Safety and durability of civil structural engineering

1. Safety of civil structural engineering Structural safety is the ability of the structure to prevent damage and collapse, and is the most important quality indicator of structural engineering. The safety of structural engineering is mainly determined by the design and construction level of the structure, and is also related to the correct use (maintenance, inspection) of the structure, which in turn is related to the reasonable setting and application of civil engineering regulations and technical standards (specifications, procedures, regulations, etc.). 1． The safety setting level of my country's structural design specifications. For the design of structural engineering, the safety of the structure is mainly reflected in the safety of the load-bearing capacity of the structural components, the overall solidity of the structure, and the durability of the structure. The safety setting level of my country's design codes for civil structures such as buildings and bridges in these aspects is generally much lower than similar codes abroad. 1.1 The two factors that have the greatest relationship between the safety setting level of component load-bearing capacity and the safety level of structural components are: 1) The code stipulates how much load the structure needs to bear (load standard value). For example, for the same office building, our country's code has stipulated that the live load of the floor slab is 150 kilograms per square meter since 1959 (it has been determined that it will be changed back to 200 kilograms in the new code), while the United States and Britain are 240 and 250 kilograms. ； 2) The size of the load partial coefficient and the material strength partial coefficient specified in the specification. The former is a coefficient that amplifies the standard value of the load when calculating and determining the effect of the load on the structural component. The latter is a coefficient that reduces the standard value of the strength of the component material when calculating and determining the inherent load-bearing capacity of the structural component. These coefficients expressed in quantitative values ​​reflect the safety of structural components under a given standard load. They are called safety factors in the safety factor design method (such as my country's highway bridge and culvert structural design specifications) and reflect the need for safety reserves. ； In reliability design methods (such as my country's building structure design specifications), it is called a partial coefficient, which reflects a certain nominal failure probability or reliability index. The larger the safety factor or sub-factor, the higher the safety level. my country's building structure design regulations stipulate that the partial coefficients of live load and dead load (such as the structure's self-weight) are 1.4 and 1.2 respectively, while those in the United States are 1.7 and 1.4 respectively, and the United Kingdom's 1.6 and 1.4 ； In this way, when designing an office building according to Chinese specifications, the floor design load (the product of the standard load value and the load component factor) is only about 52% (considering live loads such as people and facilities) and 85% (dead load such as the structure's own weight) of the British and American standards. However, the load-bearing capacity of the components (related to the material strength component coefficient) used to determine the load during design is 10 to 15% higher than that of the British and American codes. Both of them reduce the safety level of component bearing capacity. The design codes of Japan and Germany are more conservative than those of the United Kingdom and the United States in some aspects. some developing * * The structural design is mostly based on developed * * standards, just as our country’s structural design methods before liberation and in the early days of the founding of the People’s Republic of China were based on American standards. As for China’s * * and Taiwan, which are still based on British and reference to American norms respectively. What needs to be explained here is that the standard values ​​of live load of other buildings are not as different from those abroad as they are in office buildings, apartments, and dormitories. Different materials and different types of structures have different international safety standards. For example, the gap in steel structures may be relatively small. The situation of highway bridge structures is also similar to that of house building structures. In addition to the vehicle-mounted standards, the load sub-item safety factor (my country's specification takes 1.4 for vehicle load, which is about 25% lower than the 1.75 of the internationally renowned American AASHTO specification) and material strength sub-item safety factors are both relatively low. Although the safety reserve set by my country's design specifications is low, the material consumption of some projects is higher than that of similar foreign projects. The main problem here is that the design is rigid and lacks innovation in structural plans, material selection, analysis and calculation, and structural construction. 