In this decade, several 4-span suspension bridges were constructed or being under construction as a choice of economical bridge type. Although, there is a problem that the deflection of the center tower and the stiffening girder becomes large under eccentric live loads. In this study, we conducted the trial design of 4-span suspension bridge with the center span length of 1000m, considering buckling check and stiffeners of towers. In addition, the design method based on the rational live load is applied. Then, the dominant load condition when determining the cross section of towers and the influence of buckling strength curves to tower design is discussed.

In this study, the buckling load capacity of steel circular pipes with carbon fiber was investigated to repair or strengthen them when they are damaged by corrosion or need reinforcement. In addition, to establish repair and reinforcement methods, a method for evaluating the load-bearing capacity was studied using various design formulas for buckling load-bearing capacity. And then, the optimal range of bonding according to the amount of reinforcement was also verified. As a result, we found that carbon fiber bonding is adequate for both repair measures to restore performance and reinforcement measures to increase the bearing capacity.

Compression tests were conducted on CFRP strengthened short steel columns with H-shaped and rectangular cross-sections to target the seismic retrofit of truss bridges. The experimental parameters were each member’s thickness and the CFRP’s stacking direction, and the number of CFRP layers was designed from local buckling prior to yielding. The results showed that the maximum load and local buckling load were improved in all cases and that the existing design equation was applicable by considering the increased cross-sectional area of the CFRP.

This study is a development of the exterior diaphragm whose protrusion depth is smaller than conventional ones for applying to the high strength steel box-section column. In this paper, the static behavior of three dimensional cruciform frames with the developed exterior diaphragm is investigated by 45-degree directional loading test. From test results, it is revealed that the elasto-plastic behavior of frames is stable and there is no crack around the exterior diaphragm until the ultimate state. The yield strength obtained from test exceeds the calculated value by the evaluation method of reference [3] .