• Smart Structures and Systems
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• v.24 no.1
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• pp.27-35
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• 2019

This paper proposes a quantitative criterion for optimization of actuator placement for the Prestress-Accumulation Release (PAR) strategy of mitigation of vibrations. The PAR strategy is a recently developed semi-active control approach that relies on controlled redistribution of vibration energy into high-order modes, which are high-frequency and thus effectively dissipated by means of the natural mechanisms of material damping. The energy transfer is achieved by a controlled temporary removal of selected structural constraints. This paper considers a short-time decoupling of rotational degrees of freedom in a frame node so that the bending moments temporarily cease to be transferred between the involved beams. We propose and test a quantitative criterion for placement of such actuators. The criterion is based on local modal strain energy that can be released into high-order modes. The numerical time complexity is linear with respect to the number of actuators and potential placements, which facilitates quick analysis in case of large structures.

• Journal of the Korea institute for structural maintenance and inspection
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• v.25 no.5
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• pp.8-14
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• 2021

Post-tensioned prestressed concrete members experience immediate prestress losses as well as time-dependent prestress losses such as creep, dry shrinkage and relaxation. In addition, the stress of the upper and lower parts of the member changes due to the change in dead load due to the replacement of the upper slab and/or pavement. Such changes in fiber stress may affect the safety of the member, and it is necessary to adjust the prestressing force. Therefore, in this study, a screw type of re-tensioning post-tension kit is proposed, and it is verified that the safety against load and the stability against strain are satisfied through the load transfer test specified in EAD160004 and KCI-PS101.

• Computers and Concrete
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• v.21 no.3
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• pp.269-278
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• 2018

In this paper, two methods M1 and M2 to determine long-term deflection through finite element analyses including the effect of creep and relaxation are proposed and demonstrated for a PSC box-girder. In both the methods, the effect of creep is accounted by different models from international standards viz., ACI-209R-92, CEB MC 90-99, B3 and GL2000. In M1, prestress losses due to creep and relaxation and age adjusted effective modulus are estimated through different models and have been used in finite element (FE) analyses for individual time steps. In M2, effects of creep and relaxation are implemented through the features of FE program and the time dependent analyses are carried out in single step. Variations in time-dependent strains, prestress losses, stresses and deflections of the PSC box-girder bridge through M1 and M2 are studied. For the PSC girder camber obtained from both M1 and M2 are lesser than simple bending theory based calculations, this shows that the camber is overestimated by simple bending theory which may lead to non-conservative design. It is also observed that stresses obtained from FEM for bottom fibre are lesser than the stresses obtained from bending theory at transfer for the PSC girder which may lead to non-conservative estimates.

• Journal of the Korea Concrete Institute
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• v.28 no.6
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• pp.723-731
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• 2016

In pretensioned concrete members, a certain bond length from the end of the member is required to secure the effective prestress in the strands, which is defined as the transfer length. However, due to the complex bond mechanism between strands and concrete, most transfer length models based on the deterministic approach have uncertainties and do not provide accurate estimations. Therefore, in this study, Adaptive Neuro-Fuzzy Inference System (ANFIS), a Neuro-Fuzzy System, is introduced to reduce the uncertainties and to estimate the transfer length more accurately in pretensioned concrete member. A total of 253 transfer length test results have been collected from literatures to train ANFIS, and the trained ANFIS algorithm estimated the transfer length very accurately. In addition, a design equation was proposed to calculate the transfer length based on parametric studies and dimensional analyses. Consequently, the proposed equation provided accurate results on the transfer length which are comparable to the ANFIS analysis results.

• Journal of Korean Society of Steel Construction
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• v.18 no.6
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• pp.783-792
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• 2006

This study investigates developments in a prestressing method using the thermal-expanded cover-plates that increases the load-carrying capacity of structures by installing it on upper or lower flanges in case that huge flexural stiffness according to the increase in span length or load are required of steel structure, such as rolled H-beam or built-up beam. This method applies multi-stepwise contraction forces generated by the contraction of cover plates as prestressing forces after joining the cover plate applied by multi-stepwise thermal expansion that was applied to induce prestressing to structures. To perform a theoretical investigation of the prestressing force applied to a structure due to the thermal expansion and induce a multi-stepwise prestressing method using multi-stepwise thermal distribution, this study proposes a theoretical heat transfer solution for the multi-stepwise thermal distribution of cover plates and analyzes the effect of a multi-stepwise prestressing method using the multi-stepwise thermal expansion.

