NUMERICAL CHARACTERIZATION OF INNOVATIVE DEMOUNTABLE BEAM‐COLUMN CONNECTIONS USING LONG BOLTS

Circular design strategies such as adaptivity, detachability, and reusability represent new challenges and increased demands on the future‐proof buildings sector. To contribute to the circularity and sustainability of the steel construction sector, an innovative solution of beam‐column moment‐resisting joints with long bolts and short steel elements (links) is proposed, enabling an extension in the service length of existing steel members. This paper assesses the influencing connection parameters (e.g. bolt preload, bolt length and diameter, end plate thickness, etc.) on the moment (flexural) resistance, stiffness, rotation, ductility, and failure modes of base configurations of these joints.


Introduction
Bolts are widely used as fasteners in various types of steel and composite connections, representing a reliable and efficient way of connecting structural elements.High-strength bolts, which transfer the load by friction of the bolt body, are covered by the European standard EN 14399 (CEN, 2015) [12].These bolts are produced in two different systems, the HR (strength class 8.8 and 10.9) and HV (only strength class 10.9) system.These systems are practically equivalent, with the only difference relating to the mode of failure at high tensile stresses.HR systems use nuts with a smaller thickness, so their failure occurs first, while HV systems contain bolts with a longer body, so the dominant failure mode is precisely due to excessive plasticity of the bolt body.For HV systems, the standard EN 14399 defines the available length l which depends on the size of the bolt diameter.The actual length of the bolt Σt (Fig. 1) is less than the length l because the thickness of the nut and the thickness of the washers 2tw must be subtracted from it.A list of bolt diameters and corresponding lengths can be found in Table 1.For example, it can be seen that for M20 bolts there is no length exceeding 125 mm.For most joints in structures, these lengths are sufficient, however, in some cases a longer bolt body length is necessary.One possible solution to this problem is the use of the so-called "long" bolts, which can be classes 8.8 and 10.9.They are compatible with HV10 nuts (according to EN 14399-4) and HV washers (according to EN 14399-6).The long bolt, nut, and washer assembly is shown in Fig. 2. Along the body of such bolt there may be end plates or other structural elements, whereby their stiffness may be significantly less compared to conventional bolts, which is represented by springs in Fig. 2.
The body of long bolts can be partially or completely threaded, but due to the simplicity of manufacture, full threads are more often used, and they will be the only ones covered in this paper.The usual diameters of these bolts range from 16 mm to 36 mm, but it is also possible to use larger diameters, while the intended length is 1000 mm.However, the body of the bolt can also be shortened by cutting, which must be done very carefully because the material of the bolt is sensitive to temperature changes.

Fig. 2: Long bolt
Long bolts have a wide potential application.Their long length makes them ideal for solving problems with large tolerances or for adapting existing steel beams to different lengths.This is precisely the idea of the European project "CONNECT4C", led by the University of Coimbra, in Portugal.Within the project, long bolts are used as fasteners for demountable joints to extend the required length of the existing beams.

Description of the innovative demountable beam-column moment connections using long bolts
A new type of connection in steel structures introduces numerous novelties compared to conventional connections, especially regarding the geometry of the connection considering the use of long bolts and the addition of new structural elements, but also regarding the behavior of the connection, and different redistrebution of stresses.In this paper one-sided beam-column moment connection was analyzed (called "TEST OF1" in the literature) and its layout is shown in Fig. 3.

Fig. 3: Innovative beam-column moment connection with long bolts
A new structural element, called "link", is a short element inserted between the flange of the column and the beam, providing great adaptability of steel elements.In this case, long bolts pass through several elements of the connection, including the end plate on one flange of the column, then through the entire column, i.e. its flanges, then through the end plate on the other flange of the column, which is welded to one end of the link, and finally through two end plates, one welded for the opposite end of the link and the other for the beginning of the beam.The column and beam are made of hot-rolled cross-sections HEB220 and IPE300 respectively, and the link is made of the same cross-section as the beam.All end plates have the same dimensions, 20x180x320 mm.Four long bolts LM20...10.9 are used.The material chosen for the column, link, and beam is S275JR, while the material chosen for the end plates is S355J2.

