Double shear bolted bracket moment connections, part 2: Four-bolt configuration response evaluation

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Highlights

  • The hysteresis curve of the DSBB connection indicates ideal and efficient performance.

  • The total energy dissipation of the DSBB connection happened in beam plastic hinge.

  • The damage probability in the protected zone of the DSBB connection is almost zero.

  • The failure risk in the DSBB bolts is lesser than the prequalified end-plate connection.

  • The failure risk in the DSBB bolts is lesser than the prequalified double-tee connection.

Abstract

The design methodology of a new Double Shear Bolted Bracket (DSBB) moment connection was introduced in Part I of this research. The DSBB connection is a bolted bracket connection that profits from double shear performance of stem bolts. In Part II of this research, a response evaluation is conducted by designing and modeling various parametric study specimens to evaluate the efficiency of the proposed design method. In the finite element modeling of the parametric study specimens, the adequacy of size and thickness of the bracket flanges, stems, and stiffeners, and the adequacy of bolt diameter of the bolted bracket are evaluated. In order to check the qualification of the DSBB connection based on AISC 358–18 connection code criteria, the double-tee and end-plate prequalified moment connections in the aforementioned code are also considered as reference connections beside the DSBB connection. For all specimens, the cyclic responses are provided and compared. In the following, the energy dissipation in the beam and connection of the specimens is compared. Eventually, stress state indices including rupture index and plastic equivalent strain index, in the defined critical lines and points of all specimens are calculated and discussed. Comparing the performance of the DSBB connection specimens with the end-plate and double-tee connections indicates that the risk of failure, concentration of stresses and strains, and out of plane and buckling deformations in the DSBB connection are less than that of the prequalified end-plate and double-tee connections; therefore, DSBB can be a qualified moment connection.

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):PEEQIndex=23εijpεijpεy=εpεy

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 (σmσeff1.5), brittle failure of metals occurs. In addition, a large reduction in rupture strain is expected when TR is high (0.75σmσeff1.5) [46].TR=σmσeff

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):RI=εpεyexp1.5σmσeff=PEEQ Indexexp1.5TR

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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