OS-C501

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Offshore Standard DNV-OS-C501, November 2013 Sec.6 Failure mechanisms and design criteria – Page 104 2.2 Design criteria for single loads 2.2.1 The general design criterion in the case of a single load for the 'Load and Resistance Factor' design format is: Rk γ F . γ Sd. Sk < . where, γ F γ Sd S k R k γ M γ Rd, partial load effect factor partial load-model factor local stress or strain based on characteristic load effect characteristic resistance partial resistance factor partial resistance-model factor. 2.2.2 The selection of the partial safety factors shall be determined according to Sec.8. 2.2.3 The characteristic value of the local stress or strain based on the characteristic load shall be determined according to Sec.3 [9.4]. Non-linear effects in the analysis should be considered as described in Sec.9 and Sec.8 to obtain the proper value and distribution of the local stress or strain. Guidance note: In the case of a linear analysis load distributions and local stress distributions are the same. ---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e--- 2.2.4 The characteristic value of the resistance shall be determined according to the Sec.4 [1.6] and Sec.5 [1.6]. 2.2.5 The load and environmental conditions for time-dependent design checks shall be selected in accordance with Sec.3 [11]. 2.3 Design criteria for combined loads 2.3.1 The general design criterion, in the case of a combination of loads, for the Load and Resistance Factor design format is: If the design load corresponds to the combination number j as follows: then, the design criterion is written: where, N ⎡ ⎡ ⎤ h h i i i ⎤ j j i i i S d = γ Sd .max⎢γ F . Sk + ∑γ F . Sk . Ψ ⎥ = γ Sd . ⎢γ F . Sk + ∑γ F . Sk . Ψ ⎥ h= 1 ⎣ i≠h ⎦ ⎣ i≠ j ⎦ S d design load effect S i k local stress or strain based on characteristic load effect i γ i F partial load effect factor for load i Ψ i combination factor for load i γ j F , γj M partial load effect and resistance factors for load - j - . γ Sd , γ Rd as defined in [2.2.1]. S d ⎡ = γ + ∑ ⎣ 2.3.2 All explanations of [2.2] apply also to these criteria for combined loads. 2.3.3 The load combination factors Y shall be determined according to Sec.3 [11.2.9]. Guidance note: In the equation above, it is important to see that the partial resistance factor γ j M , corresponding to the load j alone, is used as the common partial resistance factor. For example, when combining a wave load and a snow load one should determine first the maximum of the following two load combinations. For clarity, the load model factor is not shown in the equations below. It should however be considered in real problems. wave wave snow snow snow ⎧γ F . S k + γ F . S k . Ψ (1) S d = max⎨ wave wave wave snow snow ⎩γ F . S k . Ψ + γ F . S k (2) γ M γ Rd ⎤ Ψ ⎦ j j i i i Sd . ⎢γ F . Sk γ F . Sk . ⎥ < i≠ j R γ k j M . γ Rd DET NORSKE VERITAS AS
Offshore Standard DNV-OS-C501, November 2013 Sec.6 Failure mechanisms and design criteria – Page 105 If combination (1) is the worse, then the design criterion to be checked is given by: If combination (2) is the worse, then the design criterion to be checked writes: A conservative value of the load combination factor Ψ = 0.7 can be used for Ψ snow and Ψ wind ---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e--- 2.4 Time dependency and influence of the environment 2.4.1 Design criteria are given as static design criteria in this section. It is assumed that time or influence of the environment does not change the design criteria themselves. However, the properties used in the design criteria shall be the appropriate properties for the point of time where the analysis is carried out. The change of properties with time is described in Sec.4 and Sec.5. 2.4.2 Time dependency is seen here in the widest possible sense. Time dependency considers influence on properties due to permanent static and cyclic loads, due to environmental effects, and all possible combinations. 3 Fibre failure 3.1 General γ γ wave wave snow snow snow F . Sk + γ F . Sk . Ψ < S wave wave wave snow snow F . k . Ψ + γ F . k < 3.1.1 Fibre failure is defined here as the failure of a ply by fracture of fibres. The fibre strength or strain to failure is based on test results from plies or laminates as described in Sec.4. Ply failures are measured as rupture of the ply in fibre direction. 3.1.2 The maximum strain criterion should be used to check fibre failures. 3.1.3 Other design criteria may be used if it can be shown that they are equal or conservative compared to the maximum strain criterion given here. See for example [3.3]. 3.1.4 Fibre failure should be checked at the ply level, not at the laminate level. 3.1.5 Laminates are made of plies with different fibre orientations. If the minimum angle between fibres at any position in the laminate is larger 45 o , matrix cracking or deformation due to in plane ply shear stresses may cause rupture of the laminate. In this case matrix cracking due to ply shear should also be checked to avoid fracture, burst or leakage (see also [4.5] and [3.3]), unless it can be shown that matrix cracks or deformations can be tolerated by the laminate under the relevant loading conditions. Guidance note: A pipe made of ±55 laminate with a liner can tolerate matrix cracks and shear deformations, as long as the pipe sees only internal pressure. If the pipe must carry axial loads or bending moments in addition to the pressure, fibres would want to reorient themselves to a different angle, a complicated condition. This is only avoided as long as the shear properties of the pipe are intact. A pipe made of a 0/90 laminate can tolerate matrix cracks and shear deformations under internal pressure and axial loads. This pipe would have problems with axial torsion, since the stresses due to torsion have to be carried by the matrix. ---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e--- 3.1.6 Regardless of the analysis method used, these laminates should always be analysed with non-degraded in-plane shear moduli G 12 . 3.1.7 Laminates are made of plies with different fibre orientations. If the minimum angle between fibers at any position in the laminate is larger than 70 o , matrix cracking or deformation due to in plane ply shear stresses or stresses transverse to the fibres may cause rupture of the laminate. In this case matrix cracking due to all possible stress components should also be checked to avoid fracture, burst or leakage (see also [4.1] to [4.3]), unless it can be shown that matrix cracks or deformations can be tolerated in by the laminate under the relevant loading conditions. Guidance note: This condition is typical for UD laminates where all fibres run parallel in one direction throughout the thickness of the laminate. Great care should be taken when using such laminates due to their low properties in all other directions than the fibre direction. ---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e--- S γ γ R k wave M . R γ k snow M . γ Rd Rd DET NORSKE VERITAS AS
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OS-C501

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