To reduce scour and to avoid impeding stream flow, it is generally desirable to orient piers with centerlines parallel to direction of flow; therefore skewed spans may be required. Truss construction does not lend itself to bridges where piers are not at right angles to the superstructure (skew crossings). Hence, these should be avoided and this can generally be done by using longer spans with normal piers. In economic comparisons, it is reasonable to assume some increased cost of steel fabrication if skewed trusses are to be used.
If a skewed crossing is a necessity, it is sometimes possible to establish a panel length equal to the skew distance W tan , where W is the distance between trusses and the skew angle. This aligns panels and maintains perpendicular connections of floorbeams to the trusses (Fig. 13.17). If such a layout is possible, there is little difference in cost and skewed spans and normal spans. Design principles are similar. If the skewed distance is less than the panel length, it might be possible to take up the difference in the angle of inclination of the end post, as shown in Fig. 13.17. This keeps the cost down, but results in trusses that are not symmetrical within themselves and, depending on the proportions, could be very unpleasing esthetically. If the skewed distance is greater than the panel length, it may be necessary to vary panel lengths along the bridge. One solution to such a skew is shown in Fig. 13.18, where a truss, similar to the truss in Fig. 13.17, is not symmetrical within itself and, again, might not be esthetically pleasing. The most desirable solution for skewed bridges is the alternative shown in Fig. 13.17.
Skewed bridges require considerably more analysis than normal ones, because the load distribution is nonuniform. Placement of loads for maximum effect, distribution through the floorbeams, and determination of panel point concentrations are all affected by the skew.
Unequal deflections of the trusses require additional checking of sway frames and floor system connections to the trusses.
