Benchmark footbridge for vibration serviceability assessment under vertical component of pedestrian load

Vibration serviceability criteria are governing the design and determining the cost of modern, slender footbridges. Efficient and reliable evaluation of dynamic performance of these structures usually requires a detailed insight into the structural behaviour under human induced dynamic loading. Design procedures are becoming ever more sophisticated and versatile and for their successful use a thorough verification on a range of structures is required. The verification is currently hampered by a lack of experimental data that are presented in the form directly usable in the verification process

Quantification of dynamic excitation potential of pedestrian population crossing footbridges

Due to their slenderness, many modern footbridges may vibrate significantly under pedestrian traffic. Consequently, the vibration serviceability of these structures under human-induced dynamic loading is becoming their governing design criterion. Many current vibration serviceability design guidelines, concerned with prediction of the vibration in the vertical direction, estimate a single response level that corresponds to an "average" person crossing the bridge with the step frequency that matches a footbridge natural frequency. However, different pedestrians have different dynamic excitation potential, and therefore could generate significantly different vibration response of the bridge structure. This paper aims to quantify this potential by estimating the range of structural vibrations (in the vertical direction) that could be induced by different individuals and the probability of occurrence of any particular vibration level. This is done by introducing the inter- and intra-subject variability in the walking force modelling. The former term refers to inability of a pedestrian to induce an exactly the same force with each step while the latter refers to different forces (in terms of their magnitude, frequency and crossing speed) induced by different people. Both types of variability are modelled using the appropriate probability density functions. The probability distributions were then implemented into a framework procedure for vibration response prediction under a single person excitation. Instead of a single response value obtained using currently available design guidelines, this new framework yields a range of possible acceleration responses induced by different people and a distribution function for these responses. The acceleration ranges estimated are then compared with experimental data from two real-life footbridges. The substantial differences in the dynamic response induced by different people are obtained in both the numerical and the experimental results presented. These results therefore confirm huge variability in different people's dynamic potential to excite the structure. The proposed approach for quantifying this variability could be used as a sound basis for development of new probability-based vibration serviceability assessment procedures for pedestrian bridges

Vibration serviceability of footbridges under human-induced excitation : a literature review

Increasing strength of new structural materials and longer spans of new footbridges, accompanied with aesthetic requirements for greater slenderness, are resulting in more lively footbridge structures. In the past few years this issue attracted great public attention. The excessive lateral sway motion caused by crowd walking across the infamous Millennium Bridge in London is the prime example of the vibration serviceability problem of footbridges. In principle, consideration of footbridge vibration serviceability requires a characterisation of the vibration source, path and receiver. This paper is the most comprehensive review published to date of about 200 references which deal with these three key issues. The literature survey identified humans as the most important source of vibration for footbridges. However, modelling of the crowd-induced dynamic force is not clearly defined yet, despite some serious attempts to tackle this issue in the last few years. The vibration path is the mass, damping and stiffness of the footbridge. Of these, damping is the most uncertain but extremely important parameter as the resonant behaviour tends to govern vibration serviceability of footbridges. A typical receiver of footbridge vibrations is a pedestrian who is quite often the source of vibrations as well. Many scales for rating the human perception of vibrations have been found in the published literature. However, few are applicable to footbridges because a receiver is not stationary but is actually moving across the vibrating structure. During footbridge vibration, especially under crowd load, it seems that some form of human–structure interaction occurs. The problem of influence of walking people on footbridge vibration properties, such as the natural frequency and damping is not well understood, let alone quantified. Finally, there is not a single national or international design guidance which covers all aspects of the problem comprehensively and some form of their combination with other published information is prudent when designing major footbridge structures. The overdue update of the current codes to reflect the recent research achievements is a great challenge for the next 5–10 years

Hoop of the fan mill

While passing through the fan mill beater wheel, coal particles are mutually ground both by collision and friction. Having left the beater-wheel, under the influence of transport fluid flow in the mill chamber spiral, the path of coal particles turns towards the spiral's exit, providing that the collision with the chamber armored plates occur under the low angle. The process of the coal grinding inside the fan mill is relatively rough, therefore the coal powder separator should be installed behind the mill. The efficiency of after-grinding process in the fan mills may be increased after the installed fixed, hoop-shaped barriers, into the chamber spiral, around the beater wheel. The increase in the hoop's transpar­ency coefficient, which depends on the number and size of barriers, as well as reciprocal value of the mean hoop's diameter, may influence the increase of the grinding efficiency. The barrier s shape audit's installment angle towards the beating wheel also make a significant influence on the grinding quality. As the consequence of the hoop installment there appears the decrease in ventilating efficiency of the corresponding fan mill. They may complete the coal grinding process without coal powder separators because of the increased rate of the grinding quality

