Ductility of short columns in building structure design

Ductility of short columns in building structure design

In daily study, work and life, everyone has written papers, and with the help of papers, we can achieve the purpose of discussing issues and conducting academic research. I believe that writing a paper is a headache for many people. Below is a paper on short column ductility in architectural structure design that I have compiled for you. It is for reference only. I hope it can help you.

For short columns, their flexural bearing capacity and shear bearing capacity are much larger. Therefore, if the shear bearing capacity is damaged under the action of an earthquake, no matter how high the flexural strength is, the Play it out.

Abstract:

With the development of social economy, high-rise buildings continue to emerge in many urban constructions. When designing high-rise buildings, most designs can be designed using calculation software, which reduces the workload of designers. However, there is still a part of the work that requires designers to perform operations, that is, calculating the stress state of the building based on the software calculation results. Design the structural measures of the building. This article mainly analyzes and studies the improvement of ductility design of short columns in elevated buildings.

Keywords:

High-rise buildings; structural design; ductility of short columns; analysis

In the construction of high-rise buildings, the application of short columns has become more common, and When the floor height is designed to be certain, in order to improve the ductility of the building, it is necessary to increase the column cross-sectional area and reduce the axial pressure ratio. The smaller the axial pressure ratio, the larger the column cross-sectional area. Therefore, in the structural design of high-rise buildings, in order to meet the axial compression ratio limit, it is often necessary to increase the cross-sectional area of ??the column, resulting in a short column structure or even an ultra-short column structure. Under the requirements of seismic performance, short columns are required to have sufficient seismic performance, and the ductility of short columns needs to be improved. This article also analyzes methods for improving ductility in building structure design.

1. Method of determining short columns

According to relevant requirements, the definition of short columns is the ratio of the net height (H) of the column to the section height (h), that is, when H/h ≤ 4 , this column is called a short column. In construction, construction technicians mostly follow this judgment method when judging short columns. The parameters used in this determination method are only the relationship between the floor height and the column section, but do not apply to the internal force relationship of the column itself. According to the theory of material mechanics and structural mechanics, the shear span ratio (λ) can also be used as the basis for measuring short columns, that is, when λ = M/Vh ≤ 2, the column is also a short column, but it is different from the floor height and column cross-section. Compared with the short column determination method under the relationship H/h ≤ 4, under this condition, the value of λ may not be less than 2, that is, it may not be a short column. In most designs, designers use H/h≤4 to judge short columns. The main principles include the following aspects: first, λ=M/Vh≤2; secondly, because most of the inflection points of frame columns have been The intersection is close to the midpoint of the column, so the value of M is 0. 5VH, then λ ≤ 2 at this time, that is, H/h ≤ 4. However, in high-rise buildings, due to the relatively small linear stiffness of columns and beams, especially at the base of the building, the impact of column embedding is relatively large, and the column's restrained bending moment by the beam is small, and the height of the inflection point is greater than half of the column height. , even if the inflection point does not exist, it is unreasonable to still use H/h≤4 to judge the short column at this time, and λ=M/Vh≤2 should be used for judgment. If the reverse bending point is not at the midpoint of the column, the bending moment values ??of the upper and lower sections of the column are different, that is, Mh≠Mt.

Therefore, the shear span ratio of the upper part and the lower part is also different, that is, λh=Mh/Vh≠λt=Mt/Vt. At this time, when judging short columns, which section shear span ratio should be used to judge is an important issue to consider.

After analysis and research, it is believed that the larger value of the two should be used as the basis for determining short columns, that is, λ = max (λh, λt). The reasons include the following aspects: First, the frame column can be regarded as a continuous beam, subject to a given The value of axial pressure and column height (Hn) are similar to the shear span of a continuous beam. Relevant experimental studies show that when the shear span of a continuous beam is constant, the same longitudinal reinforcement will be configured at the lower and upper parts of the section, and the bending moment will appear in the section with a large bending moment. Shear failure; secondly, in frame columns, critical diagonal cracks can also occur in sections with larger bending moments. In fact, within the range of continuous beam shear spans or column heights, the maximum shear span ratio will occur in the section with larger bending moment.

