With the increase of steel production and the decrease of steel price in China, it is more and more common to build houses with steel, which is not only reflected in various large-span projects in the ascendant, but also in various small and medium-span projects. It has become a fashion to choose steel as building materials or decorative materials. The diversified choice of materials not only provides architects with more means of expression, but also brings more design requirements.
Although the choice of structural form is usually the work of structural engineers, architects must also have a certain understanding of the factors affecting the design of structures and connections; Even if most of the design of steel connection members in China is completed by manufacturers, architects must have a clear understanding of the connection mode, control principle and leading factors of structural members of modern steel structures.
A, steel structure components and characteristics
Another feature of steel structural members is the diversity of sections: on the one hand, the isotropic homogeneity of steel makes it possible to be processed into various shapes, which is significantly different from the isotropic heterogeneity of wood: as two linear structural members, the diversity of steel structural sections is incomparable to that of wood structures, which is why there are I-beams instead of I-beams, which can also be explained by the fact that almost all connecting members in the bar structure are steel; On the other hand, the cross-sectional shape of steel structure members also has a great influence on the connection design. When the stress range allows, the choice of different steel can lead to different connection forms.
Second, the constraints of connection design
There are many kinds of steel, and the resistance is different. Connection design is usually limited by the following factors:
1. Source of components: In theory, steel structural components or connecting components have arbitrary machinability, but in concrete projects, structural components and connecting components are always restricted by realistic conditions. Experienced designers usually choose section steel that is easy to obtain and install, and design simple and effective connection methods and components.
2. Limitation of connection means: One of the characteristics of steel structure construction is factory processing and on-site assembly. This is different from the traditional masonry method, which will produce a large number of nodes. There are three main ways to connect various types of steel: riveting, welding and bolting. Riveting is often used in the early stage of steel structure buildings, which is simple in construction, but it is necessary to dig holes in the components, which reduces the cross-sectional performance and is easy to produce concentrated stress at the joints. It's rarely used recently. The welded joint has simple appearance and high continuous force transmission efficiency, but the construction requirements are higher. The high-strength bolt connection that appeared in the later period can also meet the strength requirements of similar welding and is widely used in modern steel structures.
3. Connecting members are hierarchical: there is a complex logical hierarchical relationship between the structural systems of steel structures. At the connection level, this hierarchical relationship is reflected in the different component sizes and installation orders. The purpose of connection is to realize hierarchical transformation, and it is also the key to realize the transformation of force from three-dimensional to two-dimensional and finally to one-dimensional components. Complex connection is usually completed by the combination of three-dimensional connection components and plane connection components.
4. Plane where the connecting members are located: Two linear structural members are always in the same plane, which is the plane where the members are stressed. In order to effectively resist the stress, bending moment or shear force in this plane, connectors are often designed in this plane, such as the welded joints of steel pipes and cables are always in the plane they form; In the case of connecting multiple members, the combined plane members can correspond to the three-dimensional stress situation.
Thirdly, the prototype of the connection object and the classification of the connection.
The structural structure of steel structure buildings includes two basic types: rod and cable, and the resulting connection modes can be divided into cable-to-cable connection, rod-to-rod connection and cable-to-rod connection. Whether it is a plane structure or a spatial structure, the complexity of the structural system will not lead to more types of connections because of the linear characteristics of component units, and complex connections can be regarded as a combination of these three basic types.
1. Cable to cable connection
In the structural system, the cable only bears tension, the internal force is constant along the axis of the cable, and the cross section of the cable does not change with the length. Its connection features are as follows:
A. the terminal connection must be realized through cable sleeve.
B. The connection between cables can be completed by various clamps.
C. The connection of multiple cables needs to design new nodes, which are different according to different cable combination methods: for example, the common single-layer or double-layer cable structure can appear in parallel or radiation form according to different arrangement methods. The light and effective cable net caters to people's universal pursuit of lightness and transparency. Piano designed the glass roof of the outdoor square of the headquarters of Bancapopolaredi. The stressed structure consists of two groups of cable nets, with glass sandwiched in the middle of the structure, reinforced and stabilized by horizontal connecting cables and fixed on the surrounding concrete walls. All nodes are connected by cables. The light and transparent roof shows the advantages of cables in specific occasions.
2. Rod to rod connection
Rod appeared earlier than cable and was widely used. Because members can be in tension and compression at the same time, the design of connection between members is more complicated and changeable. There are two main factors that determine the design of steel bar connection:
A. the angle between bars.
B. the respective cross-sectional shapes of the rods. The former is determined by the structural system, and the latter can choose different sections or combinations in the connection design stage, which is convenient for the design of connection components.
When the intersection angle is not perpendicular, the section of the member should be carefully selected to reduce the difficulty of joint design. In the beam-column connection of an information center in Berlin, the circular steel tube column was abruptly cut off and then welded to the I-beam with several steel plates. This connection is very mechanical and rigid. In contrast, in the design of the beam-column connection of Hanover Exhibition Hall designed by Hertzog, on the one hand, cables are selected to avoid non-vertical connection between components; On the other hand, the decomposition and cyclic changes of square steel tubular columns at the joints make the connection transition between them and circular steel tubular beams natural and reasonable.
The roof frame of Meixiu Museum designed by I.M. Pei did not adopt the conventional ball joint method, but designed multi-directional flat cast steel joint members, with transverse steel pipes running through it, and the cross section remained unchanged. Other members were converted into steel plates at the joints and connected with the joint members by bolts, which ensured that the structural plane was parallel to the roof and achieved a concise and flat effect. If ordinary spherical joints are used, the huge joints will inevitably make the roof truss look heavy, and it will not reflect the simplicity of tenon-mortise connection in traditional wooden buildings, and it is even more difficult to achieve such artistic effects.
