uaceg2015 rsbs belev full paper

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УНИВЕРСИТЕТ ПО АРХИТЕКТУРА, СТРОИТЕЛСТВО И ГЕОДЕЗИЯ

ПЪРВА НАУЧНО-ПРИЛОЖНА КОНФЕРЕНЦИЯ С МЕЖДУНАРОДНО УЧАСТИЕ „ВЪЗСТАНОВЯВАНЕ И УСИЛВАНЕ НА СГРАДИ И СЪОРЪЖЕНИЯ“ /ВУСС2015/

5 - 6 НОЕМВРИ 20155 - 6 NOVEMBER 2015

FIRST SCIENTIFIC - APPLIED CONFERENCE WITH INTERNATIONAL PARTICIPATION

„REHABILITATION AND STRENGTHENING OF BUILDINGS AND FACILITIES

STRUCTURES“ /RSBS2015/UNIVERSITY OF ARCHITECTURE, CIVIL ENGINEERING AND GEODESY

APPLICATION OF DAMPING SYSTEMS FOR SEISMIC UPGRADE

OF BUILDINGS AND FACILITIES

B. Belev1, I. Mualla2, A. Alaee3

Keywords: seismic upgrade, damping systems, building structures

Research area: earthquake engineering

ABSTRACT

The paper presents briefly the philosophy of capacity design adopted in the modern

earthquake-resistant design codes and its evolution towards the structural fuse concept. The

major types of damper devices used in the contemporary seismic protection systems are

overviewed. The advantages of the passive energy dissipation systems over the conventionalapproaches for seismic retrofit of buildings and facilities are summarized.

A few seismic retrofit projects implementing friction dampers of European supplier

are presented.

1. Introduction

Originating from the 70-ies of the last century, the capacity design concept has now

become a worldwide-accepted approach in earthquake-resistant design. It assumes that the

structural system must be made as insensitive as possible to the strongly-variable and

difficult-to-predict characteristics of seismic actions. This can be achieved only if the

structural engineer introduces a hierarchy in the resistances of the structural members in a

way which results in a well-predictable and ductile seismic performance. The capacity design

concept is also named “failure mode control” implying that the designer must provide

favourable pattern of plastic (dissipative) zones which does not endanger the overall safety of

the building/facility. It is also recognized that even the structures made of very ductile

1 B. Belev, Prof. Dr., Dept. of steel and timber structures, UACEG, Sofia, e-mail:

[email protected] I. Mualla, Dr., CTO, Damptech, Lyngby, Denmark, e-mail: [email protected]

3 A. Alaee, Dr., CEO, B.A.T. Engineers, Tehran, Iran



materials, such as structural steel have “brittle” components which must be protected from

overstressing through providing extra-strength. In moment-resisting frames, this conceptresulted in a design strategy called “strong-columns-weak-beams”, implying that the ductile

frame beams must be purposely made weaker than the frame columns in order to avoid

formation of “weak” storey and/or total collapse initiated by premature column failure.

Based on the vast experience and lessons learnt from many major earthquakes it

became clear that the conventional structures relying on their ductile response could reallysurvive and save human lives but in general they failed to limit the extent of damage which

resulted in heavy financial losses and business interruption. In this connection, the

development of the structural fuse concept (SFC) can be viewed upon a further step in the

evolution of the capacity design philosophy. A similar concept named “Damage-tolerant

structures” was proposed by Prof. Akira Wada.

The SFC employs a principle from the field of electrical engineering where a cheapand replaceable fuse is inserted into the circuit in order to protect its more important

components from damage in case a sudden surge of voltage. This is why the structural fusesmust be the weakest components of the structural system which must dissipate via hysteretic

response a significant part of seismic input energy and keep the response of the primary

gravity-load-resisting members within elastic range.

The eccentrically-braced frame (EBF) is considered the first implementation of theSFC in seismic engineering. Other systems related to the concept are the so-called buckling-

restrained braces (BRB) and “rocking systems”. However, the true implementation of SFC

which is superior in comparison to EBFs and BRBs is seen in the various forms of the so-

called passive energy dissipation systems (PED-systems).

2. Passive energy dissipation and types of supplemental damping

devices

According to the classification of Soong and Dargush [7], three major structuralprotective systems could be used – (1) seismic (base) isolation, (2) passive energy

dissipation, and (3) semi-active and active control systems. Damper devices of different

types could be used in any of these three options, but they are typically the key component of

the PED-systems. Depending on the response of the dampers to imposed relative

displacements and/or relative velocities at their end points of attachment, three major types

of devices could be identified according to the classification of FEMA 273 [1]: (a)

displacement-dependent devices (e.g. metallic dampers and friction dampers); (b) velocity-

dependent dampers (fluid viscous dampers, solid visco-elastic dampers, etc.); and (c) other

types (shape-memory alloys, self-centering devices, etc.). Comprehensive details of the

energy-dissipation mechanism and mathematical modelling of these devices could be found

elsewhere [1, 7].

