CHAPTER 2 2.0 LITERATURE REVIEW 2.1 INRODUCTION During the last few decades composite bridge superstructures utilizing pre-stressed concrete beams has become a widely accepted alternative to in situ deck construction. There are different basic types of beams suitable for spans up to 36m. There are five standard bridge beams developed by the pre-stressed concrete association in UK. They have wide variety of applications. Although there is a recognized range of spans for which each standard section is generally used, the fields of application for the various types overlap and the final selection for of a particular type is largely a matter of experience related to a specific situation. The most popular types are inverted T, M, U, I, and the box beams. In Sri Lanka inverted T and M beams are widely used. Further, for 30m span simply supported bridges space rectangular box beams and spaced trapezoidal box beams were used. For the continuous bridges big spine beams also have been used with post tension pre-stressing system.The above beams and bridges are described below. 2.2 STANDARD BEAM SECTIONS 2.2.1 INVERTED T BEAM SECTION Inverted T beam was introduced in 1951 by Pre stressed concrete development group (PCDG) in United Kingdom. Inverted T beams are placed side by side at a nominal spacing of 510mm, with the space in between filled with in situ concrete to cover the precast beams by a minimum of 75mm, form a solid slab-type superstructure suitable for spans from 6 to 18 m .Transverse reinforcement through the holes at the bottom of the webs and the mesh reinforcement on the top of the beams are incorporated in the in situ concrete to ensure fully composite action to support the subsequent loads. In Sri Lanka, polythene displacers are used between beams to reduce dead loads. Service ducts also can be accommodated within the space between beams. The span/ depth ratios achieved by this form of construction vary between 17 and 20. 2.2.2 M BEAM SECTION The M beam was accepted as standard bridge beam in late 1960s by Pre stressed concrete development group (PCDG). M beams are normally placed side by side at a nominal spacing of 1 m, with the concrete slab of about 160 mm thickness cast on permanent formwork on the tops of the beams forming their top flange. This top slab also affects global and local distribution of live loads between the beams. This type of beam-and- slab superstructure normally achieves span/depth ratios of between 18 and 20. Another variant with this type of beam, called a pseudo-box super structure. It has transverse reinforcement which passes through holes in the 4
Transcript
CHAPTER 2
2.0 LITERATURE REVIEW
2.1 INRODUCTION
During the last few decades composite bridge superstructures utilizing pre-stressed concrete
beams has become a widely accepted alternative to in situ deck construction. There are
different basic types of beams suitable for spans up to 36m.
There are five standard bridge beams developed by the pre-stressed concrete association in UK. They have wide variety of applications. Although there is a recognized range of spans for which each standard section is generally used, the fields of application for the various types overlap and the final selection for of a particular type is largely a matter of experience related to a specific situation.
The most popular types are inverted T, M, U, I, and the box beams. In Sri Lanka inverted T and M beams are widely used. Further, for 30m span simply supported bridges space rectangular box beams and spaced trapezoidal box beams were used. For the continuous bridges big spine beams also have been used with post tension pre-stressing system.The above beams and bridges are described below.
2.2 STANDARD BEAM SECTIONS
2.2.1 INVERTED T BEAM SECTION
Inverted T beam was introduced in 1951 by Pre stressed concrete development group (PCDG) in
United Kingdom. Inverted T beams are placed side by side at a nominal spacing of 510mm, with
the space in between filled with in situ concrete to cover the precast beams by a minimum of
75mm, form a solid slab-type superstructure suitable for spans from 6 to 18 m .Transverse
reinforcement through the holes at the bottom of the webs and the mesh reinforcement on the
top of the beams are incorporated in the in situ concrete to ensure fully composite action to
support the subsequent loads. In Sri Lanka, polythene displacers are used between beams to
reduce dead loads. Service ducts also can be accommodated within the space between beams.
The span/ depth ratios achieved by this form of construction vary between 17 and 20.
2.2.2 M BEAM SECTION
The M beam was accepted as standard bridge beam in late 1960s by Pre stressed concrete
development group (PCDG). M beams are normally placed side by side at a nominal spacing of 1
m, with the concrete slab of about 160 mm thickness cast on permanent formwork on the tops
of the beams forming their top flange. This top slab also affects global and local distribution of
live loads between the beams. This type of beam-and- slab superstructure normally achieves
span/depth ratios of between 18 and 20. Another variant with this type of beam, called a
pseudo-box super structure. It has transverse reinforcement which passes through holes in the
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beam webs and is provided with 50mm cover of in situ concrete bonded to the bottom precast
flange. This improves the load distribution properties of the super structure. However, because
of the extra site operations and additional material involved, this does not necessarily lead to
economy of construction. The pseudo -box type of construction commonly achieves
span/depth ratios of betweenl9 and 21 . These two types of M-beam superstructure are
appropriate for spans ranging from 15 to 30m. Diaphragms of in situ concrete at beam ends are
generally incorporated with these forms of construction. These Diaphragm members serve to
restrain the ends of beam torsionally and make the jacking of the superstructure easy during
replacement or repair of the bearing.
