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D12.B.1 Member Resistances

The implementation of the NORSOK N-004 code in STAAD.Pro considers sections 4, 5, 6 & 7 in of that document. The details of the various clauses implemented from these sections is presented here for member checking and design.

D12.B.1.1 General Provisions

The general safety check is per Section 4. Checks are made to ensure that the design action effect (Sd) is less than or equal to the design resistance (Rd):

Sd ≤ Rd

The design resistance is evaluated for each condition and this check is applied as described in the following sections.

D12.B.1.2 Steel selection and non destructive testing

Section 5 deals with the choice of “design class” for structural joints and components. The choice of design class will determine the choice of steel grade & quality and also the determination of inspection category for fatigue. The choice of design class (as per Table 5-1 of the code) is left to you and does not have any direct impact on how STAAD.Pro performs design checks.

D12.B.1.3 Ultimate Limit States

Clause 6.1 primarily deals with the section of material factors to be used in the various conditions or checks. The material factors chosen are dependent on the "section class" of a cross section. N-004 does not explicitly specify how to classify various cross sections. Therefore, the section classification is made as given in Section 5.5 of EN 1993-1-1:2005, except when specified explicitly along with member checks (See Member Subject to Axial Compression).

Also, N-004 does not specify steel grades to be used. Therefore, this STAAD.Pro uses the steel grades per EN 1993-1-1:2005 for designs per N-004.

Note: Ring stiffener design to CL. is not included for this implementation.

D12.B.1.4 Tubular Members

Clause 6.3.1 deals with the general considerations while using tubular members.

CAUTION: Only tubular sections can be used with the N-004 code in STAAD.Pro. A warning is presented for any other section type.

The dimensions of the tubular sections are limited as follows:

  • The thickness t ≥ 6 mm.
  • The thickness t <150 mm.
  • The slenderness ratio of the cross section D/t < 120.

    Where D is the diameter and t is the wall thickness of the section.

  • The yield strength for tubular member ≤ 500 N/mm2.

If any of these conditions are not met for a member selected for design, a warning will be issued by the engine and the design of that member is aborted.

Note: N-004 uses "Y" to define the action effects that is in plane and ‘Z’ to define out of plane effects. This is the opposite to what STAAD uses, where ‘Z’ defines the in plane effects and ‘Y’ the out of plane effects. This document will follow the STAAD.Pro convention for the Z and Y axes.

The N-004 code also segregates members into those that are subject to hydrostatic pressure and those that are not subject to hydrostatic pressure. The program allows you to specify whether a member is subject to hydrostatic pressure or not and, if so, to specify the hydrostatic pressure for the element. By default the program will assume that all members are not subject to any hydrostatic pressure. The design parameter HYD is used to specify the maximum water level with respect to the origin.

If the HYD parameter is specified, the program will take that to be the water level and will evaluate the pressure distribution on each element assuming a linear increase in pressure with depth (The density of water is assumed to be 9.8 KN/m3). Also, if the HYD parameter is specified, the program will assume that the hydrostatic loads have not been included in the analysis. For members that are subject to a combination of loads (i.e., bending plus compression) along with a hydrostatic pressure, the design will be done according to Clause 6.3.9 of the code. In the absence of any hydrostatic pressure on the member the design will be performed in accordance with Clause 6.3.8 of the code.