Innovative technologies of construction

--Ar. A. B. Farook Ahmed

8. AB Faruq Ahmed

Innovative technologies in housing construction are not used as frequently as the more standard construction technologies which involve the use of masonry,timber, and concrete. However, as with other innovations, it is expected that over time these newer technologies will gain wider acceptance. For purposes of the world housing Encyclopedia, technologies included seismic isolation and passive-energy dissipation devices.

Seismic Isolation

Seismic isolation is a relatively new concept in earthquake engineering, having been introduced in the early 1980s in the USA and New Zealand, and as early as 1975 in the former Soviet Union.

Quite simply, the idea underlying the technology is to detach the building from the ground in such a way that earthquake motions are not transmitted up through the building, or” at least greatly reduced. Seismic isolation is most often installed of the base level of o building and is called base isolation. This new concept meets all the criteria for o classic modern technological innovation: the necessary imaginative advances in conceptual thinking, new materials available to the industry’ and a s con be seen in the WHE reports using-isolators, simultaneous development of the ideas worldwide.

The principle of seismic isolation is lo introduce flexibility of the base of o structure in the horizontal plane, while at the same time introducing damping elements to restrict the amplitude of the motion caused by the earthquake. The concept of seismic isolation become more feasible with the successful development of mechanical energy dissipaters and elastomers with high damping properties. Seismic isolation can significantly reduce both floor accelerations and interstory drift and provide a viable economic solution to the difficult problem of reducing nonstructural earthquake damage

There are three basic elements in any practical seismic isolation system. These are as follows:

l A flexible mounting so that the period of vibration of the total system is lengthened sufficiently to reduce the force response

l A damper  or energy  dissipater  so that the relative deflections between building and ground can be controlled to a practical design level

l A means of providing rigidity under low (service) load levels, such as wind and minor earthquakes

Seismic isolation achieves o reduction in earthquake forces by lengthening the Period of Vibration  in which the structure responds to the earthquake motions. The most significant benefits obtained from isolation are thus in structures for which the fundamental period of the building without isolation is short-less-than one second. Therefore, seismic isolation is most applicable for low-rise and medium-rise buildings and becomes less effective for high-rise structures.

The WHE reports describe the applications of two different isolation systems:

l Rubber-based isolation system

l Sliding-belt isolation system

The rubber-based isolation system has been widely used in china  The system consists of laminated  rubber bearings, with a diameter of 350 mm lo 600 mm and o thickness of ,l60 mm to 200 mm. The isolators ore reinforced by thin steel sheets.

The isolators ore installed on top of the basement walls or the columns, or at the plinth level in buildings without a basement. The most common application in China is for those

buildings where the superstructure consists of brick multistory, brick-masonry walls with reinforced concrete floors/roof.

By the end of 2003, the system had been used in over 460 residential buildings in China. Sliding-belt isolation systems are installed of the base of the building between the foundation and the superstructure. The sliding belt consists of the following elements: (a) sliding supports, including the 2-mm-thick satin less steel plates attached to the foundation and 4-mm Teflon (PTFE) plates attached to the superstructure, (b) reinforced rubber restraints for horizontal displacements (horizontal stop), and (c) restraints for vertical displacements (uplift)-vertical stops. Once the earthquake base shear force exceeds the level of the friction force developed in the sliding belt, the building (superstructure) starts to slide relative to the foundation. A typical large-panel building with plan dimensions 39.6 m x 10.8 m has 63 sliding supports and 70 horizontal and veridical restraints. The sliding-belt scheme was developed in CNIISK. Kucherenko (Moscow) around ,l975. The first design application in Kyrgyzstan was mode in ‘1982. To date, the system has been applied in over 30 buildings in Bishkek, Kyrgyzstan. The applications include 9-slory. large, concrete panel buildings and 3-story brick masonry wall buildings.

ln the USA, New Zealand, Japan, and Italy, base-isolation technology has been used primarily to protect critical facilities. such as bridges. hospitals, city halls, courthouses, and heritage buildings. The most popular devices for seismic isolation of buildings in the USA ore lead-rubber bearings, high-damping rubber bearings, and the friction pendulum system (FPS). In Japan, as of I 999, over 300 residential buildings were projected  with base-isolation devices Typical residential buildings are reinforced concrete frame or wall construction, more than 5 stories, perhaps containing hundreds of apartments. The majority of base-isolated residential buildings in Japan were built after the 1995 Kobe earthquake (M7.3),which caused over 5,000 deaths, mainly as a result of vulnerable older wood housing.