1.2 The overall solidity of the structure In addition to the structural components having sufficient load-bearing capacity, the structure must also have overall solidity. The overall soundness of a structure is its ability to prevent local damage somewhere in the structure from causing widespread continuous damage and collapse, or in other words, the structure should not have damage consequences that are disproportionate to its cause. The overall robustness of the structure mainly relies on the structure's good ductility and necessary redundancy, which can be used to cope with disaster loads such as earthquakes and explosions or disaster consequences caused by human errors, and to reduce disaster losses. The huge casualties caused by the Tangshan earthquake are closely related to the lack of overall solidity of local housing structures. In 2001, a vicious explosion occurred in Shijiazhuang, causing damage to a residential building due to soil damage. * * The partial damage to the wall caused by the explosion actually led to the continuous collapse of the entire building, which is also a manifestation of the insufficient solidity of the house design. 1.3 Durability and safety of structures? The design and construction specifications of civil structures in my country focus on the structural strength requirements under various loads, while the durability requirements under the effects of environmental factors (such as atmospheric erosion such as dryness, humidity, freeze-thaw, and erosion by harmful chemical media in water and soil around the project) are relatively less considered. Structural safety accidents in concrete structures caused by corrosion of steel bars or corrosion of concrete are far more serious than the harm caused by low safety levels of the bearing capacity of structural components, so this issue must be paid special attention to. Some requirements related to durability stipulated in our country's specifications, such as the minimum thickness of the concrete protective layer to protect the steel bars from corrosion and the minimum strength grade of the concrete, are significantly lower than those in foreign specifications. Impairing the safety of a structure's load-bearing capacity is only one of the consequences of insufficient durability. ； Improving the safe installation level of the load-bearing capacity of structural members will also be beneficial to the durability and service life of the structure in some cases. 2. Different opinions on adjusting the level of structural safety settings. The low level of safety settings in my country's structural design specifications is related to the historical conditions of a long-term shortage economy and planning system after the founding of the People's Republic of my country. However, being able to adopt a lower safety level for civil construction structures basically met the production and life needs at that time, and it has gone through a long period of testing. This is a major achievement achieved by domestic civil engineering scientific and technological personnel after tremendous efforts. ； However, due to the low safety margin, the ability to withstand unexpected effects is relatively insufficient. If the safety setting level is appropriately improved, it will help reduce the frequency of accidents and improve the ability of the project to resist disasters. A large number of engineering safety accidents in China are mainly due to management failures. * * and bad intentions and serious human error. The current proposal to re-examine the safety level of structures is mainly based on changes in the objective situation. It is because the infrastructure construction we are currently engaged in must lay the foundation for future modernization and meet the needs of the development of people's production and living standards in the next few decades or hundreds of years. Some civil structures, such as commercial houses, must meet the needs of commodity attributes under market economy conditions. In recent years, China has organized several discussions on the setting level of building structural safety, and there are large differences of opinion on how to adjust it. These different opinions were also reflected in this science and technology forum.: 1) It is believed that the safety setting level of my country's current specifications is sufficient and has been proven by long-term practice, but there is no such experience abroad. This successful experience our country has gained must not be thrown away easily, and we cannot follow the high standards of the United Kingdom and the United States in terms of safety. ； If the safety level is too high, it is a waste. Except for individual adjustments, there is no need to change it in general. 2) It is believed that although the safety setting level of my country's norms is not high, under the full compliance with the relevant provisions of the standards and norms, that is, under the "three normal" conditions of normal design, normal construction and normal use, the vast majority of tens of billions of square meters of buildings built accordingly are still in safe use, indicating that the levels stipulated in these norms are still applicable ； However, the ideal "three normals" are difficult to achieve. At the same time, in order to narrow the gap with advanced international standards and in view of the need for sustainable development and improved durability, under market economic conditions where material supply conditions have improved, the safety setting level of structures should be appropriately improved. This increase can only be moderate because our country is still a developing country. * * . ????3) It is believed that my country’s standardized safety settings should be generally close to international standards and need to be significantly improved. This is because with the continuous improvement of my country's economic development and living standards, the consequences of risk losses caused by accidents in civil engineering projects, especially major infrastructure projects, will become more and more serious. The proportion of funds required to improve project safety