The main objective of this work is to present a methodology to design (S-W-S) Steel-Wood- Steel connections in double shear, with internal dowel steel fasteners, in fire conditions. Analytical methodologies according Eurocodes were used to determine the load-carrying capacity of the connections. To analyse the load-carrying capacity, different wood densities and steel dowels diameters will be investigated. The number of dowels is not considerably affected by the wood density of the species. To predict the fire exposure, a thermal and transient numerical analysis was performed to obtain the temperature field results in all studied connections.info:eu-repo/semantics/publishedVersio

Fonseca, E.M.M.

Piloto, P.A.G.

Silva, Vânia

English

Biblioteca Digital do IPB

Proceedings IRF2020: 7th International Conference Integrity-Reliability-Failure. J.F. Silva Gomes and S.A. Meguid (editors), INEGI-FEUP (2020), pp.557-562. https://paginas.fe.up.pt/~irf/Proceedings_IRF2020/   -557- PAPER REF: 17377   LOAD-CARRYING CAPACITY OF HYBRID STEEL-WOOD-STEEL DOWEL CONNECTIONS IN DOUBLE-SHEAR UNDER FIRE  Elza MM Fonseca1(*), Vânia Silva1, Paulo AG Piloto2 1INEGI/LAETA and Instituto Superior de Engenharia (ISEP), IPP, Porto, Portugal 2INEGI/LAETA and Instituto Politécnico de Bragança (IPB), Bragança, Portugal (*)Email: elz@isep.ipp.pt    ABSTRACT The main objective of this work is to present a methodology to design (S-W-S) Steel-Wood-Steel connections in double shear, with internal dowel steel fasteners, in fire conditions. Analytical methodologies according Eurocodes were used to determine the load-carrying capacity of the connections. To analyse the load-carrying capacity, different wood densities and steel dowels diameters will be investigated. The number of dowels is not considerably affected by the wood density of the species. To predict the fire exposure, a thermal and transient numerical analysis was performed to obtain the temperature field results in all studied connections. Keywords: S-W-S connection, load-carrying capacity, fire, Eurocodes.  INTRODUCTION The load-carrying capacity of the S-W-S dowel type fasteners connections can be described by the European Yield Model (EYM) based on a plastic limit state analysis. Johansen (1949) was the first researcher that assumed this model has an ideal rigid-plastic behaviour of the wood and steel dowel. This method predicts the strength of two or three-member dowel type connection, (Wilkinson, 1992).  Wood-steel connections are the fragile member for any structural construction. When submitted to fire conditions, an increased weakness occur due to the heat flow into the wood, driven by the steel fasteners or plates which cause faster wood charring (Martins and Fonseca, 2018; Abderrahim et al., 2018). Both steel and wood materials behave differently and the interaction between the elements is even more complex to analyse.  The heat flow in wood leads to a thermochemical decomposition (pyrolysis) which results in the char layer formation, accompanied by mass loss. In wood members exposed to fire or high temperatures, char layer appears and increases in depth with the heat progression. According to Eurocode 5 (CEN EN1995-1-2, 2004) the isotherm of the char layer coincides to 300ºC. All of these phenomena were verified by different researches and also wood connections have been widely investigated to ensure their structural safe and integrity. Several researchers have presented experimental and analytical methods to estimate the wood degradation due to fire conditions, (Fonseca and Barreira, 2009; Fonseca and Barreira, 2011; Fonseca et al., 2013). Also, empirical models for determining the charring rate and the heat transfer conditions were developed by (White and Nordheim, 1992). Track-I Civil and Structural Engineering Applications -558- The main goal of this work is to present analytical and numerical methodologies to predict the safe of S-W-S connections submitted to a fire, determining the time of fire resistance. Different constructive models (S-W-S connections in double-shear, joined by steel dowel and outer steel plates) will be analysed. This study brings results to the previous investigations by the authors (Fonseca et al., 2020a; Fonseca et al., 2020b; Fonseca and Leite, 2019), looking for the effect of the steel dowels and outside steel plates in the temperature evolution inside the wood member. The designed connections were evaluated for different applied tensile loads, parallel to grain, and different dowels diameters. The main dimensions of the connection are: width, height and thickness of the wood and steel plates, minimum spacing and edge/end distances between the dowels and the plates.  