Civil Engineering Reference
In-Depth Information
are likely to require heavy reinforcement along the entire length of the deck and to lead
to the webs being cracked also along their full length. Furthermore this eccentricity
creates rotations of the bottom corner of the box, creating stress concentration at
the outside of the launch bearings. In order to minimise these problems, one option
is to widen outwards the base of the web. In trapezoidal box girders that results in a
short vertical plinth to the web, Figure 15.38 (b), while in rectangular box girders an
'elephant's foot' thickening results, Figure 15.38 (c).
When this is not desirable, the web may be thickened internally with a vertical fi llet,
Figure 15.38 (d). This reduces the eccentricity at the base of the web and creates a kink
in the axis of the web, causing a horizontal resultant under the effect of the launch
bearing reaction that induces additional bending moments in the web, Figure 15.38 (e).
In general, the concentrated reaction of the launch bearing will have spread over a
suffi cient length of web to make these moments acceptable. It is essential to carry out
this analysis carefully, on the complete box, as signifi cant bending moments are also
created in the bottom slab.
Figure 15.38
Eccentricity of launch bearings beneath web
