How to design steel structure space truss buildings?

Space truss structure is a kind of building structure which is composed of bearings, members, spherical joints and enclosure system. It is one of the fastest growing and most widely used spatial grid structures. Different from the primary and secondary relationship between the components in the plane structure system, the space truss structure is composed of a whole continuous space or expanded by many members. Each member of the structure is a part of the overall structure. It shares the bearing load according to the geometric characteristics of the space. The internal force transmission is simple, and the structural integrity is good. Therefore, the large span can be realized with small section, which reduces the dead weight of the structure and improves the overall economic effect.

1. Design of bearing node

The steel structure space truss is generally placed on the lower supporting structures such as column top and beam top, and the support node refers to the steel truss structure node on the supporting structure. The detailed design of the space truss structure starts from the bearing node. The elevation and position of the bearing node determine whether the steel grid structure construction can be carried out normally. The following points shall be paid attention to in the detailed design of bearing node:

a. The top elevation of the lower concrete structure or steel structure and the position of the embedded parts shall be accurate.

b. The joint form of the bearing shall be determined. The common types are rubber plate bearing and pot bearing, and the restraint direction of the bearing shall be noted.

c. The side bearing node is often close to the parapet, and the clear distance between the bearing node and the parapet should be noted, which is generally more than 50 mm.

d. The bearing capacity of the middle bearing is large, and the size of the bottom plate is often large. It is necessary to pay attention to whether the section of the lower concrete column or steel column meets the requirements. If it does not meet the requirements, the expanded column head node shall be used.

e. For the top chord supported grid structure, the member at the support position is often easy to collide with the lower support structure, and it needs to be rechecked. The distance between the bottom of the support ball node and the support base plate should meet the requirement that the diagonal web member does not collide with the column or coupling beam.

f. Limiting measures shall be taken for the outer side of the grid support.

2. Ball joint design

The grid sphere joints are divided into two types: welded sphere and bolt sphere. The bolt sphere is fastened with high-strength bolts, and the welding sphere is connected with on-site welds. The following points should be paid attention to in the detailed design of spherical joints:

a. The bolt ball joint fittings are different from manufacturer to manufacturer, and the fitting data shall be obtained before design.

b. The materials of steel ball, high-strength bolt, sleeve, fastening screw, cone head or sealing plate in the bolt ball joint shall meet the requirements of design and specification.

c. The grades of high-strength bolts in bolt sphere joints shall be selected according to the specifications.

d. The diameter of bolt ball shall ensure that adjacent bolts do not collide in the ball and shall meet the requirements of sleeve contact surface.

e. The cone head or sealing plate at the end of the member in the bolt sphere joint shall be connected by welding, and the bearing capacity of the connecting weld shall not be lower than that of the connecting steel pipe. The cone head section and seal plate thickness shall be calculated and determined according to the actual stress.

f. The welding ball is generally a hollow ball welded by two hemispheres. It is necessary to determine whether to set stiffeners according to the force.

g. The ratio of the outer diameter to wall thickness of the welding ball, the ratio of the outer diameter of the ball to the outer diameter of the main steel pipe, and the ratio of the wall thickness of the ball to the wall thickness of the main steel pipe should meet the specifications: the ratio of the outer diameter of the ball to the wall thickness should be 25~45, the ratio of the outer diameter of the ball to the outer diameter of the main steel pipe should be 2.4~3.0, the ratio of the wall thickness of the ball to the wall thickness of the main steel pipe should be 1.5~2.0, and the wall thickness of the ball should not be less than.

h. Groove weld shall be used to connect the welding ball and steel pipe, and a gap shall be left between the steel pipe and the ball to ensure that the weld penetration reaches the same strength as the steel pipe.

i. When there are too many connecting members of the welded ball node, the intersection of some members is allowed to reduce the ball diameter, but the structural requirements specified in the specification shall be met: the axis of all intersecting members must pass through the center line of the ball; When two rods meet, the rod with large sectional area must be welded on the ball in full section, and the groove of the other rod must be welded on the intersecting rod, but 3/4 of the section shall be welded on the ball, and the stiffener shall be set as required; Support plates can be set for the bars with large force as required.

3. Design of supporting node

The supporting node is the link between the truss structure and the enclosure structure, which is often distributed on the upper chord and side of the grid. The following points shall be paid attention to in the design of supporting joints:

a. On the premise of meeting the stress requirements, high-strength bolts shall be used as far as possible for the connection between the support and the ball to reduce the amount of high altitude welding on site.

b. The support top plate should be set into a circle to ensure that the support can rotate 360 degrees along the rod axis for positioning and installation.

c. When the support height is large, diagonal bracing should be set to ensure the lateral stability of the support.

4. Design of purlin gutter enclosure system

The steel structure space truss purlin enclosure system often adopts the form of primary and secondary purlins connection. The main purlins often use I-shaped steel and rectangular pipe sections, and the secondary purlins often use C-shaped sections. The detailed design of purlin enclosure system needs to pay attention to the following points:

a. The connection between the main purlin and the bracket, and between the main purlin and the secondary purlin should be bolted.

b. When rectangular pipe section is adopted, attention shall be paid to the end sealing of purlin to avoid corrosion caused by inconvenient anti-corrosion treatment inside.

The grid structure has a variety of forms, which can present the building plane of various shapes, and the building shape is light and beautiful, the sense of space is sufficient, and at the same time, it can meet the requirements of large span with low steel consumption, so it is widely used in large buildings such as stations, airports, exhibition halls, etc. The design of grid structure is different from other steel structure types. It needs to be completed by combining different software such as Tekla, 3D3S, MST, etc. The designers are required to be more comprehensive and can use different tool software to seamlessly connect the detailed design.