EARTHQUAKE RESISTANT TECHNIQUES

The conventional approach to earthquake resistant design of buildings depends upon providing the building with strength, stiffness and inelastic deformation capacity which are great enough to withstand a given level of earthquake-generated force. This is generally accomplished through the selection of an appropriate structural configuration and the careful detailing of structural members, such as beams and columns, and the connections between them.
But more advanced techniques for earthquake resistance is not to strengthen the building, but to reduce the earthquake-generated forces acting upon it.
Among the most important advanced techniques of earthquake resistant design and construction are:

• Base Isolation
• Energy Dissipation Devices

Base Isolation

A base isolated structure is supported by a series of bearing pads which are placed between the building and the building’s foundation. (See Figure 1.) A variety of different types of base isolation bearing pads have now been developed.
The bearing is very stiff and strong in the vertical direction, but flexible in the horizontal direction.

Earthquake Generated Forces

To get a basic idea of how base isolation works, examine Figure 2. This shows an earthquake acting on both a base isolated building and a conventional, fixed-base, building. As a result of an earthquake, the ground beneath each building begins to move. In Figure 2, it is shown moving to the left. Each building responds with movement which tends toward the right. The building undergoes displacement towards the right. The building’s displacement in the direction opposite the ground motion is actually due to inertia. The inertial forces acting on a building are the most important of all those generated during an earthquake.
It is important to know that the inertial forces which the building undergoes are proportional to the building’s acceleration during ground motion. It is also important to realize that buildings don’t actually shift in only one direction. Because of the complex nature of earthquake ground motion, the building actually tends to vibrate back and forth in varying directions.

Deformation and Damages

In addition to displacing toward the right, the un-isolated building is also shown to be changing its shape-from a rectangle to a parallelogram. It is deforming. The primary cause of earthquake damage to buildings is the deformation which the building undergoes as a result of the inertial forces acting upon it.

Response of Base Isolated Building

By contrast, even though it too is displacing, the base-isolated building retains its original, rectangular shape. It is the lead-rubber bearings supporting the building that are deformed. The base-isolated building itself escapes the deformation and damage–which implies that the inertial forces acting on the base-isolated building have been reduced. Experiments and observations of base-isolated buildings in earthquakes have been shown to reduce building accelerations to as little as 1/4 of the acceleration of comparable fixed-base buildings, which each building undergoes as a percentage of gravity. As we noted above, inertial forces increase, and decrease, proportionally as acceleration increases or decreases.
Acceleration is decreased because the base isolation system lengthens a building’s period of vibration, the time it takes for the building to rock back and forth and then back again. And in general, structures with longer periods of vibration tend to reduce acceleration, while those with shorter periods tend to increase or amplify acceleration.
Finally, since they are highly elastic, the rubber isolation bearings don’t suffer any damage. But the lead plug in the middle of our example bearing experiences the same deformation as the rubber. However, it generates heat. In other words, the lead plug reduces, or dissipates, the energy of motion–i.e., kinetic energy–by converting that energy into heat. And by reducing the energy entering the building, it helps to slow and eventually stop the building’s vibrations sooner than would otherwise be the case–in other words, it damps the building’s vibrations.

Energy Dissipation Devices

The second of the major new techniques for improving the earthquake resistance of buildings also relies upon damping and energy dissipation, but it greatly extends the damping and energy dissipation provided by lead-rubber bearings.
As we’ve said, a certain amount of vibration energy is transferred to the building by earthquake ground motion. Buildings themselves do possess an inherent ability to dissipate, or damp, this energy. However, the capacity of buildings to dissipate energy before they begin to suffer deformation and damage is quite limited. The building will dissipate energy either by undergoing large scale movement or sustaining increased internal strains in elements such as the building’s columns and beams. Both of these eventually result in varying degrees of damage.
So, by equipping a building with additional devices which have high damping capacity, we can greatly decrease the seismic energy entering the building, and thus decrease building damage.
Accordingly, a wide range of energy dissipation devices have been developed and are now being installed in real buildings. Energy dissipation devices are also often called damping devices. The large number of damping devices that have been developed can be grouped into three broad categories:

• Friction Dampers: these utilize frictional forces to dissipate energy
• Metallic Dampers : utilize the deformation of metal elements within the damper
• Viscoelastic Dampers : utilize the controlled shearing of solids
• Viscous Dampers: utilized the forced movement (orificing) of fluids within the damper

Fluid Viscous Dampers

General principles of damping devices are illustrated through Fluid Viscous damper. Following section, describes the basic characteristics of fluid viscous dampers, the process of developing and testing them, and the installation of fluid viscous dampers in an actual building to make it more earthquake resistant.

