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OUTLINE OF THE CONSTRUCTION SCHEME Each of the 12 masts and the pyramids that support them were assembled and fully site welded in an area adjacent to the permanent site. The pyramids were carried to the site and were placed on the concrete piled foundations with a crawler crane. The masts were also carried with the same crane and laid out adjacent to their final position. Whilst there, they were fitted out with the temporary erection gear that was subsequently used for the cable pulling and also had the restraint cables attached, before being lifted into position with a large 1000 tonne capacity strut jib crane. W M R A Hi it Fig 1 A 90 metere mast is reared up prior to being placed on the pyramid The masts were guyed off with four restraining cables before the crane was released. The front two of the cables were temporary but for the rear two, the permanent backstay cables, which ran directly to ground anchors could be utilised. In order to restrict the movement of the masthead under wind loads the four restraining cables were post tensioned to a predetermined force in the order of 200KN before the crane was released. This operation was further complicated by the fact that the mast head position in space was monitored and was required to be within 150mm of theoretical when the stressing was completed. The mast was mounted on a rubber bearing located on top of the pyramid which allowed this small amount of rotational movement.
159 PRINCIPLES OF CONSTRUCTION FOR WIDE-SPAN STRUCTURES WITH EXAMPLES FROM THE MILLENNIUM DOME Peter W Miller Watson Steel Limited SYNOPOSIS There are a number of key issues that need to be considered in planning the construction of any complicated structure and the Millennium Dome was no different. This paper gives a brief description of the construction method adapted for the Dome and then describes the major issues that had to be dealt with in the planning and construction of the Structural steelwork and cable net that forms the structural framework for the Dome. An explanation of the thought processes leading to the eventual construction scheme is given and it will be shown that many of the principles described here also apply to any wide-span structure INTRODUCTION In late 1996 Watson Steel were invited to submit a tender for the supply and construction of the steel and cable frame for the Millennium Dome. In order to begin to estimate the construction costs of such an unusual and large-scale structure a workable and economic construction scheme had to be developed. The construction scheme produced at that time, whilst just a series outline sketches, was fundamentally the same as that which was eventually used. A great amount of detailed development post contract award took place however and some of the attention to detail that was required will be demonstrated in the following pages. OUTLINE OF THE CONSTRUCTION SCHEME Each of the 12 masts and the pyramids that support them were assembled and fully site welded in an area adjacent to the permanent site. The pyramids were carried to the site and were placed on the concrete piled foundations with a crawler crane. The masts were also carried with the same crane and laid out adjacent to their final position. Whilst there, they were fitted out with the temporary erection gear that was subsequently used for the cable pulling and also had the restraint cables attached, before being lifted into position with a large 1000 tonne capacity strut jib crane. WMR/A Hi it Fig 1 A 90 metere mast is reared up prior to being placed on the pyramid The masts were guyed off with four restraining cables before the crane was released. The front two of the cables were temporary but for the rear two, the permanent back- stay cables, which ran directly to ground anchors could be utilised. In order to restrict the movement of the masthead under wind loads the four restraining cables were post tensioned to a predetermined force in the order of 200KN before the crane was released. This operation was further complicated by the fact that the mast head position in space was monitored and was required to be within 150mm of theoretical when the stressing was completed. The mast was mounted on a rubber bearing located on top of the pyramid which allowed this small amount of rotational movement. 160 Fig 2 These three sketches show the rearing procedure for the masts The cable net, which consisted of over 2600 cables was assembled and lifted in four main sections. Each section formed a large concentric circle which when pulled simultaneously at 36 positions was elevated to its final height. The missing infill cables between the circular sections were installed individually using a combination of abseiling techniques for the higher locations and powered access equipment where practical. Fig 3 The second cable net being reared up with 36 pulling jacks. When all cables had been installed they were tensioned to their final design stress by progressively jacking at the anchor points of the 72 pairs of radial cables. The stressing operation was carried out in a balanced manner around the dome in three stages. The erection of the masts commenced on 15 th October 1997 and the stressing was completed by the end of March 1998. A period of only 16 working weeks. PLANNING The key to all successful site construction projects is detailed planning and the more complicated the project the more important detailed planning becomes. Inevitably wide-span projects tend