Extended end-plate connections for replaceable shear links

Highlights

• Behavior of end-plated replaceable links studied through experiments and numerical analysis.

• North American and European design guidelines and specifications were evaluated based on experimental findings.

• The US Guidelines were found to provide conservative capacity estimates.

• Thinner end-plates than required by the specifications can provide acceptable performance.

• A modified yield line mechanism was proposed to more accurately estimate the end-plate strength.

Abstract

Extended end-plate moment connections are used in a number of applications including the beam-to-column connections in seismic moment resisting frames (MRFs) and replaceable link-to-frame connections in eccentrically braced frames (EBFs). While the extended end-plate connections have been extensively studied for MRF applications, little is known about their performance in EBFs. The loading conditions and the acceptance criterion are different for these connections when they are used in MRFs or in EBFs. This paper presents an experimental and numerical study undertaken to investigate the performance of extended unstiffened and stiffened end-plate connections used in replaceable shear links. Pursuant to this goal, 10 nearly full-scale EBF tests were conducted where the thickness, width and stiffening of the end-plate were considered as the variables. The results showed that end-plates designed according to the AISC guidelines or Eurocode provisions show acceptable performance in terms of the target link rotation angle. Due to strain hardening effects, thinner plates than the ones suggested by the codes were also found to show satisfactory performance. Finite element simulations were conducted to investigate the bending strains for different plate thicknesses and to study the levels of axial forces developed in the links. Modifications to the AISC design guidelines have been proposed to determine the plastic resistance of end-plated connections more accurately.

Introduction

Eccentrically braced frames (EBFs) combine the advantages of moment resisting frames (MRFs) and concentrically braced frames (CBFs) to form a structural system with high elastic stiffness as well as high energy dissipation. In EBFs, links are the primary source of energy dissipation and the beam outside the link (collector beam), the braces and columns are designed to remain elastic during a seismic event. The link length ratio (ρ = e/(M_p/V_p)) is the single most important parameter that influences the behavior of links, where e is the link length, and M_p and V_p are the plastic moment and plastic shear capacities of the link, respectively. Short shear yielding links (ρ ≤ 1.6) are generally preferred in practice due to their superior energy dissipation [1].

Traditionally, the links and the collector beam are designed as a continuous member having the same cross-section. This approach has two major drawbacks. First, the capacity design of the collector beam is directly influenced by the forces developed in the link. A change in the size of the collector beam, results in a change in the link size. Second, the post-earthquake repair or replacement of the link is an onerous process if the link and the collector beam are from a continuous member.

The 2010–2011 series of Christchurch earthquakes in New Zealand resulted in yielding and fracture of links in EBFs which had to be replaced with new ones [2]. There have been significant improvements in the development of replaceable links for EBFs in the last decade. Bolted end-plated replaceable links in the form of flush end-plated (Fig. 1a) and extended end-plated links (Fig. 1b) have been developed. The flush end-plated replaceable link behavior differs from conventional shear links due to pinching as a result of end-plate bending and bolt thread stripping [3]. It is recommended to limit the link length ratio to ρ < 0.8 to have improved behavior. The bolted extended end-plated replaceable link performs in a similar manner to a conventional link and an additional limit on the link length ratio is not required [4]. Very short bolted extended end-plated replaceable links were also tested by Ji et al. [5] for use as coupling beams in reinforced concrete coupled wall systems.

The sizing of the end-plate is an important step in the design of replaceable links. End-plates and bolts should be designed to transmit the forces produced in the link safely to the adjoining members. More importantly, the weight of the parts to be transported and erected has a significant influence on the ease of the replacement process. In conventional construction, the increase in the weight of an end-plate does not adversely affect the hoisting and maneuvering of steel members. On the other hand, weight becomes the primary factor for replaceable steel members due to the constraints in the hoisting and maneuvering inside existing structures. For this reason, the size of the end-plate, which determines the total weight of the part to be replaced, should be minimized in order to come up with weight optimized designs and to facilitate the replacement process.

End-plated connections can be used in a number of applications such as beam-to-column connections or beam-to-beam connections. Research to date has mostly focused on the performance of these connections where bending was the dominant action. Different design philosophies can be adopted for seismic and non-seismic applications to beam-to-column connections. In the case of seismic design, the end-plate can be designed to remain essentially elastic, or to dissipate energy through yielding. Widely used guidelines (AISC 358 [6]) or specifications (EN 1993-1-8 [7]) can be adopted for sizing the end-plate connection. It should be noted that there are marked differences in the approaches given in these documents as explained in the following sections.

The end-plated beam-to-column connections for seismic applications have been extensively studied in the past [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18]. On the other hand, little is known about the performance of these connections when used in replaceable shear links. Their behavior and the acceptance criteria are different for the two cases. In beam-to-column connections, the primary action is bending, whereas, in replaceable link connections, bending moment is accompanied by significant amounts of shear and axial forces. The beam-to-column connections are expected to satisfy a certain level of story drift angle when tested under the qualifying test protocol [19]. On the other hand, the EBF links should satisfy a certain level of link rotation angle when tested under a different test protocol [19]. Furthermore, parametric limitations are imposed on prequalified end-plated moment connections [6] which were developed based on experiments conducted to date. The size of the replaceable links is smaller when compared with conventional beams used in MRF applications. Therefore, the end-plated connections for replaceable shear links have geometrical parameters that are not within the limitations given in AISC358 [6].

A combined experimental and numerical study was undertaken to investigate the behavior of end-plated connections in replaceable EBF links. In this paper, the US and European design practices for sizing the end-plates are reviewed. The details of the experimental study on 10 nearly full-scale EBFs are interpreted. The test results are evaluated in terms of the design recommendations provided in the US guidelines and European provisions. The test specimens are analyzed using the finite element method to investigate the bending strains developed in the end-plates. Modifications to the US guidelines are developed based on the finite element analysis of T-stub models.