alFaisalahCenter

8/9/2019 alFaisalahCenter

1/4

Introduction

Reinforced concrete is the most popular

construction material in Saudi Arabia. Generally, its use is confined to structures with a maximum of 15 floors. The recent pioneering work on reinforced concre

te high-rise towers in Riyadh, the capital, has necessitated the rapid assimilation of new knowledge and construction methods by the local Saudi contractors. The first of the Riyadh high-rise developments to be completed was the Al Faisaliah Centre, designed by Foster & Partners and Buro Happold Consulting Engineers.Design and engineering

This landmark development of the King Faisal Foundation includes a signature 267m-high office tower, a 224-bedroom five-star hotel, banqueting and conference facilities, a residential apartment building, and a retail mall, all around an urban public plaza. The office tower is 47 x 47m in plan at its base, soaring to atapering point in a smooth giant arc. The floor area decreases from 1500m^sup 2^at the base to 500m^sup 2^ at the top, providing a total floor area of 56,600m^

sup 2^. The lower two-thirds of the tower is of reinforced concrete, clad with silver-anodised aluminium and glass. The top 90m is made from partially clad openstructural steelwork intended to carry telecommunications dishes. At the transition of the two is a 24m diameter glass-clad geodesic sphere in structural aluminium. This encloses three floors of exclusive dining above an external observation platform.

With little local experience of structural steelwork fabrication and construction and structural steelwork expensive to import, Buro Happold selected concrete as the most cost-effective structural material for the tower. A `high-tech' concrete solution was developed, embodying state-of-the-art prestressing techniques.These contributed considerably to the visual appearance and success of the development.

To minimise the depth and weight, floors are constructed as post-tensioned ribbed slabs spanning between a central core and the perimeter elevation. The centralcore provides virtually all the horizontal wind stability and carries 65% of the vertical load. It also houses the lifts and escape stairs. Foster & Partners was keen to maximise the visual transparency of the elevations. To achieve this,the tower is arranged as three buildings of nine-, ten- and eleven-storeys placed above each other and separated by K-braced technical floors. The K-braces transfer the vertical loads from the perimeter columns of the block above to the massive corner columns. In this way, the perimeter columns are kept exceptionally light for a tower of this height, at 450 to 600mm diameter. The horizontal thrusts induced by the K-brace compressions are resisted by large tension forces (25,0

00kN) within the tie beams that girdle the tower at the base of the K-braces. These tension forces are provided by prestressing bars, stressed in stages as theblock above is constructed.

One problem that had to be overcome was the differential axial shortening of thecorner and perimeter columns relative to the central core. The combined effectof elastic shortening and time-dependent shrinkage and creep for the highly stressed slim perimeter and corner columns had to be estimated and compared with that of the moderately stressed, relatively stiff central core walls. The largest vertical deflection due to these factors was estimated as 135mm in the corner columns during the life of the building. The largest core deflection was estimatedat only 85mm. Physical height compensations were then made during construction.For example, the floor slabs at each level would be approximately horizontal bot

h immediately after completion of construction and remain within an acceptable limit until demolition of the structure.


2/4

The design of the top of the tower was changed during construction, as a 24m diameter sphere was introduced at the base of the top steelwork section. The central concrete core was transformed into a reduced circular section that extended vertically through the sphere to provide horizontal stability. It also operates asthe vertical support for the three levels of cantilevered concrete floors within, while the core acts as a vertical support to the central steelwork mast above. The spherical structure thus derives horizontal restraint from the concrete co

re at both its top and base and is reduced to a light aluminium geodesic structure giving maximum visibility over the city below. The hotel, shopping mall and apartment building are of conventional reinforced concrete construction. They have prestressed elements, both for larger spans and to create the transfer structures and cantilever canopies.

IMAGE PHOTOGRAPH 11Figure 1:


Banqueting hall

Beneath the public plaza in front of the tower lies a spectacular banqueting hall. Designed to accommodate 2000 diners under a clear spanning concrete arch-supported roof, the interior measures 57 x 92m with no internal columns. The banquethall is a multifunctional space and can be divided into numerous configurationsby movable walls. When configured as a single conference space, it can seat 4500 delegates. The concrete slab roof is carried by eight pairs of inclined arches, whose feet are tied together by multi-strand prestressing tendons within the floor slab and resisting lateral thrusts.

Construction

Construction of the Al Faisaliah Centre started in April 1997 when the Saudi Binladin Group (SBG) took over the completion of the bulk site excavation from thedemolition, site clearance and enabling works contractor. The central core and corner columns of the tower are supported on a contiguous raft foundation. The central portion under the core is 4m deep and 31m^sup 2^ in area and under each corner column 3m deep and 10m^sup 2^. The raft foundation for the tower is believed to be the largest continuous concrete pour to have been placed in Saudi Arabia. Over 6000m^sup 3^ of concrete were placed in 17 hours.

