The document details the force analysis of steel trusses, explaining the roles of top chords, bottom chords, and webs under external loads. It describes various methods for determining internal forces in trusses, including the nodal method and the cross-section method, tailored for simple and complex trusses. The internal force distributions in different truss types (triangular, rectangular, trapezoidal, and parabolic) illustrate material efficiency and construction considerations.

1. Steel truss force
analysis
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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

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.
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