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CONTENTS
Volume 31, Number 5, May 2023
 

Abstract
Precast roofing systems employing prestressed elements often serve as smart structural solutions for the construction of industrial buildings. The precast concrete elements usually employed are highly engineered, and often consist in thin-walled members, characterised by a complex behaviour in fire. The present study was carried out after a fire event damaged a precast industrial building made with prestressed beam and roof elements, and non-prestressed curved barrel vault elements interposed in between the spaced roof elements. As a consequence of the exposure to the fire, the main elements were found standing, although some locally damaged and distorted, and the local collapse of few curved barrel vault elements was observed in one edge row only. In order to understand and interpret the observed structural performance of the roof system under fire, a full fire safety engineering process was carried out according to the following steps: (a) realistic temperature-time curves acting on the structural elements were simulated through computational fluid dynamics, (b) temperature distribution within the concrete elements was obtained with non-linear thermal analysis in variable regime, (c) strength and deformation of the concrete elements were checked with non-linear thermal-mechanical analysis. The analysis of the results allowed to identify the causes of the local collapses occurred, attributable to the distortion caused by temperature to the elements causing loss of support in early fire stage rather than to the material strength reduction due to the progressive exposure of the elements to fire. Finally, practical hints are provided to avoid such a phenomenon to occur when designing similar structures.

Key Words
computational thermal-fluid-dynamics; fire safety engineering; forensic; industrial buildings; precast concrete

Address
Bruno Dal Lago: Department of Theoretical and Applied Sciences, Università degli Studi dell'Insubria, via Dunant 3, 21100 Varese, Italy
Paride Tucci: Professional Fire Safety Engineer, via Monte Golico 14, 21100 Varese, Italy

Abstract
Displacement is an important element for evaluating the stability and failure mechanism of hydraulic structures. Digital image correlation (DIC) is a useful technique to measure a three-dimensional displacement field using two cameras without any contact with test material. The objective of this study is to evaluate the behavior of stacked geotextile tubes using the DIC technique. Geotextile tubes are stacked to build a small-scale temporary dam model to exclude water from a specific area. The horizontal and vertical displacements of four stacked geotextile tubes are monitored using a dual camera system according to the upstream water level. The geotextile tubes are prepared with two different fill materials. For each dam model, the interface layers between upper and lower geotextile tubes are either unreinforced or reinforced with a cementitious binder. The displacement of stacked geotextile tubes is measured to analyze the behavior of geotextile tubes. Experimental results show that as upstream water level increases, horizontal and vertical displacements at each layer of geotextile tubes initially increase with water level, and then remain almost constant until the subsequent water level. The displacement of stacked geotextile tubes depends on the type of fill material and interfacial reinforcement with a cementitious binder. Thus, the proposed DIC technique can be effectively used to evaluate the behavior of a hydraulic structure, which consists of geotextile tubes.

Key Words
cementitious binder; digital image correlation; displacement; geotextile tube; water level

Address
Dong-Ju Kim and Jong-Sub Lee: School of Civil, Environmental and Architectural Engineering, Korea University, 145, Anam-ro,
Seongbuk-gu, Seoul, 02841, Republic of Korea
Dong Geon Son and Yong-Hoon Byun: School of Agricultural Civil & Bio-Industrial Engineering, Kyungpook National University, 80, Daehak-ro, Buk-gu, Daegu, 41566, Republic of Korea
Thomas H.-K. Kang: Department of Architecture and Architectural Engineering, Seoul National University, 1, Gwanak-ro, Gwanak-gu, Seoul, 08826, Republic of Korea
Tae Sup Yun: School of Civil and Environmental Engineering, Yonsei University, 50, Yonsei-ro, Seodaemun-gu, Seoul, 03722, Republic of Korea

Abstract
The paper investigates the effect of two parameters - sand content (SC) and grain migration during shearing - on the mechanical properties of gravel-sand mixtures. Consolidated undrained (CU) triaxial tests were carried out on eight series of mixtures containing gravel (1(d(16 mm) and sand (0.1(d(1 mm). The prepared mixtures have sand contents of 0, 10, 15, 20, 40, 54, 94 and 100%, and a relative density of 60%. The transition sand content (TSC) is experimentally defined and marks the transition from gravel-driven to sand-driven behavior. For SC(TSC, the dry density of the mixture increases with SC. This induces an increase in undrained peak strength and dilative trend. The slope and position of the critical state line (CSL) are also deeply dependent on SC. At SC=TSC, the mixtures exhibit the largest dry density and yield the highest undrained peak strength and the largest dilative trend. During shearing, large internal migration of grains was observed at the TSC, causing heterogeneity in the sample. Analysis of the CSL deduced from the final points of the triaxial tests shows that, at the TSC, failure appears to correspond to the behavior of the coarsest fraction of the soil. This fraction is located in the upper part of the sample, where the sand particles had been eliminated by suffusion. On the other hand, in the more stable materials, the CSL is consistent with the bulk grain size distribution of the soil.

