DOES ARCHITECTURAL EXTERNAL STRENGTHENING OF REINFORCED CONCRETE STRUCTURES POSSIBLE?

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1 DOES ARCHITECTURAL EXTERNAL STRENGTHENING OF REINFORCED CONCRETE STRUCTURES POSSIBLE? Serra Zerrin Korkmaz, Selcuk University, Architecture Department, Konya-Turkey Abstract Countries like Turkey, Greece, Iran, India share the same unlucky destiny, earthquakes. Seismic actions cause extensive damage and life and property loss for developing countries. The major structural type in Turkey is reinforced concrete framed structures and if they don t have enough ductility and strength, they suffer from lateral accelerations. The existing structures in the urban parts must be seismically improved for the future earthquakes. Fast, feasible, economical and effective strengthening techniques are very important. In this study, external strengthening strategies of reinforced concrete (RC) structures are discussed. For civil engineers and architect in the subject of construction and design, the new and innovative visions are highlighted. Keywords Earthquake, strengthening, reinforced concrete, external applications. I. INTRODUCTION Turkey is situated in a very active earthquake of the world. In the last 30 years several earthquakes caused extensive structural damage and life loss. The economical loss of the country is also reached to huge amounts. In 1999, Marmara earthquake caused more than life loss. This was one of the most tragic event in the world during the last 20 years. In Table 1 major earthquakes in the country and number of loss of life s are listed. TABLE I MAJOR EARTHQUAKES IN TURKEY Time and Location Magnitude Loss of life 1939 Erzincan M= Erbaa M= Bolu M= Varto M= Adapazarı M= Çaldıran M= Erzurum-Kars M= Erzincan M= Marmara M= Torbalı M= Ayvalık-Dikili M= İzmir-Karaburun M= Söke-Aydın M= Alaşehir M= Gediz M= Çaldıran M= Bingöl M= Lice M= The most common structural type in Turkey is reinforced concrete (RC) framed structures. In relation to the country s economic and technological conditions, ready mix concrete was not common before 1990 s and workers and governmental officers didn t have enough consciousness about the reinforcement and structural details and seismic resistance. Due to the poor construction quality control system in the country, old RC structures are far away from possessing satisfactory seismic performance. The civil engineers, architects, government s formal officers, city municipalities and even the prime minister of the country couldn t realize the earthquake risk and precautions to be obeyed. City planners didn t realize the seismic risks and they just allow to be urbanized in the areas of high risk or unsuitable site conditions. Officers or governors of the city municipalities couldn t understand the facts and they just increase the number of storeys in seismically active 228

2 regions. Ministers and prime ministers couldn t have enough visualization about the possible loss of life in case of a strong earthquake and they allow to unauthorized persons (neither civil engineer, nor architect) to construct residential houses. Government just allowed to everyone to construct huge RC structures in the seismic zones and thousands of families were settled in them. The only target of the past managements was to construct fast and economical houses for the incoming rapid population increase in the urban part. Engineers also couldn t realize the importance of their duty. Due to the economical competitions between the design officers to get more design project and unethical discount they applied resulted in faster project productions. The less time for a design process resulted in less selective design decisions involved. Also the design civil engineers couldn t realize the lateral rigidity and importance of shear walls and vertical elements orientation. One way columns were frequently involved in the structural systems. The lack of shear walls is the common problem in old (20 years or more) structures. The importance of steel detailing couldn t be explained in the universities and site engineers couldn t force the workers to obey stirrup spacing in confinement zones. Nearly in none of the sites, the stirrup hooks were bent 135 degrees and it was supposed to be 90 degree. The duty and importance of stirrups were not known. There was no control mechanism in construction process in the sites. The formal responsible staff generally didn t know the place or address of the site and they just signed at the desk to earn money. The constructed structural system and the one in the design sheets could alter frequently. There was no governmental response for these type incompatibilities. Since there was no consciousness in civil engineers in the subject of seismic capacity, it was unrealistic for architects to have earthquake knowledge. The strength calculations, structural configurations issued were supposed to be out of their subject. Architect still don t have enough knowledge about the relation of seismic behavior and their design decisions. II. CONDITION OF EXISTING STRUCTURE The past habits in construction industry in Turkey resulted in building of huge amounts of RC structures to satisfy societies shelter demand. But these structures also share the same property. They have low lateral rigidity, lateral displacement capacity, poor lateral displacement 229 capacity and ductility, and consequently low seismic performance. In the recent earthquakes, 1995 Dinar, 1998 Adana, 1999 Marmara, 2003 Bingöl and finally 2011 Van, the same scene was observed. RC structures failed or heavily damaged causing thousands of life loss. The construction practice oriented factors contributing to the collapses can be listed as; Poor construction practice Changing or reducing the member sizes from what is given in the official design drawings. Inferior material quality, poor concrete quality, and improper mix-design. Changes in structural system by adding/removing components. Poor steel detailing which do not comply with the design drawings. o Reducing quantity of steel from what is required and shown in the design o Use of smooth undeformed reinforcing bars (fyk=220 MPa) o Insufficient lap splice length o In the beam-column joint regions no transverse ties are present o Use of 180-degree hooks at the end of longitudinal bars o Use of bent-up longitudinal reinforcing bars in beams o Inadequate transverse reinforcement Wide spacing of the ties Insufficient lateral ties at the beam column joints 90 o degree hook in the lateral reinforcements instead of 135 o hooks Not following Turkish earthquake code provisions Poor architectural design solutions Soft first stories Irregularies in plan and elevation Large and heavy overhangs Short columns Unconfined gable walls Columns with broken axis and irregular column configurations Beams with have broken-axis Orientation of columns stronger in one direction only

