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Earthquake, one of the most destructive and harmful forces in nature, have the ability to alter the course of rivers, wipe away landmass from maps and destroy manmade structures. All of this happens in just few seconds and in a considerable distance from earthquakes initial start point. Earthquakes are the origin of additional hazards such as tidal waves also known as tsunami, soil liquefaction and fire by damaging electrical power or gas lines. All of these lead to a principal consequence of any major earthquake, the loss of life and money. (SFA, 2003)
In the past 50 years a lot of researches and developments have been done in the field of design and construction methods, in order to produce houses, which are more resistible against seismic forces and can withstand any side-to-side or up-and-down movements. These progresses lead to construction of safer houses. One of the recent solutions is the use of light gauge steel framing or LSF, (Procter, 2006). The use of this system of house construction has increased significantly throughout the world, especially in countries such as Canada, USA, Japan, Turkey and Iran, which have high probability of earthquake. Also European countries are using light steel framing as a method of house construction in recent years, (Coskun, 2004). For instance, in the UK, the market of steel has reached about 5% of house and apartment building, (Amundarain et al., 2007).
This report will give a general idea about the light steel frame structures with a brief description about its main system, advantages and disadvantages in comparison to other systems and then by studying the behavior of buildings during an earthquake, the benefits of using such a system will be addressed. In order to support the proposed structural system and help with understanding the issue two real cases are revealed in this report.
Figure - LSF usage in construction (www.casfa.org)
Figure 2 - Courtesy of Mega Building Systems (Procter, 2006)
An Introduction to Light Steel Framing
Steel is one of the most popular and commonly used materials in the building construction. Steel has been used on a massive scale for over hundred years. Most skyscrapers or suspended bridges are structures associated with steel. In recent years steel was a versatile material in various commercial, residential or industrial buildings and has a lot of potential to be improved, (Coskun, 2004). One of its main advantages among other systems is that it has sufficient strength in both compression and tension, so there is less need to combine it with other materials, (Amundarain et al., 2007).
Light Steel Frame, also known as LSF, is an improved method of using steel for construction purposes. Despite steel, which is usually, knows as a heavy construction material, cold-light steel profiles are not in this category. In fact light steel profiles are lighter than comparable wood studs and have a better weight to strength ratio than wood or concrete of similar dimensions. Galvanized light steel profiles are strong, lightweight, durable and cost-competitive materials that have long life and can be recycled. Light steel frame (LSF) construction can either be used as a main construction system or as part of internal partitions. (Amundarain et al., 2007)
In comparison to traditional construction methods, steel profiles are economical and are comparable in price to timber systems and usually more economical that reinforced concrete or standard steel structures. A light steel framed structure has the advantage of precision, strength and stability for a durable building system. LSF can be used for the frame of a building up to five floors high or in a combination with other systems for building a skyscraper. (Lawson and Ogden, 2006)
All elements of a LSF building are pre-engineered and pre-assembled in high-tech factories based on the desired designed dimensions and will be transported to site. Fully automatic machines form and shape the steel elements and based on the design and blue prints the place of the bolts and connections will be indicated and cutout. The erection process is rapid and needs less labor in comparison to other systems especially in comparison to reinforced concrete system, which takes days for the concrete to stiff. The only process which take place in the site is connecting the prefabricate elements together, which will allow for quicker construction time and efficient outcome. Also because of the simple erection process, the frames can be easily disassembled and moved. This gives the advantages of extension which can be performed easier than other systems specially reinforced concrete. (Lawson and Ogden, 2006)
From sustainability side of view, steel is 100% recyclable and it can be transformed to the same product with the same function and quality as before. About half a million tons of steel are recycled each year worldwide. Predictions show that the iron ore resources will last up to seven million years with current mining activities, so there is no need to be worried about its extinction. Also in case of sustainability and environmental impacts, it will take roughly six junk scrap cars to construct a 200 m2 and the steel elements are totally recyclable, while to construct a similar house with wood it may need up to forty trees. (Coskun, 2006)
The total weigh of a LSF building is less than one made of wood or concrete and there is fewer loads placed on the foundation and the applied stress is greatly reduced. So the buildings are highly resistible against earthquakes and high winds or any seismic forces and if the building collapses less weight will fall down and cause harm. It is a known fact that human loss during an earthquake is mainly caused by collapsing heavy structures and buildings.
In addition LSF buildings have a great isolation against fire and thermal transfer. The steel structure will not burn and is non-combustible. Polystyrene vapor barrier are the common isolation used in LSF, this forms a building wrap that insulates the whole building and prevents any thermal transfer, this characteristic allows for a significant savings in energy costs. (Lamont 2001; Bielat et al., 2002)
Some features of light steel frame structures can be mentioned as follow; (Dofas 2003; Bielat et al., 2002)
Reduction in dead load of the building up to half the weight of a traditional system resulting in lower cost.
