I. Introduction
Compared with traditional houses, light steel structure houses have outstanding advantages:
(1) The manufacturing of light steel structural fittings is highly industrialized, mechanized and commercialized.
(2) The site construction speed is fast, mainly for dry operation, which is beneficial to civilized construction.
(3) Steel structure building is a sustainable and environmentally friendly product.
(4) Light weight and good seismic performance.
(5) The comprehensive economic index is not higher than the reinforced concrete structure.
With the rapid growth of steel production in China
For a long time, the policy of steel use has changed from restricting the use of steel to rational use of steel and then to active use of steel. Document No.72 issued by the State Council 1999 proposed that the coastal cities should stop using clay bricks within a time limit to develop the steel structure housing industry. Therefore, the development of light steel structure residential system has become the focus of current residential structure research. However, the research on multi-story light steel structure is still in its infancy, and the research intensity is not enough. There are still many disputes and problems in actual design and construction. These problems need to be solved urgently to promote the healthy and rapid development of light steel residential buildings in China.
Second, the structural system selection
For low-rise and multi-storey houses, the structural systems commonly used at home and abroad at present mainly include:
(a) Cold-formed thin-walled steel system
The members are cold-formed into C-shaped and Z-shaped members with thin steel plates, which can be used alone or in combination. Self-tapping screws are used to connect members. This system is not easy to ensure the joint stiffness and poor lateral force resistance, and is generally only used for 1 ~ 2-story houses or villas. The system has been used in several old building storey-adding reconstruction projects such as Jixian State Taxation Bureau and Tianjin Port Police Station, and the effect is good.
(2) Framework
At present, this system is the most widely used in multi-storey steel structure residence. Vertical and horizontal steel frames and flexible doors and windows can provide a larger bay, which is convenient for users to redesign and meet various life needs. Considering the interaction of floors, steel frames can generally meet the requirements of lateral force resistance when used in low-rise residential buildings. However, due to the fact that the current frame columns are mainly H-beam, it is difficult to ensure the stiffness of the beam-column connection in the weak axis direction, so it must be handled carefully in design and construction.
(3) Frame support system
It is very effective to increase axial support or eccentric support to improve the lateral stiffness of the system in areas with large wind load or earthquake action. This system is a multi-lateral-force resisting system. The beam-column joints and column foot joints can be designed as hinged and semi-rigid. The construction structure is simple, and the foundation mainly bears axial force, so it has become a popular object.
(4) Frame shear wall system
In low-rise residential buildings, traditional shear wall systems, such as reinforced concrete shear wall or steel plate shear wall, can be applied. The cavity structural plate being studied at present is an ideal lateral force resistant structure. Cavity structural board is a new type of lightweight board, which is made of yellow paper into a plate-like base frame with many equilateral cavities, and then impregnated. A new shear wall can be formed through the reliable connection between the plate and the steel frame. In addition, the United States, Australia and other countries have also developed staggered truss systems, which are relatively novel.
Third, the main components design
(1) column
As mentioned earlier, steel structure houses are generally large bays, and the frame columns bear large bending moments in two directions. At the same time, the requirements of strong columns and weak beams should be considered. At present, the widely used welded H-beam or I-type hot-rolled beam has a ratio of the moment of inertia between the strong axis and the weak axis of 3 ~ 10, which is bound to cause material waste. Therefore, for frame columns with large axial compression ratio, close bidirectional bending moment and high beam section, bidirectional steel tubular columns or concrete-filled square steel tubular columns with equal strength should be adopted. For concrete-filled square steel tubular columns, not only the stress on the section is reasonable, but also the lateral stiffness of the frame can be improved, and the column will not buckle quickly when the structure is damaged. Therefore, although it is reasonable to choose H-beam or I-beam in plane stress structure, box-shaped steel tubular columns, especially concrete-filled square steel tubular columns, should be widely used. Concrete-filled square steel tubular column will be the main development direction of steel structure residence, but it is seldom used in residence at present because of the lack of corresponding norms and regulations. In particular, the connection between concrete filled steel tubular beams and columns is complex, which is not conducive to industrial production and on-site construction. Therefore, more efforts should be made in development and research.
(2) Floor
In multi-storey light steel buildings, the choice of floor structure is very important. In addition to distributing the vertical load directly to the wall column, its main function is to ensure the spatial coordination with the lateral force resisting structure. In addition, from the seismic point of view, corresponding technical and structural measures should be taken to reduce the weight of the floor. Commonly used floor structures include profiled steel plate-cast-in-place concrete composite floor, cast-in-place reinforced concrete slab and steel-concrete composite slab, of which the first one is the most commonly used. At present, in the overall analysis of multi-storey light steel buildings, the interaction between floor slab and steel beam is generally not considered, and even setting shear key is a conservative assumption.
The steel structure bears all loads, which not only increases the material consumption and structural weight, but also causes the disadvantages of strong beams and weak columns. There is an example of a six-story building. Table 1 and Table 2 respectively reflect the influence of floor combination on beam stiffness and overall stiffness of the structure.
Note: The annual combination effect is considered in brackets.
The example shows that the stiffness of the main beam is greatly increased after considering the interaction, which makes the deflection of the beam and the lateral displacement of the column top greatly reduced under the earthquake. This consideration of joint action should be paid attention to. In order to minimize the floor height and provide more space, it has become a trend to combine flat beam floors.
(3) Support system
There are two kinds of braces: axial cross braces and eccentric braces developed in recent years. The former has poor seismic performance, while the latter has good energy absorption performance under strong earthquakes, which provides favorable conditions for the layout of doors and windows. At present, it is rarely used in China, so it is suggested to give priority to eccentric bracing in high intensity areas. Stiffening ribs of shear-resistant and energy-consuming beams are designed according to the following formula:
a=29tw-d/5，(γp= 0.09rad)
a=38tw-d/5，(γp= 0.06rad)
A=56tw-d/5, (γp = 0.03 rad)
Where a- stiffener spacing, d- beam height,-web thickness, γp- plastic rotation angle; Stiffening ribs are needed at the outer side of the bending energy dissipation beam end1.5bf.
(4) Seismic design of joints
There are generally two connection modes of beam-column joints in frame, which are designed according to the "common design method", that is, the flange connection bears all the bending moment and the beam web only bears all the shear force. The earthquake damage shows that this design can not effectively meet the seismic requirements of "strong joints and weak members", and there are great hidden dangers in high intensity areas. In the improved frame joint design, welding wedge-shaped cover plates on the upper and lower flanges of beam ends, or locally widening the area of cover plates or increasing the cross-sectional area of flanges are mainly determined by the checking formula under strong earthquakes:
Where: refers to the maximum bending bearing capacity of the joint connection based on the minimum ultimate strength, which is all borne by the locally enlarged flange connection; Is the full plastic bending bearing capacity of the beam; It is the maximum shear capacity of the joint connection based on the minimum ultimate strength, which is only borne by the web connection; Is the net span of the beam; It is the design value of the shear force of the beam end section analyzed by the simply supported beam under the representative value of gravity load.
Four. conclusion
1) Considering the interaction of floor slab and steel beam, the deflection of main beam and the displacement of column top can be significantly reduced in low-rise and multi-storey light steel structure residence.
2) In order to improve the seismic performance of frame joints, wedge-shaped cover plates can be welded or widened at the upper and lower flanges of beam ends.
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