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Edited by: George C. Tsiatas, University of Patras, Greece

Reviewed by: Mario D'Aniello, Università degli Studi di Napoli Federico II, Italy; Massimo Latour, Università degli Studi di Salerno, Italy

This article was submitted to Computational Methods in Structural Engineering, a section of the journal Frontiers in Built Environment

This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

Hybrid steel to timber connections are found in many buildings and bridges. These connections offer several advantages such as ease of construction and energy dissipation. This research paper aims to study the mechanical behavior of bolted hybrid connections that consists of a square hollow steel column (SHS) and a glulam timber beam. The connection between the two structural members is achieved by means of angles and preloaded bolts. A reference model is constructed and verified by comparison to experimental and numerical data from the international literature. Additionally, several parameters that affect the response of the connection are modified in order to investigate and quantify their effect, resulting in seven different case studies. These parameters are the size of the bolts, the thickness of the angles and the addition of stiffener. The moment-rotation curve of each case study is constructed and the results are commented. Finally, a proposed optimal configuration of the hybrid connection is presented.

Steel and wood are materials that have been extensively used in constructions worldwide, even from ancient times. Nowadays, the need for a sustainable built environment encourages structural engineers to think of new design options that can maximize the benefits of the aforementioned materials. By combining the strength and ductility of steel with the low weight of wood, environmental friendly structures are achieved. The structural members in this type of constructions are connected using hybrid solutions. In countries like Norway where the access to wood resources is easy, these hybrid structures could become the norm for future lightweight structures.

Connections between steel and concrete or steel and timber structural elements are referred as hybrid. In general, the design of connections between members is always a challenge for the structural engineers. Loads and stresses need to be transferred from one structural member to another. In the presented paper, finite element modeling is used as a tool to investigate the behavior of hybrid bolted steel- timber connections considering several proposals made in technical literature, such as Amara and Embaye (

Rendering of the reference connection, figure based on Karagiannis et al. (

In structural engineering, the behavior of a certain structure under design load situations is represented by unique action-deformation curves. If the structure is examined at member level then the moment-rotation (M-ϕ) curve represents the behavior of a certain connection between structural members. The rotational stiffness of the connection is equal to the slope of the curve and the initial value of this stiffness is used for the classification of the connection, which is important for the design of MRFs. According to Eurocode 3 part 1.8 (EN 1993-1-8,

where:

S_{j, ini} is the initial rotational stiffness of the connection (kN/mrad)

EI_{b} is the bending stiffness of the beam (kN/mrad)

L_{b} is the span of the beam (mm)

The classification of the joint according to the initial rotational stiffness presented in Eurocode 3 (EN 1993-1-8,

Connection classification by initial rotational stiffness, figure from Eurocode (EN 1993-1-8,

The general procedure for determining the rotational stiffness of a connection proposed by Eurocode (EN 1993-1-8,

where:

ϕ is the rotation of the connection (mrad)

Δe is the elastic deformation of the beam (mm)

d_{b} is the depth of the beam (mm)

Then, the reaction force P_{e}, as shown in Figure

Analytical solution of semi-rigid angle connection, figure based on Lee and Moon (

where:

P_{e} is the reaction force (kN)

g_{1} is the gauge distance parallel to the column (mm)

g_{2} is the gauge distance parallel to the beam (mm)

Finally, the elastic moment of the connection is found, using the Equation (8):

where:

M is the elastic moment of the connection (kNm)

l_{e} is the effective length of the angle (mm)

Equations (7) and (8) together with the corresponding Figure

The final step is to construct the moment-rotation curve and to calculate the initial rotational stiffness, which is equal to the slope of the M-ϕcurve. The calculation is done using the Equation (9):

where:

S_{j, ini} is the initial rotational stiffness of the connection (kN/mrad)

The reference beam-column hybrid connection consists of glulam beam and a steel SHS column. The beam has a height of 405 mm and width of 140 mm, whereas the column is 150 mm wide and 10 mm thick. Top and seat angles are 150^{*}200 mm, 15 mm thick and they are connected using M12 HR bolts to the glulam beam and M16 HR bolts to the tubular steel column, respectively. The steel grade of the column is S355, of the top and seat angle is S275 and of the bolts. Glulam is of category GL28h. The model is constructed according to the geometric and mechanical characteristics of an experimental specimen found in the international literature by Karagiannis et al. (

Verification of numerical model.