2.2.3 U BEAM SECTION
The U beam voided construction was introduced in 1973 by Pre stressed concrete development
group (PCDG). The U beams are placed at a nominal spacing of 2m, and a concrete slab of about
160mm thickness cast on the tops of the beams forms their top flanges. The slab also closes the
U sections, thus enhancing the torsional stiffness of the beam sections which in turn improves
the load distribution properties for the superstructure. This type of arrangement is used for
spans ranging from 13 to 34m. In this type of superstructure, end diaphragms of half depth
between the beams requiring boxed-out top-end corners of the webs of the U beams are more
commonly used. This type of bridge construction normally achieves span/depth ratios of
between 15 and 19. Services can be accommodated through the beams or below the footways.
Figure 2.1 Standard U beam section
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2.2.4 BOX BEAM SECTION
Box beam was introduced in early 1960s by Pre stressed concrete development group (PCDG)
in United Kingdom .Box beams which cover spans ranging from 12 to 36m are placed at a
nominal spacing of l m and the narrow spaces between the beams are filled with in situ
concrete. A regulating layer of in situ concrete may be applied on the deck. Precast diaphragms
200 mm or more in thickness are provided at 2440mm centres along the length of the beams
through which holes of a suitable size are provided at the required depth selected for
transverse post tensioning. The span /depth ratios achieved with this form of construction vary
between 21 and 23. The aesthetic treatment of transverse post tensioning anchorages and the
provision of service ducts in this type of superstructures require special consideration. Further,
inside surfaces are uninspectable. For this reason, and because of the need for site post
tensioning, this form of construction has tended to be replaced by other forms during the last
decade. In this type, minimum amount of in situ concreting is involved and it is relatively fast
speed of construction.
Figure 2.2 Standard box beam section
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2.2.5 I- BEAM SECTION
I- beam was introduced in early 1960s for long span beam and slab bridges. I- beams are used
for spans ranging from 12 to 36m. They are usually placed at a spacing of 1.5 to 2m, with a top
slab of in situ concrete of 175 to 225mm thickness cast to form the top deck of the bridge. In
situ diaphragms of 200 to 300 mm thickness can be provided as required at 3050 mm spacing
by passing reinforcement through 140 mm square holes at the junction of the bottom flange
with the web. The span/ depth ratios achieved with this form of construction vary from 14 and
16. In this type, generally deeper construction depth and complicated form work for in situ
concrete are required.
2.3 SPACED BOX BEAM SECTIONS
Generally box beam has higher torsional capacity because of its closed geometry. It has higher
bending carrying capacity and requires reduced beam height compared to other beam section
for a particular span. Hollow spaces in box beams can be used for services and it is also
aesthetic.
Standard box beams require transverse post tensioning, special consideration on provision of
service ducts and inside surfaces are uninspectable. For this reason, spaced box beams super
structure was introduced. This superstructure consisted of hollow box sections at a suitable
space and cast in situ reinforced concrete slab for composite behaviour.
In this type of super structure, load distribution is depended on beam spacing, span and carriage way width. Design charts are available to find this load distribution in Concrete Bridge Design by Rowe.R.E and Bridge deck analysis by Hambly. Presently structural softwares can do this job easily by grillage analysis.
2.3.1 RECTANGULAR SPACED BOX BEAM SECTION
Rectangular spaced box beams have been used for a simply supported bridge at Mattakulia in
Sri Lanka. This bridge has 7 spans with each 30m. The used spaced hollow box beam has the
outer dimension of 1300 mm height and 1040 mm width. The inner hollow dimension is
1060mm height and 680mm width. Vertical webs are with 180mm thickness. This web
thickness is decided based on the post tensioning duct diameter, size of shear link
reinforcement and cover. Normally Webs can take shear stress. Both top and bottom flange
has been provided with 120mm thickness. This thickness is also controlled by above parameters
and it can be increased to satisfy the required section modulus to satisfy the stress limits in the
section. The corners of the hollow opening were provided with haunches of 100mm. This
rectangular box beam was designed with post tensioned pre stressing.
