SECTION  13   FATIGUE
13.1 General - This section applies to the design of structures and structural elements subject to loading which could lead to fatigue failure. The following effects are not considered in the section.
a) Corrosion fatigue
b) Low cycle (high stress) fatigue
c) Thermal fatigue
d) Stress corrosion cracking
e) Effects of high temperature (> 150o C)
f) Effects of low temperature (< brittle transition temperature)
13.1.1 Basis - For the purpose of design against fatigue, different details (of members and connections) are classified under different fatigue class. The design stress range corresponding to various number of cycles, are given for each fatigue class. The requirements of this section shall be satisfied with, at each critical location of the structure subjected to cyclic loading, considering relevant number of cycles and magnitudes of stress range expected to be experienced during the life of the structure.
13.2Definitions
Constant Stress Range - The amplitude between which the stress ranges under cyclic loading is constant during the life of the structure or a structural element.
Cumulative Fatigue - Total damage due to fatigue loading of varying stress range.
Cut-off Limit - The stress range, corresponding to the particular detail, below which cyclic loading need not be considered in cumulative fatigue damage evaluation (corresponds to 108 numbers of cycles in most cases).
Design Life -Time Period for which a structure or a structural element is required to perform its function without fatigue damage.
Design Spectrum - Frequency distribution of the stress range from all the nominal loading events during the design life.
Detail Category - Designation given to a particular detail to indicate the S-N curve to be used in fatigue assessment.
Fatigue - Damage caused by repeated fluctuations of stress, leading to progressive and consequent cracking of a structural element.
Fatigue Strength - Stress range for a category of detail, depending upon the number of cycles it is required to withstand.
S-N curve – Curve, defining the relationship between the number of stress cycles to failure  (Nsc)at a constant stress range (Sc), during fatigue loading parts of a structure.
Stress Cycle Counting - Sum of individual stress cycles from stress history arrived at using any rational method.
Stress Range - Algebraic difference between two extremes of stresses in a cycle of loading.
Stress Spectrum - Histogram of stress cycles produced by a nominal loading event.
13.3 Design
13.3.1 Reference Design Condition - The standard S-Ncurves for each detail category are given for the following conditions:

• The detail is located in a redundant load path, wherein local failure at that detail alone will not lead to overall collapse of the structure.
• The nominal stress history at the local point in the detail is estimated/evaluated by a conventional method without taking into account the local stress concentration effects due to the detail.
• The load cycles are not highly irregular.
• The details are accessible for and subject to, regular inspection.
• The transverse fillet or butt weld connects plates of thickness not greater than 25 mm.           

The values obtained from the standard S-N curve shall be modified by a capacity reduction factor mr, when any of the above conditions do not apply. The capacity reduction factor, mr to account for the detail in a non-redundant load path shall be taken as 0.70. To account for thickness of plates greater than 25 mm and joined together by transverse fillet welding, the capacity reduction factor shall be modified by the factor
mr = (25/tp)0.25  £ 1.0
where
tp = actual thickness in mm of the thicker plate being joined
The two factors are applied cumulatively when the both the conditions are encountered. Fatigue need not be investigated if condition in 13.3.2.3, 13.6.1, or 13.7 is satisfied.
13.3.2 Design Spectrum
13.3.2.1 Stress Evaluation- The design stress shall be determined by elastic analysis of the structure to obtain stress resultants and the local stresses may be obtained by a conventional stress analysis method. The normal and shear stresses shall be determined considering all design actions on the members, but excluding stress concentration due to the geometry of the detail. The stress concentration, however, not characteristic of the detail shall be accounted for in the stress calculation.
In the fatigue design of trusses made of members with open sections in which the end connections are not pinned, the stresses due to secondary bending moments shall be taken into account, unless the slenderness ratio, (KL/r), of the member is less than 40.
In the determination of stress range at the end connections between hollow sections, the effect of connection stiffness and eccentricities may be disregarded, provided
a) The calculated stress range is multiplied by appropriate factor given in Table 13.1(a) in the case of circular hollow section connections and Table 13.1(b) in the case of rectangular hollow section connections.
b) The design throat thickness of fillet welds in the joints is greater than the wall thickness of the connected member.
TABLE 13.1(a) MULTIPLYING FACTORS FOR CALCULATED STRESS RANGE
(Circular Hollow Sections)
(Section 13.3.2.1)

TABLE 13.1(b) MULTIPLYING FACTORS FOR CALCULATED STRESS RANGE
(Rectangular Hollow Sections)
(Section 13.3.2.1)

