Civil Engineering Reference
In-Depth Information
Dead load and structure depth are reduced, but dynamic forces can be large. Direct
fixation decks are generally not used in freight railway bridges and require careful
design and detailing (Sorgenfrei and Marianos, 2000).
3.3.2.2
Bridge Framing Details
Open deck through plate girder, through truss, and some deck truss spans have floor
systems composed of longitudinal stringers and transverse floorbeams. Ballasted
deck through plate girder spans generally have the concrete or steel plate decks sup-
ported on closely spaced transverse floorbeams framing into the main girder or truss
(Figure3.14b).
Insomecases,stringerswithlesscloselyspacedtransversefloorbeams
are used.
∗
Stringers should be placed parallel to the longitudinal axis of the bridge, and
transversefloorbeamsshouldbeperpendiculartomaingirdersortrusses.Stringersare
usuallyframedintothefloorbeamsandhaveintermediatecrossframesordiaphragms.
Floorbeams should frame into the main girders or trusses such that lateral bracing
may be connected to both the floorbeam and main member. The end connections
of stringers and floorbeams should generally be made with two angles
†
designed to
ensure flexibility of the connection in accordance with the structural analysis used.
Due to cyclical live load stresses, welded end connections should not be used on
the flexing leg of connections (see Chapter 9). Freight railway bridge spans should
have end floorbeams, or other members, designed to permit lifting and jacking of the
superstructure without producing stresses in excess of 50% of the basic allowable
stresses (see Chapter 4). Multiple beams, girders, and stringers should be arranged to
equally distribute live load to all the members.
Redundancy, particularly for FCM, is an important consideration in modern steel
railway bridge design.Although costly from a fabrication perspective, internal redun-
dancy can be achieved by the use of bolted built-up members. Structural redundancy
can be achieved through establishing alternate load paths with additional members.
For example, an open deck steel truss designer may decide to use two rolled beam
stringers per rail instead of one stringer per rail. The nonredundant system of a single
stringer per rail will require higher material toughness, and more stringent welding
procedures and inspection if a built-up member is required.
3.3.2.3
Bridge Bearings
Freight railway spans that are 50 ft or greater in length should have fixed and expan-
sion bearings that accommodate rotation due to live load and other span deflections.
‡
All spans should also have provision for expansion to accommodate horizontal
∗
For example, when ballasted decks are used on through truss spans.
†
Where floorbeams frame into girder webs at transverse web stiffener locations, it is often acceptable
to use an angle connection on one side of the floorbeam and on the other side to directly connect the
floorbeam web to the outstanding leg or plate of the girder web stiffener. This requires careful coping
(or blocking) of the top and bottom flanges of the floorbeam.
‡
For example, rotations due to bridge skew, curvature, camber, construction misalignments and loads,
support settlements, and thermal effects.
