Earthquake engineering is used to ensure that manmade buildings and structures are not damaged from earthquakes. It is frequently described as studying how different structures react when under seismic loads and requires both knowledge in geotechnical and structural engineering. The main objectives of such an engineering position are to foresee the possible results on infrastructure and urban areas due to strong earthquakes and to design and construct structures that can function and stand under earthquakes as well as adhering to the specified building codes.
This type of performance specifically refers to how a building or structure will respond when under seismic forces from earthquakes. This type of performance analysis is required in areas that experience frequent earthquakes. During an earthquake a structure has the potential to move in a back and forth “wave” motion. There are several different design elements that can counteract this lateral movement, which can be potentially very damaging. Structures need to meet the prescribed performance specifications to remain undamaged during earthquakes.
Most current analysis assessments and tools use computer based seismic analysis, first developed in 1970. There are five main analysis methods used to determine the seismic performance of structures. Response spectrum analysis allows for several different response modes, in which different “wave” motion frequencies refer to different modes. A computer will show how the structure will respond when the different modes are applied, as motion varies depending on the structure mass and the chosen frequency. Types of calculations used to estimate the performance results include the square root of the sum of squares, the absolute values and the complete quadratic combination. This analysis is best used on structures that are regular in design and not overly tall.
The equivalent static analysis works by applying several forces onto a building in such a way that the forces mimic the effect of earthquake motion. In this analysis it is assumed that the building will only move using the fundamental mode. This assumption only works on low rise buildings that do not twist, as several factors must be included into taller buildings that twist slightly.
Non-linear static analysis is used for structures that are very inelastic and do not respond in a linear manner. This non-linear analysis is also known as pushover, in that several non-linear forces are included into the models, one non-linear model being the inelasticity of steel. The application of these linear and non-linear forces results in a capacity curve that shows the building response depending on the force.
Linear dynamic analysis is used for very tall structures that may move at higher modes and have torsional irregularities. Within the response model the structure contains multi-degree-of-freedom points, with a stiffness and viscous damping factor in the building model. A time domain is used to calculate the structural response when ground motion is present.
Non-linear dynamic analysis is the fifth type of analysis used for seismic performance and produces results that have very low uncertainty factors. Many details of the structure are incorporated into the model. This is like a combination of linear dynamic and non-linear static analysis as non-linear properties are all calculated using a time domain. This analysis is typically used for buildings that have an unusual shape and form or buildings that are very important. Several analyses are typically used as the intensity of motion and severity will have different structural effects.
Seismic Vibration Control
Within engineering, this type of control attempts to disperse the impact of ground motion on structures. There are three different types of vibration control; active, passive or hybrid. Active controls contain real time instrumentation that integrates with the earthquake processing equipment and actuators in the structure. Passive devices are those that have no feedback and a hybrid device will have both passive and active features in the designs.
Most earthquake forces will greatly decrease when the force hits the structure. As much as 10% of the force can still travel through the building causing significant damage. The vibration control will use dampers to dissipate the energy. These controls may also disperse the energy to a wide range of wave frequency or to absorb the energy that causes the building to move.
Dry stone walls control are a type of passive control that originates back to the Incas and the building of Machu Piccu. The stones are fit together in a wall without mortar in that the stones could move slightly and resettle without collapsing. This control suppresses resonant frequencies and dissipates energy. Lead bearing rubber is one type of base isolation applied to the foundation of a building. The lead rubber bearing has a heavy dampening effect. The lead core and rubber bearing will suppress vibrations. This was created by a New Zealander and used in many of the important buildings in New Zealand, as the control has a fault line running through the center. Springs-with-damper base isolator that is similar to lead rubber bearing and has been used on town houses.
Skyscrapers typically used tuned mass dampers composed of very large concrete blocks. When movement occurs the blocks move in an opposite direction to the wave frequencies using a spring mechanism. This helps to decrease the resonant frequencies. This is best thought of as a pendulum within the building and also helps to reduce movement due to wind. Friction pendulum bearing uses three pillars. One pillar functions as a spherical concave sliding surface; one as a friction slider and the third is an enclosing cylinder that displaces lateral movement.
