Civil Engineering Reference
In-Depth Information
less frequently used due to uplift considerations (related to the large live load to dead
load ratio of steel railway bridges) and stresses imposed by foundation movements
that may not be readily detected.
Box girders are also relatively rare in railway bridge construction due to welded
stiffener fatigue issues but may be used where large torsional stiffness is required
(e.g., curved bridges).
HybridgirdersusingbothHPSandHSLAsteels(seeChapter2)maybeeconomical
for some long span applications (particularly over supports of continuous spans where
flexural and shear stresses may be relatively high).
However, most ordinary steel railway span designs will be governed by deflection
(which is stiffness related) and fatigue requirements (see Chapter 5), which are both
material independent for structural steel. These criteria and fabrication issues must
be carefully considered when assessing the use of hybrid girders with plate elements
of differing steel type and grade.
Themainelementsofaplategirder(flangeplates,webplates,mainelementsplices,
bearing stiffeners, and their respective connections) are designed to resist tensile and
compressive normal axial and bending stresses, and shear stresses in the cross section.
In ASD, secondary elements (stiffener plates) are designed to provide stability to the
main elements of the girder cross section.
7.2.6.1
Main Girder Elements
Flanges of modern welded plate girders are made using a single plate in the cross
section. The required thickness and width of the flange plates will be governed by
strength, stability, fatigue, and serviceability criteria. Cover plates should not be used
as flange plate dimensions may be varied along the length of the girder as required.
The thickness of flange plates may be limited by issues relating to steel-making and
subsequent fabrication effects on lamellar tearing and toughness.
Modern steel-making processes such as TMCP may alleviate many of the metal-
lurgical concerns relating to thick plates but designers should carefully review typical
welding details, materials, and procedures used for fabrication when considering
maximum plate thickness. Most modern railway girder bridges of span less than 150
can be designed with flanges less than 3 in. thick, which should generally preclude
concerns with respect to welded fabrication and through-thickness restraint.
Webs of plate girders are generally relatively thin plates § when designed in accor-
dance with shear strength criteria. However, because of slenderness, the required
thickness of the web plate also depends on flexural and shear stability considerations.
Therefore, in order to avoid thick webs, the plates are often stiffened longitudinally
Designers should note that it is often less costly to fabricate flanges without changes in dimension due to
butt welding and transitioning requirements. Designs that utilize varying flange plate widths and lengths
may be desirable and economical for long span fabrication, but consultation with experienced bridge
fabricators is often warranted.
Lamellar tearing occurs in thick plates due to large through-thickness strains produced by fabrication
process effects such as weld metal shrinkage at highly restrained locations (i.e., joints and connections).
Ductility is affected by the triaxial strains created by weld shrinkage and restraint in thick plates (see
Chapter 2).
§ Particularly for longer spans typical of railway plate girders.
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