S-W-S CONNECTION IN ACCORDANCE WITH EUROCODE 5  The S-W-S connections were designed with the simplified equations from Eurocode 5 (CEN EN1995-1-1, 2004) and Eurocode 3 (CEN EN1993-1-1, 2005), at ambient temperature. Different wood species in homogeneous glued laminated specimens were considered for the S-W-S designed connections. To reduce the risk of failures modes, a minimum edge distance, and end spacing criteria for the connections, using different dowel diameters, were considered.  According to the simplified equations from Eurocode 5 (CEN EN1995-1-1, 2004), the characteristic load-carrying capacity per shear plane per fastener, v,X, is determined according to equation 1. This equation is only used for thin steel plates as the outer members of a double shear S-W-S connection.  v,X = ;ÕY 0,5 ½	,/, ¢/ ­1,15 2 ,X ½	,/, ­ + v·,X4  (1)  ¢/ represents the thickness of the wood middle member; ½	,/, is the characteristic embedment strength in wood member; ­ is the dowel diameter; ,X  is the characteristic yield moment of the fastener; calculated according the dowel diameter and the material strength; v·,X represents the characteristic withdrawal capacity of the fastener. With the obtained characteristic load-carrying capacity it is possible to determine the number of dowels according the applied load design. The arrangement of the dowels will be in lines and columns. In this study, the number of columns was fixed to 3. The edge distances were calculated following the imposed standards.  Different parameters were considered: three dowel diameters (6, 8 and 10mm), three applied tensile load (10, 15 and 20kN) and three wood materials (GL24h, GL28h and GL32h), each one with different densities (380, 410 and 430 kg/m3, respectively). The results are presented in figure 1, which represents the relation between the number of dowels depending on the applied load “Ed” and the dowel diameter. The wood density does not affect considerably the number of dowels. The same conclusions were reported by the authors of this work in previous Proceedings IRF2020 7th International Conference Integrity-Reliability-Failure -559- publications, but for W-S-W and W-W-W designed connections (Fonseca et al., 2019; Aissa, et al., 2017; Fernando et al., 2018; Martins and Fonseca, 2018).    Fig. 1 - Number of dowels depending on the applied load, for S-W-S connections  S-W-S NUMERICAL MODEL  To simulate the thermal resistance of the S-W-S connection, a numerical program was used to produce simulations focused on thermal and transient analysis. This methodology allows to obtain the temperature field in the studied connections under fire and allows to calculate the char layer in the members with different wood densities.  All thermal properties (thermal conductivity, specific heat and density) used in the numerical model are based on standard codes. Steel behaviour is defined according to the Eurocode 3 (CEN EN1993-1-2, 2005), Eurocode 5 (CEN EN1995-1-2, 2004) is used for wood with different densities (GL24h, GL28h and GL32h). The material non-linearity due to the temperature effect was considered in the numerical simulation. The numerical models were developed, using the designed connections obtained from the previous calculation at ambient temperature. According to the symmetry of the model connection, only a 2D quarter model of the S-W-S connection will be studied. The numerical calculation was performed for a plane of the connection including the dowels location, where the number of columns is fixed to 3. The