Design idea of steel structure engineering?

(A) to determine whether the structure is suitable for steel structure

Steel structure is usually used for high-rise, long-span, complex structure, heavy load or crane lifting weight, large vibration, high plant temperature, high sealing requirements, movable or frequently disassembled structures. Intuitively speaking: buildings, gymnasiums, 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

This is just a brief introduction. For details, please refer to relevant professional books. Due to the wide range of structural selection, structural selection and layout should be carried out under the guidance of experienced engineers.

Conceptual design should be emphasized in the whole process of steel structure design, which is particularly important in the stage of structural selection and layout. For some problems that are difficult to make accurate and reasonable analysis or are not specified in the code, the layout and specific measures of the control structure can be determined from the overall point of view according to the design ideas obtained from the mechanical relationship between the overall structural system and subsystems, failure mechanism, earthquake damage, experimental phenomena and engineering experience. Conceptual design can be conceived, compared and selected quickly and effectively in the early stage. The obtained structural scheme is often easy to calculate by hand, with clear concept and correct qualitative analysis, which can avoid unnecessary complicated operations in the structural analysis stage. At the same time, it is also the main basis for judging whether the output data of computer internal force analysis is reliable.

Steel structure usually includes frame, plane (raft) frame, grid (shell), cable-membrane structure, light steel, tower mast and other structural types.

Its theory and technology are mostly mature. Some problems have not been solved, or there is no simple and practical design method, such as the stability of reticulated shells.

When choosing structures, we should consider their different characteristics. In the light steel industrial workshop, when there is a large hanging load or moving load, we can consider giving up the portal frame and adopting the grid structure. In areas with high basic snow pressure, the roof curve should be conducive to snowfall (snow load should be considered when the tangent is 50 degrees), such as the three-center circular reticulated shell used in the limestone shed of Yadong Cement Plant. Nearly half of the total snow was released. There are similar considerations in areas with heavy rainfall. If the building permits, it will be more economical to arrange the support in the frame than the frame connected by simple nodes. However, in buildings with large roof span, cable or cable-membrane structure system with tensile members as the main component can be selected. Steel-concrete composite structures are often used in the design of high-rise steel structures. In high-rise buildings with high or irregular earthquake intensity, we should not choose the form of core tube plus outer frame which is not conducive to earthquake resistance simply for the sake of economy. It is suggested to choose a structural system with giant SRC columns around it and a supporting frame at the core. More than half of the senior officials like China are the former. It is not good for earthquake resistance. [ 19]

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 should be clear, and the influence range of large load or moving load should be limited as far as possible, so that it can be transmitted to the foundation by the most direct route. Lateral bracing between columns should be evenly distributed, and its centroid should be as close as possible to the line of action of lateral force (wind vibration). Otherwise, the torsion of the structure should be considered. The lateral force of a structure should have multiple lines of defense, such as a braced frame structure.

The layout of secondary beams in the floor plane of frame structure can sometimes adjust its load transfer direction to meet different requirements. Usually, the secondary beams are arranged in the short direction to reduce the cross section, but this will increase the cross section of the main beam and reduce the clear height of the floor, and sometimes the top side columns will be unbearable. At this time, supporting the secondary beam on the shorter main beam can save the main beam and columns at the expense of the secondary beam.

(3) estimating the cross section

After the structure is arranged, it is necessary to make a preliminary estimation of the cross section of the member. It is mainly the assumption of the cross-sectional shape and size of beams, columns and bearings.

The steel beam can be channel steel, rolled or welded H-beam, etc. According to the load and stress, the section height is usually selected between 1/20 and 1/50 of the span. When the flange width is determined according to the l/b limit according to the distance between transverse supports between beams, the complicated calculation of the overall stability of steel beams can be avoided. This method is very popular. After determining the section height and flange width, the thickness of the plate can be estimated according to the structural provisions of local stability in the code.

Column section is estimated according to slenderness ratio. Usually 50

Novices should pay attention to the fact that the structural requirements of section in the code vary greatly for different structures. For example, the local stability of plates unique to steel structures. The limit values in General Steel Structure Code and Light Steel Code are quite different.

In addition, there is no fixed requirement for the selection of cross-section form of members, and structural engineers should reasonably choose safe, economical and beautiful cross-sections according to the stress situation of members.

structural analysis

At present, in the actual design of steel structure, structural analysis is usually linear elastic analysis, and P-δ and P-δ are considered when conditions permit.

Some recent finite element software can partially consider the geometric nonlinearity and elastoplasticity of steel, which provides conditions for more accurate analysis of structures. Not all structures require software:

The internal force and deformation of typical structures can be obtained directly from reference books such as mechanics manuals.

Simple structure is analyzed by hand.

Complex structures need modeling, running programs and detailed structural analysis.

(5) engineering judgment

In order to use structural software correctly, we should also make "engineering judgment" on its output results. For example, the period, total shear force and deformation characteristics in each direction are evaluated. According to the "engineering judgment", choose to modify the model for reanalysis or modify the calculation results.

Different softwares have different applicable conditions. Novices should be fully aware of this. In addition, there is always a certain distance between the calculation in engineering design and the accurate mechanical calculation. In order to obtain a practical design method, the assumption of large error is sometimes adopted, but for this error, the safety of the structure will be ensured through "applicable conditions, concepts and structures". In steel structure design, "applicable conditions, concepts and structures" are more important than quantitative calculation.

