Existing buildings refer to buildings that have been completed and put into use, and these buildings generally pass the final acceptance inspection. The detection and evaluation of the safety of existing building structures generally require on-site review of structural layout and load conditions, material performance testing, crack damage detection, settlement deformation measurement, structural verification and analysis, evaluation of structural safety, and proposal of necessary reinforcement treatment suggestions. When the following situations occur, it is necessary to test and evaluate the safety of existing building structures, and the structural safety testing and evaluation should focus on different situations:
　　 1) When cracks, damages or tilting deformations occur in a house due to surveying, design, construction, use, and other reasons. In addition to evaluating structural safety and proposing treatment suggestions, such projects generally require analysis of the causes of damage, including investigation, design, construction, and use, to determine which link caused the existing damage and provide a basis for liability determination. Residential quality improvement and arbitration appraisal mostly belong to this type of project.
　　 2) When the house shows signs of deterioration or aging that affect safety or use during its designed service life due to material, environmental, and other reasons. For concrete structures, material factors may include active ingredients such as MgO in concrete aggregates, high alkali content in cement, poor cement stability, excessive Cl - in mixing water, etc. Environmental factors may include chemical substances, freeze-thaw cycles, excessive Cl -, etc. These factors may cause deterioration and aging signs such as concrete cracking, steel corrosion, chemical erosion, alkali aggregate reaction, freeze-thaw damage, etc. The main aging signs of steel structures are steel corrosion, the main aging signs of masonry structures are brick wall weathering, and the main aging signs of wooden structures are insect corrosion and decay. This type of structural safety testing and evaluation generally requires material and environmental analysis to identify the main causes of deterioration or aging, predict the degree of further deterioration or aging, and propose effective treatment measures and recommendations.
　　 3) When cracks, damages, or tilting deformation occur in the house due to the influence of adjacent projects. This type of structural safety testing and evaluation focuses on distinguishing whether the crack damage or tilting deformation of the inspected building is caused by the building itself or by the construction of adjacent foundation pits, evaluating the structural safety, and proposing reasonable treatment measures and suggestions. Due to the fact that such projects are often commissioned after damage or deformation occurs, there may have been conflicts between the parties involved, so there are also many courts commissioning arbitration appraisal projects.
　　 4) When the functional or local structural changes of the house have an impact on structural safety. During the use of a house, there may be changes in its functional use, such as the conversion of a factory building to an office building, an office building to a shopping mall, etc. Local structural changes such as opening door openings, floor openings, and beam and column extraction may also be necessary. These factors have an impact on the safety of the structure and require safety testing and evaluation. Structural components should be checked and evaluated according to the new functional use and structural layout, and the safety of the structure should be assessed. When there are significant changes in functionality and structure, seismic performance evaluation is still necessary.
　　 5) When the house continues to serve beyond its designed service life. Generally speaking, when a house continues to serve beyond its designed service life, it will show signs of durability aging to varying degrees, and its structural functions will deteriorate to different degrees. Testing and evaluation are required, with a focus on predicting the structural service life, setting the next target service life, and proposing durability treatment recommendations in addition to routine testing and evaluation. When an existing building is rated as a dangerous building according to relevant standards, the inspection report must be sent to the Shanghai Housing Inspection Center for technical review.