Chapter 5Chapter 5

Fatigue

FATIGUE STRESSESFATIGUE STRESSES

1- General 2- Definitions 3- Fatigue Loads

1. General1. GeneralThis 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.

2. Definitions2. 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.

3. Fatigue Loads3. 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.

       For Roadway Bridge with design lives > 50 years, the fatigue loads should be increased by factors M.

No of year

50 80 100 120

M 1.0 1.10 1.15 1.20

   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.

 

   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.

   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.

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) > 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.