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Ancient Concept, Modern Context

The site in Alexandria, Egypt, of the most celebrated library in classical antiquity is seeing a worthy replacement rise in splendid contemporary form.

Mamdouh Hamza, P.E., and Mashhour Ghoneim, P.E. Photographs by James Willis

The library burned to the ground under mysterious circumstances, and while historians disagree about who was responsible for its destruction, most blame Julius Caesar, who sent fire ships in 48 B.C. to destroy shipping in the nearby harbor. It is believed these ships also accidentally set fire to the city's royal quarter.

With the intent of reviving the most famous library in antiquity and housing and preserving some 8 million books, the new Bibliotheca Alexandrina is being constructed on almost the same site as its predecessor. The idea of reviving the library was first proposed in 1974 but was quickly shelved. The proposal was revived in the late 1980s, when the United Nations Educational, Scientific, and Cultural Organization (UNESCO) issued an international appeal for contributions. Egypt's president, Hosni Mubarak, set up the General Organization of the Alexandria Library (GOAL), and an international design competition was organized. The $60,000 first prize was awarded to the architecture firm Snohetta, of Oslo, Norway, in September 1987. GOAL proposed six consulting engineers as candidates to form a consortium with Snohetta to develop the design, but the architect rejected all of them, preferring instead to nominate the consulting engineer Hamza Associates, of Giza, Egypt.

The new Bibliotheca Alexandrina, previous spread, occupies almost the same site as the famed library of antiquity. It features a tilted circular roof, above, oriented to minimize the amount of direct sunlight entering the building. The planetarium, in the bottom right of this photograph, is an 18 m diameter sphere that projects through a 24 by 24 m pit.

The design of the new library incorporates four basement levels and six floors under the highest point of a dramatic sloping circular roof (see figure 1[a]). The inspiration for this unusual shape was Snohetta's. And while the building has been designed to have an estimated life span of 200 years, there is little doubt that the most immediate threat to its longevity comes, as before, from its proximity to the Mediterranean Sea, only 40 m away. The library's orientation is dictated by the crucial importance of keeping the amount of direct sunlight entering the building to a minimum.

The library building is supported on a reinforced-concrete raft supported on piles. A planetarium and a science museum adjacent to the library building were constructed and supported on a raft without the need for piles.

The library is 10 stories high and is contained in an elliptical envelope with a major axis of 160 m. The four basement levels are below the water table. The geometry of the building follows the functional arrangement of bookshelves and reading desks in identical structural bays measuring 14.4 by 9.6 m (see figure 2). Under the 16-degree slope of the roof, the bays rise by a series of 4.2 m high stories. However, because the cylinder is tilted at an angle of about 8 degrees to the vertical, the floor plate and the roof form true circles in the horizontal floor plate and the roof plane.

Like London's British Library, the bulk of the building is sunk underground to protect its precious contents from the external environment. The dominant element of the Alexandria library is an enormous open-plan reading room incorporating 2,500 reading sections that cascade down seven terraces. For easy access, books are stored below the terraces.

The groundwater level-0.8 m above sea level-is 11 m above the basement slab level. The designer, Hamza Associates, briefly considered constructing the basement within a circular sheet pile cofferdam, but the sheer size-at least 160 m diameter-ruled out internal propping and Egyptian law would have banned the use of ground anchors beneath the existing buildings in close proximity. So a bold decision was made. A diaphragm wall, believed to be the largest ever attempted, was constructed to a total depth of 35 m. The wall was continuously reinforced, a technique that has never before been tried outside of the Far East.

The library superstructure consists of a reinforced-concrete frame comprising a flat floor of 400 mm deep waffle (ribbed or hollow-block) slabs, with ribs at 1,000 mm centers. The ribs span to central hidden beams that are 400 mm deep, which in turn span to columns and walls at 7.2 and 9.6 m centers. At the lower floor levels, where heavy live loads are applicable, the floor construction relies on 600 mm deep waffle slabs with corresponding 600 mm deep hidden beams. Reinforced-concrete walls were incorporated into the construction to account for the lateral loading associated with wind and earthquakes.