Damping Devices and Bracing Systems

Damping devices are usually installed as part of bracing systems. Figure 3 shows one type of damper-brace arrangement, with one end attached to a column and one end attached to a floor beam. Primarily, this arrangement provides the column with additional support. Most earthquake ground motion is in a horizontal direction; so, it is a building’s columns which normally undergo the most displacement relative to the motion of the ground. Figure 3 also shows the damping device installed as part of the bracing system and gives some idea of its action.
 Engineering forum
Seismic Design Shake Test

                                                                                                                                                      

Sources of Earthquake Damage

Vibration of the structure in response to ground shaking at its foundation is the concern of the structural engineer, and which is taken into account by codal provisions of the different seismic-resistant design codes. However, these codes do not include any provision due to other effects, which may even exceed that due to vibration, as the procedure of their estimation and the needed steps for the design are outside the scope of the structural engineering discipline. Even then, it is essential that the structural engineer should be aware of the different seismic hazards in order to advise the client of potential damage involved in selecting sites in such zones. Hence, the first step in the design procedure of a future structure should be the analysis of the suitability of the site selected with proper consideration for the potential of any one of the following types of damage.
The different ‘Direct’ and ‘Indirect’ seismic effects are as follows
Direct effects:
• Ground failures, which include Surface faulting, Vibration of soil (or effects of seismic waves), Ground cracking, Liquefaction, Ground lurching, Differential settlement, Lateral spreading and Landslides.
(A) Damage due to surface faulting:
These damages to buildings and facilities along the fault scarps vary widely from completely demolished houses to rupture of the foundations, tilting of the foundation slabs and walls. Sometimes houses also have minor damage.
(B) Damage due to liquefaction:
The instability of the soil in the area affected by internal seismic waves can cause significant damage. The mechanical characteristics of the soil layers, the depth of the water table and the intensities and duration of the ground shaking influence the soil response. Deposits of loose granular materials if present in the site may be compacted by the ground vibrations induced by the earthquake. This will cause large settlement and differential settlements of the ground surface. Further, the compaction of the soil may result in the development of excess hydrostatic pore water pressures of sufficient magnitude to cause liquefaction of the soil, resulting in settlement, tilting and rupture of structures. The seismic-resistant design provisions of most codes only assure an effective design and construction of structures against damage due to the possible vibratory response of the structure to the shaking introduced at their foundation by the ground. However, it may not be possible to have success in all such cases. The only option remains in those areas is to prohibit the construction of building structures there.
(C) Damage due to ground shaking:
Integrated field inspection and post-earthquake analyses of structural damage due to earthquake shaking is one of the most effective means of having expertise knowledge on seismic response with a view to improving the state of the art and of the practice in seismic-resistant design and construction. Such integrated inspection and analyses revealed that in addition to the soil conditions mentioned above, the seismic performance of a structure is very sensitive to type of foundation; configuration of the structure; structural material; and design and construction detailing.
(D) Damage due to sliding of superstructure on its foundation:
One of the basic guidelines in the seismic-resistant design and construction of structures is that the whole structure-foundation system should work as a unit, and that the superstructure be tied or anchored properly to the foundation.
• Vibrations transmitted from the ground to the structure.
(E) Damage due to Structural Vibration:
I. Wood-Frame Houses:
The inertia forces develop during the vibratory response of a structure to earthquake ground shaking whose intensity depends on the product of the mass and acceleration. Hence, it is of the utmost importance to reduce the mass of the structure to a minimum. It is obvious that timber is the most efficient earthquake-resistant material for low-rise buildings among the traditional structural materials – timber, masonry, concrete, steel and aluminum. However, provision of proper lateral bracing and tying of all components together from the roof down to the foundation are to be followed.
II. Concrete structures:
Concrete is a comparatively heavy material and have a very good compressive strength. Due to its very small tensile and flexural strengths steel reinforcement is provided when used in structures. Such reinforced concrete can be used effectively in seismic-resistant construction. To overcome its relatively low strength per unit weight when normal weight aggregates are used, the use of lightweight aggregate concrete offers a significant advantage in seismic regions. members carefully: the proper amount and correct detailing of the reinforcing steel plays an important role in the seismic response of a reinforced concrete structure.
III. Steel structures:
Steel comes out of steel plants having excellent quality control. The stiffness per unit weight of steel is practically the same as of any other traditional constructional material. However, its strength, ductility and toughness per unit weight are significantly higher than concrete and masonry materials. Accordingly, the slenderness of steel structural members usually exceeds significantly that of other structural members. Hence, buckling becomes a serious problem. The danger of buckling becomes much more at higher yielding strength of the steel. Further, the plate elements used extensively to form the structural shapes are prone to local buckling, particularly when strained in the inelastic range. Accordingly, the compactness requirements for the cross section of the critical regions of structural members are more stringent in earthquake-resistant design than that of normal condition. Moreover, the field-connection of the structural members is another problem in achieving efficient seismic-resistant construction.
Indirect effects (or Consequential Phenomena):
i. Tsunamis
ii. Seiches
iii. Landslides
iv. Floods
v. Fires 

