Heggade Board of Management, Gammon India Limited ABSTRACT: In case of bridge building by precast segmental construction method, the precastelements that are cast in the casting yard thr
Trang 57Multi-Span Large Bridges – Pacheco & Magalhães (Eds.)
© 2015 Taylor & Francis Group, London, ISBN 978-1-138-02757-2
Segmental precast technology for multi-span bridges
(production, transportation and launching)
V.N Heggade
Board of Management, Gammon India Limited
ABSTRACT: In case of bridge building by precast segmental construction method, the precastelements that are cast in the casting yard through specially designed molds are transported to thelocation and post tensioned together The small segments are made transversely which is in contrast
to precast girder bridges where the overall lengths of the precast girders are almost same as that ofspans
Generally the joints which are either glued by specially designed epoxy formulation or in somecases dry are orthogonal to the longitudinal axis of the bridge Normally the reinforcement isnot continuous through joints unless it is a must for from structural reasons The decisions onthe permanent structure designs are dictated by shape of segments, weight of precast elements,arrangement of pre-stressing cables and methods of casting, transportation and erection
Generally there are two principle methods of construction which are span by span constructionand cantilever method of construction For both these method of construction the precast segmentshave to be realized in casting yard either by long line casting method or short line method ofcasting In span by span construction the erection or launching of the segments can either berealized by under slung erection method or overhead launching system Balanced and even freecantilever method of construction can be realized both by form travelers or overhead launchingsystem, while the progressive cantilever construction a variation of free cantilever constructionmay require temporary towers
In the detailed paper below, the above nuances and subtleties of pre casting, transportation anderection methods are discussed by way of illustrations and evolution of precast segmental bridgeengineering in India
Long multi-span bridges have always been designed taking into account certain aspects such asgeographical location, available materials, known and practical construction methods imbibingdurability, speedy delivery and economic and aesthetic considerations The design approach tolong bridges consisting of multi-span has an inherent distinction from short bridges involving lessnumber of spans as the repetitive production characteristic in multi-span bridge provides with anopportunity of mass industrial production In addition, concrete by its very nature renders to bemoulded to produce exciting shapes.it is a versatile material that can reproduce the texture fromsteel forms This quality of the material can be put to use in obtaining exciting shapes and texturesthat enhance the appearance of the built environment It is the material most suited to express theideas of form, which is true to its function, both efficiently and economically
Precast segmental construction grew out of mass production concept to prefabricate off site asmuch of the bridge deck as possible which has many advantages that include:
– Enhanced quality control of casting in factory conditions
– Ability to adjust the rate of casting to suit the rate of erection by increasing the number of mouldsthereby flexibility to control the construction programme
– Reduction of disruption to the existing users of the site
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Trang 58Figure 1 First pre stressed concrete bridges in
India, Assam railway link bridges (1948).
Figure 2 First cast-in-situ cantilever construction
in India, Barak bridge at Silchar (1961).
Figure 3 Top: span-by-span construction method Bottom: cantilever method of construction.
Source: Precast segmental bridges, Guide to good practice for structural and site engineers.
Prepared by fib Task Group 10.3.
– The flexibility to run the casting of the deck in parallel with the construction of foundations andsub-structures
– Overall reduction in construction programme due to possible parallel activities and hence costsavings
The technological evolution of segmental construction has never been unremitting The sion is hall marked all over the world by heaps, leaps and also significant plateaus The economicand competitive necessities had given rise to innovations through incremental improvisations byvirtue of new structural forms, new methods of analysis and designs, new material properties andmaterials itself, new pre stressing ways, new construction methodologies and value engineeringenabled contracting environments The history of segmental construction in India is not very dif-ferent The technology was pioneered for bridges (Fig 1) way back in 1948 for Assam railway linkbridge load testing, which incidentally holds record for first time application of pre stressed con-crete technology for bridges in the country The technology was extended first times to cast-in-situcantilever construction for Barak bridge at Silchar (Fig 2) in 1961
progres-In precast segmental construction method, the precast elements that are cast in the casting yardthrough specially designed moulds are transported to the location and post tensioned together Thesmall segments are made transversely which is in contrast to precast girder bridges where the overalllengths of the precast girders are almost same as that of spans
Generally the joints which are either glued by specially designed epoxy formulation or in somecases dry are orthogonal to the longitudinal axis of the bridge Normally the reinforcement isnot continuous through joints unless it is a must for from structural reasons The decisions onthe permanent structure designs are dictated by shape of segments, weight of precast elements,arrangement of pre-stressing cables and methods of casting, transportation and erection
Generally there are two principle methods of construction (Fig 3) which are span by spanconstruction and cantilever method of construction For both these method of construction the
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Trang 59Figure 4 Top left: Krishna bridge at Deodurg (1971–72); Top right: Long Key bridge in US (1981); Bottom left: Ganga bridge at Buxer (1971–75); Bottom right: Ramp I bridge Florida (1985).
