Use of High Strength Steel Grades for Economical Bridge Design

Bridges offer great potential for the use of high strength steel grades (HSS). The main advantages are generally a result of reduced weight and cross-sectional dimensions. Design stresses can be increased and plate thickness may be reduced, resulting in significant weight savings. Reduced plate thickness can also save on welding costs as well as on fabrication, erection and transportation costs. Simplified structural components and construction techniques are often possible, particularly for large structures, and foundation costs may also be reduced due to lower dead weight. High strength steels can be delivered as quenched and tempered (Q&T) or as thermo-mechanically controlled processed (TMPC). In the first case, high strengths can be achieved with minimum yield strength up to 1100 MPa, which can lead to considerable weight savings, while in the second case moderate strengths (min yield strength up to 500 MPa) accompanied with excellent weldability are possible. Especially quenched and tempered high strength steels may offer big weight savings when used for bridges. However, quenching and tempering production method poses limitations to the product length. The most economical and efficient use of Q&T steels is in members stressed in tension where the high strength can be fully exploited, and in projects where dead load is predominant (e.g. long span bridges). In compression they are most effective in heavily loaded, stocky columns or in stiffened compression elements where buckling is not the controlling criterion. Furthermore, hybrid steel girders are more economical than homogeneous girders. Hybrid steel girders are welded girders with different steel grades in flanges and web (usually high strength steel for the flanges, e.g. S550 or S690 and mild steel grades for the web, e.g. S355). Higher steel grades (e.g. S690) are usually applied in steel members and/or in bridge regions with very high static stresses in order to reduce the cross sectional dimensions and plate thicknesses of these members. As a result the overall steel self-weight of the bridge will be reduced leading to a more economical design in comparison to the case where the same (equivalent) design is made out of mild steels (e.g. S355) only. This study aims to present the potential advantages that high strength steels (HSS) have to offer in case of bridges, but also possible disadvantages. Special attention is being paid to high strength steel grades up to S700 (700 MPa minimum yield strength) in quenched and tempered condition as they are expected to offer maximum weight savings. This thesis is divided into two main parts (Part 1 and Part 2): In Part 1, a literature survey is initially performed (Part 1A) based on scientific documentation and relevant sites found on the Internet. Its purpose is to collect information from previous studies, experimental projects and fabricators, utilizing HSS for application in bridges, around the world. Then in Part 1B, a long span (L= 105 m) roadway bridge is chosen as a case study (the ‘Schellingwouderbrug’ in the Netherlands) and preliminary designs for three bridge types are presented (a single box girder bridge, a warren type truss girder bridge and arch girder bridge with vertical hangers). High strength steel S690 with minimum fy = 690 MPa is applied in members with very high stresses (e.g. chord members in the truss bridge) and S355 everywhere else (hybrid design concept). The design criteria that have been studied are strength, stability and fatigue. In Part 2, the preliminary design alternatives are compared on a cost basis (based on calculated steel self-weight and required maximum plate thicknesses) and one is chosen and designed in more detail. It is then checked, by estimating total costs, whether the hybrid design with high strength steel grade S690 will lead to a more economical bridge solution in comparison to an equivalent homogeneous (completely out of S355 steel grade) bridge design. European standards have been used throughout the whole design phase. Comparing costs between the two hybrid alternative designs (for the same bridge type) and their equivalent homogeneous designs, it has been found that the developed hybrid designs (combination of S355 and S690) for the ‘Schellingwouderbrug’, result in significant weight savings in comparison to their equivalent homogeneous (only S355) bridge designs (even up to 65% in some cases). The high price for S690 (currently ? 70-75% more expensive than S355) leads to higher material costs (up to 4% higher) for the hybrid designs. Nevertheless, the weight reduction in hybrid designs has a positive impact on the reduction of total costs (up to 6% lower) including fabrication, transportation, erection and maintenance costs.