First, the three main periods of the development of modern construction industry
(1) large-scale construction period. In the late 1920s, the climax of building high-rise buildings and infrastructure such as expressway appeared in the United States, which revived the Great Depression. After World War II, in order to meet people's basic production and living needs, the construction industry of all countries in the world has ushered in an unprecedented period of large-scale construction, during which the building scale is large but the standard is relatively low.
(2) There is a period of both new construction and maintenance. During this period, on the one hand, in order to meet the needs of social development, new buildings are constantly being built, and at the same time, due to the improvement of human production and life requirements for buildings, the low-standard buildings built in the past can not meet the needs of society after decades of use, and need to be maintained, strengthened and modernized, thus making the construction industry transition to a development period of paying equal attention to both new construction and maintenance.
③ Maintenance and renovation period of existing buildings. With the further development of society, people's requirements for architectural functions are getting higher and higher, and the structural safety problems caused by the gradual weakening of the scale and functions of existing buildings have begun to attract people's attention. However, due to the high cost of demolition, it has a serious impact on the living environment and hindered the rise of the new upsurge, so people turned their attention to the maintenance, reinforcement and modernization of existing houses, with less investment, little impact and quick results, which not only has considerable economic benefits, but also has great social benefits.
At present, China's construction industry is developing from the first phase to the second and third phases, and the maintenance, reinforcement and transformation of existing buildings have attracted the attention of relevant state departments.
Second, the reasons for structural reinforcement
(1) The service life of the building has approached or exceeded its design service life, and the bearing capacity of the components cannot continue to work due to creep, harsh environmental corrosion and other reasons.
(2) The change of building function, such as the transformation of industrial workshop into supermarket, causes the change of column spacing and load, and the original columns, beams and other components can no longer meet the bearing capacity requirements, so they need to be rebuilt and strengthened.
(3) Due to the defects in survey, design and construction, the structural members have potential safety hazards and are prone to sudden damage and instability, so they need to be reinforced.
(4) Natural disasters such as earthquake, flood, fire, explosion, snowstorm, etc., which cause structural members to break and damage, affect the overall stability, and need to be tested and reinforced.
⑤ Improvement of seismic fortification. With the deepening of earthquake research, it is found that the original fortification intensity is formulated.
The distribution area is quite different from the actual situation, and the fortification intensity in many areas needs to be improved, which leads to the need for seismic reinforcement of the original buildings.
Three. Principles, procedures and methods of structural detection and reinforcement
1. Structural detection and reinforcement principle
① The design and construction of detection and reinforcement must be undertaken by professionals. Because there are many complicated uncertainties in the reinforcement design of building structures, and the requirements of the owners should be considered, the personnel who undertake the reinforcement design and construction should not only have strong structural theory and clear structural concepts, but also have rich engineering experience, so as to comprehensively and systematically analyze the problems and put forward a more reasonable structural reinforcement design and construction organization plan.
② The technical and economic effects of detection and reinforcement should be considered comprehensively to avoid unnecessary removal or replacement. If the safety reserve at design time can be used, the repair can solve the problem without reinforcement. If the damage of the structure affects the safety of the structure, but it has the value of repair and preservation, it should be strengthened.
(3) detection and reinforcement construction should follow the scientific order. Usually, it should be treated first and then reinforced; Reinforce first and then dismantle; Reinforce first and then open the hole; Foundation first, then column, beam and slab; It is better to protect the important parts first, and then protect the secondary parts.
(4) Check the seismic capacity of the building structure and do a good job in seismic strengthening design. After the structure is strengthened, there should be no local strengthening or sudden change of stiffness to form new weak parts, and the influence of the increase or change of seismic action effect caused by the increase or change of structural stiffness should also be considered.
2. Structural inspection and reinforcement procedures
The procedure of structural detection and reinforcement is reliability appraisal-reinforcement scheme selection-reinforcement design-construction organization design-construction-acceptance.
① Reliability appraisal is a process of checking, measuring, analyzing and judging the load, function, structural resistance and their relationship of existing buildings, and drawing conclusions. The purpose is to solve the problems of identification, reinforcement, reconstruction and expansion of old buildings without data, or damage to buildings caused by overload use, excessive deformation and cracks, corrosion, fire, explosion and earthquake. So as to provide a basis for identification, reinforcement and reconstruction.
