The Sun Arena at Time Square in Menlyn Maine is the biggest live entertainment venue in Pretoria. The purpose of the arena is to create an event and performance space where some of the biggest music concerts in South Africa will be held.
The multi-faceted brief to the architect was to create a performance venue that could seat 8 500 people and could be adapted to accommodate 1300 banquet guests and up to 18000 delegates in a school room format. The client wanted a continuous roof span without any columns and the arena had to incorporate all of bells and whistles that would make it an arena of international standard. The fly tower height of the arena, which is the framing around the stage, is of international standard which means that many international performers will be able to perform at the Sun Arena.
The project wasn’t envisaged in steel from the start. The columns on which the edged gutter and roof wedges sit were originally conceived in concrete, but due to steel offering faster construction times, this was later changed to steel. The roof structure, which is a tubular truss frame roof, plus a large gutter were done in steel. The gutter of the roof has a steel construction tension ring and there is a compression ring in the center of the roof trusses.
The design team had to create an acoustic sandwich out of the cladding because the performance arena had to be insulated from noise from traffic and weather, and it also had to prevent interference from the concert to neighbouring facilities. The cladding also had to be watertight so that the arena would be kept dry during bad weather. Global Roofing Solutions supplied 86 tonnes of cladding to cover the 1300m2 cladding area for the project. The company’s widely popular KlipTite system was specified for the project.
The geometry of the cladding of the Sun Arena is particularly interesting because the roof was designed as a series of wedges. A curved gutter edge, however, meant that when an edge intersected with a curve, it would lead to a varying height at the bottom. During the project, the team had to resolve how they were going to marry the varying heights at the bottom of the cladding that resulted from the combination of curved gutter edges and roof wedges.
The roof has a 96m, column-free span, which is unusually large. While there are many long-span roofs in warehousing projects, the unique acoustic envelope that the team had to create makes it an exceptional project.
The entire project team worked together from the start to conceive the structure and decide on the appropriate materials for the arena. The main contractor was involved in all stages of the project so that the goal of creating an economic, structurally efficient and aesthetically pleasing structure could be achieved. The team also worked in a 3D modeling program called REVIT, which led to digital design-led decision making and information sharing.
The external box gutter of the Arena was originally planned in concrete, however looking at the complexity of building scaffolding, supporting of a concrete gutter and fixing re-bar at > 15m high levels and weight limitations, as well as programme, it was decided to re-design the concrete gutter to a steel lattice curved box gutter approx. 4m high, with internal 3mm plate, formed gutter, These items were fabricated as complete units from column to column and lifted into position using a 220-ton crane. Rest of the Arena roof the only possible way of spanning 100m was possible with the dome type steer of with compression ring in the center.
The Design was driven by three elements
- a) Client Budget
- b) Buildability with site constraints being the construction of 100m roof and seating structure on a complete basement structure as the footprint of the site
- c) The programme, with end dates not moving due to events already booked year in advance for the opening
Detailing was done in Tekla, hand-in-hand with the engineers to determine lifting sizes and weights to ensure the steel contractor could erect the steelwork using the most economical plant.
It was important to see the 3D model for details such as sheeting and cladding and to model the lifting procedure and method statement.
Tekla was used to colour in the lifting elements with element weights, which made it clear to the engineer when approving the loads on the slabs and determining the crane positions on the slabs.
Fabrication was done in complete assemblies as far as possible, due to CADCON premises close to site we could transport abnormal loads not having to travel to far on the main highways.
We detailed in Tekla temporary jigging structures to simulate real-life fit positions, with this we could ensure that the assemblies fit with high precision on site.
Complete gutters were, trusses in two sections, back section complete and front section bolted on site, was fabricated.
Trial assembly of all elements in the workshop was done before shipping to site.
A further requirement, all the reinforcement for the main oblong columns were installed in the steel contractor yard and we had to include the weight of the reinforcing in the overall lifting weight of the main columns when doing the crane studies.
Erection was the most critical element. The Steel roof was located on a complete basement three levels down and the main roof had to be erected in just over 3 months excluding sub-structures.
We had to design the elements to be lifted with cranes which the slab could handle from a design point of view.
The process was as follows:
- Max. Lifting weights were determined for all main assemblies
- Crane positions were pinned all around the structure taking concrete structures, lift shafts, roadways into consideration
- Max radius for crane lifts determined, which determined sizes of cranes
- Back propping of the slabs had to be designed for each and every position
- Back propping was erected in sequence to synchronize with steel erection programme, we could not back prop the whole basement due to the costs involved.
