# Temperature Effects on the Natural Frequencies of Composite Girders
## 1 Introduction  
Over the past several decades, various studies have been conducted to monitor the health conditions of bridges based on modal parameters such as natural frequencies and mode shapes [1; 2; 3]. However, modal parameters can be affected by environmental factors, which limits their application as health monitoring indicators. Temperature, humidity, and wind speed have been considered as potential environmental factors influencing modal parameters [4]. Among these, temperature has been reported to be the primary factor affecting the modal parameters of bridges. Bridges are typically exposed to temperature variations caused by solar radiation, heat conduction, and convection. Temperature changes can influence the elastic modulus of materials and size of structural members. In the case of a statically indeterminate structure, secondary stress can also be induced by temperature [5]. These factors can affect the stiffness of bridges. In other words, modal parameters can be changed by temperature effects.  
Many studies have indicated that natural frequencies decrease as temperature increases [1; 6; 7; 8]. For example, experimental tests conducted on a simply supported beam to investigate the impact of temperature variations on its natural frequency in a temperature range from \(-40\) to \(60\)\({}^{\circ}\)C demonstrated that the natural frequency decreased by up to \(0.148\%\) as the temperature increased [5]. [PERSON] and [PERSON] [9] observed the daily correlations between temperature and the frequencies of steel girders in a cable-stayed bridge and found that modal frequencies decreased with increasing temperature. A similar tendency has also been observed in laboratory tests on steel and concrete beam specimens [10].  
Composite bridges composed of reinforced concrete decks and steel girders are commonly used in bridge construction based on their structural efficiency. In this type of bridge, a temperature gradient, which refers to an uneven distribution of heat, can occur as a result of the different heat properties of concrete and steel. Such temperature gradients have been considered as important factors affecting modal parameter variations in bridges [6; 11]. Various studies have been conducted to interpret the temperature gradient effects of bridges based on in situ data measured over long periods of time [12; 13; 14]. However, long-term data from bridges in service include the effects of traffic loads, which may not be negligible. Therefore, for accurate interpretation, temperature gradient effects have to be investigated based on data measured under controlled conditions.  
In this study, we investigated the effects of temperature gradients on the natural frequencies of composite bridges. A laboratory setup was developed using a composite girder specimen composed of a concrete deck and a steel beam. The investigated temperature range on the top of the specimen was 20-50 \({}^{\circ}\)C. Modal tests were conducted to obtain natural frequencies and mode shapes at different temperatures. Additionally, finite element analysis (FEA) was conducted to verify the test results and analyze temperature effects.