Focus on the earthquake in Ya 'an, Sichuan, and pray: Sichuan will hold Ya 'an and be strong.
Is the quality of the damaged school wall not up to standard?
—— Our reporter connected the civil engineering experts who are being evaluated in Lushan.
After the Lushan earthquake, some media photographed and released a large number of photos of the damaged walls of school buildings rebuilt after the Wenchuan earthquake in the disaster area, which caused many netizens to question that the quality of school buildings was not up to standard. Does the wall damage mean that the building quality is not up to standard? Why are school buildings that are said to be resistant to an earthquake of magnitude 8 destroyed by an earthquake of magnitude 7? What is the seismic fortification standard of buildings? To this end, the reporter interviewed Lu Xinzheng, a professor of civil engineering at Tsinghua University, and Li Yingmin, a professor of civil engineering at Chongqing University.
Is the wall cracking unqualified?
Only when the main structure is damaged to a certain extent can it be recognized as a dangerous building. Only cracks in non-bearing walls cannot be simply judged as quality problems.
On the morning of April 2 1, Lu Xinzheng, a professor from the Department of Civil Engineering of Tsinghua University, went to Lushan earthquake-stricken area in Sichuan with the first group of experts from the Ministry of Housing and Urban-Rural Development to conduct an emergency investigation on the safety of key buildings in the disaster area. Together with two other teachers in Tsinghua University, he completed the emergency assessment of all primary schools and public kindergartens in Lushan County, with a total area of about 22,000 square meters.
"At present, important progress has been made in the emergency assessment work, and the report has been reported to the Ministry of Housing and Urban-Rural Development and local construction departments." Lu Xinzheng told reporters that the damage of school buildings after the earthquake is related to earthquake load, earthquake intensity and local geological structure. The seismic quality of school buildings can not pass, mainly depending on whether the damage of school buildings occurs in the main structure. If there is no problem with the main structure and it meets the requirements of seismic fortification, it should be qualified. If there is a problem with the main structure, it needs to be further evaluated to see if there is a potential safety hazard.
Li Yingmin, a professor at Chongqing University, also said that you can't say that the house is damaged as soon as you see it. The key is to see where the damage or destruction occurs. The design of a building cannot resist any possible earthquake, which is neither scientific nor economical. The scientific seismic fortification strategy is that buildings show different performances and abilities when they are subjected to different degrees of earthquakes. In general, when buildings are affected by a major earthquake, it is acceptable to cause some damage to them. As long as it does not collapse, casualties and property losses can be minimized. The school buildings rebuilt after the disaster are basically reinforced concrete frame structures. Judging from the damage of Lushan earthquake, the main structures of these buildings are basically not damaged, and there are few visible cracks in frame beams and columns. However, many maintenance structures such as infilled walls in these buildings have been destroyed, which looks "very scary". In fact, the cracking and damage of the maintenance structure will not affect the seismic capacity of the main structure of the building, and the damage of these parts is foreseeable and acceptable, which cannot be simply said to be a problem of building quality. As for the damage and collapse of the maintenance structure, it may cause property losses and casualties, which is a technical problem to be solved in the future seismic design code.
What is the standard of seismic fortification in schools?
School buildings are built according to key fortification buildings, and their seismic capacity is higher than that of ordinary buildings in this area.
"Now the society and the media often say that it is unscientific for buildings to resist earthquakes of magnitude 7 and 8." Li Yingmin said, "Generally, the seismic design of our buildings follows the intensity rather than the magnitude. Intensity and amplitude are two different things. Magnitude is used to measure the energy released by an earthquake. There is only one earthquake. Intensity is a measure of the degree to which an area is affected by an earthquake. After the earthquake, the intensity in different areas is different. In addition to the magnitude, there are also factors affecting the intensity, such as focal depth, distance from the focal point, topography, and site characteristics. "
Li Yingmin told reporters that each region will determine different fortification intensities according to the possible earthquake risk level, and use this as the basis for building seismic fortification. According to the Code for Seismic Design of Buildings, the seismic fortification intensity of Baoxing is 8 degrees, and that of Lushan and Tianquan is 7 degrees. Both general buildings and school buildings are designed according to the local fortification intensity. For example, the design standard of general buildings in Lushan is that buildings (including maintenance structures) will not be damaged when they are subjected to earthquakes below 7 degrees (such as 6 degrees), but they can be damaged when they are subjected to earthquakes of 7 degrees, but they can continue to be used after being generally repaired or not, and they cannot collapse when they are subjected to earthquakes of about 8 degrees. Therefore, the seismic capacity cannot be measured by the magnitude of the earthquake. If the source is in Lushan, the building may be destroyed when the magnitude reaches 6, but if the source is far away from Lushan, even if the magnitude reaches 9, the building may still be intact. After the Wenchuan earthquake, China further improved the seismic fortification standards of school buildings. Compared with ordinary buildings, school buildings have better seismic performance during earthquakes.
