When the world watched the opening of the Athens Olympics this August, it beheld an engineering marvel.
The Athens Olympic Stadium has a suspended arched roof construction, designed by Spanish architect Santiago Calatrava. The roof is easily the most visually striking part of the total refurbishment of the original stadium.
This “jewel in the crown,” as the stadium’s roof has come to be known, is sure to be the trademark of the 2004 Olympic Games. Its two giant arches have a total span of 1,000 ft. (305 m) and soar to a maximum height of 262 ft. (80 m).
Each arch supports the cables that hold the polycarbonate roof panels, which weigh a total of 17,000 tons (15,422 t) and cover an area of 107,500 sq. ft. (9,987 sq m), covering the 75,000 spectators and athletes alike. Ninety-five percent of the stadium’s seating will be covered, as compared to 35 percent in the original stadium.
The graceful sweeping roof structure is constructed of metal and glass with a special coating designed to reflect 60 percent of the sun — a feature very important during the Greek summer. Its unique architectural and structural design makes it a particularly unique project, because the existing stadium was refurbished while the roof and arches were being constructed and erected.
Building the Roof
The roof structure — the design of which is a double bowstring tied arch — was assembled close to the sides of the stadium. Arches and roof were constructed separately in two halves, positioned 230 ft. (70 m) from either side of the stadium. They were built remotely so as not to interfere with the underlying stadium.
This remote construction, however, gave rise to an important question, namely how to move and position the two roof halves, each weighing 8,500 tons (7,710 t).
The roof structure was assembled close to the sides of the Oaka Olympic Stadium. Each arch consists of two 11 ft. (3.5 m) diameter steel tubes with a span of 997 ft. (304 m) supporting the new state of the art roof.
Moving 17,000 Tons of Roof
The arched roof is built on two shoes supported by four Teflon sliding pads. The original plan was to move each roof assembly by pulling it step-by-step using long strands, chucks and short-stroke hydraulic jacks. This technique meant the structure’s shoes had to slide over thin stainless steel plates welded onto special rails resting on huge concrete beams.
However, computer simulations highlighted potential problems using strands to pull the roof into position. It was calculated that, because of significant elastic energy stored by the strands, the difference between the static and dynamic frictions of Teflon and stainless steel could create a stick-slip issue related to the relatively high acceleration and stops during each step of the pulling process. This was unacceptable to the designers, who sought another solution.
Enerpac, integrated solutions specialist, was consulted because of its expertise in major construction and civil engineering projects. Enerpac is currently involved in a number of world-first engineering projects, including the 1,200-ft. high Millau Viaduct in France, the highest bridge in the world.
For the Athens Stadium, Enerpac proposed long stroke-pulling cylinders powered by PLC-controlled hydraulic pumps. Used for the step-by-step pulling process, these would have very little impact on the existing design.
These double-acting pull cylinders are attached to shoes in a sliding and guiding system pulling the arched roof assembly step by step into the final position.
Four pulling cylinders were proposed for each shoe of the arched roof assembly. Because each arch had two shoes and each shoe had four sliding pads, this meant a total of eight hydraulic cylinders were necessary to move and position each half of the roof.
A two-stage PLC-controlled pump was specified to power each group of four cylinders. This arrangement offered full control of speed, acceleration and deceleration during hydraulic movement.
The PLC control also was necessary to ensure digital monitoring of the pump parameters. This integrated hydraulic system was designed to provide smooth movement and positioning of the arch.
Controlling the Hydraulic Movement
During the roof positioning, which took place in May and June, the PLC-controlled pumps were located on the sides of each arch and carried with them during the pulling process. Each pump supplied all four cylinders with the same hydraulic pressure and the same pulling force while the stroke synchronization is assured by the shoe structural strength. Both pumps were connected with a 1,300-ft. (396 m) cable and synchronized by the PLC-unit to control the start and stop of the hydraulic movement of the 8,500-ton (7,711 t) roof half.
Very smooth starts and stops (while maintaining an acceptable speed) were facilitated by using a microprocessor-controlled variable-frequency AC motor drive. This unit controlled the electric motor speed, thus controlling the pumps’ oil flow and therefore the retracting speed of the pulling cylinders.
The PLC-control unit was used to vary motor speed (rpm) linearly, within the operator’s defined time intervals, from a minimum to a maximum and vice versa. In this way, a very low operational speed was achieved once the arch movement was started. The speed was kept low for a period in order to stabilize the system.
In the next step, the pumps’ oil flow was increased linearly (acceleration ramp) to ensure a constant and carefully controlled acceleration up to the pulling speed as defined by the roof design team.
The pulling process continued with the cylinder stroke at a constant speed. Once the cylinder mounted micro switch detected the end of the plunger stroke, the hydraulic system automatically decreased linearly (deceleration ramp). This reduced cylinder retraction speed to a minimum, then stopped the pulling process altogether.
The process started again after reloading (extending) all cylinders at high speed.
Advantages of Integrated Hydraulic Solutions
The Athens Olympic Stadium project showed how integration of hydraulics and electronics into one system for controlled hydraulic movement can solve important problems where conventional and traditional methods fail. With the use of PLC-controlled systems, hydraulic movements become very simple, reliable and safe. Every step in the process can be monitored at any time. Advantages of an integrated solution include:
• Use of double-acting, long stroke pulling cylinders considerably increases system rigidity, compared with using strands. This reduces the amount of elastic energy stored during the start of structure movements.
Furthermore, double-acting cylinders, with their powered extending and retracting, improve the control of heavy load in case of unexpected external forces (wind, inertia due to sudden stops, etc.).
• PLC-controlled integrated hydraulic solutions make synchronization of movement and speed control simple and accurate, reducing the risk of additional stress in the structure.
• Pulling instead of pushing makes the movement more stable, cancels the risk of buckling and contains the cylinder size.
• Two-stage pumps with a high oil flow consistently reduce the cylinder reload time.
For more information, visit www.enerpac.com.