플랫 플레이트 외부 접합부의 전단보강에 관한 연구

Abstract

플랫 플레이트 구조는 다른 구조형식에 비하여 매우 경제적인 구조형식이며 보가 없는 2방향 슬래브를 의미한다. 특히 층고절감, 평명의 가변성, 공사 기간 및 공사비 절약 등의 장점이 있다. 그러나 국내에서는 삼풍백화점의 붕괴에 따른 구조안정성 문제로 인해 지하주차장 등에 제한적으로 사용되어 왔으나 최근에는 플랫 플레이트 구조시스템이 주거용 고층 건축물에서 활발히 사용되고 있다. 보가 없는 구조형식으로 보-기둥 골조에 비하여 구조적인 취약점을 갖게 되는데 대표적인 것이 접합부에서의 뚫림전단파괴이다. 슬래브에서 기둥으로의 직접적인 하중전달은 기둥 주위에 큰 응력을 유발하여 접합부 전단파괴를 유발하며, 접합부의 파괴는 구조물 전체로 연쇄적인 붕괴를 유발하게 될 수도 있으므로 접합부에서의 뚫림전단파괴 메커니즘 규명과 전단내력의 산정을 위한 연구 및 전단보강방법 등에 관한 연구가 매우 활발히 진행되어 왔다. 일반적으로 뚫림전단파괴에 대한 저항성은 더 큰 크기의 기둥, 더 큰 유효 춤, 더 많은 휨보강근, 그리고 더 높은 콘크리트 압축강도의 사용 또는 추가적인 전단 보강근의 사용 등을 통하여 증가될 수 있다. 하지만, 큰 기둥의 사용은 보통 건축가들에 의해 선호되는 해결책이 아니며, 큰 슬래브의 유효춤 또한 고정하중과 기초 및 기둥의 비용을 증가시킨다. 또한, 휨보강근 비율은 증가와 콘크리트 강도의 증가는 기존 연구로부터 그다지 효과적이지 못하다는 사실이 밝혀졌으며 많은 경우에 실용적이지 못하다. 결국, 유효한 전단보강근의 공급이 이러한 상황 하에서 가장 경제적인 해결책이다. 그러나 플랫 플레이트 구조의 취약점인 뚫림전단파괴에 있어서는 횡하중이 작용하는 경우에는 접합부의 불균형모멘트 중 슬래브의 휨 저항으로 전달된 나머지가 위험단면에 전단응력으로 작용하여 중력하중과 함께 접합부의 전단응력으로 작용되므로 불균형모멘트의 전달과 중력하중+횡하중에 의한 전단응력의 크기를 적절히 규명할 필요가 있다. 그러나 현행 기준(ACI 318-05, KCI) 에서는 횡하중 작용에 대한 편심전단응력모델은 이론적인 탄성 플레이트 해석에 기초한 가정에 의해 정립되어 사용되고 있으므로, 철근비의 증가에 따른 슬래브-기둥 접합부에 대한 검토가 필요하다. 이에 본 연구에서는 전단보강에 따른 기둥-슬래브 접합부에 대하여 횡력이 작용하는 조건에 대하여 실험적 연구를 수행하여 기존의 뚫림전단 보강기법보다 기둥-슬래브 접합부의 가장 취약한 취성적거동을 완화시킴과 동시에 뚫림전단 저항력도 증가시키는 새로운 보강상세의 전단성능평가를 하고자 한다. 또한, 횡하중 조건으로 수행된 기존 실험자료에 기초하여 유효폭내 철근비가 고려된 불균형모멘트 전달계수식을 수정함으로써, 전단보강된 슬래브-기둥 접합부의 강도예측을 위한 자료를 제공하는 것이 본 연구의 목적이다. 이러한 목적을 위하여 본 논문은 총 5장으로 구성되었으며, 제 1장 서론, 제 2장 설계기준 및 선행연구, 제 3장 슬래브-기둥 외부 접합부의 횡하중 실험, 제 4장 설계기준식의 분석, 제 5장 결론으로 구성 되었으며, 본 연구의 결과로 얻은 결론은 다음과 같다. 1) 총 3개의 슬래브-기둥 외부 접합부에 대한 실험결과, 전단보강체의 형상에 따라 발현되는 횡저항성능이 서로 다르게 분포하는 것으로 나타났으며, 강도 및 변형능력은 기준 실험체에 비해 23%, 13% 증가되는 것으로 나타났다. 2) 동일 층간변위비 (drift = 2.75%)에서 강성감소율은 기준 실험체에 비해 FP-CS/bar, FP-SS/Bar는 10%, 54%정도 낮게 나타났다. 3) 동일한 층간변위비에서 무보강 기준 실험체는 철근의 변형이 기둥 중심부에 집중된 반면, 곡선형 철근을 사용하여 보강한 두 실험체(FP-CS/Bar, FP-SS/Bar)의 철근 변형률은 슬래브 전폭에 걸쳐 고르게 분포함으로써 슬래브-기둥 접합부에서 모멘트 소성 재분배에 의하여 슬래브 길이 방향으로 분산시켜 접합부 성능 향상에 기여하는 것으로 나타났다. 4) 무보강 실험체의 전단강도는 현행기준(ACI 318-05, KCI)이 비교적 정확하게 예측하고 있으나 보강접합부의 펀칭전단강도는 상대적으로 작게 나타났다. 이는 동일한 성능으로 설계한 보강접합부의 펀칭전단강도는 보강체의 형상적 특성으로 인하여 경사철근의 전단 균열과 만나는 각도 및 위치에 따라 불균형모멘트에 대한 저항성능이 다르기 때문으로 사료된다. 5) 기존 접합부 실험의 불균형모멘트 분석결과 전체적으로 무보강 실험체는 비교적 정확하게 예측하였으나 보강체의 경우 실험값을 과대평가하는 것으로 나타났다. 6) 제안된 불균형모멘트 전달계수에 대하여 기존 수행된 121개 실험체를 대상으로 수정 제안한 불균형모멘트 전달계수는 접합부 강도예측에 있어서 그 효용성이 있는 것으로 나타났다.; The purpose of this research was to study the response of slab-column connections containing various types of shear reinforcement when subjected to the combination of gravitational and lateral cyclic loads. The three test specimens were full-scale representations of exterior slab-column connections of a prototype apartment building in Korea. The control specimen had no shear reinforcements, while the other specimens had CS-Bar and SS-Bar as shear reinforcements. The control specimen failed due to the punching shear around the slab-column connection at 4.0% drift. None of the specimens with shear reinforcement experienced punching shear failure up to 4.4% drift. The two types of slab shear reinforcements proved to be equally effective in resisting punching shear failure of these connections subjected to relatively low levels of gravity load. The presence of shear reinforcements significantly increased the lateral load ductility of the connections. The test results showed that the strength and ductility of the specimens with SS-Bar and CS-Bar were improved by 23% and 15% compared to the specimen without shear reinforcements. Introduction Flat plate structure which is 2-way slab system without beams is more economical structure than other structures. Specifically this structural system has benefits of reducing the story height, using the space in various ways, and saving the time and money for construction. However, it is weaker structural system than beam-column frame. The main problem is punching shear failure. Little study was conducted for exterior slab-column connections compared to their interior counterparts. This exterior connection usually has short critical