The reporter learned that according to the Classification Standard for Seismic Fortification of Building Engineering revised in 2008, the seismic fortification standard of all teaching rooms, student dormitories and canteens in kindergartens, primary schools and middle schools in China has been raised from Class C to Class B. Class B seismic fortification buildings refer to lifeline-related buildings whose functions cannot be interrupted during an earthquake or need to be restored as soon as possible, as well as buildings that may cause a large number of casualties and other major disaster consequences during an earthquake and need to raise the fortification standard. Their seismic measures should be strengthened according to the requirements higher than those of ordinary buildings in this area.
Lu Xinzheng told reporters that the requirements of China's building seismic design are "minor earthquakes are not bad, moderate earthquakes can be repaired, and major earthquakes cannot collapse". For example, if the seismic fortification intensity of a certain place is 8 degrees, the general building can't be seriously damaged beyond repair under the intensity of 8 degrees (that is, moderate earthquake), and it should not collapse or endanger life under the intensity of 9 degrees (that is, large earthquake). The seismic fortification measures of school buildings are stricter than those of ordinary buildings, and they have higher safety reserves, so as to realize "not falling down in a big earthquake".
It is understood that after the Wenchuan earthquake, according to the post-disaster reconstruction plan, the primary and secondary schools rebuilt in Sichuan earthquake-stricken areas were built according to the building standards higher than those of key earthquake-resistant buildings in the region.
How was the building damaged in this earthquake?
Most of the collapsed houses are old houses, and the main structure of the school is basically intact.
Li Yingmin rushed to Lushan on the day of the earthquake to evaluate the damage of the building, including the earthquake damage assessment of the school building in Lushan Middle School. He said: "At first glance, the school building of Lushan Middle School has withstood the test of the earthquake as a whole. Only the parapet on the roof of one building fell down and was seriously damaged. In addition, many classroom maintenance walls (non-load-bearing walls) were damaged. When the Lushan intensity value of this earthquake reaches 9 degrees, the damage of these maintenance walls meets the design requirements and is not a serious injury that affects the seismic capacity of the structure. "
The reporter learned from Xinhua News Agency that the field headquarters of China Seismological Bureau drew the intensity map of Lushan earthquake with magnitude 7.0, and the intensity in some areas of Lushan County reached 9 degrees, and the intensity in Baoxing and tianquan county reached 8 degrees. Experts said that although the intensity of Lushan earthquake reached 9 degrees in some areas, the main structure of public buildings built by new technologies in Lushan County was basically intact after the Wenchuan earthquake.
Zhou Bengang, a researcher at the Institute of Geology, Seismological Bureau of China, said: "Most of the collapsed houses in the disaster area are old houses, while the buildings restored and strengthened by Wenchuan earthquake and the farmers' self-built houses are basically intact or slightly damaged. No obvious collapse was found in the buildings of schools and hospitals. "
Zhang Peng, director of the Anti-seismic Division of the Engineering Quality and Safety Department of the Ministry of Housing and Urban-Rural Development, said that the main structures of newly-built public buildings such as hospitals and schools in the disaster area are basically intact, and the damaged parts are mainly non-main structures such as non-load-bearing walls and ceilings, which can be repaired.