in the cost of the entire project (especially construction projects) is now getting lower and lower, and the supply of materials is also very abundant. The low safety level in the past only adapted to the needs of the past shortage-based planned economy era, but it is by no means without risks. If the safety level of the standard was higher, some safety accidents that had occurred could have been avoided. This flaw in the standard was to a certain extent covered up by the formulation of the "three normals". Projects under construction must serve the future modern society and must have high safety standards. Low safety quality standards will be difficult to be recognized in future international competitions, even if the safety setting level of structural design can be improved to that of developed countries. * * Likewise, due to the overall poor quality of our construction, there will still be gaps in the safety of the structure. ?? ??3. The probabilistic reliability design method of structural design specifications has been used since 1984 * * Construction Committee and * * Since the Ministry of Construction promulgated the unified standards for building structure design, my country's building structure design specifications have abandoned the traditional multi-safety factor design method since the late 1980s, and have uniformly adopted the reliability design method based on probability theory. ； The structural design specifications of other engineering departments such as highways, railways, ports, and water conservancy are also undergoing or planning to undergo such changes. Our country's standardized reliability design method is implemented with reference to the corresponding international standard ISO2394 and through the efforts of domestic scientific and technological personnel. Applying reliability design methods to structural design specifications is generally regarded as a development trend in the international academic community, but there are different views within the engineering community. Despite the existence of ISO2394, few important or well-known structural design codes abroad have directly adopted the reliability design method. The multi-safety factor design method or the load resistance factor method is still used today. In our country, although the engineering design community has criticized the reliability design method in the building structure design specifications and the attempt to unify various structural design specifications in various industries in our country with the reliability method, the academic community is in favor and affirmation. However, from time to time, some people still question the applicability of the reliability method in design specifications. At this science and technology forum, the differences of opinions on the standard reliability method were more concentratedly reflected. Experts who hold positive opinions on my country's standardized reliability design method believe that this is a major scientific and technological progress. The probability definition of safety in the reliability method is clearer, more scientific, and more reasonable than the fixed value safety factor. Of course, the probabilistic reliability design method itself still has many flaws and needs to be further modified and improved. Those who hold the opposite opinion believe that structural design specifications are oriented to diverse types of complex groups, and the uncertainties and uncertainties that need to be considered in terms of safety are very complex and cannot be scientifically described or handled by "probability definitions from a statistical mathematical perspective". ； The practice of the normative reliability method in my country for more than ten years shows that it does not bring obvious effectiveness to the safety of structural design, but instead causes some confusion in the safety concept. ； For engineering and technical personnel, the safety of a structure becomes more unfathomable and unclear after being expressed by reliable indicators and false failure probabilities. It is not as good as the safety factor, which is a measurement method based on safety reserves that is more intuitive and convenient for handling the safety issues of specific projects. ； The reliability methods in current design specifications are very immature and have many fundamental flaws. ； They believe that the semi-probabilistic multi-safety factor method is more suitable for specifications, and they do not exclude that the results of reliability analysis can be used as a reference to be considered when comprehensively judging the reasonable value of the safety factor. 2. Durability of Civil Structural Engineering The durability of civil structural engineering is related to the service life of the project. It is the ability of the structure to maintain normal functions during the service period. This normal function includes the safety of the structure and the applicability of the structure, and is more reflected in the applicability. 