Repair Of Bridge No. 422 (2008)

This bridge repair project clearly portrays the art and science of civil engineering in terms of technical excellence.  It demonstrates how innovative answers were found for the challenging demands of structural stability, safety, and economy.  The repair technique implemented is unique and displays engineering ingenuity in the practical, efficient and structurally sound approach followed.  The bridge is an important link between the Free State, Transkei and Eastern Cape that is now restored to its original historic glory.  Notwithstanding the fact that the bridge had to be closed for traffic during repairs, good public relations ensured that no incidents took place.

Description Of The Project

Bridge No. 422 is located on Route P53/1 (R726) between Zastron and Sterkspruit where it crosses the Orange River near Sterkspruit on the farm Mayaputi.  It was constructed in 1934 when pre-manufactured steel was imported from the United Kingdom and erected in South Africa. These are historic bridges and concerted efforts should be made to maintain them for posterity. 

The superstructure consists of two simply supported, steel truss spans of 61 m (200 ft) each supported by a concrete substructure with a roadway width of 3 m (10 ft).  The truss members consist of laced, riveted sections.  The bridge was seriously damaged as a result of vehicular impact to certain of its structural members. 

THM Engineers Free State cc was appointed to evaluate the deficiencies, find structural solutions and compile contract documentation for the urgent repair and rehabilitation of the bridge.

Design Approach

Since this is a narrow single lane bridge, the bridge had to be closed for the whole construction period.  The contractor had no other space than the bridge deck to work from for scaffolding, rigging equipment, welding machines etc.  Closure of the bridge would have had a major impact on the local community and long-distance haulers.  Therefore, the point of departure was to complete the repair work in the shortest possible time. 

In a statically determinant truss bridge, the removal of any compression or tension member for repair will lead to an immediate and complete collapse of the bridge.  The traditional engineering solution would have been to construct very costly coffer dams and prop the complete bridge 30 m high from the river bed to make the removal of the damaged members possible.  Considering the risk of the known frequent flash runoff of the Orange River the traditional approach was not an option.

THM displayed innovation and engineering ingenuity with a unique “repair in the air” design solution satisfying the challenging demands of structural stability, safety and economy.  The challenge was to find a repair method that allows for the safe removal and replacement of damaged structural members while the damaged bridge supports the contractor’s workers and construction loads.

Repair Technique

The following “repair in the air” technique was followed to replace all damaged vertical compression members while they were carrying the full loads due to the bridge’s own dead load and applied construction loads.  Two temporary compression struts capable of supporting the full load due to dead weight and the construction loads were installed adjacent to each damaged member.  To ensure that the full load was carried by the temporary struts a simple though effective and functional support bracket was designed to pre-stress the struts in a very controlled manner before the damaged vertical laced member was removed.  Due to the complexity of the riveted connections, it was not possible to install a replacement member as a unit.  The four angle iron legs of the laced member had to be built up one by one.

The diagonal laced members are tension members and do not need composite action for stability as each angle iron leg of the member is in tension.  Tension force influence lines that were obtained from analyses were used to cleverly position heavy construction loads such to reduce the forces in the member under repair. The result was that it was possible to replace the damaged diagonals by removing and replacing the legs one by one without the use of expensive temporary tension cables while they had to resist the influence of the full dead weight and construction loads. 

An elegant steel solution

Due to the fact that the bridge is a single lane structure with limited workspace, it was, from the outset, envisaged that repair could not take place under traffic conditions.  Long distance traffic could preplan for this eventuality; however, it was problems associated with the locals that proved a thorny issue with potential political implications.

Many locals travel between the two towns on a daily basis due to work commitments as well as for the purchasing of goods.  The only practical alternative which could be considered was to accommodate pedestrian traffic over the bridge at certain stipulated times and under certain conditions as determined by prevailing construction activities.

This arrangement allowed for Taxi operators from one town could transport their passengers to the bridge; passengers could then walk across the bridge; taxi operators from the other town could then pick them up to complete the journey. Persons making use of their own transport could leave their vehicles at the bridge and arrange for transport on the opposite side.

Being fully aware of the generally aggressive attitude adopted by Taxi Associations, the Engineer and the Contractor realised that this issue had to be sorted out prior to the closing of the bridge.  A meeting was arranged with the Taxi Associations of Zastron, Sterkspruit and Aliwal North, during which all the implications and eventual advantages of the Project were discussed at length and in detail.  

By observing the daily “crossings” of the bridge one became amazed at the vital role a bridge such as this plays in the lives of a local community.  

This was indeed an out of the ordinary technical project, but also one which had close ties with the local community.   