Initially the PED-systems were meant to add supplemental damping only andconsidered mainly as a tool for enhanced control of the interstorey drifts. It is believed,

however, that the inclusion of a damping system adds much more merits and advantages. In

new-built buildings and facilities it will enhance the overall seismic performance in terms of

reduced ductility demands, which implies less damage to structural and non-structural parts

and more economical design of members and connections. While the available ductility ofthe conventional structures is problematic following a strong earthquake, a building/facility

with a properly designed damping system will better preserve its energy dissipation capacity

and will not be vulnerable to the common aftershocks.



STRUCTURAL

PROTECTIVE

SYSTEMS

PASSIVE ENERGY

DISSIPATION

SYSTEMS

SEMI-ACTIVE

AND ACTIVE

CONTROL

SEISMIC

(BASE)

ISOLATION

STRUCTURAL

PROTECTIVE

SYSTEMS

PASSIVE ENERGY

DISSIPATION

SYSTEMS

SEMI-ACTIVE

AND ACTIVE

CONTROL

SEISMIC

(BASE)

ISOLATION

Figure 1. Types of structural protective systems according to [7]

When applied for seismic retrofit of existing structures, the PED-systems could be aeconomic and time-saving alternative to the conventional upgrade measures (e.g. addition of

new RC shear walls), and an efficient engineering tool for correction of seismic deficiencies

such as irregularities, suppression of torsional response, etc.

The displacement-dependent dampers (metallic dampers, friction devices, etc.) are

relatively cheap, durable and show well-defined predictable response which is not sensitive

to the frequency of excitation. The storey shear forces can be limited to predefined levels

irrespective of the intensity of the ground motion. Therefore, the supporting members can be

safely designed according to the capacity design rules. However, the force-displacement

response of these dampers is nonlinear which complicates the analysis and design. These

dampers are sensitive to temperature effects and long-term deformations (shrinkage, creep,

etc.) of the primary structure. They add both damping and stiffness but may not be activated

by small earthquakes. The metallic dampers, in particular, may have potential low-fatigue

problems, while the friction dampers may suffer from degradation of the contact surfaces andvariation of the friction coefficient in long term.

3. Rotational friction dampers (RFDs) for seismic protection and

vibration control

The original configuration of the rotational friction damper (RFD) developed by the

first author consists of steel plates clamped together by a prestressed steel bolt to form a T-

shaped device. In-between the steel plates circular friction pad discs made of high-techcomposite material are inserted. In order to maintain a constant pressure at the friction

interfaces several disc springs, external steel plates and hardened washers are used. The slip

capacity of the device and its energy dissipation potential can be easily increased by adding

more layers of steel plates and friction pads. Complete description of the device and review

of parameters influencing the dynamic response of structures with friction dampers alongwith discussion of efficiency criteria can be found in [5].

In 2001, an international team conducted intensive research program on a three-storey

building equipped with RFDs at the advanced large-scale shake-table testing facility of the

NCREE in Taiwan. The test building was a steel moment-resisting frame structure with 3.0

m storey height and 4.5 m bay width in the direction of shaking (Fig. 2).



Figure 3. T- and V-shaped RFDs with single friction hinge

Figure 4. Multi-joint large-capacity RFDs

Figure 5. Damper arrangement in the tallest building of Japan

4. Major seismic deficiencies of existing structures and basic

approaches to seismic upgrade

Many publications have summarized and made classifications of the basic issues facedby the structural engineers during a rigorous seismic assessment of existing buildings and

facilities. One of the most comprehensive classifications of the seismic deficiencies is given

by FEMA 547 [2]. In order to develop efficient strategies for seismic rehabilitation FEMA



547 has placed these deficiencies into categories most of which are common to all building

types. These deficiencies are related to problems with:(1) Global strength

(2) Global stiffness

(3) Configuration (irregularities in plan and along the height)

(4) Load path

(5) Component detailing (brittle or fast-degrading post-elastic behaviour)(6) Horizontal diaphragms

(7) Foundations

In addition, FEMA 547 has included the category “Other deficiencies”, related to

geologic hazards, pounding hazard of adjacent buildings and deterioration (damage) of

structural materials produced by various factors.

When the major seismic deficiencies for a specific building are identified, itsrehabilitation could implement one ore several classes of measures listed below [2]:

(1) Adding new elements (e.g. RC shear walls, steel bracing, etc.)(2) Improving the performance of existing members in terms of strength and

deformation capacity

(3) Improving the connectivity and integrity between the components

(4) Reducing the seismic demand by removing masses or by seismic protection(supplemental damping systems or seismic isolation).

In addition to the above measures FEMA 547 suggests that selective removal of

components may be carried out in order to avoid damaging interaction with other

components or reduce available irregularities.

A good illustration of the basic approaches to seismic retrofit can be made in ADRS-format charts, where ADRS stands for “Acceleration-Displacement Response Spectrum” [4].

The comparison of the conventional structural strengthening (Fig. 6(a)) with the more

advanced enhancement of seismic energy dissipation (Fig. 6(d)) reveals that in the

conventional retrofit the increase of global strength is usually accompanied by unwantedincrease of global stiffness which results in further increase in lateral seismic forces.

