Construction Techniques and practices

1. CONSTRUCTION TECHNIQUES,
EQUIPMENT AND PRACTICE
UNIT – IV
SUPER STRUCTURE CONSTRUCTION

2. SYNOPSIS:
Launching girders
Bridge decks
Offshore platforms
Special forms for shells
 Braced domes and
 Space decks
High Rise Structures
Material handling
Transmission Tower
Sky Scrapper
Cooling Tower
Erecting light weight
components on tall
structures
Support structure for heavy
equipments and conveyors
Erection of articulated
structures

3. SITE ERECTION METHODS
Slide Slowing Method
A method of construction of the super structure is to erect girders,
whether steel or precast concrete girders, over temporary supports by
the side of the pier, opposite to the span and when ready, slewing
same into position.
Move the unit, full or part of the deck, if any, is added after the basic
girder structure with adequate bracings is slewed in
This method is adopted when the erection and casting of the
girders is being done simultaneously with the construction of the
piers as in new construction, in order to avoid delay later
Where pier height is too much it is adopted and in existing bridges
case, this method is adopted to replace the existing structure with
stronger unit

4. Common for both railway and road bridges but one difference is
deck, except for linking of rails in case of railway bridges and
weathering course & parapet in case of road bridges
For new bridges the staging for casting will have to be put up by the
side of the piers on one side
In case of replacement, similar staging will have to be erected on the
other side also for receiving the old girders and dismantling them into
parts before being taken away to stores
In both cases, some temporary arrangements over the piers or
adjacent to them in the alignment for the purpose of slewing in will
also be necessary

5. LAUNCHING METHODS
End Launching
This method is adopted mainly for long span prestressed concrete and
steel girders and mostly on new construction
The girder is cast or built up on the approach bank, and it is
longitudinally traversed over the opening it has to span and lowered in
position
For this purpose, a small temporary intermediate staging has to be
provided in the gap between piers for taking the girder across the gap
For steel girder bridges , staging are arrange them one behind the
other, linked with temporary links and launch them together

6. Launching Girders
Fully cast prestressed concrete girders are not launched independently
as the cantilevering stress developed is considerable and the design
difficult
The method adopted is to first launch a steel or aluminium supporting
frame or girder so that it spans over the gap
Once the launching of this girder over the gap on one side is over, the
first main girder is moved over this temporary girder or frame, supported
at intervals or pulled across
When the full length of the main girder has come over the span, it is
jacked up and temporarily held in position and the launching girder can
be side – slewed to take the position of the next girder over the span.

7. The main girder launched earlier is then lowered into position with
the helps of jacks
The next girder cast and ready on the approach is then launched
longitudinally over the temporary girder again and the process
repeated till all the permanent girders are put in position
The launching girder can then be moved over to the next span
and it can take the position required for taking on the first girder of
the next span
The girders of the next span can then be longitudinally taken over
the girders erected in the previous span and then over the
temporary girder on the next span and the process repeated

8. Erection of Concrete Girders with Cranes/ Derrick
If the bed is dry, the girders can be cast on the bed by the side of the span
and they can be erected by using two mobile cranes one on either and with
the help of a suitable derrick in the centre or one derrick each on either end
If the height of the pier is not much and girders are too heavy to be handled
by the available crane or derrick, the girder can be jacked up from either
end on temporary trestles to pier top level and then side – slewed in position
launched and slewed has the minimum weight
The full deck slab can be cast subsequently
In the case of prestressed concrete girders there will be transverse
prestressing also involved

9. The diaphragm with necessary ducting should be cast after all the
girders are launched correctly and adjusted in position
Part prestressing is done before individual girders are lifted or
launched and remaining cables are tensioned, some before and
balance after or all after the deck is cast according to the design
This stage prestressing is done to take the maximum advantage
of prestressing
In doing such post – tensioning, extreme care has to be exercised
in following the sequence that has been given by the designers
and any small change can cause a crack in the system or
unwanted lateral deflection in the individual girder

10. Cantilevering Method
For very large spans, the method which is now used is the
cantilevering method
In this method, the erection starts from the abutment end and the
erection of the members ahead is done by using a crane which
travels on the top boom of the previously erected part of the structure
in case of steel and by using the support on the previously erected
part structure in case of prestressed concrete construction
Types : Howrah Bridge – Steel cantilevers
Sydney Harbour Bridge – Steel arch
Second Godavari Bridge – Steel simply supported

