Double shear bolted bracket moment connections, part 2: Four-bolt configuration response evaluation
Graphical abstract
Introduction
After destructive earthquakes such as the 1994 Northridge earthquake [[1], [2], [3], [4]], defects in the previously used moment connections naturally became lessons learned from the earthquake. Many researchers have attempted to improve the behavior of existing connections [[5], [6], [7], [8], [9], [10], [11], [12], [13]] and to develop new moment connections [[14], [15], [16], [17], [18], [19], [20], [21], [22], [23]]. Some of these new moment connections have been incorporated into the US prequalified connection code, AISC 358–18 [24] and others can be used as options for retrofitting steel moment frames [[25], [26], [27], [28]]. Prequalified connections must sustain more than 80% of the plastic bending moment of the beam up to 4% story drift angle [29] and must have ductile failure mode. Furthermore, in order to be classified as fully restrained moment connections, they must have a rotational stiffness greater than 20EIbeam/L0 [30].
Steel moment connections are usually studied using experimental methods or Finite Element (FE) simulation. Certainly, experimental methods yield more reliable results because there are complex behavioral phenomena in the performance of moment connections such as crack propagation and rupture initiated at points with fine defects, local buckling, rupture of bolts and welds, and bearing interaction of surfaces in compressive or shear contact. In addition to the experimental studies before conducting the experiment, numerical simulations using FE also yield important data and can predict the connection behavior to an acceptable level, provided that the FE modeling accuracy is high. Modeling of both nonlinear behavior of steel materials and nonlinear geometric behavior in FE is practical and has had good results in previous research studies [23,[31], [32], [33], [34], [35], [36], [37], [38], [39], [40], [41]]. Although it is difficult to model failure, such as steel rupture in the FE and it is avoided, a good estimation of the critical situation ranges can be obtained using indicators such as equivalent plastic strain demand (PEEQ index) [43,44], Rupture Index (RI) [45], and stress Triaxiality Ratio (TR) [46]. The PEEQ Index represents the local inelastic strain demand, which is a criterion for comparing the distributed plastic strain demand in different yielded zones. This criterion is calculated according to Eq. (1):
TR is a parameter for considering the ductile rupture of metals, which is calculated in accordance with Eq. (2). If TR is in a very high range (), brittle failure of metals occurs. In addition, a large reduction in rupture strain is expected when TR is high () [46].
RI, which was first introduced by Hancock and Mackenzie [45], is used to compare likelihood of ductile fracture in critical regions of materials and is defined as Eq. (3):
Many researchers [23,[31], [32], [33], [34], [35], [36], [37], [38], [39], [40], [41]] have used these indices to evaluate the fracture potential of the three-dimensional state of stress in steel materials used in connections and have presented operational information.
This study (Part II) attempts to accurately evaluate the structural performance of the proposed DSBB moment connection (Fig. 1) introduced in Part I of this research [42] and uses FE simulation for the parametric study of this study. The DSBB moment connection is shown in Fig. 1. DSBB connection profits from double shear performance of bracket stem bolts and has reduced bolt usage and installation time compared to the current prequalified moment connections of the well-known code, AISC 358–18 [24]. Therefore, the DSBB connection has the benefit of fast erection and the economic choice of moment connection. In addition to the four-bolt configuration, the DSBB connection also has six-bolt and eight-bolt configuration for strong beams that will be presented in the next phases of this project. Four-bolt configuration has the limitation of being applicable for short beams because it has only four bolts to connect to the column. In addition to modeling the DSBB connection specimens, other reference connection specimens of double-tee [[47], [48], [49], [50]] and end-plate [51] connections (Fig. 2) are also considered from the prequalified moment connections group of AISC 358–18 [24]. The reason for choosing these two connections in addition to the DSBB connection is their structural performance similarity to the new DSBB connection. First, hysteresis curves of all the parametric study specimens are acquired and the effects of pinching, strength deterioration and stiffness degradation in hysteresis curves are discussed. In the assessment of DSBB connection, damage to connection segments and the protected zone of connection is evaluated by the damage and response indicators (PEEQ index, RI, TR). These indices are compared in terms of critical lines and points on brackets, T-stubs, end-plates, and bolts. In order to evaluate the DSBB connection performance out of plane deformations, buckling deformations, longitudinal strain in bolts and energy dissipation are also considered. Finally, conclusions are drawn on the efficiency and accuracy of the proposed design method of the DSBB connection.
Section snippets
Definition and design of DSBB parametric study specimens
In order to control the accuracy of the proposed design method for the DSBB connection, various specimens were designed for the parametric study. In Part I of this research [42], after verification of the FE modeled Double-tee (DT1 and DT2) and End-plate (EP3 and EP4) connections with experimental evidences [[47], [48], [49], [50], [51]], the corresponding DSBB connection specimens (DSBB1, DSBB2, and DSBB3) with the aforementioned reference specimens were designed and built with FE modeling. In
Hysteresis results of the DSBB specimens and discussion
All parametric and main DSBB specimens are subject to standard cyclic loading [29] and hysteresis results are presented in Fig. 3, Fig. 4, Fig. 5. All diagrams of Fig. 3, Fig. 4, Fig. 5 in each group are plotted in such a way that the behavior of the parametric study DSBB specimen is placed beside the main associated DSBB specimen. The specimens were subject to cyclic loading instead of monotonic loading so that it was possible to precisely consider the exact effects of buckling, yield, stress
Investigation of response and damage indices in critical zones of DSBB connection
In addition to global hysteresis response of connection, the steel material failure due to stress concentration and strain accumulation, brittle fracture and ductile rupture formation in critical points of the connections are important microscopic phenomena that can affect the overall performance of the connections. Evaluation of the parameters described is presented in this Section. In order to accurately evaluate the material behavior of critical points and unfavorable deformations of
Summary and conclusions
Here, in Part II of the present research, which is a continuation of developing the new DSBB connection introduced in Part I [42] of this research, the accuracy of the proposed design method of DSBB connection of Part I [42] is assessed by the parametric study of design parameters using damage and response indices in the critical zone of the connection. The advantage of the DSBB connection is its smaller diameter of beam bolts consumed and consequently less time and lower installation costs. In
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.
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2022, Engineering StructuresCitation Excerpt :This FE verification was conducted to assure the precision of the numerical study of the present research. In the FE simulation, all assumption and input data of the DSBB specimens were similar to the study of four-bolt configuration [43–44], i.e., the previous phase of this research. Therefore, for the sake of brevity, only some important details are presented here.