Some relevant aspects of footbridge vibrations

Abstract

Considering the contemporary structural inaterials that are becoming more resistant, having higher strength to weight ratio, and the fact that live load of footbridges is low, the design based on static analysis only, respecting ultimate limit states requirements, leads to slender bridge structures for pedestrian and cycle track use. As a consequence, stiffness and masses decrease, facing lively, easy to excite structures, with smaller natural frequencies. The excitation of a footbridge by a pedestrian passing over it can be unpleasant for a person walking or standing on the bridge, but usually not destructive for the structure itself. Recent experiences regarding dynamic behavior of slender footbridges have especially shown that vibration serviceability limit states are very important requirements in any structural design. We are presenting a general algorithm for analytical testing of dynamic parameters of structures, calculation of deflection, thus speed and acceleration of superstructure under human-induced excitation, as predicted by Eurocode, British and Canadian standards in use, since no Yugoslav code deals with the problem. The evaluated system is a footbridge in a system of a simply supported concrete girder. The presented model is used to show correspondence of results, obtained by the algorithm, with the results obtained using the simplified methods suggested by the Codes of Practice, since the latter exists only for certain structural systems.Prateci trend izgradnje elegantnih konstrukcija, posebno mostovskih, susrecemo se sa novim izazovima u pogledu projektovanja i proracuna. Kada je rec o pesackim mostovima vibracije prouzrokovane ljudima mogu se kalsifikovati kao pitanje stanja upotrebljivosti iz razloga sto je ljudsko telo vrlo osetljivo na vibracije, a nivo vibracija koji prouzrokuje smetnju pesacima je nedovoljan da prouzrokuje konstrukcijska ostecenja. Imajuci u vidu cinjenice da je korisno opterecenje pesackih mostova relativno malo, a da savremeni materijali omogucavaju vecu nosivost, proracunom zasnovanim na iskljucivo statickoj analizi, postujuci granicna stanja nosivosti, dobijamo konstrukcije male krutosti i mase, koje se, dakle, lako pobudjuju. Stoga je narocito vazno ispuniti uslove granicnih stanja upotrebljivosti konstrukcijskih elemenata, posebno granicna stanja upotrebljivosti u pogledu vibracija indukovanih pesacima, kako u vertikalnom tako i u bocnom pravcu. Vazeci jugoslavenski propisi ne tretiraju ovaj fenomen, te je u radu sprovedena analiza prema nekoliko aktuelnih svetskih standarda za ovu oblast: Evrokod, Britanski standardi i Kanadski standardi. Prikazan je algoritam za odredjivanje dinamickih karakteristika konstrukcija i sracunavanje ugiba, odnosno brzine i ubrzanja, tocaka konstrukcije pod pokretnim harmonijskim opterecenjem. Testiran je model pesackog mosta sistema proste grede, sa ciljem provere tacnosti algoritma s obzirom na postojeca priblizna resenja za ovaj sistem u navedenim standardima.