The height of steel truss is determined by the requirements of economy, stiffness, use and transportation. Increasing the height can reduce the section and deflection of chord, but it will increase the number of web members and the building height. The height-span ratio of steel trusses is usually1/5 ~112; Steel trusses with high steel strength and strict stiffness requirements should adopt relatively high values. The height of triangular steel roof truss is usually determined by the height of roof slope; When the general roof slope is 1/2 ~ 1/3, the height-span ratio is 1/4 ~ 1/6.
The web system of steel truss usually adopts herringbone or monoclinic shape. The herringbone web is widely used because it has fewer web members and nodes. In order to reduce the internode size of loaded chord or compressed chord, some vertical rods are usually added. Single diagonal web members are usually arranged with longer diagonal members in tension and shorter vertical members in compression, sometimes used for long-span steel trusses. If it is necessary to further reduce the length of chord and web, subdivision web system can be adopted, and when the height of steel truss is large and the internodes are small, K-shaped or rhombic web system can be adopted. When supporting trusses and towers, cross web members are often used, which can bear the changing load well, and cross diagonal members are usually designed as tie rods. The inclination angle of the inclined web relative to the chord is usually in the range of 30 ~ 60.
The centroids of each member section of steel truss should intersect at one point at the node, and the internal force calculation is generally carried out according to hinged truss. When the truss only bears node load, all members only bear axial tension or pressure; If the load is also borne on the internodes of the members, the members will bend at the same time. Steel truss members are generally thin, so local bending moment should be avoided or reduced as much as possible when arranging nodes. For steel trusses with large section height and length, the secondary stress of members caused by joint stiffness should be considered when necessary.
In order to ensure the stiffness and stability of the plane steel truss out of the plane of the truss, reduce the calculated length of the out-of-plane chord of the truss, and bear the possible lateral load, the steel truss should be laterally arranged with supports (Figure 2). Braces can usually be divided into horizontal braces (upper and lower chord planes, horizontal and vertical braces), vertical braces (both ends and middle of the truss) and tie rods. Paired steel trusses can be provided with horizontal lateral supports along the lower chord plane and the upper chord plane respectively, and vertical supports are provided at appropriate intervals at both ends and in the middle of the steel trusses. There are many steel trusses in the roof structure, and the horizontal and vertical supports of the upper and lower chords can only be arranged at regular intervals at both ends and between two adjacent trusses, while the upper and lower chords of other trusses are only properly arranged at intervals; When there is a heavy crane or when necessary, longitudinal horizontal support can be added between the lower chord ends of the truss. In a four-sided or multilateral tower, diaphragms should be set at regular intervals to ensure the rigidity of the tower and the geometric invariance of the section.
The section form of steel truss members should be selected according to the conditions of saving steel, convenient connection and simple manufacture, and the slenderness ratio of the members on the two main shafts (the ratio of the calculated length of the members to the radius of gyration of the sections) should be as close as possible. Steel truss tie rods shall meet the requirements of strength and allowable slenderness ratio; The compression bar shall meet the requirements of strength, stability and allowable slenderness ratio.
When calculating the strength and stability of the bar, the internal force is considered as axial force; When the bar bears axial force and bending moment at the same time, the * * * interaction should be regarded as eccentric force. When calculating the stability and slenderness ratio of members, the in-plane and out-of-plane directions of trusses or the unfavorable directions with large slenderness ratio should be considered. The allowable slenderness ratio of steel bars shall be specified according to conditions such as compression or tension, static load or dynamic load.
For steel trusses with a slightly larger span, it is usually stipulated to arch in advance during manufacture to offset all or part of deflection under self-weight and load. The pre-camber (f) of the roof truss is generally 1/500 of the span.