Post on 14-Apr-2018
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Untuk contoh jika dari sebuah bawah jembatan diatasnya terdapat pembukaan yang dangkal ,
bentanganya lebih pendek dengan member tabung yang akan melebihi panjang bentang
pada berat pendeknya sebuah dermaga .ratio kedalaman bentangan juga berhubungan dengan
ketinggian Dari sebuah batasan . beton atau bangunan dermaga dngan beton baja campuran
atau RCCT-beam, dan bentuk lemping yang dapat digunakan untuk bentangan yang pendek.Untk lurus panjang atau tikungan bentangan lebih dalam pembukaannya,baja atau kotak jarring
beton menyediakan lebih halus dan bagian luar yang kelihatan simple.untuk kedalaman
lembah dan berakhir dengan sungai kecil anak sungai dan sungai yang membutuhkan
pembukaan yang besar untuk navigasi , besara bentangan dengan kabel tetap atau jenis kabel
gantung membuat suatu pilihan yang bagus. Ketika celah itu mengharuskan menyempit
medim atau bentangan yang pendek, satu baja atau lengkungan beton jenis dari struktur
kenaikan rendah akan menyediakan sebuah kesenangan dan kebebasan economical. Khusus
Beberapa saran untuk dasar balok penopang pada estesisyang diberikan oleh leonhard8
adalah
area ilustrasi
Superstructure
The Superstructure forms the dominant and dynamic member of the bridge carrying the load
directly and passing on to supports. The may deflect under loads. The soffit should preferably
be horizontal even under load or cambered up so as to provide a good view from below. For
single or multiple medium spans, flat graders with slight positive haunch of small depth at
supports are preferred. For longer bridges with multiple spans, haunched box continuous over
the supports can be adopted. The most important factor in design of superstructure is the
continuity of planes and lines and even colour. In case of unequal spans, there will be difficultyin carrying through the continuity in the soffit line. This difficulty can be overcome by varying
depths of interior girders but maintaining constant depth of fascia members. Abrupt changes in
lines at transitions can be avoided by tapering the fascia member at the piers and at abutment
ends.
Shallow segmental arches in concrete can also be used for shorter multiple span bridges
with pleasing appearance. Open spandrel concrete arches provide a good choice for medium
spans across creeks and deep valleys and even as multiple spans where there is no minimum
clearance restriction. They are common in span range 50 to 150 cm, but much longer spans
have been used. Wanxian Yangtze Bridge in china is the longest concrete arch in the world with
a single span of 425 and 45 m rise. Open spandrel steel arches can be used for long spans in
similar setting, with good rock for founding the abutments. Longest true steel arch bridge with
a span of 518 m is at evile, west Virginia, USA. Some of these shapes are shown in fig. 1.4 (a)
Tied arches, Otherwise known as bow string arches, in concrete have been popular
across streams for low heights (vide Fig.1.4.(b). hybrid tied arches with steel arches and
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concrete decks with slender prestressed steel cable suspenders are becoming popular also. A
recent example is a railway bridge on thee Qinghai-Tibet line, worlds highest railway which was
opened in july,20069. Railways in India have built perhaps one of the longest multi-span (26
numbers of 90 m spans) railway bridge with tied is across River Godavari near Rajahmundry.
Plate 1.12 shows a part of this bridge while under construction.
Pier
The function of piers and abutments is to transfer loads and forces from the
superstructure to the foundation and the design of the shaft, caps, bearings and pedestals ,if
any, should be expressive of this function. Short piers can be in masonry, where good stones
are available, or of mass concrete. Brick is masonry is rarely used in bridge building now as they
are labor intensive and call for more maintenance effort. Piers and medium height piers are
made of solid or hollow RRC section. RRC framers made of braced columns or bents are also
used for medium heights. Pier from can be made responsive to the material used specially ingrade separators, e.g., by textural treatment on concrete. Steel trestles have bee in
conjunction with steel girder superstructure for tall viaducts over deep valleys in hilly tracts for
railway bridges resulting in efficient structures with good aesthetic blending with surroundings.
They are quick to be erected also. Such trestles will have to be erected over concrete bases
taken above normal water level to avoid frequent wetting and drying, steel trestles for regular
scraping and painting to prevent corrosion.
