From the Editor’s Desk
NEED OF GUIDELINES FOR WIDENING OF
EXISTING BRIDGES
(ii) non-functional bearings (iii) poor status of
railings/broken steel railings (iv) poor condition
of wearing coat and (v) disintegrated kerbs. Such
bridges can be taken up with advantage for widening
by undertaking strengthening, retro-fitting and
repairs/rehabilitation.
Under National Highways Development Programme
(NHDP), NHAI is undertaking a large number of
2x3-Lane Road Projects which invariably requires
widening of existing 2-lane bridges in terms of
adding one lane. Central and State Governments
too are undertaking upgradation of their roads to
meet the increased traffic requirements in terms of
upgrading single-lane roads to two-lane, and adding
paved shoulders to 2-lane roads which also requires
widening of existing bridges, otherwise, narrow
carriageway width of bridges shall be an impediment
in smooth flow of traffic, an hazard and result in
reduced capacity of the road. MORT&H vide circular
dated 24th March, 2009 have revised configuration of
2-lane bridges in terms of 10.5 m carriageway width,
0.75 m raised kerb and 0.5 m crash barrier so as to
match with the carriageway of 2-lane road with paved
shoulders. IRC have revised loading standards and
after catastrophic effect of earthquakes in recent past
causing major damage, the seismic factors/criteria for
seismic forces has been revised. Bridges built in past
have steel/RCC railings, whereas, the requirement of
RCC crash barriers have been adopted from safety
consideration. With economic development, the raised
footpath as provided on bridges in rural reaches are
being hardly put to any use. Most of these bridges do
not have specific provision for carrying services.
The first step to consider an existing bridge for
widening would be site investigations, study of
designs and drawings as had been followed for its
construction, detailed condition survey and evaluation
of structural adequacy for present day design criteria.
With increased width, the load distribution shall
change which also requires checking of sub-structure
and foundations.
The existing bridges of inadequate carriageway width
which are showing signs of distress and have lived
their life can be taken up for reconstruction as per
latest design standards. Many of the existing bridges
(of inadequate width as per present day standards)
are structurally safe to carry present day traffic
loads, showing no signs of distress, have enough
balance life and their approaches are meeting the
geometric requirements too. However, most of such
bridges do not have RCC crash barriers as is being
provided from safety consideration and any specific
provision for carrying services too. Further, few of
such existing bridges may have problems in terms of
(i) mal-functioning/damage to expansion joints
In terms of widening the alternates include concentric
widening or eccentric widening limited on one side
depending upon availability of land, structural
arrangements and site conditions. For widening typical
structural arrangements in terms of super-structure can
be: (i) widening in the form of new superstructure in
RCC slab/slab-beam system/other structural system
with zero joint between existing and widening portion
(ii) widening in the form of new superstructure in
RCC slab/slab-beam system/other system integrated
with existing superstructure. It shall also require
decision by the Design Engineer whether to follow
the existing span arrangement or to go in for larger
spans.
2
The structural features of existing bridges invariably
refers to: well foundations, pile foundations or open
foundations with flooring, flooring with curtain walls
both on upstream and downstream, substructure in
brick masonry or RCC, rocker and roller bearings,
superstructure in brick masonry arches, reinforced
concrete slabs, reinforced concrete slab-beam
arrangement
or
prestressed
concrete beams
supporting reinforced concrete slabs, steel girders
supporting reinforced concrete slabs, RCC box-girder
etc. The deck slab invariably is cantilevering beyond
supporting girder and supporting footpath, RCC/steel
railing.
INDIAN HIGHWAYS, April 2012
EDITORIAL
The option among the independent new structure and
integrated structure shall also require consideration
of compatibility of new and existing concrete and
reinforcement which would require making some
assumptions. Further there is a need to consider aspect
of deflection at edges of the existing structural deck slab
and that of the new structural deck and what tolerance
can be allowed in terms of differential deflection at
their joint. For substructure and foundations too, there
would be mainly two alternatives (i) pile foundations
(with span arrangements consistent with existing
or larger spans) (ii) open foundations (in line with
existing substructure and foundations). Particularly
for existing well foundation structure, pile would be
an ideal and practical solution. Other arrangements
can be considered depending on configuration of
structural system and arrangements of existing
bridge.
The task of widening an existing bridge has to
be completed in a time bound manner, as during
construction the existing carriageway further stands
reduced which requires careful consideration of
construction planning and methodology. Otherwise,
extra expenditure has to be incurred in terms of
providing diversion which may be difficult in
certain cases on account of inadequate existing
right of way.
The process of widening of an existing bridge
would lead to several technical issues. Presently,
no such guidelines are available and in view
of financial constraints, the widening is being
planned and undertaken in a manner as considered
appropriate by the Design Engineers. There is a
need to frame detailed guidelines for planning,
designing and construction towards widening of
existing bridges in terms of: (i) whether to have
concentric or eccentric widening as preferred option
(ii) whether the widened portion of superstructure has
to be kept independent or to be integrated with existing
structure (iii) for bridges with well foundations,
option of providing bored piles for widening portion
(iv) extension of existing open foundation and
founding level thereof (v) necessity to provide/shift
curtain wall on upstream and downstream and criteria
to be followed (vi) extension of river training and
protection works and criteria thereof, etc.
It shall be worthwhile to examine and study behavior
of bridges which have been already widened to
study their time dependent behavior to arrive at most
appropriate guidelines in terms of concept, planning,
design philosophy and construction methodology
for widening part that would be consistent and
compatible with structural arrangement of existing
bridge. There is reference on this subject even in all
old books on bridges. Widening of existing bridges is
an accepted practice and has been followed in many
countries. There is need to study and examine if same
can be adopted in our country. Definite conclusions
in the form of “State of the Art Report on Widening
of Bridges” followed by an International Seminar on
the subject and later a Special Publication by IRC
shall help in arriving and laying down an acceptable
practice, to bring in economy and confidence in the
Design and Construction Engineers in undertaking
assignment of widening of existing bridges.
I hope that both Design and Construction Engineers
in bridges would come up to accept the challenge
and would exchange their experience in drafting
guidelines for widening of existing bridges which
would go a long way in overall infrastructure
development which is under implementation in the
country and would give it further impetus.
(Arun Kumar Sharma)
Secretary General
Comments on this Editorial is invited and may be sent by E-mail: secretarygen@irc.org.in
INDIAN HIGHWAYS, April 2012
3