section area, and tends to have unbalanced moment caused by their asymmetric geometry and eccentric shear occurred by both gravitational and lateral loads. Main objective for this study is to evaluate the earthquake resistance capability of exterior slab-column connections with aspect ratio of 2.5. Main objectives are represented below - Evaluation of structural performance of the exterior slab-column connection without shear reinforcement. - Reinforcing effects on different shear reinforcing methods. Experimental Program Test Specimens. A series of three specimens was tested in this study. Two specimens were specially reinforced for shear, while the other one contained no shear reinforcements. As shown in Fig. 1 test specimens were produced based on exterior connections of a flat plate residential structure built in Korea. The slab specimens were constructed in full scale and had the size of 4640mm×2820mm. Although column height was originally designed to have 2.7m, it was reduced to 1920mm because of the limitation of the testing facilities. The specimens were designed to satisfy KBC 05 code and they were designed as a dual system which has a combination of flat plate and moment resisting frame. 25% of the loads were resisted by flat plate system while rest of the loads was resisted by moment resisting frame system. All re-bars were D13. Upper re-bars were placed in column strip of the slab and lower re-bars were distributed uniformly in all slab width. In order to satisfy structural integrity in the standards, two lower re-bars were placed through the column. Special Shear Reinforcement. Fig. 2 (a) and (b) shows specially designed shear reinforcements. Curvilinear shear reinforcement is made up of curved upper anchorage bent bars which will be placed on the bottom of the top longitudinal reinforcements, middle shear reinforcements which are welded to upper and lower anchorage bent bars, and lower anchorage bent bars which have the same shape of upper bent bars. Upper and lower bent bars work as anchorages for slabs and middle shear reinforcements resist shear. These bent bars maintain a uniform distance. This distance between them is determined based on the thickness of the slabs and the space for flexural reinforcements. ◁그림 삽입▷(원문을 참조하세요) Material Test and Test Setup. Test results of the compressive strength for concrete and the tensile strength for re-bar is shown in Tables 1 and 2. Test setup is illustrated in Fig. 1. Test specimens were installed with a hydraulic jack in lower part of the column, and a 2000kN actuator for lateral loads in upper part of the column. Uniform gravitational loads on the slabs were represented using load blocks. These loads were controlled using a hydraulic jack which was installed edge of the slab. According to the past researches (Hawkins et al.), shear-moment ratio at the column surface affects the behaviors of slab-column connections significantly. In this test, load blocks were adjusted to simulate the shear-moment ratio of the original building. Lateral loads were applied through 2000kN actuator using displacement control. Displacements were represented in rotational angle which is lateral displacement of top of the column over length of the column. 13 steps (0.2%, 0.25%, 0.35%, 0.5%, 0.75%, 1%, 1.4%, 1.75%, 2.2%, 2.75%, 3.5%, 4.5%, 6%) of displacement were applied and each phase had 3 cycles. Data were gathered by load cells installed to the actuator, slab supports, and hydraulic jack, wire strain gauges attached to the reinforcements, and LVDTs installed under the slab. ◁표 삽입▷(원문을 참조하세요) Experimental Results Crack and Failure. Crack patterns of each specimen are illustrated in Fig. 3. All specimens were failed in flexural-shear and showed similar patterns except crack distribution. Based on the observation of all specimens, initial cracks on the slabs were flexural cracks developed due to the gravitational loads. As lateral displacements applied, additional flexural cracks were generated orthogonal to the loading direction