Civil Engineering Reference
In-Depth Information
6.12 Dead and live loads
Dead loads include the self weight of the deck and any deck fi nishes such as road
surfacing, footpaths and parapets.
For this exercise the density of concrete is assumed to be 25 kN/m
3
; consequently
the self weight is 25 × 5.66 = 141.5 kN/m.
The fi nishes may be assumed for the fi rst stages of a preliminary design to be
represented by a uniform thickness of concrete of 150 mm, and hence their dead load
is 25 × 0.15 × 12 = 45 kN/m.
Live loads are principally the weight of traffi c which must include the appropriate
dynamic enhancement. Other effects of traffi c loading are centrifugal force on curved
decks, braking and traction forces that are particularly signifi cant on railway bridges,
and lateral nosing on railway bridges.
For this example, the live load is assumed to be a distributed load of 100 kN/m, of
any length, which is assumed to include the dynamic enhancement.
6.13 Bending moments
6.13.1 Bending moments due to live loads
The 100 kN/m live load is arranged to maximise the effect on the design section being
considered by loading only areas of the infl uence lines that are of the same sign. For
mid-span and support sections, the loaded lengths are always complete spans. For
some other design sections, part spans may be loaded.
6.13.2 Bending moments due to differential settlement
The bending moments in continuous bridges are affected by differential settlement of
the foundations. The settlement of a foundation is made up of several components.
a) Thefi rst is the purely geotechnical component of settlement due to the response
of the ground beneath the foundation. This not only concerns the soil or rock
immediately beneath the foundation, but also may involve deeper layers.
In principle, a reasonable calculation of the total and differential settlement due
to the response of the ground beneath a foundation may be made, albeit with a
degree of uncertainty inherent in all geotechnical calculations. If the foundation
conditions are extremely variable, for instance when adjacent piers are founded on
different strata, or foundations alternate between piles and pads, the uncertainties
in the calculation of differential settlement will be much greater.
For instance, the River Nene Bridge on the Nene Valley Way in Northampton
was founded on pads resting on a layer of gravel, Figure 6.9 and Figures 11.16
and 11.17. (The bridge is further described in
11.6.
) Although the gravel was
relatively consistent, and would not have given rise to signifi cant differential
settlement, it was underlain by a thick layer of clay. The pressure bulb beneath
the relatively small pier foundations was confi ned to the gravel, giving rise to
only small settlements. However, the pressure bulb of the 30 m wide approach
embankments extended deep into the clay, causing substantial settlement of the
bridge abutments. The design of the bridge was dominated by the differential
