Design equations for thermal bowing and composite degree of concrete sandwich panels

Concrete sandwich panels (CSPs) are oftentimes subjected to extreme exterior temperatures, causing thermal gradients between inner and outer surfaces, and eventually bowing. In addition, the composite degree (kp) of CSPs has been conventionally estimated using methods that require costly and time-consuming experimental tests or rigorous numerical models. The lack of research and understanding on the effects of kp and bowing oftentimes result in overly conservative and uneconomic design, particularly for partially composite (PC) panels. This study develops simple design equations for thermal bowing and kp in CSPs based on a comprehensive parametric study using a transient thermal-structural coupled finite element (FE) model developed earlier by the authors. Key parameters considered are span (L); connector configuration, material, and reinforcement ratio; wythe and insulation thicknesses; concrete strength and thermal differential (& UDelta;T) on both sides. In panels with L larger than 13 m, particularly those featuring steel and CFRP truss connectors and subjected to temperature gradients (& UDelta;T) higher than 30 degrees C, bowing was larger than the deflection serviceability limit of L/360. Cracking in concrete wythes and yielding of steel connectors were also observed and were mainly dependent on & UDelta;T and kp. Two existing theoretical models were evaluated using results of the parametric analysis and were found to overestimate thermal bow, particularly for panels with low kp. A proposed bowing model is presented which incorporates the effects of composite degree and yields much better predictions than these from literature.