12. BRIDGE DECKS
The grade line of highway or railway track and the clearance
required under the bridge decide the use of deck type or through
type bridge
Incase the sufficient clearance is unavailable under the bridge,
the deck type bridges are the advantages over the through type
bridges
The deck type bridges are relative economical
The height of piers and abutments are reduced are in the deck
type bridges

14. SPACE DECKS
Of recent years a range on standard section mild steel tubes has been
manufactured in increasing quantity .
The advantages of a tubular section are that it has a uniform cross
section and its position does not have to be adjusted for maximum
strength as does an angle section and the surface area of a tubular
section is considerably less than standard tubular sections welded
together.
Erection of Space decks
The standard units of this type of roof consists of pyramid frames, the
tray and base of each unit consisting of light steel angles welded
together to which tubular steel diagonals are welded.

15. The diagonals are welded to the socketed couples at the apex of
each pyramid
The units are joined by bolting the angles of adjacent trays
together and threading tie bars between couplers of adjacent
units. By adjusting the tie bars the top of the deck may be flat
The finished deck is extremely rigid and can be provided for
comparatively long spans with few internal columns
This type of roof is more expensive than a pitched lattice steel
roof structure
The deck is connected to internal and external I section or hollow
rectangular section columns. The cheapest and most commonly
used form of coverings consists of wood wool laid on and dipped
to the deck and covered with felt roofing.

17. SPECIAL FORMS FOR SHELL
A structural shell is covered surface structure. It is generally
capable of transmitting loads in more than 2 directions to
support
These structures are highly efficient structurally when they are
so shaped, proportioned and supported that they transmit the
loads without bending or twisting
A shell is defined by its middle surface halfway between its
outer surface and inner surface
Depending upon the geometry of the middle surface, shell may
be classified as Dome, Barrel arch, Cone, Hyperbolic
paraboloid

18. A thin shell has relatively small thickness compared to other
dimensions
It should not be so thin that the deformation would be large
compared with the thickness
The shell shearing stresses normal to the middle surface should
be negligible
Thin shells usually are designed so that normal shears, bonding
moments and torsions are very small except for relatively small
portions.

19. DOMES
Ribbed domes are the earliest type of braced domes .
A ribbed dome consists of a number identical meridional solid girders or
trusses, interconnected at the crown by a compression ring.
The rings are also connected by concentric rings to form grids in a
trapezium shape
It is stiffened by a steel or reinforced concrete tension ring at its base.
SCHWEDLER DOMES
A schwedler dome also consists of meridional ribs connected together to
a number of horizontal polygonal rings to stiffen the resulting structure.
So that it will be able to take unsymmetrical loads

20. Each trapezium formed by intersecting meriodional ribs with horizontal
rings is subdivided into two triangles by a diagonal member
Sometimes the trapezium may also be subdivided by two cross – diagonal
members
This type of dome was introduced by a German engineer J.W.Schwelder in
1863
The great popularity of schwedler domes is due to the fact that, on the
assumption of pin – connected joints, the structure can be analyzed as
statically determinate
In practice, in addition to axial forces, all the members are also under the
action of bending and torsional moments
Many attempts have been made in the past to simplify their analysis, but
precise applied to find the actual stress distribution

21. HIGH RISE STRUCTURES
TRANSMISSION TOWER
Transmission towers are tall structures with relatively small cross
section and with a large ratio between the height and the maximum.
Towers are also called as masts or pylon.
Tower is a single cantilever freely standing , self-supporting
structures fixed at its base.
Transmission towers structures that are pin – connected to its
foundation and braced with guys or other elements. Similar examples
are water towers, radio, television towers, radio relay system etc.,
These towers of power transmission lines are used to support
transmission cables carry transmitting voltages exceeding 1,32,000
volts

22. Over long distance, since these cables carry heavy voltage, they
provide necessary clearance, where the transmission cables have
maximum sag
On general the towers may be built up with three or more legs. But
generally all towers are built up with four legs and spared suitably.
This is because four leg gives sufficient stability to the structures.
There are different types of them:
Self supporting type
Flexible type
Semi-flexible
Line tower
Angle tower
Self supporting wide base
Guyed type