Key Words
critical state line; deviator stress; granular materials; internal stability; transition sand content

Address
Zhongsen Li: 1) LMPS, Paris-Saclay University, CentraleSupélec, ENS Paris-Saclay, CNRS, 91190, Gif-sur-Yvette, France, 2) Department of Civil Engineering, Aalto University, 02150 Espoo, Finland
Jean-Marie Fleureau: LMPS, Paris-Saclay University, CentraleSupélec, ENS Paris-Saclay, CNRS, 91190, Gif-sur-Yvette, France
Hanène Souli: LTDS, University of Lyon, CentraleLyon-ENISE, CNRS, 58 rue Jean Parot, 42023 Saint Etienne, France
Jean-Jacques Fry: Centre d'Ingéniérie Hydraulique, Electricité de France, 73073 Le Bourget du Lac, France
Tariq Ouahbi and Saïd Taibi: LOMC, Normandy University, CNRS, rue Philippe Lebon, 76600 Le Havre, France

Abstract
The submerged floating tunnel (SFT) is tethered by mooring lines anchored to the seabed, therefore, the structural integrity of the anchor should be sensitively managed. Despite their importance, reaction forces cannot be simply measured by attaching sensors or load cells because of the structural and environmental characteristics of the submerged structure. Therefore, we propose an effective method for estimating the reaction forces at the seabed anchor of a submerged floating tunnel using a structural pattern model. First, a structural pattern model is established to use the correlation between tunnel motion and anchor reactions via a deep learning algorithm. Once the pattern model is established, it is directly used to estimate the reaction forces by inputting the tunnel motion data, which can be directly measured inside the tunnel. Because the sequential characteristics of responses in the time domain should be considered, the long short-term memory (LSTM) algorithm is mainly used to recognize structural behavioral patterns. Using hydrodynamics-based simulations, big data on the structural behavior of the SFT under various waves were generated, and the prepared datasets were used to validate the proposed method. The simulation-based validation results clearly show that the proposed method can precisely estimate time-series reactions using only acceleration data. In addition to real-time structural health monitoring, the proposed method can be useful for forensics when an unexpected accident or failure is related to the seabed anchors of the SFT.

Key Words
deep learning, forensic; LSTM; pattern recognition; reaction forces; structural health monitoring; submerged floating tunnel

Address
Seongi Min, Kiwon Jeong, Donghwi Jung and Seungjun Kim: School of Civil, Environmental and Architectural Engineering, Korea University, Seoul 02841, Korea
Yunwoo Lee: School of Civil Engineering, Chungbuk National University, Cheongju 28644, Korea

Abstract
Beam-column joints are a critical component of reinforced concrete frame structures. They are responsible for transferring forces between adjoining beams and columns while limiting story drifts and maintaining structural integrity. During severe loading, beam-column joints deform significantly, affecting, and sometimes governing, the overall response of frame structures. While most failure modes for beam and column elements are commonly considered in plastic-hinge-based global frame analyses, the beam-column joint failure modes, such as concrete shear and reinforcement bond slip, are frequently omitted. One reason for this is the dearth of published guidance on what type of hinges to use, how to derive the joint hinge properties, and where to place these hinges. Many beam-column joint models are available in literature but their adoption by practicing structural engineers has been limited due to their complex nature and lack of practical application tools. The objective of this study is to provide a comparative review of the available beam-column joint models and present a practical joint modeling approach for integration into commonly used global frame analysis software. The presented modeling approach uses rotational spring models and is capable of modeling both interior and exterior joints with or without transverse reinforcement. A spreadsheet tool is also developed to execute the mathematical calculations and derive the shear stress-strain and momentrotation curves ready for inputting into the global frame analysis. The application of the approach is presented by modeling a beam column joint specimen which was tested experimentally. Important modeling considerations are also presented to assist practitioners in properly modeling beam-column joints in frame analyses.

Key Words
beam-column joint; bond slip; exterior joints; global frame analysis; interior joints; nonlinear analysis; rotational spring; shear

Address
Department of Civil and Environmental Engineering, The University of Toledo, 2801 W Bancroft St. MS 307, Toledo, Ohio 43607, USA

Abstract
This study presents the visually observed behavior of fibers embedded in concrete samples that were subjected to a flexural bending test. Three types of fibers such as macro polypropylene, macro polyethylene, and the hybrid of steel and polyvinyl alcohol were mixed with cement by a designated mix ratio to prepare a total of nine specimens of each. The bending test was conducted by following ASTM C1609 with a net deflection of 2, 4, and 7 mm. The X-ray computed tomography (XCT) was carried out for 7 mm-deflection specimens. The original XCT images were post-processed to denoise the beam-hardening effect. Then, fiber, crack, and void were semi-manually segmented. The hybrid specimen showed the highest toughness compared to the other two types. Debonding based on 2D XCT sliced images was commonly observed for all three groups. The cement matrix near the crack surface often involved partially localized breakage in conjunction with debonding. The pullout was predominant for steel fibers that were partially slipped toward the crack. Crack bridging and rupture were not found presumably due to the image resolution and the level of energy dissipation for poly-fibers, while the XCT imaging was advantageous in evaluating the distribution and behavior of various fibers upon bending for fiber-reinforced concrete beam elements.

