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

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

issue: 1-2articleIncreasing 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

Efekti regionalnog planiranja u Hrvatskoj na primeru demografskog razvoja Banije

U Hrvatskoj je 1996. godine donet ,,Zakon o područjima posebne državne skrbi (brige)” u cilju otklanjanja posledica rata i podsticanja ravnomernijeg regionalnog razvoja. Njime je određeno područje sa specijalnim statusom i sve pogodnosti koje unutar tog područja važe. Jedan od glavnih ciljeva iniciranja ovog planskog pristupa bila je demografska obnova devastiranih prostora. Sve jedinice lokalne samouprave na Baniji su ušle u sastav pomenutog ,,posebnog područja”. U radu se razmatraju rezultati ovog regionalnog planiranja na prostoru Banije posle 20-ogodišnjeg perioda primene i dalje perspektive demografskog razvoja regije

Značaj i uloga gradskih centara u procesu regionalne integracije Banata

Na prostoru Banata izdvajaju se četiri dominantna gradska centra: Pančevo, Zrenjanin, Kikinda i Vršac. Njihov disperzni razmeštaj i velika međusobna udaljenost (više od 50 km) značajne su prepreke u ostvarivanju intenzivnijih međusobnih funkcionalnih odnosa. Iz ovog razloga, proces regionalne integracije Banata ima policentrični karakter i odlikuje ga prostorno-funkcionalna dinamika koja se najčešće ostvaruje na relaciji: gradski centar – njegovo gravitaciono područje. Cilj ovog rada je analiza prostorno-vremenske dinamike interregionalnih odnosa razvijenih u ovakvim okolnostima. Prilikom analize biće korišćeni podaci o dnevnim migracijama na nivou naselja, kao i podaci o saobraćajnom opterećenju određenih deonica puteva. Za potrebe utvrđivanja pravaca i dinamike prostornih odnosa, kao i uloge i značaja gradskih centara u regionalnoj integraciji okolnog prostora biće primenjen “model funkcionalne zavisnosti”. Ovaj model ima za cilj da odredi u kojoj meri su ostala naselja zavisna od funkcionalnog uticaja dominantnog gradskog centra i kakva je uloga tih centara u procesu regionalnih integracija Banata. S druge strane, analiza dinamike saobraćajnog opterećenja na deonicama puteva koji vode ka gradskim centrima, ali i onim deonicama koje ih međusobno povezuju, važna je prilikom utvrđivanja stepena razvoja ekonomskih aktivnosti u određenim subregionalnim celinama

Efekti regionalnog planiranja u Hrvatskoj na primeru demografskog razvoja Banije

U Hrvatskoj je 1996. godine donet ,,Zakon o područjima posebne državne skrbi (brige)” u cilju otklanjanja posledica rata i podsticanja ravnomernijeg regionalnog razvoja. Njime je određeno područje sa specijalnim statusom i sve pogodnosti koje unutar tog područja važe. Jedan od glavnih ciljeva iniciranja ovog planskog pristupa bila je demografska obnova devastiranih prostora. Sve jedinice lokalne samouprave na Baniji su ušle u sastav pomenutog ,,posebnog područja”. U radu se razmatraju rezultati ovog regionalnog planiranja na prostoru Banije posle 20-ogodišnjeg perioda primene i dalje perspektive demografskog razvoja regije