Fast erection and quick installation with efficient construction cycle time.
Easy extension and simple generation.
Prefabricated system, all built if factory under controlled manufacturing processes, which results in higher quality and marketing advantages.
No need for deep foundation and pile construction.
Quick and easy transportation to site.
High fire resistance in comparison to other structural systems.
100% recyclable and environmental friendly.
Highly resistible against high intensity earthquake and wind due to lighter structure and stronger connection.
Low cost of maintenance because steel is resistible against rot, molds, termite and insect infestation.
Lower shrink, split, warp and crack which will result in more age for the structure.C:\Users\Arash\Desktop\Picture5.jpg
Figure 4 - Ease of Transportation (www.casfa.org)
Figure 3 - Lighter weight and more strength in LSF (www.steelframehousing.org)
Figure 5 - Durability of light steel framing is assured by corrosion resistant galvanized steel (Dofas 2003)
What happens to a building during an Earthquake
The reference of below paragraphs is (SFA, 2003).
Considering the natural disasters, which cause structural failure in buildings and result in loss of human's life; earthquake is one of the most critical and deadliest of all regarding its damages throughout the history of mankind. Countries that are placed on a seismic zone experience earth vibrations every day, sometimes the major seismic faults and vibrations are so massive that create a very serious damage to buildings and cause harm to humans.
Figure 6 - Simple illustration of movement during an earthquake (SFA 2003)Depending on the scale of an earthquake a structure will start to move in different directions depending on the movement of earth surface. Side-to-side or up-and-down movements in ground cause these activities. In some cases combination of both horizontal and vertical movement occur, which is more destructive. These are the seismic forces that are applied to a structure during an earthquake and can destroy a building. Inertia is the main cause of structural damage during an earthquake. Inertia in a building can be defined as reluctance of upper parts of the building to begin moving while the ground starts to shake, and then conversely, to stop its moving when the earthquake is finished and the structure is beginning to discontinue its movement. This is when everything goes wrong and a structure fails to resists the applied stress from the earthquake. C:\Users\Arash\Desktop\6654.PNG
When the shift is in sideways, the effect of inertia will cause cracks in the structure of the building. If the movements are in the vertical direction, the building will start to experience compression as the earth rises and tension while the earth stops its move.
The design of structures must be in a way to absorb the energy that is produced by earthquake and resist the stress caused by inertia. A solution for overcoming this criterion is to design building in a way to flex with the movement of ground during the earthquake. So the building has the ability of flexibility and will adopt the ground movements in varying degrees. This can be achieved by the right chose of material and the design of structure or the way the elements are connected to each other. Other factors, which affect the resistance of building, are the quality of construction, the level of engineering and the used standards and codes.
Engineering of Structure against Earthquakes
The seismic forced produced by earthquake subjects the building to sliding and racking. In order to control and minimize the effects of these motions, all the elements of a building must be connected and tied together in a way to transfer all the applied stresses from upper roofs and floors all the way down to the foundation. Roofs and floors are the initial parts of any structure, which generate lateral forces, so in a simpler word the roofs and floors must be tied to the shear walls and ultimately the foundation. Also in order to prevent walls from uplift or overturning, due to the lateral forces, the walls must be tied together and finally fastened to the foundation. (Rourke and Zhang, 2004)
Figure 7 - Transfer of seismic forces from the origin to the building (SFA 2003)
Comparing different structural systems and based on the recent researches all around the world, light gauge steel frame (LSF) is the most efficient and reliable structural system that can resist high degrees of earthquake in comparison to traditional structural systems such as welded steel, reinforced concrete or even timber frame structures, (Coskun, 2004). Houses with light steel frame system are designed considering all the above basic concepts. They have less weight in comparison to other systems and are more flexible against the seismic movements of the ground. (SFA, 2003)
Reviewing Real Cases
Although steel frame has been used in residential, commercial and industrial construction for decades some still consider it as a modern concept in field of construction which needs more time to be completed and advanced. In recent years number of factories has started working on this field of construction system, especially in locations that have higher possibility of earthquake occurrence. There are plenty of cases that after an earthquake different authorities and governments choose light steel frames as their first choice for redeveloping the destructed buildings. In order to understand whether using LSF is really a good solution for resisting earthquake, two real cases will be discussed. The first case is in turkey and the second one is related to deadly earthquake of Bam city in Iran.
