7/18/2019 Abstract Tesi Di Master Filippo Conti

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TITLE THESIS: Structural Design of a Derrick Supported Chimney for ENI VERSALIS :

Optimization of structural Dynamic behavior using a Tuned Mass Damper.

AUTHOR: Ing. Filippo Conti

TUTORS: Prof. Ing. Pietro Crespi, Ing. Diego Bruciafreddo

INTERNSHIP: Thermoengineering s.r.l.MASTER: Master a.a 2014/15

The subject of this work is the structural design, according to the Eurocode and the Italian national

Regulamentation NTC08, of steel chimney Derrick Supported which will be built inside of the petrolchemical

facility of ENI Versalis in Ferrara City. The chimney is 90 m tall from the ground and it has a circular cross

section of different diameter along the height. In order to control the stability of the chimney against the lateral

displacement a Derrick structure 65 m tall is used. In this way it is required that the part of the chimney above

the end of the derrick structure works well itself as self supported. While the proposed configuration is an

optimum to reduce the total cost of the building, since the whole structure can be looked as a continuous

tapered section from the bottom to the top, it makes the structure very slender. This fact, together with a proper

low structural damping ratio of steel, makes the structure sensitive to the dynamic effect due to wind and

seismic load.

In order to mitigate and control the effect of the dynamic action, a TMD tuned on the 1st  natural frequency ofthe structure were designed and installed. In this way it was possible to increase enough the equivalent

structural damping of the structure to lower the actions and to obtain an optimum as regard the total cost of the

structure. A Tuned Mass Damper is a device composed by mass, spring and dashpot that is installed on a

structure with the goal to control the dynamic motion. The natural frequency of the TMD is tuned to be the

same of a selected natural frequency of the primary structure in a way that when resonance condition occurs the

device vibrates in opposite phase respect to the motion of the primary structure and dissipates energy by its

inertia forces. In addition to the TMD, the design of the chimney involved many other different topics such the

calculation and the check of the wind action considering proper dynamic effects, the design of thin shell, the

design of thin cold formed section, the evaluation of local effects caused by nozzle load and many other. Once

the design of the TMD was done reducing a MDOF structure in a SDOF structure, the effective behavior of

the TMD was checked by using different Time History Analysis modeling both structure and TMD. The first

goal of the analysis was the check the equivalent structural damping when resonance condition occurs with adifferent load shape. The results showed that the designed damping on an equivalent SDOF structure of the

 primary structure is exactly the same that the structure + TMD shows and that the TMD is not effective on the

vibration modes different from the one tuned. This behavior can be seen as a problem when the structure is

subjected to a dynamic load with a wide range of frequency.

Once the TMD offers an increased structural damping only on the tuning frequency it has the maximum

efficiency when the action has a narrow band spectrum on the tuning frequency (i.e. wind action) . Since the

seismic action has usually a wide band of frequency, the second goal of the time history analysis was to check

the effective behavior against seismic signal. According to the relevant code, a set of seven seismic records

where selected and applied on the structure in seven different analysis. The results showed that the reduction

offered by the installation of the TMD is related to the dominant frequency of the seismic action and the

dynamic properties of the primary structures.