chapter4-allowable stresse.ppt

7/30/2019 chapter4-allowable stresse.ppt

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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 thefatigue of structural elements that are subjectedto repeated fluctuations of stresses. Memberssubjected to stresses resulting from fatigue load

shall be designed so that the maximum stress donot exceed the allowable stress given in chapter(2) of the Egyptian steel code of practice andthat the stress range does not exceed theallowable fatigue stress range given in chapter(3) of this code. Wind bracing does not considersubjected 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 betweentwo 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 > 50 years, the

fatigue loads should be increased by factors M.

No ofyear50

80

100

120

M

1.0

1.10

1.15

1.20


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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 andsub-diagonal.

For crane

Depending on the average daily application (ADA) for 50years 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 39for 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) > 2,000,000 and the detailcategory class (A) for rolledsection, 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.

chapter4-allowable stresse.ppt

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