Modelling human actions on lightweight structures : experimental and numerical developments

This paper presents recent, numerical and experimental, developments in modelling dynamic loading generated by humans. As modern structures with exposure to human-induced loading, such as footbridges, building floors and grandstands, are becoming ever lighter and more slender, they are increasingly susceptible to vibration under human-induced dynamic excitation, such as walking, jumping, running and bobbing, and their vibration serviceability assessment is often a deciding factor in the design process. While simplified modelling of the human using a harmonic force was sufficient for assessment of vibration performance of more robust structures a few decades ago, the higher fidelity models are required in the contemporary design. These models are expected not only to describe both temporal and spectral features of the force signal more accurately, but also to capture the influence, psychological and physiological, of human-structure and human-human interaction mechanisms on the human kinematics, and consequently on the force generated and the resulting vibration response. Significant advances have been made in both the research studies and design guidance. This paper reports the key developments and identifies the scope for further research

Hoop of the fan mill

While passing through the fan mill beater wheel, coal particles are mutually ground both by collision and friction. Having left the beater-wheel, under the influence of transport fluid flow in the mill chamber spiral, the path of coal particles turns towards the spiral's exit, providing that the collision with the chamber armored plates occur under the low angle. The process of the coal grinding inside the fan mill is relatively rough, therefore the coal powder separator should be installed behind the mill. The efficiency of after-grinding process in the fan mills may be increased after the installed fixed, hoop-shaped barriers, into the chamber spiral, around the beater wheel. The increase in the hoop's transpar­ency coefficient, which depends on the number and size of barriers, as well as reciprocal value of the mean hoop's diameter, may influence the increase of the grinding efficiency. The barrier s shape audit's installment angle towards the beating wheel also make a significant influence on the grinding quality. As the consequence of the hoop installment there appears the decrease in ventilating efficiency of the corresponding fan mill. They may complete the coal grinding process without coal powder separators because of the increased rate of the grinding quality

How Does Social Media Feed Our Beliefs?

Nowadays, social media has become part of the daily routine for most people. Recently, it has also become the main source of news for many people, and as such, it could influence their beliefs in real life. It is known that social media has become a place where misinformation is spread and where governments try to influence their citizens. This review examines the processes by which social media may affect our beliefs. In general, beliefs are formed based on heuristics - mental shortcuts which, although adaptive, reveal flaws in our reasoning that can lead to unwanted biases. One of them is the mere exposure effect, often used in marketing for changing attitudes about certain products. Others, such as confirmation bias and bandwagon effects, are encouraged by the way social media platforms work (e.g., algorithms recommending personalized content). All of these can result in illusory truth effects, encouraging the spread of misinformation. The combination of these innate biases and the way social media works, creates echo chambers, situations where we are only presented with information we already believe in. Attempts at reducing these effects are discussed as well, such as designing digital nudges, flagging suspicious content, and reducing the data shared on platforms

The role of nodal centers in achieving balanced regional development

Uneven regional development is characteristic of a large part of the society, from micro to macro regional level. One of the ways of overcoming this problem is effective spatial and functional connection of the largest nodal centers of the region, among themselves and with the settlements in the gravitational sphere. The paper analyzes the spatial and functional impact of nodal centers in the Serbian part of Banat. This region is located in Vojvodina Province, Northern Serbia. Nodal centers are presented as poles of convergence of the population and functions, from which development impacts - the main factors of socio-economic transformation of the region are spreading out. The paper also deals with the possibilities for achieving balanced regional development presented through the prism of the functional impact of nodal centers in that process. Nodal (functional) region is a theoretical basis of the work. Also, the nodal region is a starting point of functional-process paradigm, resulting from modern processes which change the structure of space by their functional influence. Functional-process paradigm is one of the basic characteristics of modern regional geography

Određivanje sezone šumskih požara analizom klimatskih uvjeta na području sjeveroistočne Srbije