- As we erected, the back propping was moved in the same sequence
- Temporary cabling was used to stabilize elements where the complete structure not yet working as a whole
- The biggest challenge was to have fabricated elements of this size off-site, bring to sit in sequence and have a civil contractor aligning them 100% with the steelwork methodology and giving their support throughout and all trusses between the gutter and main compression ring fitting perfectly with only a 50mm tolerance gap over a 100m span.
Main Arena, a total of 1375.699 tons and a bolt count of 17 939.
- An internal diameter of 93.4m
- External diameter 96.0m
- Total roof tonnage 570.510 tons
- Outer tension ring ‘box gutter’ size 2.5m wide x 3.9m deep with segment lengths of 15.5m and oblong steel column ‘hammerhead’ being 2.5m wide x 3.9m deep x 4m in length. Total of 16 segmented sections of 19.4m. Total tonnage 215.216 tons.
- Internal cylindrical ‘compression’ ring; 7.95m x 5.975m high at 20.469 tons
- Main arena and staging sub-grids and catwalks/cat ladders at 132.877 tons
- Main arena plate girders and raking seating support beams with a total of 143.844 tons
- 16 number of Oblong tube steel / concrete composite columns and tension ring ‘hammerhead’ at 292.783 tons
Mobilisation and erection challenges: While continuing with the main concrete superstructure, structural steel erection had to overlap with the concrete works in order to keep to the very challenging and demanding programme. This entailed back-propping of the newly constructed lower ground concrete floors down to the –B3 level, to allow access for multiple construction laydown zones and mobile cranes in excess of 80 ton, and in some instances 220 ton, to be positioned on the concrete floors for assembly of the compression ring, tubular space frame roof trusses, rigging sub-grids, catwalks, stage mechanics support structure and the placement of the raking steel beam structures to facilitate the final finished bond-dek seating structures.
- Erection Methodology and Sequence of ARENA Roof and Seating: The steel contractor developed his Erection Methodology and Sequence to suit the main contractors concrete programme, concrete pour sequence, striving for minimum radius of lifting weights, maximum permissible slab loads using the minimum amount of back propping to determine the most economical choice of cranes to lift the Main Roof assemblies.
- The Main Roof Assemblies included the following heavy lifts to be erected in the most economical way:
- 16 No Oblong Columns
- 16 No Radial Plate Girders > 1m deep
- 32 No Hammer Head Lattice Box Girders
- 16 No Radial Box Gutter Girders with 3mm Internal folded Gutter plate
- 1 No Central Compression Ring approx. 6m high rigged as one assembly
- 16 No Main Trusses spanning over 40m (this was spliced in 1/3 and 2/3 sections due to lifting weights)’
Originally two erection philosophies were considered:
- Philosophy 1: Erection of all heavy lift assemblies as above being lifted from external lifting positions outside the Arena perimeter with a 600-ton crane.
This option was very costly, for the following reasons:
- Standing time in between the heavy lifts whilst waiting for infill steelwork to be installed using two tower cranes before next heavy lift.
- Transport logistics involved removing and re-installing mega cranes’ counterweights each time crane repositioned to new lifting position.
- Establishment and de-establishment of mega crane.
- Philosophy 2: Erection of all heavy lift assemblies at a shorter radius from inside the Arena perimeter, but off the concrete slabs.
Advantages of this option:
- Majority of lifts were possible using an 80-ton crane
- Shorter radius lifts possible due to cranes standing inside Arena perimeter.
- Hook time of 80-ton crane vs 600-ton crane is much quicker during lifting operations.
- Back propping of the internal Arena slabs was required, which ensured heavier lifts being done using maximum 220-ton crane where 80-ton crane lifting capacity limited.
- All 220-ton crane lifts were sequenced as 1st priority, after which back propping could be removed and use of this crane time limited as far as possible.
- Infill steelwork was done again using two tower cranes.
- Main assemblies were also spliced in such a way to limit lifting weights and temporary props designed to support the assemblies at the spliced positions. i.e. Main trusses spliced as a welded back segment 3rd of truss and front segment 2/3rd as bolted assembly on site.
Erection Philosophy 2 proved to be the most advantageous in terms of cost and time for contractor and client.