to be unique and challenging where the need for lateral thinking combined with attention to detail becomes even more important. On the Dome an internal system for planning and developing the construction method was established within Watson's that proved to work very well. This was based on regular brainstorming sessions where the most experienced and practical engineers within our business with both fabrication and erection experience debated specific topics and put all the ideas on the table. The project team would go away and examine all the options and develop the ideas. The project team would then present their conclusions back to the gathered engineers for critique. Meetings were held weekly at the outset of the project when the construction scheme was formed and thereafter as and when required. When the options had been narrowed down to a few options they would all be costed out to determine which was the most economic. Fig 4 A full scale trial was carried out on the first mast section to 'prove' the planned net pulling method 161 Quite often practical trials were required to determine the best options. When the stage had been reached where the scheme was finalised on paper it was decided to carry out a full-scale trial. This was carried out in the Bolton factory using the first of the mast head sections that was fabricated in July 1997. This trial proved to be a good investment. The outcome of this planning / development stage was a detailed method statement which was developed gradually over several months. PRE-ASSEMBLY For any construction scheme to be successful it must be: - a) Safe b) Economic c) Fit within programme constraints d) Comply with specification One method commonly employed to ensure that these criteria are met is to pre-assemble as much of the structure at ground level as possible. In the case of the Dome this principle was applied to the pyramids, the main masts and to the erection of the cable net that forms the structural framework of the Dome and supports the fabric covering. The final decision on which elements and to what extent they should be pre-assembled depends on many factors including cost, availability of large cranes, programme, height above ground, and the alternative safe means of man access. In the following paragraphs the decisions that were taken on the pre-assembly of the principle elements of the Dome are described. PRE-ASSEMBLY OF PYRAMIDS The four-legged pyramids that support the 12 masts are over 8 metres wide and 10 metres high and therefore had to be constructed on site. The design forces and architectural requirements meant that site welding of the node joints was the only feasible option. The overall construction programme however could not be achieved if the construction of the pyramids was delayed until the permanent foundations were available. The solution was to pre-assembly the pyramids on a temporary foundation in a separate area away from the main construction zone. The completed pyramids were then stored and eventually carried to their permanent location, using a large crane, once the foundations were released. The advantage of pre-assembling all the twelve pyramids in a specific assembly area was that once the assembly jig was set up and checked it could be used twelve times and all the pyramids were sure to be identical and therefore interchangeable. The fact that a large crawler crane in excess of 200 tonnes nominal capacity was required was not a disadvantage because the crane was planned to be on site anyway throughout the period to assemble the mast sections. Fig 5 One of the completed pyramids being placed on to the foundations PRE-ASSEMBLY OF MASTS It was a fairly obvious solution to build the main 90 metre masts on the ground and to lift them with a large crane. The original cross sectional diameter of the masts however was greater than we could transport on the public roads by conventional trailers. This would mean that there were either very expensive transport costs or else only half the mast cross section was fabricated in the factory and the remaining fabrication completed on site. Both options were considered undesirable and following discussions with the Engineers it was agreed that the overall diameter of the masts could be reduced in size and the design compensated by increasing the wall thickness of the eight tubes that made up the octagonal cross section of the mast. Fig 6 May 1997 A completed mast section ready for dispatch to the painters 162 This small structural change had a significant effect on the costs since it was now possible to fabricate the masts almost entirely within the factory leaving just five joints along the length to be completed on site in the pre-assembly yard. Fig 7 A Fully painted section arrives in the site assembly area. As in the case of the pyramids described in the previous paragraph, the decision was taken to pre-assemble the masts away from their permanent location. This was due to the fact that the foundation works for the masts had to be carried out in parallel with the mast build because of the overall programme constraints. The moving of the completed masts, however, which were 90 metres long and weighed 95 tonnes, was a much more difficult problem. Various options were considered and evaluated. These included using special multi-axial transporters, using a bogey system on a track etc. The solution which was eventually selected was to pick and carry the masts with the large 200 tonne capacity crawler crane. This was only