The　typical　slenderness　and　lightness　of　modern　footbridges　make　them　very　sensitive　to　human-induced　vibrations,　compromising　their　serviceability.　Hence,　the　vibration　serviceability　assessment　is　a　key　aspect　in　the　design　and　verification　of　modern　footbridges.　A　sound　prediction　of　the　induced　vibrations　relies　on　the　reliability　of　both　the　structure　dynamic　model　and　the　load　model.　In　this　context,　the　present　paper　investigates　the　dynamic　behavior　of　a　timber　arch　bridge　subject　to　pedestrian　crowds.　The　bridge　presents　six　natural　modes　(identified　from　ambient　vibration　test)　with　frequencies　below　4　Hz,　highlighting　its　meaningful　sensitivity　to　human-induced　vibrations.　The　behavior　of　different-sized　pedestrian　crowds　is　simulated　through　the　Social　Force　Model,　which　represents　each　pedestrian　in　the　crowd　as　guided　by　the　so-called　＇social　forces＇,　namely　motivations　and　environmental　influences.　The　Social　Force　Model　enables　the　evaluation　of　the　instant　position　and　velocity　of　each　interacting　pedestrian　in　the　crowd.　Based　on　these　outcomes,　a　walking　force　is　defined　for　each　pedestrian　adopting　a　single-step　load　model,　and　the　footbridge　response　to　different　crowd　densities　is　evaluated.　Results　focus　on　the　footbridge　vibrations　in　vertical　direction,　obtained　accounting　for　the　contribution　of　both　vertical　and　torsional　modes.　The　estimation　of　torsional　mode　contributions　is　made　possible　thanks　to　the　pedestrian　positions　on　the　bridge　deck,　simulated　through　the　Social　Force　Model.