When plate girders are to be used for a bridge, costs generally can be cut by using flanges with higher yield strength than that of the web. Such construction is permitted for highway bridges under AASHTO specifications if the girders qualify as hybrid girders. Such girders are cost effective because the web of a plate girder contributes relatively little to the girder bending strength and the web shear strength depends on the depth/ thickness ratio.
Hybrid girders, in general, may be designed for fatigue as if they were homogeneous plate girders of the flange steel. Composite and noncomposite I-shaped girders may qualify as hybrid.
Noncomposite girders must have both flanges of steel with the same yield strength. Yield strength of web steel should be lower, but not more than 35% less. Different areas may be used at the same cross section for top and bottom flanges. If, however, the bending stress in either flange exceeds 0.55Fyw , where Fyw is the specified minimum yield stress of the web, ksi, the tension-flange area should be larger than the compression-flange area. In composite construction, the transformed area of the effective concrete slab or reinforcing steel should be included in the top-flange area.
Composite girders, in contrast, may have a compression flange of steel with yield strength less than that of the tension flange but not less than that of the web. Yield strength of web
steel should be lower, but not by more than 35%, than the yield strength of the tension flange.
Criteria governing design of hybrid girders generally are the same as for homogeneous plate girders (Arts. 11.15 and 11.16). Those that differ follow.
Web. Average shear stress in the web should not exceed the allowable for the web steel.
The bending stress in the web may exceed the allowable for the web steel if the stress in each flange does not exceed the allowable for the flange steel multiplied by a reduction factor R.
In computation of maximum permissible depth-thickness ratios for a web, Æ’b should be taken as the calculated bending stress, ksi, in the compression flange divided by R.
In design of bearing stiffeners at interior supports of continuous hybrid girders for which 0.7, no part of the web should be assumed to act in bearing.
Flanges. In composite girders, the bending stress in the concrete slab should not exceed the allowable stress for the concrete multiplied by R.
In computation of maximum permissible width-thickness ratios of a compression flange, Æ’b should be taken as the calculated bending stress, ksi, in the flange divided by R.