Jank, the chief planner of the Housing and Construction Department of Sichuan Province, also said that the post-earthquake reconstruction project in Wenchuan stood the test in Lushan earthquake, especially the public buildings such as schools and hospitals where people concentrated did not collapse, which ensured the safety of people's lives. (Reporter Li Ling, Beijing, April 26)
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Seismic fortification of primary and secondary school buildings and medical buildings
Improving the seismic capacity of various construction projects is the basic and fundamental countermeasure to reduce earthquake disasters, which is the experience summary of earthquake prevention and disaster reduction projects since Xingtai 1966 earthquake in China. According to the degree of casualties, economic losses and social impacts that may be caused by earthquake damage and the role of building functions in earthquake relief, and according to the actual situation of China's existing technical and economic conditions, it is an important strategy to classify and treat buildings differently and adopt different design requirements, including seismic measures and seismic action calculation requirements, to reduce earthquake disasters and reasonably control construction investment, and it is also an anti-earthquake and disaster reduction countermeasure widely adopted in seismic design codes all over the world.
After the Wenchuan earthquake, the safety of primary and secondary school buildings and medical buildings attracted attention. Classification standard for seismic fortification of building engineering (GB50223-2008) improves the classification standard of primary and secondary school buildings and medical buildings (outpatient building, medical technology building, ward building, etc.). After the implementation of the Classification Standard for Seismic Fortification of Building Engineering (GB50223-2008), Beijing and other places have successively carried out seismic reinforcement and energy-saving renovation projects for primary and secondary school buildings, and the buildings with the original fortification classification standard of C have been reinforced according to B. The Engineering Quality and Safety Supervision Department of the Ministry of Housing and Urban-Rural Development also organized relevant units to compile the National Architectural Standard Design Atlas 09SG 19- 1 Seismic Strengthening of Buildings (I) (Seismic Strengthening of Primary and Secondary Schools) and National Seismic Appraisal and Strengthening Examples of Primary and Secondary Schools, which provided technical basis for the seismic strengthening projects of primary and secondary schools. This set of national standard atlas is suitable for primary and secondary school buildings with masonry structure and frame structure that need seismic strengthening in 6-8 degree areas. All new buildings are implemented according to the Classification Standard for Seismic Fortification of Building Engineering (GB50223-2008). This is the corresponding seismic fortification measures taken by the engineering community for primary and secondary schools and medical buildings after the Wenchuan earthquake.
After the Wenchuan earthquake, the Seismological Bureau of China issued the Notice on Determining the Principles of Seismic Fortification Requirements for Construction Projects of Schools, Hospitals and Other Crowded Places (Zhong Zhen Fangfa [2009] No.49) on April 22nd, 1999, requiring that the main buildings of schools include teaching rooms for kindergartens, primary schools and middle schools, as well as student dormitories and canteens, and the main buildings of hospitals include outpatient, medical technology and inpatient rooms. To improve the peak acceleration of ground motion, the following requirements should be met: "If the peak acceleration of ground motion is less than 0.05g, the peak acceleration of ground motion will be increased to 0.05g; The peak acceleration of ground motion is increased to 0.10g; If it is located in the area where the peak acceleration of ground motion is 0.05g; The peak acceleration of ground motion is increased to 0.15g; If it is located in the partition of 0. 10g; Located in the sub-region of 0. 15g peak ground acceleration, the peak ground acceleration is increased to 0.20g；; If it is located in the partition of 0.20g, the peak acceleration of ground motion increases to 0.30 g; If it is located in the sub-zone of 0.30g, the peak acceleration of ground motion increases to 0.40 g; If it is located in a zone with a peak ground acceleration greater than or equal to 0.40g, the peak ground acceleration will not be adjusted. " This practice does not meet the requirements of the Classification Standard for Seismic Fortification of Building Engineering (GB50223-2008) and the Code for Seismic Design of Buildings (GB 50011-2010). The former (Moderate Earthquake Prevention [2009] No.49) mainly improves the calculation of earthquake action, while the latter mainly improves the anti-seismic measures of structures. This brings confusion to engineering design. Which standard is more suitable for implementation?
