Energy Dissipation Bracing Systems in Steel Frames Using FRP Composite Materials

Passively damped bracing systems in steel frames, in which the conventional materials of the joint are replaced by high damping viscoelastic materials, have the potential of being effective practical means for passive vibration control of dynamically loaded civil engineering structures. However, this potential can be realized only if the associated structural penalties are reduced within acceptable limits. This paper describes a rational methodology for the development of an advanced joining type for structural systems capable of providing enhanced dissipation of vibration energy without serious penalties in strength, stiffness, or weight characteristics. One such configuration is that of a V-type bracing system with a joint which provides a beneficial deformation coupling between the direction of load transfer and less critical offset directions. A comprehensive parametric study has been carried out in order to establish design guidelines for favorable tradeoffs between damping benefits and the associated stiffness and strength penalties in an FRP V-type joint. The results are compared with the corresponding tradeoffs for a V-type joint made from conventional materials. INTRODUCTION Bracing systems are commonly used in steel frames in order to resist lateral loads. Many types of bracing systems such as V, K and X-type bracings have been developed in order to comply with structural design requirements as well as architectural demands. Design guidelines for bracing systems are readily available in modern codes [1,2]. Moreover, advanced bracing systems such as eccentrically braced frames [3] and chevron-braced frames [4] have been also developed in order to resist transverse dynamic loads. Eccentrically braced frames rely on the yielding of a link beam between eccentric braces, which provides ductility and energy dissipation under dynamic loads. In a chevron-braced frame, energy dissipation solely depends on the nonlinear cyclic response of the braces. Consequently, numerous research studies have been initiated in recent years to improve the performance of bracing systems through the introduction of new structural configurations [5], the use of high performance materials [6] as well as passive energy dissipation devices such as friction [7] and viscous fluid dampers [8]. In this work, a V-type bracing system utilizing an advanced joint made from fiber-reinforced-polymer (FRP) with viscous properties is introduced. Among all the above dynamic performance improvement techniques, using viscous dampers in a structure has the unique advantage of reducing the structure base shear force and deflections at the same time since the velocity-dependent maximum viscous force is out of phase with the maximum deflection of the structure. Furthermore, the addition of an FRP joint into a steel framed structure with V-bracings alters the force–displacement relationships and thus, the dynamic modal characteristics of the structure. Consequently, FRP joints seem to be powerful *Address correspondence to this author at the Department of Civil Engineering, National Technical University of Athens, Athens 15780, Greece; Tel: +30-210-7722454; Fax: +30-210-7722482; E-mail: [email protected] tools for improving the dynamic performance of V-braced steel frames. Many research studies concerning the effect of V-type bracings with dampers on the dynamic performance of steel building structures have been conducted in the past [9]. However, there is no comparative research study on the dynamic performance of V-bracing steel frames with and without FRP joints as a function of dynamic motion and damper parameters. Thus, this study focuses on comparing the dynamic performance of V-bracing steel frames with and without FRP joints as a function of the intensity and frequency characteristics of the dynamic motion as well as the damping ratio and velocity exponent of the FRP material. The results from such a research study may then be used to measure the efficiency of FRP joints for improving the dynamic performance of V-bracing systems as a function of the ground motion characteristics and their parameters and arrive at important decisions related to the dynamic retrofitting and design of V-bracing steel frames using advanced FRP joints. In typical V-braced frames, the beams are incapable of performing as a ductile link for the steel bracing system that is inserted in the frame bays. A vertical steel shear link can be introduced forming a Y-bracing pattern. In this case, the vertical shear link can be attached to the beam of the steel frame. Special consideration should be given to the connection between the vertical shear link and the beam. This connection, though, should have sufficient capacity to ensure effective transmission of forces when subjected transverse loads. In this work, details of a proposed link connection of an FRP-steel bracing system inserted in the bays of a steel frame are presented. The link connection is located at midspan of the steel girder and is attached to its bottom flange. The bracing forces are transmitted to the girder through an FRP plate-frame system. The proposed link connection has not yet been subjected to testing but is expected to provide the fixation to the link end as has been modeled in the analysis. 138 The Open Construction and Building Technology Journal, 2008, Volume 2 Ioannis G. Raftoyiannis FRP-JOINT STIFFNESS PROPERTIES The joint presented herein consists from an FRP plate with thickness t and dimensions a and b. The FRP plate is bolted into a steel frame consisting from pairs of unequal-leg angle-sections connected with hinges at all 4 corners that is attached to the bottom flange of the girder at its mid-length. The braces are rigidly connected to the joint frame with welding. In Fig. (1), one can see the details of the proposed FRP plate-frame system. Since the dimensions of the frame are relatively small, the flanges of the angle sections are expected to provide enough transverse stiffness to the system, thus preventing out-of-plane instability of the FRP plateframe system. Out-of-plane buckling of the braces alone can be prevented through special considerations provided in the codes [1,2] Fig. (1). Details of proposed FRP plate-frame system. As a consequence, the FRP-joint plate is subjected to membrane forces. Composite plates are usually thin-walled with a maximum total thickness in the order of 2.0 cm and are produced with various cross-sectional dimensions and lay-ups. In the present case, the hand lay-up procedure is employed to produce the FRP plate with the desired properties. Two cases of laminates are considered herein for the plate: [(0/90)n]S and [(±45)n]S orientation angles with respect to the horizontal direction forming a symmetric cross-ply (soft) or angle-ply (stiff) laminate, respectively. The plate consists of an even number (total 4n) of E-glass/Epoxy layers. All layers have a constant fiber volume fraction vf=65%. The material properties for all layers are E1=42.3 GPa, E2=12.4 GPa, v12=0.24 and G12=6.2 GPa, where 1 denotes the fiber direction and 2 the transverse direction. Using the Classical Lamination Theory [10], the stiffness components of a generally orthotropic plate can be determined. The constitutive equation for such a laminate is