23. Self – supporting type towers are generally rigid in both the
transverse and longitudinal directions
Flexible type towers are not rigid in the longitudinal direction ie., in
the direction along the transmission cables
Line towers, when there is a straight portion of the power line, line
towers are preferred
Angle towers, when the direction of the power line is changed angle
towers are provided
The height of these towers should be within 20 – 40m
The towers of such height provide 6 – 10m clearance from the
ground surface to the point of maximum sag of the cables
These towers are also known as single, double or multiple circuit
towers depending upon the no of circuits supported by the towers

24. Single diagonal bracing tower have long free length and double diagonal
bracing or cross diagonal tower
Radio and television towers are self – supporting towers. For there
towers rigid diaphragms are provided at the top and at several
intermediate sections to the increase the stiffness of cross sections.
In masts they are guyed with wire ropes at one or more levels
The vertical loads acting on towers are the dead load and live load
The vertical loads on the power transmission line towers include self
weight of towers insulators, fittings, ice coating, line man with tools
The self weight of towers is found by a general existing Ryle’s empirical
formula (For suspension , dead load towers)

26. SKYSCRAPERS
Skyscraper (high rise) is a tall habitable building.
Emporis data committee defines skyscraper as a building which is
35 meters or greater in height , and is divided at regular intervals
into occupiable levels.
Some structural engineers define a skyscraper as any vertical
construction for which wind is a more significant load factor than
weight.
The first skyscrapers is considered the Home Insurance Building in
Chicago
Ten storey structure was constructed in 1884 – 1885. Another
contender for the title is the 1892 ten – storey Wainwright
Building by Louis Sullivan, which still stands in St. Louis, Missouri.

27. The crucial developments for skyscrapers were steel, reinforced
concrete, water pumps, elevators.
Until the 19th century, buildings of over six stories were rare. It was
impractical to have people walk up so many flights of stairs, and water
pressure could only provide running water to about 50 feet (15m)
The weight-bearing components of skyscrapers differ substantially
from those of other buildings.
Their walls can support buildings up to about four stories, while
skyscrapers are larger buildings that must be supported by a skeletal
frame
The walls than hang off this frame like curtains hence the architectural
term curtain wall for tall systems of glass that are laterally supported by
these skeletal frames.

28. Rank Building, City Year Storyes Height in
Meters Feet
1 Burj Khailfa
2 Freedom Towers, New York 2005 121 541 1776
3 Taipei 101, Taipei, Taiwan 2004 101 509 1671
4 Petronas twin tower, Kuala lumpur,
Malaysia
1998 88 452 1483
5 Sears tower, Chicago 1974 110 442 1451
6 Jin Mao Buildings, Shanghai 1999 88 421 1380
7 Two International Finance Centre, Hong
Kong
2003 88 415 1362
8 CITIC Plaza, Guangzhou, China 1996 80 391 1283
9 Shun Hing Square, Shenzhen, China 1996 69 384 1260
10 Empire State Building, New York 1931 102 381 1250
Worlds 10 Tallest Buildings

30. COOLING TOWER
Cooling towers are widely used for cooling large quantities of water in
thermal power station , refineries , atomic power plants , steel plants , air
conditioning and other industrial plant.
A cooling towers incorporates a draft tower distributing and spraying
devices and a cold water basin.
A typical section of hyperbolic cooling tower of height 100m
The draft is induced naturally or mechanically.
Hot water is pumped to a certain height and then distributed through a
piping system of nozzles, where it splashes over a system called “Filling or
Stacking”
The filling may be of wood or transite material placed in several layers

31. The water splashes drips and flows through the layer of filling
The air which is forced upward either mechanically or by atmospheric
pressure difference rises through the driplets or films of the hot water and
consequently cools the water through evaporation and convection
The cooled water is collected in a pond at the bottom of the towers and then
recirculated for industrial use
Hyperboloid of revolution
The hyperboloid of revolution of one sheet is generally used for cooling
towers of thermal station
The great advantage of this type of shell is that it is generated by two
families of intersecting straight lines and the form work can be achieved by
straight boards warped only slightly over the lengths
The intersecting grid of straight lines forms rhombuses of intersection

32. The shell surface can also be built of pre-cast rhombic elements which are
repeated along the complete circumferences at fixed heights
The generation of the hyperboloid of revolution is by intersecting straight
lines

