Civil Engineering Career

Earthquake Engineering

Earthquake engineering is one of the more recent additions to the civil engineering specialties. While the need for earthquake engineering has always existed, the concepts and technology are a much more recent development. While all structures have a need to be designed to be earthquake resistant, it is the proliferation of high-rise buildings which has sparked the interest in developing earthquake survivability technology.

Seismic events create a number of separate, but interrelated problems for buildings and other structures. The earthquake itself can move both laterally and vertically, providing forces to which the structure is not normally subject.

Additionally, earthquakes can cause soil liquefaction, where the soil under a building flows out from under the foundation, eliminating the structural support that the building relies on. Other events, such as landslides can be caused by earthquakes, adding additional hazards.

Earthquake engineering consists of two basic parts: the first is understanding the effects of earthquakes on buildings and other structures. The second is designing structures which can withstand the forces brought to bear during an earthquake and remain safe and serviceable.

Being earthquake safe or serviceable does not mean that the structures will not suffer any damage whatsoever. An earthquake safe structure is one which will not endanger the lives and well-being of people in and around it, in the event of an earthquake. Although superficial damage will occur, the building will not collapse partially or totally. To be earthquake serviceable would mean that he structure would still be able to be used for its intended purpose, after a major earthquake.

Essentially, earthquake engineering deals with the structure of the building, not the fascia, wall covering or other decorative items. Damage to these is considered superficial, while structural failure can cause serious injuries and death.

Studying Earthquakes

A large part of earthquake engineering is studying the effects that earthquakes have on structures. Every earthquake causes damage to existing structures, providing a wealth of information to earthquake engineers. Teams of engineers analyze the damage caused by earthquakes, comparing the damage to the structures with seismic data on the force and direction of the earthquake.

Their goal in these studies is to determine the exact cause of any structural failures. They are also looking to determine the reason for the success for any structures which survive the earthquake with minimal damage. This data is essential for future design developments, in an effort to build structures which are even more survivable in the event of earthquakes.

Earthquake engineers depend extensively on testing, both actual physical testing of models and structures on shaker tables, and computer modeling. The data developed through shaker testing validates computer simulations and helps to further develop improved computer models.

Since buildings and other structures can’t be tested, this computer modeling is an important part of the design of new structures. Earthquake engineers are able to input a building’s design into the simulator program and virtually simulate the effects of earthquakes on the building. Changes to materials and construction methods can be tested in this manner, to determine the most earthquake resistant design.

Designing for Earthquake Resistance

For a structure to be earthquake resistant, it doesn’t necessarily have to be extremely strong or extremely expensive. Survivability has a lot more to do with the quality of the construction, specifically joints between various components, than it does with overall strength.

An important part of earthquake sustainability is dependent upon the flexibility of the materials used in construction. Concrete, a common material used in construction, is not very earthquake resistant. That’s because it is extremely strong under compression, but very weak under tension. Earthquakes cause both compression and tension, creating cracks in the concrete.

This is why concrete structures are reinforced with steel rods (re-bar), because steel is strong under tension. Pre-stressing concrete can help make the concrete more resilient to earthquakes, as the constant stress on the concrete structure helps prevent it from coming under tension.

Steel structures, such as steel truss bridges are some of the most earthquake proof structures that exist. Not only is steel strong both under tension and compression, but it is somewhat flexible as well. The elasticity inherent in steel allows the structure to flex and still return to its original shape.

One of the technologies which have been developed to help high-rise structures withstand the forces of earthquakes is the Tuned Mass Damper, also known as a Harmonic Absorber. This mass of this weight is determined by careful calculation of the building’s weight and design. The intent of the damper is to work on the resonance frequency of the building, not the weight of the building.

Located in the upper floors of the building, the weight is coupled to the building with shock absorbers or springs. As earthquakes and other lateral forces (such as high winds) act upon the building, the weight acts according to the first law of physics, not swaying with the building. This helps to dampen the lateral movement of the building.

Taipei 101, the world’s second tallest building has one of the largest tuned mass dampers ever installed in a building. The 660 metric ton damper is installed between the 87th and 88th floors and suspended from the 92nd to the 88th floor.

At the foundation of most skyscrapers, a number of technologies are employed to control how the base of the building interfaces with the foundation. In its simplest form, base isolation has the building sitting on top of the foundation, but not actually attached to it. As the ground and foundation moves, the building resists movement, attempting to stay in one place, according to Newton’s first law.

Base isolation can be coupled with a variety of dampers, which act as giant shock absorbers to help isolate the building from the vibrations happening in the ground. Lead rubber bearings, invented by Bill Robinson from New Zealand in 1974 are the current state-of-the-art in base dampening of buildings. These work under the same principle as smaller rubber shock mounts used for motors and other mechanical devices.

Conclusion

As civil engineers strive to design bigger buildings, bridges and other structures to meet mankind’s growing needs, the need for earthquake engineering is increasing all the time. The devastating effects of recent earthquakes have demonstrated the need for improved earthquake engineering design. This is one civil engineering field that still has plenty of room for growth and development; providing ample opportunity for the ambitious engineer.