THE REASONS AND RESULTS OF EARTHQUAKE DAMAGES IN REINFORCED CONCRETE BUILDINGS

Transcription

1 THE REASONS AND RESULTS OF EARTHQUAKE DAMAGES IN REINFORCED CONCRETE BUILDINGS Z. Canan Girgin 1 D. Güneş Yılmaz 2 70 % of population in Turkiye is situated in first and second degree earthquake zones. Great human fatalities and economic losses have occured due to high seismic activity for long years. Human and property losses are not limited with rural earthquakes, on the contrary losses concentrates in the urban earthquakes that the great majority of existing building stock consists of multi-storey reinforced concrete (RC) buildings (Erzincan 1939, Erzincan 1992, Kocaeli 1999 etc). This work mainly focuses on the most common defects made during the design, construction period and service life of RC buildings and their results after a moderate or severe earthquake. Earthquake damages can be classed in three categories: Design deficiencies (soft-storey, insufficient lateral stiffness, short column, strong beamweak column design, vertical or horizontal irregularities, etc.) Construction defects (bad workmanship, poor quality control, insufficient wrapping with stirrup, insufficient mechanical properties of material, insufficient adherence length etc.) Deficiencies in service life (cut-off or removed columns, soft story formation due to total or partially absence of masonry infill walls etc.) If we analyze these causes of damage in separate titles, the most important ones are: A. Soft story: The story having less rigidity due to reduced masonry infill walls is called as soft story. This soft story is the reason of a major weakness due to large retail spaces without masonry infill walls. The soft story is often in the ground level of a building. If non-ductile beam-column connections also exist the soft story sustain severe structural damage or even to total collapse in a strong ground motion. Figure 1: A soft-story retailed as auto-gallery 1 YTÜ, Faculty of Architecture, Structural Systems Dept. zcgirgin@yildiz.edu.tr 2 YTÜ, Faculty of Architecture, Master of Science Student. didgyil@yahoo.com.tr Figure 2: The collapse of soft-story

2 Figure 3: Visual model of transparent ribbed slabs Picture 1: Collapsed soft stories in earthquake [1] The second picture is a typical collapse mechanism B. Ribbed / Flat slabs : RC systems including ribbed slabs or flat slabs without stiffening by shear walls are susceptible to large lateral deflections and may suffer to heavy structural damages under strong earthquake excitations. Under extreme conditions the structural system may totally collapse due to resonance condition. Especially, in Turkiye, ribbed slabs are preferred due to moulding ease despite of severe earthquake risk. Figure 4: Great lateral deflections are anticipated due to seismic loads if the structure are unstiffened with shear walls Picture 2: A damaged flat slab example [1] Figure 5: This type of structure non-stiffened by shear walls may collapse

3 C. Confinement deficiencies: Structural system and its elements having non-ductile details not-well confined with transverse reinforcement (stirrup) sustain heavy brittle damage pattern under a strong ground motion and even to total collapse. Otherwise, in the structural systems and elements having ductile details well confined with stirrup, no or little damage occurs even in a strong ground motion. The main aim of this concept is to provide damage formation in beams prior to columns. Thus it is prevented brittle collapse under a strong ground excitation. Figure 5: Confinement deficiency in column and expected brittle damage in an earthquake Picture 3: Damage patterns of non-ductile and ductile columns in earthquake D. Vertical irregularity: Irregular structures including vertically changing column axes, cut-off or removed columns may suffer heavy damage under strong ground motions and even to total collapse. Figure 6: Effectively confined column and expected ductile behaviour during earthquake In addition, new structural systems must be designed according to the strong column-weak beam concept of Turkish Earthquake Code (2007). Picture 4: Examples to changing column axe and removed column [1]

4 Figure 7: Collapse may occur due to irregularity E. Planar irregularity: Planar irregularities can cause poor seismic performance. If centre of mass and centre of rigidity do not coincide, the distance between them may cause to excessive torsion and total collapse. Not coinciding beam-column axes lead to heavy damage even in a moderate earthquake as well. Figure 10: Damage pattern and possible collapse due to torsion as a result of planar irregularity Picture 5: [1] Figure 8: A regular plan coinciding the centre of mass and centre of rigidity Figure 9: Irregular plan not coinciding the centre of mass and centre of rigidity and irregular axes F. Quality control defects of material: If the following factors for concrete is under consideration o the ratio of water to cementitious material is high, o the curing conditions are insufficient, o the quantity of fine aggregate is low, o unwashed sea sand with shells is used, then these factors cause early deterioration (corrosion) of reinforcement and low durability of concrete.

5 Reinforcement must undergo large plastic deformation (ductile behaviour) before rupture during a very strong earthquake. Otherwise, if steel has low deformation capacity (brittle behaviour) before rupture the structure may not stand in a severe earthquake. This low-quality steel should never be used due to economic causes or quality control defects. Correct mix design of concrete and correct firm choice as well as the preference of a qualified firm in RC reinforcement production are very important in the earthquake resistant RC building construction. In addition an intensive quality control should be carried out during construction. As a final result, even in very strong earthquakes, a ductile performance is observed in the regular structures designed and built in accordance with regulations. Otherwise, the brittle collapse due to above mentioned deficiencies and defects is under consideration. Horizontal earthquake loading Ductile performance Brittle collapse Horizontal movement of top floor in building relative to ground Figure 11 Performance types of RC buildings during a strong earthquake References: (1) Topçu A., Structural System Irregularitiesphotos, Civil Engineering Faculty web page, Eskisehir Osmangazi University. (2) Turkish Earthquake Code Picture 6: Low-quality concrete and reinforcement steel should never be used in buildings subjected to a possible strong earthquake