Concrete structures

This section explains some of the design and construction techniques and also devices frequently used to produce a low-damage building using concrete.

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Base isolation

Base isolation systems place specially design bearings between the foundations and the superstructure, decoupling the building and also removing its rigid horizontal link to the ground. During a seismic event, horizontal floor accelerations room not or are only partially moved upwards v the bearing, avoiding or reducing damage to the structure.

Base isolation have the right to be imagined together a simple building relaxing on perfect rollers. Once the ground moves all of sudden to the left or right, the rollers turn, however the building itself continues to be in the exact same position. This is since the horizontal force of the ground activity is not transferred through the rollers, and also no force is acting on the building.

If the same building were placed on a flexible rubber pad and also the ground moved in the exact same way, the pads would partially resist the lateral forces. This would transfer few of the pressure through the pad, and the structure would move. However, the force would be much much less than if the structure were fixed directly to the ground. This is a key principle of basic isolation.


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Comparison of a fixed structure building (left) and also a base-isolated building (right) under lateral load.


As fine as enabling movement during an earthquake, base isolators space often likewise designed come dissipate energy. This add to damping come the system and also further to reduce the seismic an answer of the building.

While basic isolation have the right to add far-ranging expense come the price of a building’s foundation, the full design and also construction expense is often only slightly an ext than a traditional building. This is since base isolation helps to defend the contents and also structure of everything over the isolation plane. Significant seismic resilience can therefore be achieved even through a fairly economical building. The technique can additionally be supplied to retrofit currently buildings. In practice, part lateral resistance is still essential to oppose everyday wind forces.

Although basic isolation is now one of the oldest and also most widely provided damage-avoidance methods for concrete structures, it is not perfect for every buildings. Basic isolation as a concept works due to the fact that it alters (reduces) the horizontal stiffness the the building so the it does no respond v amplification the means a conventional building does. It is most effective for short to medium-rise structures on certain ground. Taller high-rise structures have higher overturning pressures that are not quickly resisted by basic isolation systems. Buildings on really soft floor are additionally not suitable to basic isolation.

It is vital to note that, although basic isolation is an efficient strategy to minimize the impacts of horizontal ground acceleration, the has little effect on upright acceleration. Upright acceleration reasons some of the most disastrous and difficult to mitigate forces in building design. During the February 2011 earthquake, the Christchurch suburbs experienced some of the greatest vertical accelerations ever before recorded, in overabundance of 2 g, or double the acceleration as result of gravity.

http://www.youtube.com/watch?v=JflunS4AYi0

The base isolators take countless forms, consisting of sliding bearings, solid steel plates and also some that actually look like big rubber pads, but they all autumn into two categories – elastomeric bearings and sliding isolators.

Elastomeric bearings

An elastomeric bearing typically consists of alternate layers of metal plate and also a thin elastomer, such as natural rubber or neoprene.

In a laminated rubber bearing, the layers are bonded with each other in a method that provides as lot vertical rigidity and horizontal adaptability as possible. The upper and lower deals with of the bearing are commonly fitted with steel plates that attach to the optimal of the foundation and the underside the the building.

A similar version, sometimes referred to as a command rubber bearing, has a solid lead core running v the centre, i beg your pardon acts as an energy dissipater together it deforms horizontally.


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Common species of base-isolation bearings.


Sliding isolation systems

A sliding isolator works by permitting the structure to move individually from the foundations, lot like the rolling example defined above.

A widely used sliding isolator, called a spherical slide bearing, provides a concave surface and a low-friction spherical bearing or pad to support the building. During an earthquake, the spherical bearing slides across the concave surface, moving both horizontally and vertically according to the concave confront of the surface. The force required to lift the structure near the exterior edges multiply by the lift elevation is the power that is dissipated. This is sufficient to limit the lateral forces that space exerted on the building. 


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Base isolation system at Christchurch Women"s Hospital. (Fairfax NZ/The Press)


Christchurch Women’s Hospital

Linked to the existing main hospital facilities, this 10-storey base-isolated structure was opened in 2005. The hospital is a devoted essential services facility, selected to carry out on-going restore services automatically following a major natural disaster, hence the decision to adopt base isolation.

The hospital was totally operational instantly following both the September 2010 and February 2011 earthquakes.

PRESSS post-tensioning

A post-tensioned device is a technique to strengthening a obelisk or beam by placing that under compression through unbonded high-strength stole bars or tendons. The tendons space tensioned so the they continue to be in a long-term state of tension throughout the life the the building. The technique adds toughness by clamping and holding the members tightly together.

The PREcast Seismic structure System, or PRESSS, is a type of low-damage system that offers post-tensioned technology.

The system uses a collection of precast concrete members joined along with unbonded post-tensioning tendons or steel bars to develop a minute frame.

During a young earthquake, the extr stiffness the the frame reduces motion of the structure. As soon as exposed to larger lateral seismic forces, the elastic action of the tendons permits gaps come open between the separation, personal, instance members within the frame. The framework will oscillate or rock side to side in a regulated manner. The tendons pull the joints and also therefore the structure back into its original position together the shiver subsides.