finite element chosen has eight nodes and three degrees of freedom per node. The initial temperature in the numerical model was considered equal to 20ºC. The external surface of the connection is exposed to the standard fire curve ISO834 during 1800s and the convection coefficient is taken equal to 25W/m2K. The fire emissivity is taken constant and equal to 1 for exposed side.  02010 20Number of dowels, NApplied load Ed, kNGL24h d=6mmGL28h d=6mmGL32h d=6mmGL24h d=8mmGL28h d=8mmGL32h d=8mmGL24h d=10mmGL28h d=10mmGL32h d=10mmTrack-I Civil and Structural Engineering Applications -560- Figure 2 represents the connections in study (a quarter of S-W-S), with the applied mesh, time of fire resistance and temperature at 1800s.   Wood type and dowel diameter Mesh Temperature and time  of fire resistance Max. and Min. temperature  at end of fire (time=1800s)     GL24h, d=6mm    20ºC                          300ºC time=315s   121ºC                        821ºC Min=386ºC      Max=813ºC GL28h, d=6mm    20ºC                          300ºC time=315s   121ºC                        821ºC Min=239ºC      Max=811ºC GL32h, d=6mm    20ºC                          300ºC time=315s   121ºC                        821ºC Min=204ºC      Max=811ºC     GL24h, d=8mm    20ºC                          300ºC time=285s   121ºC                        821ºC Min=240ºC      Max=817ºC GL28h, d=8mm    20ºC                          300ºC time=285s   121ºC                        821ºC Min=197ºC      Max=816ºC GL32h, d=8mm    20ºC                          300ºC time=285s   121ºC                        821ºC Min=174ºC      Max=815ºC     GL24h, d=10mm    20ºC                          300ºC time=270s   121ºC                        821ºC Min=213ºC      Max=820ºC GL28h, d=10mm    20ºC                          300ºC time=270s t=1800s   121ºC                        821ºC Min=176ºC      Max=819ºC GL32h, d=10mm    20ºC                          300ºC time=270s   121ºC                        821ºC Min=151ºC      Max=818ºC Fig. 2 - Time of fire resistance for S-W-S connections, and maximum and minimum temperature at the end of fire exposure  Proceedings IRF2020 7th International Conference Integrity-Reliability-Failure -561- RESULTS AND DISCUSSION A procedure with simplified equations was presented to assess the dimensions of the connections for any applied tensile load. The number of fasteners increases with load, according to standards at room temperature. Lower dowels diameter has a higher pronounced effect in the required number of fasteners. The mechanical effect due to the strength of material for GL24h, GL28h and GL32h is not significant, but the wood density variation affects the thermal behaviour.  A finite element program was used to produce the numerical simulations of S-W-S connections with the steel members exposed to fire, and including the non-linear material in transient analysis. The main objective is to obtain fire resistance (time) of the connection when the temperature reaches 300ºC in the wood material. The temperature field was also investigated in all connections, measuring the maximum and minimum temperature in the connections, where the material effect (steel and wood) and the timber density are important parameters to be considered. The numerical method shows a good solution process to understand the behaviour of the connection (both materials steel and wood) under fire exposure.  As a general conclusion, the combination of the largest wood density with the largest dowel diameter allows lower minimum temperatures inside the connection in a fire condition. However, a smaller dowel diameter in the connections leads to a delay in start the charring rate. The time of fire resistance is higher when compared with S-W-S connections with large dowels diameters. This is because the fire resistance time is measured when the first layer of wood charred, and in this way is independent of the material density.  