1. Introduction
With the rapid development of social economy and the improvement of people's living standards, the design specifications of housing buildings are becoming more and more perfect, and people's requirements for living safety are getting higher and higher. There are certain security risks in masonry structures of many old buildings. But most of these houses are built in relatively advantageous areas. Compared with the new buildings after demolition, they have to go through various uncertain factors such as pre-approval and planning. Many owners prefer to choose the structural reinforcement design of masonry houses to meet the needs of structural safety. In this paper, the reinforcement design of a masonry structure building will be deeply analyzed, hoping to attract colleagues' attention in future design work.
2. Mechanical characteristics and failure forms of masonry
2. 1 masonry structure has the following main features:
2. 1. 1 Masonry members are mainly used as compression members such as walls and columns. Generally, their rigidity is high, but their strength is low, especially their shear strength, bending strength and tensile strength are low.
2. 1.2 Masonry houses have a large self-weight and the foundation is prone to uneven settlement. However, the foundation of the building usually adopts the strip foundation under the wall and the independent foundation under the column, which has limited adjustment effect on the uneven settlement of the foundation, and the wall often cracks due to the uneven settlement of the foundation.
2. 1.3 Masonry structures usually adopt reinforced concrete buildings and roofs. Due to the significant difference of thermal expansion coefficient between masonry materials and concrete materials, the walls in masonry structures often crack due to excessive temperature difference.
2. 1.4 Bricks and mortar in masonry structure are porous materials, which are easy to be affected with moisture and are prone to durability damage such as weathering, freezing and thawing, and corrosion. Under the long-term effect of unfavorable factors of nature and use environment.
2. 1.5 Masonry structure needs manual masonry, which has heavy workload, high labor intensity and great variability of construction quality. The mechanical properties of masonry are sensitive to construction quality, so the performance of masonry members is often reduced due to quality defects.
2.2 Masonry members are mainly used to bear pressure, and some members are also used to bear shear, bending moment or tension. For the ultimate state of bearing capacity, the failure forms of masonry members include:
2.2. 1 Axial and eccentric compression failure. This is the main failure form of compression members such as walls and columns.
2.2.2 Local compression failure. It mainly occurs in the destruction of vertically concentrated compression members, such as walls and columns supporting reinforced concrete beams.
2.2.3 The shaft is damaged by tension. Appear in the destruction of tensile members, such as the wall of a circular pool.
2.2.4 Bending and tensile failure. Appear in the destruction of flexural members such as retaining walls.
Shear failure. It mainly appears in the destruction of flexural members and brick arch bearings.
2.2.6 Overturning damage. Mainly manifested in the destruction of cantilever beam.
2.3 In addition to the loss of bearing capacity, masonry members may also affect the normal use of buildings due to damage, cracking, inclination, vibration and other factors, which belongs to the failure form of structural members in the limit state of normal use.
3. Comparative analysis of architectural structure schemes
The reinforcement of masonry structure can be divided into direct reinforcement and indirect reinforcement. When designing, we can choose the appropriate method according to the actual situation and use requirements.
3. 1 The direct reinforcement methods applicable to masonry structures generally include:
3. 1. 1 reinforced concrete outer layer reinforcement method: this method belongs to a combined section reinforcement method. Its advantages are simple construction technology, strong adaptability, greatly improved bearing capacity after masonry reinforcement, and mature design and construction experience. Suitable for strengthening columns and walls with walls. Its disadvantage is that the wet operation time of site construction is long, which has a certain impact on production and life, and the clearance of reinforced building is reduced to a certain extent.
3. 1.2 reinforcement method of reinforced cement mortar additional layer: this method belongs to a composite section reinforcement method. Its advantages are similar to those of reinforced concrete external reinforcement method, but its bearing capacity is not as good as the former. It is suitable for the reinforcement of masonry walls, and sometimes used to seal the stirrups on both sides of the wall when the reinforced concrete outer layer is strengthened with pilaster.
3. 1.3 strengthening method with buttress: this method is one of the strengthening methods with enlarged section. Its advantages are similar to that of adding stories to reinforced concrete, but its bearing capacity is limited and it is difficult to meet the seismic requirements, so it is generally only used in non-seismic areas.
3.2 Indirect reinforcement methods applicable to masonry structures are generally as follows:
3.2. 1 Bondless steel-clad reinforcement method: This method belongs to the traditional reinforcement method. Its advantages are simple construction, less on-site workload and wet operation, and more reliable stress. It is suitable for the reinforcement of masonry columns that are not allowed to increase the section size of the original members, but require a substantial increase in the section bearing capacity. Its disadvantage is that the cost of reinforcement is high, and it is necessary to take protective measures similar to those of steel structures.
3.2.2 Prestressed strut reinforcement method: This method can greatly improve the bearing capacity of masonry columns, and the reinforcement effect is reliable. It is suitable for strengthening masonry structures with high stress and high strain. Its disadvantage is that it cannot be used in the environment above 600 degrees.
