Classification of steel truss structures
From the analysis of mechanics, the bending moment diagram of truss shape similar with the beam, axial force of upper and lower chords of the uniform distribution of abdominal axial force is small, the material; from the analysis of materials and manufacturing, wood truss made of triangular, trapezoidal or parallel chord steel truss shape, reinforced concrete and prestressed concrete truss or polygonal the ladder is appropriate.
Usually the truss height and span ratio, 1/12~1 /16, three-dimensional truss arch, three-dimensional 1/20~1/30, tensile three-dimensional arch is 1/30~1/50, specific provisions are in the design manual and specifications. The use of a wide range of truss, truss should be integrated into the application, materials and supporting way, construction conditions in the choice of truss form, its form is the best selection to meet the application requirements under the premise, and strive to the manufacturing and installation materials and the amount of labor is minimal.
Triangular truss
The triangular truss node load evenly distributed along the span, the axial force on the bottom chord of the biggest at the endpoint, to gradually reduce the cross; the axial force is opposite to the belly bar. Triangular truss chord due to internal differences, the material consumption is not reasonable, for the roof tile roof.
Trapezoidal truss
Compared with trapezoid truss and triangular truss, the member stress situation has improved, but also for the roof can be more easy to meet the process requirements of some industrial plants. If the trapezoid truss of the upper and lower chords is parallel parallel chord truss, the member stress is trapezoid slightly worse, but the abdominal rod type is greatly reduced, for bridge and Zhanqiao.
Polygonal truss
Polygonal truss, also known as polygonal truss. The node is located two parabola, such as bending arched winding can reduce the load of the internode is more complex, but manufacturing. Under uniform loads, bending moment graph truss shape beam and so on similar chord axial force distribution, small abdominal axial force, the material, is a commonly used form of truss in engineering.
Open web truss
Take the basic truss of polygonal truss shape, no oblique abdominal rod, only in the vertical and the upper and lower chords are connected. The axial force distribution of the rod is similar to that of the polygonal truss, but the bending moment at the end of the asymmetrical load is greatly changed. The advantages of the node in the intersection of the rod is less, the construction of convenient.
Truss bridge
1, truss bridge is a form of bridge.
2, the truss bridge general in railway and highway; divided into wind stress and bottom stress two.
3, by the first quarter, and the lower abdomen truss rod; abdominal rod form is divided into straight abdominal oblique abdominal rod, rod; the rod itself due to large slenderness ratio, although the connection between rods may be "fixed", but the actual moment of the rod end are generally small, therefore, the design and the analysis can be simplified as a "hinged". When the calculation is simplified, the rod is a "two force bar", which is under pressure or tension.
4, since the bridge span is large, and the single frame truss "plane" stiffness is relatively weak, therefore, the "plane" to support. Design of the bridge, the "plane" is generally designed to form a truss, so that the bridge will form a two-way have a very good overall stiffness.
5, some of the bridge deck is arranged in the first quarter, so the force is mainly through the winding transfer; some deck is arranged in the last quarter, due to out of plane stiffness requirements, still need to calculate the length of the connection between the winding to reduce the winding plane.
6, chord truss in midspan stress is relatively large, to support the direction gradually decreases; and the stress in the main truss support attachment in a cross section, brace force was small, or even zero pole theory .
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