The present work focuses on the tensile strength of reinforced concrete high yield steel bars from Rwanda and Turkey, commonly used in the construction in Goma, recycled in Rwanda from metal waste from off-road vehicles and others collected in Goma. Metal waste is collected in Goma largely manually and transported to Rwanda where a lathe machine tool is used to manufacture steel specimens which undergo a tensile test using the WP 300 universal testing machine for tensile test. The yield strength of the ordinary high yield steel bars and that of the standard high yield steel bars all recycled in Rwanda are 420.6 and 392.8 MPa respectively while the yield strength of the recycled high yield steel bars from Turkey is 443 MPa. These values ​​are very close to the values ​​recommended by French standard N F EN 1992 (Part 1-1) which must be between 400 MPa and 600 MPa and are of high ductility (Class C). The Young moduli of ordinary high yield steel bars recycled from Rwanda, that of standard high yield steel bars recycled from Rwanda and finally that of high yield steel bars recycled from Turkey are respectively 27.6, 27.6 and 31.2 GPa, very low values compared to the value of 200 GPa recommended by the standards, hence the importance of taking them into consideration in the design calculations.

The present work proposes an economic-structural study of a bridge girder with two variants: the metallic Vierendeel girder and the Warren type steel truss girder formed of top and bottom chords and diagonals in order to propose the girder that fulfills the conditions of Ultimate Limit State (ULS), Serviceability Limit State (SLS) and economically profitable for the bridge over the Kahira River in Sake, Goma in the Capital of North Kivu in the Democratic Republic of Congo (DRC). In order to analyze structural elements, the French method of loads on bridges was used, the Naylor method derived from the Cross moment distribution method was used for the analysis of the Vierendeel girder, the method of joints and method of sections for the Warren truss girder; the French code «Béton Armé aux Etats Limites 91 modified 99» was used for the design of the reinforced concrete parts; the Eurocode 3 and the «Construction Métallique 66 (CM66) » were used to determine the cross-sectional area of different elements of the steel girder. An economic study was carried out passing the bill of quantities and cost estimate for each type of beam giving a total of $2485.1 for the Vierendeel beam, a total of $5866.95 for the Warren truss beam, the total cost of the Vierendeel beam representing 42.36% of the total cost of the Warren truss beam. The Vierendeel girder meeting the structural and economic conditions is proposed for the bridge over the Kahira River in Sake.