Lynnwood Glen Pedestrian and Pipe Bridge
The new Lynnwood Glen Pedestrian and Pipe Bridge replaces the existing concrete bridge, which is not wide enough for the new road upgrades passing underneath the bridge.
The widening of the Atterbury and Lynnwood Interchanges comprises four through traffic lanes, in each direction, plus two collector-distributor lanes totalling 12 lanes in all.
The existing pedestrian and pipe bridge did not meet the required horizontal and vertical clearances of the new freeway layout necessitating development of a new bridge.
The Gauteng Freeway Improvement Project (GFIP), though causing motorist teeming headaches with current construction efforts, will ultimately relieve stresses of traffic congestion upon its completion. Arching over a section of the GFIP is a beautiful bridge erected for the South African National Roads Agency (SANRAL) as part of the GFIP between the Atterbury and Lynnwood Interchanges.
BEFORE AND AFTER
The existing bridge consists of two separate adjoining decks, one supporting the two pipelines and the other for pedestrian access. Each deck comprised a two-span continuous post-tensioned, cast in-situ beam and slab structure. After analysing a various options, a structural steel arch bridge with diagonal members proved to be the best solution.
The new structural steel bowstring arch bridge, as proposed by SANRAL was adopted by the design team as the preferred and optimum solution considering overall risk, cost and aesthetic appeal. The arch configuration is a hybrid between a warren truss girder bridge and a bowstring arch bridge and included the advantage of quick erection within the constrained working space.
The process of lifting the 142 ton bridge made use of the biggest crawler crane in South Africa, with the lifting capacity of 850 tons. Once the bridge was placed, its weight will increase to about 400 tons through the installation of handrails, a concrete walkway and two water mains, which will all be in place by May 2010.
The bridge is now 72.5m long with a 6m wide deck and 13 m high (at midspan). The pedestrian walkway is a 150mm thick composite steel and concrete deck spanning between the cross bracing and the main bottom chord members. The water pipes from the existing bridge will be replaced with steel pipes on the new bridge. One of the newer and larger pipes will be used as a supply main to the Lynnwood Reservoir, while the other will form part of the potable water reticulation system feeding Lynnwood Glen from the Lynnwood Reservoir.
MAKING THE BRIDGE
The structure was created in six abnormal segments consisting of four off 6m wide by 31 m long and two 6m wide by 10m long. The sheer scale of the fabricated segments, alterations had to be made to the fabricator’s plant doors for loading and transporting the bridge’s segments out of the premises.
Bolted end connections to the ends of the plate box segments with internal bolted connections were introduced to simplify assembly of the huge segments on site. After the assembly and lining up of segments on site, the internal bolted connections were closed with external plates, which gave a smooth external finish on the bridge structure with no visible bolted connections.
After completing all the bridge connections on site, it was time to erect the bridge into place over an active highway. Traffic was stopped, while the 142-ton bridge was lifted into place. Heavy lifting specialist were brought in to achieve one of the ‘heaviest lift’ ever on South African national roads to date. It took a mere 30 minutes for the crawler crane, with a lifting capacity of 850 tons, to achieve this task.
PIONEERING DESIGN
SANRAL’s special requirement for oblong diagonal hangers, though adding a pleasing aesthetic curved appearance to the bridge, proved quite challenging to the design team and steel fabricator, as this request was a first in the South African industry. Another bright idea for the bridge will be visible in early evenings and nights, as a specialist lighting designer used effect LED lights on the main structural elements to enhance the appeal and increase the bridge’s visibility.
The design of the bridge is relatively unique in South Africa. The bottom chord of the bridge is usually supported from the main arch via cables, but in this case made use of oblong shaped diagonals to give the arch. The oblong members were purpose manufactured using an outer skin rolled from 3mm plate cladded around a 165 by 6mm internal pipe performing the structural work.”
The Lynnwood Glen Pedestrian and Pipe Bridge, while being a structure that satisfies ‘form fitting function’, it is also an aesthetically pleasing and landmark structure for both the motorist and the pedestrian alike. It is unique, in that it there is no other bridge like it across a national highway in the country.
PROJECT TEAM
| Client | Sanral (South African National Roads Agency Limited) |
Conceptual Design & Architecture |
SANRAL and ARQ |
Detail Design of Steel Arch & Engineer |
ARQ Consulting Engineers |
Steel Fabricator |
CADCON |
Main Contractor (joint venture) |
BRCD (Basil Read, Roadcrete, Chavani and Dipcivils) |
SANRAL contact person |
Mr.Edwin Kruger +27 12 426 6038 |
ARQ contact personrs |
Mr. Tiaan Kramer +27 12 348 6668 |
