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Steel structure is usually used in the structure of high-rise, large-span, complex shape, heavy load or crane lifting, large vibration, high temperature workshop, high tightness requirements, movable or frequently assembled and disassembled. Intuitively speaking: buildings, stadiums, opera houses, bridges, TV towers, barns, factories, houses and temporary buildings. This is consistent with the characteristics of the steel structure itself.

(2) Structural selection and layout

What should be emphasized in the whole process of steel structure design is "concept design", which is especially important in the stage of structural selection and layout. For some problems that are difficult to make a rational analysis or unspecified in the specification, the design ideas obtained from the mechanical relationship between the overall structural system and the sub-system, the failure mechanism, earthquake damage, test phenomena and engineering experience Angle to determine the layout and detailed measures of the control structure. The use of conceptual design allows for rapid, efficient and effective early conception, comparison, and selection. The resulting structural scheme is often easy to calculate by hand, the concept is clear, the qualitative and correct, and can avoid unnecessary tedious calculation in the structural analysis stage. At the same time, it is also the main basis for judging the reliability of the output data of computer internal force analysis. (No matter how powerful the structural software is, solid structural concepts and mechanical analysis, and reliable hand calculation ability are the excellent qualities.)

Steel structure usually has frame, plane (wood row) frame, net frame (shell), cable membrane, light steel, tower mast and other structural types.

The theory and technology are mostly mature. There are also some problems that have not been solved, or there is no simple and practical design method, such as the stability of the reticulated shell.

When selecting structures, their different characteristics should be considered. In the light steel industrial plant, when there is a large hanging load or moving load, it is possible to consider abandoning the portal rigid frame and adopting a grid frame. In areas with heavy snow pressure on the roof, the roof curve should be conducive to the snow falling (the snow load must be considered within 50 degrees of the tangent). For example, the limestone warehouse shed of the Yadong Cement Plant uses a three-centered round mesh shell. Almost half of the total snow load was released. Areas with heavy rainfall are similarly considered. When the building permits, it is more economical to arrange supports in the frame than a simple frame with just-connected nodes. In buildings with a large roof covering span, a suspension cable or cable-membrane structure system with mainly tensioned members can be selected. In the design of high-rise steel structures, steel-concrete composite structures are often used. In high-rise buildings with high ground intensity or very irregular structures, the form of unfavorable seismic core tube and outer frame should not be selected solely for economic reasons. The surrounding giant src columns should be selected, and the core is the structural system supporting the frame. More than half of such senior executives in China are the former. Not good for * shock. (Arranging the load-bearing elements as far as possible outside the structure is the key to making full use of the material's properties, just like hollow bamboo, so the external strength and internal weakness are important.)

The layout of the structure should be comprehensively considered according to the system characteristics, load distribution and nature. Generally speaking, the stiffness should be uniform. The mechanical model is clear. Limit the influence range of the maximum load or moving load as much as possible, so that it can be transmitted to the foundation by the direct route. The distribution of anti-side braces between columns should be even. Its centroid should be as close as possible to the line of action of the lateral force (wind shock). Otherwise, the torsion of the structure should be considered. There should be multiple lines of defense on the anti-side of the structure. For example, with a supporting frame structure, the column should be able to bear at least 1/4 of the total horizontal force alone.

The layout of the floor secondary beams of the frame structure can sometimes adjust its load transfer direction to meet different requirements. Generally, the secondary beams are arranged along the short direction in order to reduce the cross section, but this will increase the main beam section and reduce the net height of the floor. Sacrifice the secondary beam to keep the main beam and the pillar.

(3) Estimated cross section

After the structural arrangement is completed, a preliminary estimate of the section of the member is required. It is mainly the assumption of the cross-sectional shape and size of beams, columns and supports.

The steel beam can be selected from channel steel, rolled or welded * h steel section, etc. According to the load and bearing conditions, the section height is usually selected between 1/20 to 1/50 of the span. When the width of the flange is determined by the l / b limit according to the spacing of the lateral supports between the beams, this method can avoid the complicated calculation of the overall stability of the steel beam. After the section height and flange width are determined, the thickness of the plate can be estimated according to the locally stable construction regulations in the code.

The column section is estimated according to the slenderness ratio. Usually 50 <λ <150, the simple choice is around 100. According to the difference of axial compression, bidirectional bending or unidirectional bending, steel pipes or h-shaped steel sections can be selected.

(4) Structural analysis

Some recent finite element software can partially consider geometric nonlinearity and the elastic-plastic properties of steel. This provides conditions for a more accurate analysis structure. Not all structures require software:

The typical structure can be obtained directly from internal reference books such as mechanics manuals.

The simple structure is analyzed by hand calculation.

Complex structures require modeling and running procedures and detailed structural analysis.

