▁ ▁thick , ▁with ▁the ▁front ▁and ▁back ▁walls ▁both ▁having ▁ 1 1 ▁dog - leg ged ▁vent ilation ▁holes ▁set ▁high ▁in ▁the ▁wall ; ▁and ▁the ▁roof , ▁set ▁flush ▁with ▁the ▁side ▁of ▁the ▁walls , ▁was ▁ 6 " ▁to ▁ 7 " ▁thick . ▁No ▁concrete ▁floor ▁is ▁indicated ▁on ▁the ▁plan . ▁The ▁two ▁ent ran ces ▁were ▁ ▁wide , ▁and ▁were ▁located ▁at ▁each ▁end ▁of ▁the ▁front ▁face ▁of ▁the ▁shelter ; ▁re cess ed ▁ ▁back ▁from ▁the ▁front ▁wall . ▁In ternal ▁bl ast ▁walls ▁form ▁entrance ▁cor rid ors ▁at ▁each ▁end ▁that ▁make ▁a ▁ 9 0 ▁degree ▁turn ▁into ▁the ▁main ▁internal ▁space ▁of ▁the ▁shelter , ▁which ▁was ▁ ▁long ▁by ▁ ▁wide , ▁being ▁ ▁from ▁floor ▁to ▁ce iling . ▁This ▁design ▁is ▁similar ▁to ▁the ▁" p ill box " ▁sh el ters ▁used ▁in ▁the ▁main ▁streets ▁of ▁Br is b ane . ▁ ▁The ▁drawing ▁was ▁signed ▁by ▁Charles ▁Da ▁Costa , ▁the ▁Princi pal ▁Railway ▁Arch itect . ▁Da ▁Costa ▁had ▁been ▁a ▁special ist ▁in ▁rein for ced ▁concrete ▁design ▁for ▁some ▁time . ▁He ▁trained ▁as ▁a ▁pup il ▁of ▁T . ▁S . ▁Martin ▁of ▁Sydney ▁from ▁ 1 9 0 5 , ▁joining ▁Queensland ▁Rail ways ▁in ▁ 1 9 0 7 ▁as ▁a ▁Junior ▁D raft s man . ▁After ▁retr en ch ment ▁in ▁ 1 9 2 1 , ▁he ▁began ▁private ▁pract ise