A regular building inspection and condition assessment have been deemed as a necessary measure to maintain the functionality and integrity of buildings and civil infrastructures. Since there is an increasing number of old buildings, the exposure of façades persistently experiencing adverse outdoor environmental conditions catalyses the degradation [1]. The percentage of public residential buildings in Singapore exceeding the age of 20 years was 74% [2] (see Figure 1). The city has reported more than 90 incidents in the past three years where parts of the façades fell off. It is expected that more and more façade defects and incidents of falling objects from heights will incur, leading to serious public safety issues [2]. As such, structural health monitoring is becoming an indispensable inspection task during façade condition assessment as falling objects from tall buildings can cause potential damage to the public and trigger structural safety considerations [3]. Periodic monitoring and building inspections are necessary to rationally secure the safety of building components [4]. This leads to the necessity of new knowledge about the types and characteristics of façade falling objects, the critical factors affecting the falling and the effectiveness of various inspection techniques.

The current practice relies on visual inspection by certified inspectors. The surface defects detected during each inspection are documented by photos and sketches. As such, the conventional inspection practices are insufficient to holistically understand the building condition at the reviewing stage. To resolve this problem, researchers have leveraged unmanned aerial vehicles (UAVs) to support automatic visual inspection [5]. Since UAVs have relatively lower payloads, unmanned ground vehicles (UGVs) or ground robots were more easily stabilised to carry advanced sensing devices such as Light Detection and Ranging (LiDAR) laser scanners for point cloud acquisition [6]. Such 2D images or 3D point clouds were further used to identify building defects and analyse potential damages. This included the identification of concrete spalling defects using laser scanning [7], concrete surface defect quantification with UAV-based laser point clouds [8] and change detection and deformation monitoring [9]. For instance, image data obtained with UAVs were used to detect different types of concrete cracks on buildings [10]. The use of infrared thermography to capture delamination defects before crack formation was also investigated ^[11][12][11,12]. Recent studies have also focused on using point clouds for quantifying building defects [13]. Furthermore, image-based 3D reconstruction was explored to support building condition evaluation and damage assessment [14].

2. Façade Defects and Inspection Practices

2.1. Types of Façade Defects and Anomalies

The serviceability of the building façade is affected by the physical property of the building materials as well as the exposed environment. Table 1 summarises the common defects and anomalies from different types of façades which potentially cause falling objects from tall buildings. The typical problems highlighted include cracking, water penetration, misalignment, discolouration, efflorescence, corrosion, etc. Concrete is one of the most common construction materials for building façades, in which case cracking, spalling, biological growth, drying shrinkage and delamination are typical surface defects that cause falling objects. The localisation and quantification of concrete cracking and spalling defects have been studied with various sensing techniques ^[7][10][7,10]. Other types of façade materials include brick masonry, plaster and tiling, which would lead to falling objects. In particular, their defects such as cracking, rising dampness, biological growth, efflorescence and delamination are common in tropical climates with high temperate and humidity ^[15][17]. However, a study on the design and maintenance at the outset during the planning stage for façade components is still lacking in the literature. One other potentially high fatal falling object is cladding. This involves stone cladding, metal cladding and glass cladding. The main reason for falling includes damage and cracking on the façade materials, joint or connection failures and the inadequate design and maintenance of the support system. Investigations showed that casement windows constitute 80% of the fallen windows because of the corrosion of aluminium rivets, as well as improper design, installation and maintenance [2]. As such, there is a research need to improve the identification and classification of common façade defects and anomalies.

2.2. Overview of Façade Inspection Practices and Regulations

Table 2 shows the relevant global standards and legislations worldwide for façade inspection. Chicago’s (US) Department of Buildings ^[16][18] requires frequent inspections between 4 and 12 years for high-rise exterior walls and enclosures for buildings 80 feet tall and higher. The consideration of building service life relating to maintainability is incorporated into the inspection standards and protocols. For example, Cincinnati’s (US) General Inspection Programs ^[17][19] require an inspection schedule of 8 or 12 years for buildings with five stories and that are 15 years old. Likewise, buildings of five or more stories must be inspected every 5 years in compliance with Quebec’s (Canada) Safety Code from the Building Act ^[18][20]. In general, buildings with more than five stories or that are more than 75–80 feet tall require a regular inspection schedule of 4–12 years. Such inspection applies to buildings varying from 15 to 30 years old. Façade inspection consists of two stages. The first stage is to assess the general condition of the building under inspection. Visual aids such as binocular cameras and infrared thermography cameras mounted on a drone ^[10][19][20][10,21,22] are some of the methods used for inspection. Specifically, it involves a visual inspection of the entire façade area to detect anomalies for the entire building from the ground level. To streamline the management of UAV-collected information, the aerial images are integrated with a geographic information system (GIS) ^[21][23] or building information modelling (BIM) ^[22][24] to support the automated detection of the dilapidation of façade elements. The retrieval and analysis of the images are performed for detecting and documenting façade anomalies. Airborne images are processed with different image processing and detection algorithms, from which the surface detections of buildings (such as concrete cracks) are extracted and identified [10]. Following the visual inspection, the second stage emphasises the hands-on inspection of each elevation. In practice, at least a 10% inspection shall be conducted for each building face ^[23][25]. This requires the application of non-destructive and destructive tests to examine the severity of the defects and anomalies ^[24][26]. The inspection may include different kinds of measures (such as tapping, the partial removal of façade elements and material testing). Recommendations of remedial and maintenance measures shall then be provided based on the evaluation of façade elements.