Pipe truss structure
A, definitions, In this case, the truss is truss beam, is a kind of lattice girder structure. Truss structure is often used in large span factory building, exhibition hall, sports centres and other public buildings and Bridges. Because of the mostly used in construction of roof structure, truss is usually referred to as roof.
Second, the main structure
Each bar stress are given priority to with one-way pull, pressure, through the rational layout of bottom chord and CHS, can adapt to the bending moment and shear force distribution within the structure. Due to the horizontal direction, pressure internal force to realize their balance, the whole structure not horizontal thrust bearing. Flexible structure arrangement, wide application range is very wide. Truss beam and plain girder beam) (that is, we usually see, compared in terms of bending, because will be in tension and compression section concentrate on both ends up and down, increase the inside lever, made with the same material consumption, to achieve the larger bending strength. In shear, through reasonable decorate, CHS can pass shear gradually to support. So both bending and shear, truss structure can give full play to the strength of materials, and thus is suitable for various span of building roof structure. More important significance is that it will be under the action of lateral bending of the solid abdomen beam within the complex stress state into truss rods in simple tension and compression stress state, enable us to intuitive understanding of the power distribution and transmission, to facilitate the change of the structure and composition.
Fourth, truss type
Truss can be classified according to different characteristics.
According to the shape of the truss can be divided into: parallel chord truss (easy to decorate a double-layer structure; standardization production, but the pole force distribution is not uniform), fold chord truss (such as parabolic truss beam, the appearance of the bending moment diagram which is under the uniformly distributed load, uniform rod force distribution, material use economy, complicated structure), triangular truss (bar force distribution is uneven, the structure arrangement is difficult, but cant meet the roof drainage need).
In truss geometry of way points: simple truss (basic hinged by a triangle, in turn, increase the dual body composition), joint truss (according to the geometric invariant system composed of several simple truss of a combination of simple rules), complex truss (different from the first two other statically determinate truss) (chart).
By the horizontal thrust points: no thrust of beam and truss (compared with the corresponding real beam structure, empty rate is large, the lower chord bending, CHS is mainly shear, reasonable force, material economy), a thrust of arch truss (arch ring and arch structure etc on the integral sex is good, convenient for construction, strong spanning ability, save steel material).
Five, the characteristics of internal force of truss
The stress characteristic is the internal force of the structure only axial force, bending moment and shear force and No. The stress characteristics of the main factors reflecting the real) structure, the main force of axial force called truss. The actual structure (such as reinforced concrete roof truss, riveting (bolt) is connected or welded steel truss bridge node) due to non ideal hinge and other reasons, also exist at the same time the shear and bending moment of tiny, also has very little effect on the axial force (vary according to size, rigid and truss rod cross-sectional area and inertial moment ratio decreases generally 5% ~ 0.1%), known as the secondary internal force.
Considering the balance of truss each node, node under concurrent force system function, establish the projection equilibrium equation of each node can be obtained by successive, unknown rod force all, this method is called node method, the most suitable for simple truss. The solution should be based on the composition characteristics of first determine zero bar, and as far as possible to avoid the solution of simultaneous equations. Sometimes it only needs a few internal forces of rod or for combined truss and complex truss, node method cannot work, with the section method. Selectively truncation rods (generally not more than three bar truss) with local to balance the object, the equilibrium equation can be obtained by the required pole axial force. For some truss (such as K type truss), the combined application of the node method and section method is more efficient. For many complex truss rod or space truss, the best choice is to computer method.