13.3.2.2 Design Stress Spectrum - In the case of loading events producing non-uniform stress range cycle, the stress spectrum may be obtained by a rational method, such as “Rain flow counting” or an equivalent method.
13.3.2.3 Low Fatigue - Fatigue assessment is not required for a member, connection or detail, if normal and shear design stress ranges, f, satisfy the following conditions:

or if the actual number of stress cycles, NSC, satisfies

where
gmft , gfft = partial safety factors for strength and load, respectively (13.3.3)
f = actual fatigue stress range for the detail
13.3.3 Partial Safety Factors
13.3.3.1 Partial Safety Factor for Actions and their effects (gfft) - Unless and otherwise the uncertainty in the estimation of the applied actions and their effects demand a higher value, the partial safety factor for loads in the evaluation of stress range in fatigue design shall be taken as 1.0.
13.3.3.2 Partial Safety Factor for Fatigue Strength (gmft) - The partial safety factor for strength is influenced by consequences of fatigue damage and level of inspection capabilities.

Based on consequences of fatigue failure, component details have been classified as given in the Table 13.2.

• Fail-safe structural component/details is the one where local failure of one component does not result in the failure of the structure due to availability of alternate load path (redundant system).
• Non-fail-safe structural component/detail is the one where local failure of one component leads rapidly to failure of the structure due to its non-redundant nature.

TABLE 13.2 PARTIAL SAFETY FACTORS FOR FATIGUE STRENGTH (gmft)

13.4 Detail Category
Table 13.3 indicates the classification of different details into various categories for the purposes of assessing fatigue strength.
13.5 Fatigue Strength
The fatigue strength of the standard detail for the normal or shear fatigue stress range, not corrected for effects discussed in 13.3.1, is given below (Fig 13.1,13.2):

Normal stress range

when

when

            Shear stress

where
ff, tf = design normal and shear fatigue stress range of the detail, respectively, for life cycle of NSC 
ffn,tfn = normal and shear fatigue strength of the detail for 5 x106 cycles

TABLE 13.3 (1) DETAIL CATEGORY CLASSIFICATION GROUP 1 NON-WELDED DETAILS

(Section 13.4)

   Note: The arrow indicates the location and direction of the stresses acting in the basic material for which the stress range is    to be calculated on a plane normal to the arrow

TABLE 13.3 (2) DETAIL CATEGORY CLASSIFICATION GROUP 2 WELDED DETAILS-NOT IN HOLLOW SECTIONS 
(Section 13.4)

Note: The arrow indicates the location and direction of the stresses acting in the basic material for which the stress range is to be calculated on a plane normal to the arrow
(continued)

TABLE 13.3 (2) (continued)

Note: The arrow indicates the location and direction of the stresses acting in the basic material for which the stress range is to be calculated on a plane normal to the arrow                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            (continued)

TABLE 13.3 (2) (continued)

Detail Category	Constructional Details
	Illustration (See Note)	Description
66		TRANSVERSE BUTT WELDS (COMPLETE PENETRATION) Welds run-off tabs to be used, subsequently removed and ends of welds ground flush in the direction of stress. Welds to be made from two sides (19) Transverse splices of plates, rolled sections or plate girders. (20) Transverse splice of rolled sections or welded plate girders, without cope hole. With cope hole use Detail Category 52, as per (15). (21) Transverse splices in plates or flats being tapered in width or in thickness where the taper is £1:4.
59		TRANSVERSE BUTT WELDS (COMPLETE PENETRATION) Weld run-off tabs to be used, subsequently removed and ends of welds ground flush in the direction of stress. Welds to be made from two sides. (22) Transverse splices as per (21) With taper in width or >1:4. £1:2.5.
52		TRANSVERSE BUTT WELDS (COMPLETE PENETRATION) (23) Transverse butt-welded splices made on a backing bar. The end of the fillet weld of the backing strip shall be greater than 10 mm from the edges of the stressed plate. (24) Transverse butt welds as for (23) With taper on width or thickness <1:2.4.

TABLE 13.3 (2) (continued)

Note: The arrow indicates the location and direction of the stresses acting in the basic material for which the stress range is to be calculated on a plane normal to the arrow

(continued)

TABLE 13.3 (2) (continued)

Note: The arrow indicates the location and direction of the stresses acting in the basic material for which the stress range is to be calculated on a plane normal to the arrow
(continued)

TABLE 13.3 (2) (continued)

TABLE 13.3 (3) DETAIL CATEGORY CLASSIFICATION GROUP 3 BOLTS
(Section 13.4)

Note: The arrow indicates the location and direction of the stresses acting in the basic material for which the stress range
is to be calculated on a plane normal to the arrow.