② The choice of reinforcement scheme is very important. The quality of reinforcement scheme not only affects the investment of funds, but also affects the quality of reinforcement.
(3) Reinforcement design, including checking the bearing capacity of reinforcement components, component treatment and construction drawings. The reinforcement design unit must have the corresponding special qualification for reinforcement design, otherwise it cannot engage in such design work.
(4) When designing the construction organization, it should be considered that the reinforcement construction is carried out under the condition of load or partial load, so the construction safety is very important, and safety measures should be taken.
⑤ In the process of construction, when removing the original waste or cleaning up the original components, special attention should be paid to observe whether there is anything inconsistent with the original inspection. Engineers and technicians should visit the site and observe whether there is any emergency at any time.
⑥ Acceptance: After the construction is completed, the owner, supervisor, designer and contractor shall be organized to conduct acceptance together, and the supervision organization shall supervise the acceptance process.
3. Structural detection and reinforcement methods
There are many ways to strengthen reinforced concrete structures, such as enlarging cross section, wrapping steel bars, pasting steel plates, increasing prestress, adding fulcrums and pasting carbon fiber materials. Now, briefly introduce these methods:
(1) Reinforcement method of enlarging the cross section: the cross section area of the original concrete structure is increased by using reinforced concrete or steel mesh mortar layer to improve the bearing capacity of the structure. It can not only improve the bearing capacity of the reinforced member, but also increase its section stiffness and change its natural frequency, so that the performance required for normal use can be improved to a certain extent. This method is simple, mature and reliable, and is mainly suitable for the reinforcement of reinforced concrete flexural members and compressive members. However, the disadvantage is that on-site wet operation needs long-term maintenance, and the increase of size causes a large increase in important space and a decrease in space area, so its application is limited to a certain extent.
(2) Steel-wrapped reinforcement method: it is a reinforcement method that steel is wrapped around the four corners of concrete members. This method is simple in construction, less in-site workload and more reliable in stress, and is mainly suitable for members that are not allowed to increase the cross-sectional size of concrete, but need to greatly improve the bearing capacity.
(3) External bonding steel plate reinforcement method: It is a reinforcement method that 2-6mm steel plates are bonded on the surface of concrete members with high-strength structural adhesive to make up for the lack of reinforcement inside the members and make the steel plates work together with the original concrete members. It is mainly suitable for reinforced concrete flexural, eccentric compression and tensile members, but not for plain concrete members.
④ Prestressed reinforcement method: it is a method to reinforce the original structural members with externally prestressed steel tie rods or steel struts. This method changes the internal force distribution of the original structure by prestressing the post-prestressed tie rod or steel compression bar, eliminates the stress lag phenomenon of the reinforced part, and enables the post-prestressed part to work well with the original member, thus improving the bearing capacity of the original structure, reducing bending deformation and narrowing the crack width. It is mainly suitable for beams, plates, columns and other components with small cross-section or need to increase their service load, and the original component is in a state of high stress and strain and it is difficult to directly unload its load.
⑤ Strengthening method by adding fulcrums: It is a method to reduce the calculated span of flexural members by adding fulcrums, so as to reduce the load on the strengthened members and improve the bearing capacity of the structure. It is mainly suitable for the reinforcement of beams, slabs, trusses, space grids and other structures. This method is simple and reliable, but it is easy to destroy the original appearance and use function of the building, and may reduce the use space, so its use is limited.
⑥ Bonding carbon fiber material reinforcement method: This is a new reinforcement technology, which uses epoxy resin adhesive to bond carbon fiber cloth to the structure to be reinforced along the tensile direction or vertical crack direction to form a new composite material, so that the reinforcement patch bears the same force as the original reinforced concrete, and the bending and shearing capacity of the structure is improved, thus achieving the purpose of strengthening the structure. This method has the advantages of corrosion resistance, moisture resistance, almost no increase in structural weight, durability and low maintenance cost. However, its fire resistance is poor and special fire prevention treatment is needed.
Conclusion With the development of society, people's living standards are constantly improving, which puts forward higher requirements for housing construction. Land is a non-renewable resource, and the transformation of the old city-adding stories, expanding and changing uses through the reinforcement and transformation of buildings, so as to rejuvenate the old buildings and create an economical society, is one of the inevitable choices for urban construction. We need to study, explore, summarize and summarize some engineering experiences to promote the rapid development of structural detection technology.
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