Discussion on the safety of the hottest concrete s

2022-10-20
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Discussion on the safety of concrete structures

Abstract

the safety of concrete structures depends on human factors such as design, construction and daily maintenance, as well as natural factors such as natural disasters. Improving the safety setting level of structural design, perfecting the design standard of structural durability, and strengthening the safety inspection in the use stage are effective ways and methods to improve the safety of concrete structures

key words: concrete structure safety durability

1 Introduction

the primary function of concrete structure is to bear the self weight of the structure and external loads (static and dynamic loads), which is usually composed of concrete and reinforcement placed therein. The reinforcement in the concrete structure mainly bears the tension and gives the structure ductility to compensate for the defects of low tensile capacity, easy cracking and brittle fracture of concrete; The concrete mainly bears pressure and protects its internal reinforcement from corrosion. Both of them work together to play their structural functions

the safety of concrete structure is the ability of the structure and its components to prevent damage and collapse under the action of external forces, and to protect personnel and equipment from damage. For a period of time, the safety of concrete structures has been widely concerned by the society and the government due to the frequent occurrence of concrete structure safety and quality accidents. Analyze the safety status of concrete structures, seek the existing problems, gaps and root causes, explore the ways, methods and countermeasures that have opened a new era of domestic injection molding machines, and provide suggestions for revising or formulating relevant technical policies, with a view to the safety of concrete structures

2 concrete structure safety accidents and their causes

the safety of concrete structures depends on the normal maintenance and repair in the process of design, construction and use, and is also related to the technical level and professional quality of engineering technicians and managers. The design and construction depend on the safety setting level specified in the laws, regulations and specifications of the project. The design safety factor is low, the construction quality is poor, and the bearing capacity and overall firmness of the concrete structure are congenitally deficient, which lays hidden dangers for the safety accidents of the concrete structure; The maintenance and repair are not timely, and the structural safety cannot be guaranteed. In China, safety accidents often occur due to design, construction, maintenance and other reasons. In addition, safety accidents caused by natural disasters occur frequently, causing huge losses to the country and people's lives and property. The reasons are mainly shown in the following aspects:

2.1 there are differences in the current technical standards, and the setting level of safety factor is low

according to the reliability design method adopted by China's current structural design codes, the reliability of structural safety is defined as the strength assurance rate under the "specified" load, which does not reflect the great differences in the standard value of load in different design codes, nor the differences in the overall structure. The structural reliability in the design code is only for the members of the structure, and the safety of the members largely depends on the value of the load. The setting level of the safety factor in the design is related to the value of the load factor. According to relevant data, the safety factor of dynamic load in China's code is 14% - 21% lower than that in the United States, Britain and other countries, and 7% lower than that in the European code; The static load safety factor is 17% lower than that of the United States, Britain and other countries, and 13% lower than that of Europe; In terms of strength safety factor, the safety factor of concrete strength in China's specifications is 15% lower than that in Europe and the United States, and the safety factor of steel strength is 6%. In this way, the dynamic load and static load ratio of the column designed according to the Chinese code is assumed to be 1:2. Due to the influence of load and material, its bearing capacity is 35% lower than that of the column designed according to the American and British code. The control mode: computer control is 28% lower than that in Europe; The safety factor of beams and slabs is 24% lower than that of the United States and Britain and 18% lower than that of Europe

2.2 human errors or errors in design and construction

structural safety accidents caused by design errors, plastic bending machine detection methods or errors are mainly caused by insufficient bearing capacity of structures or components, resulting in structural cracking and structural collapse. However, the vast majority of human errors or errors are not subjective, intentional and malicious. Design errors or errors often occur due to poor quality of designers, poor design ability, careless investigation and research, incomplete basic data, unreasonable selection of design parameters, poor calculation ability, and lack of design experience; Secondly, due to administrative intervention, taking charge of the will, the lack of a reasonable design cycle; Again, the whole process supervision of survey and design has not been fully carried out. Even if there is design supervision, it is a mere formality and lax

a considerable part of concrete structure safety accidents are caused by poor construction quality. Under the current bidding management system of engineering project construction in China, there are more or less high-quality bid winning and low-capacity construction. The project is subcontracted layer by layer, and the construction materials are shoddy, Jerry built, laying a major hidden danger for the project quality. At the same time, due to the low level of construction management and the low quality of employees, it is also a prominent problem in China's construction industry. It is difficult to find and effectively eliminate human errors in time, which eventually leads to safety accidents

2.3 fire and explosion accidents

fire and explosion may lead to disastrous consequences. The explosion in Shijiazhuang, which shocked the whole country in 2001, caused the continuous collapse of all units of the whole house; The 113 major fire accident in Hengyang caused the collapse of Hengyang Building and heavy casualties. These are all due to the reduction of the compressive strength of concrete and the tensile strength of reinforcement after the fire of concrete structure, resulting in the reduction of the bearing capacity of the structure. Especially in the explosion, the instantaneous huge impact force far exceeds the ultimate bearing capacity of the structure, causing the instantaneous destruction of the structure

