Haiti Earthquake 2010

Every year, earthquakes take the lives of thousands of people , and destroy property worth billions. The 2010 Haiti Earthquake killed over 1,50,000 people and destroyed entire cities and villages. Designing Earthquake Resistant Structures is indispensable. It is imperative that structures are designed to resist earthquake forces, in order to reduce the loss of life. The science of Earthquake Engineering and Structural Design has improved tremendously, and thus, today, we can design safe structures which can safely withstand earthquakes of reasonable magnitude.

Index of all posts on Earthquake Resistant Structures

1. Design Earthquake Resistant Buildings | Engineering Tips
2. Earthquakes and Natural Calamities
3. Types of Seismic Waves
4. Hazardous Effects of Earthquakes
5. Effect of Earthquakes on Structures
6. Building Stiffness and Flexibility | Earthquake Engineering
7. Inertial Forces in a Structure
8. Effects of Deformations in Structures
9. Horizontal and Vertical Shaking of a Structure
10. Flow of Inertia Forces to Foundations
11. How Earthquakes affect Reinforced Concrete Buildings
12. Building Planning | Earthquake Resistant Buildings
13. Earthquake Resistant Structures by Planning and Design Approach
14. Design Philosophy of Earthquake Resistant Designs
15. Building Construction Materials for Earthquake Resistance
16. Concept of Earthquake Resistant Engineering
17. Seismic Base Isolation Technique for Building Earthquake Resistance
18. Energy Dissipation Devices for Earthquake Resistant Building Design
19. Active Control Devices for Earthquake Resistance

People standing in swaying train or bus try to maintain balance by unintentionally bracing their legs or by relaying on the mussels of their spine and stomach. By providing a similar function to a building it can dampen immensely the vibrations when confronted with an earthquake. This is the concept of Dynamic Intelligent Building (DIB).

The philosophy of the past conventional a seismic structure is to respond passively to an earthquake. In contrast in the DIB which we propose the building itself functions actively against earthquakes and attempts to control the vibrations. The sensor distributed inside and outside of the building transmits information to the computer installed in the building which can make analyses and judgment, and as if the buildings possess intelligence pertaining to the earthquake amends its own structural characteristics minutes by minute.

See Active Structural Control Research at Kajima Corporation (PDF)

Active Control System

The basic configuration of an active control system is schematically shown in figure. The system consists of three basic elements:

1. Sensors to measure external excitation and/or structural response.
2. Computer hardware and software to compute control forces on the basis of observed excitation and/or structural response.
3. Actuators to provide the necessary control forces.

Thus in active system has to necessarily have an external energy input to drive the actuators. On the other hand passive systems do not required external energy and their efficiency depends on tunings of system to expected excitation and structural behavior. As a result, the passive systems are effective only for the modes of the vibrations for which these are tuned. Thus the advantage of an active system lies in its much wider range of applicability since the control forces are worked out on the basis of actual excitation and structural behavior. In the active system when only external excitation is measured system is said to be in open-looped. However when the structural response is used as input, the system is in closed loop control. In certain instances the excitation and response both are used and it is termed as open-closed loop control.

Control Force Devices

Many ways have been proposed to apply control forces to a structure. Some of these have been tested in laboratory on scaled down models. Some of the ideas have been put forward for applications of active forces are briefly described in the following:

Active-tuned Mass Dampers (TMD)

these are in passive mode have been used in a umber of structures as mentioned earlier. Hence active TMD is a natural extension. In this system 1% of the total building mass is directly excited by an actuator with no spring and dash pot. The system has been termed as Active Mass Driver (AMD). The experiments indicated that the building vibrations are reduced about 25% by the use of AMD.

Tendon Control

Various analytical studies have been done using tendons for active control. At low excitations, even with the active control system off, the tendon will act in passive modes by resisting deformations in the structures though resulting tension in the tendon. At higher excitations one may switch over to Active mode where an actuator applies the required tension in tendons.

Other Methods

The liquid sloshing during earthquakes has assumed significance importance in view of over flow of petroleum products from storage tank in post earthquakes. One of the important consideration with sloshing is that is associated with a very low damping. The wave height was controlled through force applied to the side wall by a hydraulic actuator. The active control successfully reduced wave heights to the level of 6% of those without control, for harmonic excitations at sloshing frequency. For earthquake type excitation the wave heights were reduced to 19% level.

Conclusion

Conventional approach to earthquake resistant design of buildings depends upon providing the building with strength, stiffness and inelastic deformation capacity. But the new techniques like Energy Dissipation and Active Control Devices are a lot more efficient and better.