or iety ▁for ▁their ▁use ▁of ▁concrete ▁rig id ▁frames ▁in ▁the ▁early ▁ 1 9 2 0 s . ▁At ▁the ▁time , ▁rein for ced ▁concrete ▁was ▁commonly ▁used ▁in ▁bridge ▁design ▁but ▁the ▁super structure ▁was ▁designed ▁with ▁bear ings ▁on ▁the ▁sub structure . ▁In ▁concrete ▁rig id - frame ▁design , ▁there ▁are ▁no ▁bear ings . ▁Instead ▁the ▁super structure ▁is ▁cast ▁mon ol ith ically ▁with ▁the ▁sub structure ▁and ▁the ▁entire ▁bridge ▁from ▁deck ▁to ▁foot ing ▁is ▁continuous . ▁ ▁Engine ers ▁have ▁found ▁this ▁type ▁of ▁design ▁advantage ous ▁for ▁many ▁reasons . ▁M om ents ▁at ▁the ▁center ▁of ▁the ▁deck ▁of ▁a ▁rig id - frame ▁bridge ▁are ▁smaller ▁than ▁the ▁corresponding ▁moments ▁in ▁a ▁simply ▁supported ▁deck . ▁Therefore , ▁a ▁much ▁shall ower ▁cross ▁section ▁at ▁mid - span ▁can ▁be ▁used . ▁Add itional ▁benefits ▁are ▁that ▁less ▁space ▁is ▁required ▁for ▁the ▁approaches ▁and ▁struct ural ▁details ▁for ▁where ▁the ▁deck ▁be ars ▁on ▁the ▁ab ut ments ▁are ▁not ▁necessary . ▁Engine ers ▁have ▁also ▁noted ▁some ▁dis adv antages ▁of ▁rig id ▁frame ▁brid ges . ▁The ▁pla cement ▁of ▁steel ▁rein for cing ▁bars ▁( reb ars ) ▁can ▁be ▁very ▁difficult ▁and ▁the ▁forming / p lacement ▁of ▁the ▁concrete ▁is ▁complicated . ▁Furthermore , ▁rig id ▁frames ▁are ▁stat ically ▁ind et erm inate ▁and ▁the ▁analysis ▁is ▁more ▁challeng ing ▁than ▁that ▁of ▁simply ▁supported ▁structures . ▁ ▁Types ▁of ▁rig