possible if a flat and well-compacted route could be provided and a great deal of investigation was carried out to select and subsequently prove the route. This exercise was further complicated because of limitations to the ground loading pressure that could be applied in the region around the Blackwall tunnel that crossed the site. Fig 8 A full mast being carried from the assembly area on to the site PRE-ASSEMBLY OF CABLE NETS There are over 2600 separate cables that form the 'web' structure of the dome. This presented, perhaps, the biggest challenge to the Watson Steel construction team. The initial objective in developing the scheme for the installation of the cables was to assemble the complete net at ground level and lift it in to position in one operation. This would have the massive advantage of removing almost all the risk of the high level work. It is also many times faster to install a cable in to a net at ground level under zero load, than it is to install it into a existing framework of cables at high level. This initial objective however was found to be impracticable and over ambitious due to the weight, and the many complications, technical difficulties and costs that it introduced. It was next considered splitting the net into two sections and lifting these individually and just mstalling the 72 radial cables between them at high level. Again it was found that the technical issues were too difficult and so the next preferred option of three sections was investigated. This iterative process was continued until, after much debate, the eventual decision was to opt for the pre-assembly of four separate rings and to complete the infill between these rings by lifting one cable at a time at high level. This sort of compromise is necessary and indeed often essential, when developing any complicated erection method. Having to satisfy Safety, programme, and budget considerations inevitably involves compromise. It was possible using this chosen method to assemble over 75% of the cables at ground level under a zero load condition. The difference in terms of man-hours between a cable laid out on the ground and one installed at high level is estimated to be at least six-fold. The saving in terms of cost and time of maximising the pre-assembly of a cable structure is therefore enormous. The following figures 9,10,11 show the lifting arrangement for the first two nets. This was complicated because the temporary restraint cables for the masts passed over the top of the second net where it was pre-assembled on the ground. The temporary restraint to the masts therefore had to be re- diverted via the previously erected central cable truss, once the initial central net was lifted. Masts restrained by temporary forestays with first cable ring ready for lifting Fig 9 163 SITE WELDING NIERNN. WORMNG FWFOHM .MMIfW JMPowWl : \ /IB*OMW\ /\ gwyj || ' 1HQW' Central ring lifted - mast restraint transferred to new tie downs Figure 10 Temporary forestays removed masts now restrained by tie downs to central ring Fig 11 The handling of cables is a key issue that needs careful consideration. It is very easy to cause accidental damage during laying out and handling that may necessitate having to replace the cable. The sequence of the assembly operation has to be planned in-depth to ensure that access routes are maintained and that the site equipment that is being used to handle the cables does not have to run over previously laid cables. The cables used on the Dome were of the spiral strand type, which are highly susceptible to damage caused by kinking or squashing. And any cables that showed signs of distortion had to be replaced. On the Dome, a method for laying out the cables using a forklift truck and a turntable on a flat wagon was developed. The cables were delivered in coils of a standard inside diameter. The coils were placed on a turntable on the back of a small flat bed wagon. The loose end of the cable was restrained using the fork lift truck while the wagon drove slowly away allowing the cable to unwind on to ground in a predetermined position. Some of the larger cables which ranged up to 90mm diameter also required auxiliary craneage to assist in the laying out. Many engineers tend to avoid site welding wherever possible. This may be due to preconceptions about quality, time or cost. In reality site welding can often be more economic and can provide a better engineering solution than bolting. The difficulty however is that there is no golden rule and the only way to determine which is actually the 'best' method for a particular application is to carry out a detailed comparison on a job by job basis. On the Dome, for example, the original specification was for the mast joints to be site bolted using a pipe flange detail as it was considered to be the more economic solution. The architect however preferred a smooth site welded detail and so an option was included within the tender for the contractor to specify the 'extra-over' costs he would require to site weld, grind and paint the 480 joints in-lieu of bolting. When the actual cost of the options was calculated cost it was cheaper to give the architect and engineer what they preferred and to site weld the complete mast! A good example of a win-win solution! Fig 12 One of the 480 mast joints being welded on site using a flux cored wire process. The reason why site welding is sometimes cheaper is because it can dramatically simplify the shop fabrication element of the works. If the site operation is