Concrete is the most economical building material that can withstand high compressive strength, but cannot exhibit sufficient tensile forces. On the other hand, steel is considered a good building material in the context of tensile strength. It can withstand tensile strength much more than concrete. A properly designed concrete building structure hold increased strength and ductility even in areas prone to seismic conditions. The performance of buildings during an earthquake depends mainly on design instead of the material used in the process of construction. In conducting comparison between concrete and steel buildings, on can find benefits in both types of structure depending upon utilization and geography or soil properties of the area.
Turkey is situated in one of the most active earthquake zones having numerous moving tectonic plates. These Anatolian Tectonic Plates are surrounded by the African, Arabian and Eurasian Plates. The movement of the plates resulted into many earthquakes every month. Consequently performance of the structures especially building must be identified considering earthquakes in the design stage and be controlled throughout the life time.
Strengthened concrete is one of the most largely used modern building products. Concrete is acquired by blending cement, aggregates and sand with water. Fresh concrete can be molded into any shape, which is an integral benefit of it over various other products. Nonetheless, its minimal stress resistance stopped its wide use in building construction. Steel frames structures perform well against high wind loads due to its ductility by increasing its ability to bend without breaking. Strength and ductility of steel make a building safe even in seismic zone by good design and solid engineering. The steel bars are embedded in concrete for development of the composite structure known as reinforced concrete. The globally usage of reinforced concrete construction comes from the broad accessibility of reinforced steel in addition to the concrete ingredients. Concrete building calls for a specific degree of modern technology, expertise, and craftsmanship, especially during construction. The expense of construction changes with the region and highly depends upon local methodology. Reinforced concrete structures include vertical elements called columns and straight elements called beams that attached by rigid joints. They are organized by monolithically casting in order to act in harmony. Improved concrete building provides resistance to both gravity and side loads with bending in column and beams. (Bayraktar et al., 2013; “Concrete VS Steel," 2005).
.A Synopsis on Building Structures in Turkey
As per study conducted byTapan et al., (2013) a good inspection system with seismic design code and construction practice is in use for seismic damages, in Turkey. The main factors need to be addressed for buildings are inspection system, general construction practice, and seismic design code in Turkey. These failures of existing structures are associated with the issues in Van as well as other parts of Turkey.
Inspection system
In Turkey, most of the existing structures around the afflicted region had not been properly designed subject to intensive examination throughout their design and building. The most recent building evaluation regulation in Turkey licenses independent design companies for inspection of design and construction of structures. According to this legislation, independent assessment firms are accountable for any negligence that could take place during design or building stages. This legislation was imposed in 2001 for 19 districts of Turkey after the tragic earthquakes of Duzce and Kocaeli in 1999. Nevertheless, Van district was not among these 19 pilot provinces, and new building assessment legislation was executed in Van district by the start of 2011. (Bayraktar, et al.2013; Tapan et al., 2013).
Method of Construction
For ordinary structures, like in several parts of Turkey, the proprietor or the investor of the land is the builder of the area. The builder, directly, is not needed to have specific knowledge or experience on construction. While the architectural designs are carried out by designers and civil engineers, the in situ application is performed by an uneducated supervisor, without correct evaluation. The non-engineered buildings, on the contrary, are created and constructed completely by ignorant employees. Unfortunately, properly engineered, inspected buildings with formal settlement allow composing minor section of already existing buildings in Van and other cities of Turkey. In terms of structure; the buildings in Van city center mainly consisted of 4-5 levels reinforced concrete structure buildings, whereas there were also numerous non-reinforced and confined masonry building of two to three levels. In Eric's, while there was a bunch of improved concrete high rise building having 4-5 levels, the proportion of masonry structures is fairly greater with respect to Van city center (Tapan et al., 2013 ).
Seismic design code
In Turkey, the first seismic design code was released in 1940 after the Erzincan quake in 1939, which has triggered a fatality toll of 39,968. After this preliminary seismic design code, the laws on construction and seismic design have been updated frequently. The last three models of Turkish seismic design code cover almost all elements of the most superior international seismic design codes of the time for usual sorts of structures in Turkey. Due to this reason, it could be stated that if the buildings have been developed by considering about the seismic design code, the damages and fatalities can have been reduced (Bayraktar et al.2013; Tapan et al., 2013).
Characteristics of Building Structures
Concrete
The utilization of mixed concrete was common in early 1990, and this ready mixed concrete has not been used during construction in Van until the year 2000. In addition, the utilization of hand mixed concrete remained persistent in the construction of the low cost buildings even after the year of 2000.Tapan et al. (2013) analyzed the samples collected from reinforced structures to evaluate the quality of concrete by considering the age of the building. They reported that the average concrete strengths ranged from 10 to 15 MPa for the buildings that were constructed before the year 2005, and the average concrete strength were made possible to exceed 25 MPa due to widespread usage of ready mixed concrete. The results were based on compression tests of 110 concrete samples taken from 22 different buildings.
