Steel Structure Design basic principle

 GRADE 4.6

 fultimate stress = 4 x 100 = 400 N/mm2

 fyield stress = 400 x 0.6 = 240 N/mm2


 fultimate stress = 8 x 100 = 800 N/mm2

 fyield stress = 800 x 0.8 = 640 N/mm2

 For M 20 Bolt, Diameter of Hole = D + 2 = 20 + 2 = 22 mm.

 Minimum size of the fillet weld = 3 mm.

 Minimum end return of weld is = 2 x size of weld.

 Safety factor for field weld is = 1.50.

 Safety factor for shop weld is = 1.25.

 Type of weld used for butt joint is the Groove Weld.

Component of Structure Slenderness Ratio
Lacing 145
Strut 180
Dead/Live Load 180
Wind/Earthquake Load 350
Tension Member 400

 Tdn = ∝ Anfu / γm

Where, ∝ = 0.6 for One or Two bolt

∝ = 0.7 for Three bolt

∝ = 0.8 for Four or more bolt.                  Beam Type           Effective Length
Both end Fixed 0.65 L
One end Fixed & One end Hinged 0.80 L
Both end Hinged 1.0 L
Laced Column 1.05 L
Battened Plates 1.10 L
Back to back plate 0.85 L

 Imperfection factor for Class ‘c’ = 0.49.

 A beam with a number of regular opening in its web is typically  called Castellated Beam.

 A steel beam which supporting loads from floor slab as well as wall is called Spandrel Beam.

Shear bucking of web occur if,  d / tw > 67ε.

  • γm0 = 1.10 by yielding.
  • γm1 = 1.25 by stress.
  • βb = 1.0 for plastic & compact section.


 Purlin shall be designed as a Flexural Member.

 Sag rods are used in case of roof truss for connecting purlins.

 Lug angles are used to reduce the length of the connection & used at joint to the outstanding leg of the member.

 Bolt are most suitable to carry the shear and bending stress.


 Box section is built up from perforated pipe member.

 It is provided where torsion is occur.

 Capacity of short column depends upon the yield stress & cross-sectional area.


  1. Lacing joints is used in built-up columns to carry the transverse shear force.
  2. Lacing bar of compression member is basically designed to resist the  transverse shear equal to 2.5 % of axial load in the member.
  3. Angle of inclination of lacing bar with axis of column is = 40° to 70°.

 Bearing Stiffeners are provided at the plate girder at support & point of application of the load.

 Load Factor for Dead Load is = 1.7

 Shape Factor for Square & Rectangle are= 1.5 & for Circle = 1.7.

 Plastic hinged are formed at Fixed end,

 B.M is always Maximum & Below point load.

 Maximum Deflection for Beam = L/250.

 Area of opening for building of large permeability is more than 20% of wall area.


 Gantry Girder are designed to resist the lateral, longitudinal and vertical loads.

 Effective depth of gantry girder = L/12.


 Compression member in roof truss is treated as Strut.

 In case of  roof truss for principal rafter used section is two angle section placed back to back with each other.

 FOR Fu = 410 N/mm2

 Design strength of fillet weld = 158 N/mm2

 Design strength of shop weld = 189 N/mm2


 Batten plates are used to connect the components of the built-up column.

 Batten plates are design to resist the Bending moment and shear value due to the transverse shear.

 Number of battens should be such that member is divided not less than three parts longitudinally.

 Effective length of battened column increased by 10 %.

  • One dimensional structure:- Width and thickness of structure is small in comparison to its length is called One-dimensional structure.
  • Two dimensional structure is known as the Surface structure.
  • Three dimensional structure is known as the Space structure.
  • Skeleton structure :- The structure in which the member is represented by the  line is called Skeleton structure.


 As I – section is classified into 5 series.

 It is used to resist axial forces bending and in built up sections of column.

 Junction between flange and web = Fillet.


 As Channel section is classified into 5 series.

 This is subjected to torsion because of the absence of symmetry.

 It is used where transverse loads are to be carried and combinations of two channels are used in the column.


As per, Angle section is classified into 2 series.

 It is used as tension and compression member in trusses.


 It is used to transmit the bracket loads to column.

 Also used with flat strips to connect the plates in steel rectangular tanks.


 It is used as tension member.

50 ISF 8 = 50 mm width & 8 mm thick.


  1. Also known as the  rivet line or back or gauge line.
  2. In rivets, round bar of steel or wrought iron provided with head on one side & tail at the other side.
  3. Portion between head and tail is called Shank of rivets.
  4. Size of rivets expressed by diameter of the shank.
  5. Tail ends formed as head by the process of the  forging.
  6. Size of the rivet hole kept larger than diameter of rivet.
  7. If the diameter of rivet is up to 24 mm than hole of rivet kept 1.5 mm larger.
  8. If the diameter of rivets is more than 24 mm than hole of rivet is 2 mm larger.

 Cold driven rivets

  1. Rivets at atmospheric temperature are known as cold driven rivets.
  2. Used for structural joint.
  3. Need large pressure to form the head and complete the driving.
  4. Diameter of rivets = 12 to 22 mm.

