최소철근량 이하로 보강된 강섬유보강 보의 휨성능 고찰

강덕만,박용걸,문도영 한국구조물진단유지관리공학회 2017 한국구조물진단유지관리공학회 논문집 Vol.21 No.3

본 연구에서는 국내 콘크리트구조기준(2012)에서 규정하고 있는 최소철근량 이하로 보강된 보에 강섬유를 혼입한 강섬유보강철 근콘크리트보의 휨파괴 실험을 수행하였다. 실험변수는 철근비와 강섬유의 혼입량으로 하였다. 철근보강비는 최소철근량의 44%, 66%, 78% 와 100%로 하였으며, 강섬유의 혼입량은 0.25%, 0.50%, 0.75% 및 1.00%이다. 실험결과, 강섬유는 균열저항성능을 크게 개선시키는 것으로 확 인되었다. 또한, 하중저항성능의 관점에서 강섬유는 항복하중의 증가에 기여하지만 극한하중의 증가에는 거의 기여하지 못하는 것을 확인하 였다. 강섬유로 인한 항복하중의 증가량은 철근 감소로 인한 항복하중의 감소량에 비하여 미미한 것으로 나타났다. 최소철근보에서 강섬유의 사용은 오히려 연성을 크게 감소시키는 것으로 확인되었다. 따라서 최소철근 휨부재에 강섬유를 사용하기 위해서는 연성도 확보를 위하여 철 근비를 증가시켜야 하는 것으로 확인되었다. In this study, steel fiber-reinforced concrete beams with ordinary steel reinforcements, that are below minimum steel reinforcement amount specified in domestic concrete structure design code, were tested in flexure until failure. Steel reinforcement ratio considered were 44%, 66%, 78% and 100% of the minimum steel reinforcement. Considered steel fiber volume fractions were 0.25%, 0.50%, 0.75% and 1.00%. In results, it is confirmed that steel fibers greatly improve crack performance. Also, the steel fibers contributed to increment in yield load not in ultimate load. But the increment was not greater than the reduction by steel reinforcement reduction. The use of steel fibers in RC beams lightly reinforced below the minimum reinforcement ratio specified design code reduced ductility greatly. Consequently, steel reinforcement ratio in steel fiber-reinforced beams lightly reinforced below the minimum steel reinforcement should be increased in order to enhance proper ductility.

Use of steel fibers as transverse reinforcement in diagonally reinforced coupling beams with normal- and high-strength concrete

Jang, Seok-Joon,Jeong, Gwon-Young,Yun, Hyun-Do Elsevier 2018 Construction & building materials Vol.187 No.-

<P><B>Abstract</B></P> <P>This study investigates the effects of using steel fibers as transverse reinforcement on the seismic performance of diagonally reinforced coupling beams that are composed of normal- and high-strength concrete. Four types of coupling beams were fabricated and tested under quasi-static reversed cyclic loading. A normal-strength reinforced concrete (RC) coupling beam with the compressive strength of 40 MPa was designed with the full confinement of a beam section according to ACI 318-14. A second RC coupling beam specimen with the same reinforcement details and concrete with the compressive strength of 80 MPa was fabricated also to evaluate the effects of strength. In addition, steel fiber was used in an attempt to simplify the complex details of transverse reinforcements for diagonally RC coupling beams. To this end, two fiber volume fractions of 1.0% and 1.25% were used for 40 MPa and 80 MPa steel fiber-reinforced concrete (SFRC) coupling beams, respectively. Test results indicated that the shear strength of the coupling beams increased with an increase in compressive strength, whereas the energy dissipation capacity was similar for the normal- and high-strength concrete RC coupling beams. Furthermore, this study found that the inclusion of steel fiber prevented buckling of the diagonal steel rebar as well as provided additional transverse reinforcement. The overall performance, including strength, stiffness degradation, and the energy dissipation capacity, of the SFRC coupling beams was similar to that of the conventional concrete coupling beams. Comparisons of the conventional concrete and SFRC beam specimens indicate that the use of steel fiber allows for the simplification of transverse reinforcement construction details for diagonally reinforced coupling beams.</P> <P><B>Highlights</B></P> <P> <UL> <LI> The inclusion of steel fibers improves flexural behavior and toughness of concrete. </LI> <LI> SFRC significantly contributed to prevent the buckling of the diagonal reinforcement. </LI> <LI> Using fibers as transverse reinforcement improves constructability of coupling beam. </LI> <LI> Shear strength is well predicted considering contribution of diagonal concrete strut. </LI> </UL> </P>

