Engineering Projects Research Articles

Prediction of the mechanical performance of polyethylene fiber-based engineered cementitious composite (PE-ECC)

In recent years, extensive research has focused on applying machine learning (ML) techniques to predict the properties of engineered cementitious composites (ECCs). ECCs exhibit crucial characteristics such as compressive strength (CS), tensile strength (TS), and tensile strain (TSt). Accurate forecasting of these critical properties can reduce material waste, lower construction expenses, and expedite project timelines for engineers and designers. This study investigates mixture design components and corresponding strengths of ECCs based on only polyethylene fiber drawing from existing literatures. Artificial neural network (ANN) models are developed to predict CS, TS, and TSt using a dataset of 339 experimental results with twelve input variables. The ANN models, implemented in MATLAB, consider various hidden layers and neurons to optimize accuracy and validation metrics demonstrate the model's high accuracy. Sensitivity analysis explores individual parameter impacts. Drawing inspiration from this study, it would be advantageous to enhance the predictive modeling toolkit by leveraging the progress made in existing technologies, thereby driving the green and low-carbon development of civil engineering. This approach not only improves the efficiency and sustainability of construction practices but also aligns with global environmental goals by reducing the carbon footprint associated with civil engineering projects.

Intelligent prediction of unfrozen water content of artificially frozen clay along one-dimensional column based on LF-NMR

The unfrozen water content (UWC) plays a crucial role in frozen soil engineering, however, traditional unfrozen water measurement or calculation methods are time-consuming and costly, and it is difficult to express the non-linearity among the influencing factors. Currently, LF-NMR is widely recognized as an effective tool for measuring unfrozen water. In this study, a calibration curve that can describe the relationship between the NMR signal and water content was derived. Moreover, the effects of temperature, initial water content, and soil height on UWC are explored along one-dimensional large-diameter columns based on LF-NMR data. Combined with the ML algorithm and the UWC test data under different factors based on LF-NMR, an intelligent prediction method that can consider the nonlinear characteristics of multiple factors is proposed. The results showed that the NMR accuracy is 176.64 semaphore per 1% water content, the error between the water content calculated from the calibration curve and that obtained from the drying method is small (average error is 0.97%), and the linearity degree of the calibration curve is good (R2 = 0.999). Moreover, due to the strong correlation between unfrozen water and temperature, initial water content, and other factors, the results indicated that the GPR model can better describe this correlation and has the best prediction effect, which was evaluated with the quantitative indicators: R2 = 0.96, RMSE = 1.07, MAE = 1.00, and again verifies the superiority of this model in combination with other literature data. Overall, this paper offers a technical basis for controlling and preventing freezing and thawing disasters in cold regions and artificial freezing engineering projects.

High-frame rate, large-depth-range structured light projector based on the step-designed LED chips array

Among numerous mature optical 3D measurement techniques, phase-shift profilometry (PSP) has been widely used because of its high precision and insensitivity to ambient light, and high-speed PSP has become a research hotspot in recent years. Current mainstream high-frame rate PSP projection techniques employ binary defocusing projection schemes, which limit the available measurement depth. We propose a high-frame rate, large-depth-range sinusoidal fringe projection technique based on step-designed LED chips array. In principle, on the one hand, the LED chips array still produces a binary pattern, so high-frame rate switching can be achieved, on the other hand, whether focusing or defocusing can generate sinusoidal fringes, avoiding the limit of defocusing projection on the depth range of measurement. A PSP projector is designed and manufactured, and 3D reconstruction of static human face mask and dynamic rotating fan is carried out at 1 kHz frame rate. In another experiment, the PSP projector projected the fringes at a 100kHz frame rate and detected the fringes with a single point photodetector, and the output waveform showed that the projection technique had the potential to be much higher than the 100 kHz frame rate. These results show that the PSP projection technology has the advantages of high-frame rate and large-depth-range, and is very useful for three-dimensional measurement of moving targets.