REFERENCES [1] Aissa A, Fonseca EMM, Alvear PM Daniel, W-W-W Connections in Double-Shear at Ambient Temperature: Effect of the Applied Tensile Load and Dowels Diameter. Proceedings of the 6th International Conference on Integrity-Reliability-Failure IRF2018, J.F.Silva Gomes and S.A. Meguid (Eds.), Publ. INEGI/FEUP, 2018, pp.1349-1356. [2] Aissa A, Fonseca EMM, Lamri B, Protected and Unprotected W-W-W Connection at Elevated Temperatures. Research and Advanced Technology in Fire Safety, Daniel Alvear (ed.), Editorial Universidad Cantabria, D.L. SA 739-2017. 2017, p.407. [3] CEN, EN1995-1-2: Eurocode 5: Design of timber structures. Part 1-2: General Structural fire design, Brussels, 2004. [4]- CEN, EN1993-1-1: Eurocode 3: Design of steel structures. Part 1-1: General rules – and rules for buildings, Brussels, 2005.  [5] CEN, EN1993-1-2: Eurocode 3: Design of steel structures. Part 1-2: General rules - Structural Fire Design, Brussels, 2005. [6]- CEN, EN1995-1-1: Eurocode 5: Design of timber structures. Part 1-1: General Common rules and rules for buildings, Brussels, 2004. [7] Fonseca EMM, Leite PAS, Silva Lino, Wood Connections Under Fire Conditions Protected with Gypsum Plasterboard Types A and F. Chapter No:7, Book: Advances in Fire Safety Engineering. CILASCI 2019. P. A. G. Piloto et al (Eds), Lecture Notes in Civil Engineering, vol 1. Cham, Springer Nature Switzerland AG 2020, 2020a, pp.93-106. Track-I Civil and Structural Engineering Applications -562- [8] Fonseca EMM, Silva Lino, Leite PAS, Numerical model to predict the effect of wood density in wood–steel–wood connections with and without passive protection under fire. Journal of Fire Sciences, in Special Issue: 5th Iberian-latin-american congress on fire safety, P. A. G. Piloto et al (Eds): CILASCI 2019, 2020b, pp.1-14. (in press) [9] Miranda FM, Fonseca EMM, Góes JLN, Load-Carrying Capacity in W-S-W Connections in Double-Shear at Ambient Temperature. Proceedings of the 1st Iberic Conference on Theoretical and Experimental Mechanics and Materials / 11th National Congress on Experimental Mechanics, Editor J.F. Silva Gomes. Publisher: INEGI/FEUP, 2018, pp.261-270. [10] Fonseca EMM, Barreira LMS, Charring rate determination of wood pine profiles submitted to high temperatures. WIT Press, 3 Int. Conf. on Safety and Security Eng., 2009, 108. pp.449-457. [11] Fonseca EMM, Barreira LMS, Experimental and Numerical Method for Determining Wood Char-Layer at High Temperatures due an Anaerobic Heating. International Journal of Safety and Security Engineering, 2011, 1(1). pp.65-76. [12] Fonseca EMM, Couto D, Piloto PAG, Fire safety in perforated wooden slabs: a numerical approach. WIT Press, 5 Int. Conf. Safety and Security Eng., 2013, 134. pp.577-584. [13] Fonseca EMM, Leite PAS, (2019), Fire design of wood connections loaded in shear. MATTER: International Journal of Science and Technology, GRDS Publishing, 2019, 5(2). pp.68-84.  [14] Johansen KW, Theory of timber connections. International Association for Bridge and Structural Engineering, 1949, 98. pp.249-262. [15] Martins RDAR, Fonseca EMM, Double-Shear W-S-W Connections at Ambient Temperature, with Different Applied Tensile Loads and Steel Dowels Diameter. Proceedings of the 6th International Conference on Integrity-Reliability-Failure IRF2018, J.F.Silva Gomes and S.A. Meguid (Eds.), Publ. INEGI/FEUP, 2018, pp.1341-1348.  [16] Martins RDAR, Fonseca EMM, W-S-W Connections with a Steel Plate as the Central Member and Different Dowels Diameter at High Temperature. Proceedings of the 1st Iberic Conference on Theoretical and Experimental Mechanics and Materials / 11th National Congress on Experimental Mechanics, Editor J.F. Silva Gomes. Publisher: INEGI/FEUP, 2018, pp.239-248.  [17] White RH, Nordheim EV, Charring rate of wood for ASTM E119 exposure. Fire Technology. 1992, 28 (1). pp.5-30. [18] Wilkinson TL, Strength of Bolted Timber Connections With Steel Side Members. Forest Products Laboratory, Research Paper FPL-RP-513, Madison, Forest Service, 1992, 10 p.

Load-carrying capacity of hybrid steel-wood-steel dowel connections in double-shear under fire

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Load-carrying capacity of hybrid steel-wood-steel dowel connections in double-shear under fire

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