Analysis and Design of Structures - short answer questions from AMIE exams (Winter 2021)

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Answer the following (4 marks each).

Explain various components of gantry girder through neat sketches.

The plate girder consists of the following components:
  •    The web plate
  •     Flange plate (addition of cover plate curtailed at proper sections)
  •     Stiffeners like bearing stiffeners, longitudinal stiffeners, and intermediate stiffeners
  •     Splices for flange plate and web plate
  •     End connections
Generally, gantry girders are supported on brackets connected to columns for lighter loads and are directly supported on columns for heavier loads. 
A suitable seat is fabricated to support and connect the bottom flange of the gantry girder. The gantry girder is connected to the column near the top flange to provide lateral support and torsional restraint by channel sections. 
 
Some of the typical sections are:

Advantages of HSFG bolts over bearing bolts.

The high strength bolts are also called high strength friction grip bolts. Briefly, these are called HSFG bolts. The high strength friction grip bolts have many advantages over rivets. 
  • Whereas the rivets are subjected to shear and bearing stresses, the bolts are subjected to uniform tensile stresses only. 
  • The high strength friction grip bolts have higher fatigue strength because there is no concentration of the stress in the hole. 
  • The bolts do not bear against the plates. Therefore, the uneven distribution of stress does not occur. 
  • These bolts are advantageously used in bridges and machine foundations subjected to vibrations. 
  • These bolts also simplify the problem of alterations and additions to structures as they can be assembled more easily than rivets.

Discuss wind load calculation for design of roof truss.

In order to determine the wind load on roofs and other inclined surfaces, the direction of the wind is assumed as horizontal. The roof surfaces are also inclined to the horizontal. The normal component and tangential component of wind on inclined roof surfaces may be found. It is to note that the tangential component of wind pressure is small and it is neglected. The normal component of wind pressure can be found from the empirical formula.

In order to find the wind load on individual structural elements/members (e.g., roofs and walls, and individual cladding units and their fittings, it is essential to consider the pressure difference between the opposite faces of such elements or units. For cladding, it is, therefore, necessary to know the internal pressure as well as the external pressure. The wind load, WL acting in a direction normal to the individual structural element or cladding unit may be determined from the following expression

{W_L} = ({C_{pe}} - {C_{pi}}).A.{p_d}

where,
Cpe = external pressure coefficient
Cpi = internal pressure coefficient
A = surface area of a structural element or cladding unit, and
pd = design wind pressure

Enlist advantages of bolted connections over riveted connections.

The advantages of bolted connections are as follows:
  • There is silence in preparing bolted connection. In riveting, hammering is done. The hammering causes noise in the riveting.
  • There is no risk of fire in bolted connection. The rivets are made red hot in riveting and there is a risk of fire.
  • The bolted connections may be done quickly in comparison to the riveting.
  • Though the cost of bolts is more than the cost of rivets, the bolted connections are economical to use than rivets because fewer persons are required for installation, and the work proceeds quickly.
  • The bolted connections facilitate the erection because of the ease with which these connections can be done.
The following are disadvantages of bolted connections:
  • If bolted connections become loose, their strength reduces considerably.
  • Tire unfinished bolts are not uniform in diameter and they have less strength.
  • The bolted connections have less strength when they are subjected to axial tension because the area at the root of the thread is less.
  • Generally, the diameter of the hole is kept 16 mm more than the nominal diameter of the black bolt. The bolt does not fill the hole and there remains a clearance in bolted connections.
Advantages of riveted joints
  • A riveted joint is more reliable than welded joints in applications which are subjected to vibrations and impact forces.
  • Riveted joints can be used for non-ferrous metals like aluminium alloy, copper, brass or even non-metal like plastic and asbestos.
  • Riveted joints are free from thermal after-effects because no heat required in this joint.
  • Quality inspection is easy in a riveted joint.
  • When the riveted joint is dismantled, the connected components are less damaged as compared to a welded joint.
Disadvantages of riveted joints
  • The material cost of a riveted joint is more.
  • The labour cost of riveted joints is also more than that of the welded joint.
  • Overall cost if the riveted joint is also high.
  • The riveted assembly has more weight than the welded assembly.
  • The riveting process creates more noise because of hammer blows.
  • Holes required to insert rivets cause stress concentration.
  • Production time is more for assembly.
  • Riveted assemblies are not tight and leak proof.
  • The projection of the riveted head adversely affects the appearance of the riveted structure.

State indeterminacy with example of a beam, truss and frame.

A Statically Indeterminate structure can be said if the equilibrium equation is insufficient for finding the structure's unknowns. These unknowns can be the Deformation, Support reaction, Shear force, bending moment, etc. For analysing such a structure, there is a need for some special equations called compatibility equations.

Statically indeterminate beams are the beams that require compatibility conditions along with available equilibrium equations for their analysis. Compatibility conditions of beams depend on the support conditions and type of loading etc. Compatibility conditions are based on the internal structure of the beams, like deformations and slopes of the beam. Fixed beams, propped cantilever beams, Continuous beams, etc., are examples of indeterminate beams.

Statically indeterminate frames/trusses are the frames that require some compatibility conditions for their analysis. Compatibility conditions of frames include horizontal deflection, the slope of supports, etc.

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