13.2Design

13.2.2.1 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.

13.2.2.2 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.

13.2.2.3 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:

$f \leq 27 / \gamma_{\mathrm{mft}}$

or if the actual number of stress cycles, $N_{\mathrm{SC}}$, satisfies

$N_{\mathrm{sc}}<5 \times 10^{6}\left(\frac{27 / \gamma_{\mathrm{mft}}}{\gamma_{\mathrm{ft}} f}\right)$

where

$\gamma_{\mathrm{mft}}, \gamma_{\mathrm{ft}}$ = partial safety factors for strength and load, respectively (see 13.2.3), and

$f=$ actual fatigue stress range for the detail

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

(Clause 13.2.2.1)

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

(Clause 13.2.2.1)

13.2.3.1 Partial safety factor for actions and their

effects $\left(\gamma_{\mathrm{ffl}}\right)$

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.

13.2.3.2 Partial safety factor for fatigue strength (γmft)

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

13.2.3.3

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

• a) Fail-safe 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-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 25 Partial Safety Factors for Fatigue Strength (γmft)

(Clause 13.2.3.3)