Extended end-plate connections for replaceable shear links
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/(Mp/Vp)) is the single most important parameter that influences the behavior of links, where e is the link length, and Mp and Vp 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.
Section snippets
The US guidelines
In the US, Design Guide 16: Flush and Extended Multiple-Row Moment End-Plate Connections [20], hereafter referred as DG16, provides guidelines for end-plated connections. The strength of the end-plate is calculated using assumed yield line mechanisms. Two types of end-plate behavior are defined, which are designated as “thick” and “thin” [21], [22], [23]. In the “thin” end-plate approach, the full strength of the end-plate is utilized. In the “thick” end-plate design approach, the applied
Experimental program
The experimental study concentrated on four-bolt unstiffened (4E) and stiffened (4ES) extended end-plate connections. These connection types employ two bolts per row, which is standard for most beam-to-column connections. Four-bolt extended end-plate connections will be sufficient for less than one-half of the available hot rolled beam sections assuming that full link bending strength is to be resisted with a maximum bolt diameter of M36 [24]. The eight-bolt stiffened connection (8ES) is
Experimental results
All specimens were loaded to failure, and sudden fractures in various forms were responsible for the failure of specimens. A summary of experimental results is given in Table 5 where plastic shear (Vp) and bending (Mp) capacities determined using the measured geometrical and material properties, the maximum shear (Vmax) and bending (Mmax) resistances obtained during the experiments, overstrengths, the link rotation angle at failure (γmax) and the failure mode are reported. The plastic shear and
Assessment of US guidelines and European provisions
A typical moment rotation response (M-θ) of an end-plate connection is given in Fig. 14, where My is the elastic/yield moment resistance which represents first yielding moment, Mj,Rd is the plastic moment resistance, and Mmax is the maximum flexural strength [33]. The plastic moment resistance (Mj,Rd) can be considered as the point at which the yield line mechanism forms completely along all the yield lines and is calculated from the intersection of the two lines corresponding to the initial
Finite element studies
Finite element analyses were conducted to investigate the amount of yielding in the end-plates and also to study the effect of end-plate width on the response. The specimens investigated in the experimental program were not instrumented with strain gauges and the finite element analysis results provide further information into the level of yielding that takes place in end-plates with different thicknesses and widths. In addition, the effect of axial forces is investigated.
Proposed modifications to the AISC guidelines
Recommendations were developed to improve DG16 and AISC358 guidelines with regard to calculation of the bending capacity of end-plates. The yield line mechanisms can be independently studied using unstiffened and stiffened T-stub models. For this purpose, the strength of T-stubs was studied using finite element analysis. End-plates having a width of 145 mm were analyzed. The largest differences between experimental and code estimates are for Specimens 7 and 10, which employ 10 mm thick
Conclusions
End-plated connections for replaceable links were studied by conducting 10 nearly full-scale EBF tests. The width, thickness, and stiffening of the end-plates were considered as the prime variables. The experimental study was complemented with numerical analysis to investigate the variation of the bending strains, effect of plate width and axial forces produced in the links. The following conclusions may be drawn from this study:
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The “thin” end-plate behavior can be adopted in the design of
CRediT authorship contribution statement
Yasin Onuralp Özkılıç: Investigation, Validation. Cem Topkaya: Methodology, Resources, Project administration, Supervision, Writing - review & editing.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The study was supported by the Scientific and Technological Research Council of Turkey (TÜBİTAK) through grant number 114M251 and by the College of Engineering of the Middle East Technical University through grant number GAP-303-2018-2858.
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