Discussion on the structural design of the hottest

Discussion on the design of glass curtain wall and door and window structure

I. preface

glass curtain wall is a building curtain wall that has been applied in China since 1985. It is a lightweight building envelope developed with the rise of high-rise buildings on the basis of aluminum alloy doors and windows

in 2002, China began to implement the newly revised {unified standard for reliability design of building structures (GB), load code for building structures (GB) and code for seismic design of buildings (GB) Three national standards. Curtain wall and doors and windows, as the building envelope and components that have a significant impact on the physical function of the building and the safe use of people, must be designed and calculated according to the requirements of these standards and their mandatory provisions to ensure their sufficient reliability

aluminum alloy glass curtain walls and doors and windows are the most mature and widely used metal frame building curtain walls and doors and windows in the world. China's "technical code for glass curtain Engineering" (JGJ) is currently being revised, and the "technical code for aluminum alloy doors and windows Engineering" began to be prepared in August 2002, and there is no design code for building doors and windows engineering. Conscientiously summarize the technology and experience at home and abroad, carry out structural reliability design research on them, correctly compile China's technical standards and specifications for glass curtain moisture and doors and windows, so as to gradually establish a theoretical system for the reliability design and evaluation of various materials and types of building curtain moisture and doors and windows structure, which has important practical significance and far-reaching historical significance for the engineering practice and technical development of building curtain moisture and doors and windows in China

II. Reliability analysis and design principle of building structure

1. reliability of structure

building structure is a load-bearing skeleton system that constitutes industrial and civil housing buildings, including foundations. The basic functional requirements that must be met are: (1) safety: it can withstand various possible effects during normal construction and normal use: when and after the accidental events specified in the design (such as earthquake, fire, etc.) occur, It can still maintain the necessary overall stability: (2) suitable rigidity: it has good working performance in normal use; (3) durability: it has sufficient durability under normal maintenance

structural reliability is the general term of structural safety, applicability and durability. It is the ability of a structure to complete its predetermined functions within a specified time and under specified conditions

2. Structural reliability

(1) limit state design requirements of the structure

various random factors affecting structural reliability can be summarized into two comprehensive variables that are both random variables, namely, the structural action effect s and resistance R, and the structural function z=g (R, 5) =r-s is also a random variable. When Z 0, the structure is in a reliable state; when Z 0, the structure is in a failure state: when z=r-s=0. The structure is in the limit state. The limit state design requirement of the structure is: R-S =o, that is, the resistance of the structure should be greater than or equal to its action effect

(2) probabilistic reliability of structure

because the effect of load and action in various factors affecting structural reliability is variable, and the resistance R of the structure is also uncertain (uncertainty of component material performance, uncertainty of geometric parameters, uncertainty of calculation mode), therefore, the reliability target of z=r-s =0 required by structural design cannot be absolutely guaranteed, and can only be met in a certain probability sense, that is, P (R =s) =p, Is the reliability probability of the structure. Therefore, the reliability of the structure is a quantitative description of the structural reliability, that is, the probability of the structure to complete the predetermined function within the specified time and under the specified conditions. And the failure probability of the structure pf=1-pi. Since the failure probability of the structure is generally a small probability event, and the failure probability has a good grasp of the structural reliability to meet the requirements of cycle time and cost structure, the history is intuitive, so the reliability analysis of engineering structures generally calculates the failure probability of the structure

3. reliability analysis and design of structures

according to people's basic functional requirements for building structures with different safety levels, the probabilistic reliability design of normal use limit state and bearing capacity limit state is carried out for structures, components and their connections under different design ratios (permanent state, transient state and accidental state) within a certain design reference period, To ensure that the building structure has sufficient reliability within the specified design service life

4. Necessity and possibility of reliability design of curtain wall and door and window structure

building curtain wall is an expanded wall structure outside the building that is composed of various wall panels and supporting systems (supporting devices and supporting structures) and does not bear the gravity of the main structure. Doors and windows are functional components (composite components) at the opening of the light bulb lighting structure surrounded by the house. Curtain walls and doors and windows are permanent non structural components of buildings (fixed components and parts in buildings other than load-bearing skeleton system, mainly including decorative components and parts attached to the main structure such as non load-bearing walls). Therefore, although the unified standard for reliability design of building structures and the load code for building structures are applicable to various building structures and components, considering that curtain walls and doors and windows are peripheral expansion structures and components that have a significant impact on the physical functions of buildings, the safety of users and the public, and their load effects and mechanical characteristics are similar to the main structure of buildings, Therefore, curtain walls, doors and windows should be necessary and can be designed and calculated according to the above standards and specifications, but the difference between non structural members and main structural members should be mastered