Main causes of accidents caused by insufficient bearing capacity of masonry structures
4. 1 calculation error
4. 1. 1 is mainly manifested in the small cross section, low strength grade of bricks and mortar used, no beam pad at reinforced concrete beam support, no joists at doors and windows, and the height-thickness ratio of masonry does not meet the specification requirements. Some of these problems are due to the negligence of designers and draftsmen, such as not knowing the purpose of the building, using a small calculation load, or making mistakes in calculation, or noticing the wrong size and strength grade when drawing and tracing. And a lot of problems are caused by people who don't understand technology, and they "design" at will without following the basic construction procedures and designing by the design unit. .
4. 1.2 In another case, in the absence of scientific calculation, according to the administrative orders of the leaders or some people's subjective imagination, stories are added at will in the completed or under construction projects, which increases the load of the substructure and leads to insufficient bearing capacity of the substructure.
4.2 poor construction quality
The strength of masonry structure is closely related to the quality of masonry. Poor construction management and poor quality control are the main causes of masonry structure accidents. For example, in the process of construction, unskilled workers are hired to build bricks, and the masonry technology does not meet the requirements of the specification, resulting in upper and lower joints. Brick columns are built by core-core method, the mortar strength is too low, and the wall is arbitrarily opened, which weakens the cross section too much.
Design example analysis
The project was built in 1970s, with one basement and three floors above ground, with 370mm external wall and 240mm internal wall. The floors are all precast concrete slabs. The results of seismic appraisal show that the strength of underground to three-story masonry is 4.5 MPa, 3.5 MPa, 5.0 MPa and 4.5 MPa respectively, and the strength of mortar is 1.70 MPa and 65438+. There are no constructional columns in some positions, and there are local small cracks in the wall.
The calculation results of the original structure modeling are as follows: the seismic checking calculation and compressive bearing capacity calculation of a large number of walls on the first floor and the first floor do not meet the requirements. Because the owner of the building is still using it locally, the final scheme is to use single-sided cement mortar and steel mesh mortar to strengthen the wall with insufficient seismic checking calculation, which can appropriately reduce the damage to the original wall decoration surface and plumbing pipes, and use single-sided 100 thick plate wall to strengthen the wall with insufficient local compressive bearing capacity. The above two methods are difficult to meet the strength requirements of the wall at the entrance of local doors and windows. Finally, structural columns are added at both ends of the wall to meet the calculation.
5. 1 strength accounting
According to Formula 5. 1.4 in Technical Code for Seismic Strengthening of Buildings, the comprehensive seismic capacity index β s is calculated.
η is calculated according to formula 5.3.2.2.
η = η 0 was brought into 5.3.2.2 because of two = 240mm.
Look up table 5.3.2- 1 surface thickness 40mm, φ 6 @ 300.
The mortar strength of the basement and the first to third floors of the original wall is 1.7, 0.7, 1. 1.7 respectively.
Therefore, η of the first underground layer and the first to third underground layers are 1.35, 1.835, 1.49 and 1.835, respectively.
ψ 11/T689-2009 is used to calculate ψ1and ψ 2.
ψ 1 lookup table 5.3.1-kloc-0/,ψ 1 = 1.0.
Look up table 5.3 for common wall ψ 2.1-2, ψ 2 = 1.0.
The comprehensive seismic capacity index β s of single-story reinforcement is 1.35, 1.835, 1.49, 1.835 for ordinary walls of the first to third floors respectively.
The comprehensive seismic capacity index β s of double-sided reinforcement is 1.54, 2.1,1.68, 2. 1 1 respectively.
After review, the thick wall reinforced on one side can meet the requirements of seismic checking calculation;
5.2 Basic accounting:
According to the calculation, the original structural load and self-weight are as follows:
6469+5559+5634+4696 = 22358 kn
Weight of new parts:
Total weight of external column: 0.3x0.2x14.4x34x25 = 735kn.
Total weight of ring beam: 0.2x0.24x 102x4x25=490KN.
Total weight of mortar layer:176x0.05x3.6x25x2 =1584kn.
Total weight of new parts: 2809KN
2809/22358 = 12.6% & lt； 15%
The original foundation can continue to be used without special reinforcement;
6. Conclusion
In this paper, various masonry reinforcement methods are analyzed and compared, and some problems existing in the reinforcement design of masonry houses with low block strength and poor mortar strength due to long-term weathering and erosion are put forward. Through a design example, the feasibility of single-sided or double-sided cement mortar and steel mesh mortar surface reinforcement for specific projects is summarized, and this reinforcement method has certain advantages compared with the cost of simple plate wall reinforcement project. Finally, some design details that should be paid attention to in the design are considered, hoping to be helpful to designers engaged in masonry structure reinforcement.
The detailed information of "Discussion on Key Points of Masonry Reinforcement Design" can be found in Zhong Da Consulting Design Link, and all relevant building structure information you want is available.
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