There are two additional reinforced-concrete walls on the interior of the diaphragm wall. One of these two walls stops at the ground level and is separated from the diaphragm wall by a continuously drained waterproof membrane; this is called the outer wall (see figure 3). It carries vertical loading from the basement floors and is capable of resisting water pressure should the diaphragm wall leak at any stage during the structure's 200-year life expectancy. In some areas the outer wall is restrained from lateral movement by the floor construction, whereas in others, counterfort cross walls link it to the second, or internal, wall. The internal wall is a partially sunken cylinder that creates a circular plan shape and supports etched granite cladding panels.

Because of the critical ground conditions, the complex has required extensive foundation and ground engineering work, including excavation, dewatering, bored piles, and jet grouting improvements (see figure 1[b]). All of the geotechnical work was performed from the working platform at 2 m above sea level. The groundwater table is about 1 m below the working platform and is influenced by the sea tide.

Fig. 1 Longitudinal Sections

Fig. 2 Inside Alexandria's New Library

1. Reading room with main book storage below 2.Viewing platform 3. Restaurant 4. Public entrance 5. Offices and ancillary spaces 6. Existing conference center 7. High-level walkway 8. Comiche road

The earth and water retention required for the deep basements has been accounted for by the diaphragm wall construction. The diaphragm wall extends to the sandstone at a depth of 35 m. In conventional diaphragm wall design, the reinforcement of panels is effected through discrete vertical cages with no connections between the reinforcement of the adjacent panels. When the soil inside an unsupported, unanchored wall is excavated, the wall acts, in effect, like a series of individual cantilevers. But unless these concrete cantilevers are very thick and very heavily reinforced, water leakage can occur. Hence, the innovative idea of constructing a circular wall that would have continuity in its horizontal reinforcement looked very promising. This wall will act like a homogeneous cylinder, resisting earth pressure through the development of hoop stress. The wall will be a much more efficient structure and will significantly reduce the weight of the reinforcement.

Unfortunately, the building is not a complete circle in plan. In the straight part of the wall, vertical prestressing was used to guarantee watertightness and reduce the amount of vertical reinforcement to values that ensure the quality of the concrete work. At the junction between the circular and straight sections, the large hoop forces required box-type wall segments to safely transmit straining actions between the two parts. In addition, the segments of the circular part near the junctions needed to be stiffened by a web.

Because of the asymmetrical design of the internal superstructure, variations occur in the intensity of loading to the piled raft. Large dead loads occur to the south of the site, placing the piles in compression. At the north section of the site tension piles are necessary to account for the uplift created by the water pressure as a result of minimal dead loads. In the central area of the site, the piles are in tension or compression, depending on loading conditions. There are 131 piles 1,500 mm in diameter with a single underream, or bell-shaped bottom, that are mainly under compression; 325 piles 1,200 mm in diameter with two underreams that are mainly under tension; and 143 piles 1,000 mm in diameter with two underreams that can be under compression or tension.

The compression and compression/tension piles were arranged in a grid of 4.8 by 7.2 m. The tension piles were arranged in a grid of 4.8 by 4.8 m. All the piles were reinforced along their full length. In the compression piles, however, the amount of steel was reduced in the lower half of the pile.

Piling was carried out to very tight limits. Steel castings were driven down to limestone before boring commenced. An innovative feature of the piling operations was the underreaming tool used, perhaps the first ever to feature hydraulic operation and an in-cab readout of depth and diameter. The single or double underreams it produced were checked for shape by an ultrasonic profiler developed in Japan by the firm Koden. This assessment also gave a quick readout of the alignment and cross section of the main shaft before the reinforcement cage was lowered. Sonic integrity testing was carried out after the concrete was placed by means of steel pipes cast into the piles. Where any defects were discovered, holes were drilled through the pile to the level of the defect and the resultant void was grouted. The pile was then retested.

