Classification and stability design of steel structure buildings

In the design of the building, the designer must first classify the building, various codes also according to the type of building, the number of layers, the type of load, etc., targeted to the design of valuable, sometimes can simplify the design of the classification provisions, such as the provision of seismic design when the response spectrum base shear method, response spectrum mode decomposition method and time history analysis and so on. The purpose of the various classifications is to establish a conceptual framework for rational and convenient design. This paper introduces the classification related to the calculation of the stability of steel structures and clarifies some concepts.

The concept of sidesway frame and non-sidesway frame – 88) 1 There is a note at the end of Article 5.2.2: Non-sidesway frame refers to a frame with a support structure such as a support frame, shear wall, elevator shaft, and its anti-sidesway stiffness is equal to or greater than 5 times the anti-sidesway stiffness of the frame itself; A sideshift structure is one in which no such support is provided in the frame, or the anti-sideshift stiffness of the supporting structure is less than 5 times that of the anti-sideshift stiffness of the frame itself. Although the magnitude of the lateral displacement of the building is directly related to the lateral stiffness of the structure, the above concept involves the problem of “having” and “not having”, which has caused misunderstanding in the engineering and academic circles to a certain extent.

How should the correct answer be introduced, if it is realized that all structures shift sideways under horizontal forces such as wind loads, the definition of non-shifting frames must adopt a relative and comparative approach. Eurocode3 stipulates that in a double lateral force resistant structure, if the total horizontal force borne by the space frame is less than or equal to 20% of the total shear force, it can be assumed with sufficient accuracy that all the horizontal forces are borne by the support frame (or shear wall), and the frame itself does not bear the horizontal force, so that the frame can be used as a non-lateral movement frame; Frames that do not meet the above requirements – the frame in the support structure system is a frame with lateral displacement. According to this provision, it can be known that the non-lateral displacement frame is a relative concept, which is obtained in the comparison with the lateral stiffness of the support frame and the shear wall, so that only the frame in the double lateral resistance system can be distinguished with or without lateral displacement and with lateral displacement.
(a) If the main frame has much greater stiffness than the small frame, then the small frame can be designed as a non-lateral frame, and all horizontal forces are borne by the large frame, including the horizontal forces acting on the small frame. (b) The frame can be designed without lateral movement when the stiffness of the support frame is large, and all horizontal forces are borne by the support frame.

Pure frame if there is no main frame and sub-frame, the pure frame is a side-shifting frame.

The above classification method does not involve the stability calculation of the frame, but only carries out a classification of the frame to understand the relative proportion of the horizontal force borne by each substructure of the building, and can be simplified mechanical analysis when the horizontal force shared by the frame is small to a certain extent.

In the era of computer design, the distinction between sideshift and non-sideshift frames does not bring computational benefits, but it is still meaningful to understand the preliminary design and the qualitative classification of the designed frames.

The two concepts of lateral flexible structure and lateral rigid structure have not been introduced in China, and some provisions related to these two concepts have been adopted in the new design Code for Steel structures under revision and the Technical regulations for Steel structures in high-rise civil buildings that have been promulgated. The definition of the lateral rigid structure and the lateral flexible structure is as follows: 0, then the structure is the lateral rigid structure, V, h, and each h are the total vertical load of a certain layer, the layer shear force, the lateral movement between the layers under the action of the layer shear force, and the height of the storey. The structures that do not meet the above conditions are lateral flexible structures. The physical meaning of the term on the left of the above formula needs to be explained. Interlayer lateral stiffness is generally: Today, designers may think that the “rigid” and “soft” of the structure side should be differentiated according to S, (or according to the size of the lateral shift under the horizontal force), but this is only one aspect of the lateral rigidity of the structure. From the perspective of stability, the gravity load is the factor that causes the instability of the structure, and the instability means that the structure loses the ability to further resist the load, that is, the structure reaches the state of zero stiffness. What causes the stiffness of the structure to change from the initial stiffness S to zero is the gravity load. So gravity is a negative stiffness with a value of.

According to the above definition, a slender cantilever column that does not bear vertical forces is a laterally rigid structure. This classification of slender cantilever columns confuses designers who are not familiar with stability theory. Here we can only emphasize that there is no second-order effect in this column, and the concepts of lateral rigidity and lateral flexibility should not be equivalent to the calculation of lateral displacement, but should be linked to the calculation of the stability of the structure.

The reason why these two concepts are introduced is mainly to facilitate the designer to have an intuitive understanding of the stability of the structure. For a laterally rigid structure: 1) the internal force analysis of the structure can only be carried out first order analysis, because its second order effect is small;) Because the second order effect is small, the tendency of lateral instability is small, and the height of the layer can be used as its calculation length to simplify the design. Although for pure frame, it is more reasonable to calculate the stability of frame column by taking the calculated length with lateral instability.

