Mid-Continent Earthquakes and Their Effect on Concrete Structures Disasters come in many shapes, sizes, and forms yet all are detrimental to people and structures of all types. The most terrifying of all disasters are natural disasters, the force of nature is rivaled by no man made disaster. One of the natural disasters that has terrorized as well as fascinated humans through out history is the earthquake. Most earthquakes are associated with areas such as Southern California or China, but the most detrimental aspect of disaster is the element of suprise. In this case areas such as Central North America and North Turkey have been visited by the violent shock of a mid-continent earthquake. These earthquakes are violent and cause mass amounts of structural damage and total destruction of many structures. Concrete being a very brittle material is affected greatly during an earthquake but through technology many advancements have been made to compensate for the damaging effects caused by an earthquake. This paper will discuss mid-continent earthquakes of the past, their effect on concrete structures, problems effecting concrete structures, and alternative materials and solutions to bypass those problems during future earthquakes.
A series of earthquakes occurred between December 1811 and February 1812 in the New Madrid Seismic Zone which is in the Mississippi River Valley. “These earthquakes may have been caused by a series of buried faults and anomalous rock formations that formed 500 million years ago when tectonic forces tried and failed to split North America in two. This structure known as the Reelfoot Rift is the zone of weakness that could account for the earthquakes during this period (Reducing Losses 1995).” This series of earthquakes was characterized by ground warping, fissuring, severe landslides, and caving stream banks. An area of over 600,000 square kilometers was damaged during the 1811-1812 period of earthquakes (USGS 2000).
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During the earthquakes ground shaking occurred as the land moved up and down and caused trees to bend, chimneys to fall, and log cabins to be thrown to the ground. Liquefaction was also a problem during the mid-continent earthquakes where the shaking transformed water saturated soil or sediment into a thick quicksand like slurry (Reducing Losses 1995).
Sand bars and points of islands in the Miss. River gave way while some whole islands disappeared. This period of earthquakes was also characterized by deformation which is either elastic or inelastic. Both types of deformation were apparent during the earthquakes. When the rocks snapped back after movement during the elastic deformation large cracks in the ground were opened up. Inelastic deformation occurred when land was uplifted while other parts sunk and remained in that position. This uplift of the ground and the waves moving during the ground shaking gave the appearance that the Miss. River was flowing North.
A very recent mid-continent earthquake occurred in Turkey called the Izmit Earthquake. “The rupture appeared to be about 110 km in length and occurred during August of 1999. This earthquake occurred along a strike slip fault where there were recorded offsets of 1.2m to 4.9m (USGS 1999).
There was a mass destruction of buildings and as a result about 300,000 people were killed or injured (USGS 1999) while many were left homeless in campsites where diseases were rampant. Ground shaking was tremendous as a result of the 7.5 Richter scale rated earthquake where many buildings were flattened. “Deformation was evident in the Izmut Bay where the fault leaves the eastern side of the bay and slumps along the crack formed scarps (USGS 1999).
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Sag ponds were a result of the liquefaction that occurred along the strike slip fault. This earthquake was devastating to the residents of northern Turkey but a lot of valuable information was extracted in order to prevent that magnitude of disaster in the future.
Effects and Problems of Earthquakes on Concrete Structures Earthquakes are very detrimental to concrete structures which are very brittle and struggle and often lose to the violent shaking of the earth and the many ramifications that are brought about by the shaking. “Damage to the structures are caused by the materials inertia and resistance to movement that causes the concrete to fail and ultimately the collapse of the building. Ground acceleration is the key aspect from an engineering point of view because it is directly related to the force that is transmitted to the structure (Wright 1984).” Buildings lacking adequate strength or ductility are generally damaged severely or most often destroyed. During an earthquake the basement and or first floor of the building move with the shaking ground, but because the building is quite rigid the top wants to stay put. This combination puts a great stress on the walls and often results in damage. But the most widely feared is that of collapse in which the whole entire building is destructed killings most of the occupants. Structural damage is when beams are twisted, structural members fail but the building is left standing.
Non-structural damage only leaves damage that is only visually seen and does not affect the structural soundness of the building as a whole. Reinforced concrete has continuity on its side which is the transfer of load from a failed component to sound components which holds the structure together. Because of this continuity bad components such as a wall can be removed as the column holds the structure up and then a new wall can replace the removed bad concrete. Continuity can also work against the building if all of the components are not designed properly to provide continuity which is in turn called discontinuity. A prime example is if a columns are not designed to hold the entire load of the building in case the walls fail or vice versa. In this case the building has discontinuity and will probably fall due to the lack of support. (Ghosh 1991) Earthquakes with deformation, liquefaction of the soils, and ground shaking also have an impact on the regularity of concrete structures. Regularity deals with the distribution of load over the entire structure to maintain uniform soundness. Ground shaking and deformation can cause a sudden change in the stiffness, strength, or mass on either horizontal or vertical planes resulting in the redistribution of lateral loads. Irregularity results in setbacks of appendages, changes in story height or even the participation of nonstructural components. An example of this is the columns of tall building pounding on the roof of a shorter building. Strength and stiffness irregularities often result in torsional response. Torsion due to asymmetric failure of infill panels also contributes to building failures (Ghosh 1991).