Figure 5 Top left: Fort Lauderdale-Hollywood International Airport expansion required the removal of three precast segmental concrete bridges in 2002 Top right: dismantling of one span of Ganga bridge at Patna in 2014.
precast segments have to be realized in casting yard either by long line casting method or short linemethod of casting In span by span construction the erection or launching of the segments can either
be realized by under slung erection method or overhead launching system Balanced and even freecantilever method of construction can be realized both by form travelers or overhead launchingsystem, while the progressive cantilever construction a variation of free cantilever constructionmay require temporary towers
It is very interesting to note that first application of span by span precast segmental constructionwas realized in India for Krishna bridge at Deodurg in 1971 while the technology was used inUnited States first time for Long Key bridge in 1981 Precast segmental for balanced cantilevermethod (BCM) of construction was adopted first time for Ganga bridge at Buxer in 1971–75 while
in United States technology using BCM adopted first time for Ramp I bridge Florida in 1985 Onthe other hand the dismantling of pre cast segmental bridges started in United states as early as
2002 for Fort Lauderdale-Hollywood International Airport expansion while in India the same hasbegun in 2014 for Ganga bridge at Patna It is bit surprising that precast segmental constructiontechnology for both span by span and Balanced cantilever method of construction started in Indiaalmost a decade earlier and dismantling began almost a decade later
In the recent years, segmental construction technology has been extensively adopted for span fly-over, bridges and elevated transportation structures, though there was a significant hiatusfrom 1980 to 1995 in India, in exploitation of this technology
Pre cast methods of construction have the following process of value addition:
– Pre casting of segments
– Transportation & Handling of pre cast segments
– Erection of pre cast Segments
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Trang 60Principally there are two methods of construction in precast segmental bridging, VIZ, span byspan method of construction (SSMC) and cantilever method of construction (CMC) Span by spanconstruction have similarities to that of cast in situ simply supported method of construction, thebroad distinction being instead of whole span being cast at the location, the segments are cast infactory environment somewhere else, transported to location and segments are assembled and posttensioned at location to make a span or spans.
2.1 Span by span method of construction (SSMC)
In SSMC span length is determined by the gantry which depends upon the deck width In India amaximum span of approximately 50 m is achieved Typically two spans per week is possible in case
of simply supported schemes while cast in situ span of 50m is achieved in six weeks Erection speedcould be as fast as 3-4 spans per week for 35 to 40 m spans When large deck elements working atheight supported on slender piers, extra stability precaution is warranted as overhead gantries arepotentially susceptible to instability also foundation design is governed by this construction stateparticularly under high winds
This method of construction can be realized by a larger range of types of available erectionequipment, such as overhead, underslung or falsework systems This gives flexibility in the reuse ofalready available equipment Depending upon the equipment chosen, the delivery of each segmentcan take place over or below the deck which needs to be taken into account during the equipmentselection process Invariably, overhead gantries are self-launching which involves more complexityand the consequence cost Overhead gantries are designed and fabricated by some contractors whohave separate enabling structures design department complimented by plant engineering and thecontractors who don’t have in house design facilities procure the same from specialized suppliers.The underslung trusses are launched from one span to the next with the support of cranes or whereaccess is restricted the trusses can be self-launching whose design being simple are normally carriedout by the contractors themselves The false work or ground supported trestles are used in urbanenvironment where access is easily available or where the radius of curvature of span is too sharp(lesser than 300 m) as the same introduces much complexities of geometric control during pre-casting and transverse adjustments in overhead gantries during erection of segments The heaviestpier segments are stiffened with diaphragms as the same are used for anchorage, deviation oflongitudinal external post-tensioning cables and to transfer the loads to the bearings In additionthe large concentrated forces are acting on this segment during erection, particularly when overheadlaunching system is adopted as the front leg is supported on this segment in one variation of SSCM.Obviously, reduction in weight is very important for precast segmental bridges than cast-in-situSSCM bridges The same is the derivative of aspects like transportation of the segment, capacity
