Straight and curved steel I-girder bridges - Computational Analysis and Design of Bridge Structures - page 195

Assumed stress distribution
across the effective width

b e

Actual stress distribution across slab

(a)

b e

b tr = b e / n

s = beam spacing

(b)

Figure 7.2 Definition of effective width and transformed width of a steel composite section.
(a) Actual stress distribution and (b) effective width and transformed width.

beams are more economical than welded girders. Other considerations are
delivery or specific requirements, such as camber and curvature. Typical
diaphragms or cross frames as shown in Figure 7.2 are designed for

• Lateral loads transferring.
• Stability of the bottom lange for all loads when it is in compression.
• Stability of the top lange in compression prior to curing of the deck.
• Live loads distribution.

Diaphragms or cross frames can be specified as either of the following:

• Permanent —if they are required in the bridge's final condition
• Temporary —if they are required only during construction

The difference between diaphragms and cross frames is that diaphragms
consist of a transverse flexural component, whereas cross frames consist of a
transverse truss framework; both carry vertical shear and moment from one
beam to the others. For straight bridges, the general recommendation is to
place cross frames either parallel to skewed supports or normal to the gird-
ers if the deflection between girders is constant at cross-frame connections