LIFTING ANALYSIS
QUERY :
After analyzing
Duringtheanalysisof a STRUCTURE suspended byfrom a pivot, Staad reports aInstabilityaninstability warning message.
Answer :
The concerned
Whena structure is being suspended throughfrom a pinnedsupportwhichactssinglepointdefined as a RIGIDBODYmechanism
andStaadcouldnothandletherigidbodymechanism,whichiswhytheprogram
reportssomeinstabilitywarningmessages .
Asthesupportis pinned support, it canswingaboutthatpointlikeapendulumsinceapinnedsupport cannot resist anymomentlikethemoments MX , MY or MZ , resortingthestructureto.Thisisarigidbodymotion.Rigidbodymotionsarenothandledduringstaticanalysisandarereportedas an instability condition.
One may assign a weak spring intended to stabilize the structure rotationally at thethat support. Modify the support as shown below.
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SUPPORTS
93 FIXED BUT KMX 0.05 KMY 0.05 KMZ 0.05
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This wouldshould avert the instability messagecondition. Although the moment would be developed at support, but its magnitude will be inconsequential.
Staadonlyconsiders
Alternatively,additionalsupportsdefinedat the initialloadingconditionlowerlevelsthatpreventtranslationalongFX and calculatestherotationaldisplacementwhichcanFZmayalso be resistedbytheweakspringaremedytothisproblem.
Note that hasbeenassignedatonly the supportinitialpositionforgeometryandloadingareconsideredduringthistypeoflinearstaticanalysis.
So,despiteIf the introduction
ofstructuredoesrotateasarigidbodyat the weakspringsite, thestructurethatcondition is still not being analyzed atwith the equilibrium
positionabovearrangement.
In the practical scenario, the CGstructure will try to pushthecompletearrangementtowardstheattaina position where theverticalloadactsalongavertical linepassingthroughthesupportsothat there will be no eccentricity(equilibriumposition),i.e. Thatis, the support and C.G coordinates sharelieon the same vertical line .
Take
Consider the small example shown below.
The complete arrangement is being suspended from a single top supportanddue.Due to eccentricity of Centre
of Gravity withrespecttothesupport, the initial loading condition impartsinduces an initial rigidbody rotation.
Theinitialforcei.e.mg(sinΘ)willinduceinitialdisplacement,butastheC.Gapproaches
Ifonewishes to
equilibriumpositionavoidusingstabilizingrotationalsprings, sinΘalsoconvergesthestructurehas to zero.
ThischangeinΘvalue
duringdisplacementisnotconsideredbyStaad.
However,thistypeofsystemcan be indirectlysimulatedanalyzed in Staad.
Onehastoanalyze the structure
in equilibrium position.
For this, the eccentricity needs to be determined (Thethe horizontal distance of CG from the coordinates of suspension).
The Centre of Gravity of the whole system can be knownfromfoundinthe output file byinvokingwiththehelpofthe PRINT CG command .
From thesethis data, Θ be easily computed .
NowNext, rotate the structure by that angle “Θ” , so that the samegravityloadisalignedwitha line passespassing through the support coordinateandCenterOfGravity . InotherwordsThus, you are now analyzing the arrangement at the equilibrium position.