India's Longest Bridge: The Dhola-Sadiya Bridge

the dhola sadiya bridge
The Dhola-Sadiya Bridge

  • On 26th May 2017, Indian Prime Minister Narendra Modi will be inaugurating India’s Longest Bridge, Dhola Sadiya Bridge 
  • The bridge over Brahmaputra River is 9.15 kilometre long and 12.9 metre wide connecting Dhola and Sadia Ghats. 



locaton of Dhola-Sadiya Bridge



  • The Dhola-Sadiya Bridge is 3.55 km longer than the Bandra Worli Sea Link in Mumbai, making it the longest bridge in India.

Interesting technical features of the project:

  • Length: 9.15 km
  • Width: 12.9 m
  • No. of spans: 183 (50 m each)
  • Project Design Speed is 100 kmph.
  • 1.7 m & 1.5 m diameter, 40 metre long bored cast in situ piles were used for foundation of main bridge and viaduct.
  • Concrete: M50 grade pre-stressed concrete
  • Precast segments were made using Hyper-plasticisers to achieve early strength.

Construction of the Bridge:


  • Construction began in 2011 under the umbrella of the Ministry of Road Transport and Highways in Public Private Partnership (PPP) with Navayuga Engineering Company Ltd, Hyderabad.
  • The bridge was planned to become operational from December 2015, but the construction works was delayed.
  • The project cost around 876 crore rupees and took around 4.5 years to complete

   

Benefits of the Bridge:

  • The bridge will be one of the crucial link in this Region which will not only give a relief to the existing transport facilities but will also invites various developmental schemes. Increased developmental activities will increase the vehicular movement.
  • It will promote tourism in the region.
  • This project  will serves important logistics for Military personnel for Indo-China border at Arunachal Pradesh.
  • Reduction in travel time.

Concrete operations

Mixing, Placing, Compacting, and Curing of Concrete


Mixing of Concrete:

The mixing of constituents materials for concrete should ensure that the mass becomes homogeneous, uniform in color and consistency. Hand mixing of concrete is not desirable, machine mixing should be adopted for better quality.


If there is segregation after unloading from the mixer, the concrete should be remixed


While using conventional tilting type drum mixture, the mixing time should be at least two minutes and the mixer should be operated at a speed recommended by the manufacturer (normal speeds are 15-20 revolutions per minute.





Transportation of Concrete:

Concrete can be transported from the mixer to the formwork by a variety of methods and equipment such as mortar pans, wheel barrows, belt conveyors, truck mixer mounted conveyer belts etc.
As there is  a risk of segregation during transportation, care should be taken to avoid it.


Placing of Concrete:

It is also important that the concrete is placed in the formwork properly to yield optimum results. Prior to placing, reinforcements must be checked for their location and size, splices and anchorage requirements and any loose rust must be removed.

The formwork must be cleaned, adequately supported, joints plugged and the inside of the formwork applied with mold releasing agents for easy stripping.

Concrete should be placed in thicker members in horizontal layers of uniform thickness (about The concrete should be deposited and 150 mm for reinforced concrete members). Each layer should be thoroughly consolidated before the next is placed.

Compacting of Concrete:


Right after placement, concrete contains about 20 % entrapped air. The concrete should be deposited and compacted before the initial setting of concrete and should not be disturbed subsequently.

Curing of Concrete:


All newly placed and finished concrete members should be cured and protected from drying and from extreme changes in temperature.
Wet curing should be started as soon as the final set occurs and should be continued for a minimum of 7-15 days.


Removal of Forms:

It is advantageous to leave forms in place as long as possible to continue the curing period.

The vertical supporting members of formwork (shoring) should not be removed until the concrete is strong enough to carry at least twice the stresses to which the concrete may be loaded at the instant of  removal of formwork.



Prestressed Concrete

What is Prestressing?

Prestressing is the application of an initial load on a structure, to enable it to counteract the stresses arising from subsequent loads during its service period.




  • Prestressing denotes a method in concrete construction in which the member is subjected to initial compression at the time of manufacturing itself. Thus, prestressing induces compression on the lower side of a beam and when the load is applied, tension so produced neutralize the compression already set up by prestressing.



Advantage of using Prestressed Members: 


1. In the case of RCC, the section below the neutral axis is ineffective (as all the tensile stresses are taken by steel). But in the case of prestressed concrete, the entire section is under compression and thereby increasing the load carrying capacity of the member.

2. Because of the entire section being in compression, the structure is crack free.

3. As the self-load is taken care of by additional eccentricity, the concrete required is about one-third the quantity required for corresponding RCC structure.

4. Thus because of reduced section and reduced weight, the cost of foundation etc. is less.

5. Most of the large span prestressed concrete structures are prefabricated under controlled conditions, the cost of shuttering and centering is considerably reduced.

Limitations in the use of Prestressed Structures:


The main limitation in the use of prestressed concrete is the comparatively expensive equipment and skilled operation.The method is, therefore, economical in large projects.