Fig. 3 Foundation Sections

The complex grouting operation began within a few hours of the concrete placement. The aim of the grouting was to improve the base bearing and the shaft bond capacity. French style tubes à manchette were used. Each pile had four plastic tubes cast into the concrete cover zone, and each tube was drilled with four circumferential holes every 500 mm along its length. These holes were sealed with rubber collars, and the tubes were connected in pairs to form flat U shapes at the bottom of the piles. Pumped water broke the collars and cracked the concrete cover. Grout then flowed between the pile and the surrounding shaft. The grout also filled any fissures in the sandstone and limestone and ensured contact between the concrete shaft and the surrounding concrete. The bases of compression piles were grouted by connecting the grout tubes to the base.

The thickness of the raft supported on the compression or the compression/tension piles is 1,275 mm. The raft was connected to the fourth basement slab by a system of vertical webs and short columns. The space between the top of the raft and the bottom of the fourth basement slab has been left open for future inspection. To reduce the amount of tension in the tension piles, additional dead load has been incorporated into the structure by increasing the thickness of the raft from 1,275 to 2,400 mm in the area where the raft is supported on tension piles (see figure 3[a]). The increased raft thickness also increases the flexural capacity of the raft in case any damage occurs to the tension piles in the future. To minimize the bending moments in the raft, all of the supporting columns are lined up nearly vertically with the piles in a one-column/one-pile concept. The shear walls and cores, however, transmit their loads indirectly to the supporting piles through the bending action of the raft.

The reinforced-concrete raft was cast on top of a 200 mm thick raft of plain concrete on which a waterproofing membrane had been placed. The raft is structurally separated from the diaphragm wall. To ensure full insulation for the library building, the edges of the raft are connected to the outer wall. The outer wall is connected to the diaphragm wall by a very stiff capping beam to ensure that the diaphragm wall works together with the other elements of the building in resisting any actions caused by earthquakes (see figure 3).

The inner face of the diaphragm wall is covered by high-density polyethylene (HDPE) sheets. The fact that about one-third of the total number of piles will be permanently subjected to tension created a challenge, especially when a life span of 200 years was considered. Part of the solution involved limiting the widths of potential cracks in the tension piles by limiting the stresses in the steel reinforcement. Special care has been taken to ensure that the connections between the tension piles and the raft foundation are watertight. All steel rebars in the top 1.2 m of the piles were placed inside HDPE sleeves with cementitious, nonshrinking grout in the space between the bar and the sleeve.

The granite exterior of the library, above, is etched with letters from as many of the world's alphabets as could be found. Slender concrete columns with capitals, below, are spaced on a 14.4 by 9.6 m grid to support the roof.

Despite the fact that the library building has floors with very large spans and walls that have a perimeter of about 570 m, no expansion/contraction joints were provided. This required the designers to conduct a sophisticated thermal analysis, one that regarded the entire building as a single unit under the effect of uniform as well as nonuniform changes in temperature. A three-dimensional finite-element computer model was developed to idealize the library building, the raft foundation, and the piles. Three-dimensional shell elements were used to model the raft, the floor slabs, and the walls. The columns were modeled using three-dimensional frame elements. The piles were modeled using one-dimensional spring elements. The stiffness of the piles varied depending on the diameter of the pile and on whether it was subjected to tension or compression.

Because of the necessity of providing large spaces that are free of columns, the spans of the library floors are relatively large and the dimensions of the concrete columns are relatively small-1,000 mm in diameter. Waffle slabs were used to cover the large spans. The small dimensions of the columns have been compensated for by increasing the amount of vertical reinforcement and using relatively high strength concrete. Shear walls and cores were designed to provide the required lateral load resistance.

It is only from the inside of the library that the scale of the project becomes apparent. Its tall, slender columns and high roof create a cathedral-like atmosphere. The elimination of columns in some floors required the use of hangers with which the floors above are supported. A sophisticated structural analysis was required to examine the stability of the structure supported on reinforced-concrete hangers under various load cases. The concept of having alternative load paths was also adopted in such cases. Should the hangers fail, the hanging floor will still be stable, but it will suffer some serviceability problems, such as deflections or cracks. These problems, however, will be correctable.