3 Formation of framing systems which is much stiffer and stronger in the direction perpendicular to the street Use of joisted floor slab framing Improper structural design solutions Strong beam-weak columns Poor lateral rigidity of the buildings Inadequate cross-sectional dimensions of columns Inadequate gaps between adjacent buildings The eccentricities in column-to-beam connections Several damage and failure cases are illustrated in Fig. 1. Turkey is separated on five EQ zones where 1st and 2nd zones are the most dangerous ones. In 3rd and 4th zones moderate earthquakes have occurred. 95% of the Turkish geographical land is located on the considerably risky 1 st and 2 nd earthquake zones and most populated region of Turkey, Marmara and Aegean regions, falls in to 1 st earthquake zone. %80 of its population is imposed upon to the large scale earthquakes. Under these facts, majority of the old structures requires strengthening. The conventional, popular or classical strengthening techniques like construction of RC walls inside the structure and column jacketing are proved to be effective. On the other hand their application time is long and requires to evacuation of the building. This cost secondary economical loss due to loss of the function. If this structure is a residential building, then the house occupants must stay in different locations during the construction. This creates resistance to strengthening decisions. If the structure has economical function like hospitals, then the economic loss due to loss of customers are unfeasible. The third type of economic loss is the renovation of the internal finishings. The cost of the repair of pipes, networks, electric nets costs more than the expected. If the construction coincides with the kitchen, bath or places with ceramics and fixed boards, then the water, heating and other network erection and installations cost and removal and reconstruction of ceramics can take major part in the budged. These secondary cost creates resistance of the building owners to decide to strengthening. III. POSSIBLE ALTERNATIVE APPLICATIONS Fig. 1 Failure of framed structures after earthquakes in Turkey. 230 If a structure is seismically deficient, then, most probably, it is an older structure or its owner s economic condition is not so good to get a suitable supervision during the construction and design. In both cases, for old structures and structures whose owner s economic condition is poor, to get a strengthening decision is very difficult. If the structure is used for commercial purpose, the strengthening time is a main obstacle. The author came across several cases in which the owners or occupants reject the strengthening and prefer to live with