지지부재로 이형철근을 설치하는 띠형 강보강재의 인발성능 평가

정성규(Sung-gyu Jung),김주형(Juhyong Kim),조삼덕(Samdeok Cho),이광우(Kwangwu Lee) 한국지반신소재학회 2013 한국지반신소재학회 논문집 Vol.12 No.4

본 연구에서는 지지부재가 설치된 띠형 강보강재에 대한 실내인발시험을 수행하였다. 주문진 표준사를 사용하여 상대밀도 80%인 모형지반을 조성하였고, 지지부재의 개수를 0∼2개로 구분하여 실내인발시험을 수행하였다. 상재압은 50kN/m²∼200kN/m²까지 4단계로 구분하여 적용하였고, 1mm/min 속도로 강보강재를 인발하였다. 표면이 매끄러운 띠형 강보강재의 인발저항력은 보강재 표면과 지반 사이에서 마찰저항만 발현되기 때문에 인발 초기에 급격히 증가하다가 지속적으로 감소하는 경향을 나타낸다. 반면, 지지부재를 설치한 강보강재의 인발저항력은 마찰저항뿐만 아니라 수동저항도 함께 발현되므로 계속적으로 증가하는 것으로 나타났다. 보강재의 형태에 관계없이 최대인발저항은 상재압이 증가함에 따라 선형적으로 증가하는 것으로 나타났다. 지지부재를 1개 설치한 경우에 비해 지지부재를 2개 설치했을 때의 최대수동저항은 작게 나타났다. 이는 지지부재의 설치 간격 및 위치에 따라 지지부재에서 발현되는 수동저항의 크기가 다르기 때문에 나타나는 현상으로 판단되며, 지지부재 설치 위치 및 간격에 따른 추가 인발시험을 통해 확인할 필요가 있다. Laboratory pullout tests were conducted to evaluate pullout performance of steel strip reinforcements with deformed steel bars as transverse members. The steel strip reinforcement has an installation hole to assemble a deformed steel bar. Jumunjin standard sand is used to form a relative density of ground model to 80%. Frictional resistance of steel strip reinforcement without transverse member increases sharply at the initial displacement and quickly decreases with displacement. Maximum frictional resistance increases linearly as normal pressure increasing, and soil-reinforcement interaction friction angle(P_peak) of a steel strip reinforcement is estimated to 14.64°. Passive resistance increases with displacement and converge into maximum passive resistance in most cases. Maximum passive resistance increases linearly as normal pressure increasing irrespective of shape of the steel reinforcement. Pullout force of steel strip reinforcements with installation holes or transverse members largely increases about 4 to 7 times compared to frictional resistance force of steel strip reinforcements when embedment length(L_e) of steel strip reinforcements is 500 mm. In the case of using 2 transverse members, interference effect is observed due to the spacing of 2 transverse members and location of assembly holes and transverse members.

Study on Mechanical Properties of Press-Bending Members Based on Transmission Towers Diagonal Member Reinforcement In-Situ

Zhenke Xin,Huanhuan Wei,Yunhe Liu,Gang Liang 한국강구조학회 2022 International Journal of Steel Structures Vol.22 No.4

According to the reinforcement requirements of load-bearing members of existing transmission tower, a new type of in-situ reinforcement scheme for the angle steel T-shaped composite section is proposed. Through the compression test of 10 groups of members, the infl uence of slenderness ratio, the number of fi xtures, the number of bolt rows and other parameters on the failure mode and bearing performance of members was studied. A simulation model was established using ABAQUS software, the load-displacement curve, failure models and other aspects verify the reliability and validity of simulation results. The results show that when the slenderness ratio of member reaches a certain value, the ultimate bearing capacity of member reinforced with the reinforcement angle steel of same specifi cation and strength can be multiplied and increased several times, the slenderness ratio has little eff ect on the force transmission effi ciency of reinforcement angle steel. The number of bolt rows has a small eff ect on the ultimate bearing capacity of member, the increase rate of ultimate bearing capacity, the failure mode and the force transmission effi ciency of reinforcement angle steel, the infl uence of strength of reinforcement angle steel, the specifi cation of reinforcement angle steel on the ultimate bearing capacity, and the increase rate of ultimate bearing capacity of member obviously, the higher strength and specifi cation of reinforcement, the greater the ultimate bearing capacity of reinforced member, and the higher the rate of improvement of ultimate bearing capacity, the smaller the thickness of fi ller plate, that is the smaller the eccentricity, the higher force transmission effi ciency of reinforcement. The research results can provide experimental and theoretical basis for the reinforcement of tower structure and engineering design.

Feasibility of replacing minimum shear reinforcement with steel fibers for sustainable high-strength concrete beams

Yoo, Doo-Yeol,Yuan, Tianfeng,Yang, Jun-Mo,Yoon, Young-Soo Elsevier 2017 ENGINEERING STRUCTURES Vol.147 No.-

<P><B>Abstract</B></P> <P>This study aims to investigate the feasibility of eliminating the minimum shear reinforcement in reinforced sustainable high-strength concrete (HSC) beams by incorporating 0.75% (by volume) of hooked steel fibers. To do this, five large reinforced HSC beams, with and without stirrups and steel fibers, were fabricated and tested. In order to have minimum shear reinforcement, the reinforced HSC beams were designed with longitudinal steel bar ratios of 0.64–0.72%. Test results indicate that the use of 0.75vol% of steel fibers (instead of stirrups) leads to higher flexural strength but lower ultimate deflection and ductility. The failure mode of lightly-reinforced HSC beams was transformed from concrete crushing to longitudinal steel bar rupture by including the steel fibers. However, both the reinforced HSC and steel-fiber-reinforced concrete (SFRC) beams exhibited flexural failure modes; as a result, it was concluded that the minimum shear reinforcement for reinforced HSC beams can be efficiently eliminated by including 0.75vol% of hooked steel fibers. In addition, the flexural behavior of reinforced SFRC beams was successfully simulated based on sectional analysis by considering fiber orientation factor.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Minimum shear reinforcement in HSC beam can be eliminated by adding 0.75% hooked steel fibers. </LI> <LI> Adding steel fibers increases flexural strength but decreases ductility of HSC beams. </LI> <LI> Reinforced SFRC beam without stirrup is failed by premature rupture of steel rebar. </LI> <LI> The use of ultimate ductility index is appropriate for reinforced SFRC beam. </LI> <LI> An appropriate equation of fiber orientation factor for high-strength SFRC is proposed. </LI> </UL> </P>

초근접 병설터널의 필라 안정성 확보

김동규,고성일,이정용,이철희 사단법인 한국터널지하공간학회 2022 한국터널지하공간학회논문집 Vol.24 No.6

The objective of this study is to suggest a safe and economical pillar reinforcement method when very near-twin tunnels with a minimum interval of 1 m passes through a soft zone such as weathered soil or weathered rock. A standard cross-sectional view of a two-lane road tunnel was applied to suggest a pillar reinforcement method for the very near-twin tunnels. The thickness of the pillar was 1 m. The ground condition around the tunnel was weathered soil or weathered rock. There were four reinforcement methods for pillar stability evaluation. These were rock bolt reinforcement, pre-stressed steel strand reinforcement, horizontal steel pipe grouting reinforcement, horizontal steel pipe grouting + prestressed steel strand reinforcement. When the ground condition was weathered soil, only the pillar reinforced the horizontal steel pipe grouting + prestressed steel strand did not failed. When the ground condition was weathered rock, there were no failure of the pillar reinforced the horizontal steel pipe grouting or the horizontal steel pipe grouting + prestressed steel strand. It is considered that the horizontal steel pipe grouting reinforcement played a role in increasing the stability of the upper part of the pillar by supporting the upper load applied to the upper part of the pillar. 본 연구의 목적은 2개 터널의 간격이 최소 1 m이내인 초근접 병설터널이 풍화토 또는 풍화암같은 연약대를 통과하는 경우에 대해 안전하며 경제적인 필라보강방법을 제시하는데 있다. 초근접 병설터널의 필라부 보강방법 제시를 위하여 2차로 도로터널 표준단 면도를 적용하였다. 필라부의 두께는 1 m로 가정하였다. 터널 통과 주변 지반 조건으로 풍화토 또는 풍화암으로 가정하였다. 필라부 안정성 평가를 위하여 4가지 보강 방법, 록볼트 보강, pre-stress 강연선 보강, 필라부 상부 수평강관 보강 + 그라우팅 보강, 수평 강관 보강 + 그라우팅 + pre-stress 강연선 보강 조건에 대하여 검토하였다. 터널 주변 지반조건이 풍화토인 경우 수평강관 보강 + 그라우팅 + pre-stress 강연선 보강 조건만 필라부에서 파괴가 발생하지 않았다. 터널 주변 지반조건이 풍화암인 경우 수평강관 보강 + 그라우팅 조건과 수평강관 보강 + 그라우팅 + pre-stress 강연선 보강 조건인 경우에서만 필라부에서 파괴가 발생하지 않았다. 수평강관 보강 + 그라우팅은 필라부 상부에 가해지는 상부하중을 지지하여 필라부 상부의 안정성을 증가시키는 역할을 수행한 것으로 판단된다.

Axial behavior of the steel reinforced lightweight aggregate concrete (SRLAC) short columns

Mostafa M. A. Mostafa,Tao Wu,Xi Liu,Bo Fu 국제구조공학회 2021 Steel and Composite Structures, An International J Vol.39 No.5

The composite steel reinforced concrete (SRC) columns have been widely used in Structural Engineering due to their good performances. Many studies have been done on the SRC columns' performances, but they focused on the ordinary types with conventional configurations and materials. In this study, nine new types of steel reinforced lightweight aggregate concrete (SRLAC) short columns with cross-shaped (+shaped and X-shaped) steel section were tested under monotonically axial compressive load; the studied parameters included steel section ratio, steel section configuration, ties spacing, lightweight aggregate concrete (LWAC) strength, and longitudinal bars ratio. From the results, it could be found that the specimens with larger ties ratio, concrete strength, longitudinal bars ratio, and steel section ratio achieved great strength and stiffness due to the excellent interaction between the concrete and steel. The well-confined concrete core could strengthen the steel section. The ductility and toughness of the specimens were influenced by the LWAC strength, steel section ratio, and longitudinal bars ratio; in addition, larger ties ratio with smaller LWAC strength led to better ductility and toughness. The load transfer between concrete and steel section largely depends on the LWAC strength, and the ultimate strength of the new types of SRLAC short columns could be approximately predicted, referring to the codes’ formulas of ordinary types of steel reinforced concrete (SRC) columns. Among the used codes, the BS-5400-05 led to the most conservative results.

Retrofitted built-up steel angle members for enhancing bearing capacity of latticed towers: Experiment

Jian-Tao Wang,Xiao-Hong Wu,Bin Yang,Qing Sun 국제구조공학회 2021 Steel and Composite Structures, An International J Vol.41 No.5

Many existing transmission or communication towers designed several decades ago have undergone nonreversible performance degradation, making it hardly meet the additional requirements from upgrades in wind load design codes and extra services of electricity and communication. Therefore, a new-type non-destructive reinforcement method was proposed to reduce the on-site operation of drilling and welding for improving the quality and efficiency of reinforcement. Six built-up steel angle members were tested under compression to examine the reinforcement performance. Subsequently, the cyclic loading test was conducted on a pair of steel angle tower sub-structures to investigate the reinforcement effect, and a simplified prediction method was finally established for calculating the buckling bearing capacity of those new-type retrofitted built-up steel angles. The results indicates that: no apparent difference exists in the initial stiffness for the built-up specimens compared to the unreinforced steel angles; retrofitting the steel angles by single-bolt clamps can guarantee a relatively reasonable reinforcement effect and is suggested for the reduced additional weight and higher construction efficiency; for the substructure test, the latticed substructure retrofitted by the proposed reinforcement method significantly improves the lateral stiffness, the non-deformability and energy dissipation capacity; moreover, an apparent pinching behavior exists in the hysteretic loops, and there is no obvious yield plateau in the skeleton curves; finally, the accuracy validation result indicates that the proposed theoretical model achieves a reasonable agreement with the test results. Accordingly, this study can provide valuable references for the design and application of the non-destructive upgrading project of steel angle towers.

