6.5 Верифікації

6.5.1 Загальні положення

(1) Трубопроводи, прокладені в стійкому і досить однорідному грунті, можуть перевірятися тільки на деформації грунту, обумовлені проходженням хвиль.

(2)P Підземні трубопроводи, що перетинають ділянки, на яких можливі порушення або концентровані спотворення грунту, такі, як бічне поширення, розрідження, зсуви і рухи по скиданню, повинні проектуватися, щоб витримувати зазначені явища.

6.5 Verifications

6.5.1 General

(1) Pipelines buried in stable and sufficiently homogeneous soil may be checked only for the soil deformations due to wave passage.

(2)P Buried pipelines crossing areas where soil failures or concentrated distortions are possible, like lateral spreading, liquefaction, landslides and fault movements, shall be designed to resist these phenomena.

6.5.2 Buried pipelines on stable soil

(1)P The response quantities to be obtained from the analysis shall include the maximum values of axial strain and curvature and, for unwelded joints (reinforced concrete or prestressed pipes) the rotations and the axial deformations at the joints.

(2)P In welded steel pipelines the combination of axial strain and curvature due to the design seismic action shall be compatible with the available ductility of the material in tension and with the local and global buckling resistance in compression:

- allowable tensile strain 3%;

- allowable compressive strain:
min {1%; 20t/r (%)};

where t and r are the thickness and radius of the pipe respectively.

(3)P In concrete pipelines, under the most unfavourable combination of axial strain and curvature due to the design seismic action, the limiting strains specified in EN 1992-1-1 for concrete and steel shall not be exceeded.

(4)P In concrete pipelines, under the most unfavourable combination of axial strain and curvature due to the seismic action relevant to the damage limitation state, the tensile strain of the reinforcing steel shall not exceed values that may result in residual crack widths incompatible with the leak-tightness requirements.

(5)P Under the most unfavourable combination of axial and rotational deformations, the joints in the pipeline shall not suffer damage incompatible with the specified damage limitation requirements.

6.5.3 Buried pipelines under differential ground movements (welded steel pipes)

(1)P The segment of the pipeline deformed by the displacement of the ground, either due to fault movement or caused by a landslide or by lateral spreading, shall be verified not to exceed the available ductility of the material in tension and not to buckle locally or globally in compression. The limit strains shall be in accordance with 6.5.2.

6.6 Design measures for fault crossings

(1) The decision to apply special fault crossing designs for pipelines where they cross potentially active fault zones depends upon cost, fault activity, consequences of rupture, environmental impact and possible exposure to other hazards during the life span of the pipeline.

(2) In the design of a pipeline for fault crossing, the considerations in (3) to (9) will generally improve the capability of the pipeline to sustain differential movements along the fault.

(3) Where practical, a pipeline crossing a strike-slip fault should be oriented in such a way as to place the pipeline in tension.

(4) The angle of intersection of reverse faults should be as small as possible, to minimize compression strains. If significant strike-slip displacements are also anticipated, the fault crossing angle of the pipeline should be chosen to promote tensile elongation of the line.

(5) In fault zones the depth at which the pipeline is buried should be minimized in order to reduce soil restraint on the pipeline during fault movement.

(6) An increase in pipe wall thickness will increase the pipeline's capacity for fault displacement at a given level of maximum tensile strain. Within 50 m on each side of the fault relatively thick-walled pipe should be used.

(7) Reduction of the angle of interface friction between the pipeline and the soil increases the pipeline's capacity for fault displacement at a given level of maximum strain. The angle of interface friction can be reduced through a hard, smooth coating.

(8) Close control should be exercised over the backfill surrounding the pipeline over a distance of 50 m on each side of the fault. In general, a loose to medium granular soil without cobbles or boulders will be a suitable backfill material. If the existing soil differs substantially from this, oversize trenches should be excavated for a distance of approximately 15 m on each side of the fault.

(9) For welded steel pipelines, fault movement can be accommodated by utilising the ability of the pipeline to deform well into the inelastic range in tension, in order to conform without rupture to the ground distortions. Wherever possible, pipeline alignment at a fault crossing should be selected such that the pipeline will be subjected to tension plus a moderate amount of bending. Alignments which might place the pipeline in compression should be avoided to the extent possible, because the ability of the pipeline to withstand compressive strain without rupture is significantly less than that for tensile strain. Any compressive strains should be limited to that strain which would cause wrinkling or local buckling of the pipeline.

(10) In all areas of potential ground rupture, pipelines should be laid in relatively straight sections, avoiding sharp changes in direction and elevation. To the extent possible, pipelines should be constructed without field bends, elbows and flanges that tend to anchor the pipeline to the ground.

a) циліндричну форму, з вертикальною віссю та круглим або прямокутним поперечним перерізом;

b) жорсткий або гнучкий фундамент;

c) повне або часткове анкерування до фундаменту.

Розширення для надземних резервуарів або циліндричних резервуарів з горизонтальною віссю обговорюються cтиснено.

Ретельний аналіз явища динамічної взаємодії між рухом рідини, що міститься в резервуарі, деформацією стінок резервуара і розташованого під фундаментом грунту, включаючи можливий підйом, являє собою завдання значної аналітичної складності, що вимагає надзвичайно високих обчислювальних ресурсів та зусиль. Запропоновані деякі розрахункові процедури, які застосовуються до конкретних проектних ситуацій. Так як їх точність залежить від завдання, правильний вибір вимагає від проектувальника певного обсягу спеціалізованих знань. Необхідно звернути увагу на важливість однорідного рівня точності протягом всього процесу проектування: було б нелогічним, наприклад, використовувати точне рішення для визначення гідродинамічних тисків, а потім не використовувати

ANNEX A

(informative)

SEISMIC ANALYSIS PROCEDURES FOR TANKS

А.1 Introduction and scope

This Annex provides information on seismic analysis procedures for tanks subjected to horizontal or vertical seismic action, having the following characteristics:

a) cylindrical shape, with vertical axis and circular or rectangular cross-section;

b) rigid or flexible foundation;

c) full or partial anchorage to the foundation.

Extensions for elevated tanks or cylindrical tanks with horizontal axis are briefly discussed.

A rigorous analysis of the phenomenon of dynamic interaction between the motion of the contained fluid, the deformation of the tank walls and that of the underlying foundation soil, including possible uplift, is a problem of considerable analytical complexity requiring unusually high computational resources and efforts. Several analysis procedures have been proposed, valid for specific design situations. Since their accuracy is problem-dependent, a proper choice requires a certain amount of specialized knowledge from the designer. Attention is called to the importance of a uniform level of accuracy across the design process: it would not be consistent, for example, to use an accurate solution for the determination of the hydrodynamic pressures, and then not to use a correspondingly refined mechanical model of the tank (e.g., a finite element model) for evaluating the stresses due to the pressures.