III. reliability design and analysis of curtain wall and door and window structure

1. Design service life and design reference period of curtain wall and door and window

1) design service life

gb stipulates that the design service life of building structure is divided into four categories

referring to the above provisions on the design service life of structures and components, considering that curtain wall and door and window are important persistent non structural components, the design service life of curtain wall should be 25A, The design service life of doors and windows shall be 10A. The newly revised national standards "aluminum alloy doors" and "aluminum alloy windows" stipulate the repeated opening and closing performance of doors and windows, with 100000 times for doors and 10000 times for windows, which is determined by referring to the same provisions of the Japanese industrial standard Jisa "metal windows", according to the 10A design service life of 30 times a day for doors and 3 times a day for windows

2) design reference period

design reference period is a time parameter selected to determine the variable load and action on the structure and the value of material performance related to time. It is not equal to the design service life of the structure. Glass curtain walls and doors and windows are light and thin-walled building envelope structures and components, which are vulnerable to instantaneous wind damage. In order to ensure their safe use, they should be determined according to the design reference period of 50a, that is, they can bear the maximum value of variable load and action in the return period of 50A. China's "classification and test method of wind pressure resistance of building external windows" (GB) was formulated with reference to the British door and window design wind load standard. The instantaneous wind speed and wind pressure with a return period of 50A were used as the difference between the safe design wind load and the corresponding safe detection pressure, which was used as the classification value of wind pressure resistance of windows

2. Loads and actions on curtain walls and doors and windows

1) direct action (a group of concentrated or distributed forces imposed on the structure)

① deadweight load: the gravity of component materials constituting the curtain wall and door and window structure generated by the earth's gravity, and its standard value is calculated according to the design size and the standard value of material gravity density. It is a kind of permanent action that the quantity value does not change with time in the design reference period, or its change is negligible compared with the average value. In engineering design, the self weight is converted into the standard value of the average area dead load (n/m2) distributed by area

② wind load: refers to the velocity pressure of the horizontal wind acting vertically on the surface of building curtain walls and doors and windows. The magnitude changes with time within the design reference period, or its change is a variable effect that cannot be ignored compared with the average value. There are two kinds of representative values of wind load on curtain walls and doors and windows: 1) the standard value WK2 is the standard value of wind load on surrounding structures calculated according to the code for the design of building loads (GB 50009), which is the maximum wind load that may occur on curtain walls and doors and windows during their design service period. The probability of occurrence of the 50A return period in the design reference period is 2%, that is, the probability (assurance rate) of wind load not exceeding the maximum value is 98% O 2) the frequency value wd2 is curtain walls The larger wind load value wd=0 of doors and windows sometimes occurs during their design and use period. 4wk, the ratio of the total duration T1 of wind load exceeding the frequency value to the design reference period T is TL/T ≤ 0.1, that is, the probability is 10%, which is equivalent to a return period of Loa

2) reasons for external deformation or constrained deformation of the structure caused by indirect action

① seismic action: it is the dynamic action of the structure caused by ground motion. According to the code for seismic design of buildings GB), the seismic action of non structural members only considers the horizontal seismic action caused by its own gravity and the additional action caused by the relative displacement between supports. a) Frequent Earthquake: it belongs to variable action, and the exceedance probability within 50A of seismic intensity is about 63%, and the return period is 50A. b) Earthquake with fortification intensity: the exceedance probability within 50A of the earthquake intensity is about 10%, and the return period is 475a. C) Rare earthquake: it belongs to accidental action, with an exceedance probability of 2%-3% within 50A of seismic intensity and a return period of about 2000 a

2 temperature effect: the change of ambient temperature causes the length change of thermal expansion and cold contraction of components, and when this change is constrained, stress will be generated. For this kind of variable stiffness, the maximum value should be considered when the construction method is adopted according to experience in the project. Technical code for steel structures of high rise civil buildings (GJ) takes the temperature effect of curtain wall components into account according to their maximum effect, that is, the combined value coefficient and partial coefficient are both taken as 1. 0. The standard of temperature effect is calculated according to the maximum temperature difference in a year

3. Effect and resistance of curtain wall and doors and windows

1) structural resistance and member bearing capacity

① resistance is the ability of the structure or member and its materials to bear the effect of action, such as bearing capacity, stiffness, crack resistance, strength, etc. The resistance corresponds to the action effect, and the ability of the structure to resist deformation, that is, the stiffness, is also a kind of resistance of the structure

② the bearing capacity is the maximum internal force that the member can bear, or the internal force when it reaches the deformation that is not suitable for continuous bearing

③ strength is the ability of materials to resist damage, that is, the maximum stress that the material section can bear

2) level of structural resistance

① overall structural resistance: such as the overall structure of doors, windows and curtain walls. 4. One of the most important and characteristic mechanical properties of elastic materials, the ability to bear wind load:

② member resistance: such as the bearing capacity of mullion members under the action of self weight axial force and wind pressure bending moment

③ member section resistance: member section bending and shear resistance

4. Reliability of curtain wall and door and window structural system and reliability of components