Lynnwood Glen Pedestrian and Pipe Bridge

Lynnwood Glen pedestrian and pipe bridge

PURPOSE OF THE PROJECT, APPROACH TO 1 THE DESIGN AND GENERAL FEATURES The new Lynnwood Glen pedestrian and pipe bridge is part of Package D2 of the GFIP (Gauteng Freeway Improvement Project). The bridge is located between Atterbury and Lynnwood roads on the N1 in Pretoria. The old post-tensioned concrete pedestrian and pipe bridge posed a major re- striction to the N1 freeway improvements between the Atterbury and Lynnwood interchanges. The bridge openings were insufficient and could not accommodate all the lanes of the new widened freeway. Due to the shape of the old bridge, the vertical clearance near the abutments was also insufficient. The purpose of the new bridge was to resolve these restrictions. The bridge, as well as being there for pedestrians, carries two steel water mains across the N1. These two pipes, 675 mm LYNNWOOD GLEN PEDESTRIAN AND PIPE BRIDGE (GFIP D2) diameter and 300 mm diameter respec- tively, belong to the City of Tshwane Metropolitan Municipality. The 675 mm Technical Excellence category diameter pipe is used as a supplementary supply main to the Lynnwood reservoir, mainly during the summer months when KEY PLAYERS domestic water demand is high. The Client SANRAL Professional Team DCA Joint Venture comprising 1 Aerial view of completed DEC (Pty) Ltd, Civil Concepts (Pty) Ltd, ARQ (Pty) Ltd Lynnwood Glen pedestrain and pipe Main contractor BRCD Joint Venture bridge across the N1 in Pretoria Major Subcontractors and Suppliers CADCON Steel Construction and Engineering

Civil Engineering | December 2010 79 300 mm diameter pipe is the main supply either by jacking the pipe under the to the portion of the water reticulation freeway or by relocating the pipeline system east of the N1 and can thus only over the Lynnwood Road bridge be closed down for a maximum of eight NNReplacement of the bridge with a new hours during off-peak periods. structure. Various options were looked at in The first two options proved to be prob- order to meet the requirements of the new lematic, for the following reasons: freeway widening: NNThe structural system of the old bridge NNRaising of the old deck and modifica- included ‘hold-down’ or tension an- tions of the abutment in order to pro- chors within the abutments. The option vide the desired clearances of modifying the abutments and raising NNRemoval of the pedestrian access alto- the deck was therefore not feasible. gether and relocation of the pipelines In addition, the risk of damaging the

2 Lynnwood Glen pedestrian and pipe bridge – the old and the new 3 Assembly of steel bridge structure completed alongside the N1 4 Lifting and erecting the 142-ton structure

80 Civil Engineering | December 2010 pipelines whilst the bridge was being NNThe new bridge spans the full length NNAesthetical and environmental modified would have been too great. thereby removing the need for the blending – the main arch members NNThe permanent removal of the pedes- construction of new piers in the median lean towards the centre line of the trian access was also not desired by the which would have been difficult due to bridge, the cross-bracing members are City Council and the local residents. access restrictions made up of circular tapered sections, NNJacking a culvert for the pipelines under NNThe bowstring arch bridge in addition the bridge is white with light grey the N1 would have been very difficult produced a feature which is quite fitting handrails and a darker grey pipe. All due to the depths of the jacking pits in within the surrounding areas of intense these features highlight the structure close proximity to the private proper- commercial development. during the day, while effective lighting ties in the area and the restrictions it The design approach was to attain the at night does the same. would pose on the traffic flow. following: NNDeviation of the pipelines, for example NNNecessary clearances – the new bridge CONSTRUCTION TECHNIQUES AND SPECIAL over Lynnwood Road bridge, would provides a vertical clearance of 5,8 m and MEASURES THAT WERE REQUIRED TO have been very expensive and it would the span is 72,5 m, providing the required ACHIEVE COMPLETION have been difficult to accommodate a horizontal clearance for the extra lanes. To minimise the areas of adjoining 700 mm diameter pipe over this bridge. NNRelatively easy construction – the properties that SANRAL (South African The option of replacing the bridge with a bridge was designed so that a signifi- National Roads Agency Ltd) needed to new structure was thus preferred. During cant portion could be fabricated in a expropriate, and to minimise the pipeline a workshop between SANRAL and the de- workshop and it could be transported realignments, it was decided to construct signers it was decided to opt for a steel bow- in separate segments and be assembled the new bridge as close as possible to string arch bridge as depicted in Figure 2. alongside the road. Once assembled the old one. The old bridge consisted The following were some of the contractor could lift the entire of two decks, one for the pipes and the the reasons: steel structure into its final position. other for pedestrians. It was necessary to NNThe walkway levels and pipeline invert NNMinimum traffic disruption – the demolish the old pedestrian portion first. levels remain virtually unchanged, traffic would only be disrupted The bridge was saw cut into 5 sections whilst still providing the required 3 times – twice for the demolition of and the sections were removed by crane. bridge clearance the old bridge, and once for the erec- Temporary supports were placed at calcu- NNRemoval of the old bridge and the in- tion of the new bridge. lated positions to ensure that the release stallation of the new structure are split NNMinimum water supply disruption – the in the prestressed cables from the old into independent activities, thereby re- transition of the pipelines from the old bridge would not cause a collapse. ducing the impact of disruption to both to the new bridge necessitated careful The new bridge was created from six traffic and water supply planning and construction phasing. segments which were fabricated at the

Civil Engineering | December 2010 81 5 5 Leaning arch members and tapered cross bracing members

portion of the bridge was lifted into place by one of the biggest crawler cranes in South Africa with a lifting capacity of 800 tons. The bridge was temporarily placed on jacks to allow for the curing of the permanent bearing grout. Once the new bridge was in place, the deck slab was constructed utilising concrete. Not long after, the new pipelines were installed over the new bridge and the tie-ins were made at the ends. This allowed the contractor to then demolish the pipeline-carrying portion of the old bridge.

CONCLUSION factory individually and then transported plates, rendering a smooth external finish The replacement of the Lynnwood Glen to site by abnormal-load vehicles. The six to the bridge structure. pedestrian and pipe bridge was a chal- segments were assembled at the side of the The assembled bridge was lifted into lenging project and the resulting struc- road using specially hidden bolted con- its final position utilising four lifting ture has already become a landmark in nections. After the assembly and lining hooks that were designed to take the the Pretoria area, the more so as there is up of segments had been completed, these forces. The traffic was diverted through no other bridge like it across a national bolted connections were closed with cover town while the 142-ton structural steel highway in the country.

82 Civil Engineering | December 2010