## 1 Introduction
The prestress force has a significant effect on the static flexural and dynamic behavior of the precast prestressed segmental beams, and the loss of prestress force owing to the corrosion or other causes, such as relaxation or friction, could affect the service life of precast segmental beams [22; 23]. Therefore, monitoring the prestress force loss and quantifying the prestress force in precast prestressed segmental beams is essential to ensure the integrity and performance of the prefabricated structures. In this paper, a theoretical method is proposed to quantify the prestress force in precast segmental beams, based on strain measurements to obtain the neutral axis location. This method is developed based on the static equilibrium over a specific cross-section of the segmental beam. Experimental tests on a scaled precast segmental concrete beam with post-tensioned unbonded external prestressed tendons are conducted to verify the proposed approach. The corresponding high-fidelity FEM of the tested segmental beam is also established. Parametric sensitivity analysis is conducted using the verified FEM to identify the most sensitive parameters to the prestress force change. The response surface (RS) method is used to update the initial FEM based on the measured static displacement responses and the neutral axis location, to refine and ensure that the updated FEM can better reflect the behavior and status of the real tested structure. Numerical simulations for quantifying the prestress force by using the proposed approach are conducted. Based on experimental verifications and numerical simulations, the accuracy and reliability of the proposed approach for quantifying the prestress force are validated. The reminder of this paper is organized as follows. Section 2 provides a detailed theoretical derivation for quantifying the prestress force in precast segmental beams based on neutral axis location obtained from strain measurements. Section 3 describes the experimental testing specimen of the prestressed segmental beam. This section includes a detailed description of the design, the geometry and material properties of the tested prestressed segmental beam. Experimental test setup, post-tensioning process, loading process, and strain data acquisition are described in this section. The experimental validation on the proposed approach is conducted, and the discussions on the difference between the theoretically predicted and experimental measured results are also provided in this section. Section 4 describes a corresponding high-fidelity FEM based on the design of the experimental segmental beam. In Section 5, the parametric sensitivity analysis and model updating are conducted for refining the initial model. Numerical simulations with different levels of prestress forces are conducted, and the strain measurements are taken. Studies are conducted to further validate the accuracy of using the proposed approach for quantifying the prestress force with the strain data in numerical simulations. In addition, the strain responses at the web of the bridge model subjected to a dynamic load are also used for identifying the prestress force by the proposed method. Section 6 provides a brief conclusion and discussion of the feasibility and accuracy of the proposed approach.