The columns supporting the roof of the library are spaced on a grid of 14.4 by 9.6 m. These columns, including capitals, are up to 16 m in height and 70 cm in diameter. They are rigidly connected to their lower ends and restrained from lateral movement by the roof girders at the top ends. Column capitals are precast on the ground, lifted to their specified position on top of the cast columns, and rigidly connected to the columns by welding around the steel base and sole plates.

The exterior of the new library presents a monochromatic expanse in the form of the cylinder's perimeter wall (the inner wall) as it rises above pavement level. Far from being a smooth, metallic, high-tech skin, this is a curving cliff face of roughly cleft gray granite quarried in southern Egypt in Aswan. The only interruptions in the granite face are the large letters that have been carved into its surface. To symbolize world literature, the letters have been taken from as many alphabets as could be found.

Visitors to the library will first encounter a curved expanse of rough gray granite, above, under the gleaming tilted roof, but it is only from the interior that the true expanse of the building will become evident. The roof glazing and cladding, below, are supported by reinforced-concrete girders as well as a secondary steel system.

The design of the adjacent planetarium seeks to suggest a planet in space. The structure was therefore designed as a complete sphere, 18 m in diameter, suspended in air. The upper two-thirds of the sphere is projected above the ground level, while the lower third is hidden inside a 24 by 24 m pit with the bottom slab 13 m below the center of the sphere. The sphere hangs from two orthogonal through-type bridges used for visitor access to the facility. The bridges are supported on top of the reinforced-concrete walls of the pit. The access and stage floors of the planetarium are located at the levels of the bottom and top chords of the bridges, respectively. The sphere cladding is constructed of black-striped lightweight precast glass-reinforced cement plates, while the bridges are clad with silver aluminum.

The structure of the sphere is built up from steel tube members connected to one another by patented ball joints. Each of the two bridges spans 24 m, with their two girders 3.1 m in height and spaced 2.4 m apart. Each girder is a combined truss/Vierendeel system, with the diagonal members inside the sphere eliminated to make the space in the access floor usable. Each girder has one restrained and one movable neoprene bearing. The members of the main girders have closed box cross sections. The two girders of each bridge are connected horizontally by bracing members in the planes of their top and bottom chords.

The transfer of loads from the sphere to the bridges is through two horizontal, circular ring beams provided at the levels of the top and bottom chords. The upper part of the sphere rests on the top ring beam while the tower part is hung from the bottom ring beam. The two ring beams are interconnected by tubular members to complete the sphere and to form a circular steel deck. The deck is supported on traditional secondary beams carried by the bridge chord members and the ring beams. The complete steel structure-including the members of the geodesic sphere, the two horizontal circular ring beams, and the four truss/Vierendeel girders-was analyzed by the finite-element method. Static and dynamic analyses were carried out.

The Bibliotheca Alexandrina has been a long time in the making. An archaeological investigation was begun in 1992, but no identifiable traces of the original library were uncovered. Construction of the main diaphragm wall for the library together with piles started in May 1995, and casting of the piled raft started in July 1996. The superstructure of the library was completed in February 1999, and the building will be occupied in July of this year.

Most of the project funding has come from the Egyptian government. UNESCO is also supporting the project in technical matters relating to the running of the library and the acquisition of books. The foundation works were carried out by a joint venture of the Rodio Group, of Lodi, Italy; TRAVI Foundation Specialists, of Cesena, Italy; and Arab Contractors, of Cairo. The completion of this unique and imposing building will be carried out by a joint venture of Arab Contractors and the London-based contractor Balfour Beatty Plc.

Although it took 12 years to develop, the new Alexandria library was built in strict accordance with the design that won the international competition. The new Bibliotheca Alexandrina is a testament to modern architectural and engineering design, and it could, given the texts and publications from all over the world that it will contain, become a major attraction for cultural and intellectual pursuits to rival its predecessor.

Mamdouh Hamza, P.E., is a professor of soil mechanics and foundation engineering in Egypt at the Suez Canal University and is the chairman and founder of the consulting firm Hamza Associates, of Giza, Egypt. Mashhour Ghoneim, PE, is an associate professor of structural engineering at Cairo University and a senior associate of Hamza Associates.

Source: CIVIL ENGINEERING MAGAZINE, March 2001