2.4 natural disasters

natural disasters refer to natural phenomena such as earthquakes, landslides, mudslides, hurricanes, floods and other force majeure, which will cause catastrophic damage once they occur. According to the survey and statistics, the number of structural collapses caused by global natural disasters is quite amazing, and the losses caused are immeasurable. In the 1976 Tangshan earthquake with a magnitude of 7.8, 80% of buildings collapsed or suffered devastating damage; This summer, typhoon Yunna hit Wenling, Zhejiang Province, causing economic losses of hundreds of millions of yuan, as well as heavy rains in Chongqing and Eastern Sichuan Province, and floods caused damage to a large number of houses and transportation facilities

but it is unrealistic, impossible and unnecessary to completely avoid the damage of natural disasters to the structure. However, with the economic development, the enhancement of national strength, the increase of social wealth and the progress of science and technology, it is absolutely necessary to enhance the awareness of disaster prevention, improve the level of disaster prediction and the ability of structural disaster resistance, and take this as the focus of improving the quality of structural safety

2.5 the lack of a relatively complete system of regulations and standards and management mechanism explains the brand concept of "love" carried by the gold testing machine with action

there is a lack of legislative constraints in the management of engineering construction and use in China, paying attention to project construction and ignoring the daily maintenance and maintenance in the process of use; Pay attention to the investment of construction funds and ignore the setting of maintenance funds. There is a lack of mandatory measures for the maintenance and repair of structures. Some structures are damaged due to untimely maintenance or years of disrepair, which eventually leads to structural damage. Secondly, the design, construction, maintenance and repair technologies are not matched, too dependent on the role of technical specifications, and lack of more specific and detailed technical standards such as guidelines and construction methods. Thirdly, the misplaced understanding and management of norms are limited to the existing standards and norms and lack of innovation. With the progress of technology, the revision of norms and standards is not timely and lacks the ability to keep pace with the times

3 main methods and ways to improve the safety of concrete structures

3.1 improve the setting standard of the safety factor of concrete structures

improving the setting standard of the safety factor of structures is undoubtedly conducive to improving the safety of structures, but the construction cost increases. Reasonably setting the safety factor is not only conducive to improving the overall firmness of the structure, but also can adapt to the current economic bearing capacity of the government or enterprises. From the objective reality, we should examine the safety factor setting standards, strive to improve the ability of the structure to resist emergencies, improve the seismic and fortification levels, and enhance the overall firmness of the structure. For important parts and key places, the safety factor shall be set. It is necessary to reserve the design load grade and the bearing capacity of structures and components, and consider the possible changes of load grade and service function in the long run

3.2 improve the design standard of structural durability

structural durability refers to the ability of structures and components to maintain their functions. For a long time, the problem of durability has been concerned by structural engineers all over the world. In China, due to the historical conditions of shortage economy and planned economy for a long time, the current design specifications and construction specifications are mainly limited to the safety of structures and components under load. For the impact of material performance degradation caused by environmental effects in the long-term use of structures and components, they are placed in a relatively secondary or subordinate position, mainly due to insufficient understanding of durability, The problem of structural durability under environmental action is complex, and there are many uncertainties, unlike the structural safety which can be determined by loading test. Durability can only be predicted and estimated. However, with the engineering practice and the theoretical research of engineering technicians, it is possible to predict and estimate the durability close to the reality

3.3 strengthen the supervision of project quality in the construction process

the quality of the project is directly related to the safety and durability of the structure. Strengthening the supervision of project quality in the construction process is an important means to effectively curb structural safety accidents. The government and enterprises (owners) conscientiously perform the function of supervision with the attitude of being responsible for the country, society and the people; The construction unit shall strictly comply with the product quality law of the people's Republic of China and stand the test of history, society and users; Social supervisors should conscientiously perform their supervision duties, control the process, and prohibit the use of products with potential quality hazards

3.4 formulate a scientific and reasonable project construction period

a scientific and reasonable project construction period is the premise and foundation to ensure the quality of the project. However, in some specific project construction, the artificial administrative intervention, the will of the director in advance, and the blind pursuit of the construction period and progress, especially some political projects, have the wind of "great leap forward" in the rapid development of large-scale projects, and the lack of a reasonable construction period has buried hidden dangers for the safety and durability of structures. Now, under the guidance of the scientific concept of development, a scientific and reasonable construction period is formulated with a scientific attitude, a scientific way of thinking, and a scientific organizational behavior, creating preconditions for the safety and durability of structural components

3.5 strengthen the safety inspection in the process of use

because the safety of structures and components is related to the maintenance and repair in the process of design, construction and use, as well as the level and quality of engineering technicians and managers. In the construction of structural engineering in China, the technical level and personnel quality are relatively poor, the setting of safety factor is low, the quality assurance system and quality assurance system are not perfect, the supervision function of the government has not really been brought into play, and some projects have quality defects when they are delivered for use. Therefore, it is necessary to strengthen the safety inspection of structures and components in the process of use

through effective safety detection, problems can be found immediately, rectification measures can be taken to avoid accidents and minimize the incidence of safety accidents, so as to achieve the purpose of prevention first. The safety inspection plan shall be prepared in combination with the use of structures and components, equipped with inspection instruments and arranged for inspection

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