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Comparison that a yielding shear wall surface (left) and a rocking wall surface (right) under lateral load.


The crucial to PRESSS is that the steel tendons space unbonded, unlike conventional structures where the steel is physically join to or installed within the neighboring concrete. In an earthquake, the concrete members are free to rock from side to side at the joints, while the stole tendons administer the tension pressures to hold whatever in place.


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PRESSS creates a controlled rocking motion in response to a seismic event.


Alan MacDiarmid Building The first multi-storey building using PRESSS in new Zealand is the Alan MacDiarmid structure at Victoria university of Wellington. Completed in 2010, the structure employs post-tensioned seismic frames in one direction and also coupled post-tensioned wall surfaces in the various other direction, with directly unbonded post-tensioned tendons. The building likewise features outside replaceable dissipaters in the moment frame.

Post-tensioned systems room usually linked with dissipation devices to limit the amplitude that the displacement and dampen the rocking oscillations as conveniently as possible. If the rocking i do not care too major or continues for also long, the relatively huge vertical and also horizontal accelerations together the structure rocks ago to an upright place can cause damage.

PRESSS suffers from a few drawbacks, consisting of beam elongation as the device rocks ago and forwards. As gaps open and close, the ends of the beam can separate from the support columns or floor slab, resulting in the floor to tear together the void opens at the top. Essentially, the entire structure tries to boost in size. This deserve to cause far-reaching damage if the design does no accommodate the growth in the floor and roof diaphragms and also non-structural components, together as inner linings and flooring.

Beam elongation is not restricted to concrete structures. That is likewise a common problem in tensioned steel and timber structures.

Damping tools in concrete

A damping device, or damper, is a dissipative mechanism put within a structure to increase the building’s volume to absorb seismic energy. All structures naturally dissipate energy, but dampers significantly increase the building’s ability to remove power from the mechanism quickly, thereby limiting the lot of damage it have the right to cause.

Damping devices work by suppressing the building’s an answer to lateral forces. They have the right to be provided by themselves or as part of a hybrid system to boost other low-damage systems, such as base isolation or PRESSS. Retrofitting dampers within a framework is often less complicated than retrofitting basic isolators under it. They space especially an ideal for tall buildings that space not an ideal for base-isolation techniques.

A wide variety of damping devices are easily accessible for use in concrete structures.


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Common location of damping devices in a moment frame.


Fluid dampers

A fluid damper is composed of a piston attached in a cylinder filled with a viscous fluid, such as silicone oil. The dampers job-related by moving kinetic energy from the structure into the liquid in the kind of heat as the piston is compelled through it, much like a auto shock absorber.

Fluid dampers may be design to just dissipate energy, or castle may additionally include a spring action, i m sorry acts against the pressure causing the displacement. They may be one of the best ways to mitigate the damage caused by very huge and fast ground accelerations. However, the price of viscous dampers is normally considerable.


A variant of the liquid damper is the command extrusion damper, which uses a hard lead main point as the viscous material. This maker works by advertise a obelisk with a bulge in it v a hollow main point of lead. It move kinetic power from the earthquake right into the lead, creating heat and also making the material more malleable.


Viscoelastic dampers

A viscoelastic damper offers an elastomer material, such together rubber or neoprene, sandwiched in between two metal faces. During an earthquake, power is soaked up by the controlled shear of the elastomer.

Several arrays are available, consisting of solid and also fluid viscoelastic dampers, and systems that likewise include friction effects.

Hysteretic dampers


A hysteretic damper, or productivity damper, consists of a member v a metal, frequently steel, component.

During an earthquake, the damper absorbs power by allowing the steel to yield, either by bending or in tension and compression, depending on the style of the damper.

U-shaped flexural plates and also triangular bending plates space common varieties of bending hysteretic damper provided in many species of structures, including concrete.

U-shaped flexural plates are installed in between closely spaced structural walls that room expected to absent independently.

They are designed so that the yielding that the steel is spread out over a length to avoid high strains and cycle fatigue.

Tension and compression tools are designed come yield along their length and must be restrained to protect against buckling in compression.

This is commonly accomplished by utilizing a steel tube together the yielding material and filling it through concrete or epoxy come act as a restraint.


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U-shaped flexural plates between shear walls in a concrete framework in southerly Cross Hospital Christchurch. (Buchanan, Bull, Dhakal, MacRae, Palermo, Pampanin, 2011, Fig 7.16)

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Replaceable mild steel dissipaters are supplied at the joints between columns or walls and the structure and in between columns and also beams.


Southern cross building, Christchurch The southerly Cross Hospitals’ Endoscopy Consultants’ structure in Christchurch offers PRESSS frames and walls coupled together with U-shaped flexural bowl dampers. One benefit of the combined rocking-dissipative equipment is the ability to song the level of floor accelerations to far better protect both structural and non-structural elements, consisting of the hospital’s acceleration-sensitive equipment.

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This video clip includes an check of the design and construction that the southerly Cross endoscopy building.