33. MATERIAL HANDLING
Material handling is the movement, storage, control & protection of
materials, goods & products throughout the process of manufacturing,
distribution consumption and disposal.
The focus is on the methods, mechanical equipment systems and
related controls used to achieve these functions
Material handling management consists use of a proper technique for
moving, transporting, storing or distributing materials with or without
the help of mechanical appliances.
Because of various complexities in the construction process,
management must be fully acquainted with various material handling
devices so that unit cost of construction can be down

34. All material handling activities must be simplified and must be fully
safe preventing any accidents
Also material movement planning must be economical is terms of time
and labour
Material handling objective:
The right amount
The right material
At the right time
In the right position
In the right sequence
For the right cost

35. The handling of material depends on three items. They identified
as follows
Identification of source of building material
Identification of quantity and quality of material
Identification of cost of building material
Principles
Orientation principle
Planning principle
System principle
Unit load principle
Space utilization principle

36. Standardization principle
Ergonomic principle
Energy principle
Ecology principle
Mechanization principle
Flexibility principle
Simplification principle
Gravity principle
Safety principle
Computerization principle
System flow principle
Layout principle
Cost principle
Maintenance principle
Obsolescence principle

37. MATERIAL MANAGEMENT
Material management is concerned with ensuring that the quantity
and quality required are on the job as per requirement
The construction manager is concerned about the nature of
material handling problems like quantity , quality , price , delivery
date , mode of transportation , inspection , counting , cost , storage
and protection.
Considerable time and effort is spent on a practical network that will
satisfy and resource limitations
However, such a schedule is not used unless supported by the
timely delivery of materials

38. The material for delivery to the site can be broadly classified as
follows,
Bulk items
Standard items
Fabricated items
Bulk items - like earth , bricks , cement , sand , stone chips , steel
sheet and asphalt that are obtained in a semi-processed state
Standard items - all shelf items that can be procured from the market
like pipes and pipe fittings.
Fabricated items – are the items that are specially built as per
specification and sizes for the project. Built as per specification like
roof truss, doors, windows, electrical panel etc.

39. ERECTING LIGHT WEIGHT COMPONENTS ON TALL
STRUCTURE
Tall buildings are generally multi storey structures where greater
part of construction of beams and stancheons.
Erection Procedure for Multi Storey Building
The order of erection follows pattern of columns, girder and beams
or columns, trusses and purlins. Buildings are generally erected by
cranes and beyond the reach are erected guy derricks.
The step wise operation is as follows
Guy derrick mast is assembled on ground with its base in
approximate required location. The mast is tipped up vertically and
guys are anchored to column bases. The boom is inserted and
topping lift and load lines are served. The derrick is ready to
operate. The first – tier steel is erected.

40. The bottom is removed from boom seat by picking with topping
lift falls, revolved 18degree and placed in a temporary jumping
shoe. The top of boom is guyed off with temporary guys.
The load falls are attached to mast above its center of gravity,
the mast guys are moved to top of next tier and mast is raised
to its new position. The mast guys are adjusted and load falls
unhooked.
The temporary guys of boom are removed and the topping lift
falls are used to raise the boom and place it in the boom seat.
The derrick is now ready to operate and the next tier of steel is
erected.

41. Safety Rules
See that the equipment is not over loaded. Be certain of load to be
lifted
All bolts and splice material on lattice derricks and crane sections
have been inserted
Do not open the legs of brother chains to too large an angle
The rings of chain slings is trig enough for crane hook
Avoid sudden shocks when lifting

42. SUPPORT STRUCTURE FOR HEAVY EQUIPMENT
Column Base
Foundation is necessary for a column to distribute the column load
on sufficient area of the soil so that the bearing capacity of the soil
is not exceeded, it is also equally important that the column load be
applied on sufficient area of the concrete foundation so that bearing
strength of the concrete is not exceeded
A steel base plate is therefore used to spread the column loaf on
sufficient area of the concrete foundation
Base plate used may be of following types
Slab bases
Gusseted bases

45. GUSSETED BASES

46. Slab bases : In this case the column stands directly on the base plate the
bearing end of the column is machined so that the column load is transferred
to the slab base by bearing
Gusseted base: Gusseted base plates are used in columns carrying heavy
loads.
In this case fastenings are used to connect the base plate and the column in
the form of gusset plate, angles etc.,
In case the end of the column is sufficiently machined so as to provide full
bearing on the base plate, it is usual to assume that half the column load is
liable to be transferred to the base plate through the fastenings and the
balance load is transferred to the base plate by direct bearing.
Suppose the ends of the column and gusset plates are not exactly faced for
full or complete bearing. Then it is usual to design the fastenings to transmit
all the forces to which the base is subjected.

