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Space odyssey

Alexandria Library

Alexandria Library's tall slender columns and high roof create a cathedral-like atmosphere - and wide spaces that represented a challenge for the engineer, Hamza Associates.

'Because the shelves run on rails the design required zero differential settlement, ' explains Dr Mamdouh Hamza.

'That meant there had to be 'zero' deflection over the 14m span. Also the columns had to be very long and slender in order to provide maximum column free space and beauty.

Some are nearly 16m high and only 700mm diameter.'

Roof designer and Hamza vice chairman Dr Ahmed Rashed adds: 'The columns supporting the roof are spaced on a grid 14.4m x 9.6m. These columns including capitals are up to 16m in height and are rigidly connected to their lower ends and restrained from lateral movement by the roof girders at the top ends.

'The column capitals are precast, lifted to their specified position and rigidly connected to columns by perimeter welding of steel base and sole plates.'

The roof structure is a grid of twin precast concrete beams simply supported on column capitals. The single precast roof beam weighs up to 18 tonnes.

The beams are fixed to the supporting column capitals by single anchors allowing rotation of supports, thus eliminating temperature effects.

These beams form rectangular roof panels which support a secondary steel structure under the roof steel, aluminium and glass cladding.

'The raft is structurally separated from the diaphragm wall. In order to ensure full insulation for the library building it was decided to connect the edges of the raft to an outer wall, ' says Hamza senior associate Dr Mashhour Ghoneim. 'We also had to consider the possibility of a defect in the waterproofing system and whether the outer wall could resist the expected pressure of water.'

The outer wall is connected to the diaphragm wall by a very stiff capping beam.

'We also recognised that the density of reinforcement would pose a significant challenge to the contractor in the construction process, ' Ghoneim points out. 'In a typical section of the raft there are two bottom and two top layers of reinforcement. The bars are 40mm diameter at 165mm centres.

'When we looked at lapping of bars we concluded that due to congestion of reinforcement we would have to use welding.

The average density of reinforcement was 145kg/m 3with some areas exceeding 180kg/m 3.'The design of the concrete elements was carried out according to the American Code ACI 318, ' Ghoneim says.

'Due to the importance of limiting the crack width in the raft and the elements that might be subjected to relatively severe exposure conditions, both the requirements of ACI Code and the British Standard Code of Practice 8110 were respected.

'Furthermore, in the raft, crack width was limited by reducing the allowable stress in reinforcing steel to about 900kg/cm 2in the bottom steel layers. This reduction in steel stresses is intended to keep the crack width within 0.2mm.'

The working stresses in the top reinforcing layers were about 1400kg/cm 2. 'And the reduction in steel stress to limit the crack width has resulted in an increase in the amount of steel in the raft.'

All the supporting columns are almost lined up vertically with the piles, ie a one column/one pile concept, in order to minimise the bending moments in the raft. Shear walls and cores, however, transmit their loads indirectly to the supporting piles through the bending action of the raft.

Since the foundation is subjected to uplift forces this effect was thoroughly investigated in the context of resistance to pullout and for structural capacity.

Ghoneim points out: 'To achieve the small concrete dimensions, we increased the amount of vertical reinforcement and used relatively high strength concrete. Because the library is in a Category 2 seismic zone, large amounts of transverse reinforcement were also used in the columns to provide the required ductility in earthquakes and fulfil the 'strong column - weak beam' requirement of the American Code.

'The elimination of columns in some floors led to the necessity of using hangers to support some floors from the floors above, ' he says. This called for a sophisticated structural analysis to examine the stability of the structure supported on reinforced concrete hangers under various load cases. In such cases we also examined the concept of having alternative load paths.'

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