Steel structure engineering introduces you to the classification of multi-tall steel structures and their stability design

In the design of buildings, designers must first classify buildings, and various specifications also propose classification regulations that are valuable to the design and can sometimes simplify the design according to the type of building, the number of floors, load types, etc., such as specifying when to use the response spectrum base shear method, response spectrum mode shape decomposition method and time history analysis method in seismic design. The purpose of the various classifications is to establish a conceptual framework for rational and convenient design. This article introduces the classification related to the calculation of steel structure stability and clarifies some concepts.

2. The concept of a frame with and without a side

shift frame - 88)1 There is a note at the end of Article 5.2.2: A frame without lateral movement refers to a frame with support frames, shear walls, elevator shafts and other support structures, and its anti-side shift stiffness is equal to and greater than 5 times the anti-lateral shift stiffness of the frame itself; Side-shifting structure refers to the above-mentioned supporter in the frame, or the anti-lateral shift stiffness of the supporting structure is less than 5 times the anti-lateral shift stiffness of the frame itself.

Although the size of the lateral

shift of the building is directly related to the lateral stiffness of the structure, the above concept involves the problem of "have" and "no", which to some extent has caused misunderstanding in the engineering community and academia.

The correct answer to how to introduce is that if it is recognized that all structures are side-shifted under horizontal forces such as wind loads, the definition of a frame without lateral movement must be relative, comparative. Eurocode3 stipulates that in a double lateral force structure, if the total horizontal force experienced by the frame is less than or equal to 20% of the total shear force, it can be assumed with sufficient precision that all 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 frame; A frame in a frame-support structure system that does not meet the above requirements is a frame with lateral shift. According to this regulation, it can be known that the non-lateral frame is a relative concept, which is obtained in comparison with the anti-side stiffness of the support frame and the shear wall, so that only the frame in the double anti-lateral force system will have a distinction between non-lateral and side-shifting.

The rack can be designed to withstand horizontal forces.

Two structures are depresented, (a) if the main frame has a much greater stiffness than the small frame, 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.

If a pure frame does not distinguish between a primary frame and a secondary frame, a pure frame is a frame with a side shift.

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

In the age of computer design, the distinction between framed and non-sideshift does not bring computational benefits, but it is meaningful for preliminary design and the qualitative classification of the designed framework.

Third, the two concepts of lateral flexible structure and lateral rigid structure

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 steel structure design code under revision and the technical regulations for steel structure of high-rise civil buildings that have been promulgated - 98. The definition of lateral rigid structure and lateral flexible structure is as follows: 0, then this structure is a lateral rigid structure, V, H, and each h, respectively, the sum of vertical loads of a certain layer, the layer shear force, the lateral shift between layers under the action of layer shear and the layer height; Structures that do not meet the above conditions are lateral flexible structures. The physical meaning of the term on the left of the above equation needs to be explained. The interlayer lateral stiffness is general: today's designers may think that the lateral "rigid" and "soft" of the structure should be distinguished according to S, (or according to the size of the lateral shift under the action of horizontal force), but this is only one aspect of the lateral rigidity of the structure. From the perspective of stability, gravity load is the factor that instabilizes the structure, which means that the structure loses its ability to resist further loads, that is, the structure reaches a state where the stiffness is zero. What causes the structural stiffness to change from the initial stiffness S to zero, the main culprit is the gravity load. Thus gravity is a negative stiffness with a value of , .

According to the above definition, an elongated cantilever column that does not bear vertical forces is a lateral rigid structure. This classification of slender cantilever columns puzzles designers who do not have a deep understanding of stability theory. Here we can only emphasize that this column has no second-order effect, and it is not necessary to equate the concepts of lateral rigidity and lateral flexibility with lateral displacement calculation, but to link it with the stability calculation of the structure.

The main reason why these two concepts were introduced is to facilitate designers to have an intuitive understanding of the stability of the structure. For a lateral rigid structure: 1) the internal force analysis of the structure can only be performed for first-order analysis, because its second-order effect is small; Due to the small second-order effect, the tendency to have lateral shift instability is small, and the floor height can be approximated as its calculated length to simplify the design. Although for pure frames, it is more reasonable to calculate the stability of the frame column by calculating the length with side shift instability.

For a lateral flexible structure, the influence of second-order effects must be considered in a suitable way. There are three ways to consider the effects of second-order effects: 1) frame column calculation length method; The force analysis within the framework adopts second-order analysis: approximate methods such as hypothetical horizontal force method and accurate second-order analysis methods, and column stability calculation can safely take the floor height as the calculated length or even use the calculated length coefficient when there is no lateral shift instability; 3) Amplification coefficient 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 designers to choose suitable charts and formulas to determine the calculated length coefficient of frame columns, and also to distinguish them from the traditional concept of "side shift": when to use frame columns with side shift instability to calculate the length coefficient, when to allow the use of frame columns without side shift instability to calculate the length coefficient. The note to Article 5.2.2 of the current steel structure design code (GB17-88) is also for the same purpose.