Numerical analysis in Abaqus software
A numerical model of the innovative connection was made in Abaqus [9] software and the layout of the connection is shown in Fig. 4. Material model used for this study is the model with yielding plateau and strain hardening according to the new standard prEN 1993-1-14:2022 [11].Welds were not explicitly modeled, but the connection is realized using the "Tie Constrain" option.All welds were modeled in this way, including the welds at the face plate-to-beam and end plate-to-link joints.The load was applied to the connection in two phases, the first one is the phase of prestressing the bolts and the second refers to the loading of the free end of the beam with a concentrated moment.The prestressing forces of the bolts are chosen according to the recommendations of the standard EN 1993-1-8:2005 [1], and in the case of a bolt diameter of 20 mm, its value is 172 kN.The concentrated moment applied in the second phase is 200 kNm.
When applying the FEM method of analyzing a model, the mesh of finite elements, their size, and arrangement are of crucial importance for obtaining correct modeling results.In the case of profile I cross-sections, the mesh is defined so that there are at least 4 FE per thickness of the flange and web of the element.Details of the FE mesh can be seen in Fig. 5.

Scope of the study
To evaluate the behavior of the innovative demountable beam-column moment connection and the influence of characteristics of the individual components of the connection, a parametric study was performed.This analysis involves changing certain connection parameters and observing the impact of that change on the behavior of the connection.The modified parameters are bolt diameter, level of the bolt preload, link length, and thickness of the end plate.
Also, two more models were analyzed that represent a modified innovative connection so that instead of long bolts, conventional bolts were used.The first model (Model 0_1) that contains a beam, column, link, and three end plates is shown in Fig. 6 (a), where it can be seen that the beam-link and link-column connections are made using conventional bolts M20..10.9.In the second model (Model 0_2), additional steel element, link, is completely removed and the connection practically represents a conventional beam-column connection with an end plate.Insertion of the link was done to increase reusability and adaptability of already used steel elements.Model 0_2, shown in Fig. 6, should provide a conclusion of how much impact the addition of such an element has on the connection´s moments capacity.Model 0_1 and 0_2 are identical to Model 0 in all other parameters, including the material used, geometric characteristics of the elements, boundary conditions, mesh FE, load, etc. Table 2 shows a list of 17 analyzed models, where Model 0 represents the basic model with initial geometry.In the last column of the table, the length of the bolt is given, which is not a parameter that is initially changed, but its change occurs due to the change of other parameters (link length and thickness of the front plate).

Results of the parametric study
Fig. 7 shows the M-Φ curves obtained from the software for all models individually, divided into groups according to the modified parameter, and compared with the curve of the initial Model 0. The moment load capacity for all models was determined by using the procedure of Jaspart [4], which defines the moment load capacity of the connection in the intersection of the y-axis of the diagram and the tangent to the part of the curve with post-plastic stiffness.Also, the initial and post-plastic stiffnesses of the connection were determined, as the slope of the tangent to the corresponding part of the M-Φ curve.Several conclusions can be drawn from the given diagrams.With an increase in the diameter of the bolt from the initial 20 mm to 24 mm, an increase in the moment capacity of the connection of only 2% is observed, while with a decrease in the diameter of the bolt to 16 mm, the moment capacity of the analyzed connection decreases by 34%.The change in the prestressing force in the bolts contributes the most to the change in the initial rotational stiffness of the connection.With a change in the length of the inserted steel element, the link, there is no change in the moment capacity, but the initial stiffness decreases by about 10%.By reducing the thickness of the end plates from the initial 20 mm to 15 mm, there is a reduction of the moment capacity by 17%, while increasing the thickness to 24 mm increases the capacity by only 2%.Replacing long bolts with conventional ones results in a drop in the initial stiffness of the connection by 100% in models with a link.The moment capacity of the connection decreases by 14% and 12% in models with conventional bolts with the link and without it, respectively.

