REBUILDING HISTORIC STELLENBOSCH BRIDGE WITH PRECAST CONCRETE BEAMS

October 10, 2022  409  409   

However, the growth of the Bosman’s Crossing precinct during the second decade of the 2020s, an increase in traffic and the deteriorating condition of the bridge, prompted the Stellenbosch Municipality to investigate widening and strengthening options.

AECOM was appointed to inspect the bridge in 2020 and found the structure to be neither safe nor practical for existing and anticipated traffic volumes.

Based on this finding AECOM was commissioned to evaluate alternatives which compared repair, rehabilitation and upgrading options. And because of the site’s possible historic significance, the council also appointed a heritage architect to compile a report to be used as a guideline for the rebuild.  

Prepared by Rennie Scurr Adendorff Architects and CTS Heritage in consultation with Heritage Western Cape, the report was submitted together with the permit application to upgrade the bridge. Covering the heritage, archaeological, landscape and contextual significance of the site, the report recommended that the new design should respond to the semi-rural character of its immediate surroundings rather than the commercial and residential aspects of Bosmans Crossing.

Y20 rebar spliced with extensive reinforcing.

Several design proposals were considered in the report. In each case the improvements required to improve traffic flow and safety were assessed in terms of the bridge’s alignment and positioning; its component features; load bearing capacity of the old elements; river through-flow and flooding; and the need to limit any potential damage to the surrounding landscape and environment.

An additional heritage aspect was the re-instatement of four existing handrail bollards, which serve as entry markers at either end of the bridge.

The engineering team tabled six design proposals which were workshopped with the heritage team and two options which adhered to both heritage and functional requirements were adopted and developed further by the engineering team. One proposed retaining the central stone pier as a non-structural component in a single-span bridge and the other advanced a two-span structure with a functional central pier.

The T-shaped suspension bracketing showing the tension cabling attached to the top of the vertical T-piece.

AECOM’s analysis confirmed that the bearing capacity of the existing central pier would be significantly exceeded if it were incorporated structurally in the proposed two-span design and would require another row of piles and further structural modifications. By contrast, in a single-span design using prestressed precast concrete beams supported by new abutments, the central pier could be retained in a non-structural capacity, and it was this solution which was finally chosen. The retention of the pier was recommended because, as with the old abutments, it predates the existing upper structure and could therefore serve as a visual link to the old stone bridge.

Eight metres wide kerb-to-kerb and 22.5m long (two metres longer than the old bridge), the new bridge will accommodate two lanes of traffic. In addition, pedestrians and cyclists are being given cantilevered walkways on either side of the vehicle deck. Besides their functional purpose, the walkways enhance the visibility of the central pier and reflect the character of the old bridge.

The underside girders and Econoform panelling.

The new bridge was designed by AECOM bridge engineer, Heinrich van Wijk, who proposed a precast beam design primarily because it saved time, reduced the risk of flood damage to formwork staging, and limited construction debris falling into the river bed.

“In a cast-in-situ bridge option, there is an element of flood risk when staging formwork above a river bed. Instead, we chose to specify a precast construction technique, which limits disturbance in the riverbed and enhanced our application to the Department of Water Affairs for a water-use license.” said van Wijk.

Van Wijk’s design adheres to the South African bridge design code of TMH7 for NA and NB loads. One of the design constraints was the need to keep the bridge open during construction which is why it is being built in two phases. Apart from the 1.5m wide edging which was demolished, the old bridge was left intact during the construction of Phase 1 and was only fully demolished once Phase 1 was opened to traffic.

T-shaped suspension bracketing at the Phase-1 walkway mounted on pivot-point jacks.

“The abutments and deck are being built in an integral monolithic design, unlike conventional bridges which are mounted on bearings with expansion joints at either end. The integral design approach allows the free articulation of the deck without imposing significant stresses on the abutments. Thermal expansion and contraction is the primary source of movement (elongation and shortening) and the magnitude of this movement is directly related to the length of the deck.

“For shorter decks, such as the Distillery Rd Bridge, conventional expansion joints are not needed. Instead, thermal expansion and contraction can be accommodated by buried joints located at either end of the bridge, which allows for the small movement of the flexible abutments.

“Bank pad abutments were cast onto a single row of piles in two lifts. The first abutments were cast above new piles, which varied in depth between 13m and 19m, at either abutment end. The pile diameters were designed with sufficient flexibility to accommodate deck movement and the back-ends of the abutments were backfilled with rounded aggregate, which allows for ratcheting movement within the fill and prevents the build-up of stresses behind the abutment,” explained Van Wijk.

A concrete approach slab (foreground) and the concrete deck of Phase 1 prior to the installation of T-shaped suspension bracketing.

Cape Concrete cast nine 22.8m prestressed beams for the new structure: seven internal beams weighing 25 tonnes apiece; and two edge beams at 35 tonnes each. The edge beams are non-symmetrical with thicker sides and flat face finishes.  Four beams were used in the construction of Phase 1 on the southern side of the bridge and five are being used for Phase 2 on the northern section. Both edge beams were designed to support the cantilevered walkways with protruding transverse pull-out bars.

