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Definition5
Anaxisoftwistisasumofthecentresofrotationsofallfold’scross-
sections.Oneshouldtendtoachievethesamestraightlineastheaxisof
foldandaxisoftwistforallfoldsofeachshellsheeting.
Definition6
Asheetingreferencesurfaceisaregularsurfacepassingthroughallfold’s
axesofthesheetingandrestrictedbytwolinescorrespondingtotwo
directricesandtwostraightlinescorrespondingtothelongitudinaledges.
TheshellsectorfromFig.1-2canbetakenasareferencesurfacefor
asheeting.
Definition7
Afoldreferencesurfaceisalongitudinalstripofasheetingreference
surfaceofsheetinglimitedbytwostraightlinescorrespondingtothe
longitudinaledgesofthesamefold,forexampletherulingst
i
andt
i-1
,
showninFig.1-2,limitafoldreferencesurface.
Theeffectivetransformationsofshellfoldsenableachieving:
a)greatinitialdeformationsofasheetingatthestageofassemblingthe
sheetingtotheroofdirectrices,
b)smallinitialstrains,
c)straightaxesofthefolds,
d)anglesclosetorectangularatverticesofthefoldreferencesurfaces
[6,24].
Duetotheaboveproperties,itispossibletoobtaininnovative,diversified
andveryattractivefreeformsoftheshellroofs,Fig.1-3[8,9,19,98].
Fig.1-3.Twodifferentrooffreeforms
