The GO!Durban C3 bus corridor is currently under construction in Durban and will carry BRT buses from Bridge City near Phoenix southwards parallel to the MR25 and then west along the MR577. The route required the construction of several new bridge crossings, and this article discusses two of these structures.
BHEJANE OVERPASS BRIDGE
The Bhejane underpass is a 90m long twospan continuous prestressed box girder that carries the IRPTN C3A GO!Durban BRT route over the new C9 route. The route required sufficient space for sight distances for onramps.
The deck cross section accommodates two 3.6m lanes with 0.6m shoulders. Foundations are spread footings, with the central pier founded on mass concrete backfill. The abutments are reinforced concrete and cantilevered, but the mechanically stabilised earth MSE approaches are returned behind either abutment, allowing a large reduction in the lateral force required to be resisted by the abutments. This in turn results in lower toe pressures underneath the spread footing, and hence the optimisation of footing size.
The deck itself is a typical box girder shape and is prestressed longitudinally by means of 18 tendons nine in each web, and transversely over the pier by four tendons contained in a transverse diaphragm. The deck is 2.1m deep, representing a span-depth ratio of 21, which is reasonable for prestressed box girders. The deck cantilevers are reduced slightly to accommodate larger impact forces 100kN than traditionally designed for. Concrete of 55MPa was specified for the deck, with a minimum fly ash content of 30%. The structure was analysed using SOFISTIK analysis software, capable of determining all stresses due to prestressing, and dead and live loads throughout all construction stages.
Time-dependent effects, deflections and all movements were predicted using this three-dimensional model. Secondary effects caused by the continuous prestressing were also determined. In terms of bearing fixity, the bridge was fixed at the western abutment, with multi and unidirectional pot bearings being installed at the pier and eastern abutment respectively. Movements may have been reduced at the southern abutment, had the pier position been fixed; however, this would have attracted larger loads to the slender pier, and in turn would have resulted in a larger foundation.
The construction of this bridge took approximately 12 months. The deck was cast in thirds and staged on conventional formwork. The formwork was released after all longitudinal and transverse prestressing works had been completed. Pot bearings were manufactured by Nova Bearings in Johannesburg, and a steel claw-type expansion joint was utilised.
Problems encountered and innovations
The original design in the preliminary design phase of this project assumed that piled foundations would be required.
Once the geotechnical investigations had been completed, it was discovered that spread footings were an option. It was decided to minimise footings by two innovative interventions. The first was to use a returned MSE wall behind the abutment to reduce the lateral loads of the abutments. The second was to fix the deck at an abutment, as opposed to fixing the deck at the tall central pier. This reduced the cost of the bridge by approximately R3 million.
The central pier for the structure was proposed during preliminary design as being a cylindrical prism. During detailed design this was amended to a fluted, beveled shape, improving the aesthetics of the bridge. The increasing diameter of the pier could then also easily accommodate the pot bearing at the top of the pier. The new pier shape both improves functional performance and enhances the aesthetics of the bridge.
Unfortunately, upon stressing of the deck and stripping of the formwork and scaffolding, it was discovered that several of the pot bearing base plates had deformed due to problems with the grout packing underneath the bearing adaptor plate. It was decided to jack the deck and replace the bearings in question. The contractor proposed a very innovative solution for jacking the central bearing, which required an external temporary frame being attached to the pier and founding of the spread footing. The frame was used as a prop for the hydraulic jacks, which were used to lift the deck 20mm in order to allow bearing replacement.
While this work package has been completed (see Photos 1-4), an adjacent work package near the Malendela Road Intersection is yet to be completed. The CAPEX spend on the bridge was approximately Rl2 million, forming part of a larger construction package totaling R400 million.
COROVOCA BRIDGE WIDENING
The original bridge carrying the MR577 over the passenger rail linking Duff’s Road Station and Durban was constructed in 1984. The rail follows a curved horizontal alignment before passing below the MR577 at a skew angle in excess of 45 degrees. The original design comprised a simply-supported single span, and traditional cantilever type abutments, running parallel to the rail. To account for the high skew, a rather unconventional arrangement of precast beams was utilised for the original deck.
