Civil Engineering Reference
In-Depth Information
Figure 9.32
Dong Po South Bridge (Image: David Benaim)
The bridges were to be built in balanced cantilever. All the intermediate piers
consisted of four columns that were built into the deck, providing a stable base for
construction. An 'N' truss confi guration was chosen for the webs, which allowed a
module of 5 m that was suitable for the method of construction. The thickness of the
truss members was fi xed at 800 mm. The width of the members varied according to
the shear forces acting on the deck, Figure 9.33 (b). For the 160 m long spans, the
webs became solid at a point about 30 m from the pier, and then thickened outwards
to 1,050 mm for the last 20 m. Figure 9.34 shows the shear force envelope for the
family of decks.
The inclined members of the truss were in compression and the vertical members
were in tension. The principal diffi culty in designing these trusses was the presence
of secondary bending moments in the members. These bending moments were both
in the plane of the truss, due to its defl ections and to the end moments of the top
and bottom booms, and out of plane, due to the end moments of the top and bottom
slabs. During design, it was discovered that the in-plane secondary moments could
be signifi cantly reduced by rotating the diagonal such that it met the vertical slightly
inside the point of conjunction of the truss members (below the upper point and above
the lower point of conjunction), Figure 9.35 (a).
The tension verticals were prestressed with between one and four tendons, each
of 19/15 mm strand. These tendons accomplished three tasks. Firstly they carried the
direct tension in the truss member. Secondly, they were displaced about the member
centre line in the plane of the truss to help carry the in-plane secondary moments.
Thirdly they were displaced out of the plane of the truss to help carry the out-of-plane
moments, Figure 9.35 (b).
It was found to be impractical to design the truss members under their combination
of direct load and biaxial bending to criteria of zero tensile stresses. Stresses under
working dead loads were designed to be entirely compressive, while under the effects
of live loads the members were designed at the ULS, with a limitation on crack width
at the SLS.