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This bridge has 7.4m carriage way width and two foot walks with 1.8m width. The box beams
has been place with the spacing of 1.54m such that clear spacing between beams is 500mm. A
slab of 100mm in situ reinforced concrete has been placed on top of beams and 250mm in the
clear spaces as the bridge deck to behave compositely. Seven numbers of beams have been
used for one span.
Weight of the one beam is 48 ton. Bailey panels have been used to launch these beams over
the piers.
1040
120
X
13001060 180 180
1 0 0 x 1 0 0
X 120
Figure 2.3 Spaced rectangular box beam section
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Figure 2.4 Spaced rectangular box beams in Mattakulia bridge
2.3.2 TRAPEZOIDAL SPACED BOX BEAM SECTION
Trapezoidal spaced box beams have been used for a simply supported elevated flyover at
Orugodawatta in Sri Lanka. This beam was modeled and designed by Prof. M.T.R Jayasinghe.
This flyover has 11 spans with each 30m. Total width of the bridge is 17.4m. It has a central
reserve of 1.2m. There are four lanes. Each lane is 3.5m. There are two pedestrian walk ways of
1.1m.
Trapezoidal spaced box beams has the following dimensions. The width of top flange is 3.5m.
The thickness of the cantilever top slab will vary from 160mm to 100mm. The thickness of the
top flange between tow webs will be 140mm. The width of the web is 250mm and they are
inclined at 62 degrees to the horizontal. The width of the bottom flange is 1.5m. The corners of
the hollow openings are provided with haunches. This box beam has been designed with post
tensioned pre stressing.
In this deck, the beams are placed with a gap of 800mm between two adjacent beams. This gap
is casted with in situ screed concrete. The screed acts as part of the top slab structurally. The
screed has a minimum thickness of 60mm and is laid to a slope of 1 in 60 for drainage. The
thickness of screed at the central reserve is 177mm. Four numbers of beams have been used
for one span.
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Weight of the one beam is 120 ton. A ramp and Bailey panels have been used to launch these
beams over the piers.
Figure 2.4 Spaced Trapezoidal box beams section
Figure 2.5 Spaced Trapezoidal box beams in Orugodawatta over head bridge
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2.4 SPINE BOX BEAM SECTIONS
Spine beam bridges are often used in the span range of 30-200m. Those bridges are continuous
structures. Spine beams are quite popular due to the inherent torsional rigidity that gives very
good load sharing characteristics. The spine beam consists of single or multiple cells with
cantilever overhangs on either side at the top flange. In this super structures there will be
secondary moments. It should be considered in the design.
The concept of minimum practical material quantities is of important in these concrete bridges
since the extra weight represented by the added volume of concrete is also has to be carried by
the structure. The smallest possible section should satisfy the construction technique,
constructability and stress limits. A rational design method is explained in the research paper,
Rationalization of section design philosophy for prismatic prestressed concrete beams by Prof.
M.T.R Jayasinghe.
2.4.1 SELECTION OF THE SMALLEST PRACTICALLY POSSIBLE CROSS SECTION FOR SPINE BEAM
The loads acting on spine beam bridge consist of dead loads, superimposed dead loads and live
loads. Live loads will depend on the number of notional lanes. Both HA and HB type of loadings
are considered as per BS 5400, Part 2. At the preliminary design stage, the designer has to
select a suitable section as the starting point. The section can be idealized according to the
required dimension of the bridge and can be defined by a number of parameters. Each of these
parameters is governed by one or more criteria, which are set out below.
1 . OVERALL DEPTH OF THE SECTION
It may be specified in the client's brief or it may be governed by the vertical alignment
considerations. If it is not given, the designer is free to select a reasonable value for the
span/depth ratio; values between 14 and 25 are typical. A minimum depth of 2.0 m is preferred
by considering the need for workers to stand unobstructed inside the spine beam.
As per literature, it is economical to use larger depths and a reduced prestress if allowed.
Greater depths allow a reduction of bottom flange area, but carry the penalty of increased
weight due to increased depth of the web. The best option may be to consider few alternative
solutions in order to find the minimum cross sectional area that gives the required flexural
properties.
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2.THE WIDTH OF THE TOP FLANGE
The width of the highway super structures is controlled by the number of traffic lanes, cycle
ways and foot walks. Therefore, the selection of top flange width is depended on all these
values.
3.WEB SPACING
The local bending of the top slab governs the web spacing. An increase in the web spacing will
increase the required thickness of the top slab, but will reduce the number of webs. Thus, extra
material in the flanges contributes to global bending resistance and stiffness, but, extra web
material primarily adds weight. The minimum number of webs consistent with adequate local
bending performance of the top slab is preferable. The practical maximum web spacing is
considered as 6.0m if the top slab is transversely reinforced, or 7.5m if the top slab is
transversely prestressed.