Care was taken so the temperature build-up - due to hydration heat from the large volume of concrete did not generate temperature differentials within the raftthat would compromise the concrete strength. Following studies carried out on similar pours at such developments as Petronas Towers, Kuala Lumpur, a target maximum temperature differential of 30degC between the core of the raft and base orsurface was considered acceptable.



The 28-day cube strength for the raft was specified at 40N/mm^sup 2^, with a minimum cement content of 370kg/m^sup 3^ of sulfate-resisting cement and a maximumwater/cement ratio of 0.45. Local limestone coarse (maximum 20mm) and fine aggre

gates were used, together with a water-- reducing/retarding admixture and superplasticiser. To minimise the temperature of the fresh concrete, SBG stipulated that the ready-mix suppliers should use cement that had been stored for at least 3


3/4

0 days to dissipate its heat of manufacture. The usual precautions of using crushed ice in the mix water and evaporative cooling of the aggregate stockpile werealso taken. With the daily ambient shade temperature 30-40degC, the temperatureof the concrete, as delivered to site, varied between 31deg and 27degC during the night. Nine sets of three thermocouples were cast into the raft to monitor concrete temperatures in the base, centre and top layer. The concrete at the centre of the raft peaked at 79degC after seven days and remained above 70degC for ov

er 15 days. Curing was achieved using a curing compound and a polythene sheet. The temperature of the top surface was further controlled by covering the surfaceand any exposed sides with 50mm thick insulation boards during the cool nights.These were removed during the day to allow heat to escape without exceeding thepermitted temperature differentials. After 25 days, the centre was still at 62degC and cooling at a rate of about 1degC per day. The maximum recorded temperature differential was 21degC between the centre and the base of the raft.

Another first for Saudi Arabia was the use of self-climbing formwork for the central core and corner columns. The core was cast in lifts 4m high and was advanced three levels ahead of the typical floor, with corner columns proceeding one level ahead of the typical floor. Table formwork was used for the 350mmthick post-

tensioned floor slab. SBG worked on four levels at a time, the upper level beingprepared for pouring while the level below was curing and the two levels belowthat being used for propping. A casting cycle of four to five days per level wasachieved. Post-tensioning was by strands in the form of sheathed cable anchoredwith a dead anchor in the slab adjacent to the core wall and stressed from theperimeter spandrel beam.

K-braces were constructed from reinforced concrete at the lowest level and fromwelded-plate steel box sections in the upper two levels. These were connected tothe lower tie beams by fabricated steelwork thrust blocks. Bearing plates werewelded to the apex of the struts and cast into the concrete transfer beams above. Tie beams were stressed by high-tensile steel bars to counter the thrust fromthe K-braces. SBG used a combination of mobile and static concrete pumps, workin

g in tandem, to deliver the concrete to a maximum height of 200m above street level. A traditional concrete skip suspended from a tower crane was also deployedon occasions, and was available as an emergency backup.

Construction of the concrete arches for the banquet hall presented interesting challenges. To keep the level of the finished plaza slab at a reasonably accessible height above existing street level, banquet hall arches had to be shallow. This resulted in particularly concentrated reinforcement, requiring SOmm mechanically coupled splices in many locations. The cross-sections of the leaning archesvaried from 750 x 1500mm in depth at the centre point to a combined section of 1300mm^sup 2^ at the springing points. The upper and lower curves of the arch follow slightly differing radii. The two sides of the arch also follow different curves, providing a rectangular section that smoothly varies, both in height and breadth, while maintaining a cross-sectional area of about 1.25 m^sup 2^. The main arch sections were constructed from 45N/mm^sup 2^ concrete, with 60N/mm^sup 2^concrete for the buttress sections. Superplasticising admixture minimised shrinkage in the arches and resulted in concrete that was virtually self-compacting and could be placed through the dense reinforcement.

Concrete was also used as the main cladding material for the lowrise buildings.The hotel, apartment building and shopping mall are all clad in Riyadh stone-coloured acid-- etched precast panels. To achieve the required fine surface flatness tolerance, SBG cast the panels on a form faced with float glass.

Construction was supervised by a Buro Happold/Foster & Partners site team of eng

ineers, architects and inspectors directed by the author. The result is a standard of construction that would be commended anywhere in the world.


4/4



AUTHOR_AFFILIATION

Eddie Pugh, Buro Happold

Documents

alFaisalahCenter