considered in isolation then welding will always be more expensive than bolting but when the savings in fabrication and bolts are taken into account the cost advantage often swings the other way. There are other aspects to consider as well. The site programme will often be extended if welding is involved but in the case of the Dome this was not critical because the welding was taken off the critical path by 164 pre-assembling the masts away from the main site area in parallel with the foundation works. Another factor is the corrosion protection to the welded areas which has to be applied in site conditions and can also effect the cost and programme equation. Fig 13 The semi-automatic welding equipment used on site For most site welding applications the preferred process is to use a flux cored wire with a semi-automatic hand held gun. This system is quite robust, can withstand a reasonable amount of draught and has a much higher deposition rate than conventional MMA welding. One of the major advantages that the wire feed processes has is that they do not require the baking and control systems that the MMA electrodes require. The working areas are also a lot cleaner and there is less waste because there are no leftover electrode ends. If site welding is to be considered then it must be well organised with a professional set-up. There is a significant cost to estabUshing a well-controlled site environment and usually there is a minimum scope of work below which it is not usually economic to introduce site welding. Conversely, however, once the decision to site weld has been taken, there are often many other opportunities which present themselves and site welding becomes the preferred solution for that site. The important thing to remember is that there is nothing to be fearful of by introducing site welding. Provided that it is well organised and controlled it can be a major benefit to the project. TEMPORARY ERCTION GEAR One of the common elements with wide span structures is that they usually involve complicated and unique erection methods. Where cables are involved the erection method also usually demands special equipment for lifting, jacking, pulling etc. There can be a substantial investment required in such equipment before the construction can commence, in the case of the Dome this was in the order of £0.5m. The major fabricators experienced in such operations often have large stockpiles of specialist equipment that can be adapted for future schemes. JACKS The attached sketch shows the arrangement of the pulling equipment that was developed for lifting the nets on the Dome. Most of the equipment was designed specifically for this purpose. Fig 14 Original Sketch of the proposed arrangement for pulling up the cables Each mast was equipped with three pull jacks. The jacks were each capable of pulling a six tonne force. The pulling wires were then double reeved which increased the pulling force provided by each jack to almost 12 tonne force. A pull test carried out in site conditions found that the theoretical 12 tonne force at the clamp position had been reduced to 10.5 tonnes due the friction loss in the system. The friction loss is a significant factor that should be allowed for in the design of any lifting arrangement such as the one developed for the Dome. The friction loss would normally vary between 5- 20% however it can be reduced by using special low friction bearings and divertors but this also adds significantly to the cost of the system. The actual design therefore is a trade off between the capacity of the jacks used and the sophistication of the equipment. On the Dome it was found that the most economic solution was to provide enough jacks so that there was plenty of spare capacity and hence the relatively high friction loss did not cause concern. In total 36 jacks were used which generated a combined pulling force at the clamps of 375 tonnes. 165 TEMPORARY CLAMPS The design of the clamps, which attached to the ends of the permanent cables in order to transfer the pulling force, was an important issue on the Dome. It was expected at the outset of the contract that propriety clamps would be available for each of the 3 different diameters that required pulling. It was found however, that due to the necessary restrictions on the local stresses that could be applied to the spiral strand cables it was not possible to locate clamps 'off the shelf. It was necessary therefore to design and fabricate purpose made clamps. The design was based on limiting the compressive stress to 28 n/mm2 which lead to the clamping length of 500 mm. Fig 15 The purpose made clamps used to pick up the permanent cables without damage The clamps also required a lining material to enhance the friction capacity. Various pull tests were carried out during the design period to determine an appropriate lining material. Initially a rubber-based material was used which was found to generate the required friction during the trials. During the first net lift carried out under site conditions however, It was found that the clamps tended to slip in certain circumstances. The subsequent investigation resulted in the conclusion that the friction properties of the rubber material had altered since the initial tests. This was due to the fact that the test was carried out in dry warm conditions and the actual conditions in the middle of winter on site were very different. The problem was resolved by changing the lining material to a type similar to that used in the manufacture