―88 The relevant definition of this article adopts the name of this article, because the name of this article

is confused with the lateral and non-lateral shift corresponding to the structural mechanical analysis, and the essence of this article is to judge the instability mode of the frame structure, not whether the structure actually has lateral shift. This is one of them.

The name, the content does not change, whether it can be done, it depends on whether this provision is reasonable or unreasonable. This provision is an important and fundamental issue in the theory of structural stability. However, the source of this provision cannot be seen or consulted in domestic information. GB17-88 is specified with reference to a note in Chapter VIII of the Steel Structure Stability Manual published by the European Steel Construction Association ECCS in 1977: 12. Looking at this note today, it can be noted that it does not talk about stable calculation, but has this sentence: oftheverticalloadonly Obviously, the meaning of the note is still that the effect of horizontal force can be ignored when the condition of 5 times is met. There is no problem with the note itself, the problem is that the position of this note in this manual is precisely where the stability calculation of the framework is discussed, so it can be assumed that the scholars who wrote this part at the time believed that this 5x standard was used to determine the instability mode of the framework.

3 about the division of side-shifting frame and non-side-shifting frame: when the anti-lateral stiffness of the frame is less than 1/5 of the anti-side stiffness of the support frame, the horizontal force shared is less than 20%, and this frame can be regarded as a non-lateral frame, and the frame design is carried out without considering the role of horizontal force.

According to China's norms, this frame can calculate the stability of the frame column according to the mode of no lateral shift instability. In this comparison, it can be found that the use specified in GB17-88 is currently different from that specified in Eurocode 3, so it is necessary to carry out new research.

An examination of national specifications found that the U.S. code did not specify when the length factor could be calculated using columns without lateral instability. Quoting the original text of the section C2.FrameStability of the American AISC 1993 specification, we can see the current helplessness on this issue: "Theverti-1990, introduces the following definition of frame structure: a strong support frame is a frame in which the anti-lateral stiffness of the support is large enough to make the frame unstable in a mode without side shifting. The strongly supported frame calculates the calculated length coefficient of the column according to the frame column without lateral movement instability.

A weak support frame is

a supported frame in which the anti-side stiffness of the support frame is insufficient and cannot make the frame unstable without lateral movement. The stable bearing capacity of the frame column of the weakly supported frame is calculated according to the following formula: load force, with subscript 1 indicates the bearing capacity of the frame column when there is no lateral shift instability, S is the anti-side stiffness of the support frame, Sth is the threshold stiffness of the support frame, is the judgment criterion of the strong support frame, n is the total number of columns of a certain layer, including all supported frame columns, including the columns that constitute the support frame column. Because if the support rods are crossed, these columns that were originally part of the support frame are unstable with the rest of the frame, so the stable bearing capacity of the support frame column itself is also improved by the introduction of the support rod.

When operating in the elastoplastic state, equation (2) still holds. Because the purpose of determining the length coefficient is to calculate the bearing capacity of the frame, the calculation according to equation (2) avoids the intermediate step of calculating the length of the column of the weakly supported frame, and directly determines its bearing capacity.

A pure frame is a frame that is not supported by any supports, and the calculated length factor of the column is calculated according to the frame column with lateral instability.

5. The judgment criterion of the strong support frame of the pure frame structure naturally occurs in the instability mode with lateral shift; The addition of support in the pure frame structure will improve the stable bearing capacity of the structure, according to the study of the text, the increase part of the critical load can be safely calculated according to the lateral stiffness proportional to the support, see; After setting the support, the instability mode contains the component with side shift mode and the component without side shift mode, as the stiffness of the support increases, the component with side shift becomes smaller and smaller, when the anti-side stiffness of the support is large to a certain extent, the instability mode of the structure changes radically: the instability mode does not contain any component with side shift.

Based on the above phenomenon, we can conclude that the determination criterion of the strong support frame must link the stiffness of the support frame with the difference between the bearing capacity of the column without lateral movement instability and lateral movement instability, because the support is the support of the entire structure, so all columns must be summed.

According to the author's research on the support problem and foreign research on similar problems, the bearing capacity of the structure is provided by the support, and the critical load is large as the stiffness of the supporting side.

In order for this structure to be unstable without lateral movement, the lateral stiffness of the supporting cantilever column must be achieved: such as H x) rationality and existing problems.