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Ahmet Yakut, Middle East Technical University, Turkey
BACKGROUND
Reinforced concrete is one of the most widely used modern building materials. Concrete is “artificial stone” obtained by mixing cement, sand, and aggregates with water. Fresh concrete can be molded into almost any shape, which is an inherent advantage over other materials. Concrete became very popular after the invention of Portland cement in 9 th^ century; however, its limited tension resistance prevented its wide use in building construction. To overcome this weakness, steel bars are embedded in concrete to form a composite material called reinforced concrete. Developments in the modern reinforced concrete design and construction practice were pioneered by European engineers in the late 9 th^ century. At the present time, reinforced concrete is extensively used in a wide variety of engineering applications (e.g., buildings, bridges, dams).
The worldwide use of reinforced concrete construction stems from the wide availability of reinforcing steel as well as the concrete ingredients. Unlike steel, concrete production does not require expensive manufacturing mills. Concrete construction, does, however, require a certain level of technology, expertise, and workmanship, particularly in the field during construction. In some cases, single-family houses or simple low-rise residential buildings are constructed without any engineering assistance.
The extensive use of reinforced concrete construction, especially in developing countries, is due to its relatively low cost compared to other materials such as steel. The cost of construction changes with the region and strongly depends on the local practice. As an example, a unit area of a typical residential building made with reinforced concrete costs approximately US$100 /m^2 in India, US$250/m^2 in Turkey, and US$500/m^2 in Italy.
With the rapid growth of urban population in both the developing and the industrialized countries, reinforced concrete has become a material of choice for residential construction. Unfortunately, in many cases there is not the necessary level of expertise in design and construction. Design applications range from single-family buildings in countries like Algeria and Colombia to high-rises in Chile, Canada, Turkey, and China (Figure 1). Frequently, reinforced concrete construction is used in regions of high seismic
Figure 1: Typical residential RC frame building in Turkey (WHE Report 64, Turkey)
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risk, such as Latin America, southern Europe, North Africa, the Middle East, and Southeast Asia.
REINFORCED CONCRETE FRAME BUILDINGS
Reinforced concrete (RC) frames consist of horizontal elements (beams) and vertical elements (columns) connected by rigid joints. These structures are cast monolithically— that is, beams and columns are cast in a single operation in order to act in unison. RC frames provide resistance to both gravity and lateral loads through bending in beams and columns (Figure 2). There are several subtypes of RC frame construction:
- Νonductile RC frames with/without infill walls
- Νonductile RC frames with reinforced infill walls
- Ductile RC frames with/without infill walls
The current WHE database includes over twenty reports describing RC frame construction. The most prevalent type is RC frame with masonry infill walls (Figure 3). This construction is still practiced extensively in many parts of the world, especially in developing countries. This construction comprises approximately 75% of the building stock in Turkey, about 60% in Colombia, and over 30% in Greece. Details of this construction type including regional variations are contained in the WHE reports from Cyprus (WHE Report 13), India (WHE Report 19), Palestinian Territories (WHE Report 48), Turkey (WHE Report 64), and Romania (WHE Report 71). RC frames with concrete infill walls, also known as dual systems, are very common in earthquake-prone areas. The WHE reports from Chile (Report 6) and Syria (Report 59) describe details of this construction type.
Code requirements related to design and detailing of RC frame buildings in seismic zones were significantly changed in the early 1970s. Earlier codes focused on the strength requirements—that is, on providing adequate strength in structural members to resist the lateral seismic forces. However, based on research evidence and lessons learned from earthquakes in the early 1970s, code requirements have become more focused on the proportioning and detailing of beams, columns, and joints with the objective to achieve a certain amount of ductility in addition to the required strength. Ductility is one
Figure 2: A plan of a typical RC frame building in Ahmedabad, India; note the portion that collapsed in the 2001 Bhuj earthquake (WHE Report 19, India)
SEISMIC PERFORMANCE
Earthquake performance of RC frame construction has been well documented. Damage patterns in reinforced concrete frames during the 1971 San Fernando (California) earthquake have been extensively studied. More recently, several destructive earthquakes of the last decade, including the 1999 Athens (Greece) earthquake, the 1999 İzmit and Düzce earthquakes (Turkey), 1999 Chi Chi (Taiwan) earthquake, 2001 Bhuj (India) earthquake, and the 2003 Boumerdes (Algeria) earthquake, have caused substantial damage to RC frame construction. These earthquakes have revealed the following patterns of damages and failures in RC frame construction:
- Shear failure and concrete crushing failure in concrete columns. These are the most undesirable nonductile modes of failure (Figure 5). This behavior can lead to the loss of gravity load-bearing capacity in the columns and potentially a total building collapse.
