of first order and second order plastic analysis. Beam sections are first designed to resist vertical loads and the column sections are then determined on the basis of limit analysis for the global collapse mechanism of the frame under vertical load and lateral distributed load applied statically. The distributed load is of the inverted triangle type and can be associated with the collapse prevention performance level. Using second order plastic analysis, one is able to determine during the design the plastic rotation capacity of beams and beam-columns. The whole design procedure uses simplified analytical expressions and thus, one can complete the design by hand. Numerical examples associated with the seismic design of plane steel moment resisting frames are presented and discussed.

A compete methodology for the seismic design of plane steel frames is presented. It has been successfully used for moment resisting frames, eccentrically braced frames, concentrically braced frames, special truss moment frames and buckling restrained braced frames. In a performance-based design framework, the method uses pre-selected target drift and yield mechanisms as main performance limit states, which are directly related to the amount and distribution of damage, respectively. The design base shear for a specific seismic level is obtained by solving a balance of energy equation established by equating the work done by the structure as it is pushed up to the target drift to the seismic energy input on an equivalent to the structure elastic-plastic single-degree-of-freedom system. With a known design base shear, plastic design is used for determining member forces and dimensioning members and detailing connections. Since the base shear has been computed on the basis of the design target drift and yield mechanisms, this design method is accomplished in one step without iterations in contrast to code design methods requiring a two steps approach involving strength and deformation checking with iterations. The method is illustrated by four application examples covering plane steel moment resisting frames, eccentrically braced frames and concentrically braced frames, which also serve the purpose of demonstrating its advantages.

A performance-based seismic design method for plane steel moment resisting and braced framed structures is presented. It is a force-based design method employing equivalent viscous modal damping ratios ξ_k instead of the behavior (or strength reduction) factor q (or R) to account for inelastic energy of dissipation. These modal damping ratios ξ_k are defined for an equivalent linear multi-degree-of-freedom structure to the original non-linear multi-degree-of-freedom structure, which has the same mass and elastic stiffness as the non-linear one. In addition, they are functions of the structural period, the target inter-storey drift ratio and member plastic rotation and the soil type. Empirical expressions of these ξ_k for the first few significant modes are obtained through extensive parametric studies involving non-linear time-history analyses of many frames under many far-fault earthquakes and different deformation targets. These ξ_k are used for seismic design through an elastic acceleration design spectrum with high amounts of damping. The presented method, which is illustrated by numerical examples, is more rational and provides results of higher accuracy in one step (strength checking) than code-based design methods requiring two steps (strength and deformation checkings).

A performance based seismic design method for plane steel moment resisting and braced framed structures is described. It is a force-based seismic design method employing different modal (or strength reduction) factors for the first four significant modes of the frame, instead of the same constant behavior factor for all modes as in all current design codes. These modal behavior factors are functions of the modal periods of the structure, different soil types and different performance targets. Thus, the method automatically satisfies deformation demands at all performance levels without requiring deformation checks, as in all current design codes. The method is theoretically based on the construction of the equivalent linear structure to the original nonlinear one and the equivalent modal damping ratios of the previous chapter. The modal behavior factors are determined from the equivalent modal damping ratios with the aid of the modal damping reduction factors. Empirical expressions for the modal behavior factors as functions of period, deformation/damage and soil types for the seismic design of steel plane moment resisting and braced frames are derived. These expressions are appropriately converted to ones which can be used directly in conjunction with code defined elastic pseudo-acceleration design spectra with 5% damping. The proposed method is illustrated with representative numerical examples that demonstrate its advantages over code-based seismic design methods.

A rational and efficient seismic design method for plane and space steel moment resisting frames using advanced methods of analysis is presented. This method employs an advanced dynamic finite element analysis working in time domain that takes into account geometrical and material nonlinearities and member and frame imperfections. Seismic actions are in the form of accelerograms compatible with the elastic response spectra of EC8 for three performance levels. The design starts with assumed member sections for the frame, proceeds with the checking of drifts, member plastic rotation, damage and plastic hinge pattern for the three performance levels considered here and ends with the adjustment of member sizes iteratively so as the above response parameters to satisfy their limit values for every level. Thus, the method can sufficiently capture the limit states of displacements, strength, stability and damage of the structure and its members so that separate member capacity checks through the interaction equations of EC3 or the use of the approximate behavior factor of EC8 are not required. Numerical examples dealing with the seismic design of plane and space steel moment resisting frames are presented to illustrate the method and demonstrate its advantages.

The direct damage controlled design method for seismic design of plane steel moment resisting framed structures is described. This method in its dynamic or static (pushover) version is capable of controlling damage at all levels of performance. The proposed method can be used not only for designing a structure for a given seismic load and desired level of damage, but also for determining damage in a designed structure locally or globally due to any seismic load or evaluating the maximum seismic load a structure can undertake for a desired level of damage. The above are accomplished by introducing a new seismic damage index and constructing appropriate damage performance levels. The seismic damage index takes into account axial force-bending moment interaction, strength and stiffness deterioration and low cycle fatigue. The damage performance levels are established with the aid of extensive parametric studies on a large number of frames under a large number of seismic motions in conjunction with the above damage index for damage evaluation. Numerical examples are provided to illustrate the proposed method in its dynamic and static versions and demonstrate its merits against other methods of seismic design.

This chapter presents various methods for the seismic design of steel building structures equipped at their base by seismic isolation devices. The most well-known isolation devices are the lead rubber bearings and the friction pendulum bearings. These isolation devices or isolators succeed to uncouple the seismic response of the structure from the ground motion and thus to reduce the structural seismic forces. There are basically two kinds of design methods for base isolated steel building frames: the force-based and the displacement-based ones. The design methods according to ASCE provisions are either the equivalent lateral force method or methods based in dynamic analysis including the response spectrum analysis and the nonlinear time-history analysis. The design methods according to Eurocode 8 are analogous to the aforementioned ones of the ASCE provisions. A method using an improved linear analysis is also presented. In addition to these force-based methods of design employing acceleration spectra, a displacement-based design method employing displacement spectra for high damping values is also presented. Three numerical examples involving steel building frames are presented to illustrate the design methods of ASCE and Eurocode 8 as well as the method using the improved simplified linear analysis and demonstrate the effectiveness of base isolation in seismic design.

This chapter describes various methods for the seismic design of new steel building structures equipped with supplemental dampers and the seismic retrofitting of existing structures by supplemental dampers. The most widely used dampers are the fluid viscous ones (linear or nonlinear), which enhance the seismic energy of dissipation of the structures and thus reduce the seismic forces on them. The result is lighter new structures or effective retrofitting of existing structures against stronger earthquakes. The most widely used design methods are the force-based simplified linear ones, including the equivalent lateral force method and the response spectrum one. The ASCE code provisions are based on such force-based simplified methods. Displacement-based simplified design methods are also presented. The arrangement of dampers for an optimum structural performance is also discussed briefly here by mentioning appropriate optimization procedures for that purpose. The limitations of the simplified methods are pointed out and the method using nonlinear time-history analysis is suggested as the appropriate method for final design of a structure with dampers. Four numerical examples are presented in some detail in order to illustrate methods based on force or displacement as well as methods characterized by new concepts.