# Structural Health Monitoring for Prefabricated Building Envelope under Stress Tests
## 1 Introduction
Figure 1 shows the InComEss architecture underlining the components embedded within the facade and the digital components. The first considerations presented are related to physical components such as the FBG sensors. This product represents an application for the facade demo case, and during the InComEss project, it will demonstrate the potential of its applicability for SHM in the building envelope market. An optical fiber consists of a core and cladding. The cladding of the fiber has a typical diameter of 125 \(\upmu\)m while the core has a diameter of typically \(\sim\)10 \(\upmu\)m. The core has a higher refractive index than the cladding, such that the light is captured in the core. A Fiber Bragg Grating is a periodic modulation of the refractive index in the fiber core. A representation can be seen in Figure 2.  
Figure 1: Scheme of InComEss system architecture with components integrated into facade and communication to IoT platform. The period of the refractive index modulation determines the wavelength of light that is reflected. Due to temperature or strain, the period of the modulation, and thus, its reflected wavelength, will change. Due to this principle, FBG sensors serve dual roles as both strain and temperature detectors. When mounted within a hollow capillary, an FBG sensor exclusively detects temperature changes, disregarding strain. The sensor's wavelength indicates temperature variations through a fixed, linear conversion based on glass material properties. The MonadGator interrogator is the measurement device developed to measure the shift in Bragg wavelength of the FBG sensors. The system uses a light source that emits a broadband wavelength spectrum through the fiber toward the FBG sensors. The reflected wavelength is measured by the MonadGator and converted to a strain or temperature. This MonadGator is designed to operate with low power consumption. Figure 3 reports the InComEss architecture, showing how the energy generated powered the MonadGator for the FBG sensors. An Arduino nano is used to rapidly start up the MonadGator and execute a measurement of the FBG wavelength. The broadband light source is turned on for a period of 70 ms. In that time span, the FBG sensor is read, and the data from the photodiodes are acquired by the Arduino. These data are transferred using a Bluetooth Low Energy (BLE) connection to the IOT gateway. The Arduino is powered by a power conditioning circuit, which is charged with the energy that is harvested from the energy harvester connected to this circuit. A total measurement cycle, including wireless data transfer, takes less than 150 ms and uses only 0.08 J of energy per measurement. In this scenario, the research aims to investigate the possibility of integrating a kit of sensors for structural health monitoring (SHM) into the curtain wall facade to monitor its behavior post-installation. Therefore, this paper outlines the results achieved by the SHM sensors during a performance test, wherein a full-scale facade prototype was subjected to various weather and mechanical conditions in a controlled laboratory environment. The objective was to monitor the facade's behavior under stress from weather conditions and accidental impacts. The paper's objectives are to investigate and provide original contributions in the scientific field to support analyses useful for establishing monitoring solutions integrated into building envelopes using low-consumption sensing technology. Additionally, it aims to explore potential applications within facade components to define the stress application of Fiber Bragg Grating (FBG) and Fiber Optic Sensors (FOS). Figure 3: Scheme of InComEss architecture. Figure 2: Schematic representation of a fiber and Fiber Bragg Grating inscribed into the core of an optical fiber. Furthermore, the study seeks to validate the facade's integrability and the effectiveness of the multi-parameter sensor kit for facade SHM, along with the measurement method employed.