Civil Engineering Reference
In-Depth Information
6.2.4 D
ESIGN OF
A
XIAL
T
ENSION
M
EMBERS FOR
S
TEEL
R
AILWAY
B
RIDGES
Tension members in steel railway bridges may be comprised of eyebars, cables, struc-
tural shapes, and built-up sections. Eyebars are not often used in modern bridge
superstructure fabrication, and suspension or cable-stayed bridges are not common
for freight railway structures due to flexibility concerns (see Chapter 1). Structural
shapes such as W, WT, C, and angles are frequently used for steel railway bridge
tension members.
It is often necessary to fabricate railway bridge tension members of several struc-
tural shapes due to the large magnitude railroad loads and tension members that
undergo stress reversals. Bending effects (see Chapter 8) and connection geometry
constraints may also dictate the use of built-up tension members. The components
must be adequately fastened together to ensure integral behavior of the tension mem-
ber. In cases where a box-type member is undesirable, such as where the ingress of
water is difficult to preclude, open tension members are used. Built-up open tension
members are often fabricated with lacing bars and stay (tie or batten) plates or per-
forated cover plates.
∗
Shear deformation in tension members, which is primarily due
to self-weight and wind loads, is relatively small andAREMA (2008) recognizes this
by providing nominal recommendations for lacing bars and stay plates.
Lacing bar width should be a minimum of three times the fastener diameter to
provide adequate edge distance and the thickness for single flat bar lacing bars should
be at least 1/40 of the length
†
for main structural members and 1/50 of the length for
bracing members. Stay plates should be used at the ends of built-up tension members
and at intermediate locations where lacing bar continuity is interrupted due to the
connection of other members.
‡
The length of the stay plates at the ends of laced bar
built-up tension members must be at least 85% of the distance between connection
lines across the member. The length of the stay plates at intermediate locations of
laced bar built-up tension members must be at least 50% of the distance between
connection lines across the member.
The thickness of perforated cover plates should be at least 1/50 of the length
between closet adjacent fastener lines. Perforated cover plate thickness is based on
transverse shear,
V
, at centerline of the cover plate. The maximum transverse shear
stress,
τ
V
, at the center of the cover plate is
3
V
2
bt
pc
,
τ
V
=
(6.10)
where
b
is the width of the perforated cover plate and
t
pc
is the thickness of the
perforated cover plate. Therefore, the longitudinal shear force,
V
, over the distance
between centers of perforations or apertures,
l
p
,is
3
Vl
p
2
b
V
= τ
V
(l
p
t
pc
)
=
(6.11)
∗
Perforated cover plates are most commonly used for modern built-up truss members.
†
Lacing bar length is the distance between fastener centers.
‡
For example, stay plates are used each side of members that interrupt lacing bars.