 Hot driven rivets

  1. It is used to make leak proof joints.
  2. Diameter of rivets = 16 to 30 mm.
  3. Classified as power driven shop rivets, power driven field rivets & hand driven field type  rivets.

 Two types of rivet joint

 Lap joint: – When one member is placed above the other and connected by rivets & lap joint develop is bending stress.

 Butt joint: – Whenever the two members are kept in same alignment butting each other and cover plate is provided one side or both sides of main member & riveted together.

  • Staggered Pitch: – It is the distance between the centres of two consecutive rivets in zigzag riveting.

 Shearing of rivet takes place in the one cross-section, also rivet said to be in single shear.

 Shearing of rivet takes place in the two cross-section, rivet said to be in double shear.


  1. Electric arc welding
  2. Oxy-acetylene welding

 Most widely used gas combination is oxygen and acetylene.

  1. Weld is given a reinforcement which may be taken as 10 % of the  plate thickness.
  • Effective length of fillet weld = Actual length – 2 x size of weld = 4 x size of weld.
  • Fillet weld is not used if the angle between fusion faces more than 120° or less than 60°.
  • If angle of fusion is  = 60°-90°, Effective throat thickness is = 1 2 x size of weld.


  1. Effective length of intermittent butt weld is 16 x thickness of thinner part.
  2. This type of weld is provided to resist shear stress.
  3. Failure due to shear.
  • End fillet weld = Stressed in transverse shear.
  • Side fillet weld = Stressed in longitudinal shear.
  • Vertical compression member is  = Column.
  • Compression member in the roof truss = Strut.
  • Main compression member in roof truss is  = Rafter.
  • Jib of crane which is compression member is  = Boom.


  1. 16 mm diameter for angle of 10 mm thick.
  2. 20 mm diameter for angle of 10 to 16 mm thick.
  3. 22 mm diameter for angle of 16 mm thick.
  • Web plate is totally unstiffened if Clear depth thickness of web = 85.
  • Web plate is totally Stiffened when Clear depth thickness of web > 85.
  • Failure of column depends upon the slenderness ratio of column.


 Splicing of the columns necessary when the required length of column is not available or when section is to be changed.

 Allowable working stress is = 125 N/mm2.

 Column splices are provided at ¼ of height of column.


 To support and distribute the load of column on greater surface area.


 It is used on both sides of the butt jointed members.

 It is used when the length of the section is less than that of tension member required.

 It is used when size of member changes at different length.


  • Joist

 When Beam supporting flooring but no other beams.

  • Floor beam

 When Beam supporting other beams or joists.

  • Girder

When Major beam is used for beam for floor beam.

  • Lintel

When Beam supporting wall directly over doors & windows.

  • Header

 When Beam at stair well opening.

  • Trimmer

 When One of the beams supporting header.

  • Stringer

 When Beam supporting stair steps.

  • Girt

 When Beam supported by external wall columns & supporting wall covering.

  • Purlin

 When Horizontal beam spanning between two adjacent roof trusses.

  • Rafter

When Roof beam resting on purlins.

  • Butt weld: – When structural member subjected to direct tension or compression.
  • Fillet weld: – It is the joining  of two surfaces approximately at right angles to each other.
  • Slot weld: – A hole is made in component & welding around periphery of  the hole.
  • Seam weld: – In this case pressure is applied continuously.
End Condition Effective Length
Held in position and restrained in direction at both ends.            0.65 L
Held in position at both ends restrained in direction at one end.            0.80 L
Held in position at both ends not restrained in direction.                 L
Held in position and restrained in direction at one end and other end restrained in direction but not held in position.              1.2 L
Held in position and restrained in direction at one end and another end partially restrained but not held in.              1.5 L


 Web crippling due to failure of web under concentrated load.

 Web crippling & Web buckling prevented by increase of  the thickness of web and increasing length of bearing plate.

 Filler Joists: – Steel structured beam of light sections placed in plain cement concrete is called Filler Joists.


 In case of plate girder, the vertical stiffners are provided when the ratio of depth to thickness of web exceeds 85.

 The economical depth of the plate girder is defined as depth at which gives minimum weight of plate girder.


 Vertical stiffners are provided to prevent the local buckling of web of member.

 Vertical stiffners spacing should not be greater than 1.5d times and should not be less than 0.33d.


 It is provided when depth of web exceeds 85 tw.

 It is provided in plate girder to eliminate web buckling.

 Net shear area of bolt is  = 0.78 x (π/4 d2 )

 Edge of plate cut by,

Hand flame than, edge distance is = 1.7 d0

Machine flame than, edge distance is  = 1.5 d0

Diameter of Bolt Single Shear Double Shear
16 mm 29 kN 58 kN
20 mm 45.3 kN 90.6 kN

 Permissible stress value in Bending is maximum in Base Plate.

%Opening in roof truss wall Cpi (Internal Pressure Coefficient)
< 5% ± 0.2
5 – 20% ± 0.5
> 20% ± 0.7