(1) Determining whether the structure is suitable for steel structures
Steel structures are commonly used in structures with highrises, large spans, complex shapes, heavy loads on cranes or cranes, large vibrations, requirements for active or frequent assembly and disassembly. Intuitively speaking: buildings, stadiums, opera houses, bridges, TV towers, sculptures, warehouses, factories, houses, mountain buildings and temporary buildings. This is consistent with the characteristics of the steel structure itself.
(2) Structural selection and structural arrangement
Structural selection and layout are qualitative to the structure and should be carried out under the direction of an experienced engineer due to its wide range of applications. Here is a brief introduction. Please refer to the relevant professional books for details.
What should be emphasized throughout the design of steel structures is "conceptual design", which is especially important during the structural selection and placement phases. For some problems that are difficult to make accurate rational analysis or unspecified norms, the design ideas obtained from the mechanical relationship, failure mechanism, seismic damage, experimental phenomena and engineering experience between the overall structural system and the sub-system can be used from a global perspective. To determine the layout of the control structure and detailed construction measures. In the early stage of rapid, effective conception, comparison and selection, the resulting structural schemes are often easy to calculate, the mechanical behavior is clear, the qualitative is correct, and unnecessary cumbersome operations in the structural analysis phase can be avoided. At the same time, it is also the main basis for judging whether the computer's internal force analysis output data is reliable or not.
In the book "Structural Concepts and Systems", Professor Lin Tongyan introduced the method of planning structural schemes with the overall concept, as well as the mutual mechanical relationship between the total structure system and the individual subsystems and the simplified approximate design method.
Steel structures usually have the form of frame, plane truss, grid (shell), cable membrane, light steel, tower and so on.
Most of its theory and technology are mature. Some problems have not been solved, or there are no simple and practical design methods, such as the stability of the reticulated shell.
When selecting a structure, the characteristics of different structural forms should be considered. In industrial plants, when there is a large suspension load or a large range of moving loads, it is considered to abandon the portal frame and use the grid. In areas with large snow pressure, the roof curve should be conducive to the snow falling (the snow load is not required when the tangent is 50 degrees). For example, the limestone silo of the Yadong Cement Plant adopts a three-hearted round net shell, and the total snow load is compared with the slope roof. Release nearly half. Areas with large rainfall are similarly considered. When the building allows, it is better to arrange the support in the frame than the simple node just connected. In a building with a large roof covering span, a suspension or cable-membrane structure system in which the member is mainly pulled can be selected. In the design of high-rise steel structures, steel-concrete composite structures are often used, and the seismic intensity is high or
In a very irregular high-level, the core tube and the outer frame of the unfavorable earthquake should not be selected solely for the economy. The surrounding giant SRC column should be selected, and the core is the structural system of the supporting frame. More than half of these high-rises in China are the former, and they are not good against earthquakes.
The layout of the structure should be comprehensively considered according to the characteristics of the system, the distribution of the load and the nature. Generally speaking, the stiffness should be uniform. The mechanical model should be clear. Limit the influence range of large loads or moving loads as much as possible, so that the most direct line is transmitted to the foundation. The distribution of the anti-side support between the columns should be uniform. The shape of the center 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. The anti-side of the structure should have multiple lines of defense. For example, there is a support frame. Structure, the column should be able to withstand at least 1/4 of the total horizontal force.
The layout of the floor plan secondary beams of the frame structure can sometimes adjust its load transfer direction to meet different requirements. Usually, in order to reduce the section, the secondary beam is arranged in a short direction, but this will increase the section of the main beam and reduce the net height of the floor. The top side column can sometimes be eaten. At this time, the secondary beam can be supported on the shorter main beam. Sacrifice the secondary beam to keep the main beam and the pillar.
(iii) Estimated cross section
After the structural arrangement is completed, a preliminary estimate of the component cross section is required. Mainly the assumption of the shape and size of the section of the beam and the support.