4.CANTILEVER OVERHANG
The web spacing is related to the cantilever overhang. Aesthetic consideration and local
bending of the top slab also govern this. If the overhang is too large , then the thickness of the
top slab becomes governed by hogging bending at the root of the cantilever, where as if it is
too small , the design has unused hogging resistance at the root.
Generally, the ratio ( y) between the cantilever overhang and the clear spacing between the
webs is taken as 0.5.
Web spacing =( Wt - N tw) / (2 y + (N-1) ) , Where
Wt- total width
N- Number of webs
tw- web thickness
5.THE THICKNESS OF THE TOP FLANGE
The top flange thickness is governed by local bending consideration, practicability and the form
of construction. The thicknesses for the top flange, given in below table as a function of the
clear spacing between webs, are taken from research papers.
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Table 2.1 The thickness of the top flange
Clear spacing between webs Ws (m)
Top flange thickness for transversely reinforced web (m)
Top flange thickness for transversely prestressed web(m)
Up to 3.0 0.250 0.200 3.00-4.50 0.300 0.225 4.50-6.00 0.350 0.275 6.00-7.50 0.300
6.THE THICKNESS OF THE WEBS
The web primarily resists shear stresses due to torsional moments and shear force. Shear
reinforcements can be used in the webs if adequate shear resistance is required. When the
thickness of the web is increased, it will not only increase the dead weight but also the area to
be prestressed. Further, web thickness depends on constructability, type of prestressing ducts,
minimum clearances for proper concreting, cover and two planes of reinforcements. Following
table gives the web thicknesses required depending on the type of ducts.
Table 2.2 The thickness of the web
Type of ducts Web thickness (m) No prestressing ducts in webs 0.200 Small ducts for vertical prestressing 0.250 Ducts for prestressing cables 0.300 Anchors for the prestressing cables 0.350
7.THE WIDTH OF THE BOTTOM FLANGE
It depends on the cantilever overhang, inclination and number of webs, the clear spacing
between webs and the thickness of the webs. The designer should decide the inclination of the
webs which are probably governed by aesthetic considerations. In some bridges webs are
considered as vertical.
8.THE THICKNESS OF THE BOTTOM FLANGE
This bottom flange requires extensive consideration. It should satisfy a number of constraints:
1. Practical detailing considerations
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2. It should be large enough to allow the section to be prestressed at the working load and
3. It should provide adequate compressive resistance at the internal support positions
Though the minimum constructable thickness is 125 mm, 175mm is recommended as the minimum.
2.4.2 BRIEF INTRODUCTIONS ON SPINE BEAMS BRIDGES AVAILABLE IN SRILANKA
There are few bridges constructed with spine beams in Sri Lanka. They are continuous
superstructures constructed with different launching methods.
1. BRIDGE AT MANNAMPITIYA ACROSS MAHAVELI RIVER
This is a continuous bridge with six spans each at 48.5m. The incremental launching method has
been used to launch this superstructure. The geometry of this beam is as figure 2.5.
Figure 2.5 Spine beam at Mannampitiya bridge
2. SRILANKA JAPAN FRIENDSHIP BRIDGE AT PELIYAGODA ACROSS KELANI RIVER
This bridge has seven spans each at 32.5m. This continuous bridge was launched with
incremental launching method. There are two separate bridges side by side such that each
bridge has two lanes and foot walks. Initially, one was constructed keeping the old bridge to
function. Then, old bridge was demolished and newly constructed very close by. Spine beams
were used for both. In addition to the longitudinal post tensioning prestressing for the beam,
webs and top flanges also has transverse prestressing. The geometric dimensions of this beam
are shown in table 2.3 and figure 2.6. All corners of the hollow have haunches.
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Table 2.3 The geometric dimensions of Sri Lanka Japan friendship bridge
DIMENSION PARAMETERS VALUES(m)
Overall depth 2.2
Width of the top flange 11.8
Web spacing 5.5
Cantilever over hang 2.4
Top flange thickness 0.32
Web thickness 0.4
Inclination of the web to horizontal 59 deg.
Bottom flange thickness 0.28 -0.44 (from mid to support )
3 r L 5 0 0
_ ecwco p i u e » i i o o
Figure 2.6 Section view of SriLanka Japan friendship bridge
is
3. BRIDGE AT SOUTHERN HIGHWAY ACROSS KALUGANGA
This is a continuous four lane bridge with three spans such as 45m, 75m and 45m. The balance
cantilever launching method has been used to launch this superstructure. The dimensions of
this beam are shown in table 2.4. The launched bridge is shown in figure 2.7.