Uticaj klimatskih promena na migracije i populaciju ptica selica

U poslednjih 50 godina sve je izraženiji uticaj globalnih klimatskih promena na životnu sredinu planete i živi svet u njoj. Svaki pojedinačni element životne sredine posebno reaguje na promene, a jedan od najboljih indikatora poremećenih odnosa u prirodi su ptice selice. Ptice su izuzetno osetljive na klimatske promene predstavljene globalnim zagrevanjem, promenama u vlažnosti vazduha i količini padavina. Sve učestalije nagle promene vremena utiču na izmene prirodnih uslova staništa, što može dovesti i do trajne promene istog ili čak nestajanja pojedinih vrsta. Takođe, menjaju se i ustaljene putanje kretanja ptica selica između zimskog i letnjeg staništa, kao i dužina boravka u njima. Ove su promene izraženije kod onih vrsta ptica selica koje prelaze veća rastojanja u svojim sezonskim migracijama. Osetljivost ptica selica na klimatske promene ogleda se i kroz negativan trend izumiranja jedinki mnogih vrsta. Negativni uticaji vidljivi su širom sveta, a takođe se reflektuju i na prostoru Srbije. Sušni letnji periodi, koji pogađaju ornitološke stanice u našoj zemlji, utiču na smanjenje brojnosti određenih vrsta ptica. Takođe, pojedine vrste ptica selica za vreme toplijih jeseni i zima ne migriraju na jug, već se duže zadržavaju na ovim prostorima. Ovaj rad se bavi pitanjem kako klimatske promene nepovoljno utiču na ptice selice i u kojoj meri one mogu biti indikator klimatskih promena, u svetu i u Srbiji

Značaj i uloga gradskih centara u procesu regionalne integracije Banata

Na prostoru Banata izdvajaju se četiri dominantna gradska centra: Pančevo, Zrenjanin, Kikinda i Vršac. Njihov disperzni razmeštaj i velika međusobna udaljenost (više od 50 km) značajne su prepreke u ostvarivanju intenzivnijih međusobnih funkcionalnih odnosa. Iz ovog razloga, proces regionalne integracije Banata ima policentrični karakter i odlikuje ga prostorno-funkcionalna dinamika koja se najčešće ostvaruje na relaciji: gradski centar – njegovo gravitaciono područje. Cilj ovog rada je analiza prostorno-vremenske dinamike interregionalnih odnosa razvijenih u ovakvim okolnostima. Prilikom analize biće korišćeni podaci o dnevnim migracijama na nivou naselja, kao i podaci o saobraćajnom opterećenju određenih deonica puteva. Za potrebe utvrđivanja pravaca i dinamike prostornih odnosa, kao i uloge i značaja gradskih centara u regionalnoj integraciji okolnog prostora biće primenjen “model funkcionalne zavisnosti”. Ovaj model ima za cilj da odredi u kojoj meri su ostala naselja zavisna od funkcionalnog uticaja dominantnog gradskog centra i kakva je uloga tih centara u procesu regionalnih integracija Banata. S druge strane, analiza dinamike saobraćajnog opterećenja na deonicama puteva koji vode ka gradskim centrima, ali i onim deonicama koje ih međusobno povezuju, važna je prilikom utvrđivanja stepena razvoja ekonomskih aktivnosti u određenim subregionalnim celinama

CAD/CAM of mold for helicopter rotor composite blade

U radu se predstavlja programiranje primenom CAD/CAM sistema, udubljenja u kalupu za lopaticu glavnog rotora helikoptera. Pri proizvodnji ovakvih lopatica neophodno je postići visoku geometrijsku tačnost. Zbog velikih dimenzija duž jedne ose za izradu je planiran segmentni kalup. U radu je pokazana procedura programiranja za jedan takav segment kalupa. Ostvarena je simulacija putanje alata, dobijanje APT i NC koda kao i verifikacija putanje simulacijom uklanjanja materijala. Za realizaciju je korišćeno okruženje progama CATIA V5.This paper shows CAD/CAM technique of mold recess for helicopter main rotor blade. It is necessary to achieve high precision in this process. Due to the dimensions along one of the blade axis, the segmented mold is found necessary. In this paper the programming procedure is shown for one of these mold segments. The tool path simulation is shown as well as producing of APT and NC code with the verification of material removal. For realization of these processes CATIA’s tools were used.Tehnički urednici: Prof. dr Ljubodrag Tanović, Doc. dr Radovan Puzović, Mr Mihajlo Popović, Mr Goran Slavković Nenad Nešić, dipl. maš. ing

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