A weak or soft story in a building has relatively low lateral strength or stiffness compared to those of the stories above or below that story. Weak and soft stories are typically observed in the first story of reinforced concrete buildings in Turkey. In multi-story buildings the first story is used as garage, office space, or usually for commercial purposes. As a result, a typical first story has: (1) taller columns with resulting lower lateral stiffness; (2) much less infill walls between the columns leading to lower lateral strength and stiffness compared to upper stories; and (3) rectangular columns with their weak axis mostly perpendicular to the street, i.e., weaker columns, thus weaker story in one direction [10]. With less stiff, taller or weaker columns and less infill walls, the first story experiences larger lateral drifts. Soft and weak stories have been one of the most common reasons for damage or collapse of multi-story reinforced concrete buildings in Turkey [10,11,12] and in other parts of the world [13,14]. In the building shown in Figure 8, first story columns were severely damaged which led to collapse while the upper stories suffered virtually no damage. Even the windows are not broken in the upper stories because the deformation demand in the upper stories was limited.

Existing buildings with a soft or weak story can be strenghened by increasing the stiffness and strength of the soft or weak story. The TEC [9] recommends addition of structural walls as a practical retrofit method. This strengthening is currently mandatory for existing buildings in seismic zones 1 and 2, which are the two seismic zones with the highest earthquake risk with corrresponding ground acceleration a_g ≥ 0.4g and 0.3g, respectively. These buildings typically consist of reinforced concrete flat slab, bare or infill joist system, and waffle slab system with columns and beams not satisfying the requirements for high ductility level.

Reinforced concrete frames need to be designed for both strength and ductility in order to prevent severe damage that can be caused during a strong earthquake. Strength is the maximum shear, axial or moment capacity of a structural member or the structural system to resist the earthquake loading, and ductility is related to maximum deformation capacity. In order to avoid collapse during an earthquake, columns need to be designed stronger than the beams in a reinforced concrete frame. This phenomenon is called strong column and weak beam concept. The beams are supposed to be ductile and controlled damage or plastic hinges are supposed to occur at end of beams rather than in columns. In order to dissipate sufficient energy within the plastic hinges, plastic hinge regions should have special reinforcement detailing to improve the energy absorption capacity and ductility. However, reinforced concrete frames with weaker columns suffer brittle shear and axial failure while the stronger beams remains elastic. This undesirable behavior is also seen in the 2011 Simav earthquake. Figure 9 shows column failures in a building with strong beams and weak columns.

Many reinforced concrete buildings were seriously damaged in past earthquakes due to presence of short columns [10,15]. Short columns can be classified into two groups. The first group consists of columns that are designed or constructed shorter than other columns in that story. The second group includes columns that are shortened during construction by adding partial walls or panels on both or either side of the columns although their specified height is the same as other columns in that story in the original design. Damage in these short columns is often in the form of X-shaped cracking due to brittle shear failure. Examples of short column failures from the 2011 Simav earthquake are shown in Figure 10.

Figure 9 and Figure 10 show the large transverse reinforcement spacing in the columns. Widely spaced and insufficient transverse steel leads to: (1) lower column shear strength; (2) less effective confinement of the core concrete and lower core concrete strength; and (3) buckling of the longitudinal bars under increased axial and lateral loads. The effects of inadequate transverse reinforcement are further discussed below.

Unconfined gable wall damage has been common in recent earthquakes in Turkey. Although the structural integrity of the building is not compromised due to the gable wall damage, such damage results in falling hazard that leads to financial loss, injuries and even fatalities [11]. An example of unconfined gable wall damaged in the Simav earthquake is shown in Figure 11. In order to limit the gable wall damage, the Turkish earthquake regulations [9] require vertical and inclined concrete beams within the wall if the roof gable wall is taller than 2.0 m (Figure 12).

Transverse reinforcement in concrete members primarily confines concrete core, holds the reinforcement cage during construction, and provides lateral or shear strength. Typically, columns, sometimes shear walls, provide the main lateral load resisting mechanism in reinforced concrete frame buildings in Turkey. Therefore, the shear strength and ductility of columns are crtitical in seismic performance of concrete buildings. The amount and detailing of column ties are directly related to the lateral strength and deformation capacity of the columns. For example, 90-degree hooks at the end of column ties do not effectively carry shear forces and cannot confine the concrete at large displacements, usually resulting in brittle shear failure at a low lateral displacement or ductility. This is mainly because poorly detailed hooks open up as the shear force applied on the column increases. Sezen and Moehle [16] provides evidence for shear strength degradation from experimental tests of full-scale column specimens with 90-degree hook ties.

In the TEC [9], columns are divided into one central zone (near column mid-height) and two confinement zones (at column ends) in terms of transverse reinforcement spacing and detailing requirements. According to Turkish earthquake regulations, transverse reinforcement spacing in confinement zones of columns at high ductility level cannot be larger than or equal to 100 mm and one third of smaller column dimension. In columns of normal ductility level, transverse reinforcement spacing cannot be more than one third of smaller column dimension, eight times longitudinal bar diameter, or 150 mm. Transverse reinforcement spacing in the middle of columns can be less than or equal to 200 mm, half the smaller column dimension, or twelve times longitudinal bar diameter. Thus, the minimum transverse reinforcement spacing is limited to 100 mm in the column confinement zone, and 150 mm in the middle zone. The observed transverse reinforcement spacing in buildings damaged during the Simav earthquake were much larger than these limits.