Assessment of the behavior of innovative demountable beam-column moment connection according to the standard EN 1993-1-8:2005
The practical application of the innovative demountable beam-column moment connection requires the existence of mathematical calculation models that can be used to estimate its loadbearing capacity and stiffness accurately and reliably, so a proposal for the calculation of an innovative connection according to the currently valid standard EN 1993-1-8:2005 [1] is given.The idea was to determine the weakest component of the connection, i.e. to estimate the occurrence of the first plastic deformations, and then calculate the moment capacity according to the standard for that component and connect it with the moment capacity of the entire connection.This approach was applied to all models from the parametric study.By observing the development of plastic zones in Model 0, it can be seen that the development of plastic deformations first occurs on the end plate welded to the beam.The failure of the T-element of the end plate was adopted as the authoritative one and the momentbearing capacity of the connection was calculated according to the moment-bearing capacity of this component.The same case was in almost all other models with long bolts, except for Model 3 (the model in which the bolt diameter was modified to 16 mm compared to the initial 20 mm) where plasticity initially occurs at the web of the column in the pressure zone, which can be seen in Fig. 8.The specificity of Model 3 was seen through the parametric analysis, where, according to the diagram in Fig. 7, a significant drop in load capacity was observed compared to all other models.In Model 0_1 and Model 0_2, the first plastic deformations occur at the Telement of the column flange, which is a component that was not relevant in any model of the innovative connection, which is also shown in Fig. 8.
Since the relevant failure modes have been identified for all analyzed models, it is now possible to calculate the moment capacities and initial rotational stiffnesses for all connection models according to EN 1993-1-8:2005 [1].These values are shown in Table 3 together with the values obtained in the numerical analysis in the Abaqus software [9], as well as the ratio between moment capacities and initial stiffnesses according to the analytical and numerical analysis.

Conclusions
Considering that the innovative connection with long bolts is completely new, in this paper only the surface is "scratched" in their analysis.A parametric analysis was performed to see the effects of changing certain connection components on its behavior and to compare it with the behavior of conventional connections.Also, an attempt was made to evaluate the innovative connection using the standard EN 1993-1-8:2005 [1], based on the determination of the appearance of the first plastic deformations in the connection and the assessment that the loss of load capacity of that connection component means the moment load capacity of the entire connection has been reached.Several conclusions can be drawn: 1. Defining the boundary conditions and a satisfactory FE mesh is crucial in terms of the accuracy of the obtained results of numerical calculations; 2. The biggest decrease in moment capacity (of even 34%) was achieved in the Model 3, with a bolt diameter of 16 mm.3. Modification of the level of bolt preload shows a difference in initial stiffness of about 32%. 4. For all link lengths M-Φ curves were almost identical, with only the difference in initial stiffness of about 10%.Hence, this is an influential parameter that certainly needs to be studied more. 5. Use of links in innovative connections is a good solution in terms of durability and renewability of the structure while causing a drop in the moment capacity of the connection by only 2%.6. Performed assessment of the calculation of the innovative connection according to the currently valid standard shows a discrepancy with the numerical analysis by about 35%.Through this paper, only a small part of the assessment of innovative demountable moment connections was processed, and the performed analyses only opened the door for new potential ideas regarding the examination of these connections.The upcoming experimental tests will certainly be of great use for more precise evaluations of the behavior of innovative connections.

Fig. 4 :
Fig. 4: Numerical model of the innovative connection with long bolts

Fig. 6 :
Fig. 6: Layout of moment beam-column connections with conventional bolts (a) Bolt diameter modification (b) Bolt pretension modification (c) End plate thickness modification (d) Link length modifications (e) Comparison of models with long and conventional bolts Fig. 7: M-Φ curves of the models processed in a parametric study

8 :
(a) Models 0-2, 4-14: Endplate in bending (b) Model 3: Column web in compression (c) Models 0_1 and 0_2: Column flange in bending Figure Appearance of the first plastic deformations in analyzed models

Table 1 :
Diameters and lengths of conventional bolts

Table 3 :
Results of numerical and analytical analysis