Figure 4: Features of nonductile RC frame construction in Taiwan (WHE Report 61)
Figure 3: RC frame construction with hollow- clay tile masonry infill in Algeria (Credit: S. Brzev)
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- Partial ductile design and detailing. Systems that exhibit some (limited) yielding behavior can eventually form dangerous collapse mechanisms as a result of stiffness or strength degradation at sections without ductile detailing.
- Conceptual design deficiencies. This includes such deficiencies as incomplete load path and architectural planning deficiencies such as vertical and/or horizontal irregularities. Architectural features play an important role in the performance of RC frame buildings.
- Inappropriate column/beam relative strengths. This can lead to failure of individual members and connections when the “weak column-strong beam” mechanism develops.
- Inadequate detailing of reinforcement.
- Soft-story effects. In many applications, architectural considerations result in a taller first story, which causes a soft-story formation due to drastic change in the stiffness between adjacent stories (Figure 6). The presence of a soft story results in a localized excessive drift that causes heavy damage or collapse of the story during a severe earthquake (Figure 7). Another typical case of soft story arises when the first floor is left open (that is, no infills) to serve a commercial function (stores) or as a parking garage (very common in Turkey, India, and Cyprus), while upper floors are infilled with unreinforced masonry walls. A relatively rare case results when the strength of the two adjacent stories is significantly different (weak story) leading to localized deformations similar to the soft-story mechanism.
- Short-column effects. The short- or captive-column failure occurs due to partial restraining of the columns that are, in turn, subjected to high shear stresses and fail in shear if unable to resist these stresses.
Figure 5: Shear failure of a reinforced concrete column in the 2001 Bhuj earthquake (WHE Report 19, India)
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Earthquake resistance in RC frame structures can be enhanced by either of the following approaches:
- Strengthening the components, such as columns and beams, by jacketing with concrete, steel, or fiber wrap overlays (see Figure 8)
- Increasing the overall capacity of the structural system by installing new concrete infill walls or steel bracings.
The most common rehabilitation measure is installation of new reinforced concrete infill walls (Figure 9) along with jacketing the columns to increase the strength of the existing structure. These new walls are reinforced in such a way as to act in unison with the existing structure. However, careful detailing and material selection is required to ensure that bonding between the new and the existing structure under earthquake loads is effective.
Figure 8: Jacketing of RC frame members (WHE Report 11, Colombia)
Figure 9: Illustration of seismic strengthening with addition of RC infill walls (WHE Report 62, Taiwan)
An alternative procedure which has been recently developed for RC frames with unreinforced masonry infill walls proposes the use of carbon-fiber, reinforced polymers (CFRP) applied on existing unreinforced masonry infill walls (Figure 10) to increase the overall lateral load capacity. Although its cost is higher, this method is easy to apply and much faster when compared to the installation of new concrete infill walls (Ozcebe et
al. 2 ).
Figure 10: Strengthening of brick- infilled RC frame with CFRP
ENDNOTES
(^) Applied Technology Council (ATC), 1996. Seismic Evaluation and Retrofit of Concrete
Buildings. Vol.1, Report No. SSC 96-01, (ATC-40). (^2) Ozcebe, G., Ersoy, U., Tankut, T,, Akyuz, U., and Erduran, E., 2004. Rehabilitation of
existing RC structures using CFRP fabrics. Proceedings of the 13th^ World Conference on Earthquake Engineering, Vancouver, Canada, Paper No. 1393.
