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* Corresponding author: andrew@hho.co.za
Olifants River Bridge Widening
Andrew Rowan1 and Les Thomson1
1HHO Consulting Engineers, Cape Town, South Africa
Abstract. The Olifants River Bridge B3611 carries the N11 over the Olifants River, just North of
the Loskop Dam. This structure was originally built in 1979 and was recently widened as part of the
South African Roads Agency Limited (SANRAL)’s upgrade to the N11. At the time of design, very
little was known about the bridge as no ‘As Built’ drawings were available. Due to the remote
locality of the structure, exploratory investigations were reserved until the construction phase. The
final design solution was therefore amended during the construction phase in order to account for
the reinforcement found within the structure. In addition to the heavier dead weight of the new
widened deck, the bridge would be required to carry higher loads under modern loading codes.
Widening works included new widened cantilevers with new reinforced concrete balustrades, tying
into existing reinforcement. Strengthening for bending was provided to the main deck beams by
means of longitudinal FRP plates epoxied to the soffit. Transverse pierhead strengthening using
DYWIDAG bars was installed to counter increased moments, and pier strengthening using a
reinforced concrete jacket was implemented to strengthen the piers. Durability concrete was
specified in accordance with current SANRAL regulations and the durability performance of the
concrete, even in this remote location was excellent. This paper summarises the work that was
completed as part of this project.
1 Introduction
B3611 Olifants River Bridge was constructed in 1979 to
the North of the Loskop Dam in the Mpumalanga
Province of South Africa. The bridge is a seven span
151m long reinforced concrete structure that carries the
National Route 11 over the Olifants River. The 22m
spans are simply supported and rest on elastomeric
rubber bearings. Cylindrical Piers are 1,5m in diameter
and are founded on pad footings set into competent rock.
Abutments are vertical reinforced concrete cantilever
type, with return walls running parallel to the road
centreline. The deck level is approximately 9-10 metres
above the founding level.
While the structure was in good condition at the time
of initial inspection, the required road cross section for
the upgrade N11 necessitated the widening of the bridge.
2 Preliminary Design
Unfortunately no As-Built information was available at
the time of preliminary design. As such, it was unknown
what original design loads were allowed for, and what
reinforcement was provided.
While investigating the structure in more detail was
an option, the cost, time delay and remote location of the
bridge site resulted in the designers opting for a more
conservative design, which could then be rationalised
and possibly reduced during the construction phase, once
the structure could be more fully investigated.
Fig. 1. B3611 Olifants River Bridge prior to Widening.
3 Existing Bridge Condition
An Under Bridge Inspection Unit (UBIU) vehicle was
used to inspect the bridge prior to the design phase. A
full inspection according to the SANRAL Bridge
Management System (BMS) was performed by an
accredited senior bridge inspector. Defects were rated in
terms or degree, extent, relevance and urgency. It was
found that in general the structure was in good condition
MATEC Web of Conferences 199, 10007 (2018) https://doi.org/10.1051/matecconf/201819910007
ICCRRR 2018
© The Authors, published by EDP Sciences. This is an open access article distributed under the terms of the Creative Commons Attribution License 4.0
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and showed little sign of damage. Some minor shear
cracking in the longitudinal beams was present.
Expansion joints needed replacement and other minor
improvements (eg painting handrails, cleaning drainage
scupper and replacing manhole covers) were noted as
being required.
4 Original Design / As Built Information
As the structure was constructed in 1979, it was assumed
that the original design had adopted the BS 153 (1954)
loading system in its structural analysis. This called for a
uniformly distributed linear traffic load of 2200 pounds
per foot to be applied along with a single heavy axle line
load of 2700 pounds per foot. By comparison, current
loading requirements prescribed by TMH7 Parts 1 and 2
have requirements of up to 20-60% higher that the BS
153 loading code. The live loads that the structure would
need to handle are therefore higher. In addition to this,
the dead loads from the widened cantilevers are
increased.
The consequence of this is that even if the original
design was conservative, the new loads (live and dead)
would require strengthening to the majority of the
structure.
5 Deck Widening
It transpired early on in the design phase that the client’s
requirements for a ‘full’ upgraded geometric cross
section for the entire N11 route would require several
structural widenings along the length of the project. This
also presented an opportunity to upgrade the existing
balustrade (Steel balustrade) to a SANRAL standard F
Shape Parapet capable of resisting current design impact
loads.
6 Cantilevers
The existing 1,5m cantilevers were demolished by means
of hydraulic impact hammers, while retaining the
reinforcement to be lapped onto the new longer
cantilever reinforcement (see section 13). Where
moments were increased due to longer cantilevers,
increased traffic and impact loads, this was accounted
for by increasing the depth of the cantilevered section.
The original proposal (later revised) was to include
additional main reinforcement that would be tied into the
remaining deck, anchored into recesses cut in to the top
slab of the deck. There were several questions at the time
of design regarding the amount of reinforcement present
in the existing section, the configuration, the cover
depths, even the material strength of the reinforcement
itself was assumed. These would be later confirmed in
the construction phase, but a conservative approach was
taken in the design phase.
Fig. 2. Reinforcement preparation prior to casting on new
cantilevers.
