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Hidden giant

Alexandria Library

It is not until you are inside the Alexandria Library that you realise how big it is. Outside, most of the structure is hidden below ground and its dimensions are disguised by its circular shape and sloping roof.

Once inside, it is a different story. The main reading area is a massive, 10-storey high, concrete cavern. The 10 floors step like terraces all the way from the fourth basement level up to the sixth floor. And, high above, the 160m diameter roof falls sharply across the whole space. Its long, sloping concrete beams are supported by rows of even longer columns. Shafts of coloured light from tinted windows in the roof give the feeling of being in a cathedral.

The huge dimensions, long beams and tall, elegant columns are not the only things that have made the Alexandria Library challenging for the Arab Contractors/Balfour Beatty (AC-BB) joint venture. The unusual geometry of the building added its own complications.

One of the first issues was how to achieve the high-quality finish required by the designers on the exposed concrete elements. Initially the JV wanted to precast the elements off site, but this was not possible.

'The longest columns are 16.15m long and 700mm in diameter and weigh nearly 13t, ' explains AC-BB chief engineer Iain Wilson. 'They would have had to be cast vertically off site and carried there flat by lorry before being lifted vertically on to site using two cranes. Lack of space on site for so much craneage and the risk of damage meant the columns had to be cast insitu.'

The heavy reinforcement needed in the slender columns made pouring difficult. The contractor used three separate lengths of tremmie pipe and internal and external vibrators on each column to ensure the concrete was properly compacted. Though even the biggest columns required only 6m 3of concrete, each took two to three hours to pour.

Sikament NN superplasticiser was used with a retarder to ensure there were no construction joints. Formwork was manufactured in the UK by Chart, the Kent-based specialist manufacturer.

'The quality of formwork was the subject of one of our early discussions with the JV board, ' says project director Jack Thomson. 'But the problem was understood by them and the right decision was made.'

During Phase 1 the contractors had been unable to pump the mix because their design for the diaphragm walls used a high HSBF cement content of 470kg/m 3. On Phase 2, the joint venture changed supplier and found a slightly higher strength cement. This enabled the cement content to be reduced to 420kg/m 3which allowed the concrete to be pumped. This mix was adjusted with different dosages of superplasticisers.

'Everything comes back to geometry, ' says Adel Rizk. 'We were limited with the tower cranes so we had to pump the concrete a lot more than we would have done. This meant we had to be careful with the mix. On Phase 1 the contractor used Rotec conveyor belts to get around pumping but that was cost-prohibitive to us.'

Eighty-eight splayed column capitals connect the columns with the sloping roof beams.

These had to be precast because their awkward asymmetric shape and textured finish would have been extremely difficult to achieve through insitu casting at height.

But their 9t weight was outside the capacity of the site's tower cranes, which meant they could not be precast off site.

'We did consider casting them hollow to reduce the weight to 6t, ' says Wilson. 'But that was rejected because it made colour matching and handling more difficult. And it would have been hard to achieve the weight saving required for the tower cranes.'

The solution was to cast the column heads immediately adjacent to the columns, then raise them into position with bespoke lifting frames designed by British consultant Tony Gee.

These gallows-shaped frames are fixed to the columns and were fabricated in the UK by Derby-based manufacturer Butterley. They were designed in a way that meant they could be broken down into 3t segments. This allowed them to be placed anywhere on site by the existing tower cranes.

The capitals were cast in steel form boxes specially fabricated in the UK by Chart.

The four sides of the coneshaped boxes peel apart to make stripping easier. The base of the box consists of a circular steel plate with starter bars welded to the top side which are cast into the capital. This is welded to the top of the column. Tapered circular shims measuring 4mm to 8mm are used to correct the alignment of the capital on top of the column.

Completion of the watertight roof was crucial to the programme so that watersensitive finishing and M&E work could be carried out. This made installation of the long span roof beams critical.

But once again the size of the elements - each beam is 14m long by 1.8m deep by 300mm wide and weighs about 20t - and the 80m radius of the building meant the largest beams could not be lifted in.

They are outside the reach of mobile cranes at the perimeter and outside the capacity of the site tower cranes.

'At tender stage we proposed lifting the beams with a large gantry crane on heavy steel rails which could move across the hall to install the 429 beams sequentially, ' says Wilson. 'We later rejected this because there were a number of drawbacks with this plan. The gantry itself weighed over 20t and required a 350t crane to move it from bay to bay. Progress of the roof depended upon the performance and reliability of a single bespoke piece of equipment. And, thirdly, the roof would have had to be constructed entirely in a single sequence, restricting programming.'

Casting the beams insitu was also considered. But casting concrete on the roof slope at an angle of 16¦ would have been extremely difficult.

There was also a need to achieve the 5mm tolerance required to tie in with the complex secondary steelwork frames that support the aluminium roof cladding. This course of action would have required a massive amount of formwork and falsework which would have blocked large areas of the site.

The solution was to reuse the lifting frames employed for the column heads. Because the capacity of each of these frames was only 10t - half the weight of the beams - they had to be used in tandem. To ensure safe co-ordination of the tandem lifts the mini cranes were controlled remotely with the operators standing together at ground level.

Five gallows frames were made so that progress was not dependent on a single piece of equipment. It allowed installation of the roof to be carried out at any point, providing flexibility in the sequence should any problem occur. It also meant that the beams could be cast on the ground adjacent to the lift to minimise any risk of damage during handling.

Sequencing the 429 roof beams and 88 precast column head lifts was complex. With 30 lifts a week, planning was crucial. The site management held weekly meetings with the site foremen and planners to work out the following week's lifts.

Egyptian engineers were responsible for drawing up the plans for the lifts.

'They were responsible for everything right down to details of the anchorages and lifting points in Balfour Beatty's structural drawings, ' Wilson says. 'There were over 100 separate orders for precasting from Switzerland, Scotland, England and locally from Egypt.'

Wilson says the joint venture was able to start the programming planning during the course of the Phase 1 foundations package, long before the jv came to site. 'So we were able to hit the ground running, ' he says.

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