Project team

Developer/ Owner:  Department Public Works, Roads & Transport, Directorate Land Transport Planning
Structural Engineer:  THM Engineers Free State cc
Project Manager:  THM Engineers Free State cc
Main Contractor:  Tanekk cc
Steelwork Contractor:  Tanekk cc
Other:  Rand Sandblasting Projects (Pty) Ltd

7th Avenue Bridge (2010)

Designed as a cable-stayed bridge, the unique feature of this design is that it does not have a counterweight rear span. Typical cable stayed bridges – like, for example, the 2003 Steel Awards overall winner, the Nelson Mandela bridge –  have a long steel clear span with a short counterweight span in concrete.

In the case of the 7th Avenue Bridge, the traditional short span concrete counterweight span is replaced with an outwards leaning concrete tower column which transfers the forces in the cable stays both axially and by bending moment into the footing. According to the submission, the absence of a counterweight span gave the team an opportunity to do something ‘very elegant’.

 “When a functional concrete arch bridge spanning the N1 highway in the North-west corner of Johannesburg needs to be replaced and the replacement bridge is a combination of steel and concrete where the strengths of both materials are used exactly as they should be, we regard it as a coup for the industry,” the submission added.

The Steel Awards, held on the 15th September 2010 concurrently in Johannesburg, Durban and Cape Town was hosted by the Southern African Institute of Steel Construction (SAISC) with The Aveng Group the main sponsor.

In its brief, the client, South African National Roads Authority Limited (SANRAL) said, inter alia, that the 7th Avenue Bridge contract was an opportunity that should be used to create a special structure that would be seen as a ‘gateway’ to Johannesburg.

The judges said that the result certainly did justice to this request. “This is one of the most utilised pedestrian bridges in the greater Johannesburg area used, in the main, by scholars crossing the busy highway. We have no doubt that both SANRAL and Johannesburg will be proud of it from both aesthetic and engineering points of view.

Structurally, the walkway and cable stay ends are tied to a Toblerone circular hollow section truss creating a clear arched span over the freeway and while the absence of a counterweight span did give the team an opportunity to do something very elegant, a load path for all the forces at the top ends of the cable still had to be created.

In this regard, the concrete tower leans back from the road, creating an axial path down the concrete that redirects all those sloping cable forces through the concrete structure down to the ground. Another advantage of the concrete is its bulky mass with the ability to withstand the rogue forces a vehicle smashing into the concrete work would impart to the structure.

The use of a concrete (composite) walkway surface (deck) makes sure that concrete loads are all compressive in their nature to suit the strengths of the concrete. The steel carries the tensile forces where it is strong as well as some of the compressive forces.

The judges noted that dealing with the ends of the cables is one of the intricate issues in designing, detailing and building a bridge of this nature. Each slope is different and each angle of intersection between the cable and the concrete tower is different. “This requires immaculate attention to detail, which is an omnipresent feature of this job,” they said.

They added that the 7th Street Bridge was a great example of ‘the right material in the right place’ and this, combined with the quality of the steelwork and the excellent presentation overall made it a high quality and elegant solution.

“This is a wonderful addition to Johannesburg’s North Western gateway and the lighting at night makes it truly exceptional. It is clearly an example of excellent use of steelwork truly deserving this award in the bridge category,” they concluded.

Project Team

Developer/ Owner: SANRAL
Architect: Professor Glen Mills / SANRAL  
Structural engineer: SSI Engineers and Environmental Consultants
Main contractor: WBHO
Steelwork contractor: Omni Struct Nkosi (Pty) Ltd
Detailer: Omni Struct Nkosi (Pty) Ltd

Wupperthal Pedestrian Bridge

Safe and reliable pedestrian access is a frequently neglected aspect of rural life in South Africa. In many areas, adults and children are forced to traverse rivers via unsafe paths along river courses, low level bridges prone to flooding, or unmaintained and dangerous pedestrian bridges. The Western Cape Department of Rural Development and Land Reform has identified this as an area requiring intervention after numerous accidents and fatalities caused due to unsafe river crossings and poorly designed and maintained infrastructure.

Near Clanwilliam in the Cederberg Mountains is the small town of Wupperthal, located in a pristine valley in an unspoilt part of the Country. Wupperthal is split by the Dassieboskloof River.  The existing pedestrian bridge over the river was poorly maintained and is no longer in a usable condition. For most of the year, the river is easily traversed on foot, however, during heavy rains, the river floods and crossings are no longer safe. The Western Cape Department of Rural Development and Land Reform appointed iX Engineers (Pty) Ltd to assess whether the existing bridge was adequate to meet its intended purpose. It was found that the bridge was not safe and a new bridge was required.

SMEC South Africa (Pty) Ltd were appointed as a sub-consultant to iX Engineers (Pty) Ltd to propose  possible replacement options for the existing bridge and to provide the detailed design of the approved proposal.

