# Thermomechanical Analysis of Steel-to-Timber Connections under Fire and the Material Density Effect
## 3 Results and Discussion
### Temperature, Load-Bearing Capacity, and Stress Level in Connections with Different Wood Densities  
The load-bearing capacity of the connection when submitted to tensile load and exposed to fire requires verification of the stress level and the temperature reached at the  
Figure 16: Temperature in the middle of the cross-section of the connection for the time of 300 s.  
Figure 17: Temperature in the plane of wood in contact with the steel plate for the time of 300 s.  
beginning of the wood char layer formation. According to Eurocode 5 part 1-2 [14], the point that the char layer appears in the wooden material corresponds to the temperature of 300 \({}^{\circ}\)C.  
The temperature-time curves for points K1, K2, and K3 and the three wood species are represented in Figure 18.  
The curves show a similar tendency for the same point of analysis, with an early rise in temperature being consistently observed for the GL20H wood. The connection made with GL20H wood presents the highest temperatures over time, being this followed by the connection with GL24H wood and the connection with GL32H wood. After 500 s, the temperature reaches 300 \({}^{\circ}\)C at the external point K3 for connection with GL20H wood, with a delay of 90 s to the connection with GL32H wood and 60 s to the connection with GL24H wood. The temperature difference is evident due to the wood density effect, with a delay between the connection in GL20H and the other wood species being observed. The connection in GL32H has lower temperatures until the exposure to fire in the connection reaches 900 s, where all connections start to have the same behavior. The average temperature at 900 s is 688 \({}^{\circ}\)C for the connection with GL20H wood, 664 \({}^{\circ}\)C for the connection with GL24H wood, and 656 \({}^{\circ}\)C for the connection with GL32H wood. In the work presented by [PERSON] [2], the results of the temperature obtained on the timber element submitted to the fire curve has a similar tendency compared with the results in our study. The use of the ISO834 fire curve resulted in higher temperatures in the outer regions and in the steel elements. In Figure 19, it is possible to analyze the load-bearing capacity of the studied connection for different times of exposure to fire and for the three wood species under study. These results are based on the material's failure strength according to the material behavior at different temperature levels. Figure 18: Temperature–time history on the connection. The results presented in Figure 19, at room temperature, correspond to the initial instance. The results only show the effect due to the applied mechanical load in the connection. These values are in accordance with the calculated load-bearing capacity given by Equation (2), Eurocode 5, part 1-1 [16]. The numerical value is 48.76 kN for connection with GL20H wood, 55.19 kN for connection with GL24H wood, and 63.09 kN for connection with GL32H wood. In this graphic, the results also show a decreasing load-bearing capacity with the exposure time to the fire. The GL32H connection has more resistance than other connections with lower wood density. On average, the connection with GL32H wood presents a superior load-bearing capacity of 16 kN compared to the connection with GL20H wood, and more than 6 kN more than the connection with GL24H wood. The decrease in load-bearing capacity in all connections is slightly linear with the increasing fire exposure time. These connections do not withstand more than 300 s at high temperatures. At the end of the fire exposure, and in relation to the applied load at ambient temperature, it was possible to observe that the GL20H connection has an 83% decrease in the load-bearing capacity, the GL24H connection has a reduction of 71%, and the GL32H connection decreases by 68%. The duration of fire exposure changes the load-bearing capacity in the connection because both materials (steel and wood) are also changing, a conclusion also reported by [2].