Two reinforced precast edge beams span the skew distance between the abutments of approximately 18m. Smaller precast beams are placed perpendicularly to the abutments. The majority of the smaller square spanning beams are supported on rubber bearings upon the abutment seating. However, due to the irregular deck shape, the outer square spanning beams tie in monolithically where they intersect with the larger edge beam. This unconventional beam arrangement can be appreciated for its efficient design; however, future widening of this bridge was always going to present some challenges. Thus, in 1994 when the MR577 was upgraded, a similar but independent bridge was completed adjacent to the 1984 bridge, leaving a clear gap of 1.5 m between the respective decks.
Required bridge works
The 1984 bridge accommodated three westbound lanes, while the 1994 bridge accommodated two eastbound lanes. The project scope required two additional dedicated BRT lanes to be accommodated, without compromising the number of existing lanes. The project strategy for accommodating the proposed BRT lanes was to shift the eastbound lanes inwards by closing the median gap. This allowed the BRT lanes to run adjacent to the eastbound carriageway, with minimal impact on the existing road prism. The limited space and topography of the route were determining factors in selecting the BRT alignment. The chosen alignment resulted in significant savings in bulk earthworks. Thus, the scope of works at the Corovoca Bridge required that the median gap between the two existing bridges be closed and that the eastbound bridge be widened by approximately 4m. See Photo 5 for an aerial view of the bridge site.
The most significant challenge was that the top portion of the existing edge beam formed an integral part of the existing sidewalk, and was approximately 300mm proud of the road level. The existing raised sidewalk was within the proposed BRT lane. Thus, widening the bridge meant that a portion of the edge beam needed to be demolished to suit the road levels. A review of the as-built drawings, together with the additional traffic loading, revealed that the beam would not have sufficient strength.
Strengthening the beam for flexure could have been achieved through externally bonded steel plates or carbon fibre strips. However, in this instance, removal of the upper 300mm of the edge beam impacted on the anchorage of the shear stirrups. Thus, a robust and practical means to strengthen the beam for both flexure and shear was required.
The design intent was for a support girder that would provide additional passive resistance without impacting adversely on the existing bridge. There was ample clearance to the tracks below to accommodate a 1m deep steel fabricated girder. The girder was galvanised for durability. Due to its length this required a double dip in the galvanising bath. The steel girder was supported on fabricated steel seating brackets that were bolted to the existing abutments. The steel girder and seating brackets are shown in Photo 6.
After the girder had been erected, a series of elastomeric bearings were installed between the concrete beam soffit and the steel girder top flange. Any gaps between the beam soffit and elastomeric bearing were filled by injecting an epoxy grout. After the steel girder had been erected, the top portion of the concrete beam was demolished. This construction sequence ensured the safe transfer of loads into the steel girder through the elastic bearings, while also allowing for sufficient site tolerances.
Further design challenges included converting existing wing walls into abutments. This required partial demolition works and the construction of abutment seatings. Existing foundations and walls were strengthened by bounding additional concrete to the existing sections. Numerous shear studs were anchored into the existing concrete to ensure composite action. This method of utilising the existing concrete walls mitigated the need for formwork directly adjacent to the rail tracks.
Traditional post-tensioned precast beams and in-situ top slabs cast on permanent formwork were utilised for the widened decks. Given that the existing deck was constructed from reinforced concrete, it would have been problematic to connect the post-tensioned beams monolithically to the existing deck. Thus, longitudinal deck joints were installed to insure that the new decks acted independently. Further, the longitudinal deck joints allowed for marginal long-term differential deck deflections. It is noted that the longitudinal deck joints are located outside the typical wheel path. The prestressing force in the beams was designed to effectively balance the deck self-weight, minimising the hogging associated with post-tensioned beams. Furthermore, a minimum time period was specified between stressing the beams and casting the in-situ top. Thus, the early creep and shrinkage effects were not locked into the composite deck.
The design and construction of new bridges in the middle of existing infrastructure requires innovation and exact problem-solving. The widening and retrofitting of bridge structures that have been in service for several decades require even more careful thought and design. These two structures bear testimony to the ability of structural engineers to work through challenges and come up not only with optimised fit-for-purpose solutions, but solutions that are also aesthetically pleasing.