Shapes Of Structural Steel

Lesson 24 : Structural Steel Work

Shapes Of Structural Steel

The cross-sectional properties of a member are identified by its shape. The various names given to the members of different shapes are Angles; T-section; I-section, Z-section, Channel, H-section and so on. Each shape has its particular use in steel structures. The various sections are fabricated from white hot steel by passing it through rolling mills or other machines. An angle section is named by writing the two leg lengths and the thickness of the plate forming the angle. An equal angle-section having length of its each leg as 8 cm and its thickness equal to 12mm will be written thus : 80x80x12mm. In case of I-section the depth of the section is written first. Thus, an I- section weighing 34.2 kg/m and having a depth equal to 200mm and flange width 100 mm will be written as 200mmx100mmx34.2kg/m. Similarly, the other shapes of steel section are named by writing their dimensions and weights.

Rivets: 1

The sections used in steel structures are connected to each other by riveting or bolting. Welding is an alternative method of connecting the structural sections. The various forms of riveted connections are shown below.

The rivet is rendered soft by heating and then it is placed in the rivet hole. The shank extends out of the prepared rivet hole and the extended part of the shank is given the shape of a second head by riveting machine. The usual form of rivet head which is cup-shaped in appearance is termed as snap head. Thus snap head always extends out from the face of the riveted plate. In case, it is desired to have the second head flush with the plate or member to be riveted, countersunk head is employed.
Important rules to be observed while designing the riveted connections.

  • The centre to centre distance between the rivets should never be less than three times the nominal diameter of the rivet.
  • In case of members subjected to tension, the maximum centre to centre distance between the rivets should never exceed 10 times the thickness of the thinnest outside plate or angle or 20 cm whichever is less.
  • In case of members subjected to compression, the maximum centre to centre distance between the rivets should never exceed 16 times the thickness of the thinnest outside plate or angle or 15 cm whichever is less.
  • The distance between the edge of a rivet and edge of the plate and so on should never be less than the diameter of the rivet.

Beams2

I-sections or rolled steel joists are commonly used as simple beams. They are suitable for moderate loading conditions and spans. But for heavy loads and longer spans, compound beams are used. They are fabricated from one or more rolled steel joists or riveted with plates and angles. In some cases only plate and angle are riveted together to form a compound beam. Plate girder and box girder are two examples of compound beams.



Compression members 3

A compression member is one, which is subjected to compression stresses in a direction parallel to its principle axis. The compression stresses tend to buckle the member or shorten it. Hence, in the design of a compression member, its shape, area of cross- section and its length are of vital importance. Stanchions and columns are the examples of compression members.

Columns and stanchions

Column is a vertical compression member which is circular in cross- section. It may be hollow or solid in section. Vertical compression member may be termed as stanchion. Depending upon the height of stanchions, and the loading condition; they may be fabricated in various shapes. Long column tends to fail in bending and as such, from economical consideration, the column should be so shaped that the variation between the maximum and minimum radii of gyration about its two principle axis and the value of its area divided between least radius of gyration should be as small as possible. Hence, single sections of rolled steel joists, angles or channels etc. are commonly adopted for columns subjected to light loading conditions. For columns required to carry heavy loads, two or more sections of angles channels, rolled steel joints, etc. are joined to form a compound section.

Framing long spans

For auditoriums, theatres, hangers, large industrial buildings where greater clear distance is necessary, girders, trusses, arches, so on fabricated from steel section are used. Girders are used in situations where the depth of framing is limited to 1m. When the designed depth of the beam works out to a more value, plate girders are employed. But in places where depth of framing is not restricted from architectural consideration, trusses are used.

Trusses:

A trusses consists of an assembly of rigid but elastic members jointed in the form of triangles to act as a beam. The safe working tensile stress of mild steel is about 20 times that of structural timber. Thus steel trusses work out to be economical, specially for bigger spans. Out of the various shapes of steel sections, angels are considered most suitable for steel roof truss. This is on account of the fact that angles can resist both compressive and tensile stresses effectively. In addition angels can be produced economically and can be jointed easily.

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5

  • Steel trusses are stronger than timber trusses.
  • Steel sections forming the truss are light in weight and can be fabricated in any desired pattern to suit the architectural requirement.
  • There is no danger of the material being eaten away by white ants or other insects.
  • Steel trusses are more fire – resisting than timber trusses.
  • They do not have restrictions and as such steel trusses can be used for industrial buildings and other such structures where large areas are required to be covered without obstructions due to columns and so on.
  • The sections forming a steel truss are easy in transportation.

Stanchion base

Stanchions are riveted to specially built bases. The stanchions base consists of an arrangement of base plate, gusset, plates, angle cleats, stiffeners, fastenings etc. The stanchion is fully connected to the base plate with the help of the above fixtures and finally the base plate is fixed to the foundation of stanchion (which may be in most of the cases a simple cement concrete block foundation) through rag bolts. The width of the base plate varies from 2 to 4 times the width of the stanchion and the heights of the gusset plates vary from 1 ½ to 2 times the width of the stanchion. To avoid shear and bending, the base plate should not project more than 8 times its thickness beyond the stanction face.

Welding

It is a process of joining two or more structural members by introducing fused metal into fillets between them or by raising the temperature of their 6surfaces to the fusion temperature and then applying pressure. It may thus be said to be an alternative method of connecting structural steel sections. With the development of welding it has become possible to render architectural beauty to steel structures also. Structural members of the desired pattern can be joined flush to a smooth and pleasing surface.
There are two different methods of welding commonly adopted.

  • Electric arc welding
  • Oxy – acetylene welding

Electric are welding is economical and is commonly used in places, where electricity is easily available. In case of oxy- acetylene welding, a flame of high temperature is produced by burning a mixture of acetylene and oxygen.

There are three different types of welds, namely, Butt welds, Fillet welds and Lap welds.

  • Butt weld: In this type, the edge of on plate is brought in line with the edge of a second plate and the joint is filled with welding metal or the two edges are heated to their fusion temperatures are pressed together.
  • Filled weld: In this type, the edge of one plate is brought against the surface of another (not in the same plane) and welding metal is fused in the corner between the two plates. Thus the joint can be welded on one or both the sides.
  • Lap weld: In this type, the edges of plates are lapped one over the other and the edges of one plate are welded to the surface of the other.
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Last modified: Friday, 20 April 2012, 7:17 AM