Engineering guide
The 18T4 module performs the shell buckling safety check for cylindrical, conical and spherical shells to DIN 18800-4 (the German steel structures standard on stability – shell buckling). The standard has been withdrawn and superseded by Eurocode 3 – for shell buckling, DIN EN 1993-1-6 with its National Annex now governs. For re-assessing existing structures, silos, tanks and stacks built to DIN 18800-4, and for comparative calculations, the withdrawn standard nevertheless remains relevant.
Unlike the pure external pressure checks of the pressure vessel codes, DIN 18800-4 addresses the interaction of several load components: axial compression from constant and varying axial load (such as silo wall friction), bending moment with a circumferentially sinusoidal axial stress, circumferential compression from external pressure or wind stagnation pressure, and shear from torsional moment and lateral force. For each component the ideal (elastic critical) buckling stresses are determined, reduced by knock-down factors and combined in the interaction check.
The module accounts for wall thickness undertolerance, ring stiffeners (width, cross-sectional area and spacing of the stiffeners) and the material-dependent transition from elastic to plastic buckling via yield strength and modulus of elasticity. This makes it possible to calculate cylindrical shells under axial load and external pressure as they occur in plant engineering on column shells, silos and thin-walled tanks.
Scope
- Cylindrical, conical and spherical shells under external pressure
Typical workflow
Define geometry, material and imperfection assumptions
Length, diameter and wall thickness of the cylinder are specified; the wall thickness undertolerance from manufacturing tolerance and corrosion reduces the effective wall thickness. Yield strength and modulus of elasticity at design temperature determine the material side of the check.
Determine the applied stresses per load component
The total axial compressive stress is formed from the constant axial load, the bending moment and the longitudinally varying axial load (e.g. silo loading); the circumferential compressive stress per Eq. (49) from the external pressure or stagnation pressure with the factor δ; and the total shear stress from torsional moment and lateral force.
Calculate the ideal buckling stresses
For each component the ideal buckling stress of the perfect shell is determined – axially per Eq. (26), circumferentially per Eq. (34) or (36) – taking into account the boundary conditions, the factor ψ for very short cylinders, and any ring stiffeners with their area and spacing.
Determine the real buckling stresses via reduction factors
Via the relative slenderness, the ideal buckling stresses are reduced to the real buckling stresses using imperfection-dependent reduction factors (κ values); the standard also states from which slenderness values a check is required at all.
Perform the interaction check
The utilization ratios from axial compression, circumferential compression and shear are combined in the interaction criterion of the standard and checked against the required buckling safety. If the check is not satisfied, wall thickness or stiffening are adjusted.
Frequently asked questions
May DIN 18800-4 still be used for new structures?
No, the standard has been replaced by the Eurocode in the building regulations; new verifications must follow DIN EN 1993-1-6 (strength and stability of shell structures) with its National Annex, supplemented for silos and tanks by EN 1993-4-1/-4-2. The withdrawn DIN 18800-4 remains important for re-assessing existing structures, modifications to older plants, and for retracing old structural calculations.
How does the buckling check per DIN 18800-4 differ from the external pressure check per AD 2000 B 6 or ASME UG-28?
The pressure vessel rules primarily consider the external pressure load case and work with fixed safety margins against buckling. DIN 18800-4 is a general shell buckling verification with interaction of axial compression, circumferential compression and shear, and with slenderness-dependent reduction factors. For vessels with significant axial and wind loads – tall columns or silos, for instance – it captures load combinations that the pure pressure vessel checks do not cover.
What role do ring stiffeners play in the check?
Ring stiffeners shorten the governing buckling length for circumferential compression and raise the ideal circumferential buckling stress. The module takes the width, cross-sectional area and centre-to-centre spacing of the stiffeners as inputs; with a spacing of 0 the unstiffened shell is calculated. The stiffeners themselves must be sufficiently rigid, otherwise they may not be counted as buckling boundaries.
Why is the wind stagnation pressure treated differently from a uniform external pressure?
Wind pressure does not act axisymmetrically but is concentrated at the stagnation point. The standard converts the non-uniform pressure into an effective uniform equivalent pressure via the factor δ per Eq. (47). Applying the full stagnation pressure as an all-round external pressure instead significantly overestimates the circumferential loading.
Technical content source: LV-Solved-Problems-Content 628d313247e7