Structures with Controlled Seismic Response

According to seismic design philosophy of current practice and codes, based on bal-ance between strength/stiffness/ductility requirements, controlled damage of buildings’

structural and non-structural elements under design earthquakes are accepted (“life-safety’’ goal). Even though this goal has been reliably achieved by the buildings designed according to modern codes the cost of losses related with damage repair and with interruption of building function remains considerable. Consequently, development of global strategies for seismic risk management based on new, innovative concepts and technologies became an evident priority.

The main idea, borrowed from industrial and mechanical applications, was to decou-ple the building from the soil or to increase significantly its damping capacity using mechanical devices added to its structure.

A building subjected to earthquake motions can be considered as a dynamic system.

Its response to external excitation depends on three major parameters: mass, damping and stiffness. Through appropriate strategy, intervention on any of these parameters can advantageously modify the response. Intervention on parameters which govern the seismic response in order to minimize its effects leads to the concept of structures with controlled seismic response.

Solutions for structural control can be grouped in three main categories:

1. Systems of base isolation 2. Damper systems and 3. Systems with tuned mass.

5.3.1 S y s t e m s o f B a s e I s o l a t i o n

Seismic base isolation is one of the most frequently used technique of structural control.

The idea is to “split’’ the structural system vertically into parts decoupled from each other through special devices, so that the seismic movement of the foundation is no longer transferred to the upper part (or parts) of the building. The procedure uses different types of seismic isolators.

The isolators are bearing devices which have low lateral stiffness. They allow sub-stantial horizontal displacements (about 20 to 30 cm) and can develop restoring forces.

Accordingly, the upper part of the building will respond to the horizontal seismic excitation through quasi-rigid body displacements, with only minor distortions. So, substantial seismic damage will be prevented.

Numerous structural control devices have been imagined, then laboratory tested and, finally, implemented into new or existing structures.

Some frequently used seismic isolators will be briefly described below.

• Elastomeric Isolator (laminated rubber bearing) is a low cost isolation solution, having simple constitutive laws and, accordingly, simple analysis model, easy to understand and implement. Their main disadvantage is related with low damping ratio (about 2–3%) which involves large displacements, causing possible second-order (stability) effects.

• Seismic Isolators with Lead Rubber Bearings (LRB) are similar to elastomeric bearings but provided with a lead core which dissipates energy due to plastic

deformations. Their damping ratio is high, up to 30% of critical damping, and show a good restoring capacity due to their elastomeric component.

• Sliding bearing devices with controlled friction – are provided with two metal com-ponents, in contact with each other, which dissipates energy through dry friction (Coulombian energy dissipation). An example of such device is that called friction pendulum system (FPS). They have the advantage of a pure mechanical system with mechanical properties which don’t depend upon the time. The damping capacity is high (10 to 50% of critical damping ratio).

5.3.2 D a m p e r S y s t e m s

These solutions imply devices which significantly increase the building damping capac-ity. Dampers reduce vibration amplitude similar to the hydraulic shock absorbers of an automobile. Damping devices could be passive, with constant mechanical properties, or with variable characteristics (tuned dampers) adjustable in function of the seismic excitation.

Passive systems are hydro-mechanic devices similar to those used for automobiles, guns, etc. They are triggered by the seismic input signal and their characteristics remain constant during building lifetime.

Active systems adapt their action on the building according to seismic excitation in order to optimize the effect on the building response. They generate additional forces acting onto the structure being strictly related to an exterior energy source.

As an alternative, semi-active systems combine the advantages of passive and active devices, by providing adaptive damping according to seismic in-put magnitude. They improve the seismic response with minimal exterior energy supply. Even in absence of external power these devices perform a passive control.

Tuned damper systems – both active and semi-active – use full automatic devices, computer controlled, according to the seismic in-put characteristics supplied by specific sensors.

5.3.3 S y s t e m s w i t h T u n e d M a s s

These systems act on a building through an additional mobile mass (generally at roof level) generating forces of opposite sense to those induced by the seismic action. The existing applications refer mainly to high rise buildings subjected to wind actions (example: John Hancock Building in Boston, Ma.) but applications for seismic prone buildings do exist too.

The systems with controlled response are an extremely promising solution from the building behavior point of view. They open the perspective of obtaining intelligent constructions able to adapt their response to external actions so that the potential damage is minimal or even avoided.

5.4 Infrastructure

When subjected to lateral forces the whole building acts as a vertical cantilever. Thus, the structure, considered as a whole, respond to the seismic action by important over-turning moments and shear forces. In order to transfer to the soil earthquake-generated reactions as well as the gravity loads the whole structure has to be provided, at its base, with a bearing system.

The structural system component that transfers to the soil the seismic overturning moments and shear forces, resulted from the “cantilever effect’’, as well as the gravity loads of the whole building is called infrastructure. The infrastructure can be provided (not always necessarily) with local foundations that ensure direct contact with the soil.

Generally, infrastructure is identified as the structural component which has a considerably greater lateral stiffness and resistance than the superstructure.

The infrastructure is currently provided at basement level. Sometimes the infrastruc-ture is extended over the first floor(s) of the building too.

The infrastructure stiffness and resistance is generally obtained through addition of supplementary structural walls at the basement level. Other solutions can also be implemented like a system of braces on the first floor of the building.

Due to the change in stiffness at the infrastructure level (in comparison with stiff-ness distribution above this level) substantial redistribution of internal forces among different components occurs. The total lateral force acting at interface superstructure/

infrastructure is shared to the infrastructure components proportionally to their stiff-ness. The horizontal diaphragm just above the infrastructure has to ensure the transfer of total lateral force to the infrastructure components.

Specific recommendable solutions for infrastructure of different types of structures (framed, wall or dual systems) and their analysis, design and detailing features will be presented within specific chapters of the book.

As a general rule, robust detailing solutions have to be chosen for the infrastructures since its analysis involves many uncertainties.

Conclusions

Within the present chapter the need of considering the structural system, with its three components, in determining the real seismic response of concrete building is highlighted. Comparison between three main structural systems for buildings is made and optimum use for each is identified.