Civil Engineering Reference
In-Depth Information
A rule of thumb for making a preliminary estimate of the total web thickness
required for a continuous bridge when designing to the UK loading code (45 units of
HB loading) is
t
=
LB
/200
H
, where
t
is the total web thickness,
L
is the span,
B
is the
width of the deck and
H
is the height of the deck at the pier. This assumes that the
concrete cube strength is at least 45 MPa and that prestress ducts are in single fi le in
the web. For stronger concrete and external prestress the formula is conservative.
For codes of practice that consider shear at the SLS, the minimum thickness is
governed by the principal tensile stress. The rule of thumb is not applicable to such
cases.
d) Width of webs near the quarter points of continuous box girders
Between the pier and mid-span, the thickness of the web may vary. The rate at which
the thickness can be reduced depends critically on the detailed design of the prestress.
At the supports the prestress centroid is high, while at mid-span it is low. This gives
the opportunity to incline the tendons in elevation, creating shear of a sign opposite to
that due to applied loads. The profi le of selected tendons may be designed such that it
is possible to shorten the length of web that needs to be thickened, saving weight and
materials, Figure 9.29
.
The changes in thickness of the webs of box girders along their length add signifi cantly
to their cost of construction. One of the main benefi ts of using very high strength
concrete for bridge decks is the possibility of keeping the web thickness constant along
the span, without adding too much to the deck weight at mid-span.
9.5.3 Vertical prestressing in webs
When designing to codes of practice that require the principal tensile stress to be
limited, it is sometimes cost effective to reduce it by the addition of vertical web
prestress. However, prestressing strand extends during stressing by 6 mm/m, and
prestressing bars by 3.5 mm/m. Thus for a 2.5 m deep web, prestressing strand will
Figure 9.29
Use of inclined prestress to shorten web thickening