5. Examples of seismic retrofit with RFDs

One of the first applications of the Damptech friction dampers was in two ancientJapanese temple buildings with timber structure. Both temples had a soft basement storey

and by adding dampers and bracings both stiffness and supplemental damping were

increased. The first temple with RFD devices was the Yakuri-ji Temple in Kagawa

Prefecture (Fig. 7).

The passive energy dissipation concept was implemented by the authors for the seismic

protection of an industrial facility in Greece which had to be erected over an existing RC

substructure. The design PGA for the site was 0.24g. The target was to reduce the designbase shear to levels below 1120 kN, for which the supporting RC sub-structure was

originally designed. Preliminary calculations indicated that a conventional steel structure

with chevron braces was not acceptable due to the very high base shear (about 3000 kN

according to the Greek seismic code). The design solution involved RFDs of small slip

capacity (50-60 kN per device), which resulted in total slip capacity per each major direction

smaller than 600 kN (Fig. 8).

An example of rooftop extension of existing residential building (2+2 stories) with

light-weight steel structure and application of Damptech dampers is shown on Fig. 9.



Figure 6. Comparison of basic retrofit measures in ADRS-format [4]

Figure 7. Japanese temple upgraded with RFD devices [8]

Figure 8. Industrial facility with V-shaped RFD devices (Greece)



Figure 9. Steel rooftop extension with RFDs (Greece)

Passive energy dissipation systems and devices can be successfully used for seismic

upgrade of single-storey buildings with precast RC structure. Typical problems associated

with this structural system are its relatively low lateral stiffness, low inherent damping, lowstructural redundancy and connections between primary structural members which are not

suitable for seismic areas. The set of these drawbacks often results in severe damage in

strong ground shaking. A study made in Turkey has revealed that the majority of the existing

buildings of this type (industrial single-storey precast RC frame structures) are highly

vulnerable to seismic actions and very few of them satisfy the provisions of current Turkishdesign code [3]. An example of seismic retrofit of such building with V-shaped rotational

friction dampers is shown in Fig. 10. The major advantage of this approach was the fact that

it was implemented without any business interruption of the building.

Figure 10. Seismic upgrade of precast RC frames (Turkey)

The last example illustrates the application of rotational friction dampers for the seismic

retrofit of a twenty-storey office building in Tehran, Iran with a steel primary structure. It

was concluded by the local consulting company that the building does not meet the

requirements of the current design code. The locations of the added braces with RFDs are

indicated on Fig. 11 with thick lines. In order not to overstress the existing columns theadded braces with dampers were arranged in alternating bays along the height where



possible. Fig. 12 shows a typical detailing used for this project, developed by the third

author. The efficiency of the chosen retrofit solution was confirmed by nonlinear timehistory response analyses.

Figure 11. Plan of added braces with dampers in multistorey steel structure (Iran)

Figure 12. Typical joint detailing of braces with dampers (Iran)



6. Conclusions

Based on the review of many publications on applications of passive energy dissipationsystems for seismic upgrade of existing structures and experience of the authors in research

and design, the following essential conclusions could be drawn:

1) The passive damping systems have reached the status of a mature and reliable

seismic protection technology;

2) The application of displacement-dependent damping devices (and friction dampers in

particular) complicates the design due to their nonlinear response but could be a powerful

and non-expensive engineering tool for enhancing the safety of existing buildings and

industrial facilities;

3) The retrofit solutions with RFDs may offer certain advantages over the conventional

seismic retrofit measures such as avoidance of foundation strengthening and business

interruption;

4) The major difficulty when applying this relatively new technology is the deficientdeformation capacity of the older structures. The critical point is to provide deformation

compatibility of the added damping system with the existing structure through selective

upgrade of critical members and joints with advanced materials such as FRP.

5) Despite that a new European product standard for anti-seismic devices is available

(EN 15129), a new section to Eurocode 8 with design guidance and provisions for the

implementation of damping devices in new and existing structures is needed.

LITERATURE

1. FEMA, NEHRP Guidelines for the seismic rehabilitation of buildings (FEMA 273),

Washington D.C., 1997.2. FEMA, Techniques for the Seismic Rehabilitation of Existing Buildings (FEMA

547), Washington D.C., 2006.

3. Ildirim, S., Asik, G., Erkus, B., Mualla, I., Seismic retrofit of single story precast

reinforced concrete structures with infill walls using friction dampers. Second European

conference on earthquake engineering and seismology, Istanbul, August 2014.

4. IST Group, Methods for Seismic Retrofitting of Structures, MIT, 2004.

5. Mualla I.H., Belev B. Performance of steel frames with a new friction damperdevice under earthquake excitation. Engineering Structures, 2002, 24(3): 365-371.

6. Mualla I.H., Nielsen L.O., Belev B., Liao W.I., Loh C.H., Agrawal A. Performance

of friction-damped frame structure: shaking table testing and numerical simulations.

Proceedings of 7th US National Conference on Earthquake Engineering, Boston, USA, 2002,

vol. I, pp. 287-294.

7. Soong T.T., Dargush G.F. Passive energy dissipation systems in structuralengineering. J. Wiley & Sons, 1997.

8. www.damptech.com

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