• Journal of the Korean Society of Hazard Mitigation
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• v.9 no.5
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• pp.79-86
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• 2009

With the substantial increase of the size of structure, the management of excavation becomes more difficult. Therefore, massive collapses which are related to retaining wall recently increase. However, since the study on measuring and monitoring the pre-stressing force of anchor is insufficient, behavior of anchor may not be predicted and monitored appropriately by the existing strain gauge and load cell type monitoring system. FBG Sensor, which is smaller than strain gauge and has better durability and does not have a noise from electromagnetic waves, is adapted to measure the strain and pre-stressing force of 7-wire strand, so called smart tendon. A series of pullout tests were performed to verify the feasibility of smart tendon and find out the load transfer mechanism around the steel wire tendon fixed to rock with grout. Distribution of measured strains and estimated shear stresses are compared with those predicted by theoretical solutions. It was found that developed smart tendon can be used effectively for measuring strain of 7-wire strand anchor and theoretical solutions underestimate the magnitude of shear stress and load transfer depth.

• Journal of the Korea Concrete Institute
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• v.23 no.3
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• pp.353-364
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• 2011

In a previous research performed by the authors, the allowable compressive stress coefficient (K) in pretensioned members with rectangular section at transfer was proposed based on strength design theory. In this study, a subsequent research of an enormous analysis was performed to determine the K factor for Tee and inverted Tee section members, considering the effect of section height (h), section type, amount of tendons ($A_{ps}$), and eccentricity ratio (e/h). Based on the analysis results, the allowable compressive stress coefficients (K) for Tee and inverted Tee section members at transfer were derived, which limit the maximum allowable stresses as 80% and 70% of the compressive strengths at the time of release for Tee section and inverted Tee section, respectively. And these were larger than the allowable stresses specified in domestic and other international codes. In order to verify the proposed equations, they were compared to the test results available in literature and other codes, which showed that the allowable stresses in domestic and international codes are unconservative for the cases with low eccentricity ratios while conservative for those with high eccentricity ratios. The proposed equations, however, estimate the allowable stresses of the Tee and inverted Tee section members reasonably close to test results.

• Journal of the Korea Concrete Institute
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• v.26 no.5
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• pp.635-642
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• 2014

The weight of concrete could be reduced by using hollow core slabs instead of heavy solid slabs, leading to cost reduction. The long span be also achieved by introducing prestress in hollow core slabs. but the evaluation of shear strength of precast-prestressed hollow core slabs are needed because the cross section is reduced in web and arranging shear reinforcement is not possible. In this study, the shear strength of precast-prestressed hollow core slabs were evaluated based on experimental tests. For this purpose, six full scale specimens were made and tested. The shear strength of the specimens were compared with those evaluated from current design provision(EC2 ACI, EN1168 and AASHTO).

• Journal of the Korea Concrete Institute
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• v.19 no.1
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• pp.83-90
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• 2007

The present study reports a simple unbonded-type shear strengthening technique for reinforced concrete beams using wire rope units. Fifteen beams failed in shear were repaired and strengthened with wire rope units, and then retested to failure. Influence of the prestressing force, orientation and spacing of wire rope units on the shear behavior of strengthened beams having shear span-to-depth ratios of 1.5, 2.5, or 3.25 were investigated. Test results showed that beams strengthened with wire rope units exhibited a higher shear strength and a larger post-failure deformation than the corresponding original beams. Inclined wire rope units was more effective for shear strength enhancement than vertical wire rope units. The increase of the prestressing force in wire rope units causes the decrease of the principal tensile stress in concrete, as a result, the diagonal tensile cracking strength of strengthened beams was higher than that of the corresponding original beams. Shear capacity of strengthened beams is compared with predictions obtained from ACI 318-05 and EC 2. Shear capacity of strengthened beams having shear span-to-depth ratio below 2.5 is reasonably predicted using ACI 318-05 formula. On the other hand, EC 2 overestimates the shear transfer capacity of wire rope units for beams having shear span-to-depth ratio above 2.5.