1. Current Durability of Civil Structural Projects Most civil structures are constructed of concrete. The durability of concrete structures is a worldwide problem currently plaguing civil infrastructure projects. It is not unique to our country, but it has not yet caused problems in our country. * * The competent authorities and the majority of design and construction departments should pay sufficient attention to it. Concrete has long been thought to be a very durable material. Until the late 1970s, developed * * It was gradually discovered that the originally built infrastructure projects suffered premature damage under some circumstances. Concrete infrastructure projects and port projects in many U.S. cities deteriorate within two to three decades or even less after they were built. ； According to a 1998 estimate from the American Society of Civil Engineers, they need $1.3 trillion to deal with the problems of domestic infrastructure projects in the United States. Repairing and replacing concrete decks on highway bridges alone will cost $80 billion, and now the federal government * * The annual allocation for this purpose is only US$5-6 billion. Other data indicate that in the United States, one-quarter of the highway bridges in the United States that require load-limiting traffic due to corrosion of steel bars caused by deicing salts account for a quarter of the bridges in this situation. developed * * A large amount of scientific research funds have been invested in the durability of concrete structures and countermeasures have been actively taken. For example, in order to deal with deicing salt erosion and freeze-thaw damage on highway bridges in Ontario, Canada, the minimum thickness of the concrete protective layer of steel bars has gradually increased from 2.5cm in the 1950s to 4cm and 6c. m until 7cm after the 1980s, and the minimum grade of concrete strength also increased from C25 in the 1950s to C40 later. The bridge deck concrete did not require external air-entraining agents and no waterproof layer to require air-entraining and require the installation of advanced waterproofing membranes and the introduction of epoxy-coated steel bars. However, there are still no clear requirements for durability design of highway bridges in areas affected by salt freeze in my country. The requirements for concrete protective layer and strength are only 2.5cm and C25, which is consistent with the Canadian level mentioned above in the 1950s. A bridge designed according to this standard in China had to be partially demolished and rebuilt due to salt freeze erosion only 8 years after its completion. A survey conducted by my country's Ministry of Construction in the 1980s showed that most domestic industrial buildings require major repairs after 25 to 30 years of use, and the service life of buildings in harsh environments is only 15 to 20 years. The use environment of civil buildings and public buildings is relatively good and can generally last for more than 50 years. However, the service life of outdoor balconies, rain covers and other open-air components is usually only 30 to 40 years. The durability problem of infrastructure projects such as bridges and port construction is even more serious. Because the concrete protective layer of the steel bars is too thin and has poor compactness, corrosion of the steel bars and cracking of the concrete occur within a few years after the completion of many projects. Harbor terminals generally need major repairs due to cracking and spalling of concrete reinforcements after being used for about ten years. Due to the effects of deicing salt and freezing in winter, urban overpasses in the Beijing-Tianjin area have problems after more than ten years of use, and some have to limit their load, undergo major repairs or be demolished. Salt freeze also causes damage to concrete pavements. A high-grade highway in Northeast China suffered extensive erosion after just one winter. my country's railway tunnels use low-strength C15 concrete as lining material, which has poor compactness and impermeability, is not resistant to erosion by groundwater and locomotive exhaust gas, and suffers from serious cracking and leakage. ； A sample survey of tunnels under the jurisdiction of several railway bureaus showed that 50.4% were leaking, and 1/3 of them were leaking seriously, causing corrosion of rails and other accessories and leakage in electric traction sections, affecting normal operation. However, the railway tunnel design specifications promulgated in 1999 still failed to take appropriate countermeasures to the durability of tunnels, such as appropriately increasing the minimum strength level of concrete and incorporating chemical fibers into concrete. The seriousness and urgency of the durability problem lies in the fact that many of the projects we are building have still not learned a lot of painful international and domestic lessons and are repeating the same mistakes along the same old path. Some newly built overpasses and highway bridges in northern cities still do not have the necessary comprehensive measures to prevent freezing, thawing and salt damage in terms of material properties and structural construction. Even large-scale projects such as the Zhuhai Lotus Cross-Sea Bridge, which was put into operation in 2000, still use C30 concrete, which is not resistant to the alternating erosion of dry and wet seawater, and a 3-4cm thick protective layer in the splash zone for its main structure. Some experts estimate that the construction of my country's "big dry" infrastructure projects will * * It can still last another 20 years. Due to the neglect of durability, we will be greeted by "overhaul" for another 20 years. * * ,this * * It may not take long to come, and the cost will be doubled compared to the original investment in the construction of these projects. Reasons that further exacerbate the durability problem of concrete structures include: ????1) Due to quality inspection of concrete * The customary use of a single strength index as a measurement standard has led to the cement industry's inappropriate pursuit of cement strength, resulting in an increase in cement fineness and an increase in the proportion of early-strength mineral components, which are not conducive to the durability of concrete. Our country's cement quality inspection only requires that the strength should not be lower than the specified minimum allowable value, while foreign countries also require that the strength should not be higher than the specified maximum value. If the strength exceeds, it will be considered unqualified. This requirement is also conducive to the uniformity of cement product quality. 