Part 6Structural Design — Section 6 Steel

NBC 2016 (SP 7:2016) Volume 1

2016 · PDF page 1042

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a) Corrosion fatigue,

b) Low cycle (high stress) fatigue,

c) Thermal fatigue,

d) Stress corrosion cracking,

e) Effects of high temperature ( $>150^{\circ}C$ ), and

f) Effects of low temperature (< brittle transition temperature).

19.1.1 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.

19.2Design

19.2.1 Reference Design Condition

The standard S-N curves for each detail category are given for the following conditions:

a) Detail is located in a redundant load path, wherein local failure at that detail alone will not lead to overall collapse of the structure.

b) 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.

c) Load cycles are not highly irregular.

d) Details are accessible for and subject to regular inspection.

e) Structure is exposed to only mildly corrosive environment as in normal atmospheric condition and suitably protected against corrosion (pit depth <1 mm).

f) Structure is not subjected to temperature exceeding 150 °C.

g) Transverse fillet or butt weld connects plates of thickness not greater than 25 mm.

19.2.1.1 Thickness correction factor

The values obtained from the standard S-N curve shall be modified by a capacity reduction factor $\mu_{\mathrm{r}}$ , when plates greater than $25~\mathrm{mm}$ in thickness are joined together by transverse fillet or butt welding, given by:

\mu_ {\mathrm{r}} = (2 5 / t _ {\mathrm{p}}) ^ {0. 2 5} \leq 1. 0

where

$t_{\mathrm{p}} =$ actual thickness of the thicker plate being joined, in mm.

No thickness correction is necessary when full

penetration butt weld reinforcements are machined flush and proved free of defect through non-destructive testing.

19.2.1.2 Necessity for fatigue assessment

a) Fatigue assessment is not normally required for building structures except as follows:

Members supporting lifting or rolling loads,
Member subjected to repeated stress cycles from vibrating machinery,
Members subjected to wind induced oscillations of a large number of cycles in life, and
Members subjected to crowd induced oscillations of a large number of cycles in life.

b) No fatigue assessment is necessary, if the following conditions are satisfied.

The highest normal stress range $f_{\mathrm{fmax}}$ satisfies:

f _ {\mathrm{fmax}} \leq 2 7 \mu_ {\mathrm{r}} / \gamma_ {\mathrm{mft}}

The highest shear stress range $\tau_{\mathrm{fmax}}$ satisfies:

\tau_ {\mathrm{fmax}} \leq 6 7 \mu_ {\mathrm{r}} / \gamma_ {\mathrm{mft}}

The total number of actual stress cycles $N_{SC}$ , satisfies:

N _ {\mathrm{SC}} \leq 5 \times 1 0 ^ {6} \left(\frac {2 7 \mu_ {\mathrm{r}}}{\gamma_ {\mathrm{mft}} f _ {\mathrm{feq}}}\right) ^ {3}

where

$f_{\mathrm{feq}} =$ equivalent constant amplitude stress range, in MPa, given by:

f _ {\mathrm{feq}} = \left[ \frac {\sum_ {\mathrm{i} = 1} ^ {r _ {5}} n _ {\mathrm{i}} f _ {\mathrm{i}} ^ {3} + \sum_ {\mathrm{j} = \mathrm{r} _ {5}} ^ {r} n _ {\mathrm{j}} f _ {\mathrm{j}} ^ {5}}{n} \right] ^ {1 / 3}

where

n = \sum_ {i = 1} ^ {r} n _ {\mathrm{i}}

$f_{\mathrm{i}}, f_{\mathrm{j}} =$ stress ranges falling above and below the $f_{\mathrm{fh}}$ , the stress range corresponding to the detail at $5 \times 10^{6}$ number of life cycles.

$r_{5}$ = summation upper limit of all the normal stress ranges (fi) having magnitude lesser than $(\mu_{\mathrm{r}}f_{\mathrm{fn}}\gamma_{\mathrm{mft}})$ for that detail and the lower limit of all the normal stress ranges ( $f_{j}$ ) having magnitude greater than $(\mu_{\mathrm{r}}f_{\mathrm{fn}}\gamma_{\mathrm{mft}})$ for that detail.