47. Grillage foundation: It is provided for a column carrying heavy load when
it has to transfer its load to a soil of low bearing strength
This foundation consists of two or more layers or tiers of steel beams, the
layers being provide one above the other at right angles at each other .
The beams are completely encased in well compared concrete.
Generally only two tiers of beams are used
The column rests on a base plate through which the load is transmitted
to the upper tiers beams
These beams in turn transfer the loads to the lower tier beams. From the
lower tier beams the load will be transmitted to the soil.

48. GRILLAGE FOUNDATION

51. ERECTION OF ARTICULATE STRUCTURE
Bow String Girder Type Bridges
The bow string girder type bridges derives its name from its shape. The
arch rib and the tie respectively resemble bow and string.
The flooring of bridge rests on ties and the load is transmitted to the arch
rib through suspenders
Suitable bracing may also be provided in case of steel bow string girders
The bow string girder type bridge removes the following two
disadvantages of the arch bridges
The horizontal thrust is resisted by ties, hence the reactions at supports
are vertical and not inclined as in case of the arch bridges. As a result of
vertical reactions the supports require lighter sections

52. The bow – string girders project above the formation level of road or
railway line. Hence, the question of accommodating the rise between the
level of approaches and the springing level of arch of arch close not arise.
The bow string girder type bridges are therefore very much suitable for
multiple spans and at places where the available clearance is restricted.
Deck Type and Through Types Truss Bridges
In the deck type truss bridges, the floor system rests on the top chord.
In the through type truss bridges, the floor system rests on the bottom
chord. Through type warren truss bridge with verticals.
Although the parallel chord trusses are used for the through type truss
bridges, but the lines of various numbers of parallel chord truss bridges
are more in harmony with deck type truss bridges.

53. Component Parts of Truss Bridge
The various component parts consists of
The main vertical trusses
The floor system
The bottom lateral bracing
The top lateral bracing
The portal bracing
The sway bracing

54. Cable Stayed Bridge
Cable – stayed bridges may look similar to suspensions bridges both have
roadways that hang from cables and both have towers.
But the two bridges support the load of the roadway in very different
ways. The difference lies in how the cables are connected to the towers.
In suspension bridges, the cables ride freely across the towers,
transmitting the load to the anchorages at either end.
In cable-stayed bridges, the cables are attached to the towers, which
alone bear the load.
The cables can be attached to the roadway in a variety of ways. In radial
pattern, cables extend from several points on the road to a single point at
the top of the tower.
In a parallel pattern, cables are attached at different heights along the
tower, running parallel to one other.

55. Construction Sequence of Cable Stayed Bridge
The cable stayed bridges are similar to the suspension bridge except
that there are no suspenders in the cable stayed bridges and the cables
are directly stretched from towers to connect with the decking.
Thus no special internal anchorage is required for the cables as in case
of suspension bridges because the anchorage at one end is done in the
girder and at the other on top of tower.
Each because in the girder introduces horizontal and vertical forces.
The cables can be arranged with two plane system or one plane system.
The two plane system requires additional width to accommodate the
towers and deck anchorages.
But in case of one plane system the anchorage at deck level can be
accommodated in the traffic median and it results in the least value of
total width of the deck.

56. In principle the cable stayed bridge essentially of the following three
elements
Bridge deck
Pylons or tower
Stay cables
The stay cables are the principal structural elements because they play
an important role in the design, stability and performance of the
structure as a whole.
The different types of stay cables are
locked coil wire ropes or stranded ropes
long lay spiral strand cables
parallel or semi parallel wire cables

57. The deck in case of the cable – stayed bridges is supported by a number of
cables meeting in a bunch at the tower, known as the harp form.
The use of multiple cables facilities smaller distances between points of
supports for the deck girders and it results in the reduced structural depth.
The cable stayed decks are less prone to the wind induced oscillations
than the suspension bridges because of the damping effect of the inclined
cables.
The brick deck may be in the form of steel prone or RCC or prestressed
concrete girders.
The concrete girders possess the following advantages
As their damping effect is very high, the vibration effects are also small
They possess much higher stiffness and hence, they exhibit comparatively
less deflection

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