Passive Energy Dissipation Devices

Passive energy dissipation systems represent an alternative to seismic isolation as a means of protecting building structures against the effects of damaging earthquakes. The basic function of passive energy dissipation devices in a building is to absorb or consume a portion of the earthquake input energy, thereby reducing energy dissipation demand on primary structural members and minimizing structural damage. The means by which the energy is dissipated is either through the yielding of mild steel, sliding friction, motion of a piston or a plate within o viscous fluid, motion of on orificed viscous fluid device. or viscoelastic action of polymeric materials. The most common types of passive devices used{o-date include viscous fluid dampers, friction dampers, metallic

dampers, and tuned moss dampers. These devices can be effective against wind motions as well as against earthquakes-.

Research and development of passive devices has o 3O-yeor history. Most often, this technology has been used to retrofit existing public buildings that do not meet the seismic code requirements or were damaged by on earthquake. There are very few.

examples of the application of this technology to housing construction. ln Canada, friction dampers were used in 1988 to retrofit a two-story wood house in Montreal. ln the former Soviet Union, a unique passive seismic protection system called “disengaging reserve elements” (DRE) has been used 1o protect over .l40 residential apartment buildings in the lqst30 years, and it is the only application of passive-energy dissipation devices currently included in the WHE.

The DRE system was developed around 1970 in the former Soviet Union: A Building with the DRE system must be made with a flexible reinforced concrete frame on the ground floor while the upper stories may be made with any of the more rigid systems: typically, large precast panel construction or brick masonry wall construction. The elements are constructed within the boys of the reinforced concrete frame on the ground floor. They consist of o ,,rigid structure,” generally RC wall panels, connected to the adjacent RC frame .em5ers by means of disengaging restrains.

The DRE do not Cary any gravity load and ore only installed to act as-a part of the lateral load-carrying system. The disengaging restraints, which connect the DRE to the RC frame, ore sacrificial reserve elements-(fuses) that ore designed so that they will be the first structural  members damaged in a large earthquake. Typical restraints are made of steel plates joined together by means of rivets or steel bolts, steel bars, concrete  prisms or cubes. initially (at the lower ground motion levels), the DRE and RC frame system (at the ground floor level) work together as o rigid structure; ,at that stage, disengaging elements transfer lateral loads lo the DRE (RC panels).

However, once the lateral load exceeds the prescribed level (depending on the site seismicity and other factors’), the disengaging elements snap and disconnect from the DRE. at that stage, due to the suddenly increased flexibility , a building changes its vibration period to higher value of about 0.8-1.0 sec. As a result resonance effects are avoided and seismic demand is reduced. After an earthquake disengaging restraints need to be replaced . however, the cost is not high and the replacement is not complex.

This system was developed by Professor J. Eisenberg. The first building using the DRE system was constricted in 1972in Sevastopol, Ukraine (the former Soviet Union). The system has been widely used in earthquake-prone areas of Russia and Kyrgyzstan. ln Russia, about 140 buildings ore protected with this system. primarily in North Baykal City and Siberia. There are several dozens of buildings with this system in Kyrgyzstan, Kazakhstan, Tajikistan, and Georgia. Most of the buildings ore residential and currently occupied.

Earthquake Performance

Typically, all the buildings built with these new technologies hove performed or ore expected to perform well in major earthquakes. ln fact, unlike some of the other construction technologies described in the WHE, this technology is used to improve o building’s performance in on earthquake. ln China, where the use of base isolation for rigid masonry buildings is becoming more widespread, these buildings have been subjected to numerous strong earthquakes and hove all performed well, No damage to this building type has been observed in any of these earthquakes: I994 Taiwan Straits (M 7 .3): 1995 Yunan Province (M 6.5); 1995 Yunan Province (M 2.0), and the 20po Xingjian Autonomous Region (M 6.2).

Retrofit

Again, unlike some of the other construction technologies described in the WHE, buildings built with these advanced technologies do not need to be strengthened to improve their performance in earthquakes. Rather, these technologies con be used to strengthen buildings. Some structures ore inherently more suitable for retrofit using seismic isolation thon others; for example, bridge superstructures lend themselves to the replacement of steel bearings with elastomeric ones. Buildings ore typically more difficult to retrofit thon bridges. A Marina apartment building in Son Francisco, California, is one of the rare applications of base-isolation technology for seismic retrofit . This four-story, wood frame building was severely damaged during the 1989 Loma Prieta earthquake.

ln 1990, thirty-one Friction Pendulum (FPS) bearings were installed at the base of the new garage-level steel columns. FPS bearings “isolate” the structure from the most damaging earthquake motions by using the characteristics of o pendulum to lengthen the structure’s natural period. The total retrofit cost was less than the cost to structurally upgrade the building to the seismic requirements of the then current UBC code. interestingly, this building is considered lo be the first base-isolated building in Northern California.

References:

1.            Design of structures with seismic isolation, Mayes, R.L, and Naeim

2.            New design Technologies, Clark, P.W.

3.            Single family wooden house, Maki, N. and Tanaka

4.            World Housing Encyclopedia.