As the shear span ratio increases, the shear resistance of reinforced concrete members will decrease. Therefore, under the same conditions, the shear bearing capacity of a section with a smaller bending moment is greater than that of a section with a larger bending moment. Under the action of load, the possibility of shear failure in the section with larger bending moment is greater than that in the section with smaller bending moment. Therefore, it is in line with the requirements to use the larger shear span ratio between the upper and lower sections as the shear span ratio for judging short columns. Generally speaking, the inflection point of the frame column in the base of a high-rise building is located at the upper part of the column, that is, Mbgt; Mt.

At this time, when judging short columns, the following formula can be used, formula (1): Hn/h≤2/yn, where Hn represents the net height of the n-layer column. yn represents the height ratio of the inflection point of the n-layer column. According to the geometric relationship, it can be seen that: yn=1/(1 ψ), where ψ=Mt/Mb, 0≤ψ≤Hn. If the inflection point appears at the midpoint of the column, then ψ=1, yn=0.5, then formula (1): Hn/h≤4; if the inflection point is at the upper end of the column, then ψ=0, yn=1, Then formula (1): Hn/h ≤ 2; if there is no reverse bending point, the judgment can be made directly according to the shear span ratio λ = M/Vh ≤ 2 of the section where the maximum bending moment is applied. Generally speaking, during the calculation process, the inverse bending point height ratio yn can be determined according to the D value method, and then a preliminary judgment can be made based on the calculation of formula (1) to determine whether it is a short column.

2. Measures to improve the ductility of short columns

2.1 Use concrete-filled steel tube columns

The structure composed of thin-walled circular steel tubes filled with concrete is called concrete-filled steel tube. The steel pipe exerts lateral restraint on the concrete, and the concrete is under pressure. The compressive strength and ultimate compressive strain capacity of the concrete are improved, especially the improvement effect on the ductility of high-strength concrete is very obvious. In addition, the steel pipes in the structure not only play the role of transverse stirrups, but also play the role of longitudinal reinforcements. The ratio of pipe diameter to pipe wall thickness is less than 90, which is similar to the concrete reinforcement ratio of more than 4.6, which significantly exceeds the concrete reinforcement ratio in the seismic requirements. Gluten rate requirements.

Because this type of structure has very good compressive strength and anti-deformation ability, even under high axial compression ratio, the pressure area will not be damaged first. Compared with steel columns, it is also There will be no buckling instability. Therefore, in order to control the cross-section rotation ability, there is no need to limit the axial pressure ratio limit. The bearing capacity of a single pillar of concrete-filled steel tubes can be calculated using formula (2). Formula (2): bearing capacity ≤ Φ1ΦeNθ. In the formula, θ represents the ferrule index, and the value range is [0.3, 3]. According to formula (2), when the hoop index is appropriately selected, the use of high-strength concrete can greatly increase the bearing capacity of the column, and the column cross-section can be greatly reduced, which is at least half lower than that of ordinary reinforced concrete columns, eliminating short columns. The seismic performance can be greatly improved.

2.2 Use steel-reinforced concrete columns

Steel-reinforced concrete columns can be formed by wrapping concrete around steel frames. In half of the cases, the types of steel frames include cross-shaped sections, cross-sections, and I-shaped sections. Compared with steel structures, local buckling of steel structural members may occur, while steel-protected concrete columns will not buckle because the steel members are wrapped with concrete. Buckling phenomenon, the overall strength of the column is strengthened, and the strength of the steel can be effectively exerted. Moreover, compared with general steel structure columns, the use of steel-reinforced concrete structures can reduce the amount of steel used by more than half.

Compared with concrete structures, due to the presence of steel frames, the bearing capacity of the columns is greatly improved, and the cross-sectional area of ??the columns is also effectively reduced. Since the concrete is restrained by the steel frame flanges and stirrups, the ductility of the concrete is improved. , the ductility and energy dissipation capacity of the column are enhanced. In this structure type, the advantages of concrete and steel are maximized. It has the advantages of good ductility, small cross-section, and light weight. Its application in high-rise buildings can effectively improve the seismic performance.