2) The construction unit inappropriately speeds up the construction progress, especially * * Inappropriate interference by administrative leadership in project progress. The durability quality of concrete especially requires a sufficient construction and curing period to ensure it. The concept that premature birth is harmful to life and health also applies to concrete. The so-called projects that build a road, build a bridge, or build a high-rise building in a few months, as well as rush-work-dedicated projects that have been widely publicized in the domestic media, are probably short-lived projects that are destined to spend more money on major repairs in the future. Those who complete the construction period specified in the contract ahead of schedule will be fined abroad, because it means that the quality of the project may be damaged. 3) The environment continues to deteriorate, such as waste gas and acid rain. The area of ​​acid rain in our country has exceeded 30% of the country's land. The current urgent need is to compile technical regulations on the durability design of infrastructure projects such as bridges, tunnels, and port engineering as soon as possible, and to revise and supplement the requirements for structural durability in the current specifications. The first thing that needs to be clarified is the design working life of various infrastructure projects. There must be service life requirements and demonstrations in the design documents of important projects. One of the important reasons why many projects currently under construction are still following the same path in terms of durability is that engineering design and construction technicians have no new basis to follow in terms of durability. What is even more serious is that some provisions in the current specifications are themselves harmful to durability. In order to improve the durability of concrete, the rational use of mineral admixtures such as fly ash and slag in concrete is an important technical means. Some foreign regulations even stipulate that admixtures such as fly ash must be added to concrete structures such as bridges. However, my country's railway concrete bridge and tunnel construction specifications still explicitly prohibit the use of such admixtures. In addition, there are still some outdated views that have been formed for a long time in the engineering and technical circles, causing resistance to improving the durability performance of concrete. For example, concerns over the impact on concrete strength prevent the use of air-entraining agents, which is a routine means of improving the durability and workability of concrete. ； For another example, it is hoped to increase the cement dosage to ensure the strength of concrete, while the lowest possible cement dosage should be an important way to improve the crack resistance and durability of concrete. Regarding the durability requirements of the revised specifications, the Technical Specifications for Anti-Corrosion of Port Engineering Concrete Structures promulgated by the Ministry of Transport in 2001 have set a good example for other civil engineering industries. On the one hand, we must refer to existing information and experience at home and abroad to prepare corresponding design and construction technical documents as soon as possible to meet urgent needs. On the other hand, we must arrange systematic research projects and increase support for durability research work. ； The durability of concrete structures is one of the most important frontier research fields in structural engineering disciplines in the world, but our country is quite lagging behind in this aspect. Research on the durability of concrete cannot be separated from specific conditions such as raw materials and environment. It needs to take into account the characteristics of the country and cannot completely rely on foreign research results. Paying attention to the durability of concrete structures is also a need for sustainable development. The raw materials such as cement, sand, and stone required for the production of concrete consume a large amount of land resources and destroy vegetation and river beds. The carbon dioxide emitted by cement production has accounted for 1/5 to 1/6 of the total emissions from human activities, and the amount of carbon dioxide emitted by my country ranks second in the world. Our country currently produces more than 500 million tons of cement every year, which is accompanied by an annual consumption of more than 2 billion cubic meters of sand and gravel. It is really unsustainable in the long run. Extending the service life of the structure means saving materials, and durable concrete should generally have a lower cement content and a higher mineral admixture (industrial waste) content, so durable concrete meets the needs of environmental protection. Internationally, the design working life of civil engineering projects such as bridges and tunnels is mostly 100 years, and some, such as the United Kingdom, are 120 years. Considering the huge economic losses and resource waste caused by insufficient durability, there has been an international trend in recent years to further extend the minimum working life of these projects. For example, it is proposed that bridges in urban environments should be at least 150 years old. This post was last written by zhangya * ong edited on 2009-3-25 09:31 ]