(1) Judging whether the structure is suitable for steel structure
Steel structures are usually used for high-rise, long-span, complex body shapes, heavy loads or crane lifting capacity, large vibration, high-temperature workshops, high sealing requirements, activities or frequent assembly and disassembly. Intuitively speaking: buildings, stadiums, opera houses, bridges, TV towers, warehouses, factories, residences and temporary buildings, etc. This is consistent with the characteristics of the steel structure itself.
(2) Structural type selection and structural layout
Here is just a brief introduction. Please refer to relevant professional books for details. Since the structure selection involves a wide range, the structure selection and layout should be carried out under the guidance of experienced engineers.
What should be emphasized in the whole process of steel structure design is “conceptual design”, which is especially important in the stage of structure selection and layout. For some problems that are difficult to make precise and rational analysis or not stipulated in the code, based on the design ideas obtained from the mechanical relationship between the overall structural system and the sub-systems, the failure mechanism, earthquake damage, test phenomena and engineering experience, from an overall perspective To determine the layout of the control structure and detailed measures. Conceptual design can be used to quickly and effectively conceive, compare and select at an early stage. The resulting structural schemes are often easy to calculate by hand, with clear concepts and correct qualitative properties, and can avoid unnecessary cumbersome calculations 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 or not. (No matter how powerful the structural software is, solid structural concepts and mechanical analysis, as well as reliable hand calculation ability, are excellent qualities.)
Steel structures usually have structural types such as frame, plane (wooden row) frame, network frame (shell), cable-membrane, light steel, and tower mast.
Most of its theories and technologies are 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 a structure, their different characteristics should be considered. In light steel industrial workshops, when there is a large hanging load or moving load, it may be considered to abandon the portal frame and use the grid frame. In areas with heavy snow pressure on the roof, the roof curve should be conducive to snow falling (snow load must be considered within 50 degrees of the tangent line). For example, the limestone warehouse shed of Yadong Cement Plant adopts a three-center circular reticulated shell. Nearly half of the total snow load was released. Similar considerations apply to areas with high rainfall. Where the building permits, it is more economical to arrange bracing in the frame than a simple rigid-jointed frame. For buildings with a large roof covering span, a suspension or cable-membrane structure system with components mainly in tension can be selected. In the design of high-rise steel structures, steel-concrete composite structures are often used. In high-seismic high-rise buildings or very irregular high-rise buildings, the core tube plus outer frame that is unfavorable for earthquake resistance should not be selected purely for economical reasons. It is advisable to choose a structural system with giant src columns around and the core as a supporting frame. More than half of such senior executives in our country are the former. Unfavorable against earthquakes. (Arranging the force-bearing units as far as possible to the periphery of the structure is the key to making full use of the material properties, just like hollow bamboo, so it is very important to be strong on the outside and weak on the inside.)
The layout of the structure should be considered comprehensively according to the characteristics of the system, the distribution and nature of the load. Generally speaking, the stiffness should be uniform. The mechanical model is 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 route. The distribution of anti-lateral bracing between columns should be uniform. Its centroid should be as close as possible to the line of action of lateral force (wind shock). Otherwise, the torsion of the structure should be considered. There should be multiple lines of defense on the resistant side of the structure. For example, if there is a supporting frame structure, the columns should be able to bear at least 1/4 of the total horizontal force alone.
The layout of the floor plane secondary beams of the frame structure can sometimes adjust its load transfer direction to meet different requirements. Usually, the secondary beams are arranged along the short direction in order to reduce the cross-section, but this will increase the cross-section of the main beams, reduce the clear height of the floor, and the side columns on the top floor may sometimes be too much. At this time, supporting the secondary beams on the shorter main beams can be Sacrifice the secondary beams to keep the main beams and columns.
(3) Estimated cross section
After the structural layout is completed, a preliminary estimate of the component section is required. It is mainly an assumption of the cross-sectional shape and size of beams, columns and supports.
The steel beam can choose channel steel, rolled or welded H-shaped steel section, etc. According to the load and support conditions, the section height is usually selected between 1/20 and 1/50 of the span. When the width of the flange is determined according to the l/b limit according to the spacing of the lateral supports between the beams, the complex calculation of the overall stability of the steel beam can be avoided, and this method is very popular. After the height of the section and the width of the flange are determined, the thickness of the plate can be estimated according to the locally stable construction regulations in the code.
Column sections are estimated by slenderness ratio. Usually 50<λ<150, the simple choice value is around 100. According to the axial compression, two-way bending or one-way bending, steel pipe or H-shaped steel section can be selected.
Beginners should note that corresponding to different structures, the structural requirements for cross-sections in the code are very different. For example, the local stability of the plates that make up the members is unique to steel structures. There is a big difference between the limit values in the general steel specification and the light steel specification.
In addition, there are no fixed requirements for the selection of component cross-section forms. Structural engineers should reasonably choose safe, economical and beautiful cross-sections according to the stress conditions of components.
(4) Structural analysis
At present, in the actual design of steel structures, the 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 geometric nonlinearity and elastic-plastic properties of steel. This allows for a more precise analysis of the structure. Not all structures require the use of software:
For typical structures, the internal forces and deformations can be obtained directly from reference books such as mechanics manuals.
Simple structures are analyzed by hand calculations.
Only complex structures need to model and run programs and do detailed structural analysis.
(5) Engineering Judgment
In order to use structural software correctly, an “engineering judgment” should be made on its output results. For example, evaluate isotropic periods, total shear forces, deformation characteristics, etc. According to the “engineering decision”, choose to modify the model and re-analyze, or correct the calculation results.