Case 1: Izmir Earthquake, TURKEY
Turkey, a country located on one of the most active and highly effected seismic zone, has experienced massive highly scaled earthquakes during its existence. On average about 50 earthquakes occur in this country each year, but not all of them have the intensity to collapse buildings and cause fatality. The north Anatolian fault is one of the continental plateaus, which caused serious earthquakes in the history of region. (Coskun, 2006; Marza, 2004)
The latest earthquake that took place in this fault goes back to 17th of August 1999, in Izmir city near Marmara Sea region. With a scale magnitude of 7.8 Richter, it was the second biggest and deadliest earthquakes in the history of Turkey with thousands of life loss and huge economical impact. Based on the statistics nearly 50 thousand reinforced concrete building were heavily damaged in the five main cities which were located in the earthquake region. The destruction and failure of heavy building was the main reason of human loss. The buildings of the region were mainly not designed according to the seismic codes or there were problems in their process of construction or selection of material and structural system. It is interesting that a few number of schools and hospitals, which were built by LSF system, had no major damage. (Marza, 2004)
After the earthquake professional groups started to discuss and find an alternative building method in order to withstand future treats. The researches showed that despite the dynamic and high potential value market of construction in Turkey, steel consumption was not at a desirable level. This was mostly because of the traditional building construction, which is mostly based on reinforced concrete and masonry material. Now a day Steel structures and specially LSF system are considered as an important alternative for an earthquake resistance, flexible, light weight and environmental friendly building. (Coskun, 2006)
One of the other problems that the government and people confronted after the earthquake was demolishing of collapsed and heavily damaged buildings. Removing heavy and brittle reinforced concrete materials of a manipulated and demolished building is big economical and environmental concern and needed lots of money and time. The recycling of such materials are also limited and required very big stocks and handling areas. This is an essential issue that must be taken in to account while selecting the structural system and building material for areas with potential of earthquake. (Coskun, 2006)
Case 2: Bam Earthquake, IRAN
Another similar case has happened in Iran. On 26 of December 2003, a massive earthquake with a magnitude of 6.6 richters struck Bam city and leveled its surroundings areas. Almost the entire infrastructure of the city was districted and the estimations showed a death toll amounting to 25'000 people. Nearly 30'000 were injured and a great number have damaged their spinal cord due to impact of a structure element. (Manafpour, 2003)
The buildings were mostly made of adobe, mud brick and masonry with a few buildings made of reinforced concrete or steel. The effects of the earthquake were exacerbated because of the heavy weight structures and most of the human losses were because of this issue. One of the main losses in the earthquake was the world's largest mud-brick structure, known as Arg-e-bam, which suffered from severe damage to total destruction. In addition to this many other types of buildings exhibited similar damages. A reason behind this is use of poor materials, also the design and structural system of the buildings played an important role. (Manafpour, 2003)
There were few buildings, which survived the earthquake and their performance against the applied seismic forces were satisfactory. Among these some steel structures with braced frame load resisting systems had suffered minor damages, (Manafpour, 2003). The only problem that steel structures challenged with was due to incompatibility of masonry materials used in walls and roofs and the steel structure itself. Burnt brick and sand-cement mortar, the main masonry material used in the region, connect and fit together with a good friction amount, but the problem occurs when the adhesive agent which is cement wants to bond with the steel frame, often leaving the materials with no cohesion in between. It becomes critical when masonry walls and roofs must be connected to the steel columns and beams. So a lateral force due to the earthquake can easily spring apart the walls and roofs from the steel frame, leaving the brick mass separated from the structure, causing it to collapse. There were plenty of cases that the body has collapsed while the skeleton of steel frame remained standing. (Gharaati, 2004)
LSF overcomes this problem by using dry wall panels as the main material of walls which are bolted to the main frame and have a very light weight. Also sandwich panels are used to cover the roofs, these are great isolations and are well connected to the main frame with bolt. This shows that steel system is a great option for buildings, which are located in seismic locations. (Gharaati, 2004)
Rebuilding and recovery of the city continued for about six months after the fatal quake. Number of companies started to investigate the effect of earthquake on the local buildings. Again the results were somehow similar to those of Turkey. Light steel frames are now one the major structural systems in the city. Hospitals, schools, governmental buildings or even mass construction residential areas are using this system to build more economical and more resistible buildings against future earthquakes. (Gharaati, 2004; Manafpour, 2003)
Based on the statistics mentioned earlier and the revealed cases, steel is a suitable material in case of environmental and in comparison to wood, concrete and masonry it is a competitive and advanced material, which can be easily dismantled, repaired, fixed and reused. On the other hand, because of easy and fast construction, environmental friendly, energy efficient and resistance against applied seismic forces and earthquakes, use of Light Steel Frame structure as the main system of structure is growing day after day and presents a lot of advantages in residential, commercial and industrial field.