Stability Evaluation Method of High Fill Loess Foundation Based on Numerical Simulation

In order to understand the stability evaluation method of high fill loess foundation, the author proposes a study on the stability evaluation method of high fill loess foundation based on numerical simulation. The author first established a three-dimensional finite element model of a multi-level high fill slope using PLAXIS 3D software based on a loess high fill slope engineering project in a certain section of northwest China. The study investigated the effects of changes in fill materials, fill boundaries, slopes, and unloading platforms on slope stability. Secondly, based on the vertical and horizontal displacement of the top and foot of each level of slope under step-by-step filling, the distribution pattern of the most dangerous points of each level of slope and the overall deformation trend of the slope were analyzed. The results indicate that the cohesion and internal friction angle of the filling material are key factors affecting the stability of high fill slopes. Reducing the height of the steps at the boundary between the filling and the undisturbed soil, deepening the width of the steps, reducing the slope, and widening the unloading platform can all improve the stability of the slope. During the construction of lower slopes, there is a significant vertical displacement mutation, while the horizontal displacement mutation is relatively slow; After the construction is completed, the deformation situation is good, and the vertical and horizontal displacement of the higher slope during construction changes greatly, with uneven distribution; After the completion of construction, the consolidation settlement period is long and the deformation is large; Emphasis should be placed on strengthening deformation monitoring at high altitudes after construction is completed. Finally, the platform width can be selected within this range based on the actual engineering situation. After the platform width is greater than 3.6m, as sufficient platform width has been reached at this point, further increasing the platform width has little impact on the safety factor, and the curve gradually flattens out. The research results have determined the stability influencing factors, deformation trends, and development laws of loess high fill slopes in the northwest region, providing a scientific basis for further research on deformation control of loess high fill slopes.

The Innovative Potential Of Social Media In Green World Engineering And Technology And Project Management Institutions For Leadership Development And Learning The English Language

The Innovative Potential Of Social Media In Green World Engineering And Technology And Project Management Institutions For Leadership Development And Learning The English Language

Research on the mechanical model of anchorage resistance in deeply filled jointed rock masses

The calculation model of anchorage resistance is the core issue in the study of anchorage theory in jointed rock masses. Due to the complex geological environment of deep rock masses, the anchorage mechanism of soft rock filled in joints is even more complicated and has not yet been clearly defined. Based on the theory of structural mechanics, considering the restraining stress in the plastic extrusion deformation zone of the bolt as a rectangular distribution pattern is more in line with the interaction state between the bolt and the surrounding medium when the rod is at its limit load. Taking into account the influence of the joint filling part, the value of restraining stress is revised to include a formula expression that factors in the strength of the filling material(σcj) and the degree of filling(Δ).Develop a mechanical model of anchorage resistance for filled joint soft rock considering large deformation conditions due to turning angles. Further, conduct shear tests on anchored filled joint rock bodies under various σcj−Δ combination patterns with constant normal stiffness (CNS) boundary conditions to validate and analyze the model. The research results indicate that under the influence of deep rock mass mechanical boundaries, the ratio of the shear deformation section length to the bolt diameter differs from existing research results, with specimens l/D under different σcj−Δ combinations typically ranging between 0.7 and 1.5 times.The displacement of the bolt at turning angle θ increases with the increase of Δ and decreases with the decrease of σcj, where Δ is the main factor affecting the degree of tensile-bending deformation of the bolt. As Δ increases, the calculated contribution rates of bolt shear resistance under three different σcj conditions increase from 9.8% to 34.8%, 40.3%, and 39.4%, respectively. This indicates that an increase in Δ can reduce the increment of normal stress, further decrease the overall stress and compressive deformation zone’s normal constraint stress in the rock mass, leading to a gradual increase in the degree of tensile-bending deformation of the bolt and an increase in resistance contribution.For the specimens with three different values of σcj, the ratio of shear force to axial force providing resistance on the joint surface decreased from a maximum of 2.83 to 0.72, 1.04, and 2.01, respectively. This indicates that when σcj is relatively large, tensile-bending deformation is more likely to occur, mobilizing the axial force of the bolt to provide the main resistance. Further systematic analysis was conducted on the length of the shear deformation section of the bolt and the distribution characteristics of the anchor resistance under condition β=30°∼90°. The reasonableness of the model was verified through the comparison of model calculation results with experimental data. Finally, a series of recommendations were proposed for preventing failure of the anchored system in filled jointed rock masses in actual engineering projects.

