Design of Axial Force Steel Members - Design of Modern Steel Railway Bridges - page 230

For slip-resistant (or friction-type) bolted connections, ∗ the effective net area, A efat ,is
taken as the gross area of only the member elements that are directly connected. S rfat
is the allowable fatigue stress range depending on the number of design cycles, and
connection and fabrication details of the tension member. For design, the number of
cycles is generally assumed at greater than 2,000,000 for single-track bridges with rel-
atively short influence lines (for spans 100 ft as recommended by AREMA, 2008).

Since fatigue design is based on nominal stresses, S rfat is recommended for various
fatigue detail categories (A, B, B ,C,D,E,E , and F) depending on connection or
detail geometry (see Chapter 5). Table 6.1 i ndicates the allowable fatigue stress ranges
used for the design of tension members at connections for greater than 2,000,000
stress cycles. † The allowable fatigue stress ranges for detail categories consider stress
concentrations related to member discontinuities (such as change in section) (Fig-
ure 6.2a) or apertures in the member (such as bearing connection holes, access, or
drainage openings) (Figure 6.2b). The magnitude of the stress concentrations may be
determinedbyelasticitytheoryandfracturemechanicsmethodsorbyuseofpublished
stress concentration factors, ‡ K t , (Pilkey, 1997; Anderson, 2005; Armenakas, 2006).
Member transition fillets of usual dimensions that might be considered for steel
bridge tension members may result in stress concentration factors in the order of
K t =

≤

2.0. Figure 6.2b illustrates that, for the simple case of uniaxial tension in
a flat plate with a single circular hole, stress concentration factors may reach K t =

1.5 −

3.0.
Table6.2 indicatesAREMA(2008)fatiguedesigndetailcategoriesandcorresponding
stress concentration factors (Sweeney, 2006).
However, since the allowable fatigue stress ranges recommended by AREMA
(2008) are based on nominal stress test results, it is usually not necessary to explicitly
consider stress concentration factors in the design of axial tension members unless the
design details are particularly severe. Therefore, the designer must carefully consider
theuseoftransitions,apertures,orotherdiscontinuitiesinthedetailingofaxialtension

TABLE 6.1
Allowable Fatigue Stress Range for Number of Design
Stress Range Cycles > 2,000,000
Member or Connection Condition S rfat (ksi)

Plain member 24

Bolted slip resistant connection 16

Partial penetration groove and fillet welded connection 2.6-10

Full penetration weld connection 2.6-16

∗ Recommendedformainmemberconnectionsandallconnectionssubjecttostressreversaland/orcyclical
loading.
† For welded connections the allowable fatigue stress range depends on type of stress, direction of stress,
direction of weld, weld continuity, and transition details. AREMA (2008) makes recommendations
considering these factors.
‡ Often obtained by photo-elastic testing.