Steel Structures Interview Questions and Answers

IS 800-2007: General construction in steel – code of practice.

Reversal of stresses due to repeated loading is called fatigue. It is important in case of bridges, crane girders and platform carrying vibrating machines etc.

IS 800-2007 specifies no limit on minimum thickness requirement of steel sections but however a minimum thickness of 6 mm for main members and 5 mm for secondary members must be used in steel design and construction. The minimum thickness is required for better performance under adverse environmental conditions. In addition to that, steel in contact with water and soil and those subjected to alternative wetting and drying, an additional thickness of 1.5 mm should be provided.

In the limit state design of structures, all conditions are taken into account that makes the structure unfit for use, means it considers all the relevant conditions pertaining to limit states of strength and serviceability.

The strength limit states are based on load carrying capacity of the structure wgich includes buckling. Fatigue, fracture etc. Serviceability limit states are based on performance of the structure under the actual application of service loads and includes deflection, vibration, corrosion, Ponding etc.

Advantages of limit state design are:

1. This method recognizes that design parameters are variants.
2. This method logically deals with the fact that there exists always a possibility of variations in loads and material properties.
3. This method gives different weightages to different materials and loads.

The design load in Limit state method (LSM) of design is obtained by multiplying the working load (called as characteristic load) with a partial factor of safety and the resulting load is called as factored load or the design load. Different loads (i.e. live load, earthquake load, dead load, wind load etc.) are then combined under the most severe but realistic conditions.

The design strength of the material is obtained by dividing the characteristic strength of the material by a suitable partial factor of safety. The design strength of each material must be such that the most severe combination of design loads must not cause failure and it must also be ensured that the structure is stable against overturning, sway etc.

Factor of safety is defined combindly for the material properties and uncertainties in loads while partial safety factors are defined separately.

Factor of safety is derived from experience only and underestimated the properties of materials and forces but partial safety factors are derived using probabilistic approach for most unfavourable conditions for material properties and forces.

Riveting is a method of joining together pieces of metal by inserting ductile metal pins called rivets into holes of pieces to be connected and forming a head at the end of the rivet to prevent each metal piece from coming out.

Following are the disadvantages:

1. Removing poorly installed rivets is costly
2. Inspection of connection is a skilled work.
3. It is associated with high level of noise pollution.
4. Labour cost is high.

Advantages of bolted connections over rigid connections are following:

1. Bolted connections facilitate faster erection of structure.
2. Semiskilled labours can easily do the bolting connections work whereas riveting connection required skilled labours.
3. Bolted connection is more economical than riveted connection because cost skilled labour is reduced as well as equipment costs are also very less.

Bolts are classes as grade x.y

Where x represents 1/100^th of the nominal tensile strength and y represents the ratio of yield stress to ultimate stress.

For example – a bolt of grade 4.6 implies the ultimate strength of the bolt is 400 N/mm² and yield strength is 0.6 x 400 = 240 N/mm²

On the basis of load transfer in the connection, bolts may be classified as:

a. Bearing type

b. Friction grip type

Unfinished (black) bolts and finished bolts are bearing type since they transfer shear force from one member to other member by bearing, whereas HSFG bolts belong to friction grip type since they transfer shear by friction.

Pitch of the bolt – it is the centre to centre spacing of bolts in a row, measured in the direction of load.

Gauge – it is the distance between the two consecutive bolts of adjacent rows and is measured at right angle to the direction of load.

A. Lap Connections

in lap connection the connected members to be overlap each other. the load in a lap connection has an eccentricity and thus causes bending on the riveted connection.

B. Butt Connections

In butt connection, the connected members are placed end to end, cover plates are provided one or both sides of main plate. in double cover connection, no eccentricity of load, Hence double cover butt connection is free from bending effect.