ПРИМЕНЕНИЕ BIM-ТЕХНОЛОГИЙ ПРИ РЕНОВАЦИИ И РЕКОНСТРУКЦИИ ГОРОДСКИХ ТЕРРИТОРИЙ

With the development of modern digital technologies, many industries began to need the creation and development of tools for processing and systematization of information, their visual representation. In the architectural and urban planning sphere, digital technologies are reflected in the application of information modeling technologies (BIM modeling), which are effectively used in the design processes, during the construction of various facilities and their further operation. This study examines the effectiveness of the use of BIM technologies in the development of projects for the renovation and reconstruction of urbanized areas. Three-dimensional modeling of BIM technologies in renovation and reconstruction projects allows to simulate scenarios for the development of produced concepts and design solutions, assess the effectiveness of their impact on the territory, and predict their further development. Through the use of BIM technologies in renovation and reconstruction projects, it is possible to monitor the implementation of the developed solutions, monitor the current state of local facilities after their commissioning, identify and prevent the development of crisis situations. The authors have considered the definition of information modeling technology, and some basic ways of using BIM technologies are given. The main stages of work on the BIM model during renovation and reconstruction of the territory have been clarified and divided into four main stages. The first is the stage of collecting data on the existing situation; the second is the stage of building a model of the existing situation; the third is the stage of developing a project proposal for renovation and reconstruction and the forth is the stage of creating additional information models and detailed study of objects. Based on the analysis of the city of Belgorod, the need for renovation and reconstruction of the territory of the neighborhood "Savino" is justified, a project proposal for renovation and reconstruction using BIM technologies has been developed.

Augmented Reality Technology Framework for Engineering and Construction Projects: A Triangulation through a Qualitative Research Approach

Augmented Reality Technology Framework for Engineering and Construction Projects: A Triangulation through a Qualitative Research Approach

Influence of transformational and transactional leadership on agile project success

This study investigates the influence of Transformational and Transactional Leadership contribution in project accomplishment. It specifically evaluates the mediating role of Leadership for Agile project methodology. The data was collected from cross-sectional professionals such as Information Technology project Managers of various fields, Civil and Construction, Pharmaceuticals, Telecommunication, Higher Educational Institutions (HEIs) in Oman that comprised 384 valid samples. We used conformity factor analysis for reliability and validity of our measurement and it was valid and reliable. Partial Least-Squares Structural Equation Modelling (PLS-SEM) was used to analyze the theoretical framework. The results confirmed leadership enhances agile project methodology through the project excellence, improves end user satisfaction, value addition by achieving triple constraints. It was observed that leadership improves project performance in which team building, management commitment, unobstructed vision, daily standup meeting eases the influence of leadership on Agile methodologies.

Study on the Impact of Deep Foundation Excavation of Reclaimed Land on the Deformation of Adjacent Subway Tunnels

The objective of this research is to investigate the characteristics of the deformation response in adjacent subway tunnels caused by deep foundation excavation of reclaimed land. Focusing on a deep foundation excavation project situated in proximity to Line 11 of the subway in Shenzhen, this study employs theoretical analysis, numerical simulation, and on-site measurements to thoroughly investigate the deformation issues induced by the unloading of the excavation. The research results are as follows: using the energy method to calculate the uneven deformation of adjacent subway tunnels caused by the excavation can overcome the limitations of traditional algorithms, which treat the subway tunnel as a uniformly elastic foundation beam, resulting in more reasonable calculation results. Increasing the self-stiffness (EI)eq of the tunnel can effectively reduce the maximum displacement (wmax) of the tunnel, and as (EI)eq increases, its “weakening effect” on wmax gradually diminishes. Underground continuous walls can effectively control tunnel deformation, with tunnel displacement decreasing as the thickness and concrete strength of the continuous walls increase. “Long excavation” deep foundation excavations can impact the displacement and uplift range of the tunnel, with the maximum tunnel displacement showing a nonlinear decrease with increasing excavation depth. Tunnel displacement decreases as geotechnical parameters (elastic modulus E, internal friction angle φ, and cohesion C) increase, with the elastic modulus being the most sensitive parameter. The research findings can be applied to tunnel construction, maintenance, and safety evaluations, providing valuable references for similar engineering projects in the future.