throughout the slab. By applying more lateral displacements, tensile cracks were developed in lower part of the slabs, and diagonal torsion cracks were developed. Finally, the specimens were failed due to punching shear. As a result of the analysis using the standard strength equation, the failure of the specimens was governed by shear and flexure. The control specimen which had no shear reinforcements failed due to development of a small amount of concentrated deep and wide cracks, while two specimens which had special shear reinforcements failed with widely distributed shallow and narrow cracks. It was believed that the special reinforcements provided additional shear resistance by distributing shear stresses which develop diagonal cracks. ◁그림 삽입▷(원문을 참조하세요) Stress-Strain Relationship. Fig. 4 shows stress-strain relationship of the specimens from the test. All specimens showed story drift larger than 4.4%. It is an adequate capacity for seismic design limitation, since it is larger than limit drift ratio of 1.5%. Based on the strength measured from each specimen, strength of CS-Bar and SS-Bar specimens increased 24% and 27% compared to the control specimen. The specially reinforced specimens showed 5% story drift while the control specimen showed 4.4% story drift. The specially reinforced specimens had 13% increase of drift capacity. Table 3 shows details of the specimens and test results. In order to ensure lateral load resisting capacity of the shear reinforcements in slab-column connections, continuous placement of the shear reinforcements with top and bottom longitudinal reinforcements is important. The special reinforcements improved strength and ductility of the system efficiently by satisfying this placement condition. ◁표 삽입▷(원문을 참조하세요) ◁그림 삽입▷(원문을 참조하세요) Shear strength. Shear strength evaluation of specimens are executed based on eccentric shear stress model described in ACI 318-05. According to this model shear stress is decided from two stresses. One is direct shear stress which is applied to centroid of critical section of slab and the other is developed by unbalanced moment. Table 4 represents the shear strength of the result for this study and conventional studies. ACI strength evaluation is well predicted the control specimen shear strength. However, prediction for special reinforcing specimens’ shear strength is relatively small than test results. It is believed that different geometric characteristics of special reinforcement show different resistance capacity for diagonal crack and shear failure caused by unbalanced moment. Relationship between Gravity-Shear Ratio & Story Drift. ACI318-05 ensures drift capacity of 2-way slab connections by limiting drift for 0.035-0.05(Vg/Vc) ≥ 0.005. Fig. 5 shows influence of gravity to shear ratio (GSR). Most existing specimens satisfied gravity to shear ratio limit of 0.4 and story drift limit of 1.5%, while the specially reinforced specimens showed better performance than most specimens. Current design specification only considers gravity to shear ratio but it needs to consider other variables such as column aspect ratio and reinforcement ratio to distinguish capacity of these connections. ◁표 삽입▷(원문을 참조하세요) ◁그림 삽입▷(원문을 참조하세요) Conclusion The objective of this study is to evaluate the special reinforcing methods for punching shear failure of flat plate structure. ■ According to the test results, the lateral resistant capacity for each specimen was different. Strength and drift of the specially reinforced specimens showed 23% and 12% improvement over those of the control specimen. ■ The specially reinforced specimens showed uniformly distributed steel strain while the control specimen showed concentrated steel strain around the column. ■ All specimens satisfied limit of gravity to shear ratio of ACI, and the specially reinforced specimens outperformed existing slab-column connections with shear reinforcements. ■ Based on results, performance of flat plate structure using special reinforcements was sufficiently ensured.