The beam, the column and the angles are simulated with three-dimensional structural solid finite elements and the contact interfaces with surface-to-surface contact elements. Contact, itself, is a complex phenomenon that adds nonlinearities to a finite element simulation as presented by Bathe (

A total number of seven case studies is examined by Amara and Embaye (

Case studies examined.

1 | Reference model |

2 | Thickness of the top and seat angle equal to 18 mm |

3 | Thickness of the top and seat angles equal to 20 mm |

4 | Diameter of the vertical bolts equal to 16 mm |

5 | Diameter of the vertical bolts equal to 20 mm |

6 | Reduction of 20 mm in the width of the top and seat angle |

7 | Thickness of the top and seat angle equal to 12 mm and use of a 14 mm stiffener in the middle of the top angle |

The main results of the numerical analysis are the moment-rotation curves together with the corresponding initial rotational stiffness and the von Mises stresses that develop in the connection. The results are presented in Figures

Von Mises stress distribution in the top angle for case 1.

Von Mises stress distribution in the seat angle for case 1.

Von Mises stress distribution in the vertical bolts for case 1.

Von Mises stress distribution in the glulam beam for case 1.

Numerical moment-rotation curves.

Von Mises stress distribution in the top angle for case 7.

The numerical analysis of the hybrid steel-timber connection focuses on the initial rotational stiffness and the developing von Mises stresses in the top angle.

Effect of angle thickness

By changing the thickness of the top and the seat angles from 15 mm (case study 1) to 18 mm (case study 2) or to 20 mm (case study 3), the rotational stiffness of the connection increases. For case 2 the increase is 73% and for case 3 is 137%, in comparison to case 1. This denotes that even a small increase in thickness of the angles has a significant effect on the rotational stiffness of the connection. As expected, the connection becomes stiffer when the thickness of the angles is increased. Moreover, the maximum von Mises stress in the top angle is decreased by 12.2% in case 2 and by 15.5 % in case 3. The increase in the amount of steel used is 21 and 35.2%, respectively.

Effect of bolt diameter

By changing the diameter of the vertical bolts from12 mm (case study 1) to 16 mm (case study 4) or to 20 mm (case study 5), the rotational stiffness of the connection is not affected. The change in the diameter of the bolts is reciprocal to the change in the pretension force. Moreover, the maximum von Mises stress in the top angle is decreased by 20.7% in case 4 and by 32.7% in case 5. The increase in the amount of used steel is higher by 21 and 35.2%, respectively.

Effect of angle width

By reducing the width of the of the top and the seat angles from 150 mm (case study 1) to 130 mm (case study 6), the rotational stiffness of the connection is reduced by 14.15%. Moreover, the maximum von Mises stress in the top angle increases by 14.28%.

Effect of stiffener

By adding a stiffener at the top angle (case study 7), the rotational stiffness of the connection increases by 53.6% in relation to the reference model. The addition of stiffener leads to the development of high von Mises stresses in the seat angle near the bolt heads due to bending of the horizontal leg of the seat angle. After the pretension of the bolt, the stiffener also enhances the clamping forces at the bolt-angle interface.

Local surface crushing of the glulam beam is present in all models, due to compression from the top and seat angle. The effect is more significant when the stiffness of the angle is higher, thus resulting in a huge difference in deflection between the beam and the angle.

The connection in all the case studies remains semi-rigid, as assumed. Case studies 2, 3, and 7 have the higher values of initial rotational stiffness. The von Mises stress in case study 5 has its minimum value.

The recommended connection is a combination of case studies 3 and 5. The geometry of the joint remains the same, as in the reference model, but the thickness of both angles and the diameter of the bolts are set equal to 20 mm. This design option increases the initial rotational stiffness of the connection by 137%, whereas the von Mises stress decreases by at least 32.7%. The aforementioned benefits in the response of the connection by far outweigh the increase in steel use.

TT, YA, and SE contributed conception, design of the study, and performed the analysis; TT wrote the first draft of the manuscript; TT, YA, SE, and EN wrote sections of the manuscript. All authors read and approved the submitted manuscript version.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.