Investigation on failure characteristics of mode II crack in heat-treated granite specimen subjected to dynamic loading

In order to solve the energy crisis, geothermal energy development has been increasingly valued in recent years. In many geothermal engineering projects, hydraulic fracturing can cause many cracks to appear inside the rock mass during the extraction process of dry hot rock. At present, there have been many studies on mode I cracks and mixed mode I/II cracks. In order to ensure the smooth progress of the project, it is necessary to supplement the research on mode II crack rocks in high-temperature geothermal environments. A series of experiments were conducted using the Split Hopkinson Pressure Bar (SHPB) device and cracked granite specimens treated at different temperatures, the failure modes could be tracked by using a high-speed camera. The dynamic experiment was simulated using software coupling. The Grain Boundary Model (GBM) can well describe the force-thermal coupling damage behavior among various crystal particles. Some analyses were conducted on dynamic fracture toughness, failure mode, strain distribution, thermal damage, and changes in the number of cracks. The research results illustrated that high temperatures lead to a decrease in the mode II fracture toughness of granite, an increase in fracture time, and greater susceptibility to damage. High temperature will increase the maximum strain at the moment of failure. It was also found that the proportion of intra-grain cracks increased with the temperature. After 500 °C, the number of cracks generated during the fracture process is inversely proportional to the temperature.

Unraveling the control factors of long-term morphological evolution in the Yangtze Estuary: A synthesis of natural processes and human interventions

Quantitative research regarding the effects and contribution rates of both natural processes and human interventions on the long-term evolution of the Yangtze Estuary is currently insufficient. To address this, we employ the Mann-Kendall (M-K) test to analyze hydrological changes and utilize bathymetric maps from 1958 to 2019 to quantify erosion and deposition patterns. Subsequently, combined with the empirical orthogonal function (EOF) to separately identify the top three control factors and elucidate spatiotemporal characteristics of the main factors controlling the Yangtze Estuary's morphological evolution. The results showed that sediment load and runoff consistently rank within the top three with respective mean contribution rates of 40.5% and 19.4%. Sediment load prevails as the first control factor across all regions except for the North Passage, which experienced a transition from natural dominance to human domination in 1998. Notably, extremely events like the 1998 flood occupied more eigenweighting in the short term and progressed in ranking due to the strong channel-forming capacity. Regarding escalating human interventions, estuarine engineering projects affect estuarine shoals following initial deposition and subsequent erosion patterns. However, their influence tends to wane post-completion, often surpassed by natural processes. Specifically, since 2010, continuous navigation channel dredging has governed the morphological evolution of the North Passage. Additionally, sand mining targeting the Ruifeng Shoal has driven the post-evolution dynamics of the South Channel as the second control factor. Overall, this study enhances our understanding of the coupling driving mechanisms involving both natural processes and human interventions, thereby enabling improved strategies for future sustainable management in estuarine environments.

Three-dimensional measurement based on equal spacing binary fringe coding

Abstract Binary fringe projection technology can effectively avoid the measurement error caused by nonlinearity in structured light three-dimensional measurement system. In this paper, a binary fringe projection coding based on equal spacing is proposed firstly, the image sequence projected by binary fringes with equal spacing is corresponding to the sinusoidal intensity values in the same period one by one, and then the sinusoidal fringes are generated by weighted superposition. The experimental results show that, compared with the traditional four-step and 12-step phase shifting methods, the mean square error of the synthesized sinusoidal pattern is reduced by 36.74% and 18.24% respectively, and the mean square error of the distance between the obtained spherical point cloud of the standard sphere and the center of the fitting standard sphere is reduced by 89.36% and 77.27% respectively.

Measures for Investment Risk Management of Enterprise Technological Innovation Projects

Enterprise technological innovation project investment risk management refers to the enterprise taking a series of measures to manage and control possible investment risks when making technological innovation project investment decisions. Investment risks in technological innovation projects refer to various risks that may have a negative impact on investment due to the uncertainty of technological innovation and changes in the market environment. This article, from an industry professional perspective, analyzes and discusses measures for investment risk management of corporate technological innovation projects.

Scheduling optimization model for linear projects with resource transfer in diverse construction scenarios

Studies on linear engineering projects often overlook complex construction scenarios, neglecting global resource quantities, transfer time and cost during resource deliberation. This study addresses these gaps by employing a linear scheduling method and a multi-mode resource-constrained project model. The decision variables of the model include the construction mode, construction direction, activity priority and global resource amounts, considering linear, bar and block construction scenarios. A hybrid algorithm combining the non-dominated sorting genetic algorithm-III with differential evolution is proposed for model resolution, featuring hierarchical, multi-strategy, adaptive crossover and mutation operators. Sensitivity analysis and case studies demonstrate the effectiveness and superiority of the model and algorithm. According to the study, the unit cost of resource transfer significantly influences the optimization result more than the unit time; the optimizing cost has an upper bound; and linear projects scheduling optimization with resource transfer are more realistic and provide a better trade-off between time and cost.