Abutments and wing walls
Abutment have two function, viz., transmitting the loads and forces from thesuperstructure and containing the pressure exerted by the embankment behind. With the wing
walls, it provides the transition between the bridge and the embankment. The objective in their
design is to make positive and expressive composition out of the disparate elements.
Parapets and railings
Though the parapets are secondary members on bridges, they have dual function of
being a safety barrier for the road user and at the same time making a major visual impact on
the user and surroundings. The safety requirement calls for a heavy design while the visual
requirement calls for a lighter structure, with minimum visual obstruction. This will have to be
solved by designing them with directness, simplicity and quality of forms. Modern technological
developments in from steel crash barriers and railings have much to offer in this respect.
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1.6.4. Aesthetic Design Of bridges
Aesthetic design of any structure touches emotion and defies rational analysis. still home
broad guide lines are advantage to the designer. Leonhardt8
, who has designed a large number
of modern bridges, has done a study and formulated some guides lines illustrated with some
example in paper 8 developing Guidelines for Aesthetic Design. A few salient ones are covered
in this section. For more details, the reader is referred to the original paper. Issues involved are:
function of structure ; proportions ; order; refining the form; integration into environment;
surface texture; color; character; color; character; stimulation by variety and incorporating
nature.
Function
The structure should by its form, impose a feeling of confidence and stability to the user and
viewer about is performing is primary function. Incase of a bridge one should while looking at it,
feel confident that it will carry the anticipated load and forces safely down to the supporting
soil, be stable and provide safety to the user. The function and from will depend on the
materials used and their inherent qualities in respect of strength, ductility etc. brick or stone
masonry and timber forms different from those for steel or reinforced concrete. It should
provide adequate protection against vagaries of weather and limitation of deformation and
oscillation. It should be of good quality and be durable.
Good in Harmonious proportioning of the various elements in three-dimensional space is
important to achieve a structure of acceptable aesthetic value. Good proportions are required
in the total structure to convey an impression of balance between different parts of thestructure . such relationship on a bridge structure have to be considered between the spans
and supporting columns)piers; between spent length and depth of girder; and among height,
length and width of openings. The proportion of overhangs has to be decided based on their
influence on the light and shade effect on girder. It will be good to adopt same proportion in
the entire structure and maintain symmetry on the entire length. For example a golden mean in
proportioning adjacent openings is 2:3:2,e.g in at three span bridge the end span length should
be about two thirds that of middle span ( fig.1.3)
Order
Principle of order applies to lines and edges of a structure and placing of adjoining
opening. The directions of the lines and edges of structure should be limited. Similarly good
order should exist between adjacent objects, like proportions in adjacent units or even opening
by avoiding placing a stout rectangle or square object by side of a lean rectangle one. On the
order hand repetition of equally proportioned object can provide rhythm. In some cases,
uniformly varying proportions like increasing span lengths of a multi span bridge while
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providing an ascending gradient or increasing span length of an arch bridge tower the centre
with a hump in centre can give a pleasing elevation. Unnecessary accessories should avoid.
Refining the form
Refining of the basic form may become necessary, when structural form parallel straightlines horizontal and vertical may create an uncomfortable feeling of static and stiffness. The
soffit of girders on long spans can look as though they are sagging. Giving a pre-camber to the
soffit can remedy this situation. Continuous spans can be used with larger depth at ends with
the depth reducing toward the centre, by proving well proportioned haunches in soffit of girder
et ends. The soffit can be given a shallow convex curvature also to achieve more pleasing look.
Similarly parallel surface of tall piers on a viaduct will given an illusion of being wider on top at
bottom. This is remedied by providing a taper to the pier toward top, which is functionally and
structurally also more desirable. Skew angles on piers can also cause a disturbing of overlapping
in elevation. In such cases, it is desirable to study the light and shade effect on scale models andrefine the form.
Integration into environment
The need for integrating a structure into environment has already been emphasized. One
should avoid longspan bridges with deep beams over a lovely valley or over a river in a town
old row rise houses lining the bank. Alternative like provision of short slander arches or cable
supported structure or large tied arch with slender wire suspended decks is preferable. On
narrow deep valleys and gorges, open spandrel arch bridges provide an efficient and pleasing
solution.
