Page 396 - Forensic Structural Engineering Handbook

STEEL STRUCTURES 11.27

therefore, is accounted for using the Miner’s effective stress range method to calculate the
appropriate effective stress range (ΔS ), as follows:
eff
n
ΔS eff = ⎡ ⎣∑ γ i S ⎤ 1 n (11.14)
i⎦
where γ is the ratio of cycles at the i-th load sequence to the total number of cycles, S is
i
i
the stress range for the i-th load sequence, and n is equal to 3.0 when Miner’s cumulative
damage rule is used.
Equation (11.14) can be used to define the total variable cycles N when substituted into
v
Eq. (11.3), N replacing N, and ΔS replacing S . The greatest uncertainty in fatigue dam-
v
r
eff
age estimates lies in the actual service load history of the structural system. The resistance
estimates provided by Eq. (11.11) are better defined than the service loads to which most
systems are subjected.
Generally, members subjected to cyclic loading have mandatory values of absorbed
Charpy V-notch (CVN) energy to ensure that fatigue cracking does not result in premature
fracture. This requirement serves to screen out the use of steels that have a higher risk of
brittle fracture because of notch sensitivity. Fracture toughness is highly desirable in
weld metals as they are far more likely to have defects such as lack of fusion, slag, and
gas pockets, and crack-like discontinuities when backing bars are used. For instance,
weld toughness played a major role in beam-column connection failures during the
Northridge earthquake.
Design for Corrosion Control
Corrosion often develops in steel members when dirt and debris are allowed to accumulate
and cause active corrosion cells due to wetting and lack of water control and the presence
of other contributors to electrochemical cell activity, such as salts, bird guano, and other
contaminants. Failure to keep exposed structural elements clean has been a problem with
bridges, garages, stadiums, and many other structural systems. Atmospheric corrosion can
also lead to deterioration, but this depends on the corrosivity of the atmosphere at a specific
location. Environmental protection laws have resulted in significant changes in atmos-
pheric conditions at many locations. For example, the atmospheric conditions in the
Bethlehem, Pennsylvania, area were observed to be about the same as those in a rural envi-
ronment in the late 1980s rather than an industrial environment cited in the 1960s, 39,40 even
though an integrated steelmaking facility remained in operation throughout that period.
Weathering steel has generally performed well when the environment was not a severe
marine or industrial atmosphere and water was controlled. A protective oxide coating that
acts as a self-healing barrier to further corrosion will form and will prevent significant loss.
However, dirt and debris and lack of water control can result in a poultice and active cor-
rosion cell loss.

Redundancy

Most structures have a degree of redundancy. This can provide alternate load paths should
a local failure occur. In these cases, the damage is limited, and the local failure does not
result in collapse. In many cases, the redundancy of the system is enhanced by contribu-
tions that are not even accounted for in the design model. For example, roof decking may
act compositely with the supporting members due to friction, tack welds, and other minor
contributions.