4 collapse probability. If there is no governmental enforcing to check the seismic performance of the structure and to get a technical report to present to the governmental offices or positions, then very low number of owners cares about their earthquake safety. Considering the number of structures to be strengthened and the economic conditions of the owners, new strengthening methods must be developed for the reluctant owners. These methods must be more economical and faster. At this point external strengthening strategies are seemed to be best solution. Every framed structure has an external frame along the periphery of the building. These external frames can be laterally strengthened with reinforced concrete (RC) infill walls, external shear walls, diagonal steel bracings etc. The RC infill walls can be constructed along the frame openings of the external frames. To do that, the available brick infills must be demolished. The most difficult task in demolishing the brick infills is the transfer of the brick dump from inside to outside. If the outer bricks are demolished, then the transfer of the dumb will be very easy. In this case also damage to the floor pavements and other neighbor finishings will be minimized and labor cost will be reduced. If the RC infills are constructed along the outer periphery of the structure, then, the foundation construction is also accelerated and it will be economically better. If the foundations are constructed inside the structure close to the center or core of the building, transfer of the soil dumb will be also difficult. During the digging operation, machines can not be used and man power will be needed. Construction of the foundation outside of the structure enables machine use and saves time and money. Also construction of forms and scaffolding at the outer of the structure is more easy then the inner operations. Pouring or settling of concrete inside the structure is more difficult than the outer operations. As stated before the outer addition of RC infills destroy or damage less finishings which takes a major economical replacement cost. As an alternative to the RC infills, external RC shear walls can be more economical and further easier. The difference between external shear wall and external RC 231 infill is that, the first one is constructed outside the borders of the structure just tangent to it. The shear walls are constructed tangent to the outer frame. The connection between the old frame and new shear wall is attained with anchor dowels exerted in to the beams, columns and slabs of the old frame and they are extended to the new shear wall. In this case there is no need to demolish the existing brick infill walls. This result in save o economy, time and labor work and causes less damage to secondary objects. The lateral resistance of the existing brick infills also makes contribution to the earthquake response. The total mass accumulated within the old structure and corresponding inertial lateral load must be transferred from the old frame to the new shear walls and then to the foundations. For that reason the connection between the old and new members are important. The dowels inserted to the old RC members and extended to the new RC shear walls can transfer the lateral load with shear action. The number of dowels must be high as much as possible. On the other hand they must be too close to each other which can weaken the old members. Beams and slabs as well as the columns can be used. To achieve a satisfactory bond, the old concrete quality must be high enough. This external shear wall strengthening can create problems and may not display expected benefit in case of low concrete quality. The frictional force transfer between two different concrete members can be handled as additional capacity and may not be taken to account. The external columns of the old frame can be enveloped with concrete jackets and these jackets can be extended to the new external shear walls via stirrups. In Fig. 2, an example of external shear wall applied at the 4 corners of the structure is shown. First foundation along the building was manufactured. Column periphery was emptied and external shear walls were cast. The compressive strength of the existing concrete was low. For this reason, the column jackets also used to connect old and new concrete and members. In Fig. 3, the application of RC external shear wall to the masonry structure is illustrated. That masonry building composed of solid (no cavity or hole) bricks. Although structure was located in the 1 st earthquake zone, it has 3 storeys and the overall shear capacities of

5 the walls were not enough to resist lateral earthquake loads. The structure was a dormitory and the owner asked to find an external solution. The common or usual strengthening method for masonry structures is application of mesh reinforcement and plaster. But the engineers decided to use external shear wall in the shape of L located at the four corners of the structure. New RC foundations for the shear walls were located outside of the structure and they connected to the stone foundations of the masonry walls. RC shear walls were also connected to the structure with steel dowels exerted to the slabs and external beams (located on the top of the walls). The structure has also additional RC beams located at the top of the windows and these beams were continuous along the building. Additional connections were drilled to these beams. Plaster at the surface of the walls were cleaned and brick units become visible. Masonry walls were used as one side of the form. As a result, also frictional contact between the masonry bricks and new concrete can be used to transfer lateral forces from old structure to the new shear walls. Since the shear walls were L shaped, under the lateral load, the slab displacements can be prevented through the out of plane bending action of the RC walls. This mechanism further can transfer lateral forces to the in-plane shear walls. Since the bricks were solid and strong, additional dowels were anchored to the bricks and they extended to the new RC shear walls. Fig. 2 Example of an external shear wall application. 232

6 IV. RESULTS Fig. 3 Example of an external shear wall application in masonry structure As an alternative to the RC external shear walls, external steel bracings can be also used. In Fig. 4, a sample structure located on a risky seismic zone is strengthened with external steel bracings. At the first look, these new members can be thought as architectural elements and they improved the esthetic view of the structure. The members were fxied to the existing RC columns, slabs and beams. At the corner they also attached to the perpendicular shear walls. Some columns were jacketed with U shaped steel plates. The above mentioned strengthening methods definitely can increase the lateral load carrying capacity of the structure and improve the lateral rigidity. The rigidity of the lateral load carrying system is a major property in terms of earthquake response. The design details of the strengthening application is very important. Dynamic characteristic of the structure is completely changed and the possible adverse effects must be taken into consideration. Another important point is workmanship and in-situ application. If the connections of the old and new members are not well maintained, than the expected benefit cannot be obtained. The other important point is related to the esthetical view of the final product. The owner or users must be well informed about the final appearance of the structure and user participation must be aimed during the design stages. V. REFERNCES [1] S. Z. Korkmaz, Observations on the Van Earthquake and Structural Failures, ASCE Journal of Performance of Constructed Facilities, vol. 29, pp. 1-25, [2] S. Altin, O. Anil and M. E. Kara, Strengthening of RC nonductile frames with RC infills: An experimental study, Cement and Concrete Composites, vol. 30, pp , [3] S. Altin, O. Anil, M. E. Kara and M Kaya, An experimental study on strengthening of masonry infilled RC frames using diagonal CFRP strips, Composites Part:B Engineering, vol. 39, pp , Fig. 4. External steel bracing application on a RC structure 233