Engineers should not trust and rely too much on structured software. An American scholar once warned: "It is only a matter of time before computer abuse causes structural damage and disaster."

Paying attention to conceptual design and engineering judgment is the way to avoid such engineering disasters.

(6) Component design

The design of components is first of all the choice of materials. Q235 (similar to A3) and Q345 (similar to 16Mn) are commonly used. Usually, the main structure adopts a single steel grade for the convenience of project management. Considering the economy, composite steel with different strength can also be selected. When the intensity plays a controlling role, Q345 can be selected. Q235 is applied to stability control.

In component design, the current code uses elastic-plastic method to check the cross section, which does not match the elastic method to calculate the internal force of the structure.

The current structural software provides the post-processing function of cross-section checking calculation. Due to the progress of program technology, some softwares, such as sap2000, can increase the number of failed components by one level from a given section library, and automatically re-analyze and check until they pass. This is one of the functions of section optimization design. It reduces a lot of workload of architects. However, beginners should at least pay attention to two points:

1. When checking the section of components (mainly columns), the determination of the calculation length coefficient sometimes does not conform to the norms. At present, all the schemes can't completely solve this problem. Therefore, especially for members with complex joint connections or variable cross sections, architects should check them one by one.

2. When the estimated section in (3) above is not satisfied, the increased section should be treated differently in two cases.

When the (1) strength is not satisfied, the thickness of the cross-section plate of the member is usually increased, in which the bending bearing capacity does not meet the requirement of increasing the flange thickness and the shear bearing capacity does not meet the requirement of increasing the web thickness.

(2) When the deformation exceeds the limit, the plate thickness should not be increased, but the section height should be increased, otherwise it will be uneconomical.

It is difficult to consider the difference between the above strength and stiffness by using the optimization design function of the software to automatically increase the cross section. In fact, it is often inappropriate.

(7) Node design

Joint design is one of the important contents of steel structure design. Before structural analysis, the form of joint should be fully considered and determined. The final design of the joint is often not completely consistent with the form used in the structural analysis model, which must be avoided. According to the different force transfer characteristics, joints can be divided into rigid connection, hinged connection and semi-rigid connection. Beginners should choose the first two that can be analyzed simply and quantitatively. Common reference books [2] are rich in recommended joint practices and calculations.

Different connections have great influence on the structure. For example, although some rigid joints have no problem bearing bending moment, they will produce large rotation, which does not conform to the assumptions in structural analysis, and will lead to unfavorable results such as actual engineering deformation greater than calculated data.

There are two commonly used joint connection methods: equal strength design and actual stress design. Novices can safely choose the former. The design manual [2] usually provides designers with tables of welded joints and bolted joints, which is more convenient. It can also be done automatically by using the post-processing part of the structural software.

The specific design mainly includes the following contents:

1. welding: the specification has mandatory provisions on the size and form of weld, which should be strictly observed. The choice of covered electrode should be suitable for the metal materials to be joined. E43 corresponds to Q235 and E50 corresponds to Q345. When Q235 is connected with Q345, E43 with low strength should be selected instead of E50.

Weld shall not be enlarged arbitrarily in welding design. The center of gravity of the weld should be as close as possible to the center of gravity of the connecting parts. For other details, please refer to the provisions on weld structure in the specification.

2. Bolt connection:

Riveting is rarely used in construction projects.

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

High strength bolts are widely used. There are two commonly used strength grades: 8.8s and 10.9s, which are divided into bearing type and friction type according to the mechanical characteristics. The two types of calculation methods are different. The minimum specification of high strength bolts is M 12. The common bolts of M 16~M30 are unstable, so they should be used carefully in design.

Self-tapping screws are used for secondary connection between plate and thin-walled steel. In foreign countries, it is also often used to connect the main structure in low-rise wallboard houses.

3. Connecting plate: simply take its thickness as the thickness of beam web plus 4mm, and then check the shear capacity of net section.

4. Beam web: Check the net section shear strength of the web at the bolt hole. Pressure-bearing high-strength bolt connection also needs to check the local pressure on the hole wall.

5. The joint design must consider the installation of bolts, the construction space for on-site welding, and the hoisting sequence of components. It is a common mistake for novices to transport components to the site and fail to install them. In addition, workers should make on-site positioning and temporary fixing as convenient as possible.

6. The design of the joint should also consider the technical level of the manufacturer. For example, the cutting of intersecting lines of steel pipe joints requires equipment such as CNC machine tools.

(8) Drawing preparation

Steel structure design drawings are divided into two stages: design drawings and construction details. The design drawings are provided by the design unit, and the construction details are generally compiled by the steel structure manufacturing company according to the design drawings, and sometimes by the design unit. Due to the contradiction between the increase of 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 ability to participate in the preparation of design drawings.

1. Design drawings: the basis for the manufacturer to prepare detailed construction drawings. Depth and content should be complete but not redundant. In the design drawings, design basis, load data (including earthquake action), technical data, material selection and material requirements, design requirements (including manufacturing and installation, weld quality inspection level, painting and transportation, etc.). ), structural layout, component section selection, main node structure should be made clear. The main materials shall be listed.

2. Construction details: also known as processing drawings or lofting drawings. The depth must meet the requirements of direct manufacturing and processing in the workshop. Other parts that are not exactly the same must be drawn and represented separately, with a detailed bill of materials attached.

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