Surface texture
Surface finishes can provided good appearances in certain cases , but they have to be chosen
so hat they not wheatear badly,, do not lose their color and can be maintained well offer the
life of the structure. On bridges, rough finishes are suitable for piers and abutments and
smooth surfaces for superstructure elements and slender columns. Stone rough finished can be
used in conjunction with stone masonry columns for small and medium span arches, with
pleasing effect. Concrete piers on interchanges and elevated roads are beneficially given
special texturing apart from giving different plan shape. Typical example can be seen in recentlycompleted flyovers in Delhi, Mumbai, Bangalore and Chennai. Figure 1.6 shows an example.
Views of piers on two major flyovers are shown in Annexure 21.1.
Color
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Color is play a major role in the overall aesthetic effect of the structure, especially in urban
areas. There should be harmonious combination of color schemes on various parts of the
structure. Urge to finish with sensational or aggressive effects should be avoided. Too dark
colors tend to give an opposite effect.
Character
Each major structure should have a unique effect on the people. The effect will depend
on the type of society and their tastes in the immediate surroundings. Translation of too much
individualism and egoism of the designer on to the stricture should be avoided.
Incorporating nature
Even while building a bridge, one can give more room to nature in its built environment.
This can be done by introducing green areas on and around a structure. It is learnt that this had
been done on the first autobahns in Germany. Singapore planner have given such an effect ontheir bridges on the expressways and elevated metro rail lines by growing of creepers and
letting them fall over the parapets.
Foot bridges
In the past decade, a large number of elevated pedestrian walkways and bridges have
built all over. Trend has been to go aesthetically pleasing and light structure. The light structure
are mostly cable supported either from light towers or elegant arches on inclined planes. While
going in for light structure, care has to be taken in the design on the vibration aspect. The panic
caused by excessive vibration by the millennium bridge in London and their redesign is anexample. Solid structure mostly use arch forms blending with surroundings
Detailed design
Loading standards
Loading standards for design for bridges are specified by various countries through
either their standardization organization or recognized professional bodies. They may very
considerably country to country, depending on the type of vehicles in use or proposed for use
in their country. In UK , BSI has been issuing loading standard which cover road bridges in termsof struck axle loads and axle spacing; and for railways in terms of locomotive axle loads and
spacing and trailing loads following. Britain also has drawing available for standard lengths of
bridge beams for RCC and PSC formulated and issued by their department of transportation.
Russians have their own common standard of loading for highway bridges and specific
enhanced loading standard for some areas like Moskow metropolitan area. Russians railways
have also developed standard plans/drawing for steel girder spans of common use.
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The European countries has different specifications for each country before the
formation on the European Union. In order to have a common basis for bids emanating from
different country for the same job, they intended to have a common set of calculation rules.
The Structural (Euro) codes have been drafted as an alternative set of codes and standards for
their use by individual member countries and ultimately these standards will replace thestandards of individual countries. Part 3 of Structural Code covers loads common for all
structures and part 4 covers live loads on bridges. The codes specifies that design of bridge
should be for withstanding the cumulative effect of loads on the pass through the structure
and forces arising out of natural actions for a return period of 100 years. With regard to traffic
loads, they constitute a completely new document, which gives the required elaboration as
scientifically as possible.
The wide variation in highway bridge loading adopted by different countries, are they
were some time back in different countries in the world, can be judged from the comparative
graph evolved by Thomas [1975] and reproduced in Fig.7.10. it will be seen, as at the time, up
to 100 meter spans, the German Codes specified the heaviest loads and the AASHTO the
lightest.
The developing countries, which were under different European powers during the
industrial revolution and after followed the practice of the respective masters and most of
them are continuing to do so even after independence, as a matter of convenience. The Middle
countries appear to have been adopting the AASHTO practice. The countries like Malaysia,
Myanmar and Sri Lanka fallow the British Standards. It will therefore be necessary for those
who want to take up design jobs in these countries to be conversant with at least Euro Codesfor structures, BS 5400 and AASHTO and ACI (American Concrete institute)
The concept of design has also undergone changes. Earlier practice was to use working
stress or allowable stress concept for design of bridge structures. Most countries now follow
Limit State design concept in design of bridge structures also. The load factors assumed may
vary from standard to standard. AASHTO specifies use of LRFFD (Load and Resistance Factor
Design ) for design of highway bridges.