The aim of the new crossing was to provide a simple structure that would blend in with the natural surroundings. It was decided to traverse the river with a single span, preventing the need for piers within the river course and the consequent environmental disturbance which could arise as a result there of. A concrete beam bridge was judged to be too heavy to span the long distance, which might have appeared out of place in the natural setting. Steel provided the perfect solution to enable a simple and slender structure, yet still robust enough to withstand debris impacts during flood events. The applicability of the use of steel in rural and remote contexts was illustrated by the fabrication of the structure in Cape Town and subsequent transport to the site. The colour of the steelwork, which was chosen through a participative process with the local community, ensured that the bridge blended in with the local surroundings, and to create stewardship for the bridge among the community. A steel solution also presented the opportunity to be innovative with the design and sculpt a unique and creative form. 

The chosen solution is a tapering, 36 m single span, steel plate through girder bridge.

The deck superstructure consists of two girders created from an assembly of welded plates to form a varying depth beam.  A semi-circular void in the girders was introduced at mid span to give the bridge an arch feel, a natural form that suited the setting. A compression box was provided on the top flange at mid span to prevent buckling of the compression fibre. The transverse members are steel I-beams with diagonal circular hollow section plan bracing, both of which are bolted to the longitudinal plate girders.

The bridge is a simple and basic structural solution, consisting of a simply supported beam. However, the open form and intricate geometry required a complicated analysis and design process. A three-dimensional finite element plate model was created to perform a detailed buckling analysis and stress check on the plate elements.

The Contractor appointed for the construction of the bridge was Guerrini Marine Construction, who were responsible for the construction of the bridge substructure and transport and erection of the superstructure. The deck superstructure was fabricated in Milnerton by Just Engineering and transported to site in three separate parts with a maximum length of 13.7 m. The three parts were erected on temporary supports on the river bank where they could be spliced together. The splice joints consisted of a temporary bolted connection used to secure and position the structure. A full penetration weld could then be performed to conclude the permanent connection. The completed 36 m long deck could then be lifted and rotated into position on top of the reinforced concrete supports.

The handrail consists of an 80 mm diameter circular hollow section, with a custom-made mesh of 8 mm diameter solid bars, which could be bolted onto the bridge in modules on site. This unobtrusive design was chosen so that the handrail does not detract from the elegant lines created by the aesthetically pleasing form created by the steel plate girders below.

The bridge was positioned with a 0.5 m freeboard above the 1:100 year flood line to reduce the risk of debris impacts during a flood event on the lightweight steel superstructure. A timber walkway was provided to link the bridge with the 1:5 year flood line to ensure that the river could be safely crossed even in minor flood events. The total construction cost of the project was R5.265 million.

The bridge in Wupperthal highlights the potential for steel to be used in an innovative way to achieve social and developmental objectives for the community, combining function and aesthetics, without impacting on the environment. The steel bridge provides a positive landmark to the small town that the local community and Client can be proud of.  

Tons of structural steel used 37 tons
Structural profiles used Plates, I Beams, CHS

Project Team

Project Team Role Company
Nominator SMEC South Africa (Pty) Ltd
Client/ Developer Department of Rural Development and Land Reform
Architect Not provided by nominator
Structural Engineer SMEC South Africa (Pty) Ltd
Engineer SMEC South Africa (Pty) Ltd
Quantity Surveyor Not provided by nominator
Project Manager iX Engineers (Pty) Ltd
Main Contractor Guerrini Marine Construction
Steelwork Contractor Just Engineering
Steel Erector Guerrini Marine Construction
Cladding Manufacturer Not provided by nominator
Cladding Supplier Not provided by nominator
Cladding Contractor Not provided by nominator
Corrosion Protection
Not provided by nominator
Corrosion Protection
Paintwork Contractor
Not provided by nominator
Photographer, Photo competition SMEC South Africa (Pty) Ltd
Photographer, Other submitted images SMEC South Africa (Pty) Ltd

If you were a part of this project, and your company details are incorrect or missing – please notify the SAISC so that the error can be corrected.


Kirstenbosch Centenary Tree Canopy Walkway (“Boomslang”) – (2015)

The project kicked off in early 2012, with Mr Philip Leroux of Kirstenbosch approaching Architect Mark Thomas to design a pedestrian bridge between the treetops of the Arboretum at Kirstenbosch.  Mark requested that Henry Fagan & Partners be appointed as the structural engineers for the project.

Although the project budget was very limited, the entire project team were very excited, albeit rather apprehensive, about the prospect of adding a structure between the trees of this natural treasure.

The walkway was designed to be organic and blend unobtrusively into the forest while minimising damage to the trees.  To achieve this, a delicate structure, carefully located between the trees, and finished in colours that would blend in was needed.  Steel was clearly the material of choice, to satisfy these criteria.


Since the walkway was to be built in an existing forest with a requirement to minimise disruption and damage to the vegetation, a comprehensive survey of the area was done.  This established not only ground levels but the position, height and canopy diameter for every tree.  Mark spent many hours carefully planning a route through and over the trees together with horticulturalist Adam Harrower. 