Key Words
debonding; fiber-matrix mechanism; fiber-reinforced concrete; pullout; third-point loading test; X-ray computed tomography

Address
Seok Yong Lim and Tae Sup Yun: School of Civil and Environmental Engineering, Yonsei University, 50 Yonsei-ro, Seoul 03722, Republic of Korea
Kwang Soo Youm: GS Construction & Engineering, 33 Jong-ro, Jongro-gu, Seoul 03159, Republic of Korea
Kwang Yeom Kim: Department of Energy & Resources Engineering, Korea Maritime & Ocean University, Taejong-ro, Youngdo-gu, Busan 49112, Republic of Korea
Yong-Hoon Byun: School of Agricultural Civil & Bio-Industrial Engineering, Kyungpook National University, 80 Daehak-ro, Buk-gu, Daegu 41566, Republic of Korea
Young K. Ju: School of Civil, Environmental and Architectural Engineering, Korea University, 145 Anam-ro, Seongbuk-gu, Seoul 02841, Republic of Korea

Abstract
This is a research analyses on the bearing capacity at a pile tip embedded in rock. The aim is to propose a shape coefficient for an analytical solution and to investigate the influence of the plastic flow law on the problem. For this purpose, the finite difference method is used to analyze the bearing capacity of various types and states of rock masses, assuming the Hoek & Brown failure criterion, by considering both plane strain and an axisymmetric model. Different geometrical configurations were adopted for this analysis. First, the axisymmetric numerical results were compared with those obtained from the plane strain analytical solution. Then the pile shape influence on the bearing capacity was studied. A shape factor is now proposed. Furthermore, an evaluation was done on the influence of the plastic flow law on the pile tip bearing capacity. Associative flow and non-associative flow with null dilatancy were considered, resulting in a proposed correlation. A total of 324 cases were simulated, performing a sensitivity analysis on the results and using the graphic output of vertical displacement and maximum principal stress to understand how the failure mechanism occurs in the numerical model.

Key Words
bearing capacity; finite difference method; flow law; pile foundation; pile; rock mass; shape factor

Address
Ana S. Alencar and Rubén A. Galindo: ETSI Caminos, C. y P., Universidad Politécnica de Madrid, C/ Profesor Aranguren s/n, Madrid 28040, Spain
Miguel A. Millán: ETS Arquitectura. Universidad Politécnica de Madrid, C/ Profesor Aranguren s/n, Madrid 28040, Spain

Abstract
Buildings, bridges, and dams are examples of civil infrastructure that play an important role in public life. These structures are prone to structural variations over time as a result of external forces that might disrupt the operation of the structures, cause structural integrity issues, and raise safety concerns for the occupants. Therefore, monitoring the state of a structure, also known as structural health monitoring (SHM), is essential. Owing to the emergence of the fourth industrial revolution, next-generation sensors, such as wireless sensors, UAVs, and video cameras, have recently been utilized to improve the quality and efficiency of building forensics. This study presents a method that uses a target-based system to estimate the dynamic displacement and its corresponding dynamic properties of structures using UAV-based video. A laboratory experiment was performed to verify the tracking technique using a shaking table to excite an SDOF specimen and comparing the results between a laser distance sensor, accelerometer, and fixed camera. Then a field test was conducted to validate the proposed framework. One target marker is placed on the specimen, and another marker is attached to the ground, which serves as a stationary reference to account for the undesired UAV movement. The results from the UAV and stationary camera displayed a root mean square (RMS) error of 2.02% for the displacement, and after post-processing the displacement data using an OMA method, the identified natural frequency and damping ratio showed significant accuracy and similarities. The findings illustrate the capabilities and reliabilities of the methodology using UAV to evaluate the dynamic properties of structures.

Key Words
computer vision; displacements; dynamic properties; structural health monitoring; UAV

Address
Rithy Prak, Ji Ho Park, Sanggi Jeong, Arum Jang and Young K. Ju: School of Civil, Environmental, and Architectural Engineering, Korea University, 145 Anam-ro, Seongbuk-gu, Seoul 02841, Republic of Korea
Min Jae Park: 1) School of Civil, Environmental, and Architectural Engineering, Korea University, 145 Anam-ro, Seongbuk-gu, Seoul 02841, Republic of Korea, 2) Department of Architectural Engineering, Pukyong National University, 45, Yongso-ro, Nam-gu, Busan, 48513, Republic of Korea
Thomas H.-K. Kang: Department of Architecture and Architectural Engineering, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul, 08826, Republic of Korea

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