Civil Engineering Reference
In-Depth Information
Figure 12.6
Typical infl uence line for share of mid-span sagging bending moment carried on
rib B
For instance, when working to the UK codes of practice where the three critical loads
are uniformly distributed HA lane loads, HB vehicle loads and a Knife Edge Load, one
would draw three sets of infl uence lines, one for a full span loaded, one for a 10 m
loaded length and one for a point load.
If the load is placed on a rib immediately adjacent to the pier, clearly the loaded rib
does not defl ect, and no load is transferred to the unloaded rib. For instance, if a multi-
axle vehicle is centred on one rib, and placed with the forward axle close to the pier,
the load of that axle will be carried entirely by the loaded rib. Axles placed further
from the pier will distribute a proportion of their load to the adjacent rib.
In the early stages of a design, great accuracy is not required; it is more important that
the basic behaviour of the deck should be understood, and a few well-chosen infl uence
lines will usually suffi ce. For detailed design, the more extensive use of infl uence lines
can make it clear where the deck should be loaded for maximum effect.
12.2.4 Transverse strength and stiffness of decks without pier diaphragms
It is always necessary to check that the horizontal strength and stiffness of decks
without diaphragms are adequate. For highway decks up to about 2 m deep subjected
