Based　on　data　collected　by　weigh-in-motion　(WIM)　measurements,　truck　traffic　is　synthesized　by　type　and　loading　condition.　Three-dimensional　nonlinear　models　for　the　trucks　with　significant　counts　are　developed　from　the　measured　data.　Six　simply　supported　multigirder　steel　bridges　with　spans　ranging　from　10.67　m　(35　ft)　to　42.67　m　(140　ft)　are　analyzed　using　the　proposed　method.　Road　surface　roughness　is　generated　as　transversely　correlated　random　processes　using　the　autoregressive　and　moving　average　model.　The　dynamic　impact　factor　is　taken　as　the　average　of　20　simulations　of　good　road　roughness.　Live-load　spectra　are　obtained　by　combining　static　responses　with　the　calculated　impact　factors.　A　case　study　of　the　normal　traffic　from　a　specific　site　on　the　interstate　highway　I-75　is　illustrated.　Static　loading　of　the　heaviest　in　each　truck　type　is　compared　with　that　of　the　American　Association　of　State　Highway　and　Transportation　Officials　standard　design　truck　HS20-44.　Several　important　trucks　causing　fatigue　damage　are　found.