of car brake linings. Once the linings had been changed no further problems were experienced. Fig 16 The clamps in action at the start of a lifting operation. STRESSING OF THE CABLES The final stressing of the cables was carried out at the 72 perimeter adjustment points. Each pair of radial cables incorporated a pair of turnbuckles that were used to take up the adjustment. The cable attachment points were detailed to accommodate a 50 tonne capacity pull jack. A hydraulic pump that had an accurate oil pressure gauge operated the pull jack. The force that was being introduced into the cable was calculated from a calibrated chart based on the hydraulic pressure reading. Fig 17 Arrangement of the stressing equipment 166 EXAMPLES OF OTHER STEELWORK STRUCTURES ON WHICH SIMILAR CONSTRUCTION PRINCIPLES WERE ADAPTED. TGV INTERCHANGE, CHARLES DE GAULLE AIRPORT, PARIS Site welding was chosen as the preferred method for constructing the trusses primarily for aesthetic reasons but also because of the difficulty achieving the required force transfer between the members. The 50 metre span trusses were pre assembled in an assembly yard some 200 metres away from the construction area and transported by tractor & trailer. The unusual features on this completed structure are the inverted bowstring trusses, which are post tensioned by pulling down the perimeter cable ties. Client Aeroports de Paris Architect Aeroports de Paris Consulting Engineer R.F.R. Partnership, Paris Fig 18 One of the Bow string trusses being assembled in the factory. It was subsequently dismantled for transport to France. REEBOK STADIUM, BOLTON The steel roof trusses were pre-assembled by site welding in to sections up to 20 metres x 20 metres. The pre-assembly sizes were determined by the size of the available lifting crane. The trusses were then joined together by insitu welding at heights of up to 50 metres. The complete suspended roof was erected on a series of 72 temporary props. The roof trusses were supported from the propped rafters until all the welding was completed. The props were then struck and the trusses allowed to span the full length of 150 metres. Tie rods from the truss support the front edges of the rafters, which in turn provide lateral support to the top boom of the truss in certain circumstances. Client Bolton Wanderer EC Architect Lobb Partnership Steel Designer Watson Steel Fig 20 View of the south stand under construction. Note the temporary props to the rafters and the roof truss sections being prepared for site welding Fig 21 View on the completed stadium Fig 19 One of the four separate roofs nearing completion. 167 HULME ARCH ROAD BRIDGE, MANCHESTER The 52 metre span bridge was constructed on a series of temporary trestles. The deck sections were pre- assembled on the adjacent ground and site welded in sections up to 18 metres square. The arch sections were also partially pre-assembled and the remaining joints in the 28 metre high arch were welded insitu and ground smooth afterwards. The cables were installed individually once the welding had been completed and the props removed from the arch. The cables were then tensioned by jacking before the remaining temporary trestles were removed from under the deck. Client. Manchester City Council Architect Chris Wilkinson Architects Engineer Ove Arup & Partners Fig 22 The bridge was erected over the busy dual carriageway during a series of road closures CHEK LAP KOK AIRPORT, HONG KONG. The 490,000 square metre roof structure was pre- assembled as 129 large panels up to 36m * 36m square. Each roof panel was fully site welded and painted and then carried over one kilometre to the final location before being lifted and slid in to position. The overall construction programme could only be achieved by pre-assembling the roof in parallel with the concrete substructure. Massive amounts of temporary works were required to assemble, transport and place the roof panels into position. Client Hong Kong Airport Authority Architect Sir Norman Foster & Partners Consulting Engineer Ove Arup & Partners Fig 24 The first fully welded roof panel in position Fig 25 Aerial view during construction. The separate modules have yet to be joined together by site welding. Fig 23 Note the continuously changing cross section of the plated box section 168 THE GREAT GLASSHOUSE, LLANARTHNE, CARMARTHENSHIRE The steel and glass roof has a total area of 4300 square metres. The geometry of the complex, doubly curved roof structure is part of a torus. The roof was constructed insitu on temporary trestles by site welding. The curved tubular ribs span up to 55 metres. The site joints were full strength butt welds and were ground smooth Client National Botanic Garden of Wales Architect Sir Norman Foster & Partners Consulting Engineer Anthony Hunt Associates Fig 26 The tubular curved arches were site welded insitu. Fig 27 The completed Glass house inclined to face the south. SUMMARY On Wide-span and complicated structures each and every erection scheme will have different priorities and different conditions which have to be taken into account. The one common and essential factor however is detailed planning and attention to detail Also the principles outlined in this paper with regard to pre-assembly, welding, temporary equipment etc. can be applied to most structures and will be equally valid. ACKNOWLEDGEMANTS Client The New Millennium Experience Ltd Architects Richard Rogers Architects Ltd Engineers Buro Happold Construction Managers McAlpine / Laing J.V.