1.7.2 Practice in India
In India, during the early days of structured bridge building, i.e.. during the British period, the
Public Works Departments of different providences appear to have followed loading standards
to suit expected truck loadings in their respective areas. Many of the presidencies like Madras,
Bombay and Bengal had their, Specification ( like MDSS, Bombay PWD/Manuals), which
covered not only material and construction specification, but also axle loads and their spacing
to be used for bridge designs on different types of roads. Major inter-state highway had to take
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into consideration movement of army vehicles including armored ones on their major roads.
Madras Presidency, which was the forerunner in forming a separate department for highway,
had formulated a Highways Manuals, which covered loading standards and design and
construction specification for roads and bridges. With the advent of the Indian Roads Congress (
since middle of last century ), the loading requirement for bridges on different types of roadshave been standardized. Different IRC codes exits for design and construction of bridges in
steel, concrete and for foundation including soil investigation Separate codes have been formed
for design and testing of Elastomeric bearings and expansion joints. These codes have been
drown up modeled on international codes, though some of the provision in them are
considered too conservative. IRC have also issued a detailed Standard Specifications for Roads
and Bridges, which have become mandatory for all National Highway works. In order to assists
the departments, IRC has also issued standard drawings for box culverts, RRC culverts slabs for
small spans; also beam and slab designs with RRC girders for 12 to 18 m spans with PSC girders
for some longer span for use on State and National Highway.
The Indian Railway in early days followed either provision in different parts of old BS 153
( codes for bridges ) or loadings to suit the axle loads of heaviest locomotives used or likely to
be used on respective individual railway systems. For example, the axle load adopted was 12
towns in 1875, which was progressively increased to 18 tons 1903 standard. It was increased by
25% in 1908. It was in 1926 that the concept of Heavy Mineral load HM (28i) axle load, Main
Line MBG (22.5t axle load) and Branch line BGBL ( 17 t axle load ) was introduced. They had also
issued similar standard for MGML, MGBL and NG tracks also. Tractive force be used in design
for longitudinal effect was also specified then.
Each railway company or unit had its own standard specifications for civil engineering
works and even type drawings for different spans with steel girders. Structure for important
bridges were designed by reputed consultants from UK. With more and more railway system
being taken over by the government, the Indian Railway Board constituted a Central Standard
Organization ( CSO ) for laying down common standard to be followed by all railway systems in
India. This organization later became the Railways Research Design and standards Organization
(RSDO). The former organization ( CSO ) had brought out in 1937 Bridge Rules, Which
specified the loading arrangements to be used in design of bridges for thee three different
gauges and separately for Branch, Main and Heavy Mineral Lines. They were based on the
steam locomotive wheel arrangements. This has since undergone a number of revisions to take
car of heavier load locomotive and change of traction as well as increased speeds. CSO also
brought out Codes of Practice and Standard Specifications for design and construction of
bridges and structures in steel and steel and concrete and for design and construction of
foundation and substructures for bridges and for road-cum-rail bridges. Later the RSDO issued
standard drawing for steel girder spans up to 61 meter and steel-composite girder bridges for
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shorter spans. They have issue also issued standard drawing for RCC/PSC bridges slabs and PSC
Box girders for standard spans. The available IRS parameters have been found to be deficient
for design of long span steel arch bridges in respect of additional parameters like fatigue,
global stability, seconds order column effects, composite action etc. while design two such
bridges on Udhampur-Srinagar-Baramulla rail link. Their provisions have had to besupplemented with the provisions in BS 5400, AASTHO and Euro Codes
9
Anyone familiar with these codes and specification for road and railway bridges can
easily understand the requirements of codes and standards of other countries, especially
developing countries. A list of codes and standards applicable for design of bridges for roads
and railways in India is given under References at end of Chapter 14.