Main Challenges – Fabrication and Erection:
Planning had to be done from shop detailing stage, to ensure complete assemblies fit on site and erection weights being considered taking into account the crane/ lifting philosophy followed to limit erection costs and back propping as far as possible.
CADCON designed temporary workshop jigging, which was detailed in X-steel and built to make sure when the complete gutters girders, hammerhead structures and roof trusses built, it would fit x 100% on site.
In essence, the complete roof assemblies built in workshop, and bolted spliced where the assemblies suited best the transport route from Centurion to Menlyn, whilst also considering each assembly maximum lifting weights. Weights were pre-determined in Tekla and indicated on the Erection Methodology, also indicating maximum crane lifting radius.
This ensured that slabs and cranes were not overloaded on slabs where activity were 24/7 with labour and surrounding main contractor plant, tower cranes, etc.
- Oblong columns
The external perimeter columns – “oblong columns” consisted of 1,2m high oblong rolled plate, welded together in segments to form the external plate columns 13m-16m high. The oblong columns were fabricated from 16mm plate and internally fitted with 120 x 60 RHS sections to prop the external face of the columns to ensure all stay aligned when the 16mm plate welded and the heat added. Studs and mesh were welded internally to provide the working of a composite column.
The oblong columns were fitted with rebar internally. The steel contractor fitted with the help of the steel re-bar supplier the reinforcement inside the workshop, to avoid this activity not possible to fix if the oblong columns already erected. From a practical and programme point of view, it made sense to fit the reinforcement in the workshop.
HD- bolts were designed to receive the oblong columns and a cable/prop stay system was also developed and designed by the steel contractor and main contractor to support the oblong columns after erected and the trusses not yet installed.
It was a requirement that the full circle had to be erected, after which the cable/ prop system would remain in place during the concrete pumping of the oblong columns. To assist the pumping of the oblong columns with concrete, the steel contractor fitted nipples to each Oblong column, which used to pump concrete from bottom up in each oblong column
- Compression Ring
The Compression ring was completely built as a bolted assembly standing over 6m tall in the workshop.
After The compression ring pre-assembled x 100%, it was dismantled and sent to site in loose elements.
The compression ring had to be installed x 100% centrally to the Arena and at the correct level to ensure the roof trusses fit. To achieve this, a central scaffold tower was designed by Form Scaff in conjunction with the WSP and the main contractor.
The arena slab receiving the scaffold tower had to be back propped 3 floors down to the lower level surface bed.
At the base of the scaffold tower, sand release jacks were built and positioned under each scaffold prop by CADCON.
Main roof trusses were installed in segments supported from temporary designed columns to limit truss lifting weights and limit crane loadings on the slab.
Once all the trusses were installed in opposite sequence segments and all infill steelwork complete, the scaffold tower had to be lowered. Releasing the scaffold was done by washing the sand from, the sand jacks systematically, after which the roof lowered by approximately 80mm over the 95m spanned roof.
The engineer calculated the roof under full load with sheeting and stage sub-grids, the roof would settle 150mm lower from the compression ring scaffold platform level.
A remarkable achievement of this Arena installation, that it took only 2,5 months to install all roof and infill steelwork after the compression ring was installed and leveled on the central scaffold support tower.
The Overall Arena installation from 1st steelwork being the oblong columns to the release of the compression ring – 19th of October 2016 to 15th of May 2017, approximately 6 months.
After the main roof was released, supported off the compression ring scaffolding platform, the following installations proceeded.
- Roof Sheeting
- Main sub-grid suspended with hanger system off roof
- Lower and upper stages
What made this exceptional, was the teamwork required between the steel contractor, main contractor and engineers which all had to work in harmony, trusting each other views and coming up with the best plans to execute such a complex roof and sub-structures of over 1500 tons in this short period of time, with a client which gave their backing in all circumstances during the process to ensure the end goal achieved to open to the public on time and produce revenue.
|Tons of structural steel used||Approx. 1 800 tons|
|Structural profiles used||Tubular Steelwork up to 1000mm diameter in roof, Hot rolled in Sub-grid and Seating|
|Nominator||CADCON (Pty) Ltd|
|Client/ Developer||SUN INTERNATIONAL|
|Steelwork Contractor||CADCON (Pty) Ltd|
|Steel Erector||On Par|
|Cladding Manufacturer||Global Roofing Solutions|
|Cladding Supplier||Global Roofing Solutions|
|Cladding Contractor||Chartwell Roofing|
|Dram Industrial Coatings|
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