| Necessity for subpunching and reaming | 9 | |
| Necessity for symmetry in riveting | 22 | |
| Necessity for symmetry of section | 21 | |
| Necessity for thorough inspection | 300 | |
| Net section | 157 | |
| Net sections near pinholes | 176 | |
| Niagara cantilever bridge | 314 | |
| Nippon Railway bridges | 62 | |
| Notching ends of cross-girders | 173 | |
| Notes for triangulation-work | 323 | |
| Notes on drawings | 339 | |
| Number of columns for elevated railroads | 93 | |
| Number of rivets at ends of plate-girders | 169 | |
| Number of test-pieces | 249 | |
| Nuts, specifications for | 258 | |
| Oaks | 298 | |
| Objections to curved members | 19 | |
| Objections to large bascule bridges | 107 | |
| Objections to lattice girders | 19 | |
| Objections to redundant members | 19 | |
| Objections to skew-bridges | 18 | |
| Objections to star-struts | 28 | |
| Objections to two-rivet connections | 25 | |
| Objections to Whipple trusses | 19 | |
| Obstacles in pier-sinking | 302 | |
| Office materials | 345 | |
| Office practice | 328 | |
| Oil-grooves | 194 | |
| Omission of diagonals in lateral systems of cantilever bridges | 65 | |
| Omission of diaphragm webs in columns of elevated railroads | 100 | |
| Open-dredging process | 302 | 303 |
| Operating cables for Halsted Street Lift-Bridge | 112 | |
| Operating machinery for Halsted Street Lift-Bridge | 111 | |
| Opposition of steel-manufacturers to improvement | 10 | |
| Opposition to aesthetic reform in bridge-building | 45 | |
| Opposition to proposed methods of design | 4 | |
| Ornamental casting, mask of | 42 | |
| Ornamental hand-rails | 52 | |
| Ornamentation for bridges, appropriate | 47 | |
| Ornamentation of bridge approaches | 53 | |
| Ornamentation of elevated railroads | 52 | |
| Ornamentation of portals | 52 | |
| Ornamentation of viaducts | 52 | |
| Outlines of truss-bridges, harshness in | 42 | |