Generally, the height-span ratio of trusses is: three-dimensional truss1116, three-dimensional arch 1/20~ 1/30, and tensed three-dimensional arch 1/30. Trusses are widely used. When choosing the truss form, the purpose, material, support mode and construction conditions of the truss should be considered comprehensively. The principle of choosing the best form is to minimize the materials and labor used in manufacturing and installation on the premise of meeting the use requirements.
Triangular truss
Under the action of uniformly distributed nodal loads along the span, the axial force of the upper and lower chords of the triangular truss is the largest at the end points, and gradually decreases towards the midspan. The axial force of the web is opposite. Triangular trusses are mostly used for roof trusses with tile roofs because of the great difference in internal forces of chords and unreasonable material consumption.
Trapezoidal truss
Compared with triangular truss, trapezoidal truss improves the stress of components, and it is easier to meet the technological requirements of some industrial workshops when applied to roof truss. If the upper and lower chords of a trapezoidal truss are parallel, it is a parallel chord truss, and the stress of its members is slightly worse than that of a trapezoidal truss, but the types of web members are greatly reduced, which are mostly used in bridges and trestles.
polygonal truss
Polygonal truss is also called folding truss. The upper chord node is located on the quadratic parabola. If the upper chord is arched, the bending moment caused by the load between nodes can be reduced, but the manufacture is more complicated. Under the action of uniform load, the shape of truss is similar to the moment distribution law of simply supported beam, so the upper and lower chord axial forces are evenly distributed, the web axial forces are small and the materials are the least, so it is a common truss form in engineering.
vierendeel truss
The vierendeel truss is basically a polygonal truss with no diagonal web members, and only connected with vertical web members and upper and lower chord members. The axial force distribution of members is similar to that of polygonal truss, but the bending moment at the end of members changes greatly under asymmetric load. The advantages are that there are few intersecting members at the nodes, and the construction and manufacture are convenient.
truss bridge
Truss bridge is a kind of bridge. 2. Truss bridges are commonly found in railways and expressway; It can be divided into upper chord stress and lower chord stress. 3. Truss consists of upper chord, lower chord and web; The form of web bar is divided into oblique web bar and straight web bar; Because of the large slenderness ratio of members, although the connection between members may be "fixed connection", the actual bending moment at the end of members is generally small, so the design analysis can be simplified as "hinge". When simplifying the calculation, the members are all "two-force bars", which bear pressure or tension. 4. Because of the long span of the bridge and the weak "out-of-plane" stiffness of the single truss, it is necessary to set up "out-of-plane" bearings. When designing a bridge, the "out-of-plane" is generally designed as a truss, which makes the bridge form a whole and has good stiffness in both directions. 5. Some bridge decks are arranged in the upper chord, so the force is mainly transmitted through the upper chord; Some bridge decks are located in the lower chord. Due to the requirement of out-of-plane stiffness, the upper chord still needs to be connected to reduce the out-of-plane calculated length of the upper chord. 6. The chord of the truss bears a large force in the span and gradually decreases in the direction of the support; However, the stress of the web member is the largest near the bearing, and the stress of the web member in the middle of the span is relatively small, and even there is a theoretical "zero bar".