Manner and time of mixing. — Using of large stones in mass. — Vibration and jigging of freshly-made concrete. — Age of cement. — Protection of fresh concrete. — General remarks. — Indebtedness for suggestions to Mr. J. J. Yates ....... 386

Indebtedness for data to Mr. Frank W. Skinner. — Bridge erection is a function

of design, location, and available equipment. — Specialization of bridge erection. — Division of subject. — Steel bridges. — Completion of fabrication at bridge shops. — Use of standard plant and equipment. — Skilful and experienced labor necessary. — Complication of problem by artificial conditions. — Girder spans. — Limits in dimensions. — Derrick cars, derricks, gin-poles, and other erection apparatus. — Viaduct erection. — Ideal method is by derrick traveler or mule. — Cantilever travelers. — Strident gantry traveler. — Medium spans. — Pile foundations. — Supplementary gantry traveler. — Alternative methods. — Cantilever erection. — Erection on eccentric false-work. — Method of protrusion. — Erection on moving suspending platform. — Distribution of steel on ground. — Method of flotation. — Erection from temporary suspension span. — Long spans. — Framed-timber falsework. — Cantilever method. — Limiting lengths for cantilever spans. — Assisted cantilever method of erection. — Erection of suspended spans by flotation and hoisting. — Suspension bridges. — Methods of erection. — Arch spans. — Methods of erection. — Cantilevering arches. — Erection plant. — Steel travelers. — Structural-steel falsework. — Yokes, clamps, etc. — Replacing steel bridges. — Maintenance of traffic. — Diversion of traffic. — Replacing short spans on old sub-structure. — Use of barges therefor. — Replacing by transverse displacement. — Replacing long spans. — Erection of concrete-girder bridges and concrete-arch spans. .... 396

Data furnished for chapter. — Old rule of author's for passing upon over-stressed

structures. — Converting two old bridges into one. — Repairing bridges that should be relegated to the discard. — Criterion for determining whether to repair or to discard. — But little repair work now done by consulting engineers. — Mr. Loweth's letter and contribution. — Shifting light structures to branch lines. — Necessity in war times for repairing structures that at other times would have been removed. — "Carrying Bridges Over." — Shifting overloaded bridges to branch lines. — Strengthening bridges in place. — Example of problem in repairing or replacing. — General considerations. — Classification of bridges. — Determination of safe unit stresses for actual maximum loading. — List of factors affecting such determination. — Standard loadings. — General method of investigating a bridge. — Classification of loadings. — Speed restrictions. — Where low classification usually occurs in bridges. — Timber trestles. — Safe unit stresses for timber. — Effect of brine drippings. — Effect of locomotive smoke. — Fatigue of metal in bridges is a fallacy. — Strengthening of light steel bridges. — Strengthening timber bridges. — Strengthening versus renewal and examples thereof. — Conditions shaping the general policy concerning the keeping of light bridges in service.