Martin & East construction manager, Ricardo De Sa says that a temporary barrier was placed on the old bridge during the construction of Phase 1. 

“Once the piles had been sunk, seven in Phase 1 and six in Phase 2, they were trimmed to the bottom of the foundation level before the first-lift abutments were cast. The second lifts act as diaphragm beams, tying all the precast beams together at either end of the bridge. Moreover, we cast concrete jockey slabs between the second abutments and the feeder road sections at each end to act as an interface between the solid concrete of the bridge deck and the more flexible road surface,” said De Sa.

When the first-lift abutments had reached the requisite seven-day strength, the precast beams were lowered onto temporary steel bearings which had been cast into the first abutment lifts of both phases. In addition, matching bearing steel plates were cast into the beams’ soffit sides. After being lowered into position the beams were tied together at the bottom of the lower beam flanges with transverse reinforcing which was covered with a 150mm layer of in-situ concrete.

The soffit sides of Phase 1 beams and the old central pier.

The installed beams were capped with a permanent Nutec formwork and reinforcing for the 175mm-250mm cast in-situ bridge deck. The deck was cast with protruding rebar to tie into and support the walkways’ cantilevered reinforcing.

“Apart from some minor staging at each end of the bridge, the walkways are being constructed without ground-supported staging due to environmental concerns. This meant we had to come up with an alternative method of supporting and constructing the cantilevered platform.  

“We opted for inverted steel-girder T-shaped suspension brackets, designed and manufactured by Formscaff, from which the support work and formwork could be hung.  The brackets, which extended 2.4m off the edge of the deck, were mounted on two pivot-point jacks,” explained De Sa.

 

The edge beam in Phase 1 is lowered into position.

In addition, the brackets’ horizontal sections were secured to the underside of the deck with diwydag bars, and once all 24 T-brackets had been attached to the deck they were linked together with steel girders and poles to form an integrated support unit. Girder sections were also attached to the underside of the deck to support the Econoform panels used for casting the walkway’s sloped soffit surface.  

Six transverse cantilever beams support each walkway slab. They include voids in the concrete to save weight and make provision for future service pipes. The cantilever beam reinforcing was spliced together with the Y20 rebar rods which had been cast into the top deck. They were also spliced with R20 inclined pull-out rebar which extended from the bottom of the edge beams.

The edge beam in Phase 1 is lowered into position.

Additional formwork support was provided by tension cabling which was attached to the top of the vertical bracket sections and the lower support girders on the one side, and from the top of the bracket sections and the end of horizontal girders on the deck side. Once the walkway concrete attained an early strength of 60% of the required 40MPa rating, the cabling was de-tensioned.

After all the support work had been removed, a further 20mm of concrete was added to the top surfaces of the walkways rendering them flush with the 40mm premix layer on the bridge deck. This additional layer includes several service ducts and slopes at 1.5% for stormwater drainage. The bridge itself slopes at 1.0% from west to east for drainage.

The M-type beams were cast at Cape Concrete’s yard in Cape Town as a portal framework based on a bending schedule and rebar drawings supplied by AECOM. They comprise a wide bottom flange, a narrow web portion and a flared upper flange section.

Workers insert rebar through sleeves situated in the beams’ lower web sections.

“We used a W50 mix with a super plasticiser in a 100 slump flowable concrete which complied with three durability criteria: oxygen permeability; water sorptivity and chloride conductivity,” said Cape Concrete director Johan Nel. “The plasticiser reduced water consumption and gave us better workability. What’s more, we achieved a transfer strength of 40MPa which was reached in 18 hours using the high-strength mix design and steam curing.

“After de-tensioning, the beams assumed a vertical positive camber of between 15-20mm, but once they were positioned on the bridge and the in-situ deck slab had been cast, they assumed a neutral camber.

“Reinforcing is congested when using prestressed strands which is why we used a 13mm stone in the mix. It is vital for the concrete to flow through the rebar and fill the entire structure without air pockets, and smaller stones facilitate this process.

“We used external vibrators and pokers to remove all the air, but we only used pokers on the top flange sections.

“One of the plusses in M-type beam casting is that the bottom flange dimensions remain constant. If larger spans are required one simply adjusts the web or top flange depths. This means one can use an adjustable mould from an M2 to an M10 beam and M3 beams were used at Refinery Rd.

The second beam in Phase 1 is lowered into position onto the first abutment lift.

“In this instance, the webs were a bit higher and the top flanges a bit deeper. But because the profile did not vary the beam casting was economical. Engineers often design using available moulds and if only small adjustments are required, it saves costs and reduces lead times,” concluded Nel.

Approximately 60m of feeder road on either side of the bridge is being resurfaced and these new road sections will be bordered with new pavements.

Project team

Client                                         Stellenbosch Municipality

Consulting engineers                 AECOM SA (Pty) Ltd

Main Contactor                          Martin  & East

Precast beams                          Cape Concrete

Support work                             Formscaff

Heritage architects                    Rennie Scurr Adendorff Architects

Heritage consultants                  CTS Heritage

 

 

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