Steel beams can be selected from channel steel, rolled or welded H-section steel sections. Depending on the load and the bearing, the height of the section is usually chosen between 1/20 and 1/50 of the span. When the flange width is determined by the l/b limit according to the spacing of the lateral support between the beams, the complex calculation of the overall stability of the steel beam can be avoided. This method is very popular. Once the section height and flange width have been determined, the thickness of the panel can be estimated according to the locally stable construction specifications in the specification.
The column section is estimated by the slenderness ratio. Usually 50 < λ < 150, and the simple selection value is around 80. According to the axial compression, biaxial bending or one-way bending, steel pipe or H-section steel section can be selected.
Corresponding to different structures, the specifications have great differences in the structural requirements of the sections, such as the local stability of the components of the steel components, which are very different in the specifications of the general steel and the light steel. .
In addition, there is no fixed requirement for the selection of the section form of the component. The structural engineer should reasonably choose a safe and economical section according to the force of the component.
(iv) Structural analysis
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 software can partially consider geometric nonlinearity and the elastic properties of steel. This provides conditions for a more accurate analysis structure. Not all structures require software:
Typical structures can be found in tools such as the mechanics manual to obtain internal forces and deformations directly.
The simple structure is analyzed by hand calculation.
Complex structures require modeling to run the program and do detailed structural analysis.
(5) Project determination
To properly use the structure software, you should also make "engineering decisions" on its output. For example, evaluate the various periods, total shear, deformation characteristics, and so on. According to the "Engineering Decision", choose to modify the model to re-analyze or correct the calculation result.
Different software will have different applicable conditions. Beginners should be fully aware. In addition, the calculations in engineering design and the exact mechanical calculations often have a certain distance.
In order to obtain a practical design method, sometimes the assumption of large error is used, but for this kind of error, the structure is guaranteed by the "applicable conditions, concepts and construction".
In the design of steel structures, "applicable conditions, concepts and constructions" are more important than quantitative calculations.
Engineers who are overly trusting and relying on structural software are likely to bring structural disasters. Focusing on conceptual design, engineering judgment, and construction measures can help avoid such disasters.
(6) Component design
Component design is first of all material selection. More commonly used are Q235 and Q345. When strength is controlled, Q345 can be selected; when stable control, Q235 should be used. Usually the main structure uses a single steel grade to facilitate project management. Economic considerations, It is also possible to select welded joint sections of different strength steels (flange Q345, web Q235). In addition, the welded structure should be Q235B or Q345B.
The current structural software provides post-processing functions for cross-section checking. Some software can choose not to pass the components from the given section library to increase the level of automatic re-checking until the passage, such as sap2000. This is one of the commonly described cross-section optimization design features, which reduces a lot of work. However, we should at least pay attention to two points:
1. When the software performs cross-section verification of components (mainly columns), the calculation of the length coefficient sometimes does not meet the requirements of the specification. At present, all programs cannot completely solve this problem. Therefore, especially for components with complex node connections or variable cross-sections, we should check them one by one.
2. When the estimated section in the above (3) is not satisfied, the enlarged section should be treated differently in two cases.
(1) If the strength is not satisfied, the thickness of the plate member is usually increased. Among them, the bending resistance is not satisfied to increase the thickness of the flange, and the shear resistance is not satisfied to increase the thickness of the web.
(2) The deformation is over-limit. Generally, the thickness of the plate should not be increased. Consider increasing the height of the section, otherwise it will be uneconomical.
It is difficult to consider the above distinction between strength and stiffness using the software's aforementioned optimized design of the auto-increasing cross-section. In fact, other structural forms are often not suitable except for grid design.
(7) Node design
The design of the joint node is one of the important contents in the design of steel structure. Before the structural analysis, the form of the node should be fully considered and determined. Sometimes a situation arises in the final design of the node and structure analysis model. The form is not exactly the same, if you can't be sure that the deviation caused by this inconsistency is within the engineering permission (5%), you must avoid it. According to the different force transmission characteristics, the nodes are just connected, hinged and semi-rigid.