Environmental risk assessment of coastal dredging based on clustering of meteocean forcing

Dredging and dumping in-situ sediments is a fundamental operation for most coastal engineering projects and coastal defense projects, such as the construction of breakwaters, beach nourishment and land reclamation. Future projections in terms of coastal hazard suggest that coastal protection and land reclamation project will be more and more frequent. In this context, the assessment of the environmental and socio-economic impact of the risk induced by dredging is a fundamental step during both the design stage and the operational management. Most of the standard practices and available risk assessment frameworks rely on the numerical prediction of the sediment plume in the large field driven by coastal circulations forced by tides, winds and waves. In this study, we formulated a new risk assessment framework based on an unsupervised machine learning clustering algorithm, K-means clustering, for generating representative meteocean scenarios subsequently used to force a regional circulation model. Moreover, we introduced three criteria of hazard/risk based on the spatial and temporal evolution of the suspended sediment concentration that explained different environmental impacts and two new methods to synthetically present the risk values. The major improvement of the present framework is that the final probability of risk fully describes the statistics in terms of hydrodynamic and dredging conditions.This framework presents the probability analysis of risk spatial distribution based on representative hydrodynamic conditions and dredging scenarios, which is a major improvement of this study compared with previous risk assessment strategies that were unable to predict quantified dredging risk before field construction. Finally, to demonstrate the potentiality of the risk assessment framework, we applied this methodology to the Hong Kong Water and Pearl River Estuary (China) as a pilot case.

ЭТАПЫ ПРЕДИНВЕСТИЦИОННОЙ ФАЗЫ МЕГАПРОЕКТНОГО ФИНАНСИРОВАНИЯ ЭКОНОМИКИ

The article offers the author's description of the stages of the pre-investment phase of mega-project financing of the economy. The launch of this process was preceded by a thirty-year transformation of the socio-economic system of the Russian Federation. During these transformations, the previously functioning organizational and economic mechanism was deactivated. Its viability until the start of the special military operation was due to the ongoing policy of Reaganomics, closely related to the principles of the Washington Consensus. Determining a new vector of economic development, the Government of the Russian Federation selected 11 technological sovereignty projects for the first time in December 2023. Among them are port infrastructure projects, shipbuilding, energy, mechanical engineering, infrastructure projects, and instrument engineering. By designating banks as subjects of financing megaprojects, the state thereby promotes the development of economic relations using such a financial instrument as a state guarantee. To make a decision on their participation in large-scale investment projects, authorized government bodies organize the calculation of all performance indicators, incl. budgetary, financial, economic, social. If we are talking about projects that have passed the examination and are significant for the constituent entities of the Russian Federation, its initiators determine the structure of financing. When participating in projects of commercial banks, according to established practice, the Ministry of Finance of the Russian Federation may offer a state guarantee of the federal budget as security. In this case, issues of the effectiveness of a particular project fade into the background. Reason: if negative results are obtained from the implementation of the investment project, the commercial bank will write off from the treasury account all existing debt on loans issued to the megaproject operator. But in this case, the amount of debt on the loan will be lost from the federal budget. To prevent this from happening, it is important to maintain a balance of interests of all participants in the megaproject. The role of the state remains two-pronged. When identifying “positive” and “negative” aspects of state activity, the gradation assumes the presence of the following structure: a) the “negative” aspect was determined by the acceleration of economic growth and was the basis of structural transformation; b) the “positive” aspect was that set of socio-political circumstances that determine growth from the fundamentally possible potential to the real.

