Steel structure truss force analysis.pptx

Steel joist generally consists of top chord, bottom chord and web. Its force is different from that of ordinary beams: the top chord is compressed and the bottom chord is tensile, forming a force couple to balance the bending moment generated by external load; the vertical component of axial force of diagonal web bar is used to balance the shear force generated by external load. Each rod unit is an axial tensile or axial compressive member without bending moment and shear force, and this feature can make the strength of the material to be fully utilized.

1. Steel truss force analysis LF GROUP

2. Steel joist generally consists of top chord, bottom chord and web. Its force is different from that of ordinary beams: the top chord is compressed and the bottom chord is tensile, forming a force couple to balance the bending moment generated by external load; the vertical component of axial force of diagonal web bar is used to balance the shear force generated by external load. Each rod unit is an axial tensile or axial compressive member without bending moment and shear force, and this feature can make the strength of the material to be fully utilized.

3. According to the equilibrium theory, the nodes are subjected to convergent force system, and the projected equilibrium equations of each node are established one by one: ΣX=0, ΣY=0, and all the unknown internal forces of the rods can be found out, which is called the node method, and it is the most suitable for the simple truss. This method is called nodal method, which is most suitable for simple truss. When solving, the zero rod can be determined according to the balance theory, and avoid solving the joint equations as much as possible, and sometimes only a few internal forces of the rods are required. For joint truss and complex truss, when nodal method can not work, it is necessary to use cross-section method to selectively cut off the rods (generally not more than three) to the truss local equilibrium object, and then the equilibrium equations can be used to find the required axial force of the rods. For some trusses (e.g. K-type truss), joint application of nodal method and section method is more effective. For complex trusses with many bars or space trusses, it is better to choose the electrical method.

4. The internal force of truss structure is dominated by axial force, and the distribution of internal force of each member is not uniform. The internal force of the top chord members is axial pressure, and the internal force of the bottom chord members is axial tension, which forms a force couple to resist the action of bending moment. The internal force of vertical web and diagonal web rods may be axial tension or axial pressure (determined by the arrangement of rods), resisting the shear force. The direction of diagonal web arrangement of common trusses such as triangular truss, rectangular truss, parabolic truss, and trapezoidal truss has an effect on the web force, and the following conclusions are obtained based on the distribution of internal forces:

5. Triangle truss: the internal force distribution is not uniform, the chord has to change the cross-section, the construction is difficult, otherwise it wastes the material, the angle of the end point is small, it is difficult to fabricate, it is generally used in the small span, the span is generally below 18-24m, and the slope is large roof frame.

6. (2) Rectangular truss: the internal force is not uniformly distributed, if the rods are selected according to the internal force, there are many types of rods, poor aesthetics, construction difficulties, and material wastage, but if standard rods are used, the chords with the same cross-section can make the structure neat, and it is easy to purchase without causing wastage.

7. (3) Trapezoidal truss: The internal force is between triangular truss and rectangular truss.

8. (4) Parabolic truss: the internal force is uniformly distributed and the material used is the most economical, but the inclination angle of the upper chord bar is different, the structure is complicated and the construction is inconvenient, so it is mostly adopted in the large span structure of 60m and above, which is conducive to the saving of material. Explore the endless possibilities at https://www.cnxzlf.com/