Connecting communities with opportunities

By Eamonn Ryan

Award-winning Grayston Pedestrian Bridge connects 10 000 people a day with work opportunities. Leader of this bridge team was Darryl Klassen (PrEng), structural engineer: Transport & Planning: Africa at Royal HaskoningDHV. 

Royal HaskoningDHV Aug2018

The Grayston Pedestrian Bridge forms part of a major pedestrian and cycling corridor built to connect residents of Alexandra Township to Sandton, over the busy M1 freeway.
Image credit: ee Publishers

The design and delivery of the Grayston Pedestrian Bridge in Johannesburg won consultant Royal HaskoningDHV four awards at the 2018 SAICE Johannesburg Regional Awards: the Most Outstanding Civil Engineering Achievement – Community-based Category; the Most Outstanding Project in the Transport Engineering Category; it came second in the Technical Excellence Category; and third in the Most Outstanding Project in the Structural Engineering Category.

Klaasen


Darryl Klassen (PrEng), structural engineer: Transport & Planning: Africa at Royal HaskoningDHV.
Image credit: Royal HaskoningDHV


The Grayston Pedestrian Bridge also received a Commendation in the category ‘Projects with a value between R50-million and R250-million’ at the 2018 CESA Aon Engineering Excellence Awards, followed by the award for the Most Outstanding Project in the Transportation Engineering Category at the 2018 SAICE National Awards held in Cape Town in October.

The Grayston Pedestrian Bridge forms part of a major pedestrian and cycling corridor built to connect residents of Alexandra Township to Sandton, on either side of Johannesburg’s notoriously busy M1 freeway. Every day, in excess of 10 000 people make their way across the busy M1 highway in Johannesburg from Alexandra Township to their jobs in the affluent business hub of Sandton — this mere 5.2km journey used to take them hours across busy roads. 

The City of Johannesburg decided to make this walking route safe and convenient — and the Great Walk project was born. The Grayston Pedestrian Bridge forms an integral part of this project and now provides pedestrian and cyclist access into Sandton. It literally bridges the gap between two communities with opposite economic backgrounds while creating a visually pleasing gateway into the economic hub of the country.

Klassen presented a paper he wrote on the Grayston Pedestrian Bridge at the IABSE Vancouver conference in September 2018. “It is exceptionally fulfilling to proudly present the work one has been involved in for three years to the top structural engineering professors and consultants in the world.”

Originally from Canada, he graduated with a BSc Engineering (Civil/Structural) degree from the University of Western Ontario in London, Ontario. After graduation, he immediately started working for a civil engineering consultant in Ontario. “First, I started in the transportation geometrics group but after four months there, saw an opening in the bridge group so I put my name forward and made the switch to structures, which is where my passion is. From that point, I have never looked back and always have been in structures.”

Klassen has had a number of mentors throughout his career. “I have always enjoyed structures, probably because my father was a residential contractor and I spent many summer holidays helping him build houses. The great thing about designing structures is that you can physically see the product of what you have designed and have a chance to leave your mark on the landscape. During university, I had a professor, Michael Bartlett, who I really connected with as he made structural design interesting and fun. This really solidified my choice to pursue structural engineering,” says Klassen.

He began mentoring staff even while he himself was still on the receiving end: “Mentoring staff is so important and is a way to give back and help others along their path. I started mentoring staff probably three years into my career, even though I was still being mentored by someone else. There is always someone less experienced than yourself who can be helped. Mentoring staff will be a continuous thing until my retirement.”

A view to aesthetics

Commenting on the lessons he has learnt along the way, he says: “Over the years, I have learnt that proper detailing and considering aesthetics are very important. This project was personally important to me because it challenged and took me out of my comfort zone. I led the preliminary and detailed design of this bridge, which is the first cable-stayed bridge that I have designed. Pedestrian bridges really give designers a great chance to evaluate the site conditions and come up with the most appropriate design. They also provide more opportunities to enhance the aesthetic flair of the bridge.”

