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behavior and stresses near the crack area, whereas the local model was
used to find the stress concentration factor (SCF) at the exact crack loca-
tion, called hot spot.
Stress concentration factor.
For steel bridges, weld toes are usually the
critical fatigue damage regions. However, the monitoring sensors installed
in bridges are not necessarily located in these areas. To know the hot spot
stress near the weld toe, it is necessary to convert the nominal stress obtained
from the monitoring sensors into the corresponding hot spot stress near the
weld toe for fatigue life evaluation. The SCF is defined as the ratio of the hot
spot stress value to that of the nominal stress and can be calculated from
stress values obtained from the global model of coarse mesh and the local
model of refined mesh. With the SCF value obtained, the hot spot stress can
be obtained by multiplication of the nominal stress with the SCF value. The
SCF value of a welded joint is commonly obtained by experiment or numeri-
cal finite element method.
Global model
. A 3D model of the Southbound consisting of eight I-girders
as shown in Figure 7.37 was generated by CSiBridge (2011). The concrete
deck, eight I-girders, and connection plates, which connect diaphragms and
girder webs, were modeled by shell elements, whereas all the diaphragms
were modeled by truss elements. The translations of
x
-,
y
-, and
z
-directions
are fixed at the abutments. To locate the hot spot, a global model refined
mesh around the hot spots was built for analysis, and a detailed view of this
global model is presented in Figure 7.38.
Figure 7.37
Global model of I-270 Middlebrook Bridge.
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