# Thermomechanical Analysis of Steel-to-Timber Connections under Fire and the Material Density Effect
## 1 Introduction  
The study of the thermomechanical behavior of a wood structural connection when exposed to fire is very important [1]. In a fire situation, temperatures and thermal gradients are very high. Wood loses mechanical resistance and steel is a good conductor of temperature, which generates thermal stresses and strains because of the overall strength of the connection. The use of wood material has increased in construction and engineering activities. This growth requires a high level of knowledge regarding the behavior of such structures in fire situations. Predicting the thermomechanical response in the steel-to-timber connection with dowels subjected to fire is a highly nonlinear problem, due to the thermal and mechanical properties dependent on the temperature of the materials involved. The thermomechanical analysis of connections has been recently studied [2]. [PERSON] [2] studied a double shear steel-to-timber connection under fire using two different fire loads: a natural fire curve was created with OZone software, and a nominal temperature-time curve, ISO 834. The model was built using the ANSYS software. The authors verified the influence of the timber width cross-section and the diameter of the fastener with the same materials [2].  
Initially, many studies were developed involving analytical models to predict temperature results on the behavior of structures exposed to the fire scenario [3, 4]. Meanwhile, someresearch involving numerical and computational models was developed to study structures exposed to fire, for example, using the ANSYS software [5; 6; 7]. More recently, some of these studies included thermomechanical analyses in steel structures, concrete-filled steel columns, composite steel and concrete beams, composite concrete, and timber beams, among others [2; 8; 9]. The thermomechanical behavior of timber connections under fire is complicated because of the complexity of analyzing the wood's thermal and mechanical properties and its degradation in fire [10].  
This paper aims to present a consistent approach to accurately estimate the thermomechanical behavior of steel-to-timber connections. While some studies present structural finite element methods for predicting mechanical stresses, other work only emphasizes thermal analysis. The present methodology considers both thermal and mechanical structural analysis. The ability to obtain thermomechanical results makes these methodologies useful in comparison to other previous methods. The current study intends to evaluate, through an advanced calculation model using the finite element method, the behavior of steel-to-timber connections in fire situations, considering the nonlinearity and the elastoplastic regime of the materials. This work was developed with the ANSYS(r) software [11], where the connection model was exposed to a nominal temperature-time curve recommended by the standard ISO 834 [12; 13]. The mechanical and thermal properties were imposed on the numerical model according to the Eurocodes for each material [14; 15].  
The main objective of this manuscript is to study timber connections in tension and exposed to fire, identifying the load-bearing capacity that allows the dimensioning of safety in the connection. Simultaneously, it is intended to verify the effect of wood density on the heat transferred through the connection. To accomplish this, a three-dimensional numerical model was created to carry out a transient and thermomechanical analysis. Thus, the thermal and mechanical analysis is based on a two-step simulation process, submitting the connection to fire, and then, for each fire rated time, the load-bearing capacity is determined. Load-bearing capacity is the ability of the connection to withstand the imposed load (thermal and mechanical) without exceeding the material's yield conditions.