Engineering forum

Scientific study of the cause of the cholera epidemic proves the responsibility of the U.N Nepalese soldiers

Via Emerging Infectious Diseases: Understanding the Cholera Epidemic, Haiti. It is the most explosive report I have ever linked to on my H5N1 blog, and you need to read the whole thing—not only as a brilliant example of forensic epidemiology, but as a history of the coming of cholera to Haiti. It is also an implicit indictment of the United Nations. Excerpt from the Discussion section of the report, with my comments afterward:

Determining the origin and the means of spread of the cholera epidemic in Haiti was necessary to direct the cholera response, including lasting control of an indigenous bacterium and the fight for elimination of an accidentally imported disease, even if we acknowledge that the latter might secondarily become endemic. 

Putting an end to the controversy over the cholera origin could ease prevention and treatment by decreasing the distrust associated with the widespread suspicions of a cover-up of a deliberate importation of cholera (15,16). Demonstrating an imported origin would additionally compel international organizations to reappraise their procedures. 

Furthermore, it could help to contain disproportionate fear toward rice culture in the future, a phenomenon responsible for important crop losses this year (17). 

Notably, recent publications supporting an imported origin (7) did not worsen social unrest, contrary to what some dreaded (18–20). 

Our epidemiologic study provides several additional arguments confirming an importation of cholera in Haiti. There was an exact correlation in time and places between the arrival of a Nepalese battalion from an area experiencing a cholera outbreak and the appearance of the first cases in Meille a few days after. 

The remoteness of Meille in central Haiti and the absence of report of other incomers make it unlikely that a cholera strain might have been brought there another way. 

DNA fingerprinting of V. cholerae isolates in Haiti (1) and genotyping (7,21) corroborate our findings because the fingerprinting and genotyping suggest an introduction from a distant source in a single event (22). 

At the beginning, importation of the strain might have involved asymptomatic carriage by departing soldiers whose stools were not tested for the presence of V. cholerae, as the Nepalese army's chief medical officer told the British Broadcasting Corporation (23). 

The risk for transmission associated with asymptomatic carriage has been known for decades (24), but asymptomatic patients typically shed bacteria in their stool at ≈103 V. cholerae bacteria per gram of stool (25) and, by definition, have no diarrhea. This small level of shedding would be unlikely to cause interhuman contamination of persons outside the military camp having few contacts, if any, with MINUSTAH peacekeepers. 

By contrast, considering the presence of pipes pouring sewage from the MINUSTAH camp to the stream, the rapid dissemination of the disease in Meille and downstream, and the probable contamination of prisoners by the stream water, we believe that Meille River acted as the vector of cholera during the first days of the epidemic by carrying sufficient concentrations of the bacterium to induce cholera in persons who drank it. 

To our knowledge, only infectious doses >104 bacteria were shown to produce mild patent infection in healthy volunteers, and higher doses are required to provoke severe infections (26,27). Reaching such doses in the Meille River is hardly compatible with the amount of bacteria excreted by asymptomatic carriers, whereas if 1 or several arriving soldiers were incubating the disease, they would have subsequently excreted diarrheal stools containing 1010–1012 bacteria per liter (25). 

We therefore believe that symptomatic cases occurred inside the MINUSTAH camp.

If this peer-reviewed article is correct, the UN in general and MINUSTAH and WHO in particular are complicit in a public-health catastrophe as well as a shameful cover-up. The UN imported Nepali mercenaries (call them what they are) and failed to ensure that they were at least healthy when they left Kathmandu in the midst of a local cholera outbreak.