3. Connection between cables and steel bars
Although cable is the most suitable material property for steel, it is a flexible member with unidirectional tensile property, so the structural system composed of cable and bar can realize the dual advantages of light weight and stability, such as beam string structure, which is very popular in the long-span field in recent years. Even in small-span buildings, cable-strut combination is quite common.
According to whether the cable is continuous or not, different connectors must be used in the connection design: the connection at the end of the cable needs to be fixed with the rod through a series of components such as cable sleeve, bolt and welding lug. The connection program is simple and reasonable in structure and widely used. At the same time, you can choose two different ways: cable sleeve clamping welding joint or welding joint clamping cable sleeve. For example, in the redevelopment project of the old seaport in Genoa, Italy, the cable-membrane structure and poles are connected in different layers painted with gray and white respectively. However, it can also be directly combined with the rod through a specially designed cable sleeve. It should be noted that omitting the weldment means that the cable sleeve must undertake the function of transforming the angle between the cable and the bar, such as the connection between the cable and the foundation of Keppel Cultural Center: the cable sleeve with exaggerated scale and certain angle integrates the function of the weldment, making this joint unique in this kind of connection. For connections located in the middle of cables, cable clamps and cable clamps (or welding in a few cases) can be used to fix the position of cables and rods. The connection design of fish belly beam, upper chord and lower chord in Hanover Trade and Trade Office is fixed with Sokka.
Fourth, the method of connection design
In a considerable number of steel structure buildings, the requirements for the form and size of components are not very strict, which brings considerable design space for the design of components and connections. The architect's choice of component form and size is closely related to the connection design or connection form. For domestic designers, the actual situation is often a limited choice of section steel and a few formed connecting members, so the design method is particularly important, although these methods or means are often purely expressive.
The object of 1. decomposition is an abstract single component in the structural concept, which is often manifested as the decomposition of the number of components or the decomposition of the cross-section form in the actual connection. This technology is quite common in the design of bar connection including steel structure. For example, the compression bar in a suspended structure is often decomposed into several bar combinations; Columns are often broken down into beams and columns and so on. Some of these decompositions are for the need of structural stress, and some are only for creating gaps in the parts and facilitating connection. The decomposition design of complex nodes is helpful to extract the basic form of connection, thus simplifying the concept of connection and integrating node design.
The concept of decomposition is widely used in the connection design of steel roof named CyTwombly Gallery in Houston, USA, which skillfully combines multi-storey roof structure with sunshade system. First of all, it is a grid-like sunshade structure composed of I-beams, and the skylight structure is suspended under the sunshade structure. Because the tensile member is more in line with the mechanical properties of steel, the size is smaller, so the hanging rod is not decomposed, while the steel beam is more complicated (bending and shearing), so the size is larger, and one is decomposed into two. The logic of the whole roof design is clear, which complements the layered concept of roof function.
2. The consideration of the conversion section form and the size conversion are also the main problems in the connection design. The purpose of the transformation is to reduce the section of components and facilitate the connection design. Generally speaking, the sections of members can be divided into two categories: tubular sections (such as round steel pipes) and non-tubular sections (such as I-beams). The connection between the former often needs to reduce the cross section, such as the reduced solid steel pipe at the ball joint of grid structure. The connection between the latter usually simplifies the components, such as removing the wide wings and retaining only the height of the section steel, and often combines the decomposition design. In addition to changing the cross-sectional shape of members, it is also a common method to design new joints after cutting members along a plane perpendicular to the axis of members. For example, the roof frame joint of Meixiu Museum discussed earlier, the transformation of steel pipe section not only brings convenience to the connection, but also reflects the flexibility of joint design. While expressing the transmission of power, the use of sculpture design makes the flow of power visual and artistic.
3. Integrated design is based on reducing the types and quantity of components, simplifying the connection mode and emphasizing the integrity of structural forms, which makes such design works often have elegant images. This method has two premises:
1) All nodes are stressed in the same way.
2) The cross-sections of structural members are similar in shape and size. The same stress mode ensures the scientific selection of similar section, and the similar section form and approximate size ensure the visual integrity.
The shell of the Law Department Building of Cambridge University in England designed by Foster is made of steel and glass, which blurs the concepts of windows, walls and roofs, and the integrated design of the components just expresses this idea. Woven skin is a structure interwoven by many arched vierendeel beams with the smallest height, and all components are welded by circular steel pipes, so the whole structure appears even and elegant. Coincidentally, OMA applied the same principle to the skin design of Seattle Central Library. In order to achieve the integrated effect, all the surfaces of the whole building are completely composed of I-beams intersecting with diamonds, and the cross-sectional shapes and sizes of I-beams are completely consistent, and the connection methods are also unified through riveting. After stress analysis and calculation, it is rational and romantic to superimpose two or three I-beams where the stress is large and the section height needs to be increased, which makes the library quiet and peaceful.
Although types can be analyzed and summarized and methods can be classified and summarized, this does not mean that design has become dogma, nor does it mean that excellent design has equations. Due to the excavation of steel properties and the pursuit of the ultimate form, the connection design contains a lot of technical factors. Even so, the design of connection components should not be the responsibility of engineers or component suppliers, because the solid reserve of technology is always an insurmountable step for architects to obtain excellent connection design. Although the connection methods (riveting, welding and bolting) that conform to the properties of steel must follow these laws and be designed within the scope allowed by steel, excellent design can still find beautiful connection forms without rigid mechanical products due to constraints. Even if there are differences in the evaluation criteria of aesthetics in different times, an inherent attraction that conforms to the logic of materials will inevitably break through the limitations of style or theory and reach the true meaning of beauty.
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