The grade of concrete used: M35 and above concrete.


Methods of Prestressing:

1. Pre-Tensioning:

In this method, the tendons are stretched to the desired amount. One end of the tendon is secured to an abutment, while the other end is stretched out with the help of prestressing jack. The formwork is then erected around the tensioned tendons and fresh concrete is poured, compacted and properly cured. At the end of the curing period, concrete is hard enough to sustain the compression passed through the bond. The projecting steel wires are then cut. The method is particularly adapted in precast beams, posts and simply supported slabs.




2. Post Tensioning: 

In this method, the structural member is cast, leaving ducts inside for subsequent insertion of prestressing tendons. The ducts are formed by placing CGI sheets inside the formwork or by keeping steel spirals or sheet metal tubes or any other suitable methods. When the concrete has been fully cured and hardened, the tendons are inserted inside the ducts. One end of the duct tendon is fixed while the other end is attached to prestressing jacks. When the required stretch has been given, the jack end of the tendon is also anchored and the duct is grouted with cement.


Advantages of Concrete

Concrete is used in nearly every type of construction. Traditionally, concrete has been primarily composed of cement, water, and aggregates.

Concrete is not a homogeneous material, and its strength and structural properties may vary greatly depending upon its ingredients and method of manufacture.

Steel reinforcements are often included to increase the tensile strength of concrete; such concrete is called reinforced cement oncrete (RCC) or simply reinforced concrete (RC).

As of 2009, about 25 billion cubic meters of concrete were produced each year.

Concrete is the 2nd most used material on the earth.

Read: 15 Amazing Facts about Concrete


Mixing, Placing, Compacting, and Curing of Concrete


Advantages of Concrete:


  • Concrete can be moulded to any shape
  • For manufacturing concrete, materials are easily available.
  • Concrete structure require low maintenance.
  • Concrete is Water and fire resistant
  • Concrete has good rigidity
  • Concrete has high compressive strength
  • Concrete is economical
  • Low-skilled labour required for handling concrete


Disadvantages of Concrete:


  • Low tensile strength (one-tenth of its compressive strength
  • Concrete requires forms and shoring of construction
  • Relatively low strength (the compressive strength of normal concrete is about 5–10% steel)
  • Time-dependent strength and volume changes with variation
  • CO2 emission in production of cement.

Principles of Surveying

Surveying


Definition of Surveying:

Surveying is the art of determining the relative positions of points on, above or beneath the surface of the earth by means of direct or indirect measurement of distance, direction and elevation.

  • Surveying also includes the art of establishing points by predetermined angular and linear measurements.
  • The object of surveying is to prepare plan or map so that it represent the area on a horizontal plane.
  • Surveying differs from leveling in following ways:
Leveling is a branch of surveying the object of which is:

1. To find the the elevation of points with respect to a given or assumed datum.
2. To establish points at a given or different elevations with respect to the datum.
  • Leveling deals with measurement in vertical plane.

  • In surveying, all measurement of lengths are horizontal or reduced to horizontal distances.


Division of Surveying:

Broadly, Surveying can be divided into two classes:

1. Plane surveying:

Plane surveying is that type of surveying in which the mean surface of the earth is considered as a plane and the spheroidal shape is neglected.

2. Geodetic Surveying: 

The type of surveying in which the spheroid shape of the earth is taken into consideration.

Principles of surveying:

The fundamental principles upon which the various method of surveying are based can be stated under following two aspects-

1. Location of a Point by measurement from two points of reference:

The relative position of the points to be surveyed should be located by measurement of at least two points of reference, the position of which have already been fixed.

2.  Working from whole to part:


The second ruling principle of surveying is to work from whole to part. It is very essential to establish first a system of control points and to fix them with higher precision. Minor control points can be established by less precise methods and details can be located using these minor control points.


The idea of working in this way is to prevent the accumulation of errors and to control and localise minor errors which, otherwise, expand to greater magnitude if the reverse process is followed.

Classification of Columns

  • A Column or strut is defined as a compression member whose effective length exceeds three times the least lateral dimension.

Read in details: What are Columns?

  • A structural element that is predominantly subjected to axial compressive forces is termed a compression member.

  • When a compression member is vertical, it is called a column, and when it is horizontal or inclined, it is called a strut.

Classification of Columns

The Classification of columns can be done on following Basis:

Classification of Columns based on Cross Section




1.Rectangular Columns
2.Square Columns
3.Circular Columns
4.Hexagonal Columns
5.T, L, or + shapes Columns


Classification of Columns based on Type of Reinforcement



1.Tied columns
    Columns reinforced with longitudinal reinforcement and lateral (transverse) ties. Tied columns are applicable to all cross-sectional Shapes.

2.Spiral columns:
    Columns with longitudinal reinforcement tied by continuous spiral reinforcement. Spiral reinforcement is used mainly in columns Of circular cross-section, though they can have hexagonal, octagonal, or even square shapes.