of cranes and winches of the launcher, etc The weight reduction of cross-sections are generallyachieved by:
– Reducing weight of webs (use of steel/concrete trusses or corrugated steel plates),
– reducing thicknesses of bottom and top slabs,
– two stage construction for some parts of the segment in transverse direction,
– transverse pre-stressing to reduce number of cells in the box and increase the lengths ofcantilevers,
– provision of ribs (steel or precast) with in the box as well as outside to support cantilevers,– reducing the length of segments,
– use of high strength concrete
The variety of cross-sections (Fig 6) are used for precast segmental bridges Among the number
of aspects enumerated below, the choice principally depends on span and deck width In addition
to the torsional stiffness of a box section which provides some tolerance in the construction, thereasons why box sections are preferred in precast segmental construction are:
– during construction, temporary out of balance forces, lateral torsion deformation and bucklingeffects are taken care of by torsional stiffness,
– for curvilinear alignments closed box sections are required to cater for permanent loads,
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Trang 61Figure 6 Various structural cross sections used precast segment bridges.
– In segmental construction using overhead launching,hogging capacity of section near the pier isrequired to allow for additional hogging moment effects from a launching girder In box girder
it is possible to lower neutral axis due to wide bottom slab than an open section which allows anefficient use of prestressing and high moment capacity in hogging at the pier
– the external prestressing can be effectively used in box section, there by reduction in weightThus the governing segment dimensions are width of the top slab, depth of the box girder, width
of the bottom slab, spacing of the webs and length of the segments Normally, the width of the topslab is considered to be equivalent to the width of bridge in transverse directions and up to 12.0 mtwo webs are considered to be adequate from the premise that the girder behaves like a beam up towidth to depth ratio of 6.0, exceeding which cross section starts behaving like a slab In the casewhere the cross-section has to be analysed on the principles of beam theory for the width to depthratio exceeding 6.0, multiple celled box or single box with multiple webs is adopted The depth ofthe box section could be anywhere between 1/18 to 1/30 of the span for uniformly deep girders,which are normally used for span-by-span construction For less than 40m span in some cases,voided slab with transversal cantilevers are also used Where the roadway width is greater than theachievable maximum for single cell cross-sections also for reducing the weight of segments, thebasic box sections are over the period value engineered with number of variations such as box withribs, box with webs in steel, box with struts, box with transverse pre-stressing Fish belly closedsections with defined and undefined webs and the spine with precast cantilever ribs and strut arealso tried out in the recent past from aesthetic and economic considerations
External post-tensioning has many advantages for precast bridges constructed by SSMC Theexternal cables can be installed very quickly and a near ideal vertical PT arrangement can be realizedwithout affecting the required web thickness They also offer benefits with regard to durability, asthe corrosion protection is pre-applied under a controlled environment and can be inspected duringthe operational period of the structure The disadvantages if any could be additional anchor blocksand deviators as well as the reduced available internal lever arm and inefficient resistance to ULS
as the tension in the cables increase due to rotation of section Despite the obvious advantages, theexternal prestressing for SSMC is not caught up in India The first externally pre-stressed bridgeusing this technology in India is Noida toll bridge at Delhi (Fig 7)
Continuity of pre-stressing especially for continuous bridges in SSMC method of precastsegmental bridges can be achieved in many ways:
– long tendons over multiple spans after spans constructed
– cables terminating at or near1/4span
– crossing cables close to the pier segment
– cable Couplers
– straight cables adjacent to the pier
The bridge across River Krishna at Deodurg, constructed in 1971–72 was the first bridge wherespan by span method of construction adopting precast segments was used in India This 540 m long
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