Civil Engineering Reference
In-Depth Information
The validity of this range of limitations is demonstrated in Section 5.2.4.3, particularly by
Figure 5.7. In Chapter 5,
90
◦
−
θ
α
r
is used and
α
r
=
.
2.3.5 Design Example
The ACI design procedures for shear and torsion are given in a flow chart in Figure 2.17.
This flow chart procedure can be used to design the shear and torsional reinforcement of
a guideway girder. A 3.66-m-wide (12 ft) and 1.27-m-deep (4 ft. 2 in.) box girder with
overhanging flanges (Figure 2.18a), was designed as an alternative to the double-tee girder
used in the Dade County Rapid Transit System, Florida. The standard prestressed girder in
this 35.4 km (22 mile) guideway is simply supported and 24.4 m (80 ft) long. It is prestressed
with sixty-two 270 K, 12.7 mm (1/2 in.), seven-wire strands as shown in Figure 2.18(b). The
total prestress force is 6076 kN (1366 kips) after prestress loss. The design of flexural steel is
omitted for simplicity. The net concrete cover is 3.81 cm (1.5 in.) and the material strengths
are
f
c
=
48.2 MPa (7000 psi) and
f
y
=
413 MPa (60 000 psi).
Sectional Properties
L
=
24
.
08 m (79 ft)
h
=
1
.
270 m (50 in
.
)
d
=
1
.
016 m (40 in
.
)at0
.
3
L
from support
t
=
251 mm (9
.
875 in
.
) average of stem width
b
w
=
502 mm (19
.
75 in
.
) for two stems
10
3
mm
2
(2361
2
)
A
=
1523
×
.
4in
.
10
6
mm
4
4
)
I
=
319 800
×
(768 336 in
.
y
t
=
517 mm (20
.
34 in
.
)
y
b
=
753 mm (29
.
66 in
.
)
Loading Criteria
The standard girders are designed to carry a train of cars, each 22.86 m (75 ft) long. Each car
has two trucks with a center-to-center distance of 16.46 m (54 ft). Each truck consists of two
axles 1.981 m (6 ft 6 in.) apart. The crush live load of each car is 513.7 kN (115.5 kips). The
maximum amount of web reinforcement was obtained at the section 0.3
L
from the support
under a derailment load, which consists of two truck loads located symmetrically at a distance
3.20 m (10 ft) from the midspan (Figure 2.19b). Each axle load is taken as one-fourth of the
crush live load with 100% impact and a maximum sideshift of 0.914 m (3 ft). The load factor
is taken as 1.4.
The girder is also subjected to a superimposed dead load caused by the weight of the track
rails, rail plinth pads, power rail, guard rail, cableway, acoustic barrier, etc. At derailment,
this superimposed dead load is assumed to produce a uniform vertical load of 12.84 kN/m
(0.88 kip/ft) and a uniformly distributed torque of 3.158 kN m/m (0.71 kip ft/ft). This torque
is neglected in the calculation because the magnitude of the distributed torque is small and
because the torque is acting in a direction opposite to the derailment torque.
