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63
in the deck, shear studs in the negative moment regions pro- Bridge engineers may encounter situations beyond the
vide an essential element of the required load path (Carden range of values investigated in the parametric study. The fol-
et al., 2003). Concerns about fatigue in the shear stud welds lowing brief discussion addresses these situations.
in negative moment regions need not prevent welding to the
top flange altogether.
3.5.1.1 Span Length L > 60 m (200 ft)
3.4.5 Shear Connector Impact There is no reason to expect that spans longer than 60 m
(200 ft) would not behave similarly to 200 ft spans. The span
At first glance, one might expect a wider effective slab length limit could be relaxed, since in the parametric study
width to cause more demands on the shear connectors, in order presented herein it was found that the longer the span (or,
to develop that wider effective slab. The impact of wider beff more accurately, the greater the length/width ratio), the more
on shear connector layout, however, is surprisingly minimal. we can be sure that the full width is effective. The reason for
The reason for this minimal impact apparently stems from specifying the 60 m (200 ft) span length as a limit is that the
offsetting effects. Shear connectors are designed to resist the parametric study did not consider longer spans.
longitudinal shear flow and are typically governed by fatigue
rather than strength. At the fatigue limit state, elastic analy-
sis is performed, where the longitudinal shear flow is given 3.5.1.2 Girder Spacing S > 4.8 m (16 ft)
by the familiar equation VQ/I, where I is the moment of iner-
tia of the short-term composite section and Q is the first In the parametric study conducted herein, a small number
moment of the transformed area of the slab. A wider beff of cases were analyzed with S > 4.8 m (up to S = 7.6 m). For
increases both Q and I, thus producing offsetting effects. those few cases, there was no indication that effective width
Several bridges with wide (4.8 m = 16 ft) girder spacings should be taken as less than full width. However, they were
were investigated regarding their shear connector layout in only a few cases.
Appendix L. The most significant impact on shear stud lay-
out was for the longest spans investigated (60 m = 200 ft).
Even in this case, however, the required shear stud pitch
3.5.1.3 Skew Angle > 60º
decreased only 10 percent.
In the parametric study, no cases were analyzed with skews
greater than 60 degrees. What happened with the 60-degree
3.5 PROPOSED DESIGN CRITERIA skews analyzed was that although effective width was typi-
cally somewhat less than full width, moments extracted from
3.5.1 Slab-on-Girder Bridges the FEM model were less than moments that would be pre-
dicted by a line-girder analysis (with AASHTO 2004 skew
Based on the impact assessment of various candidate correction factors for the transverse live-load distribution fac-
effective width criteria according to Process 12-50 principles tors). Thus, if the designer assumed full width but also used
using Rating Factor as the measure of comparison, the addi- line-girder analysis, there were offsetting errors. The small
tional accuracy achieved by the more complicated formula- impact of these offsetting errors on rating factor were such that
tions is minimal. Thus, this simple formulation is recom- they allowed use of full effective width. Presumably, such off-
mended instead: "for both interior and exterior girders setting errors could reasonably be expected for skews greater
designed to be composite sections, the effective flange width than 60 degrees. Of course, if the ongoing NCHRP Project
may be assumed equal to the physical flange width." This 12-62 develops more significant skew correction factors for
recommendation should be limited to the parameter range the AASHTO LRFD transverse live-load distribution factors,
used in the parametric study on which it is based: then there may not be such offsetting effects.
· Girder spacing S 4.8m (16 ft)
· Span Length L 60m (200 ft) 3.5.2 Cable-Stayed Bridges
· Skew Angle 60°
In light of the results tallied in Table 11 for the first four ana-
The skew angle here is defined as it is in AASHTO lyzed cable-stayed bridges and the longitudinal variation of
LRFD Chapter 4, such that 0 deg skew is a right bridge align- effective slab width seen in Figures 62 through 72, a reason-
ment. Further discussion of the rationale and justification for able and conservative lower bound set of effective width val-
this recommendation is provided in Appendix M. ues for cable-stayed bridges may be summarized as follows: