Effect of In-Service Strengthening on the Axial Load Carrying Capacity of Steel Box Columns

Saeed Erfani,Mohsen Naseri,Vahid Akrami 한국강구조학회 2017 International Journal of Steel Structures Vol.17 No.1

To avoid any interruption of service and considering economic issues, strengthening of existing columns usually occurs while the member is under service loads. One of the important issues being neglected in the redesign process of retrofitted columns is the effects of significant axial load existing in the columns. This paper presents a numerical study to investigate the behavior and the ultimate bearing capacity of box shaped steel columns reinforced with continuous welded plates while under load. In the first phase of the study, it is intended to evaluate the variation of results with respect to the existing design relations. For this purpose, the ultimate bearing capacity of un-preloaded models is assumed to be in accordance with the selected design code and the corresponding geometric imperfection for each model is defined using several analyses to reach the values obtained with the design code, for further studies. In the second phase, the magnitude of initial imperfection is set to 1/500 of the studied columns. In both phases, the effect of magnitude of existing preload on ultimate bearing capacity is investigated considering two influential parameters, namely the slenderness ratio of the column and the ratio between cross sectional area of the column and reinforcing plates. Results show that by increasing the preload magnitude, the ultimate bearing capacity of reinforced columns with identical reinforcing plates, decreases. This reduction is more notable for the columns with median slenderness ratios. However, as the magnitude of preload increases, the effect of slenderness ratio on the reduction of ultimate bearing capacity becomes more evident. Also, it is found that by using a fixed imperfection ratio of L/500, even for un-preloaded models the ultimate bearing capacity of the strengthened column will be less than the values calculated as per the selected design code. Finally, an empirical relation is proposed to calculate reduction of ultimate bearing capacity for columns affected from various preload level considering different slenderness ratios.

대구경 현장타설말뚝의 대용량 양방향 말뚝재하시험 분석 및 극한지지력 추정을 위한 수치해석 연구

남문석(Nam, Moonsuk),김상일(Kim, Sangil),홍석우(Hong, Seokwoo),황성춘(Hwang, Seongchun),최용규(Choi, Yongkyu) 한국지반환경공학회 2011 한국지반환경공학회논문집 Vol.12 No.10

초고층 건축물 기초의 고용량 하중 지지능력을 확인할 수 있는 가장 현실적인 방안인 고유압방식의 고용량 양방향 말뚝재하시험을 2개 현장에서 실시하였다. 고유압 복동식 양방향 말뚝재하시험은 정재하시험 시 재하용량 한계와 현장조건의 제약을 극복할 수 있는 가장 현실적인 방안으로 볼 수 있었으며 고용량이 필요한 시험말뚝에 대한 재하시험에 매우 유용한 시험방법으로 판단되었다. 2개의 사례에서 계산된 설계하중 충족비는 각각 3.3, 2.1이었으므로 사례(P-2)에서 1방향 재하하중을 다소 작게 재하하였더라면 말뚝기초의 안정성을 실증적으로 확인하지 못하였을 것으로 판단되었다. 초고용량의 양방향 말뚝재하시험에서 설정한 최대하중까지 재하하더라도 말뚝 및 지반의 극한상태를 확인하는 것은 쉽지 않았으므로 대구경 현장타설말뚝의 극한지지력을 추정하기 위하여 2개의 고용량 대구경 현장타설말뚝에 대한 수치해석을 실시하였다. The high capacity bi-directional pile load test is an optimum pile load test method for high-rised buildings. Especially, a high pressure and double-acting bi-directional pile load testing, a special type of the high capacity bi-directional pile load test, is the most practical way to overcome limitations of loading capacities and constraints of field conditions, which was judged to be a very useful test method for requiring high loading capacities. Total of 2 high capacity bi-directional pile load tests(P-1 and P-2) were conducted in high-rised building sites in Korea. Based on the field load test results, the sufficiency ratio of loading capacities to design loads for P-1 and P-2 were 3.3 and 2.1, respectively. For P-2, the load test could not verify the design load if 1-directional loads applied slightly smaller than the actual applied load. Also, high capacity bi-directional pile load tests were difficult to determine an ultimate state of ground or piles, although the loads were applied until their maximum loads. Hence, finite element analyses were conducted to determine their ultimate states by calibrating and extrapolate with test results.

Experimental Analysis of Vertical Deformation and Bearing Characteristics of Bridge Piles in High and Steep Slopes

Yuanyuan Wang,Ronggui Deng,Tao Guo,Yi Sun,Bo Huang,Qinke Wang 대한토목학회 2023 KSCE Journal of Civil Engineering Vol.27 No.3

More and more rigid frame bridges with high piers and large spans are built in the high and steep slope areas of deep valleys in southwest China. The slow deformation of the slope in the geological sense often causes the problems of piles, which in turn causes damage of the upper bridges. The vertical bearing characteristics of bridge piles in slope still need to be conducted because of the peculiarity of slope topography. The vertical deformation and bearing characteristics of piles in the slope area were experimentally studied by considering different influencing factors and the fitting formula for the ultimate bearing capacity of the piles under vertical load is obtained. The results show that the vertical deformation and ultimate bearing capacity (defined by vertical limit settlement deformation of 0.013 times the pile diameter) of the pile are closely related to its position in the slope. The pile in the middle of the slope has the lowest vertical ultimate bearing capacity. Moreover, the side frictional resistance transfer depth of the pile in continuous slope is greater than that of the pile in unilateral slope. Additionally, the slope angle has a significant influence on the vertical bearing performance of piles. The delayed settlement of the pile top decreases approximately 40% at most and the vertical ultimate bearing capacity of the pile increases 48.6% at most as the slope angle decreases by 15°. Meanwhile, the side friction resistance of the pile increases with the decrease of slope angle. The bending moment applied to the pile top reduces the vertical ultimate bearing capacity of the pile and increases the axial force of the pile body. The results can provide data support for pile design and instability judgment with similar geological conditions.

비선형 회귀분석을 이용한 쇄석다짐말뚝의 극한지지력 예측

박준모(Park, Joon Mo),한용배(Han, Yong Bae),장연수(Jang, Yeon Soo) 한국해안해양공학회 2013 한국해안해양공학회 논문집 Vol.25 No.2

쇄석다짐말뚝의 한계상태설계법에서 신뢰성이론에 기반한 저항계수를 보정하기 위해서는 신뢰도 높은 극한지지력의 평가가 요구되고 있으며, 실무에서는 극한지지력을 예측하기 위하여 주로 정재하시험을 이용하고 있다. 정재하시험의 하중-침하량 곡선을 여러 도해법 등을 이용하여 극한지지력을 예측하는 평가법들이 설계기준에 제시되어 있으나, 기술자의 판단에 따라 극한하중이 일정하게 산정되지 못함으로써 신뢰성을 확보하기 어려운 단점이 있었다. 본 연구에서는 쇄석다짐말뚝의 정재하시험 결과를 비선형 회귀분석을 이용하여 극한지지력을 예측하고, 기존의 극한지지력 판정법과 비교함으로써 실제 극한지지력을 예측하는데 적합한 비선형 회귀모형을 제안하였다. 또한 극한지지력 판정법이 저항편향계수에 미치는 영향을 분석하고, 한계상태설계법을 위한 데이터베이스 축적을 목적으로 정재하시험을 계획하는데 필요한 시험조건을 검토하였다. The calibration of resistance factor in reliability theory for limit state design of gravel compaction piles (GCP) requires a reliable estimate of ultimate bearing capacity. The static load test is commonly used in geotechnical engineering practice to predict the ultimate bearing capacity. Many graphical methods are specified in the design standard to define the ultimate bearing capacity based on the load-settlement curve. However, it has some disadvantages to ensure reliability to obtain an uniform ultimate load depend on engineering judgement. In this study, a well-fitting nonlinear regression model is proposed to estimate the ultimate bearing capacity, for which a nonlinear regression analysis is applied to estimate the ultimate bearing capacity of GCP and the results are compared with those calculated using previous graphical method. Affect the resistance factor of the estimate method were analyzed. To provide a database in the development of limit state design, the load test conditions for predicting the ultimate bearing capacity from static load test are examined.

PHC Pile의 허용지지력 결정에 관한 연구

안종필,박주원,이광용 한국지반공학회 1999 한국지반공학회논문집 Vol.15 No.2

지금까지 말뚝기초의 이론적인 연구에 있어 수직하중을 받는 말뚝기초의 극한지지력을 산정하는 것에 초점이 맞추어져 왔으며, 이를 위한 다양한 종류의 정적 및 동적 지지력 공식들이 말뚝기초의 극한지지력 산정을 위해 제안된 바 있다. 그러나 이들 공식의 적합성은 아직 확실하게 정립되지 못한 실정이며, 정역학적 및 동역학적 공식에 의한 지지력의 신뢰도는 말뚝재하시험에 의하여 확인되어지고 있다. 본 연구에서는 4개현장 12개소의 PHC Pile재하시험의 결과를 토대로 하여 정역학적 지지력 공식중의 하나인 Meyerhof공식과 동역학적 지지력공식중의 하나인 Hiley공식으로부터 산정된 두가지 극한지지력의 값과 비교\ulcorner분석하여 봄으로써, 설계시 허용지지력의 결정을 위해 사용되고 있는 각 공식의 적합성을 검토하여 보았다. 그 결과 표준관입시험의 N치를 적용한 Meyerhof공식에 의한 정역학적 방법에 있어 안전율 3.0을 적용함은 비교적 타당한 것으로 나타났고, 항타시험결과를 적용한 Hiley공식에 의한 방법에 있어 적용안전율을 5.0으로 조정함이 타당한 것으로 나타났으며, 추후 방대한 자료의 축적과 분석 및 연구를 통해 보다 합리적인 말뚝기초의 설계가 이루어져야 할 것으로 사료되었다. Analytical studies on piles so far have been directed toward prediction of bearing capacity under vertical loads. Various static and dynamic formulas have been used in predicting the ultimate bearing capacity of a pile. Further, the reliability of these formulas has been verified by comparing the predicted values with the pile load test measurements. Accordingly, by means of the ultimate load from the data measured by the actual field load tests of PHC piles, safety factors were compared and analyzed static and dynamic formula methods applying to 4 different sites. As a result, the safety factor by Meyerhof formula method indicates 3.0 and the safety factor by Hiley formula method indicates 5.0.

항타말뚝과 비교한 현장타설말뚝의 지지력분석

이성준,정상섬,김수일,Lee, Seong-Jun,Jeong, Sang-Seom,Kim, Su-Il 한국지반공학회 1997 지반 : 한국지반공학회지 Vol.13 No.5

본 연구에서는 항타말뚝과 비교한 현장타설말뚝의 극한지지력을 하중전이함수를 적용하여 예측하였으며 이를 토대로 항타말뚝과 비교한 현장타설말뚝의 지지력을 현장재하시험 결과와 비교, 분석하였다. 비교, 분석은 하중전이함수들 중 응력 연화를 고려한 방법, 쌍곡선 함수법 및 탄소성 방법을 대표하는 Vijayvergiya의 방법, Castelli의 방법 및 2중 직선법을 적용하여 지반 종류에 따라 현장타설말뚝의 하중-변위관계를 산정하였다. 본 연구 결과 동일한 지반에 설치된 항타말뚝과 현장타설말뚝의 극한지지력은 총 주면마찰력의 차이로 인하여 항타말뚝이 더 큰 것으로 나타났으며 이러한 차이는 점토질 지반에서 보다는 모래질 지반에서 더 크게 나타났다. 같은 지지력을 발현하기 위해 필요한 말뚝 두부에서의 변위량은 항타말뚝보다는 현장타설말뚝에서 더 크게 나타났으며 현장타설말뚝이 극한지지력을 발현하기 위해 필요한 변위량과 이와 같은 크기의 지지력을 발현하는데 필요한 항타말뚝의 변위량과의 차이는 모래질 지반에서 가장 크게 나타났다. 현장타설말뚝과 항타말뚝의 말뚝직경과 길이의 비(LID)의 변화에 따른 극한지지력의 차이는 점토질 지반의 경우 일정한 것으로 나타났으나 모래질 지반에서는 LID의 비가 증가할수록 극한지지력의 차이가 증가하는 것으로 나타났다. In this study an iterative procedure for the analysis of drilled shafts was proposed on the basis of the load transfer mechanism. Special attention was given to the estimation of bearing capacity of drilled shafts which was compared with driven piles, and then with the results of pile load test. The load displacement at the pile head was calculated by load than sfer curves (t -z curves, q-z curves) by using Vljayvergiya, Castelli and hi -linear models. Bab ed on the analytical results, it is found that the behavior of drilled shafts is different from that of driven piles the smaller the skin friction mobilized at the pile-boil interface, the smaller the development of the bearing capacity. Hence the greater pile head movement is required to mobilize the same mainitride of bearing capacity. This trend is more noticeable in sand than in clay. It is also found that as the length-todiameter ratios increase, the dirtference of ultimate bearing capacity between drilled shafts and driven piles is becoming lass ger in sand, but it is minor in clay.

Performance of rock-socketed drilled shafts in deep soft clay deposits

Kim, Myung hak 인제대학교 2007 仁濟論叢 Vol.22 No.1

In designing rock-socketed drilled shaft, bearing capacity evaluation is very important because the maximum values of base and side resistance are not generally mobilized at the same value of displacement. FHWA and AASHTO code suggest different ultimate bearing capacity formular according to rock type and shaft settlement. In domestic code suggest base resistance and side resistance can be added on condition that after confirming the result of field load test with axial load transfer test. This paper shows that static load test and bi-directional load test result analysis of deep rock-socketed drilled shaft in three different sites. Load-settlement curve, t-z, and q-w curve in rock-socketed part were calculated and compared. t-z curve in weathered and soft rock showed no deflection softening behavior in pretty large strain (about 2-3 % of diameter). Ultimate resistance could be the summation of side resistance and base resistance in rock-socketed drilled shaft in domestic sites.

Ductile Fracture Analysis of Welded Hollow Spherical Joints Subjecting Axial Forces with Micromechanical Fracture Models

Yue Yin,Xinyu Che,Zhenyu Li,Jinlong Li,Qinghua Han 한국강구조학회 2019 International Journal of Steel Structures Vol.19 No.6

Two micromechanical fracture models, void growth model (VGM) and stress modifi ed critical strain (SMCS) model, were adopted to distinguish the failure mechanism of welded hollow spherical (WHS) joints under axial load based on FE analysis. Ductile fracture was successfully predicted for WHS joints under axial tension. The predicted fracture location is at the weld toe between WHS joints and circular hollow section members, which is consistent with corresponding test results. The predicted fracture load is lower than the peak load on the load–displacement curve, which indicates that the failure mechanism of WHS joints under axial tension is fracture due to inadequate strength and the fracture load should be taken as the ultimate load bearing capacity of the joints. A simplifi ed SMCS model was proposed and verifi ed for ductile fracture prediction of WHS joints under axial tension. Micromechanical fracture analysis was also conducted on WHS joints under axial compression. It was obtained by both VGM and SMCS model that no fracture would occur before the load reached its peak value, the reason of which was discussed by tracing the variation of the equivalent plastic stain and stress triaxiality at the potential location of fracture. Therefore, the failure mechanism of WHS joints under axial compression is losing stability with the depression of the sphere cap and the peak load on the load–displacement curve should be taken as the ultimate load bearing capacity of WHS joints.

Investigation on the failure type of tower segments under equivalent static wind loads

Yue Li,Qiang Xie,Zheng Yang 한국풍공학회 2022 Wind and Structures, An International Journal (WAS Vol.34 No.2

This paper presents a failure type assessment curve method to judge the failure type of transmission tower segments. This novel method considers the equivalent static wind load characteristics and the transmission tower members’ load-bearing capacities based on numerical simulations. This method can help judge the failure types according to the relative positions between the actual state points and the assessment curves of transmission tower segments. If the extended line of the actual state point intersects with the horizontal part’s assessment curve, the segment would lose load-bearing capacity due to the diagonal members’ failure. Another scenario occurs when the intersection point is in the oblique part, indicating that the broken main members have caused the tower segment to fail. The proposed method is verified by practical engineering case studies and static tests on the scaled tower segments.

The behavior of lightweight aggregate concrete filled steel tube columns under eccentric loading

Elzien Abdelgadir,Ji Bohai,Fu Zhongqiu,Hu Zhengqing 국제구조공학회 2011 Steel and Composite Structures, An International J Vol.11 No.6

This paper consists of two parts; the first part describes the laboratory work concerning the behavior of lightweight aggregate concrete filled steel tubes (LACFT). Based on eccentricity tests, fifty-four specimens with different slenderness ratios (L/D= 3, 7, and 14) were tested. The main parameters varied in the test are: load eccentricity; steel ratio; and slenderness ratio. The standard load-strain curves of LACFT columns under eccentric loading were summarized and significant parameters affecting LACFT column's bearing capacity, failure mechanism and failure mode such as confinement effect and bond strength were all studied and analyzed through the comparison with predicted strength of concrete filled steel tube columns (CFT) using the existing codes such as AISC-LRFD (1999), CHN DBJ 13-51-2003 (2003) and CHN CECS 28:90 (1990). The second part of this paper presents the results of parametric study and introduces a practical and accurate method for determination of the maximum compressive strength of confined concrete core ( fmax), In addition to, the study of the effect of aspect-ratio and length-width ratio on the yield stress of steel tubes ( fsy) under biaxial state of stress in CFT columns and the effect of these two factors on the ultimate load carrying capacity of axially loaded CFT/LACFT columns.