2.3 Use split columns

For short columns, their flexural bearing capacity and shear bearing capacity are much larger, so under earthquake action, if the shear bearing capacity After being damaged, the bending strength cannot be exerted no matter how high it is. Therefore, the flexural strength of short columns can be artificially reduced to slightly lower than the shear strength. Under earthquake conditions, the flexural strength of the columns will be exerted, showing ductile damage. In order to reduce the bending strength, a gap can be set vertically in the column, and the short column can be divided into two or four parts to form a split column. Each split column is individually reinforced and connections are provided between the split columns. seams to increase the initial stiffness of each split column. Relevant studies show that compared with integral columns, split columns have basically the same shear resistance and reduced bending load-bearing capacity. This improves the deformation capacity and ductility of the columns, and transforms the original shear-resistant failure form into a bending one. The type failure mode also eliminates short columns and turns them into split long columns, which effectively improves the seismic performance when the shear span ratio λ ≤ 2.

3. Conclusion

When judging short columns, the shear span ratio λ ≤ 2 is used as the basis for judgment. The cross-sectional size of the short column should be reduced as much as possible to make the short column short. The load-bearing capacity of the column is improved as much as possible, and various methods are used to improve the ductility of the short column to effectively improve the seismic performance of the short column. In practice, structural types such as steel tube concrete and steel reinforced concrete have a very significant effect on improving the load-bearing capacity of short columns, while split columns are very effective in improving the seismic performance of short columns. The application in high-rise buildings also effectively reduces the occurrence of short columns and ultra-short columns in the structures at the bottom of the building, avoiding the problem of reduced seismic performance of the building caused by the brittle failure of short columns.

References:

[1] Kang Hongtao, Wang Xingyang. Solution to the problem of short columns in seismic design of high-rise buildings [J]. Science and Technology Information, 2013 (12): 412.

[2] Xie Guo. On the ductility design of high-rise building frame structures [J]. Henan Science and Technology, 2013 (13): 166 170.

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Architectural Structure Internship Report

My architectural structure internship is roughly divided into two parts: first, visiting various buildings that have been put into use to understand the classification of various structures and their different characteristics; second, It is to visit the construction site and understand how the design drawings are turned into physical buildings through construction. Through the visit and study, we have a preliminary understanding of the building structure and construction, which serves as a guide for our future study and practice. Now I will summarize the two parts of the internship respectively.

1. Visit and learn about the structures of various buildings

The construction laboratory is an industrial factory building with an old-fashioned appearance. It uses a rack structure. Through the teacher’s explanation, we know , the structure of the building can be classified according to different classification standards. For example, according to the material, it can be divided into: concrete structure, masonry structure, steel structure, wooden structure, bamboo structure, etc. According to the different stress, it can be divided into: row frame, Frame, grid, arch, truss and other structures. The rack structure used in the laboratory has the advantage of convenient construction. It generally uses prefabricated components and is mostly used to build one-story factories.

The top of the laboratory uses prefabricated reinforced concrete row beams and concrete slabs. This structure is bulky and limits the span of the beam. It has now been replaced by the widely used steel trusses and steel plates. We can It can be seen that in order to reduce the dead weight of the concrete prefabricated structure, the beams in the laboratory are all made into an I-shape in the middle, and the excess concrete is dug out to reduce the dead weight.

The columns on both sides are typical industrial factory columns. There are corbels on the upper part for installing the crane rails. Similarly, they are made into an I-shape in the middle, and in order to increase stability, there are two columns on both sides. Inter-column supports are set up at intervals between the columns; in addition, on the sides of the crane rails, lateral connections with the columns are added, because during the use of the crane, the horizontal movement of the crane will generate horizontal loads.