Different software will have different applicable conditions. Beginners should be fully aware. In addition, there is often a certain distance between calculations in engineering design and accurate mechanical calculations. In order to obtain practical design methods, assumptions with large errors are sometimes used, but for such errors, “applicable conditions, concepts and structures” way to ensure the safety of the structure. In steel structure design, “applicable conditions, concepts and structures” are more important than quantitative calculations.
Engineers should not place undue trust and reliance on structural software. A scholar in the United States once warned: “Misusing computers to cause structural damage and cause disasters is only a matter of time.”
Paying attention to conceptual design and engineering judgment is the way to avoid this kind of engineering disaster.
(6) Component design
The design of components is firstly the choice of materials. More commonly used are q235 (similar to a3) and q345 (similar to 16mn). Usually the main structure uses a single steel grade to facilitate project management. For economical considerations, it is also possible to choose a combined section of steel with different strengths. When the strength plays a controlling role, q345 can be selected; when the stability is controlled, q235 should be used.
In the component design, the current code uses the elastic-plastic method to check the section. This does not match the elastic method of structural internal force calculation.
The current structural software provides the post-processing function of cross-section checking calculation. Due to the progress of program technology, some software can choose to increase one level from the given cross-section library for components that do not pass the checking calculation. And automatically re-analyze the checking calculation until it passes, such as sap2000 and so on. This is one of the functions of section optimization design that is often said. It saves a lot of work for architects. However, beginners should pay attention to at least two points:
1. When the software checks the cross-section of components (mainly columns), the determination of the calculation length coefficient sometimes does not meet the specifications. None of the current programs can fully solve this problem. Therefore, especially for components with complex node connections or variable cross-sections, structural engineers should check them one by one.
2. When the cross section estimated in item (3) above is not satisfied, the enlarged cross section should be treated differently in two cases.
(1) If the strength is not sufficient, the thickness of the plates that make up the section is usually increased. Among them, the bending resistance is not sufficient to increase the thickness of the flange, and the shear resistance is not sufficient to increase the thickness of the web.
(2) If the deformation exceeds the limit, the thickness of the plate should not be increased in general, but the height of the 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 enlarging the section of the software, and in fact, it is often not appropriate.
(7) Node design
The design of connection nodes is one of the important contents in steel structure design. Before structural analysis, the form of nodes should be fully considered and determined. It is often the case that the final design of the joints does not exactly correspond to the form used in the structural analysis model, and this must be avoided. According to the different force transmission characteristics, the joints are divided into rigid joints, hinged joints and semi-rigid joints. Beginners should choose the first two that can be easily quantitatively analyzed. The commonly used reference book [2] has a wealth of recommended node practices and calculation formulas.
The difference in connection has a great influence on the structure. For example, although there is no problem with some rigid joints bearing the bending moment, they will produce a large rotation, which does not meet the assumptions in the 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 of equal-strength design and actual force design for connecting nodes, and beginners can choose the former for safety. There are usually tables of welds and bolted connections in the design manual for the designer to check and use, 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 contents:
1. Welding: For the size and form of welding seams, etc., there are mandatory regulations in the specifications, which should be strictly followed. The selection of electrodes should be adapted to the metal materials to be connected. e43 corresponds to q235, and e50 corresponds to q345. When q235 is connected with q345, low-strength e43 should be selected instead of e50.
The welding seam shall not be enlarged arbitrarily in the welding design. The center of gravity of the weld should be as close as possible to the center of gravity of the connected members. For other details, please refer to the regulations on weld structure in the specification.
2. Bolting:
The form of riveting is rarely used in construction engineering.
Ordinary bolts have poor shear resistance and can be used in secondary structural parts.
High-strength bolts are widely 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 of the two are different. The minimum size of high-strength bolts is m12. Commonly used m16 ~ m30. The performance of oversized bolts is unstable, so they should be used with caution in design.
Self-tapping screws are used for secondary connections between plates and thin-walled steel sections. In low-rise wall panel houses abroad, it is also commonly used for the connection of 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 and so on.
4. Beam web: The net cross-sectional shear resistance of the web at the bolt hole should be checked. The pressure-bearing high-strength bolt connection also needs to check the local pressure of the hole wall.
5. The joint design must consider the construction space for installing bolts, on-site welding, etc., and the hoisting sequence of components. It is a common mistake for beginners to ship components to the site and fail to install them. In addition, workers should be able to locate and temporarily fix the site conveniently as much as possible.
6. Node design should also consider the manufacturing plant’s technological level. For example, the incision of the intersecting line of the steel pipe connection node needs equipment such as CNC machine tools to complete.
(8) Drawing preparation
The drawing of steel structure design is divided into two stages: design drawing and construction detail drawing. The design drawing is provided by the design unit, and the construction detail drawing is usually prepared by the steel structure manufacturing company according to the design drawing, and sometimes the design unit also prepares it on its behalf. 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 common for steel structure companies with design capabilities to participate in the preparation of design drawings.
1. Design drawing: It is the basis for the manufacturer to prepare detailed construction drawings. Depth and content should be complete but not redundant. In the design drawings, the design basis, load data (including earthquake action), technical data, material selection and material requirements, design requirements (including manufacturing and installation, weld quality inspection level, coating and transportation, etc.), structural layout , the selection of component sections and the structure of the main nodes of the structure, etc. should be clearly expressed, so as to facilitate the smooth compilation of construction detailed drawings and correctly reflect the design intention. The main materials should be indicated in a list.
2. Construction detailed drawing: also known as processing drawing or lofting drawing, etc. The depth must be able to meet the direct manufacturing and processing of the workshop. The other component units that are not exactly the same shall be drawn and expressed separately, and a detailed material list shall be attached.
The content expression method of design drawings and construction detailed drawings and the control of drawing depth are relatively chaotic at present, and there are differences between various design units and steel structure companies. Beginners can refer to other people’s excellent designs and related reference books, and compile according to the specifications.