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Double pendulum with splitted cables

BAKU/AZERBAIJAN, May 16, 2015 – How does one build a skyscraper in a seismic area in the shape of two burning flames? And how do you fit a 10m pendulum in a 7m space? The Munich expert of structural protection systems devised a unique solution.

The capital of Azerbaijan, Baku, is reputed as the city of winds. Currently the State Oil Company of Azerbaijan Republic (SOCAR) builds their new headquarters as a high rise building of 200m and 42 stories. Due to the slender structure vibrations have a big impact, which are to be damped. The core element is a double pendulum with 450 t mass. The only question: how to place a 10 m long pendulum if the architect only permits a maximum height of 7 m?

SOCAR tower in Azerbaijan's capital Baku

The 200 m high SOCAR Tower strives skywards like two burning flames. | photo: Maurer

Soft structure

The tower consists of a concrete core, around which the various shaped steel platforms will be mounted. As a relative soft structure it has a low natural frequency. Normal wind would already push the tower to its side and would make the tower swing. Such a triggered vibration would last around 10-15 minutes, because such high rise structures do almost not display any inner friction or damping. “This exceeds the comfort regulations to a great extent”, explains the Maurer project manager Peter Huber. “And within this period that the tower swings, the next wind force might attack, which can even reinforce the vibrations. The tower would swing permanently.”

As a result such vibrations would give the people in the building an unpleasant offshore sensation and millions of vibration cycles would cause fatigue to the structure. A service life of at least 80 years would be almost impossible.

Sketch of pendulum for SOCAR tower

Principle sketch of a double pendulum:
the total pendulum length adds
l1 and l2. | photo: Maurer


High tower, high winds

What is requested is a damping system that damps wind speeds of up to 190 km/h, for which the structure is designed. In addition it has to accommodate the load case earthquake of a magnitude 9 on the Richter scale. The recent Nepal earthquate measured 8.6 on the Richter scale. Without damping the total structure would act like a big pendulum, which would vibrate slowly, but with extremely big amplitudes of ± 50 to 100 cm. Such types of vibrations cause an especially strong material fatigue.

Challenging architecture

The optimum location of the damping system is the hollow and lightened tip of the SOCAR tower. Initially this did not even cause a problem for the design engineers: according to the natural frequency of the tower a 10 m long pendulum was calculated, which should counter-vibrate with a mass of 450 t, hung at 4 cables. But in the course of the design of the levels, the 10 m availability of height in the tip shrank.“ Finally we had just 7 m of pendulum length left”, explains Huber, “and it was questionable whether the requirements for damping could be technically met.

Splitted pendulum length

The MAURER engineers solved the problem in splitting the two cables of the double pendulum into two lengths. “Such a solution for a pendulum mass of 450 t should be unique”, assumes Huber. The two outer and shorter cables hang at the outer frame. An inner frame connects, inside which the inner and longer cables are installed. The total pendulum displays a frequency of 0.15 respectively 0.25 Hz into the 2 horizontal directions.

Cables are being used because their lengths could be fixated variably and thus the frequency could be tuned on site in a simple way. Depending on the calculated natural frequency of the structure a frequency range was defined that could be covered by the cables. After installation and measurements at site the length of the cable was tuned to the real existing frequency.

Viscous dampers and rubber dampers with a lead core

The double pendulum is being damped by a maximum of 6 viscous dampers, and in extreme case by additional 4 rubber dampers with lead cores. There is a three level safety mechanism:
1.    Service vibrations – will in reality almost never occur, because the real structure is stiffer than as per previous calculations
2.    Stronger wind attacks and storm – will create bigger mass amplitudes, but the pendulum movements will remain within the limits of the frame structure.
3.    Strong storm or earthquake with the danger of very high amplitudes (> ±30 cm)

The horizontal hydraulic dampers reduce vibrations of all 3 levels by a factor of up to 5. In addition, they prevent uncontrolled pendulum movements in case of an earthquake.
The rubber dampers with lead cores underneath the pendulum will be additionally activated only in level 3, when the pendulum would otherwise swing outside of the frame. In case of an earthquake, the damping system is capable of absorbing the large vibrations caused by earthquake under control within 3 to 4 minutes.

The Maurer team in Azerbaijan

The Maurer factory team illustrates the dimension of the steel frame for the double pendulum in Baku. | photo: Maurer

Mass with class

A further challenge in Baku was the installation the total system with its extremely tight and comprehensive steel structure into the geometrical constraints of the tip of the tower.
The 450 t for the damper mass consists of staggered steel plates. The mass plates were providing a superficial corrosion protection, and all damages of the corrosion protection during installation were rectified.