Design and Construction of Pre-Tensioned Sutlej Bridge in Punjab

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Fig. 5. Laterial stability of long prestressed concrete beams with factor of safeties
beams cannot buckle which was also taken into cognisance
during lateral stability investigations.
The lateral stability guidelines for precast beams given in
Indian Codes for prestressed concrete members are similar to
that of for RCC and steel beams in terms of span to depth ratio
and depth to top flange width ratio. However, in the case of
prestressed beams, the aspect of prestressing is a new variable
and as such the same guidelines may not be applicable for
prestressed beams. In view of this the factor of safeties
enumerated above may be included for ensuring the lateral
stability of beams during shifting, transportation and erection
of prestressed, precast beams in the prestressed concrete
codes.
6.2. Design of Superstructure
The superstructure consists of six numbers of precast
pretensioned girders spaced at 2.15 m centre to centre with 250
mm thick end diaphragms to support 200 mm thick cast-in-site
RCC deck cantilevering by 1.10 m from the centre line of external
girders on either side. The pretensioned girders in the casting
yard were specified to be prestressed after 24 hours when the
strength of the concrete was 31 MPa, while the concrete grade
for the beams were M 40.
The precast girders were transported to site and placed on
bearings followed by casting of the end diaphragm. The RCC
deck slab was cast on formwork supported on girders and the
HEGGADE, MEHTA & PRAKASH ON
same was removed after the sufficient attainment of strength in
the deck. Thereafter, for the further loads such as weight of
crash barrier, wearing coat, railings and live load, etc. the
structure was assured to be a composite section. The effect of
differential shrinkage and temperature variation were also
considered in the design.
For finding all the longitudinal beam reaction components,
the grillage analysis (Fig. 6) was used for superimposed dead
loads and live loads, the structure was idealised as a grid of
longitudinal and transverse members. The composite girders
consisting of precast beam and deck slab was descretised to
be placed along the axis of the girder while deck slab and deck
slab with diaphragm was placed as transverse grillage members
along the line of each of end diaphragm in the structure. The
slab acts to transmit applied loads to beams by spanning
transversely between them, apart from providing means for
load sharing between longitudinal beams. Therefore transverse
members having slab properties were provided to reflect the
load sharing characteristics of the deck. For the application of
the loads due to railing, the dummy longitudinal members with
negligible section properties were provided at the edges and
transverse grillage members were continued to connect them.
The flaring properties of precast beams at the end for the
distance of 2.65 m from 200 mm to 300 mm thick has been
accounted for in the descretisation.
The grillage analysis results especially for superimposed