Investigations of different steel layouts on the seismic behavior of transition steel-concrete composite connections

Qi, Liangjie,Xue, Jianyang,Zhai, Lei Techno-Press 2019 Advances in concrete construction Vol.8 No.3

This article presents a comparative study of the effect of steel layouts on the seismic behavior of transition steel-concrete composite connections, both experimental and analytical investigations of concrete filled steel tube-reinforced concrete (CFST-RC) and steel reinforecd concrete-reinforced concrete (SRC-RC) structures were conducted. The steel-concrete composite connections were subjected to combined constant axial load and lateral cyclic displacements. Tests were carried out on four full-scale connections extracted from a real project engineering with different levels of axial force. The effect of steel layouts on the mechanical behavior of the transition connections was evaluated by failure modes, hysteretic behavior, backbone curves, displacement ductility, energy dissipation capacity and stiffness degradation. Test results showed that different steel layouts led to significantly different failure modes. For CFST-RC transition specimens, the circular cracks of the concrete at the RC column base was followed by steel yielding at the bottom of the CFST column. While uncoordinated deformation could be observed between SRC and RC columns in SRC-RC transition specimens, the crushing and peeling damage of unconfined concrete at the SRC column base was more serious. The existences of I-shape steel and steel tube avoided the pinching phenomenon on the hysteresis curve, which was different from the hysteresis curve of the general reinforced concrete column. The hysteresis loops were spindle-shaped, indicating excellent seismic performance for these transition composite connections. The average values of equivalent viscous damping coefficients of the four specimens are 0.123, 0.186 and 0.304 corresponding to the yielding point, peak point and ultimate point, respectively. Those values demonstrate that the transition steel-concrete composite connections have great energy dissipating capacity. Based on the experimental research, a high-fidelity ABAQUS model was established to further study the influence of concrete strength, steel grade and longitudinal reinforcement ratio on the mechanical behavior of transition composite connections.

Experimental and numerical study on fire resistance of tubed steel-reinforced concrete stub columns under eccentric compression

Jie-Peng Liu,Yonghui Xing,Keyan Song,Wei-Yong Wang 국제구조공학회 2021 Steel and Composite Structures, An International J Vol.41 No.4

This paper presents a series of eight fire tests conducted on circular tubed steel-reinforced concrete columns subjected to eccentric loads. The cross-sectional temperature, axial displacements, fire resistance, and failure modes were recorded and discussed. The influence of key parameters-load ratio, load eccentricity, and wall thickness of the steel tube—on the deformation and fire resistance of the circular tubed steel-reinforced concrete columns were also investigated. Subsequently, the coupled thermal–stress model was developed using the ABAQUS program to investigate the effects of key parameters on both thermal distribution and fire resistance. For the thermal analysis, the considered parameters comprised the cross-section dimensions, the thickness of the steel tube, and types of concrete, and for the fire resistance analysis, they were the load ratio, load eccentricity, thickness of the steel tube, and concrete and H steel strengths. The results showed that the cross-section dimensions have a relatively larger influence than the thickness of the steel tube and the types of concrete on the temperature distribution of the columns. Regarding the fire resistance of the columns, the effects of the load ratio and thickness of the steel tube are remarkable, whereas the concrete and H steel strengths and the load eccentricity have a minor influence. The calculation methods were simplified to determine the steel temperature of a column in a fire, and the N–M curves of the eccentric members subjected to ISO 834 standard temperature are presented. Using the simplified methods, the steel temperature, and the N–M curves of the eccentric circular tubed steel-reinforced concrete columns can be evaluated for any value of the significant parameters, such as the thickness of the steel tube, load ratio, and concrete strength.