3.12 Identification and Reinforcement of Existing Building Structures
3.12.1 For the seismic fortification of new types of educational institutions, such as training institutions, community clinics, rehabilitation centers, and specialized outpatient clinics, similar to middle schools, primary schools, kindergartens, and hospitals, should the seismic fortification standards be raised? Can we refer to the practices in Suzhou and treat these renovated educational institutions, community hospitals, or elderly care institutions as Class C buildings?
Answer: New types of educational institutions, community clinics, rehabilitation centers, and specialized outpatient clinics do not belong to newly constructed middle schools, primary schools, kindergartens, or hospitals, and therefore do not need to raise the seismic fortification standards. According to local regulations in Hubei Province, the seismic fortification category for educational institutions, community hospitals, or elderly care institutions transformed from existing buildings should follow the seismic fortification categories specified in current regulations.
3.12.2 In partial renovation projects, to what extent do modifications to beams, slabs, columns, load-bearing walls, stairs, and other load-bearing components constitute a change in the main structure? Besides the modifications to load-bearing components such as beams, slabs, columns, and load-bearing walls, does the demolition, addition, or relocation of partial walls, as well as the addition of a bathroom that increases floor load, constitute a change in the main structure?
Answer: The main structure typically refers to the system that receives, bears, and transmits all upper loads of the construction project, maintaining the integrity, stability, and safety of the upper structure, including structural components such as slabs, beams, columns, and load-bearing walls. Modifications to main structural components are recognized as changes to the main structure, including changes to the original design load transfer paths, methods, or changes to the original design’s integrity, stability, and safety.
Based on the above definitions, the demolition, addition, or relocation of self-supporting walls, as well as the increase in floor load, although not directly modifying structural components, may reduce the safety of structural components and may require reinforcement. Therefore, a structural review should be conducted, and based on the review results, it should be determined whether it constitutes a change in the main structure.
3.12.3 Does the addition of an elevator to an old building require an assessment of its relationship with the original main structure?
Answer: According to the “General Specification for Maintenance and Renovation of Existing Buildings” GB 55022-2021, Article 5.2.7 and Article 5.3.7, the addition of an elevator to an old building falls under the category of renovation of existing buildings. According to Article 3.1.4 of GB 55022-2021, an assessment should be conducted before the renovation of existing buildings.
3.12.4 How should structural assessment and testing be conducted when there are missing or distorted records for existing buildings? What should be done when testing partial components is difficult?
Answer: The “General Specification for Assessment and Reinforcement of Existing Buildings” GB 55021-2021, Article 3.1.2, states that when the engineering drawings and records of existing buildings are incomplete or distorted, a detailed on-site verification and testing should be conducted. Article 3.1.3 of GB 55021-2021 specifies the content of assessment, investigation, and testing. When testing partial components is difficult or inaccurate, special methods may be used for testing, but timely repairs should be made to ensure safety.
3.12.5 The “General Specification for Assessment and Reinforcement of Existing Buildings” GB 55021-2021, Article 5.4.1, requires that the assessment of seismic measures for existing buildings should be verified according to the subsequent working years, based on building structure type, fortification intensity, site category, etc. Should this assessment be based on the original function of the existing building or the function after renovation?
Answer: The seismic assessment involves the function of the existing building after renovation, and the assessment should consider the function of the building after renovation.
3.12.6 For projects that have passed the main acceptance but have been suspended for many years and are being continued, with the main usage function unchanged but with partial layout adjustments, should the seismic design be reviewed according to current standards or the standards at the time of project implementation?
Answer: When it is indeed difficult to follow current standards, the seismic design can be reviewed according to the standards at the time of project implementation, but the reasonable working (usage) life of the structure after completion should be clearly defined. Additionally, the areas of partial adjustment should consider the actual situation regarding live loads and their combination values, frequency values, and quasi-permanent values, and should follow current standards for value selection. Refer to the “General Specification for Engineering Structures” GB 55001-2021, preface: “For renovation projects of existing buildings (referring to those that do not change the existing usage function), when conditions are not met and it is indeed difficult to follow current standards, it should not be lower than the standards at the time of original construction.”
3.12.7 Regarding the reinforcement of existing buildings, for example, if a building originally classified as Class C for seismic fortification and Grade IV for seismic resistance has recently been completed, but later due to usage requirements is converted into a school, raising its fortification category to Class B and seismic grade to Grade III, the question is, if the original stirrup diameter of the frame column is 6mm, and the current Grade III requirement is for a stirrup diameter of 8mm, does this mean that all columns need to be reinforced if only the stirrup diameter does not meet the requirement, given the widespread nature of this situation?
Answer: A recently completed building should be classified as a Class C building. For Class C buildings, when the construction does not meet current standards, it should generally be reinforced according to current standards. If necessary, performance-based seismic design may be adopted.
3.12.8 Under what circumstances is it sufficient to conduct only a safety assessment, and under what circumstances is it necessary to conduct both safety and seismic assessments?