Two wind-resistant columns are also designed on each wide side of the laboratory. These wind-resistant columns and beams set at regular intervals enable the wall to have sufficient stiffness to resist strong wind loads.

Then there is the Building Energy Conservation Laboratory, which is a brick-concrete building with a frame structure.

Frame structures are single-story and multi-story buildings composed of foundations, columns, beams, and plates, as well as high-rise buildings composed of frames and shear walls or frames and tubes. The connection between them is fixed Or called rigid connection, unlike the rack structure of the construction laboratory, the optimal floor of the frame structure building is around 15-16 floors, and the construction of the frame structure is mostly cast-in-place construction rather than prefabricated hoisting. In addition, after searching for information, I learned that the frame structure is composed of beams and columns, and the member cross-section is small, so the load-bearing capacity and stiffness of the frame structure are low. Its stress characteristics are similar to vertical cantilever shear beams. The higher the floor, the greater the horizontal displacement. The slower it goes, the high-rise frame will bear a large horizontal force in both vertical and horizontal directions. At this time, the cast-in-place floor also works as a beam, and the role of assembling the integral floor is not considered. The walls of the frame structure The body is a filling wall, which plays the role of enclosure and separation. The characteristic of the frame structure is that it can provide flexible use space for the building, but its seismic performance is poor.

According to the number of spans, the frame of the house is divided into single span and multi-span; according to the number of floors, it is divided into single-story and multi-story; according to the facade composition, it is divided into symmetrical and asymmetrical; according to the materials used, it is divided into steel frame , reinforced concrete frame, prestressed concrete frame, glulam structural frame or steel and reinforced concrete hybrid frame, etc.

The floor slab of the Building Energy Saving Laboratory is a prefabricated reinforced concrete slab, which is a one-way slab. Through the teacher’s explanation, we learned that various slabs can be divided into one-way slabs according to their length-to-width ratios. and two-way plates, and the value of the aspect ratio is different in the elastic theory and the plastic theory. The elastic theory is , and the plastic theory is . The one-way plate can only be supported in one direction during use, and the opposite is true for the two-way plate.

Next we visit the automotive laboratory, which is a steel structure building. Steel is characterized by high strength, light weight, and high rigidity, so it is particularly suitable for the construction of large-span, ultra-high, and ultra-heavy buildings; the material has good homogeneity and isotropy, is an ideal elastomer, and is most suitable for general engineering The basic assumption of mechanics; the material has good plasticity and toughness, can have large deformation, and can withstand dynamic loads well; the construction period is short; it has a high degree of industrialization and can carry out specialized production with a high degree of mechanization; high processing precision and efficiency High, good sealing. Its disadvantage is poor fire resistance and corrosion resistance.

When designing steel structures, special consideration must be given to the instability of materials. Therefore, on the steel in the automotive laboratory, we can see that at small intervals, the webs of the I-shaped steel are added A rib to solve the stability problem. The construction of steel structures generally involves prefabricating various components in the factory and then assembling them at the construction site. There are three connection methods for assembly: welding, bolts and riveting. Among them, welding is easy to rust, and bolted connections are the highest in strength, so they are less effective in handling shear loads. Bolts are used to strengthen joints with higher requirements, such as the joints between beams and columns.

Finally, we visited the gymnasium and learned about the characteristics of the truss structure. A truss is a geometrically constant structure composed of many rods connected by pins at both ends. If all the members in the truss are in the same plane, it is called a plane truss, otherwise it is called a space truss. Since the materials used in the truss structure are relatively economical, the truss itself is light in weight, and each member of the truss is only subject to tension or compression, so the material can be fully utilized. For large buildings like gymnasiums, the use of truss structures can save costs and meet load requirements.

2. Visiting the construction site

We visited the 14-story science and technology building under construction in the school.