7 Longitudinal Beams
As the amount of reinforcement in the existing
longitudinal beams was unknown, any additional loads
due to increased dead weight or additional design live
loading was accounted for by the addition of longitudinal
steel plates up to 12mm thick. These would have been
epoxied onto the underside of each of the longitudinal
beams. The external beams called for a 12 mm plate and
the internal beams called for a 7mm plate.
8 Shear Strengthening
Similarly the longitudinal beams were also given
strengthening for increased shear forces by means of
5mm vertical shear plates epoxied on either side of the
beams at varying centres. These were intended to be
anchored vertically through the deck and bolted into
place temporarily during the setting time of the epoxy
used to adhere the plates to the beams.
9 Pierhead Strengthening
The pierhead required strengthening in the form of
external prestressing. 3x40mm diameter DYWIDAG
threaded bars were used on either side of the pierhead to
counter the additional moments. These bars were then
encased in concrete.
10 Pier Jacketing
The additional moments induced into the piers,
especially during construction stages and for earthquake
design, required that the 1800mm dimeter cylindrical
piers be strengthened. Additional strengthening works in
the form of column wrapping with FRP sheets was
investigated but was not preferred by the client for
reasons of durability and long term robustness due to
potential scour in the river under flood conditions. The
final solution adopted was a simple 150mm reinforced
concrete jacketing for the full height if the pier. Anchor
MATEC Web of Conferences 199, 10007 (2018) https://doi.org/10.1051/matecconf/201819910007
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bars were drilled into the pier to improve interaction and
bond between old and new concretes.
11 Exploratory investigations
As soon as the contractor was established on site, we
were then in a position to confirm reinforcement
amounts in the various elements. Specific locations were
identified and scabbled to expose outer reinforcement.
This confirmed both the size and spacing of
reinforcement for key bridge elements.
12 Confirmation of reinforcement
strength
It was also necessary to confirm the strength of the
reinforcement, as it was unclear what material properties
were present with the existing rebar. Tests concluded
that the reinforcement was of an equivalent yield
strength to our current day high yield strength
reinforcement (i.e 450 MPa Y bars).
13 Revised design
The tests and exploratory investigations now left the
engineers in a position rationalise/reduce the original
design. The requirement for additional shear
reinforcement was removed since large amounts or shear
reinforcement was found in the longitudinal beams. The
amount of longitudinal reinforcement required in the
form of plates was left as originally specified.
14 These things happen – Cantilever
reinforcement accidentally removed
The detail proposed for extending the existing
cantilevers including utilising the existing reinforcement
in the cantilevers, and lapping new reinforcement onto
these bars (once they were exposed). Unfortunately an
incident involving a misunderstanding on the site
instruction saw the first span’s reinforcement completely
removed, which left a challenge as to how the new
reinforcement would be adequately anchored into the
existing deck. The implemented solution was to return to
the original proposal of using longer heavier bars located
in recesses, which we then bent through 90 degrees and
anchored vertically into the deck. The anchor protrudes
though the thickness of the beck and the exposed bar is
then locked off with a plate and bolt arrangement. All
exposed steel is then given corrosion protection in the
form of multiple zinc rich barrier coatings.
15 Concrete Durability Performance
“Durability’ concrete was specified in the tender
documents for this structure, and this required certain
performance criteria for Oxygen Permeability Tests,
Water Sorptivity Tests and Chloride Conductivity. For
each of these tests, the highest categorization of
performance was achieved.
16 Longitudinal Strengthening
alternative
The original specification for the longitudinal plates
required for beam strengthening called for 12mm thick
plate with a width of 570mm and length of up to 20m.
The weight of this plate would be in the order of 1 tonne,
and the application of the plates at height over a sensitive
area proved to be a challenge on site. The contractor
proposed an alternative of using FRP bands with an
equivalent strength. This was accepted by the client, and
was implemented on site.
17 Deck vibrations/ limiting traffic
During casting of the new cantilevered section, concerns
were raised by the contractor of the vibrations induced
by vehicle traffic on the bridge, and whether or not this
would have a negative effect on the curing and strength
gain of the new concrete. In order to mitigate this, traffic
was limited on the deck for the first few days after each
cast.
18 Balustrades
The original steel balustrades were removed, and
replaced with SANRAL standard F Shape Parapets
design to withstand 100kN impact loads. An updated
endblock shape was also required, in line with the latest
client standards.
Fig. 3. Finished structure showing new balustrades.
19 Joints
The existing concrete nosing joints were replaced by
asphaltic plug joints.
20 Conclusion
This paper covered the successful design and
construction of a bridge widening in the Mpumalanga
Province of South Africa. Innovative designs and
creative responses to various challenges resulted in an
attractive and structurally sound solution for the client.
MATEC Web of Conferences 199, 10007 (2018) https://doi.org/10.1051/matecconf/201819910007
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The project was completed on time and within budget,
and is a testimony to the South African ‘can do’
approach to life.
Fig. 4. Underside of finished widening showing elegant beam
strengthening and pierhead external prestressing
MATEC Web of Conferences 199, 10007 (2018) https://doi.org/10.1051/matecconf/201819910007
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