Answer: The “General Specification for Assessment and Reinforcement of Existing Buildings” GB 55021-2021, Article 2.0.2, clearly states the circumstances under which assessments should be conducted. Article 2.0.4 of GB 55021-2021 stipulates that both safety and seismic assessments should be conducted simultaneously. If the assessment is conducted only for partial renovations and does not involve structural seismic performance issues (including regulatory requirements), a seismic assessment may not be necessary; however, if the renovation involves structural seismic performance, a seismic assessment is required.
3.12.9 The “General Specification for Assessment and Reinforcement of Existing Buildings” GB 55021-2021, Article 5.1.2, states that the choice of subsequent working years should not be less than the remaining design working years; while Article 5.1.4 states that when it is technically difficult to execute according to current standards, it is allowed to lower the subsequent working years and handle it according to the requirements for Class B buildings. Are these two articles contradictory? How should they be implemented?
Answer: Article 5.1.2 provides a principle that the choice of subsequent working years should not be less than the remaining design working years and encourages the adoption of longer subsequent working years. Article 5.1.4 proposes more specific seismic assessment standards for Class A, B, and C buildings. When it is difficult to execute according to current standards, it is allowed to lower the subsequent working years. These two articles are not contradictory. Refer to the corresponding article explanation: the choice of subsequent working years should not be less than the remaining design working years and encourages the adoption of longer subsequent working years; based on subsequent working years, existing buildings are classified into Class A, B, and C; from the perspective of having the same exceedance probability within the subsequent working years, corresponding seismic assessment standards are proposed for Class A, B, and C buildings, and it is encouraged to adopt higher standards when conditions permit to improve the seismic capacity of existing buildings as much as possible.
3.12.10 Regarding the “General Specification for Engineering Structures” GB 55001-2021, which mentions “not changing the existing usage function,” if the overall usage nature of a building remains unchanged but only partial rooms change their usage function, does this situation fall under “not changing the existing usage function,” requiring only the changed function areas to be reviewed according to current standards, while other areas remain unchanged?
Answer: Even if the overall usage nature of the building remains unchanged, if the usage function of partial rooms changes and renovations are implemented, it does not fall under “not changing the existing usage function.” Therefore, the design for the partial room renovation should follow current standards. When reviewing the structural impact on other areas due to partial renovations, since their existing usage function remains unchanged, they can be reviewed according to not lower than the original construction standards.
3.12.11 Can partial renovation projects only conduct partial safety assessments? Is it still necessary to conduct seismic assessments?
Answer: Partial renovation projects can conduct partial safety assessments, and the scope of the assessment should be jointly determined by the assessment unit and the design unit based on the project’s situation. If the partial renovation involves structural seismic issues and requires understanding the seismic performance of the existing building, a seismic assessment is necessary. At the same time, the following conditions must be met for structural modifications to be defined as partial renovations, taking the modified range and its related structural components for assessment (reliability evaluation) and reinforcement design:
1) The renovation does not extend the structural design working years;
2) The renovation does not alter the original structural lateral force-resisting components, having a minor impact on the overall structural safety;
3) The building’s engineering data is generally complete and reliable;
4) Normal use in the early stages, regular inspections, no changes in usage conditions or functions, and no modifications that reduce structural performance.
3.12.12 According to the “General Specification for Assessment and Reinforcement of Existing Buildings” GB 55021-2021, Article 4.2.2, when assessing the safety of the original structure or components for the purpose of structural reinforcement, changing usage, or extending working years, calculations should be performed according to current standards. Due to the change in partial coefficients for dead load and live load from 1.2 and 1.4 to 1.3 and 1.5, and the seismic partial coefficient from 1.3 to 1.4, this leads to the need for reinforcement of the entire building when assessing and reinforcing buildings designed under previous standards, which is excessively costly. How should this issue be viewed?
Answer: First, calculations should be performed according to the “General Specification for Assessment and Reinforcement of Existing Buildings” GB 55021-2021, Article 4.2.2. Secondly, whether all buildings and structural components need reinforcement should be determined based on safety assessment ratings and other assessment results. In specific engineering design, for existing buildings designed after the implementation of the 89 standard, without extending the subsequent usage years, the following requirements may be followed:
1) During safety assessments, gravity loads and partial coefficients may be taken according to not lower than the design standards at the time of construction. For components that require reinforcement after assessment, load values should follow current standards.
2) Without changing the usage nature or function, and without altering the lateral force-resisting structural system, seismic assessments may be conducted according to not lower than the design standards at the time of construction. When the fortification category and seismic motion parameters change, current design standards should be followed. Partial coefficients and seismic measures should not be lower than the design standards at the time of construction.
