Fatigue

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Chapter 5

• Fatigue

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FATIGUE STRESSES

• 1- General
• 2- Definitions
• 3- Fatigue Loads

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1. General

• This chapter presents a general method for the
  fatigue of structural elements that are subjected
  to repeated fluctuations of stresses. Members
  subjected to stresses resulting from fatigue
  load shall be designed so that the maximum stress
  do not exceed the allowable stress given in
  chapter (2) of the Egyptian steel code of
  practice and that the stress range does not
  exceed the allowable fatigue stress range given
  in chapter (3) of this code. Wind bracing does
  not consider subjected to fatigue load.

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2. Definitions

• Fatigue damage in member (gradual crack
  propagation) caused by repeated live load (stress
  fluctuations).
• Design Life the period which the structure will
  not fail or require repair.
• Stress Range the algebric difference between two
  extreme values of stress due to fatigue load.

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3. Fatigue Loads

• 1.      Crane Full traveling crane load
  impact.
• 2.      Roadway Bridge 60 of live load
  impact.
• 3.      Railway Bridge Full standard live load
  impact.

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       For Roadway Bridge with design lives gt 50
years, the fatigue loads should be increased by
factors M.
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   For Roadway Bridges Depending on the average
daily truck traffic (ADTT) for 50 years design
life, the number of constant stress cycles (N) is
given in table 3.1a for long members and
transverse members.  
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   For Railway Bridges Divided the bridge to 3
classes Class 1 for chords and main girder
(plate girder) Class 2 for web of truss
bridge Class 3 for transverse floor beams,
vertical of truss and sub-diagonal.  For
crane Depending on the average daily application
(ADA) for 50 years design life, the number of
constant stress cycles (N) is given in table 3.1C
for different operation.
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   High strength Bolts Friction type For H.S.B
friction type and according to (N), the allowable
stress ranges (Fsr) are given in table page 39
for bolts of grade 8.8 10.9. Each structural
element has a particular detail category as shown
in table 3.3 (page 43). The classification is
divided into four parts which correspond to the
following groups Group 1 non-welded details,
plain materials, and bolted plates. Group 2
welded element. Group 3 welds and bolts. Group 4
orthotropic deck bridge details.
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Depending on the details, member (rolled or built
up), connection shape, type of weld, etc., the
code divided the details to eight categories (A,
B, B, C, D, E, E, F). Depending on these
categories and on (N), the allowable stress range
(Fsr) is given in table 3.2 (p. 41) and in Fig.
3.1 (p42). For fatigue consider the number of
constant stress cycles (N) gt 2,000,000 and the
detail category class (A) for rolled section,
class (B) for built up section, class (C) for
high strength bolts in shear, class (D) for
ordinary bolts and class (F) for bolts in
tension.