Building elevation control helps to control vibrations due to earthquakes. Specifically the design of the building is shaped differently to reduce the resonant frequencies. One such example is the pyramid shape used at the top of skyscrapers. It is mainly the tapering effect that functions as vibration control, though tapering of the stiffness and mass of the building has a similar effect as tapering the building profile.
Another base isolation control form is simple roll bearing that reduces damage due to strong earthquakes. The bearings will absorb more of the earthquake forces, so that the structure does not move as much. There are four different types of hysteretic dampers that help to dissipate earthquake energy. These four types include friction dampers, metallic yielding dampers, fluid viscous dampers and viscoelastic dampers. Each type is suitable for specific types of buildings and the strength of the earthquake motion.
Failure modes refer to the way in which structural failure occurs due to earthquakes. Many engineers will study structures and buildings that have failed to help create new and more effective seismic designs applied to structures. One of the main causes of failure is due to a lack of reinforcement for buildings made of masonry. Leaning walls and cracked walls occur when inadequate roof to wall tiles are used or low quality mortar holds the masonry together. In extreme cases the walls will separate from the building frame.
Failure known as soft story effect occurs when the shear walls that are attached to the frame are not adequate and can completely separate from the stud wall. The inadequate soft walls can cause anything above to collapse due to improper support. The building may also slide off the foundation. Base isolation systems will prevent this. This also applies to reinforced concrete columns that can buckle when shifted. These columns are found in buildings as well as for road overpasses.
Retaining wall failure will cause whatever is being held back to burst forth, whether this is water or earth. Retaining walls can also be combined with landslide failure when gravity causes land or rocks to fall. The slope stability of the ground is a main effect of this failure and retaining walls are sometimes put in place to prevent landslides. This type of failure may also be caused due to soil liquefaction. The change in the water pressure of the soil will cause structures to silt, soil to settle and can even push soil through cracks in paved areas.
Another possible failure mode is the bounding of an adjacent building that adds additional force onto the structure. The possibility for pounding is normally built in to the analysis model but if any of the buildings support frame components such as the through-ties or confinement steel should fail, then pounding can cause the building to collapse. Force from a tsunami caused by an earthquake can put undue pressure on buildings causing failure; specifically as tsunamis cannot be predicted and only occur when earthquakes occur off the coast out in the sea.
Earthquake Resistant Construction
There are specific types of structural design that are resistant to the damage of earthquakes. Not only does this design enable all structures to survive any seismic activity but the design also meets the necessary building codes for each area. Some of the main processes use earthquake resistant design including retrofitting, construction material chose, and assembling of the proper infrastructure. Earthquake damage can be direct or may be indirect and the resistant design takes both types of damage into account. Structures may appear to be very sturdy but when they are not designed to be earthquake resistant then they will easily damage due to seismic forces.
Adobe structures are very resistant to earthquakes, as the bricks are made of mud and can withstand very strong earthquakes when reinforced. This building material is used extensively in areas of the world prone to earthquakes and other natural disasters. Sandstone and limestone as earthquake resistant materials, as these materials are very strong and durable. Seismic retrofitting makes sandstone and limestone buildings have much better seismic performance, particularly those that use base isolation foundations. Another tool for earthquake resistant structures that use stone is to use steel reinforcement within the masonry joints. In earthquake areas this type of steel reinforcement is a mandatory part of building design. Reinforced concrete is also similar to reinforced masonry, as steel rebar or fibers are embedded within the concrete. This reinforcement is used in concrete bridges, floors, columns and beams. Concrete may even contain ductile joints that prevent collapses.
Light frame structures used wood diaphragms and plywood shear walls to resist earthquake forces. The ratios of all the vertical and horizontal elements as well as the give of the fasteners and connector are taken into account for earthquake resistant design. Timber framing has been used historically in homes and does offer some structural benefits to surviving earthquakes, as wood is much more flexible than stone. However the wooden and timbre components must be engineered properly to withstand earthquakes.