For a laterally flexible structure, the effects of second-order effects must be considered in a suitable way. There are three ways to consider the influence of second-order effects: 1) frame column calculation length method;) The internal force analysis of the frame adopts the second order analysis method, such as the approximate method of imaginary horizontal force method and the exact second order analysis method. The column stability calculation can take the height as the calculation length safely or even use the calculation length coefficient when there is no lateral instability. 3) Amplification factor method: Multiply the internal force and displacement by the Disdnger coefficient, similar to here). The purpose of introducing this pair of concepts is to facilitate the designer to choose the appropriate chart and formula to determine the calculated length coefficient of the frame column, and to distinguish it from the traditional concept of “lateral displacement” : when to use the frame column with lateral instability to calculate the length coefficient, and when to allow the frame column without lateral instability to calculate the length coefficient. The note in Article 5.2.2 of the current Code for the Design of Steel structures (GB17-88) serves the same purpose.

For the definition of this article in -88, the name of this article is adopted, because the name of this article is confused with the lateral and non-lateral shift corresponding to the structural mechanics analysis, and the essence of this article is to judge the instability mode of the frame structure, not whether the structure actually has a lateral shift. That’s one thing.

The name and the content do not change, which depends on whether this provision is reasonable or not. Gb17-88 is defined by reference to a note in Chapter VIII of the Stability Manual of Steel Structures: 12, published by the European Steel Construction Association ECCS1977. It can be noted that it does not talk about the problem of stable computation, but has this sentence: oftheverticalloadonly obviously, the meaning ofthe note is still that the effect ofthe horizontal force can be ignored when the condition of 5 times is satisfied. The note itself is not a problem, the problem is that the position of the note in this manual is precisely where the stability of the framework is calculated, so it can be assumed that the scholars who wrote this section at the time thought that this 5x criterion was used to determine the instability mode of the framework.

When the lateral stiffness of the frame is less than 1/5 of the lateral stiffness of the support frame, the horizontal force shared is less than 20%. This frame can be regarded as a non-lateral frame, and the frame design is carried out without considering the role of the horizontal force.

According to our standard, the stability of the frame column can be calculated according to the mode of no lateral instability. In this way, it can be found that the uses specified in GB17-88 and Eurocode3 are different at present, so it is necessary to conduct new studies.

A review of national codes found that the United States code does not specify when a column without lateral instability can be used to calculate the length factor. The original text of the American AISC1993 specification C2.FrameStability section can be seen to be currently unable to cope with this problem: “Theverti-1990, the introduction of the following definition of the frame structure: A strongly supported frame is a frame whose lateral stiffness is sufficiently large to make the frame unstable in a non-lateral movement mode.” The length coefficient of the columns for strongly supported frames is calculated according to the frame columns without lateral instability.

The weakly supported frame is a supported frame whose lateral stiffness is insufficient and cannot make the frame unstable without lateral movement. The stable bearing capacity of the frame column with weak support frame is calculated according to the following formula: Load, with subscript 1, indicates the bearing capacity of the frame column without lateral instability, S is the lateral stiffness of the support frame, Sth is the threshold stiffness of the support frame, and is the judgment criterion of the strongly supported frame, n is the total number of columns in a certain layer, including all the frame columns to be supported and the columns constituting the support column. Because these columns, which were originally part of the support frame, have lateral instability together with the rest of the frame if the support rods are crossed, the stable bearing capacity of the support column itself is also improved because of the introduction of the support rods.

Equation (2) is still valid when operating in the elastoplastic state. Because the purpose of determining the calculated length coefficient is to calculate the bearing capacity of the frame, the calculation according to formula (2) avoids the intermediate step of calculating the length of the columns of the weakly supported frame, and directly determines its bearing capacity.

The pure frame is a frame without any support, and the length coefficient of the column is calculated according to the frame column with lateral displacement and instability.

The criterion of the strongly supported frame is that the pure frame structure certainly has the instability mode with lateral movement; Adding supports to the pure frame structure will improve the stability bearing capacity of the structure. According to the study of the paper, the increase of the critical load can be safely calculated as proportional to the lateral stiffness of the support, see; After the support is set, the instability mode contains both the components with and without lateral motion mode. With the increase of the support stiffness, the component with and without lateral motion becomes smaller and smaller. When the lateral stiffness of the support reaches a certain level, the instability mode of the structure changes fundamentally: the instability mode does not contain any component with lateral motion.

According to the above phenomenon, we can draw the following conclusion: the criterion for the determination of the strongly supported frame must relate the stiffness of the support frame to the difference between the bearing capacity of the column without lateral instability and that of the column with lateral instability, because the support is the support of the whole structure, so all the columns must be summed.

According to the author’s research on the support problem and the conclusion of foreign research on the same problem, the bearing capacity of the structure is provided by the support, and the critical load is as large as the lateral stiffness of the support.

In order to make this structure without lateral instability, the lateral stiffness of the cantilever column that plays a supporting role must reach the rationality and existing problems such as H x).