Beam-column joints are designed to transfer moments and resist lateral loads within the beam-column moment frame system. Transverse reinforcement needs to be provided within the joint region to resist shear forces and confine concrete [17]. Some buildings were damaged in Simav because of insufficient lateral reinforcement in the beam-column joints (Figure 13). No confinement or lateral reinforcement exist in most joints, making them unable to achieve the required ductility. Beam-column joints of high ductility frame systems are classified as confined or unconfined joints in the TEC [9]. A beam-column joint is defined as a confined joint if beams frame into all four sides of a column and the width of each beam is not less than three quarters of the adjoining column width. In practice, it is rather difficult to satisfy this requirement. All joints not satisfying the above given conditions are defined as unconfined joints.

TEC [9] requires 135-degree hooks at the end of seismic transverse reinforcement, because 90-degree hooks easily open and lose their effectiveness. As the labor or installation is difficult, transverse steel with 135-degree hooks has not been used until recently. However, with more government oversight and better supervision at the construction site, TEC [9] requirements appeared to be carefully applied in the field within the last five years.

In general, S220a reinforcing steel with a yield strength of 220 MPa and S420a rebar with a yield strength of 420 MPa are used in Turkey. Rebar diameter ranges from 8 mm to 32 mm. It was found that mostly S220a reinforcing steel bars were used in the buildings in the region affected by the 2011 Simav earthquake. According to TEC [9] minimum rebar diameter for slabs, joists, and columns is 8, 12, and 14 mm, respectively.

As discussed in the previous section, reinforcement detailing is critical in sesimic design of reinforced concrete structures because the ultimate strength and displacement ductility highly depend on the amount and detailing of the reinforcement. The authors observed severe damage and collapse of structures mainly due to poor reinforcement detailing (Figure 14). Most common problems included insufficient or lack of longitudinal bars, and insufficient development length, lap splices, or anchorage of rebar. These problems were exacerbated by corrosion of rebar and concrete damage and spalling. Although the corrosion was typically not severe in longitudinal bars in buildings damaged during the earthquake, some corroded rebar were spotted typically at the column-foundation interface.

If the gap between adjacent buildings is insufficient, pounding between closely spaced buildings with different dynamic characteristics can be a serious threat in seismically active areas. Structural periods of two adjacent structures cannot always be the same because periods of the buildings depend on variation of mass and structural system stiffness of the building. Dynamic properties of structures, properties of ground motion and gaps between closely spaced structures have influence on the location and magnitude of pounding. In addition, soil type can affect the structural response in terms of both fixity of the building foundations and periods of the structures. The simplest and most practical way to prevent this impact and reduce damage is to provide enough separation. Corner buildings are expected to experience more damage than interior buildings because of pounding, since corner buildings are free to move in many directions. If the required sufficient space or gap cannot be provided, an alternative solution is to decrease the lateral motion and drift of buildings. Pounding of closely spaced buildings is frequently observed during earthquakes in Turkey because it is very common to have very small separation or no gap between adjacent buildings. Two pounding examples resulting from inadequate gap between adjacent buildings are shown in Figure 15.

Low material quality is an important factor that affects the seismic response of buildings. Damage resulting from low quality material was observed in many earthquakes all around the world [18]. In Simav, most of the older structures were constructed with hand-made concrete. Ready-mixed concrete has been used only recently (especially after the 1999 Turkey earthquakes) in Simav and some rural regions of Turkey. As a result, it is not uncommon to find very low compressive concrete strength in some buildings. Low quality of concrete usually results from lack of sieve analysis and proper gradation of aggregate, lack of vibration after concrete pouring, insufficient curing of concrete after construction without attention to weather conditions. Tests of concrete samples taken from buildings by Pamukkale University in Simav showed a compressive strength of 6 to 12 MPa much lower than the minimum required strength of 18 MPa [19].

Unreinforced hollow clay bricks were used in infill walls of almost all reinforced concrete buildings in the region affected by the 2011 Simav earthquake. Unreinforced infill walls have relatively low lateral strength and usually brittle once their strength limit is reached. Diagonal cracks, a sign of shear failure, in infill walls were commonly observed in reinforced concrete buildings. Depending on the magnitude of the lateral load applied on the building, the level of observed wall damage varied from light diagonal cracks in the walls to separation from the structural frame or out-of-place buckling (Figure 16).

Simav was also hit by a magnitude 5 earthquake in 2009, causing damage in some reinforced concrete buildings. Most of these damaged buildings apparently were not repaired or strengthened, instead they were plastered and painted. The 2011 Simav earthquake caused further damage in those buildings usually at or near the areas damaged during the 2009 earthquake. Figure 17 shows one such building with damaged beams and infill walls plastered after the 2009 earthquake. Large cracks caused by the 2011 earthquake are visible in the second floor, e.g., near the right end of the wall in the second story.