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Explanation: 

Underpinning:

• The process of constructing a new foundation below the existing foundation or the process of strengthening the existing foundation is called underpinning.

Shoring: 

• The process of construction of temporary structures to support unsafe structures temporarily. Shoring can be used when walls bulge out, or when walls crack due to unequal settlement of the foundation.

Types of shoring:

1. Racking Shoring 2.Flying Shoring 3. Dead Shoring

Scaffolding:

• It is a temporary structure to support a platform over which the workman can stand or sit to execute the construction work in case of height of the wall exceed 1.5 m.
• Ordinary Scaffolding consists of standards, ledgers, putlogs, guardrails, free boards, and braces.

Jacking: 

• Jacking is a term used to describe the application of a force with the aim of pushing or lifting an object.
• In the trenchless construction and rehabilitation industry, jacking is used to install subterranean pipes by pushing them through the soil.

Concept:

As shown in the above figure.

de = effective depth (Distance between C.G of comp. flange & tension flange)

D = overall depth

d = clear depth of web

Important Points

• In a plate girder webs of required depth and thickness are provided to keep flange plates at required distances, to resist shear in the beam.
• Flanges of required width and thickness are provided to resist bending moment acting on the beam by developing compressive force in one flange and tensile force on another flange

Concept:

Euler’s theory:

• This theory is valid only for long columns only.
• This theory is valid only when slenderness ratio is greater or equal to critical slenderness ratio.
• For any slenderness ratio above critical slenderness ratio, column fails by buckling and for any value of slenderness ratio less than this value, the column fails in crushing not in buckling.

Euler’s critical load formula is,

P = \(\frac{{{n^2}{\pi ^2}EI}}{{{L^2}}}\)

Euler’s formula is applicable when, Crushing stress ≥ Buckling stress

\({σ _{cr}} ≥ \frac{{{\pi ^2}E}}{{{λ _c^2}}}({λ _c};is;critical;slenderness;ratio)\)

\({λ _{min}^2} = {λ _c^2} ≥ \frac{{{\pi ^2}E}}{{{σ _{cr}}}}\)    

For mild steel,

E = 2 × 105 N/mm2

σcr = 330 N/mm2

∴ λ ≥ 80 N/mm2

∴ When slenderness ratio for mild steel column is less than 80, the Euler’s theory is not applicable. 

According to the given options, Option 3 will be correct. Euler's formula for column is valid for Large slenderness ratio.

Explanation: 

Working stress:

• Working stress is the stress which may be developed in the member without causing structural damage to it.
• Maximum stress induced in the structure should be less than the stress at which failure occurs. Stress that is capable of preventing failure is called working stress.
• \(Working \ Stress = \frac{Yield \ Stress}{Factor \ of \ Safety}\)

Channel Section as can be observed in the figure consists of one web and two flanges.

Explanation:

Surface structure: It is made from a material having a very small thickness compared to its other dimensions.

• Material for surface structure may be flexible taking any form of a tent or air-inflated structure or may be made up of rigid material such as reinforced concrete.
• These structures act like cables or arches since they support loads primarily in tension or compression, with very little bending.
• If the thickness of a structural member is small as compared to its length and width, it is classified as a surface structure or two dimension structure.
• Similarly, if the width and thickness of a structure is less than its length then it is called a one-dimensional structure

Curtailment is a way of reducing the area of tensile reinforcement at points/areas (either on a beam/slab) where bending moment is minimum or zero for the purpose of achieving an economic design.

Condition of curtailment - At least 50 % percent of   the total main reinforcement shall be carried through at the bottom on to the bearing and anchored in accordance

In simply supported slab let us find out the BM at 1/7^th of span or 0.15L from the face of the support. (Considering unit strip)

\({\rm{BM}} = \frac{{{\rm{wl}}}}{{{\rm{;}}2}}{\rm{;}} \times 0.15{\rm{l}} - {\rm{w}} \times 0.15{\rm{l}} \times \frac{{0.15{\rm{l}}}}{2}; = 0.0635 \times {\rm{w}} \times {{\rm{l}}^2}\)

Which is almost equal to 1/16 wl² or in other words it is 50 %of maximum. Moment at mid span (for udl) wl²/8

Thus we can curtail 50 % of bars or alternate bars depending upon moment requirement at app. 1/7^th   of the span from face of the support, subject to the provision of min. reinforcement.

Explanation:

Compression members:

The members subjected to axial compressive force in a direction parallel to the longitudinal axis are called compression members

Following are some examples of compression members

Strut:

• A strut is a slender bar or member in any position other than vertical, subjected to a compressive load and fixed rigidly or hinged or pined jointed at one end or both ends.

Column

• A column is a long vertical slender bar or member, subjected to an axial compressive load and fixed at both ends.

Principal strut:

• It is a top chord member of a roof truss or a truss girder subjected to heavy axial compression

A tie is a tension member

Explanation:

RCC T-beam:

(i) The beam consists of a flange and a rib in the form of a T, generally made of RC concrete or metal is known as T-beam.

(ii) The top part of the slab which acts along the beam to resist the compressive stress is called flange.

(iii) The part which lies below the slab and resists the shear stress is called the rib.

Dimension of T-beam:

• The effective width of the flange is adopted as the minimum of c/c distance of the nearby ribs or beams.
• The overall thickness of the slab crossing over the beam is taken as flange thickness.
• The breadth of the rib is taken down earth ground. It should be adequate to hold the steel zone in it, effectively. It might be taken as between 1/3 to 2/3 of the rib depth of the beam.
• The depth of T-beam is taken between 1/10 to 1/20 of the span.

Explanation:

The different types of sections are as follows: