Structural Analysis and Design of Steel Railway Bridges - Design of Modern Steel Railway Bridges - page 204

5.3.1 S TEEL R AILWAY S UPERSTRUCTURE F AILURE
Strength failure by fracture, yielding, or instability must be precluded. The von-
Mises yield criterion (see Chapter 2) is appropriate for use in elastic strength design
(Armenakas, 2006). Tension, compression, and shear yielding failure are based on the
yield criterion of this failure theory. For structural design, allowable tension, com-
pression, and shear stresses are based on the yield failure stress (typically the yield
failure stress is divided by a safety factor to obtain the allowable stress).Tension mem-
bers must also be designed considering ultimate stress fracture criteria. Compression
members may become unstable prior to yielding and the effect is incorporated into
elasticstrengthdesignproceduresaseffectivereductionstotheallowablecompression
stress (usually expressed as parabolic transition equations). The strength design of
axial members, flexural members, and connections is discussed further in subsequent
chapters.
Serviceability failure occurs as excessive elastic deformation or vibration, or frac-
ture. Allowable service live load deflection criteria based on the length of the span
are recommended by AREMA (2008), which will affect the stiffness design of the
superstructure.Vibration effects on stresses are included in the empirically developed
dynamic load increment (see Chapter 4) and vibration from wind is generally not
a concern for the usually relatively stiff steel railway superstructures. ∗ The deflec-
tion design of steel railway superstructures is discussed in greater detail later in this
chapter.
Failure by fracture can be sudden or caused by accumulated damage from cyclical
application of live loads over time. Sudden or brittle fracture is caused by preexist-
ing flaws (cracks, notches, weld discontinuities, and areas where triaxial stresses are
constrained) that create stress concentrations with high mean normal tensile stresses,
which can cause failure before yielding. † Therefore, the failure may be a sudden frac-
ture without evidence of yielding. Fracture susceptibility is generally more severe
with dynamic loads, thick plates, ‡ and low service temperatures. Design against brit-
tle fracture is accomplished through the use of steel with adequate notch toughness §
for the design service temperature (which depends on geographical location ∗∗ ) and
fabrication quality controls (see Chapters 2 and 3). AREMA (2008) recommends
steel fracture toughness requirements †† for steel members considered as primary and
for FCM. FCM are those members in tension where failure would result in fail-
ure of the entire structure (e.g., nonredundant structural members such as girders and
trusses of many typical steel railway superstructures). The fracture toughness require-
ments for ordinary bridge design are based on relatively simple CVN testing ‡‡ in lieu
∗ Wind vibration is implicitly considered in the design of steel railway spans in accordance withAREMA
(2008) by recommendation of a notional lateral load that provides for the design of sufficiently stiff
lateral bracing systems.
† The von-Mises yield criterion is independent of mean normal (or hydrostatic) stresses.
‡ At a given temperature, thicker plates exhibit lower fracture toughness in elastic-plastic regions (crack
tips) due to plane strain conditions.
§ Toughness can be interpreted as the energy required to cause fracture at a given temperature.
∗∗ Indicated as Zones 1, 2, and 3 in AREMA (2008).
†† Material with adequate toughness to initiate yielding prior to brittle fracture.
‡‡ In North America the CVN tests are generally carried out in accordance with ASTM A673.