As far as Wenchuan earthquake disaster is concerned, a large number of houses were damaged due to insufficient ductility, such as masonry structures without ring beams and structural columns (most floors are prefabricated hollow slabs) and frame structures with "strong beams and weak columns, strong bending and weak shear". A large number of houses are damaged due to insufficient seismic bearing capacity, which is mainly due to the fact that the actual earthquake action is much higher than the acceleration value corresponding to the original fortification intensity. From the analysis of this situation, simply improving the calculation of earthquake action can not solve the problems of insufficient seismic fortification ability and poor collapse resistance of buildings in vast areas of China. Although there are still some defects in the methods to improve the ductility of structures and components, these measures are relatively more targeted and effective, and have also been recognized by the majority of engineers and technicians.
Specifically, compared with Class C buildings, in Class B buildings, the seismic grade of concrete structure buildings should be determined at one time, and corresponding seismic measures (mainly internal force adjustment, etc.) should be taken. ) and seismic structural measures (reinforcement ratio, stirrup ratio, etc. ) should be taken; Brick-concrete structure, Table 7 of Code for Seismic Design of Buildings (Note 3).1-2010 "Class B multi-storey masonry buildings are still checked according to the local fortification intensity, and the number of floors should be reduced by one, and the total height should be reduced by 3m; Masonry houses with bottom frame-seismic wall are not suitable. " Article 7.65438 +0.4 "Maximum ratio of total height to total width of multi-story masonry building", Article 7.65438 +0.6 "Local size limit of masonry wall section in multi-story masonry building", Article 7.65438 +0.5 "Spacing between seismic transverse walls of building" and so on. Class B buildings are required to increase the original fortification intensity by one degree to determine the height-width ratio, local size limit and seismic capacity of masonry buildings. According to these requirements, the 6-degree seismic fortification zone is basically unchanged, the 7-degree zone is considered as 8 degrees, and the 8-degree zone is considered as 9 degrees, with great changes. In the foundation, among the anti-liquefaction measures in Table 4.3.6 of Code for Seismic Design of Buildings (GB 500110), the anti-liquefaction measures of Class B buildings are also higher than those of Class C, so the measures are not treated as high degree. For example, for the foundation with medium liquefaction grade, the anti-liquefaction measures for Class B buildings are "completely eliminating liquefaction settlement, or partially eliminating liquefaction settlement, and treating the foundation and superstructure", while the measures for Class C buildings are "treating the foundation and superstructure, or demanding measures". However, the effectiveness of seismic measures for these foundations and superstructure can not be covered and realized only by improving the calculation of seismic action.
Of course, the measures of "the peak acceleration of ground motion is less than 0.05g, and the peak acceleration of ground motion is increased to 0.05g" in No.49 of Moderate Earthquake Prevention [2009] are GB50223-2008 and GB 500 1 1-20655. At present, the level of earthquake prediction is not mature enough, so it is difficult to make accurate short-term prediction of the time, place and magnitude of the actual earthquake, and the level of medium-and long-term prediction needs to be further improved. However, judging from the distribution of earthquakes in history, there are some areas, such as Zhejiang Province, where nearly 4,000 years of data show that there have been basically no earthquakes of magnitude 5 or above. Therefore, it is unlikely that earthquakes of magnitude 5 or above will occur in areas like this in the future. However, due to the incomplete historical data of earthquakes, the interval between the next earthquakes in a certain area is long, some years, decades, hundreds or even thousands of years. It is uncertain whether there will be earthquakes above 6 degrees in the future in areas designated as non-seismic fortification areas in the current zoning map (not excluding near earthquakes and distant earthquakes), and the earthquake damage survey shows that buildings with seismic fortification of 6 degrees are uncertain. Its seismic capacity has been substantially improved, and the integrity of the earthquake-resistant building is much better, which can prevent and slow down the local damage caused by the explosion caused by gas leakage, and the local wall cracking or even the overall collapse of the building caused by uneven foundation. Therefore, it is reasonable for important buildings in non-seismic fortification areas to be fortified at 6 degrees, and the cost will not increase too much (about 2% for brick-concrete structures and about 5% for frames). However, it is a complicated problem how to carry out the corresponding work if there is no administrative organization such as the earthquake-resistant office in the non-seismic fortification area.
Generally speaking, it is feasible for primary and secondary school buildings and medical buildings to be implemented according to the Classification Standard for Seismic Fortification of Building Engineering (GB50223-2008), which is also a widely adopted method in the engineering field at present.