Steel trusses

Steel trusses are structures of steel bars connected into joints. Trusses according to structural mechanics is a bar system, geometrics of which is not changed if rigid joints are replaced with hinged ones. Only tensile and compressing stresses appear in elements of trusses if there is no misalignment of rods or out-joint loads.

Steel truss consists of two elements: chord and web.

The chord is a steel structures forming the outline of a truss. The web consists of posts and crossbeams connected to truss chords by means of welding. The Russian word «ферма» (in Russian it stands for a word “truss”) is derived form Latin word “Firmus” (firm). The main advantage of steel trusses is a reliability of structures; on this parameter they outperform any other types of structures, even those entirely made of steel. At the same time trusses weights less and consumes less steel in manufacturing. This allows reducing labor and transpiration costs. modern type of trusses can withstand huge loads and thus they are commonly used as support or floor structures.

Trusses are used wherever the huge safety coefficient is required and big weight is not acceptable.

Scope of application of steel trusses:

  • Floor structures of industrial buildings
  • Roof structures of residential buildings
  • Bridge span structures
  • Cargo cranes
  • Outside structures for advertising
  • Roof structures for trading and entertainment and sports complexes.

Steel trusses are of two types:

  • three-dimensional;
  • plane.

In the first case the axis of a truss form an axis which is capable of withstanding of multiaxial loads. Eachfaceofthisaxisisaplanetruss. Elements of a plane truss are located in one plane and thereby it can withstand significant loads directed from one side only.

Trusses can be divided into several types according to their structures: shape of the chord, dimensions of spans, structural static, type of joints, etc. Static trusses can be arch, beam, cable or framed. Other frequently used types of trusses are: polygonal, triangular, segmental, trapezoidal trusses and the ones with parallel chords.

The most common type of trusses is a combined truss. They are irreplaceable in constructions with live static loads. Structure of steel truss has no bends, thus it can cover spans with length of more than 70 m. 

According to the most widespread classification of trusses they are divided into the following types:

  • Cantilever. This type is used for construction of hanging structures.
  • Arch. This type consumes less steel, but enlarges area of a building. Because of this, the area of closure structures becomes large too.
  • Rope. Elements of this type are designed only for tensile stresses.
  • Split-type. This type is used for construction of buildings with big-sized spans in most of the cases.
  • Continuous. Used to cover several spans at a time and have reinforced stiffness.
  • Trusses for industrial buildings – rafter truss and secondary trusses. Rafter trusses are used for construction of buildings having spans of 24 m and more, and bridge cranes with load capacity more than 20 t. Secondary trusses are used as a support for the main bearing and floor structures, in case if the pitch  of columns exceeds the one of beams and constructional trusses.

Steel trusses are also divided into:

  • Light – span up to 50 calculated node loads not more than 4000 kN.
  • Heavy – span over 50 m and calculated node loads exceed the ones on light type.

Light trusses are single-web frame with one row of gusset plates and chords of different cross-sections. Angles, channels, t-beams, seamless pipes of a different configuration are used for fabrication of this type. Light trusses of shape angles forming a structural tee are the most common. Gusset plates located between angles form its base. Anglesareweldedwithsidefilletwelds.

The base of a tube trusses are hot-rolled and welded tube and pipes. Welded ones are economically advantageous in fabrication of compression elements. The most feasible type of splice connection – is the one with direct abutment of contour welded pipes. In case if it is unable to perform shape cut, the ends of pipes are usually flattened and then welded to the chords of a truss. Trusses made of closed or open section have joints of a simpler structure.

Production engineering of steel trusses

Fabrication of steel trusses is carried out in several stages. The first stage is – preparation of KMD drawings.

Cutting stage includes CNC-machine cutting of sectional and plate steel.

Assembly of truss elements is performed in several stages with the help of tack-welding. Assembly of the whole rafter structure is done is a single working area.

The next manufacturing stage is the final welding of a structure. Direct-current power supply is used for this purpose. Welding method – semi-automatic. Once all welding works are finished, each seam should be thoroughly cleaned from weld spatter and metal flows.

Anti-corrosion treatment is a final stage of manufacturing at Chelyabinsk Steel Structure Plant. Steel structures are cleaned in shot-blasting chambers and then coated with several layers of anti-corrosion paints – primer and final coating (if required).