(5) Project judgment

To use the structural software correctly, the "engineering judgment" of the output results should also be made. For example, to evaluate the isotropic cycle, total shear force, deformation characteristics, etc. According to "Engineering Judgment", choose whether to modify the model for re-analysis or to correct the calculation result.

(6) Component design

In component design, the current specification uses the elastoplastic method to check the section. This does not match the elastic method of calculating the internal force of the structure.

Current structural software provides post-processing functions for cross-sectional verification. Due to the advancement of programming technology, some software can choose to increase the * level from the given section library for the components that do not pass the check. And automatically re-analyze the calculation until it passes, such as sap2000. This is one of the functions of cross section optimization design. It reduces the workload of the architect. However, beginners should pay attention to at least two points:

1. When the software is doing the cross-section calculation of the components (mainly columns), the calculation of the calculated length coefficient sometimes does not meet the requirements of the specifications. All current programs cannot completely solve this problem. Therefore, especially for components with complicated or variable cross-sections, the architect should check one by one.

2. When the estimated cross-section in Article (3) above is not satisfied, the enlarged cross-section shall be treated differently in two cases.

(1) If the strength is not satisfied, the thickness of the plates forming the cross-section is usually increased. Among them, the bending resistance is not sufficient to increase the flange thickness, and the shear resistance is not sufficient to increase the web thickness.

(2) Deformation exceeds the limit. Generally, the thickness of the plate should not be increased, but the height of the cross section should be considered. Otherwise, it will be very uneconomical.

It is difficult to consider the above-mentioned distinction between strength and stiffness by using the aforementioned optimization design function of automatically increasing the cross-section of the software.

(7) Node design

The design of connecting nodes is one of the important contents in the design of steel structure. Before structural analysis, the form of nodes should be fully considered and determined. A situation that often occurs is that the nodes of the final design are not completely consistent with the form used in the structural analysis model, which must be avoided. According to the different force transmission characteristics, the nodes are divided into rigid joints, hinged joints and semi-rigid joints. Beginners should choose the first two that can be simply quantitatively analyzed. Commonly used reference books [2] have a wealth of recommended node practices and calculation formulas.

The difference in connection has a great influence on the structure. For example, although some rigid joints have no problem with bending moments, they will produce large rotations, which does not meet the assumptions in structural analysis. It will lead to unfavorable results that the actual engineering deformation is greater than the calculated data.

There are two commonly used methods for connecting nodes, such as equal-strength design and actual force design. Beginners can choose the former. Design manuals usually have tables of welds and bolt connections for designers to check, which is more convenient. It can also be done automatically using the post-processing part of the structural software.

The specific design mainly includes the following:

1. Welding: The size and form of the welding seam are compulsory and must be strictly observed. The choice of electrode should be adapted to the metal material to be connected. e43 corresponds to q235, and e50 corresponds to q345. When connecting q235 and q345, you should choose low-intensity e43 instead of e50.

Welding * Welding seams must not be arbitrarily enlarged in the design. The weight * of the weld should be as close as possible to the weight * of the connected member. For other details, please refer to the regulations on the construction of welds.

2. Bolting:

Riveting forms are rarely used in construction projects.

Ordinary bolts have poor shear resistance and can be used in secondary structural parts.

High-strength bolts are increasingly used. Two intensity levels of 8.8s and 10.9s are commonly used. According to the force characteristics, it can be divided into pressure type and friction type. The calculation methods are different. * Strong bolt * Specification m12. Commonly used m16 ~ m30. The performance of the oversized bolts is unstable and should be used carefully in the design.

Self-tapping screws are used for the secondary connection between sheet and thin-walled steel. In low-rise wall-slab houses abroad, it is also often used to connect the main structure.

3. Connecting plate: The thickness can be simply taken as the thickness of the beam web plus 4mm. Then check the net section shear resistance.

4. Beam web: The net section shear of the web at the bolt hole should be verified. The pressure-bearing type * strong bolt connection also needs to check the partial pressure of the hole wall.

5. Node design * The construction space for mounting bolts, on-site connection, etc. and the order of component lifting must be considered. It is a long-term mistake for beginners that components cannot be installed on site. In addition, workers should be able to locate and temporarily fix the site as easily as possible.

6. The node design should also consider the level of craftsmanship of the manufacturer. For example, the cut of the intersecting line of the steel pipe connection node needs CNC machine tools and other equipment to complete.

(8) Drawing preparation

The steel structure design drawing is divided into two stages: the design drawing and the construction detailed drawing. The design drawing is provided by the design unit. The construction detailed drawing is usually prepared by the steel structure manufacturing company according to the design drawing, and sometimes it is also prepared by the design unit. Due to the contradiction between the increase in steel structure projects in recent years and the lack of steel structure engineers in design institutes, it is also common for steel structure companies with design capabilities to participate in the preparation of design drawings.