3.12.13 Regarding the “Building Seismic Design Standards” GB/T 50011-2010 (2024 Edition), Article 8.1.5, when adding steel structure elevators to old buildings exceeding 28m (residential) and 24m (other civil buildings), can single-span frames be used? When the new elevator steel structure extends steel beams and connects with the existing brick-concrete structure, does this constitute the mixing of different structural types?
Answer: When adding steel structure elevators to old buildings exceeding 28m (residential) and 24m (other civil buildings), single-span frame structures should not be used. When the new elevator steel structure extends steel beams and connects with the existing brick-concrete structure, it should not be recognized as mixing different structural types.
3.12.14 Regarding the definition of “existing buildings,” the general regulation explains it as “buildings that have been completed and can be accepted and have been put into use.” Does “completed” refer to the main frame or the entire building? Does “can be accepted” refer to the main acceptance or the completion acceptance?
Answer: “Existing buildings” are different from “unfinished buildings” or “buildings under construction.” “Put into use” refers to buildings that are currently in service, while “completed and can be accepted” means that they meet the conditions for service. Therefore, “completed” refers to the entire building being completed, and “can be accepted” refers to completion acceptance. The “General Specification for Assessment and Reinforcement of Existing Buildings” GB 55021-2021, Article 2.0.1, states: “Existing buildings should undergo regular safety inspections, … safety inspections are part of the daily management of buildings.” This indicates that existing buildings do not apply only to the stage of main completion.
3.12.15 Currently, assessment reports are not required to be reviewed. When the content of the assessment report provided by the construction party does not fully meet the design specification requirements, is it necessary to require a new assessment report?
Answer: If the project involves reinforcement and renovation, the construction party generally determines the design unit in advance and then determines the assessment and testing unit. At this time, the design unit should propose corresponding requirements. If the assessment report does not meet the design specification requirements, the design unit should require a new assessment report. If the review agency believes that the assessment report does not meet the design specification requirements, it may also require a new assessment report. According to the “Regulations on the Management of Dangerous Buildings in Cities” (issued by the Ministry of Construction of the People’s Republic of China on November 21, 1989, Order No. 4, and amended by Order No. 129 on July 20, 2004), Article 11 states: Safety assessments of buildings should use unified terminology, fill out assessment documents, and propose handling opinions. If a building is assessed as dangerous, the assessment agency must promptly issue a notice of dangerous building; if it is a non-dangerous building, it should indicate the effective time limit under normal usage conditions in the assessment document, generally not exceeding one year.
3.12.16 In Article 4.2.2, Clause 2 of the “General Specification for Assessment and Reinforcement of Existing Buildings” GB 55021-2021, the term “changing usage” can sometimes be difficult to control. If the load decreases or remains unchanged, does this situation fall under not changing usage?
Answer: Generally, a decrease or unchanged load does not constitute a change in usage. When a decrease in load adversely affects the structure after renovation, regardless of whether it constitutes a change in usage, structural safety calculations are required.
3.12.17 Regarding the “Building Seismic Assessment Standards” GB 50023-2009, Article 1.0.5, existing building seismic assessments are classified into three categories: Class A, B, and C, based on subsequent usage years, where Class A corresponds to buildings before the 89 version standard, Class B corresponds to buildings before the 2001 version standard, and Class C corresponds to buildings before the 2010 version standard. However, this standard has been in effect since 2009, and it has been 15 years. Is it appropriate to assess seismic performance according to this standard?
Answer: The “General Specification for Assessment and Reinforcement of Existing Buildings” GB 55021-2021, Article 5.1.3, has clearly defined the subsequent working years for Class A, B, and C buildings. When the seismic assessment requirements conflict with current general specifications, the current general specifications should prevail.
3.12.18 Regarding the “Classification Standards for Seismic Fortification of Construction Projects” GB 50223-2008, Article 4.0.3, when the function of an existing building is changed to a school (such as an elderly university, continuing education training school, vocational school, or children’s training school), although it is not a primary or secondary school or kindergarten, it has a dense flow of people and some are protected objects. Should the seismic fortification category be raised?
Answer: According to the “Regulations on Seismic Management of Construction Projects” (State Council Order No. 744), schools, kindergartens, hospitals, elderly care institutions, children’s welfare institutions, emergency command centers, emergency shelters, and broadcasting and television buildings should adopt seismic fortification measures according to no less than key fortification category requirements. The scale, grade, and scope used to classify seismic fortification categories vary by industry. According to the “General Specification for Civil Buildings” GB 55031-2022, Table 1 (Classification of Civil Buildings), elderly universities, continuing education training schools, and children’s training schools are classified as training places in the commercial service category (C-5 category) and are not included in the building categories listed in the above regulations, so their seismic fortification category generally does not need to be raised; however, vocational schools belong to the education category of secondary vocational education places (A-3 category) and are included in the building categories listed in the above regulations, so their seismic fortification category generally needs to be raised. It should be noted that vocational schools that are transformed may not be conventional schools, and if determined to be small-scale training institutions, the seismic fortification category may not need to be raised.