Columns were located relative to trees and were not necessarily placed at the most efficient structural support positions.  Where it was not possible to avoid a particular tree, it was allowed to pass through the structure and was tied back to ensure that it experienced minimum interference from the structure.

The surveyor was required to accurately set out the position of each column, and the position and orientation of the two abutments on site, so that when steel was delivered to site, everything fitted. 


Rather than adding the deck and hand-railing on top of a conventional structure comprised of trusses or beams, components were designed to be multi-functional, with balustrades and safety mesh being an integral part of the primary structure.

The spine of the structure, a single tube section, forms the bottom chord of a truss.  The box section handrails double as the top chords of the left and right trusses.  Ribs cut from 8mm plate at 1 m centers serve both as stanchions and as the vertical elements of the trusses. 

The ribs are made up of 3 parts bolted together below the deck.  This ensures efficient use of material, facilitates handling in the confined areas between trees and limits the use of heavy equipment in this sensitive area.

Two longitudinal angle rails, onto which the transverse walkway planking is fixed, also serve more than one purpose.  In the interim stage, when only the lower central portion of the walkway is erected, these angles serve as top chord members of a triangular truss, with the circular hollow section being the bottom chord.

The 8 mm rods forming the safety mesh contribute to the structure as truss diagonals.  Their gradient varies with span, being steeper near the columns where shear forces are highest, and shallower at mid-span.  The curves thus introduced soften the appearance and give the structure an organic feel.


This project was more like crafting a sculpture than designing a structure.  To develop a sensitive and appropriate design given the practical constraints of the site required that Mark and Henry collaborate very closely from the outset. 

The shape of every component of the bridge was carefully tweaked and adjusted over many meetings until a solution which performed as required structurally, without excess material and which was considered aesthetically optimal, was achieved.


Hand calculations and a finite element analysis of simplified straight sections of the bridge were initially used to obtain the member sizes for the tender.  The tender specifications stipulated that all the elements of the bridge be defined in a 3-dimensional model developed by the steel manufacturer using a suitable detailing package.  Prokon Services, who fabricated the steelwork chose to use Tekla for the detailing.

The Tekla model would serve two purposes:

  • Firstly, it accurately defined the shape, orientation and position of each component of the bridge for manufacturing purposes.
  • Secondly, the file was converted and imported into the structural engineering finite element package Strand 7, setting up the geometry for a detailed analysis of the structure. The stresses in each component of the bridge could be checked under design load combinations.

An ultimate limit state design was applied to the bridge, with deflections and frequency response being checked.  Based on the FE Analysis, supplementary members were required, in particular where the section is wider.

To achieve the clean uncluttered aesthetic, and limit the impact on the forest, a slender structure was required.  Movement was thus always going to be significant.  At a late stage of the design process, the deck was raised to clear the tree canopy, optimizing the visitor experience.  The column lengths thus increased, from a maximum of 9 m to the current 12 m maximum length, more than doubling the calculated column deflections.

Conventional cross-bracing was designed to limit deflections and movement.  A dynamic finite element analysis indicated that this would reduce movements by approximately 70%.  This bracing was not installed. The approach being to first monitor actual bridge behaviour – and the response from the public.  Subsequently, cable stays have been introduced which limit movement somewhat, though less than cross-bracing would have done. Interestingly, feedback from many visitors indicated that movement added an extra dimension of excitement, enhancing the experience.


Once the structure had been fully drawn out, but before manufacturing commenced, a further detailing iteration was required, to ensure that all the transition curves were smooth.  To accommodate the changing horizontal and vertical curves, each portion of the central tube, the box sections at the handrails and the angles supporting the deck had to be rolled to the correct radius. 

The stanchions (single 8 mm plate) and central transverse frame sections (double 8 mm plate) were laser cut from large steel sheets to create the perfectly smooth curved edges one sees on site.

All components were pre-assembled in the steel yard at precisely the correct angles and slopes, with two adjacent sections being connected at a time, to check the geometry and ensure that the required smooth transitions between the curves were achieved. When installed on site everything fitted perfectly, with no on-site cutting or welding being required.

Tight controls were implemented on site to ensure that a similar level of care was taken during construction to that taken with the design.  Plant size and access were restricted and designated areas were allocated for hoarding, to protect the natural vegetation.

Since being opened to the public, the Boomslang has proven to be extremely popular.  Kirstenbosch saw an increase in visitor numbers of 31.6% from the 2013/14 to the 2014/15 financial year (829,668 to 1,091,438), and attribute this largely to the Boomslang which opened to the public in May 2014.

There is no additional fee to walk on the Boomslang; only the standard Kirstenbosch garden entry fee.  However, the increase in gate income from more visitors allowed the capital costs of the bridge to be recovered within one year of it being opened.