Some structures may be prestressed. This prestressing creates intentional stress that will then improve the structures earthquake response, as it is known how the materials and building responds. Typically prestressing techniques include post-tensioning using high-strength bonded tendons, pre-compression and the use of high-strength embedded tendons for pretensioning. This type of design is used widely for underground structures, offshore facilities, TV towers, nuclear reactor vessels and bridges. The most resistant designs are those that are made entirely of steel. Steel frame designs are created using Load and Resistance Factor Design, as steel is inelastic in some ways. However of all material it has the most give and is very strong, hence the use of steel for reinforcement in other buildings and material.
General Earthquake Resources
Earthquake Facts – Some of the science behind what causes destruction during earthquakes.
Earthquake Scenarios – Various earthquake scenario shakemaps and ground motion maps.
National Earthquake Information Center – Data from the National Earthquake Information Center (NEIC).
Understanding Earthquakes – Information on understanding earthquakes including quizzes, history, and more.
Earthquakes – A slideshow with information on what causes earthquakes.
Earthquakes: Search and Rescue – The aftereffects of earthquakes including search and rescue missions.
Earthquake Seismology – Information on earthquake seismology, seismic waves, and how we measure earthquakes.
Earthquakes – General Interest – Some general information on earthquakes including historic pictures.
Earthquake History and Science – The history of earthquakes as well as the science behind earthquakes.
What are Earthquakes? – An easy to understand explanation of what an earthquake is.
Structural Engineer Resources
International Association for Bridge and Structural Engineering – The website of IABSE, includes publications, membership information, conferences and more.
Institution of Structural Engineers – Brief information on starting a career as a structural engineer.
Structural Engineering & Design – A collection of engineering slides from around the world.
Beginner’s Guide to Structural Engineering – Study materials for structural engineering students.
Journal of Structural Engineering – Information on subscribing to the Journal of Structural Engineering and how to preview manuscripts online.
Structural Engineers Association – The websites of the International Structural Engineers Association.
Structural Engineering & Materials – An online book discussing structural engineering and materials.
Advances in Structural Engineering – Various journal entries on the advances in structural engineering.
Structural Engineers Association of California – The website of the Structural Engineers Association of California.
Structural Engineering – Lecture notes on structural engineering and design.
Geotechnical Engineering Resources
Geotechnical Engineering – A geotechnical engineering directory including books, conferences, jobs and more.
International Society of Soil Mechanics and Geotechnical Engineers – The website of the International Society of Soil Mechanics and Geotechnical Engineers.
Electronic Journal of Geotechnical Engineering – A free electronic journal for Geotechnical Engineering.
Geoengineering – A brief explanation of what exactly geoengineering is.
Geotechnical Engineering Standards - A list of geotechnical engineering standards developed by ASTM.
Geotechnical Engineering and Geomechanics – Information on geotechnical engineering and geomechanics.
Southeast Asian Geotechnical Society – The website of the Southeast Asian Geotechnical Society.
Institute for Geotechnical Engineering – The website of the Institute for Geotechnical Engineering.
Geotechnical Engineering – An article discussing the engineering properties of Earth materials.
Geotechnical Engineering Research – Information that answers the question “What is geotechnical engineering?”
Earthquake Engineering Research Institute - The website of the Earthquake Engineering Research Institute.
Earthquake Engineering – Information on earthquake engineering including a list of earthquakes and slides.
Earthquake Engineering – Information on earthquake engineering from the San Francisco Department of Building Inspection.
International Association for Earthquake Engineering – The website of the International Association for Earthquake Engineering.
Journal of Earthquake Engineering – A list of free earthquake engineering and science journals.
European Association for Earthquake Engineering – The website of the European Association for Earthquake Engineering.
Pacific Earthquake Engineering Research Center – The website of the Pacific Earthquake Engineering Research Center.
Network for Earthquake Engineering Simulation – The website of the George E. Brown, Jr. Network for Earthquake Engineering Simulation.
Geotechnical Earthquake Engineering and Soil Dynamics – Two case studies on spatial variability of soil properties.