What's more, the Nepali camp in Meille was effectively a worse slum than the post-quake tent camps, because the mercenaries' sewage wasn't contained.

Worst of all, Piarroux argues that some Nepalis had to be actively ill with cholera in Meille, or the leaking sewage couldn't have contaminated the Meille River badly enough to cause the explosive spread of the disease. 

And yet Ban Ki-Moon's Haiti spokesman Martin Nesirky publicly asserted on October 29 that MINUSTAH had collected stool samples from the Nepalis that had "negative results."

Even incompetent commanding officers and UN bureaucrats would notice if their soldiers were down with a severe case of the shits. So someone responsible knew the Nepalis had brought active cholera with them, and kept quiet. Sooner than identify the true cause of the outbreak, that person (or persons) chose to conceal it. As Piarroux's report notes, identifying the cause would have made fighting the outbreak far easier.

So for crass political reasons, the UN denied and delayed, allowing cholera to spread among an already fragile population. I doubt that anyone responsible will suffer in the slightest for it.

Extracted from Crawford Kilian's blog in Haiti Rewired

Seismic design of structures

When we speak of seismic design of structures, we mostly speak of the modern methods.
Two basic technologies are used to protect buildings from damaging earthquake effects. These are Base Isolation Devices and Seismic Dampers.

let us have a detailed idea about the base isolators first.

BASE ISOLATION:
The idea behind base isolation is to detach (isolate) the building from the ground in such a way that earthquake motions are not transmitted up through the building or at least greatly reduced. A base isolated structure is supported by a series of bearing pads, which are placed between the buildings and building foundation. The bearing is very stiff and strong in the vertical direction, but flexible in the horizontal direction.



the closer look at the rubber bearings will be as follows:




A lead–rubber bearing is made from layers of rubber sandwiched together with layers of steel. In the middle of the bearing is a solid lead "plug." On top and bottom, the bearing is fitted with steel plates which are used to attach the bearing to the building and foundation. The bearing is very stiff and strong in the vertical direction, but flexible in the horizontal direction.

           Engineering forum

New Force Powering Earth's Tectonic Plates Discovered

July 07, 2011

Scientists at Scripps Institution of Oceanography at UC San Diego have identified a new mechanism driving Earth's massive tectonic plates, bringing fresh insight into long-standing debates about how powerful geological forces shape the planet, from earthquake ruptures to mountain formations.

Scientists who study tectonic motions have known for decades that the ongoing "pull" and "push" movements of the plates are responsible for sculpting continental features around the planet. Volcanoes, for example, are generally located at areas where plates are moving apart or coming together. Scripps scientists Steve Cande and Dave Stegman have now discovered a new force that drives plate tectonics: Plumes of hot magma pushing up from Earth's deep interior.

Using analytical methods to track plate motions through Earth's history, Cande and Stegman's research provides evidence that such mantle plume "hot spots," which can last for tens of millions of years and are active today at locations such as Hawaii, Iceland and the Galapagos, may work as an additional tectonic driver, along with push-pull forces.

Their new results describe a clear connection between the arrival of a powerful mantle plume head around 70 million years ago and the rapid motion of the Indian plate that was pushed as a consequence of overlying the plume's location.

The arrival of the plume also created immense formations of volcanic rock now called the "Deccan flood basalts" in western India, which erupted just prior to the mass extinction of dinosaurs. The Indian continent has since drifted north and collided with Asia, but the original location of the plume's arrival has remained volcanically active to this day, most recently having formed Réunion island near Madagascar.

The team also recognized that this "plume-push" force acted on other tectonic plates, and pushed on Africa as well but in the opposite direction.

"Prior to the plume's arrival, the African plate was slowly drifting but then stops altogether, at the same time the Indian speeds up," explains Stegman, an assistant professor of geophysics in Scripps' Cecil H. and Ida M. Green Institute of Geophysics and Planetary Physics. "It became clear the motion of the Indian and African plates were synchronized and the Réunion hotspot was the common link."

After the force of the plume had waned, the African plate's motion gradually returned to its previous speed while India slowed down.

"There is a dramatic slow down in the northwards motion of the Indian plate around 50 million years ago that has long been attributed to the initial collision of India with the Eurasian plate," said Cande, a professor of marine geophysics in the Geosciences Research Division at Scripps. "An implication of our study is that the slow down might just reflect the waning of the mantle plume-the actual collision might have occurred a little later."

The Daily Galaxy via University of California - San Diego