Bars for steel reinforcing
The usage of ready mixed concrete and deformed reinforcing bars has become common in Van in early year of 2000. The plain bars have been used for construction of almost all reinforced concrete buildings before the year 2000. For assessing the mechanical characteristics of reinforcing bars used in construction, they collected samples from the damaged buildings and the main consideration were given to longitudinal and transverse bars having small diameter. The mechanical characteristics of reinforcing bar samples could be analyzed and evaluated for optimum performance (Bayraktar et al., 2013; Tapan et al., 2013).
Concrete structures
The industrial structures had performed better during the earthquakes having good engineering design, construction, and inspection of structures comparative to residential buildings. Moreover, structures that were built using the method of cast in place suffered limited damages and some pre-cast concrete industrial structures failed resulting in complete collapse. The complete collapse of few structures of the same type was observed due to failure of connections of the column to beams. The connections were normally being fixed by bolts and nuts in addition to grouting the holes of the beam at the support sections. On the other hand, bolts and nuts were not used in those particular pre-cast structures. Another major design error was the assumption of Z-2 type local ground condition, whereas it should have been Z-3 or Z-4, considering that an allowable soil stress of 120 kPa structural design.
According to Turkish Seismic Code, Z-1 class soil corresponds to the stiff soil condition and Z-4 is the soft soil conditions. This erroneous assumption causes a remarkable reduction in seismic design forces. Furthermore, the bottom longitudinal reinforcing bars were hooked at the end of purlins in the design. The lack of hooks was also observed for purlins that might have played a significant role in the failure. An interesting observation for these totally collapsed columns, which might shed light on nonlinear design and analysis of such structures observed was plastic hinge length that measured to be approximately twice the depth of the column. Other deficiencies were also observed; that did not play a major role on the failure as poor concrete quality, insufficient number of transverse cross-ties in the pre-cast columns and insufficient cover thickness. (Bayraktar, et al., 2013; Tapan et al., 2013).
Steel structures
There were also some silos and open sheds constructed with steel. Some of these steel structures performed well, without experiencing significant damage, and some of them totally collapsed due to poor design details. The main weakness of the severely damaged steel sections was observed to be a lack of stiffeners at the base plate connection. The total collapse of the silos was also associated with weight of the cement filled within it. (Bayraktar et al., 2013; Kelesternur et al., 2009).
Reasons for Concrete and Steel Buildings Failure
The field investigations proved that concrete, steel or reinforced concrete buildings may fail due to the following factors:
- Ground failures,
- Liquefaction of soils,
- Poor concrete strength
- Plain reinforcement steel,
- Improper designing of beam and column joints,
- Weak columns with strong beams,
- Insufficient transverse reinforcement,
- Presence of short lap splices and inaccurate end hook angle,
- Short columns,
- Feeble walls,
- Failure of concrete slab,
- Insulation materials,
- Cracks at the corners of the doors and windows
Conclusion
Performance of concrete, steel and reinforced concrete buildings was discussed and reviewed against earthquakes at different levels by many researchers and engineers. Seismic code needs have gone over and should be compared with identified details. A properly designed and constructed building structure is the best option in different conditions including earthquakes and other natural disasters. In short, while conducting comparison between concrete and steel buildings, one can find benefit in both types of structure but well designed reinforced concrete structure is the best option.
References
Bayraktar.A, Altunişik.C.A. , & Pehlivan.M. (2013).Performance and damages of reinforced concrete structure buildings during 23rd October and 9th November, 2011 Van, Turkey, Jr. Earthquake Soil Dynamics and Earthquake Engineering, 53, 49-72.13.
Bilgin.H. (2013).Fragility based assessment of public buildings in Turkey, Engg. Structures. 56, 1283-1294.
Concrete vs steel (January 06, 2005). [Online forum comment]. Retrieved from http://www.buildings.com/article-details/articleid/2511/title/concrete-vs-steel.aspx
Kelesternur.O. Kelestemur.H.M. ,& Yildiz.S.(2009).Improvement of mechanical properties of reinforced steel used in the reinforced concrete structures. Jr. of Iron and steel research, International, 16(3), 55-63.
Tapan.M, Comert.M., Sayan.Y. Ilki.A., Orakcal.KIlki.A.& Demir.C(2013).Failures of structures during the 23rd October, 2011 Tabanlı-Van and 8th November, 2011 Edremit-Van, earthquakes in Turkey, Engineering Failure Analysis, 34,606-628.
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