19.2.2 Design Spectrum

19.2.2.1 Stress evaluation

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 effect is accounted for in detail category classification (see Table 26). The stress concentration, however, not characteristic of the detail shall be accounted for separately 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 greater 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 24(a) in the case of circular hollow section connections and Table 24(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.

19.2.2.2 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.

Table 24(a) Multiplying Factors for Calculated Stress Range (Circular Hollow Sections) (Clause 19.2.2.1)

Sl No.	Type of Connection		Chords	Verticals	Diagonals
(1)		(2)	(3)	(4)	(5)
i)	Gap connections	$\left\{ \begin{array}{l} K type \\ N type \end{array}\right.$	1.5	1.0	1.3
i)	Gap connections		1.5	1.8	1.4
ii)	Overlap connections	$\left\{ \begin{array}{l} K type \\ N type \end{array}\right.$	1.5	1.0	1.2
ii)	Overlap connections		1.5	1.65	1.25

Table 24(b) Multiplying Factors for Calculated Stress Range (Rectangular Hollow Sections) (Clause 19.2.2.1)

Sl No.	Types of Joint		Chords	Verticals	Diagonals
(1)		(2)	(3)	(4)	(5)
i)	Gap connections	{K type	1.5	1.0	1.5
i)	Gap connections	{N type	1.5	2.2	1.6
ii)	Overlap connections	{K type	1.5	1.0	1.3
ii)	Overlap connections	{N type	1.5	2.0	1.4

19.2.3 Partial Safety Factors

19.2.3.1 Partial safety factor for actions and their effects ( $gamma\{fft\}$ )

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.

19.2.3.2 Partial safety factor for fatigue strength ( $gamma\{mft\}$ )

Partial safety factor for strength is influenced by consequences of fatigue damage and level of inspection capabilities.

19.2.3.3 Based on consequences of fatigue failure, component details have been classified as given in the Table 25 and the corresponding partial safety factor for fatigue strength shall be used:

a) Failsafe structural component/detail is the one where local failure of one component due to fatigue crack does not result in the failure of the structure due to availability of alternate load path (redundant system).

b) Non-failsafe 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 25Partial Safety Factors for Fatigue Strength ( \gamma_{mft} ) (Clause 19.2.3.3)

Sl No.	Inspection and Access	Consequence of Failure
Sl No.	Inspection and Access	Failsafe	Non-Failsafe
(1)	(2)	(3)	(4)
i)	Periodic inspection, maintenance and accessibility to detail is good	1.00	1.25
ii)	Periodic inspection, maintenance and accessibility to detail is poor	1.15	1.35

19.2.1 Reference Design Condition

19.2.1.1 Thickness correction factor

19.2.1.2 Necessity for fatigue assessment

19.2.2 Design Spectrum

19.2.2.1 Stress evaluation

19.2.2.2 Design stress spectrum

19.2.3 Partial Safety Factors

19.2.3.1 Partial safety factor for actions and their effects ( gamma{fft}gamma\{fft\}gamma{fft} )

19.2.3.2 Partial safety factor for fatigue strength ( gamma{mft}gamma\{mft\}gamma{mft} )

19.2.1 Reference Design Condition

19.2.1.1 Thickness correction factor

19.2.1.2 Necessity for fatigue assessment

19.2.2 Design Spectrum

19.2.2.1 Stress evaluation

19.2.2.2 Design stress spectrum

19.2.3 Partial Safety Factors

19.2.3.1 Partial safety factor for actions and their effects ( gamma{fft}gamma\{fft\}gamma{fft} )

19.2.3.2 Partial safety factor for fatigue strength ( gamma{mft}gamma\{mft\}gamma{mft} )

19.2.3.1 Partial safety factor for actions and their effects ( $gamma\{fft\}$ )

19.2.3.2 Partial safety factor for fatigue strength ( $gamma\{mft\}$ )

19.2.3.1 Partial safety factor for actions and their effects ( $gamma\{fft\}$ )

19.2.3.2 Partial safety factor for fatigue strength ( $gamma\{mft\}$ )