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Support team: Singapore sports hub

A 310m free spanning dome roof covering a whole stadium, 75m temporary towers and complex three dimensional geometries presented major challenges to the temporary works designers for the Singapore Sports hub in the latest in NCE’s series on BCIA winning projects.

Transient in nature, temporary works rarely leave traces of their existence after a project is complete. Consultant often forget to prioritise them or leave them to others to design and are all too often forgotten about by the consultancy team. Yet temporary works designers can be the unsung heroes of a project, allowing complex buildings to be constructed safely and cost effectively. The Singapore Sports Hub is a fine example of this.

The huge challenges, complexities and sheer scale of erecting the roof of this brand new 55,000 seater stadium required the development of a world leading temporary works design.

Situated on the south coast of Singapore, the new S$1.33bn (£658M), domed sports hub sits elegantly in its 35ha waterfront location. It is a multi-sports venue also created to encourage everyday public participation. Designed to host athletics, football, rugby and cricket, it has retractable seating to accommodate differing sightlines which apply to different sports.

Singapore sports hub

In action: The roof closes to protect the pitch from the wind and sun

However, the unique feature of this facility is its immense roof structure.

Roughly symmetrical about its principle axes, the dome roof is supported by a 10,000t network of intersecting steel trusses. Two arched main trusses, running east to west, span 310m to form the principle elements of the domed roof. The trusses, which are slightly further apart in the middle than at their ends, work integrally with a series of transverse trusses that guide and support the retractable roof. At either end of the stadium, more trusses curve and intersect around the spherical geometry to form diamond patterns in plan.

The pitch below can be protected from fierce sun or torrential rain by closing the roof. The moving roof opens and closes like a blinking eye, the main seam running centrally along the length of the pitch.

During the construction period, an incomplete structure is not able to develop the load paths for which it is designed in its permanent condition.
In complex jobs, each stage of an erection sequence has to be carefully modelled to ensure that the members would not be overstressed in their temporary state. So called “locked in stresses” can develop in the members during this time, depending on how the structure is constructed. Minimising these locked in stresses is key because, as the name suggests they remain in the member, adding to the stresses developed in the permanent condition. If these stresses are too high, local or even global failure of the structure may occur.

The trusses supporting the roof, are, at their highest, 75m from the ground and are typically about 5m tall. For the span this makes for an impressively thin roof structure. But how was it all built?

Singapore sports hub

Lifting work: Installing the retractable roof

The starting point for Tony Gee & Partners project director Steve Harridge, was safety. “Safety has to be paramount,” he says. “The best way to keep it safe is to keep it simple even though some of the analysis and the engineering that goes behind the structures we build can be quite complicated.”

Yongnam, the Singaporean steelwork fabricator employed Tony Gee & Partners to carry out the construction engineering for the erection of the roof structure. The package of work however, did not include a site supervision role, which made the job more complex.

Tony Gee & Partners created two models to analyse the structure and any locked in stresses which might accumulate (See box). Built in a software programme called Sofistik, the first model aimed to mimic the original design model produced by Arup for the permanent condition. This acted as a calibration, to check that the models acted in the same way and that the outputs were similar enough. Based on this, the second model was then used to break the stadium down into a series of construction stages, building it up piece by piece. By analysing the forces and stresses in the members at each stage and then comparing them to the forces in the same members in the complete model, the locked in stresses could be determined.

Locked in stresses

During the construction period, an incomplete structure is not able to develop the load paths for which it is designed in its permanent condition.

In complex jobs, each stage of an erection sequence has to be carefully modelled to ensure that the members would not be overstressed in their temporary state. So called “locked in stresses” can develop in the members during this time, depending on how the structure is constructed. Minimising these locked in stresses is key because, as the name suggests they remain in the member, adding to the stresses developed in the permanent condition. If these stresses are too high, local or even global failure of the structure may occur.

 

“It wasn’t a straightforward process,” explains Tony Gee & Partners structures director Martin Omerod. “We had to go through a number of iterations and look at positions of temporary towers, jacking loads etcetera, in order to reduce the locked in stress in the structure and control the movements of the roof truss to assist with fit up and assembly.

“The engineer had allowed a 10% margin in his design on the member usage to account for stress, which would be locked into the structure during the build process.”

Complex nodes where several steel tubes met at a point, were analysed separately using finite element analysis to make sure they were not
vulnerable to deflection during the erection process.

The basic premise of the erection sequence was quite simple. Commencing at the eastern end of the structure, the two halves on either side of the movable roof were erected independently, forming an open slot down the middle. The sequence was carefully planned to avoid blocking in the cranes.

Singapore sports hub

Scale: First main truss section supported by its springing and temporary trestles

“As the erection of the structure proceeded the cranes tended to work themselves into a corner, says Harridge. “So we had to be quite careful that they could get towards the right position to build the later parts of the roof.”

Held at four points, the trusses were lifted onto temporary towers or trestles, which supported them until the completed arch could support itself.

The ends of the transverse trusses were then joined together with the “key stone” section of the truss across the slot. Simple in theory, but more complex in practice.

“The difficulty that made it more complicated than your average arch bridge is that it was three dimensional,” explains Harridge. “We started off with two dimensional arches which then grew in stiffness and in strength as the project grew.”

Unknown to Tony Gee & Partners at the time of appointment, Yongnam had a stock of prefabricated steel props which had principally been developed for use on the cut and cover parts of the Singaporean metro system. Understandably, it was eager to reuse them for the stadium’s temporary works.

