SPINNAKER TOWER
The Spinnaker Tower is a 170 meter (558 ft) high tower in Portsmouth ,United Kingdom. It is the centerpiece of the redevelopment of Portsmouth Harbour, which was supported by a National Lottery grant and is aimed at transforming the waterfront of Portsmouth and Gosport. It adds a new international landmark to the South Coast of Britain.
Its shape was chosen by Portsmouth residents from a selection of concepts. Designed by local firm HGP architects, engineering consultant Scott Wilson, built by Mowlem, and steelwork fabricated by Butterly engineering, the tower reflects Portsmouth's maritime history by being modeled after a sail. The name Spinnaker comes from the shape of an exoskeleton which billows out like a sail catching the downwind.
The tower is 2.5 times higher than Nelson's Column( a monument in Trafalgar Square, London), making it the tallest accessible structure in the United Kingdom outside London. It is visible from miles around Portsmouth, changing the area's horizon. Spinnaker tower has won several awards including the RICS Project of the year award and the RICS Regeneration award (award recognises improvements to the built or natural environment that makes a long-term contribution to the viability and prosperity of an area.)
The tower represents sails billowing in the wind; a design accomplished using two large, white, sweeping steel arcs, which give the tower its spinnaker sail design.
DESIGN CONCEPT
At the top is a triple observation deck, providing a 320° view of the city of Portsmouth, the Langstone and Portsmouth harbours, with a viewing distance of 37 kilometers (23 miles). The highest of the three observation platforms, the crow's nest, has a wire mesh roof, allowing visitors to be in the elements. Windows are extended to above head height, so it is not possible to get a view unobstructed by glass.
Its design is similar to the Burj Al Arab in Dubai, whose structure is almost twice as tall at 321 meters. The tower has a design lifetime of 80 years and has the tallest observation tower in the UK with observation decks at 100 meters, 105metres and 110 meters providing views. One highlight of the decks is that the lower one contains a vertigo inducing glass floor allowing visitors to "walk on air”, while the upper most level is completely open to the elements and in keeping with the nautical theme of Portsmouth that has been dubbed "crows nest". These decks stand at the top of a curving exoskeleton that threads its way in between the legs of the tower creating a rib cage.
The Spinnaker Tower is lit by 28 Martin Architectural Exterior 600 colour changing lights to up light the front and top whilst there are a further 22 Martin Exterior 200 colour lights for the sides with LEDs fitted to the interiors of the viewing levels. All of this allows six different light shows including one called "Time" that cycles through the entire colour spectrum from blue to red over 125 minutes.
CONSTRUCTION
The project was over budget, with an overall cost of £35.6 million (47.2 million USD) for the tower alone. Taxpayers were not meant to fund the tower, but Portsmouth City Council eventually contributed £11.1 million (14.7 million USD) towards its construction.
PILES
Spinnaker tower stands on 1,400 millimeters concrete piles set from a floating platform in a job that took seven months to complete. During this time the piles were sunk to a depth from 36 meters to 51 meters with steel casing on the top ends measuring 18 meters to 36 meters.
BOWS
Two elegant crossed bows, fabricated at 10.8metres long and 1.5metres wide structural steel box sections, rise from the ground, crosses between the shafts at 35metres and rise to connect with the shaft at 120metres. The bow dimensions vary from 10-12meters in depth at the base and top of the tower, reducing to 2.5meters wide in the central region. Aerofoil shaped ribs span between the bows to give the distinctive ‘Spinnaker’ sail appearance.
SHAFTS
The tower comprises of two inclined slip-formed hexagonal concrete shafts, of 6 meters across, which merge into a single shaft at 70 meters. One shaft contains an internal express lift while the other shaft carries a panoramic external lift up the seaward face. A composite spire then forms the needle-like top to the concrete A-frame.
Viewing platforms
The tower provides three high-level trapezoidal viewing platforms that vary in size from 18 meters x 14 meters down to 12 meters x 4.7 meters for the highest open deck. A grid of I-section steel beams span between the concrete tower, steel bows and frontal rib beams at each level to support the 150 millimeters thick concrete floor slabs. Each platform was designed as a composite deck with vertical vibration from crowd loading considered as part of the design check.
A construction challenge was caused by the sheer amount of work that was needed on the dockside location requiring 84 piles sunk to depths of up to 50 meters, these support a weight of over 30,000 metric tonnes.
The result of the dockside construction is that the Spinnaker Tower is in the unusual position of having an identical Above ground level (AGL) and above ordinance datum (AOD)-height relative to the average sea level- as the ground level is at sea-level. Despite these challenges the tower is no doubt a roaring success with the public.
Modelling and Analysis
Static, nonlinear and dynamic analysis was all employed by Scott Wilson to analyse the model of the Spinnaker Tower. Ship impact, wind dynamics, wind-induced fatigue and the consideration of human perception of wind-induced movements at observation deck level all needed to be assessed in the design check.
To do this, a model of thick beam and thick shell elements was created and used as a basis for the three distinct analyses that were carried out. In doing this the arbitrary section property calculator built was particularly helpful in deriving the section properties required for the varying cross sections of the concrete towers and of the steel bows, helping speed-up the building of the model significantly.
Mokhtar El-Houry, Assistant Structural Engineer at Scott Wilson’s London office said: "Spinnaker Tower is quite unique in its design and required the use of finite element tools to accurately model its behaviour.
CONCLUSION
From the results of the various analysis it was seen that, despite its uniqueness and the use of different construction materials, the tower essentially behaves as a static/simple structure.
Now complete, the Spinnaker Tower has attracted more than half a million visitors annually within the few years of its opening. The build cost of the project was shared between three main financial contributors: the Millennium Commission, Portsmouth City Council, and the Berkeley Festival Waterfront Company Ltd.
PROJECT TEAM
Architect : HGP Architects
Developer : The Millennium Commission
Structural Engineer: ScottWilson
Services Engineer: Halcrow Yolles
Main contractor: Mowlem
