Engineering and Construction

The majority of bridges fall within the category of short span  because for many crossings of rivers, railways or secondary highways a single span of less than 30m is sufficient. For multiple-span viaducts a decision on span length must be made,which depends on factors shown in Table 4.4.

For long viaducts it is necessary to carry out comparative estimates for different spans to determine the optimum choice, as shown in Fig. 4.1.

Typical optimum spans are shown in Table 4.5.

Long spans are usually adopted only when a larger number of shorter spans are precluded by restrictions of the site. This is because the material content and cost per unit length of long spans is much greater.Therefore the bridging of a wide river or estuary should generally use a number of medium spans. Only if there is a high risk of shipping collision, or if the depth and speed of water flow is such as to make foundation construction very difficult, should a long-span bridge be chosen.

Table 4.4 Factors which decide choice of span for viaducts

Fig. 4.1 Choice of optimum span for viaducts

Table 4.5 Typical optimum span ranges (m) for viaducts