1.7.3 Design Practice
The design of various components of bridges is now done is most countries almost invariably
with the use of the computers. Designers are going in for longer and longer spans and adopt
different forms and geometry in alignment. At one time engineers preferred to have straight
alignments in plan and avoided vertical curves on bridges decks. Now such limitation are
looked down upon from the point of view of aesthetics and economy in construction as well as
maintenance, though such requirement call for complicated calculation. Design have to be
competitive and during conceptual and design stage,, this calls for an iterative approach to
arrive at the optimal span, type and structural arrangements. Design by hand calculation for
such cases is very difficult and time consuming, if not impossible, Naturally, this calls for use of
computers and custom made programmers. A number off standard structural packages like
STAAD III/ STAAD Pro, GTSTRUDL and SAP 2000 are used along with power full tools like
ADINA,ANSYS, NASTRAN and ABAQUS for analysis. These are utilized for determining the most
probable response of bridge structure for arrange of applied loads. The output provides the
engineer the design date for evaluating the bridge structure. By trying alternatives, most
efficient configuration can be arrived at and most reliable design alternative arrived at. By
suitable linking with older relevant computer software, plans, specifications and estimate are
drawn up.
Firms in many countries have developed specific sub programmers with in-built graphic
capabilities bridge design. One such is bridge-design-specific programs like LUCAS available inthe market. The extent of development of purpose built software can be judges from the
approach used in China in recent times1
. computer technologies have been used in China for
structural analysis in bridge design since late 1970s and many special purpose programmers
have been developed. Synthetically Bridge Programmers provided the capability of
construction stage transferring, concrete creep and shrinkage analysis, priestess calculation
etc. the first version of BICADS consists of five subsystems including the Design
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Documentation, Pre-Processing of Bridge FEM, and the Preliminary Design of Box Girder
Bridges. A new generation BICADS ( Bridges Cad System )has been developed recently. Several
detailed design subsystems of other commonly used bridges can be included by employing a
good integrating and expanding mechanism in the main system.
But very few locally developed bridge design programmers have been made
commercially available in India. A number of academic institutions including the Indian
Institutes of Technologies have been developing design software as part of academic research
programmers. Railways have developed in house programmers for design of bridge girders and
design of wells. Recently, they have availed of the services of the Indian Institute of Technology
for developing interactive design software for design of steel girders and prestressed concrete
box girder. Similarly, the Indian Roads Congress has been sponsoring development of such
software for road bridge design. Apart from these, practicing engineers develop their own
simple spread sheet programmers for design of some bridge components. Though they are of
limited use on and can not be extended for design of innovative forms of bridge components,
they facilities the engineer to do trial and error calculation for arriving at more cost effective
solutions.
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In some case , it may be necessary to provide a change of gradient on the bridge itself calling
for provision of vertical curves. These provide an opportunity to the engineer to design
structure harmoniously blending with the road alignment and surroundings. the form, span
arrangement, depth of girders and height of the piers have to result in a pleasing structure to
the one who looks at it from below and also while passing over the same. For example, an archfrom chosen for a long bridge with level roadway will look better with equal spans, with the
same with a curved vertical alignment with summit at center can have spans of varied lengths,
increasing in lengths from end towards center. Same bridge with a rising gradient will look
better with span lengths progressively increasing from lower end. This principle can be followed
while using RRC or PSC girders over viaducts across shallow valleys also, but it will be desirable
to keep uniform depths of girders of fascia members so that there is no break in the soft line .
Choice of type of structure
The factors determining the type of structure are : basic geometry of roadway/railway ;dimensions of openings spanned; topography; requirements of clearance ( for navigation or
flood flow for passage of vehicles below ); sub-grade conditions; accessibility and working
conditions (including clear working season for construction) at site. The choice is finally dictated
by other major factors of availability of materials and technologies in the area/region where the
bridge is proposed, likely costs of construction and maintenance and any time limitations.
Visually the structure has to provide a pleasing appearance ,for which the designer has to look
at the forms, lines, planes and proportions of spaces formed by various components of the
bridge like superstructure, piers, abutments, wings and embankments. Ideally objective is
lightness and openness, with least encroachment on thee motorists visual and physical space.There should be easy flow of lines and forms and abrupt changes and transition should be
avoid.