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WinNONLIN NonLinear Analysis
Options With the WinNONLIN Options module licensed, the user has two powerful but simpletouse additional analysis choices. Selection of either of these is made at the same stage that a Linear Analysis would be selected. No additional model data is needed. BUCKLING ANALYSIS OPTION Structural compression members/components and structural frames as a whole can become unstable when the intensity of loading reaches a critical level.
This instability or buckling can be sudden and dramatic and must be prevented. In theory, at the elastic critical load, the member/structure can adopt an infinity of deformed positions in which there is a balance between the externally applied loads and the resisting loads. Codes of practice which imposed reduced member stress levels, based on member effective length, go a long way to ensuring that individual members do not buckle. Procedures for ensuring that frames as a whole do not buckle are more problematic and considerable reliance on engineer experience has been vital. The arrival of design codes such as Eurocode EN1993 have changed the design emphasis, requiring elastic buckling loads to be used as a measure of when firstorder elastic linear analysis must be replaced by secondorder nonlinear analysis. The WinNONLIN Buckling Analysis Option is a program which calculates the elastic buckling factor aCR for each loadcase, that is the ratio between the elastic buckling load intensity and the design loadcase intensity aCR = PCR /P, so indicating the 'marginofsafety' against instability failure. Use of Buckling Analysis Option A Buckling Analysis performed at initial member sizing stage is a very efficient way of identifying weaknesses in the stability of a design and indicating when a NonLinear Analysis should be undertaken.
NONLINEAR ANALYSIS OPTION For many years, the structural design engineer’s calculation workhorse has been firstorder linear structural analysis. In most situations, linear structural analysis provides a sufficiently accurate calculation of displacement, internal stresses, reactions etc. which allow the engineer to make sound preliminary selection of member sizes. Having said that, it should be noted that linear analysis is clearly an approximation as it assumes that the response to loading is directly proportional to load intensity and ignores the changes in model response due to changes of the model shape during application of load. A nonlinear analysis is a more accurate way of calculating deformation, internal stresses and reactions. With an increasing pressure for more efficient structures, a greater accuracy of structural modelling and analysis is required to achieve a corresponding balanced and safe design. The WinNONLIN NonLinear Analysis Option is a design tool to help the engineer with this. The WinNONLIN NonLinear Analysis Option is an iterative secondorder analysis, which with each iterative cycle works towards establishing the equilibrium position for the current stage loading. Nodal outof balance loads are calculated at the end of each cycle and these are used in the next cycle to move the structural model towards that equilibrium position. The changing global geometry (node/memberend coordinate positions) as the loading is applied, results in changing member/element resistance. If load application points move across the direction of loading, there will be a secondorder PD effect and if frame members with axial loading flex sideways there will be a secondorder pd effect. Both effects can be either stabilising or destabilising, the first in an overall way and the second in a local way. A further secondorder effect is to do with the accuracy of calculation of the shortening/lengthening of members. It is clear that triangulated frames, flexural frames and catenary cables behave differently so the importance of each effect will vary from one structural model to another. The WinNONLIN NonLinear Analysis Option can allow for all three effects. Use of NonLinear Analysis Option