SummaryThe territory of Serbia is sensitive to forest fires, which endanger various systems and play an important role in shaping the ecosystem. The probability of the occurrence and spread of forest fires in the area of northeastern Serbia was examined, depending on the influence of climatic conditions. Changes in climatic conditions are investigated at the annual and monthly level of air temperature, precipitation and air humidity values recorded at three main meteorological stations (Zaječar, Negotin, Crni Vrh) in the area of northeastern Serbia. Changes in climatic conditions in the period 2009-2022 compared to the period 1961-1990 were analyzed, with an emphasis on 2012 and 2014.The length of the average fire season was determined through a modified version of Thornthwaite’s evapotranspiration index. The fire season is longer in the area of Negotin and Zaječar than in the area of Crni Vrh. On the basis of the monthly humidity indices, a significantly longer fire season was determined in 2012 compared to 2014, as well as the multi-year period 1961-1990.An analysis of the value of the Burning Index (B) for the period 2009-2022 was performed and a correlation was established with the data on the number of forest fires. The burning index in the area of northeastern Serbia is the highest during the months of July, August and September. Significantly higher values of the burning index are for 2012 compared to 2014, which is correlated with the occurrence of forest fires in this period.SažetakTeritorija Srbije je osjetljiva na šumske požare koji ugrožavaju različite sustave i igraju važnu ulogu u oblikovanju ekosustava. Ispitana je vjerojatnost nastanka i širenja šumskih požara na području sjeveroistočne Srbije, ovisno o utjecaju klimatskih uvjeta. Promjene klimatskih prilika istražuju se na godišnjoj i mjesečnoj razini vrijednosti temperature zraka, oborina i vlažnosti zraka zabilježenih na tri glavne meteorološke postaje (Zaječar, Negotin, Crni Vrh) na području sjeveroistočne Srbije. Analizirane su promjene klimatskih uvjeta u razdoblju 2009.-2022. u odnosu na razdoblje 1961.-1990., s naglaskom na 2012. i 2014. godinu. Duljina prosječne požarne sezone određena je modificiranom verzijom Thornthwaite-ovog indeksa evapotranspiracije. Požarna sezona duža je na području Negotina i Zaječara nego na području Crnog Vrha. Na temelju mjesečnih indeksa vlažnosti utvrđena je značajno duža sezona požara u 2012. u odnosu na 2014., kao i višegodišnje razdoblje 1961.-1990. Provedena je analiza vrijednosti indeksa gorenja (B) za razdoblje 2009. – 2022. te je utvrđena korelacija s podacima o broju šumskih požara. Indeks gorenja na području sjeveroistočne Srbije najveći je u srpnju, kolovozu i rujnu. Značajno veće vrijednosti indeksa gorenja su za 2012. godinu u odnosu na 2014. godinu, što je u korelaciji s pojavom šumskih požara u ovom razdoblju

Suggestions for calculation and construction of the fan mill barriers hoop

Čestice uglja se prilikom prolaska kroz udarno kolo ventilatorskog mlina melju udarom i trenjem. Putanja čestice ugljenog praha posle napuštanja udarnog kola se povija prema izlazu iz spirale usled čega se sudar sa pancirnim pločama kućišta odvija pod malim uglom. Mlevenje uglja u ventilatorskom mlinu je relativno grubo zbog čega se iza mlina postavlja separator. Efikasnost naknadnog mlevenja iza udarnog kola, kod ovih mlinova, može se povećati postavljanjem nepokretnih prepreka u vidu zakola, u spiralu kućišta mlina, oko udarnog kola. Povećanjem koeficijenta prozračnosti zakola, koji zavisi od broja prepreka, dimenzije prepreke i recipročne vrednosti srednjeg prečnika zakola, može se povećati efikasnost mlevenja. Značajnog uticaja na povećanje finoće mlevenja takođe ima oblik prepreke i ugao njenog postavljanja prema udarnom kolu. Pri konstruisanju zakola kod ovog mlina potrebno je izvršiti odgovarajuće profilisanje same spirale, kako bi se ostvario najmanji nad pritiska pri, strujanju sušećeg fluida i samlevenih čestica uglja. Zbog poboljšanog kvaliteta ugljenog praha kod ovih mlinova, može se ostvariti protočni proces mlevenja uglja bez separatora.While passing through the fan mill beater wheel, coal particles are mutually ground both by collision and friction. Having left the beater wheel, under the influence of transport fluid flow in the mill chamber spiral, the path of coal particles turns towards the spiral's exit, providing that the collision with the chamber armoured plates occur under the low angle. The process of the coal grinding inside the fan mill is relatively rough therefore the coal powder separator should be installed behind the mill. The efficiency of after-grinding process in the fan mills, behind the beating wheel, may be increased after the installed fixed, hoop-shaped barriers into the chamber spiral, around the beater wheel. The increase in the hoop 's transparency coefficient, which depends on the number and size of barriers, as well as reciprocal value of the mean hoop's diameter, may influence the increase of the grinding efficiency. The barrier's shape and it 's installment angle towards the beating wheel also make a significant influence on the grinding quality. While constructing the body it is necessary to profile the spiral correctly in order to achieve the smallest pressure loss during the flow of pulverized coal/conveying gas mixture. The increased rate of the grinding quality enables the coal grinding process performed with no coal powder separators