1 overview
As we all know, the reinforcement of building structure is a comprehensive project, involving a wide range, restricted by many factors, with uncertainty and complexity. As far as existing buildings are concerned, there are old buildings before the founding of the People's Republic of China and buildings in different times after the founding of the People's Republic of China. Because of the different design specifications and calculation methods adopted in different periods, the reliability is also different. If the differences of construction quality in different periods are taken into account, the actual reliability will be even different. In addition, the reliability of existing buildings changes with their service life. Since the 1980s, with the rapid development of China's market economy, a large number of urban construction projects have been completed, with numerous high-rise buildings and vigorous development of the construction industry. The relevant government departments and the public urgently demand that the target safe service life of the building structure, that is, the design service life of the structure, be defined to standardize the construction market and the temporary structure of the building for 5 years; Easy-to-replace structural parts are 25 years; Ordinary houses and structures are 50 years; Memorial buildings and particularly important building structures are 100.
Defining the design service life is the service life specified in the design. Within this specified period, the intended use can be achieved only through normal maintenance without major overhaul, and the intended function can be completed, that is, the service life of the building should be achieved under normal design, normal construction, normal use and maintenance. At the same time, the Unified Standard also stipulates that the design reference period is 50 years. In addition, (Load Code for Building Structures (GB50009-200 1) (hereinafter referred to as "Load Code") and various structural design codes all abide by the "Unified Standard", and the design service life of their respective structures is given. The design service life stipulated in the Unified Standard is the first time in the history of building structure design in China, which is of great significance to standardize the management of building market, the safe and rational use of building structures and the sustainable and healthy development of the construction industry. However, due to various reasons, there are still problems in the safe and rational use of the strengthened building structure and the proper management of the next target use. Therefore, for new projects, the current Unified Standard for Reliability Design of Building Structures (GB50068-200 1) (hereinafter referred to as the Unified Standard) draws lessons from the General Principles of Structural Reliability (ISO2394: 1998), and stipulates the design service life of different types of structures with mandatory provisions, that is, how to determine the structural reinforcement. This is an important problem to be studied and solved urgently in the steel bar industry.
2 discussion
According to the preliminary statistics of relevant departments, the total number of existing buildings (including urban residential buildings and industrial buildings) in China has exceeded 40 billion ㎡, and more than 2 billion new buildings are added every year. According to some experts' estimates, about 30%~50% of buildings have safety failure or entered a period of functional degradation. Statistics also show that about 2.5 billion ㎡ buildings are in urgent need of identification and reinforcement. Both the former and the latter show the huge workload of reinforcement. However, under such a heavy reinforcement project, the inexperienced designers account for a large proportion in the structural reinforcement design team, which leads to the occurrence of reinforcement engineering mistakes from time to time, and the service life of structural reinforcement design is even more confused, which is difficult to grasp correctly. The design service life of reinforced building structure is indeed a major problem to be solved urgently in the current reinforced industry.
Fortunately, Article 3. 1.7 of the recently promulgated national standard "Code for Design of Reinforcement of Concrete Structures (GB50367-20 13)" (hereinafter referred to as "Code for Reinforcement of Concrete Structures") stipulates that the design service life of reinforcement of concrete structures shall be determined according to the following principles:
1) The service life of the reinforced structure shall be agreed by the owner and the design unit.
2) When the reinforcement material of the structure contains synthetic resin or other polymer components, the service life of the reinforced structure should be considered as 30 years; When the owner requires the service life of the reinforced structure to be 50 years, the bonding performance of the glue and polymer used should pass the long-term stress resistance test.
3) After the service life expires, the service life of the structure can still be extended when the reliability appraisal once again considers that the structure works normally.
4) For structures and components reinforced by gluing or mixed with polymers, their working conditions should be checked regularly. The inspection interval can be determined by the design unit, but the first inspection time should not be later than 10 year.
5) When strengthening locally, the influence of the remaining design service life of the original building on the design service life after structural reinforcement should be considered.
However, this provision seems to be too principled, and it has not taken into account how to treat various complicated situations of structural reinforcement differently.
