In 2013, the City of Ekurhuleni Metropolitan Municipality embarked on an upgrade of their existing transportation route planning and infrastructure in the form of an Integrated Rapid Public Transport Network (IRPTN). The City is now set to kick off the first official run of its IRPTN service, Harambee, as part of the implementation of its Phase lA operations. The limited service will run eight buses in mixed traffic along the 38 km service route, with 33 kerbside stops, between Tembisa and Isando.
The full roll-out of the Harambee Phase lA service includes the introduction of new elements into the system until it is ready to operate as a fully-fledged BRT (Bus Rapid Transit) scheme. Integral to the project is the construction of nine new BRT Bus Stations (see the Station layout in Figure 1 below) which run into the heart of Tembisa. These new Bus Terminals and pedestrian bridges are the projects submitted here for consideration.
The overall project scope includes the upgrade of the existing road facilities, to add new BRT lanes with sidewalk and median, as well as the construction of nine new station facilities. Due to the overall road width increase, several structures are required to be extended and upgraded – these include a road culvert water crossing and several rail culverts that cross the main road (Andrew Mapheto drive) that traverses through central Tembisa.
With the exception of the pedestrian bridges, the typical layout model that has been deployed throughout the project is similar to that which has been used in the Johannesburg CBD and Cape Town BRT systems. The Stations represent central node points that facilitate the easy access of pedestrians into the BRT system, and the brief to the Architect was to design an aesthetically pleasing solution that provided such access.
The elements generally incorporated into a typical Station are the following:
- A median area where passengers are noused in a dedicated enclosed structure that provides direct access to the bus route and BRT lane,
- the pedestrian bridges that cross over the road intersections, allowing pedestrians the ability to gain safe access to the median area,
- the various interlinking walkways and stairs,
- the lift shafts that allow users to transition from the Ground level to the First-Floor level and back
The original concept developed by the Architect and Structural Engineer is very similar to the final design that is currently under construction, which involves a curved steel truss type arrangement, made out of mostly circular hollow sections. Structural steelwork was envisaged on this project due to its ease of use for the required spans, the ability for easy erection over existing roads, as well as the overall slim look and feel that arises out of its use in the design.
The bridges incorporate a composite structural steel/reinforced concrete slab design. The top and bottom girder chords are constructed from circular hollow sections and the design has a curved top chord that is set at an angle relative to the main floor structure, making the bridge took “open” as one travels from one end to the other. Due to deflection requirements, the bridges are given a slight vertical preset to allow the deflections to normalise under dead and live load conditions. The vertical girder members are given outward curved radi, and a similarly radiused plexiglass f de is attached using aluminium mullions – the fac;ade envelops the entire truss frame, giving the bridge an aesthetically appealing look and feel, To facilitate drainage on the bridges, a longitudinal fall is provided between deck interface platforms and full-bore outlets are used on the bridge ends to facilitate the discharge.
The longest pedestrian bridge has a span of 36m and therefore it is long enough for vibration considerations to play a role. A LUSAS model was therefore developed to simulate the effects of pedestrian crowds as well i:IS single dynamic pedestrian users moving over the bridge. The analysis showed that that the maximum accelerations developed on the bridges are not in excess of the 0.5m/ s2 allowed by the relevant Codes. Acceleration results were further compared with the SETRA Pedestrian Bridge design guide as well as the TMH7 guidelines.
The Lift Tower shafts and their supporting structure represent key structural elements that directly interface with an unusual curved supporting element that takes the bridge load directly down to the foundation level after first wrapping its way around the Lift Shaft Tower. The lift shaft itself is also enclosed in an aesthetic plexiglass facade, providing cover to users of the lift as well as protection for the lift equipment. Louvered vents through the facade are also required due to the heat-build inside the Lift Shaft Tower structure.
From a design point of view, the interfacing of the plexiglass facade with the bridge structure required several Iterations to ensure that a robust and workable solution was arrived at. One of the challenges of designing with plexiglass material is that all the connections to the structure need to include a gasket sealing material and gaps are required in connections to allow for a certain degree of plexiglass movement. Interfacing with the 11ft contractor also required a thorough understanding of the details of the levels and falls on the site to ensure that the lift exit and entry points corresponded with the deck slab and ground levels.
The open frame design of the bridge girder members meant that careful attention had to be paid to the stability of the main girder structure in the transverse direction – this meant that any member splicing along the lines of the main transverse framing action required the use of stiff moment connections.
A significant number of Shop drawings were required to be generated for the entire project. 3-D Tekla models were used by the steelwork contractor to facilitate the rapid development and approval of shop drawings. Due to aesthetic requirements, welding for the splicing of main longitudinal members was done on site under carefully controlled conditions.
For each bridge site, road closures were required for the erection of each bridge – this required a significant amount of coordination with the local traffic authorities who were fortunately very obliging. Some 32 tonnes per 36m span were lifted, and a typical station involved the erection of approximately 115 tonnes (Including the Lift Towers) per station.
For this project, the two main Contractors on the project (Stefanutti Stocks and King Civils) were required to think of solutions to the problem of how and when to Install the concrete deck slabs whilst allowing for the concrete creep and shrinkage to occur. This produced a set of interesting and varying opinions, and ideas suggested varied from precast concrete to in situ concrete with permanent formwork.
The design of the Station Bridge structures that are required to harmonize with Lift Shaft Towers presented an interesting challenge to the SMEC South Africa bridges design team. The project demonstrated that there is still a significant preference for the use of structural steel for pedestrian bridges in this country.
|Completion date of steelwork||± September 2018|
|Completion date of full project||± April 2019|
|Tons of structural steel used||725 Tons|
|Structural profiles used||Chs, Ub, Uc, [, Angle|
|Client||Ekurhuleni Metropolitan Municipality|
|Structural Engineer||Smec South Africa|
|Engineer/Project Manager||Lte Consulting|
|Main Contractor||Stefanutti Stocks / Khombanani Steel JV|
|Main Contractor||King Civils|
|Steel Erector||Onpar Steel|
|Cladding Manufacturer||Global Roofing Systems|
|Cladding Supplier||Global Roofing Systems|
|Dram Industrial Painters|