Singapore sports hub

Towers of strength: Red temporary trestles support the roof during erection

“The fact these props were better at propping cuttings apart as opposed to acting as trestles for a sports stadium didn’t make them absolutely ideal,” says Harridge. “But actually they weren’t as bad as they might have been.

“Yongnam didn’t mind us dismantling them a bit so they could be more efficient in the lighter towers, the heavy towers were pretty good.”

One of the quirks of this, however, was that Yongnam was subcontracting the erection work on a per tonne basis and wanted the best of both worlds. It wanted to use the minimum weight of steel, yet reuse its existing stock, so a delicate balance had to be struck.

To keep the erection of the trusses as simple as possible, the trestles were grouped together as far as was practical. Initially only three types of large trestle were designed to support the trusses. However a fourth, lighter trestle was added to keep the geometry of the trusses on the perimeter in place.

“We only ever jacked four towers at a time as we knew that was what it [Yongnam] could control”

Steve Harridge, Tony Gee & Partners

“Without the type four trestles it was quite difficult to control the bending deflections of the outermost trusses as the process proceeded,” says Harridge.

Another of the team’s considerations was the thermal movement of the trusses during construction. Although the range of temperatures in Singapore is small, it still had to be accounted for.

“The whole thing grows and then it comes back [expands and contracts], but the good thing is it does it at the same time,” explains Harridge. “Our danger would have been if one part of it was in shade, and the other part was not. But Singapore is 80km from the equator and so the sun goes pretty well over the top for the main part of the day so it’s getting pretty uniformly heated.”

Once lifted into place, the truss sections had a tendency to want to fall over sideways putting a horizontal load into the top of the trestles. To relieve the trestles of this load and the effects of the thermal movement, sliding bearings were incorporated at the connection of the trestles and the truss. Instead, the lateral forces were taken by additional temporary props. These connected the top chords of the truss and to the tops of the trestles preventing unwanted, localised lateral bending in the trusses.

“The T4 trestles were needed to keep the geometry straight and the locked in stresses under control”

Steve Harridge, Tony Gee & Partners

When complete, the trusses develop an arching action, and with it an inherent stiffness. However, individual sections of the trusses when being lifted into place, were relatively flexible. To avoid geometrical problems and to prevent the trusses sagging into an incorrect position, they were installed around 70mm above their correct locations and allowed to settle into place. This is a technique more often used in bridge construction but adapted for the project.

Installing the trusses involved lifting individual sections into place and then adjusting jacks installed on the tops of the trestles as subsequent sections were added.

As Tony Gee did not have site supervision role, controlling the process was more tricky. The three dimensional geometry also proved difficult for the contractor on site.

“The matter of sequential jacking up and down of the trestles was something which Yongnam wasn’t familiar with,” says Harridge. “At the stage where we had almost complete arches in one axis and almost complete arches in the other axis, we realised that what we did in one direction, for example the x-x axis, also affected what we did in the y-y axis. It took some time to make Yongnam realise that it didn’t have completely free rein to do whatever it wanted to do make one arch fit.”

 

Singapore sports hub

Propped: The structure was supported by steel trestles during construction

When complete, the arches no longer required the support of the temporary trestles and they could be removed in a controlled process known as “de-propping”.

Pressure in the lifting jacks could either be increased or decreased to raise or lower the trusses at the specific points.

Remove one trestle - or prop - too quickly and the forces developed in the potentially mis-shapen geometry could cause buckling, and collapse the roof. The process was geometry led, setting the intermediate points of the truss to the correct level, while checking the forces in the jacks were within the acceptable limits. There were generally eight trestles supporting each truss.

“We only ever jacked four towers at a time as we knew that was what they [Yongnam] could control,” explains Harridge.

“You start off [de-propping] with small increments and then get to bigger increments. All the time you jack down, the load on the trestle is decreasing because the structure is starting to act as a structure. Therefore the amount of tolerance you can allow between one set of nodes and the next set of nodes gets bigger because you’ve got more of an envelope in which to work.”

Delays on site and problems with the programme meant Yongnam wanted to de-prop the trusses earlier. A trench, designed to stow the retractable seating around the perimeter of the running track had to be excavated but its alignment clashed with location of the T4 trestles.

“The T4 trestles were needed to keep the geometry straight and the locked in stresses under control. The effect of this [excavation of the trench] was that they needed to change the erection sequence and we were a significant way through this when the need for change occurred. So we needed to see how urgently we could get rid of some of the trestles that were being affected.”

In the end, the erection sequence was adjusted so the trestles could be removed in time for the trench to be excavated.

With the trusses in the correct location in space and the load on the trestles released, the splices between the sections could be welded up and the temporary, bolted connections removed.

However, not all the site works went to plan. As Harridge explains, “Not everything fitted perfectly, not surprisingly. I think Yongnam went into this thinking this job was going to be easier than it turned out to be. There were some areas where it had to disconnect connections and weld them and then that did have an effect on the locked in stress on both those and the surrounding members.”

The stadium opened in June 2014.

 

BCIA judges’s view

The temporary works for the Singapore Hub won the 2014 British Construction Industry Awards temporary works award.

The judges said it was a hugely complex project which required “de-construction” and a detailed understanding before trying to build it for real. They noted that the assembly method of this ambitious project of “outstanding scale and complexity” had been “meticulously devised and specified” for maximum simplicity on site, all in order to achieve the degree of precision demanded by a retractable roof.

 

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