When we arrived at the gathering place, I saw that all the students were wearing engineering hats on their heads; at the same time, it was written on the doors of the living area and construction area of ??the construction site: Those who do not wear safety helmets are not allowed to enter the construction site; of course, in the science and technology building There is also a slogan written on the protective net outside the main body of the structure: Safety responsibility is more important than Mount Tai. It can be seen that the first issue to pay attention to during construction is safety. In the past, production companies did not pay attention to the safety of migrant workers, resulting in many work-related injuries and fatal accidents. These accidents caused great damage to workers and companies! At the same time, in order to ensure the smooth progress and safety of the construction, the construction site must be enclosed with brick walls. Only various construction vehicles and internal personnel can enter and exit, and our internship must also obtain their consent!

When we entered the construction area, we saw the main structure of the science and technology building at a glance. At that time, the main structure gave me the feeling that it did not look like a building and was not good-looking. This may be because it is different from the buildings I have seen that have been built and put into use. There is a large space in front of the main body. This space is used for stacking building materials. You can see that the building materials stacked are mainly steel bars. There are no building materials such as cement, sand, and stone. This is because finished concrete is now used. Time to pour the structure. This can ensure the quality of concrete, reduce construction waste and reduce production costs. In the steel bar stacking area, we can see that different types of steel bars are separated, and information such as the steel bar model and arrival time are also marked in front of them.

We followed the on-site manager upstairs. We stepped on a ladder made of steel pipes and iron mesh. We began to feel that it was very dangerous. There were steel pipes or iron bars protruding from all sides. The formwork and brackets on the second and third floors have been removed, and we can clearly see that the pillars supporting the upper weight are so large that we all feel that the floor height has become smaller. There are many structural columns around the load-bearing columns, which are used to increase the strength of the wall to prevent it from easily collapsing due to the wall being too long. Along the way, we saw that the supports for the upper floors had not yet been removed. These supports were made of steel pipes and formwork. The steel pipes were very dense, which shows that it takes a lot of support to withstand concrete slabs and beams that have no strength at all.

Going up to the tenth floor, we saw that workers were still tying steel bars. The steel bars of columns and beams had been tied and placed in the slots reserved for the formwork. I observed several of the beams and columns, and it was just like what the teacher said: the lower part of the beam is the first force reinforcement, there are nine main beams, and there are six secondary beams; the upper part is not the vertical reinforcement, and the main beam and the secondary beam are different; the load-bearing reinforcement Tie with stirrups between the supporting ribs. The column is different. If three or four beams are to intersect at the column, the steel bars of the beams must pass through the columns. This makes the steel bars on the column heads very dense. At the same time, attention must be paid to the density when pouring concrete. The reinforcement of the slab generally includes load-bearing ribs and supporting ribs. The load-bearing ribs are at the bottom and are divided into vertical and horizontal directions; the supporting ribs are at the top and are also placed vertically and horizontally. The placed steel bars must be tied with wires. In order to ensure that the gluten is not stepped down, horseshoe tendons must be used to raise it. When looking at the plate reinforcements, we found that there were electrical conduits laid together with the steel bars. This is a manifestation of the cooperation between the electrical and structural professionals.

Our on-site visit time was very limited. We only saw workers laying out the reinforcements. We did not see them pouring columns, beams, slabs, laying brick walls and other construction scenes, so our understanding was very one-sided. This can only be used as our perceptual understanding of construction!

Summary:

During the internship, we did come into contact with a lot of practical applications, but there is still a big gap between the actual level of production methods and the more advanced technologies. . I found that the production techniques we saw were not advanced. Just like the formwork method we watched in the video was not used in the science and technology building, but others had already started to use it in the early 1980s. This may be due to construction The unit's materials are insufficient, but advanced production technology can indeed improve construction progress and production quality.

Judging from the development trend of construction, steel structures have received more and more attention and recognition from people. Studying the stress of steel structures and enhancing the fire resistance of steel structures is an urgent topic that needs to be in-depth.

Of course, we still have to base ourselves on the study of reinforced concrete structures. Through study and practice, we can have a deeper understanding of the structure of the building, and we cannot ignore certain hidden dangers that may occur to ensure that what we build Tall buildings can truly cope with emergencies of all kinds.

