Buckling of axially compressed cylindrical shells under different conditions
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Date
10/07/2017Item status
Restricted AccessEmbargo end date
31/12/2100Author
Al lawati, Hussain Ali Redha Mohammed
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Abstract
Civil Engineering thin cylindrical shells such as silos and tanks are normally
subjected to axial compression that arises from a stored solid, wind, earthquake, self-weight
or roof loads. The walls of these shells are very thin, generally of the order of
6 to 25 mm, and massively less than the radius, which is typically 5 to 30 m. They
are thus very thin shell structures, like those of rockets, spacecraft, motor vehicles
and aircraft. The commonest failure mode is elastic buckling under axial
compression. It has long been known that the buckling strength of a thin cylindrical
shell under axial compression is very sensitive to tiny deviations of geometry,
reducing the buckling strength to perhaps 10 or 20% of the value for the perfect
structure.
A normal internal pressure usually accompanies the axial compression, caused by
stored granular solids or fluids. At relatively low pressures, the elastic buckling
strength under axial compression rises, but an elastic-plastic buckling phenomenon
intervenes at higher pressures, causing a dramatic decrease in buckling resistance
associated with an elephant’s foot collapse mode. To construct such large shells, the
fabrication technique is generally the assembly of many rolled plates or panels,
joined by short longitudinal welds and continuous circumferential welds. The
process of welding produces a distinctive geometric imperfection form at each weld
joint, which in turn is extremely detrimental to the shell axial buckling carrying
capacity.
The strength may be further reduced by slight misalignments between adjacent
panels, or in bolted construction, by vertical and horizontal lap splices. Due to the
pattern of loading, both the axial compression and internal pressure increase
progressively down the wall. Accordingly, practical construction usually uses a
stepped wall, formed from panels of uniform thickness, but with larger thicknesses at
lower levels. Since the loading varies smoothly, but each panel has constant
thickness, the critical location for buckling lies at the base of a panel. But the greater
thickness of the lower panel can usefully enhance the buckling strength of the critical
panel above it.
This thesis presents an extensive computational study that examines all the above
influences, divided into chapters that are outlined here. A full exploration of the
effect of the cylinder length on the perfect and imperfect elastic buckling strength is
presented in Chapter 3. In Chapter 4, the elastic-plastic buckling resistance of
imperfect cylinders is described, including strain hardening. These lead to many
capacity curves, for which the key parameters are extracted. The effect of internal
pressure on the buckling resistance of imperfect elastic cylinders is explored in
Chapter 5. Chapter 6 studies the effect of high pressures that produce elastic-plastic
elephant’s foot buckling at circumferential welds in imperfect shells. Next, a step
change in plate thickness is studied in Chapter 7 for imperfect butt jointed cylinders
with and without the internal pressure. Chapter 8 presents an exploration of the
effect of plate misalignments at a circumferential joint, as well as the full
misalignment of a circumferential lap joint in bolted construction. These are
investigated in both the elastic and elastic-plastic domains.
The entire thesis is conceived in the context of EN 1993-1-6 (2007) and the ECCS
Recommendations on Shell Buckling (2008). This research has shown significant
weaknesses in both the concepts and the detailed rules of these standards. Many
conditions are found where either the standard is unnecessarily conservative, or its
safety margin may be too low. Thus, some new provisions are proposed for each of
the above practical problems. These are expected to provide useful knowledge for
the design of such structures against buckling in the future.