We believe that the following points deserve further discussion:
1) service life of the reinforced structure. The residual reliability of existing buildings changes with the length of service life, and the reinforcement is too thick considering 30 years.
2) The reinforcement of existing buildings caused by adding stories or rebuilding is quite different from that required by general structural members. The former should consider how to coordinate the service life of old and new parts; The latter is relatively simple.
3) What if the government planning department or the owner requires that the service life after reinforcement is less than 30 years?
4) The standard selection of variable action, namely floor live load, wind load and snow load, should be considered. Because the Load Code is based on the design reference period of 50 years, and the reinforced structure is based on the next target service life after reinforcement (such as 30 years), the two should be obviously different. In fact, the performance values of reinforced materials are also different.
5) Emergency reinforcement caused by some accidents, such as explosion, fire and impact, should also be treated. , or some dangerous buildings that have been sentenced to death with a suspended execution, or are in urgent need of temporary reinforcement (we have been responsible for handling many projects in this area).
6) For some inexperienced reinforcement technologies, such as using adhesives or other polymers, there are inevitably problems such as aging resistance, which are very concerned by the owners and designers. Whether the performance of the strengthened structure is the same as that of the original structure needs further study.
Three suggestions
We believe that the above problems must be considered when determining the service life of the reinforcement design of the structure, and they should be as detailed as possible so that the owners and designers can correctly grasp and deal with them. For this reason, we can consider grading treatment: that is, it can be divided into four grades: emergency repair reinforcement, temporary reinforcement, short-term reinforcement and long-term reinforcement, and its design service life should be stipulated accordingly.
1. Emergency repair and reinforcement
Some buildings have been identified as dangerous buildings (whole or part) for various reasons, but they cannot be demolished and rebuilt immediately due to special reasons, which is called "suspended death sentence". Some buildings have been partially damaged due to accidents such as explosions and fires, and must be repaired and strengthened immediately. It is suggested that the service life after emergency repair and reinforcement should generally not exceed 1 year. This situation should be checked frequently after reinforcement (for example, once every three months) to prevent the danger from happening again.
2. Temporary reinforcement
In order to coordinate with the "unified standard", when some temporary structures or existing building structures are damaged by fire, earthquake and explosion, temporary reinforcement can be adopted. And the owner requires emergency reinforcement, and then it will be treated after long-term consideration. It is suggested that the design service life of temporary reinforcement is generally not more than 5 years, and it should be checked once a year after reinforcement; After the expiration, if the re-conducted reliability appraisal thinks that the structure works normally, its service life can still be appropriately extended.
3. Short-term reinforcement
Short-term reinforcement is generally applicable to buildings that have been in service for about 30 years and need to change their functions (including adding stories and renovating), or owners only need to consider demolition and renovation within this period, and the city planning department has requirements.
It is suggested that the design service life of short-term steel bars should generally not exceed 25 years. This is also consistent with the 25-year period for easily replaceable structural members stipulated in the "Unified Standard". It must be pointed out that the variable load value used in short-term reinforcement design should be reduced on the basis of the current Load Code and then multiplied by the reduction factor with the next target service life less than 1.0.
4. Long-term reinforcement
Long-term reinforcement is generally applicable to the reinforcement of buildings that have not been in service for a long time or have not been in service for more than 20 years for various reasons. Generally speaking, the design service life of its steel bars is 30~50 years or longer. In addition, the reinforcement required for the project under construction should be regarded as a new project, that is, 50 years. At this point, the reinforcement design should comply with the relevant provisions of the Code for Concrete Reinforcement.
It should be pointed out that for some commemorative buildings and particularly important building structures, the design service life can be agreed to be more than 50 years, and if the owners think their buildings are of great preservation value, the reinforcement cost is not a problem. In this way, in the reinforcement design, the variable load value should be multiplied by the load expansion coefficient greater than 1.0, and the material strength value should be subtracted by the reduction coefficient less than 1.0, except in accordance with relevant regulations. The expansion coefficient and reduction coefficient should be determined according to the agreed actual design service life, the probability distribution specified in the unified standard and the corresponding statistical coefficient.
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