Experience in architectural structure training

Internship purpose:

Through this week’s internship, let us get to know some of the structures and components mentioned in the course. Understand certain practical knowledge and recognize some problems encountered in actual projects and their solutions. Let us put the rigid knowledge in the textbook into practice and lay a certain foundation for future design courses.

Internship content:

When visiting a public building, I learned about the interior of the house and some of the components involved in the parking lot of the public building. The problem is that since this building does not require a large open space, it uses a shear wall structure. The columns on the underground floor are relatively thick but not dense, so the space does not feel crowded. The accompanying design The staff explained to us the layout and classification of pipelines on the ground floor, and how to install fire pipes, domestic water pipes and other pipes. The formation and treatment methods of expansion joints, construction joints and settlement joints are explained.

For example, the causes of cracks in concrete: Causes of cracks There are many reasons for cracks in concrete, mainly changes in temperature and humidity, brittleness and unevenness of concrete, unreasonable structure, and insufficient raw materials. Qualified, template deformation, uneven settlement of foundation, etc. During the hardening of concrete, the cement releases a large amount of hydration heat, and the internal temperature continues to rise, causing tensile stress on the surface. Later in the cooling process, due to the constraints of the foundation or old concrete, tensile stress will appear inside the concrete. The decrease in temperature will also cause large tensile stresses on the concrete surface.

When these tensile stresses exceed the crack resistance of the concrete, cracks will appear. The internal humidity of many concretes changes little or slowly, but the surface humidity may change more or drastically. If it is not maintained well and is wet and dry intermittently, the dry shrinkage deformation of the surface will be constrained by the internal concrete, which often leads to cracks. Some of our questions were explained in detail through explanations. At the same time, the teacher also pointed out to us the unreasonable aspects of the external design of the building, such as: the equipment platform is too narrow, and the staggered design of the balconies can easily cause sewage seepage, etc., and reminded us what we should pay attention to in future designs.

We entered the house and visited the functional partitions of the house. The accompanying staff also told us the misunderstandings and mistakes in the architectural design, letting us know that we should pay attention to the bathroom, elevator room and Staircase treatment. There, first of all, we watched the construction workers' excavation process of the foundation on site, and learned about the reasons that affect the depth of the foundation, as well as some issues that need to be involved and paid attention to when laying foundations in Guizhou. Then I visited the second phase of the project there. The project uses a frame-shear structure. It is a combination of the frame structure and the shear wall structure. It absorbs the strengths of each and can not only provide building layout Large use space and good lateral force resistance performance. This kind of structure arranges a certain number of shear walls in the frame structure to form a flexible and free use space to meet the requirements of different building functions. It also has enough shear walls and considerable stiffness, and the frame shear structure is subject to The force characteristics are a new force-bearing form composed of two different lateral force-resistant structures, frame and shear wall structure. Therefore, its frame is different from the frame in pure frame structure, and the shear wall is also different in frame-shear structure. Shear walls in shear wall structures. In this project, we learned in detail the construction methods of beams and columns and the issues that need to be paid attention to when constructing frame structures.

The construction staff explained to us the different construction methods of beams and columns, the binding of steel bars and the connection of broken bars. For example, the binding of steel bars and the binding of the underlying foundation steel bars must first be laid out, and each span must be laid out. The number of joints in the steel bars is only 25, that is, there is only one joint in the four steel bars. In addition, the joints should be placed in the pressure zone as much as possible.

During the process of building a wall, if the walls are to turn or intersect, the two walls should be built together. During the process of retaining the beams, you can leave the diagonal beams. If you want to leave the straight beams, then The yang cha must be left, and there must be tie muscles, and the yin cha must not be left. This knowledge is often something I rarely come into contact with or pay attention to in school, but it is very important and basic knowledge. Let us benefit a lot. I have been practicing outside for more than a month. During these days, through personal experience, I have put into practice the theoretical knowledge I learned in school. It also provides great help for actual design work and provides realistic materials for graduation projects. This avoids the disconnect between design and actual construction during the design process;