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The stiffness of the structure is also affected by earthquakes that happen along strike slip faults or failing rifts. Although some buildings survive the natural disaster there is often extensive damage which results in extensive repairs and extensive bills for repair. Stiffness is the component that allows the structure to laterally drift. The control of lateral drift is crucial during the design of any seismic structure.
One of the most important aspects of maintaining a sound structure during an earthquake is ductility. Ductility refers to the ability of a structural member to deform permanently without a significant loss in strength (Wright 1984).
Ductile behavior is often achieved by designing concrete members so that they yield in bending in overload conditions (Wright 1984).
Brittle failure is the opposite of ductile behavior is what is generally trying to be avoided. For a properly designed structure to resist earthquake damage the designer must assume that the brittle members will fail so the ductile members need to prevent the failure of the structure as a whole causing total destruction of the building. “Ductile columns generally permit the larger deformations that take place at the first story level (Wright 1984).”
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... earthquake engineering design, that the principles of capacity design of structures were developed and subsequently introduced into many modern design standards. 5. Earthquake Design ... strength or load carrying capacity. Such mechanisms have been found to generally involve the flexural response of reinforced concrete ... buckling of compression members such as columns, and the tensile failure of brittle ...
Earthquakes bring about various problems as they shake their violent head. Ground shaking, liquefaction, and deformation top the list in damage to concrete structures. A sound concrete structure, being a small patio to a high rise building starts at the bottom with a sound sub-grade and sub-base. “Liquefaction , causes the soil to lose its ability to support weight and the building can sink or topple ( Reducing Losses 1995).” The other two factors, ground shaking and deformation have multiple affects on a concrete structure and many can be avoided through a proper selection of concrete mix and reinforcement. A quality concrete mix is the key to avoiding many problems in seismic structure. The mix design of the concrete is crucial in the compressive strength and flexural strength which are both tested to the limit during the movement involved during a mid-continent earthquake. A properly designed mix would contain a low water/cement ration which is directly related to the strength of the concrete. The mix would also have properly graded aggregates which are the backbone of the concrete mix. Properly graded aggregates have a good distribution of particles, large and small, so that there is a limited amount of void spacing between the larger aggregates which are the back bone of the hardened concrete.
Concrete by itself has incredible compressive strength but is relatively weak in flexural strength or bending strength. Steel, known as rebar, is placed in the concrete structure before placing the concrete to increase the flexural strength. This combination, reinforced concrete will provide excellent strength both in compression and flexural to withstand the force exerted upon the structure by the shaking and deformation of the earthquake.
Along with the design of the concrete properties the design of the hardened concrete must also be adequate to sustain the many variables that will be placed upon it during a earthquake. Proper placement of beams and columns is crucial in order to have continuity and ductility. “The essential objectives of failure mode control for concrete structures are as follows: a. Beams should fail before columns: b. Brittle failure modes should be suppressed c. An appropriate degree of ductility should be provided (Dorwick 1990).
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“In order to ease the reinforcement design problems of the beam-column joint plastic hinges may be desirable a short distance away from the column face (Dorwick 1990).
One of the major avoidance factors is greatly dependent upon technology and predicting the seismic activity in the area. The demand and capacity of the building during the earthquake is extremely crucial in the design of the building. Over designing the structure may be costly up front but could be quite a savings in the future when compared with the price of a new building.
Technology also governs the materials and design of the structures. “Battelle’s Pacific Northwest Laboratories has found a connection between the use of steel fibers and the resistance the failure during earthquake like conditions ( Henager 1980).
“Technology has also brought the concrete industry carbon fiber for reinforcing columns as well as using steel jackets or reinforced concrete to support existing columns (Roberts 1995).” These techniques strengthen the columns allowing for better continuity.
Earthquakes seldomly rear their ugly head, but when they do there is generally mass destruction involved. Studies of past earthquakes and future predictions can give the concrete industry an advantage in designing more earthquake damage resistant structures for safer living. Technology brings disaster but it can also help prevent the damage from natural disaster.
Bibliography:
Works Cited Dorwick, David J. Earthquake resistant design: for engineers and architects. Chichester New York 1990 Ghosh, S.K. “Observations on the behavior of reinforced concrete buildings during quakes”. ACI SP-127. 1991 Henager, C.H. “Earthquake resistant connection uses steel fibrous concrete”. Concrete Construction. July 1980 Reducing Earthquake Losses. Office of Technology Assessment; U.S. Congress Washington D.C. 1995 Roberts. “Seismic Retrofit”. Concrete Repair Digest. Dec. 1995 U.S. Geological Survey 1999. “1999 Scientific Expedition of Turkey” URL: http://quake.wr.usgs.gov/study/turkey/aug25report.html U.S. Geological Survey 2000. “Largest Earthquakes in the United States” New Madrid 1 811-1812.
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URL: http://earthquake.usgs.gov/neis/eqlists/USA/1811-1812.html (2000) Wright, James K. & Berg, Glen V. “Earthquakes and reinforced concrete” Concrete Construction. May 1984
