Michael D. Engelhardt

Seismic eccentrically braced frames

Abstract This paper provides an introduction and an overview of the design and behavior of seismic-resistant eccentrically braced frames (EBFs). Within the last ten years, EBFs have become a widely recognized lateral load-resisting system for steel buildings in areas of high seismicity. The primary purpose of this paper is to present design recommendations for links and connections in EBFs. Some basic concepts on the behavior of EBFs are reviewed, and highlights of significant experimental results are presented. The important effects of link length on both the elastic and inelastic response of EBFs under lateral load are emphasized. The paper focuses on EBFs constructed with shear links, as these provide for the maximum stiffness, strength, ductility and energy dissipation capacity of an EBF. Suggested details are provided for links and for selected connections in EBFs.

On Design of Eccentrically Braced Frames

Seismic-resistant Eccentrically Braced Frames (EBFs) are becoming a widely used lateral resisting system for steel buildings, with even wider application anticipated as design requirements are put into building code format. This paper addresses a number of EBF design issues, which in the opinion of the authors are inadequately considered either in current practice or in the emerging code provisions. The overall design philosophy for EBFs is reviewed, with specific reference to the concept of “Capacity Design”. Application of capacity design principles assures that yielding will be restricted primarily to the ductile link elements, an important goal of EBF design. Further, through careful choice of frame geometry and link length at the preliminary stages, many potential design difficulties can be avoided. The paper also presents some important observations from experimental work currently underway on EBFs with long, flexural yielding links.

Retrofit of Concrete Columns with Inadequate Lap Splices by the Use of Rectangular Steel Jackets

This paper describes an experimental research program on the use of rectangular steel jackets for seismic retrofit of non-ductile reinforced concrete frame columns. Eleven large scale columns were tested to examine the effectiveness of various types of steel jackets for improving the ductility and strength of columns with an inadequate lap splice in the longitudinal reinforcement. Response of the columns before and after being strengthened with steel jackets was examined. Several types of steel jackets were investigated, including rectangular solid steel jackets with and without adhesive anchor bolts. The test results indicate that a thin rectangular steel jacket combined with adhesive anchor bolts can be a highly effective retrofit measure for reinforced concrete columns with an inadequate lap splice. Design guidelines for the use of rectangular steel jackets as a seismic retrofit for non-ductile reinforced concrete columns are presented.

On seismic steel joints and connections

Abstract A major part of this paper is devoted to a discussion of joints and connections for Moment Resisting Frames (MRFs). The demand placed of such joints for different earthquakes is examined using time-history analyses for a low-rise and a high-rise building with perimeter framing; this is compared with the experimentally determined capacities of such joints. A brief discussion of column splices indicating the proposed AISC specification for jumbo sections follows. The paper concludes with recommended details for brace-to-beam link and link-to-column connections for Eccentrically Braced Frames (EBFs) based on some recently completed analyses and tests.

Advances in Design of Eccentrically Braced Frames

Eccentrically Braced Frames (EBFs) have attained recognized status as a viable structural steel system for resisting lateral seismic forces. Sustained research at the University of California, Berkeley, since 1977 and numerous field applications provide a good database for their proper design. In this paper the different types of EBF are critically evaluated, and the kinematics of their inelastic deformation are examined with particular reference to the behavior of isolated short beam segments or links. Desirable link length and web stiffening are recommended. A preliminary design procedure for hand-calculation of EBFs is described and some suggestions for brace connection details are advanced.