3.Composite columns
     Columns reinforced longitudinally with structural steel sections, such as hollow tubes and I-sections, with or without additional longitudinal reinforcement or transverse reinforcement.




Classification of Columns based on Types of Loading




Classification of columns
Cross section of column with different types of loading (a) Concentric axial loading (b) Loading with one axis eccentricity (c) Loading with biaxial eccentricities


1.Columns with concentrically applied loads: 
   Such columns with zero bending moments are rare. In multi-storey frames, interior columns will be subjected to axial compression and shear, under gravity loads.

2.Columns with uniaxial eccentricity—ex = 0, ey ≠ 0 or ex ≠ 0, ey = 0: 
    Edge columns such as B and D in Fig. 13.3 are subjected to uniaxial bending moments.

3.Columns with biaxial eccentricity—ex ≠ 0 and ey ≠ 0: Corner columns like C in multi-storey buildings are subjected to biaxial bending moments in addition to the compressive force. When subjected to lateral loads, most of the columns will be subjected to uniaxial or biaxial bending moments.




Classification of Columns based on Slenderness Ratio


What is Slenderness Ratio?
The Slenderness ratio of a member is defined as the ratio of the effective length and the radius of gyration of the section.

Columns, struts, beams, and ties are often slender members. 

Based on the slenderness factor, columns can be classified as follows:

1.Short columns: These types of columns generally fail after reaching the ultimate load carrying capacity of columns.

2.Slender columns: These types of columns generally fail suddenly at relatively low compressive loads due to buckling.

Read also: What are Columns?




Types of Stairs


  • Stairs are an important component of a building and often the only means of providing access between the various floors of a building. 
  • The staircase essentially consists of landings and flights. Often, the flight is an inclined slab consisting of risers and treads (collectively called the going of a staircase), whereas the landing is a horizontal slab.
  • From a structural point of view, a staircase consists of slab or beam elements.

Definition of Terms:


Components of a staircase (a) Plan of staircase (b) Terminology used (c) Part section

Tread or going of step: Tread is the horizontal upper portion of a step where the foot rests. Going to step is the horizontal distance of the tread minus the nosing.

Nosing: Sometimes, the tread is projected outwards for aesthetics or to provide more space; this projection is called the nosing. Many times, the nosing is provided by the finishing over the concrete tread.

Riser and rise: Rise is the vertical distance between two consecutive treads and riser is the vertical portion of the step.

Flight or going of stair: Flight is a series of steps provided between two landings. Going of a stair is the horizontal projection of the flight.

Landing: Landing is the horizontal slab provided between two flights. It is provided every 10–14 steps for comfort in climbing. Landing is also  provided when there is a change in the direction of the stairs.

Overlap: The amount by which the nosing of a tread (or landing) over sails the next lower tread (or landing) is called the overlap.

Waist: It is the least thickness of a stair slab.

Winder: The radiating or angular tapering step is called winder.

Soffit: It is the bottom surface of a stair slab.

Headroom: The vertical distance of a line connecting the nosings of all treads and the soffit is referred to as the headroom.
Steps may be of three types as follows:
(a)Brick or concrete steps on inclined slab
(b)Tread-riser steps
(c)Isolated steps

Type of steps (a) Steps on waist slab (b) Slab-less tread-riser (c) Isolated steps


Types of Stairs

There may be following types of stairs:

1. Straight flight stairs with or without intermediate landing.

2. Quarter-turn stairs

3. Half-turn stairs also referred to as dog-legged or scissor-type stairs

4. Branching stairs

5. Open-well stairs (half-turn) and quarter-turn landing

6. Spiral stairs

7. Helicoidal stairs


Some of the most common geometrical configurations are shown in Fig, which includes the following:



Plan views of various types of stairs (a) and (b) Straight flight stairs (c) Quarter-turn stairs
(d) Half-turn stairs (e) Branching stairs (f) Open-well (half-turn) stairs (g) Open-well stairs with
quarter-turn landing (h) Part-circular stairs (i) Spiral stairs (j) Helicoidal stairs

Spiral, helical, circular, and elliptical stairs are classified under Geometrical stairs


Structural Classification of Stairs:


For design purposes, stairs are classified into the following two types, depending on the predominant direction in which the slab of the stair deflects in flexure.

1.Transversely supported (transverse to the direction of movement in the stair)

2. Longitudinally supported (in the direction of movement)

Selection of Stairs

The type of stairs and its location are selected based on architectural considerations, such as accessibility, function, comfort, lighting, ventilation, and aesthetics, as well as structural and economic considerations.


Principles to be observed while Planning and Designing a Stair

Width of the stair should not be less than 1.00 m.
Length of flight: The number of steps in a single fight should not be more than 12.
Pitch of the Stair should be 25 to 40
Width of landing should be 150 mm more than width of stair.
Hand rails should be 750 to 850 mm. in height from the top of respective step.