2.6.2.1 Principles and application rules
Like in the Eurocode (for steel structures) distinction is made between principles and application rules. Principles comprise general statements, definitions and requirements for which there is no alternative, and requirements and analytical models for which no alternative is permitted (unless specifically stated). Application Rules are generally recognised rules which follow the principles and satisfy their requirements; alternatives are allowed provided it is shown that they accord with the relevant principle.
Typical examples are the primary and the primary & secondary stress criteria of the stress categorisation approach, which are stated here, in slightly modified forms, as application rules. 2.6.2.2 Actions
This term, which replaces the old term loadings, denotes all thermo-mechanical quantities imposed on the structure causing stress or strain, like forces (including pressure), temperature changes and imposed displacements.
Actions are classified by their variation in time:
• permanent actions (G)
• variable actions (Q)
• exceptional actions (E)
• operating pressures and temperatures (p,T ) - although these are variable actions, they are considered separately to reflect their special characteristics (variation in time, random properties, etc.).
The notion variable actions encompasses actions of quite different characteristics – from those actions which are deterministically related to pressure and/or temperature, via actions not correlated with pressure or temperature but with well defined (bounded) extreme values, to actions which can be described only as stochastic processes not correlated with pressure or temperature, like wind loads. Actions with a deterministic relationship with pressure and/or temperature shall be combined in the pressure/temperature action and the relationship, exact or approximate, shall be used.
The characteristic values of actions describe the regime of actions which envelops all the actions that can occur under reasonably foreseeable conditions. The characteristic values are used in determining the design values of the actions, and they depend on the actions' (statistical) properties. The characteristic values of permanent actions are usually their mean values (or credible extreme values). The characteristic values of variable actions are defined as mean values, or p% - percentiles, of extreme values, and values specified in relevant codes for wind, snow, earthquake may be used; usually they are adapted to Eurocode concepts anyway. The upper characteristic value of pressure shall not be smaller than the lesser of the set pressure of the protecting device or the highest credible pressure that can occur under normal and upset conditions (reasonably foreseeable), and the upper characteristic value of the temperature not smaller than the highest credible temperature (under the same conditions). Therefore, the (limited) pressure excursion
(overpressure) that occurs if a safety valve opens need not be included in the (maximum) characteristic value of pressure; it is taken care of in the partial safety factors.
2.6.2.3 Partial safety factors
To allow for an easy, straightforward combination of pressure action with environmental ones, and, at the same time, to give the flexibility, expected from a modern code, to adjust safety margins to differences in action variation, likelihood of action combinations, consequences of failure, differences of structural behaviour and consequences in different failure modes, uncertainties in analyses, a multiple safety factor format was introduced, using different partial safety factors for different actions, different combinations of actions, different failure modes and corresponding resistances of the structure. Examples of partial safety factors are given in the following Table. The corresponding combination rules for e.g. Design Check GPD-OC Global Plastic Deformation – Operating Conditions are:
• all permanent actions shall be included in each load case
• each pressure action shall be combined with the most unfavourable variable action
• each pressure action shall be combined with the corresponding sum of variable actions; stochastic actions may be multiplied by the combination factor.
• favourable actions shall not be considered.
The partial safety factors of pressure and resistances are calibrated with respect to the DBF results; no attempt has been made to justify the partial safety factors by probabilistic investigations or decision theory under uncertainty; if pressure is the only action the approach can be transformed to a nominal design stress one.
Partial safety factors
Design check Actions GPD-OC GPD-HT Permanent γG Unfavourable 1.35 1.35 Favourable 1.0 1.0 Pressure γP 1.2 (1.0) 1.0 Variable γQ 1.5 (1.0) - Combination factor ψ 0.91 1.01 (stochastic actions) Resistance γR 1.25 1.05 (Temperature γ )T (1.0) (1.0) 1
If not specified differently in the relevant code of environmental actions.
2.6.2.4 Design checks – effects of actions
Design checks are investigations of the structure's safety under the influence of specified combinations of actions - the design load cases - with respect to specified limit states (representing
Design by Analysis
Page2.44 Design by Analysis
one or more failure modes). Characteristic values of the actions are multiplied by the corresponding partial safety factors to obtain their design values and their combined design effect (on the structure) is evaluated:
Ed(γGG,γpp,γQQ,... ,ad,... )
In the design checks these design effects are compared with the corresponding design resistances, obtained by dividing the resistance of the structure, corresponding to the action's combination, by the relevant partial safety factor of the resistance:
Ed ≤Rd =R G p Q( , , ,... ,ad, ) /γR
This comparison can, in general, be performed in actions, in stress resultants (generalized stresses) or in stresses.
The resistances are related to the limit states - states beyond which the part no longer satisfies the design performance requirements.
2.6.2.5 Design checks – resistances
Design checks are designated by the failure modes they deal with. The following ones are incorporated in the first issue of the standard:
• gross plastic deformation (GPD), with corresponding failure modes ductile rupture and, for "normal" designs, also excessive local strains
• progressive plastic deformation (PD)
• instability (I)
• fatigue (F)
• static equilibrium (SE).
Checks against gross plastic deformation
The design resistances are given by the lower-bound limit loads for
• proportional increase of all actions
• a linear-elastic ideal-plastic material (or a rigid ideal-plastic one)
• first-order theory
• Tresca's yield criterion and associated flow rule
• specified design strength parameters.
Design strength parameters R M and partial safety factors of the resistances γR are chosen such that
for the simplest structures and pressure action only DBA and DBF results agree. The only exception are steels, other than austenitic ones with A5 ≥30%, where the design strength parameter R M is
If the procedure used to determine the limit action does not give an (absolute) maximum in the region with maximum absolute values of principal strains less than 5%, the boundary maximum, for which the maximum absolute value of the principal strains equals 5%, shall be used.
As an application rule the "usual" primary stress criterion is given, formulated in stresses and - for structures where the concept of stress resultants is applicable - in stress resultants and local (technical) limit loads.
These checks (against GPD) are considered also to encompass Excessive Yielding, provided "usual" design details (with not too severe strain concentrations) exist.
Checks against progressive plastic deformation
On repeated application of specified action cycles PD shall not occur for
•a linear-elastic ideal-plastic material
•first-order theory
•Mises' yield condition and associated flow rule
•specified design strength parameters RM.
A slight modification of the "usual" f3 criterion is given as application rule; it is noted that this application rule, which is derived from shakedown considerations, is only a necessary condition for the fulfilment of the principle, but is considered, together with all the other checks, to be sufficient to achieve the principle's goal - avoidance of ratchetting in the structure.
Check against fatigue failure
Reference is made to the Fatigue Assessment section of the Standard. Instability
Static equilibrium
The usual checks against overturning and (rigid body) displacement are stated explicitly, using the partial safety factors given in the other checks.