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Junsheng Yang, in Shield Tunnel Engineering, 2021 3.2.1.2 Propulsion system Care should also be taken at the joints between wall shell and both roof and floor. Guidance on these and on other details is given in both AWWA D100 and API 650. Care should be taken with penetrations of the shell, floor or roof where additional stiffening or reinforcement may be needed. Minimum plate thicknesses given in the codes are 6.35 mm (1/4″) for floor and 4.76 mm (3/16″) for the roof. Buckling safety factors should be determined for spherical or ellipsoidal roofs. Under AWWA D100 roof loads should include dead weight, snow or other live loads, wind and vacuum or internal pressure commensurate with the capacity of the ventilation system. When a tall tank is empty wind loads can cause floor lift anchors should be provided to prevent this. Distortion may result but failure is rare. Sloshing of tank contents may need to be considered separately since it can occur in earthquakes, depending on the tank size and the frequency of seismic oscillations. AWWA D100 contains factors for determining increases in plate thickness for different tank heights and seismic accelerations. The factor of safety against buckling should be calculated and wind girder stiffeners included if necessary ( Rajagopalan, 1990). Such lateral loads may induce buckling in tall shell cylinders if the roof provides insufficient bracing. Shape factors are given in the standards and provide a convenient means of determining wind pressure. Loads to be allowed for in the tank walls include those from the contained liquid and either wind or seismic effects. D100 requires the plate thickness to be based on the stress in the highest loaded extremity. One difference between the AWWA and API codes is that the latter allows the excess thickness at the top of one wall plate to be taken into account in determining the thickness of the plate above it. Toughness needs to be taken into account for higher strength steels it reduces with increased thickness but is improved for fine-grained steels and for those with higher manganese content.
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The steel grade must be weldable and suitable for the stress and temperature ranges expected.
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The refined design procedures recognize the benefits of higher grade steels, an advantage for higher loaded members such as walls. With these, the steel plate selected is usually the cheapest that satisfies the rules for the intended service although a wide range of steel grades are permitted. The basic procedures use conservative allowable stresses (the same in both codes) and are based on simplified design rules. Both these codes include basic and refined design procedures. Otherwise design of tanks for storing water can be to AWWA D100 which, for certain details makes reference to API standard 650 (oil tanks).
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Reinforcement at openings is calculated by the area replacement method. Minimum carbon steel plate thickness for tank bottoms excluding corrosion allowance is to be 5 mm for butt welded plates and 6 mm for lap welded plates. Serviceability checks are to be made for deformations, deflections and vibrations. The shell is to be checked for plastic limit, cyclic plasticity, buckling and fatigue. Loadings are to be as defined in BS EN 1990, Eurocode 0. Under this Eurocode steel tanks for water come under Consequence Class 1 for which membrane theory may be used for determining principle stresses and simplified expressions may be used to determine local bending effects. By 2010 steel tank design in UK must follow BS EN, Eurocode 3, subject to issue of the UK National Annex. Large steel plate tanks are usually cylindrical with their axes vertical. Michael Johnson, in Water Supply (Sixth Edition), 2009 18.11 Welded Steel Plate Design