Beginners should choose the first two that can be easily and quantitatively analyzed. The commonly used reference book [2] has a rich recommended node approach and calculation formula.
Different connections have a great influence on the structure. For example, some rigid joints have no problem when they are subjected to bending moments, but they will produce large rotations, which do not meet the assumptions in structural analysis. The actual engineering deformation is greater than the unfavorable results of calculation data.
There are two common methods for connecting nodes with equal strength design and actual force design. For beginners, the former can be used safely. The design manual [2] usually has weld seams and bolted tables for designers to check, which is convenient. It can also be done automatically using the post-processing part of the structure software.
The specific design mainly includes the following contents:
1. Welding: For the size and form of welding seams, the specifications are mandatory and should be strictly observed. The selection of welding rods should be adapted to the metal material to be connected. E43 corresponds to Q235, and E50 corresponds to Q345. When Q235 is connected with Q345, it should be selected. Low intensity E43, not E50.
The weld seam shall not be arbitrarily enlarged in the welding design. The center of gravity of the weld shall be as close as possible to the center of gravity of the connected member. Other details may be found in the specification of the weld construction.
2. Bolting:
The riveted form is rarely used in construction projects.
Ordinary bolts have poor shear resistance and can be used in secondary structural parts.
High-strength bolts are widely used. They are commonly used in 8.8s and 10.9s. They are divided into bearing type and friction type according to the force characteristics. The calculation methods are different. The minimum specification of high-strength bolt M12. Commonly used M16~M30. Large size Bolt performance is unstable and should be used with caution.
Self-tapping screws are used for secondary connections between sheets and thin-walled steels. They are also commonly used in low-rise wall-slab houses for the connection of main structures. What is difficult to solve is the destruction of the anti-corrosion layer during self-tapping.
3. Connecting plate: It is necessary to check the net section shear resistance of the bolt hole weakening part. The thickness of the connecting plate can be simply taken as the thickness of the beam web plus 4mm, except for the short beam or the beam with large concentrated load, it is often not necessary to check the calculation. Shear.
4. Beam web: The net section shear of the web at the bolt hole should be checked. The pressure-bearing high-strength bolt connection also needs to check the partial pressure of the hole wall.
5. The node design must consider the construction space of the mounting bolts, field welding, etc. and the lifting sequence of the components. The failure to install the components to the site is a mistake made by beginners. In addition, workers should be able to carry out on-site positioning and temporary fixing as much as possible.
6. The node design should also consider the manufacturing process level of the manufacturer. For example, the incision of the intersecting line of the steel pipe connection node may require equipment such as CNC machine tools to complete.
(8) Drawing preparation
The steel structure is designed in two stages: the design plan and the construction detail. The design drawings are provided by the design unit. The construction details are usually prepared by the steel structure manufacturing company according to the design drawings, and sometimes by the design unit. Due to the increasing number of steel structure projects in recent years and the lack of design steel structure engineers, it is also common for steel structure companies with design capabilities to participate in the design of the design drawings.
1. Design drawings: It is the basis for providing detailed drawings of the construction of the manufacturer. The depth and content should be complete but not redundant.
In the design drawings, for design basis, load data (including seismic effects), technical data, material selection and material requirements, design requirements (including manufacturing and installation, grade of weld quality inspection, painting and transportation, etc.),
The structural arrangement, the selection of the members' sections, and the main node structure of the structure should be clearly stated to facilitate the smooth preparation of the construction details and correctly reflect the design intent. The main materials should be listed.
2. Construction details: Also known as machining drawings or stakeout drawings, etc. The depth must be able to meet the direct manufacturing and processing of the workshop. The different component units that are not identical must be drawn separately and should be accompanied by a detailed material list.
The content expression method of the design drawings and construction details and the control of the drawing depth are currently confusing, and the design units are not the same as the steel structure companies. Beginners can refer to the excellent design of others and refer to the relevant reference books, and prepare according to the specifications.