Determination and application study of optimal delay time for tunnel millisecond blasting based on interference vibration reduction method

In tunnel excavation, blasting is widely adopted as an efficient excavation method. However, the influence of vibration on tunnel surrounding rock and support structures during the blasting process cannot be ignored. In this study, based on the background of tunnel blasting construction, we theoretically analyze the reasonable range for selecting the optimal delay time, considering the wave superposition cancellation effect and rock fragmentation effect. We use field measured single-hole waveform and calculate superimposed predicted waveforms for different delay time through linear superposition. This allows us to determine the optimal delay time; it is then validated through numerical simulation and field experiment. The results indicate that, based on the principles of interference vibration reduction and rock fragmentation, the optimal delay time in theory should be in the range of 6.14–8.06 ms. By performing superposition calculation on the measured single-hole waveforms, we determined that the optimal delay time for continuous detonation of cut-holes is 7 ms. The delay time of 7 ms falls within a reasonable millisecond range and it is consistent with the results of numerical simulation. When the optimal delay time was applied to field blasting, the measured vibration waveforms exhibited uniform distribution. Compared to blasting without delay, the peak vibration velocity of the cut-holes decreased from 2.08 cm/s to 0.20 cm/s, and the dominant frequency band shifted from 20 Hz–60 Hz to 30 Hz–120 Hz. This achieved the desired effects of reducing vibration and enhancing frequency. These findings can serve as a reference for future similar engineering projects.

Evaluating the Positional Stability Status of the Druk Corsnet: A Preliminary Study

Abstract— The continuously operating reference station (CORS) that provides a Global Navigation Satellite System (GNSS) enables precise location and height data for various applications such as site selection of important engineering projects and constructions and hazard analysis. Similarly, the most common applications of the CORS network, in Bhutan, are for engineering, cadastral, geodetic, and topographical surveys. In this regard, the consistency and dependability of the data from the CORS station are crucial. Moreover, the ability of such applications heavily depends on precise and reliable data from the CORS network. However, Bhutan has not conducted any studies on the stability, in terms of its position, and height fluctuation of the CORS station. Therefore, the goal of this study is to determine the positional and height variation of the 9 CORS stations that are operational in Bhutan between the years 2016 to 2022. Assessing the stability and dynamic behavior of reference stations in a CORS network is part of position variation analysis. It is feasible to identify and quantify a variety of geodetic occurrences by examining the time series of positional analysis. Analysis of height variation focuses on changes in orthometric heights throughout the CORS network. In our study, all the available GNSS CORS data are processed and the displacement in height and position is computed and compared. The analysis consists of a simple method (mean value subtraction) of time series data, which is computed and compared. The major findings show that the absolute mean standard deviation in the position of the stations was 5.49 cm, 6.36 cm, and 4.24 cm vertical (height), ease, and north horizontal component respectively. The findings highlight that the proposed method of determining the variation in height and location of the CORS stations is fairly practical and will help the user community to guide them in using the CORS network across Bhutan wisely. Similarly, the result of this study will be a relevant source of information to organizations such as the National Land Commission Secretariat (NLCS) as a means to augment the system in the future.

Impact of Project Management Practices on Cost and Time Overruns in Construction Projects in Nigeria

A major contributor to economic growth in Nigeria, the construction sector is frequently beset by large cost and schedule overruns that jeopardize project profitability and impede the development of infrastructure. This study looks into how project management techniques affect these overruns in building construction projects in Nigeria. The goal of the study is to pinpoint the major project management variables that lead to overspending and delays while also offering recommendations for best practices to address these problems. A mixed-methods strategy was used, integrating qualitative case studies with quantitative data analysis. Project engineers, contractors, Architects, and Technicians were given standardized questionnaires to complete about a variety of Nigerian building projects. In-depth interviews and focus groups were also held to acquire a deeper understanding of the real-world difficulties and experiences encountered in the field. The study pinpoints several crucial project management techniques that affect time and cost performance. The results show that Inadequate construction site management and supervision ranked 1st with an RII of 0.7877, Improper construction techniques and lack of experience of the consultant came 2nd and 3rd with RII of 0.7215 and 0.7213 respectively. Late payments during work progress and Use of improper equipment are contributors to project time overrun; while, factors affecting cost overrun are fluctuation in building material costs ranked 1st with RII of 0.7354, frequent design changes, (0.7138), omissions and errors in bills of quantities (0.6754), import of construction materials (0.6754). The report suggests enhancing project management through effective resource management techniques, ongoing training and certification programs, and implementing advanced technology to improve communication, resource management, and planning precision, contributing to sustainable growth in Nigeria's construction industry. Keywords: stakeholder management, construction sector, project management, major contributor