Building Information Modelling (BIM) was used for the design and drafting of the bridge. Design is likely to become even more exciting in the near future as BIM develops. Klassen says: “BIM is really starting to take off in the transportation sector — although it has been around for a while in building infrastructure. In fact, 3D design/drafting programs like Civil 3D and Revit are now being used to communicate transportation designs and not just rely on 2D drawings. In five years, BIM/3D designs, visualisations, and virtual reality presentations will become mandatory by our clients. The rapid change of technology, 3D design, automation, and parametric design are already on our doorstep. New civil engineering graduates will not only need to be a civil engineer but also be a computer programmer to have a real edge in the future.”

Klassen was one of a team, he says, as this was a complex multidisciplinary project involving structural engineers, geotechnical engineers, wind experts, and architects. The primary concerns were safety (strength, stability, robustness), serviceability, economy, constructability, aesthetics, and low maintenance.

The project was envisaged as a community-focused initiative incorporating sustainable best practices such as local labour and design approaches intended to minimise future maintenance needs. While the project has achieved the original design goals, there were many challenges.

Tragedy struck in the early stages of the project, when the temporary works over the M1 collapsed with two fatalities. While the temporary works were not part of the overall responsibility under Royal HaskoningDHV, clearly it had a significant impact on delivery. 

“The collapse of the temporary works during construction had a significant effect on the completion of the project. It was important to respect those who lost their lives and were injured. In addition, the investigation into the accident needed to be carried out thoroughly. Although contractually the completion date was not met due to the reasons mentioned above, the permanent bridge reached practical completion on 16 March 2018, which generally met the client’s expectations,” says Klassen.

Project specs

The completed bridge has eight spans with a continuous post-tensioned concrete box girder deck. The main span over the M1 freeway is supported along its centreline by cable stays. The cables are supported by a concrete pylon with a height of 54m above deck level. The pylon was supported by backstay cables anchored into the ground. The length of the cable stays vary between 25m and 114m, with the majority of the cables around 60m in length. Redundancy was built into the design so that one cable at any location could be removed for maintenance.

At the west end of the bridge, the poor quality of the fill required piled foundations socketed into competent rock to resist uplift forces from the cable stays. Although less extreme conditions were encountered at the other founding locations, piled foundations were used for the entire bridge structure.

The pile type used was an augured cast-in situ concrete pile with minimum diameter of 900mm. These piles were socketed 1m into the hard granite or diabase rock. For the cable-stay anchor blocks and main pylon, ground anchors in the form of 32mm-diameter DYWIDAG bars were installed and grouted to a depth of 10m below the base of the piles to resist uplift forces. 

Records indicate that this is the first cable-stayed bridge in South Africa supported by back-stay anchor blocks instead of being supported within the deck itself. The main deck is supported by a single row of cables in the middle of the deck, which blends in well with the garden planter box area.

Grayston bridgeNight view of the Grayston Pedestrian Bridge.
Image credit: Royal HaskoningDHV

Meeting client expectations

For aesthetical reasons, steel railings and lighting posts were used along the edges of the deck. Additionally, steel anchor plates were used in the pylon for attaching the cable stays. Because of increased durability and long-term appearance, stainless steel was used in the construction of these components. The cable-stay anchor plates were constructed using grade 316 stainless steel. Grade 316L stainless steel was used for the railings and lighting posts.

A garden area dividing cyclist and pedestrian lanes was incorporated to enhance the user experience and incorporate a park-like theme. Seating benches were also incorporated in the garden area allowing people to rest and enjoy the surroundings.

“Overall, this was a challenging yet tremendously rewarding project. The completed bridge is both functional and aesthetically pleasing. The fact that it serves such an important socio-economic purpose is fundamental to the sense of achievement the whole team gained from the work. As a company, Royal HaskoningDHV states its vision as ‘enhancing society’ — and we have certainly achieved that with the Grayston Pedestrian Bridge,” says Klassen.

<pull-quote> Pedestrian bridges are a lot of fun and really give designers a great chance to evaluate the site conditions and come up with the most appropriate design. They also provide more opportunities to enhance the aesthetic flair of the bridge.

 

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