We’ve received many messages from our website recently that many visitors asked how to design a steel structure, so today we’ll share the design procedure and principle of steel structure designing. Hope this article will be helpful for you.
1. Determine whether steel structure is suitable for the building.
Steel structure is usually used for high-rise, large-span, complex frame, heavy load or crane lifting, large vibration, high tightness requirements, movable, or frequently assemble and disassemble structures. Intuitively speaking, buildings, stadiums, opera houses, bridges, TV towers, warehouses, factories, garages, hangars, residences and temporary buildings. This is consistent with the characteristics of the steel structure itself.
2. Structure selection and layout.
The structure selection and layout should be carried out under the guidance of experienced engineers as it involves a wide range. What should be emphasized in the whole procedure of steel structure design is "conceptual design", which is especially important in the structure selection and layout stage. For some problems that are difficult to make an accurate rational analysis or are not specified in the code, the design ideas could be obtained from the mechanical relationship between the overall structural system and the sub-systems, failure mechanism, earthquake damage, experimental phenomena and engineering experience. To determine the layout and detailed measures of the control structure from overall perspective. Conceptual design can be used to conceive, compare and select quickly and effectively at an early stage.
Different characteristics of steel structures should be considered during the selection. In areas with heavy snow pressure on the roof, the roof curve should be conducive to snow sliding. Similar considerations are given to areas with heavy rainfall. When the building permits, arranging supports in the frame will be more economical than a simple frame with just-connected nodes. In buildings with large roof spans, a suspension cable or cable-membrane structure system with main components under tension can be selected. In the design of high-rise steel structure, steel-concrete composite structure is often used.
The layout of the structure should be comprehensively considered according to the characteristics of the system and load distribution and nature. Generally speaking, the stiffness should be uniform, and the mechanical model should be clear. Limit the influence range of large loads or moving loads as much as possible so that they can be transmitted to the foundation in the most direct way. The distribution of anti-slide support between columns should be uniform. The center should be as close as possible to the action line of the lateral force (wind shock). Otherwise, the torsion of the structure should be considered. The anti-side of the structure should have multiple defense lines.
Sometimes the load transfer direction of secondary beams on the floor plan of frame structure can be adjusted to meet different requirements. Usually, in order to reduce the cross-section, the secondary beams are arranged in a short direction, but this will increase the section of main beam and reduce the net floor’s net height, which caused side columns on the top floor are overwhelmed sometimes. At this time, the secondary beam can be supported on a shorter main beam, and sacrifice the secondary beam to save the main beam and pillars.
3.Cross section’s estimation.
A preliminary estimation of the component section is required after the structural layout is completed. It is mainly the assumption of the cross-sectional shape and size of beams, columns and bracings.
Channel steel, rolled or welded H beams can be chosen as steel beams. According to the load and support, the section height is usually selected between 1/20 and 1/50 of the span. When the flange width is determined according to l/b limit of lateral support spacing between the beams, the complicated calculation of overall stability of the steel beam can be avoided. This method is very popular. After the section height and flange width are determined, the thickness of the plate can be estimated according to the local stable construction regulations in the code.
The column section is estimated according to the slenderness ratio. Usually 50<λ<150, simply choose a value around 100. According to the different axial compression, two-way bending or one-way bending, steel pipe or h-shaped steel section beam can be selected. Beginners need to pay attention that corresponding to different structures, the structural requirements of the cross-section in the specification are very different. There is no fixed requirement for the selection of the component’s section form. The structural engineer should choose a safe, economical and beautiful section based on the force of the component.
4. Structural Analysis
At present, in the actual design of steel structures, structural analysis is usually linear elastic analysis, and p-Δ, p-δ are considered when conditions permit. Some recent finite element softwares can partially consider the geometric non-linearity and the elastic-plastic properties of steel. This provides conditions for a more precise analysis structure. Not all structures require the use of software: typical structures can be found in reference books such as mechanical manuals to directly obtain internal forces and deformations.
5. Engineering Judgment
To use the structural software correctly, the output result should also be "engineered judgment". For example, evaluating the period of each direction, total shear force, deformation characteristics, and etc. According to the "engineering decision", choose whether to modify the model to re-analyze or to modify the calculation result. Different softwares have different applicable conditions. Beginners should fully understand. In addition, there is often a certain difference between calculations in engineering design and precise mechanical calculations. In order to obtain practical design methods, sometimes assumptions with large errors will be used, but applicable conditions, concepts and structures will be adopted to ensure the safety of structure. In steel structure design, the principle of "applicable conditions, concept and structure" is more important than quantitative calculation. Engineers should not rely too much on structural softwares. Paying attention to conceptual design and engineering judgment is the way to avoid such engineering disasters.
6. Component Design
The first job for components design is the choice of materials. Q235 (similar to a3) and Q345 (similar to 16mn) are generally used. Usually the main structure uses a single steel grade to facilitate project management. For economic considerations, it is also possible to choose a combination section of different strength steels. When the intensity plays a control role, Q345 can be chosen. When it is stable, Q235 will be a better choice. In the component design, the current principle uses the elastic-plastic method to check the section. This does not match the elastic method of structural internal force calculation.
All current structural softwares provide post-processing functions for cross-section checking. Due to the advancement of program technology, some softwares can choose to increase one level from the given cross-section library for the components that did not pass the check. And automatically re-analyze and check the calculation until it is passed, such as sap2000. This is one of the functions of cross-section optimization design. It reduces a lot of work for architects.
7. Node Design
The design of connecting nodes is one of the important contents in steel structure design. Before structural analysis, the form of the node should be fully considered and determined. A situation that often occurs is that the final design node is not completely consistent with the form used in structural analysis model, which must be avoided.
According to different force transmission characteristics, the nodes are divided into rigid connection, hinge connection and semi-rigid connection. Beginners should choose the first two for simple quantitative analysis. The difference in connection has a great influence on the structure. For example, although some just-connected joints could bear the bending moment, they will produce large rotation, which does not conform to the assumptions in structural analysis. It will cause the actual engineering deformation larger than unfavorable result of the calculated data. There are two common methods for connecting nodes: equal-strength design and actual force design. Beginners can choose the former in consideration of safely
8. Drawing Preparation
The steel structure design drawing is divided into two stages: design drawing and construction detail drawing. The design drawing is provided by design company. The construction detail drawing is usually prepared by steel structure manufacturing company according to design drawing, and sometimes by design company.
Design drawing: It provides the basis for the manufacture of detailed construction drawings. The drawing and content should be complete. In the design drawing, the design basis, load data, technical data, material selection and material requirements, design requirements, structural layout, component cross-section selection, and main node structure should be clearly expressed to facilitate the smooth preparation of detailed construction drawings , which could correctly reflect the design intent. The main materials should be shown in a list.
Detailed construction drawings: also called processing drawings or stakeout drawings. The drawing must be sufficient for direct manufacturing and processing in workshop. Other component units that are not exactly the same shall be drawn and expressed separately, and a detailed list of materials shall be attached.
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