In the whole process of steel structure design, “conceptual design” should be emphasized, especially in the stage of structure selection and layout.For some problems that are difficult to make accurate rational analysis or not specified in the specification, the layout and detailed measures of the control structure can be determined from the overall point of view based on the design ideas obtained from the mechanical relationship, failure mechanism, earthquake damage, test phenomenon and engineering experience between the overall structure system and the sub-system.Concept design can be used in the early stage of rapid and effective conception, comparison and selection.The obtained structural scheme is easy to calculate by hand, clear in concept and correct in quality, and can avoid unnecessary tedious calculation in structural analysis stage.At the same time, it is also the main basis to judge whether the output data of computer internal force analysis is reliable or not.

      Steel structure usually has frame, plane (wooden) frame, grid (shell), cable membrane, light steel, tower and mast and other structural types.Its 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 latticed shell.

      Their different characteristics should be considered in the selection of structures.In the light steel industrial workshop, when there is a large hanging load or moving load, it can be considered to abandon the portal frame and adopt the grid frame.In the area with high basic snow pressure, the roof curve shall be conducive to snow sliding (snow load shall be considered within the tangent line of 50 degrees), for example, three center circular reticulated shell shall be adopted.Nearly half of the total snow load is released.Similar consideration for areas with large rainfall.When the building allows, the arrangement of supports in the frame will be more economical than the simple frame with rigid joints.In the buildings with large roof span, the suspension or cable membrane structure system can be selected.In the design of high-rise steel structure, steel-concrete composite structure is often used. In high-rise buildings with high seismic intensity or very irregular, the form of core tube plus outer frame should not be selected simply for the sake of economy.The surrounding giant SRC columns should be selected, and the core is the structural system of supporting frame.More than half of such high-level in China are the former.Not good for earthquake resistance.

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

In order to meet different requirements, the load transfer direction can sometimes be adjusted for the layout of floor plan secondary beams in structural production.Generally, in order to reduce the cross-section of the secondary beam along the short direction, but this will increase the cross-section of the main beam, reduce the floor clear height, and sometimes the top side column will not be able to bear it. At this time, supporting the secondary beam on the shorter main beam can sacrifice the secondary beam to keep the main beam and the column.

The section selection of steel structure design, after the completion of the structure layout, it is necessary to make a preliminary estimation of the member section.It is mainly about the assumption of cross-section shape and size of beam column and support.

       Channel steel, rolled or welded H-section can be selected for steel beam.According to the load and bearing conditions, the section height is usually between 1 / 20 and 1 / 50 of the span.When the flange width is determined by the L / b limit according to the distance between the lateral supports between the beams, the complex calculation of the 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 structural regulations in the code.

        The section of column is estimated by slenderness ratio.Generally, 50 < λ < 150, and simple selection value is near 100.According to different axial compression, two-way bending or one-way bending, steel pipe or H-section can be selected.

       In addition, there is no fixed requirement for the selection of member cross-section form. Structural engineers should reasonably select safe, economical and beautiful cross-section according to the stress conditions of members.