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WinNONLIN Non-Linear Analysis Options

With the WinNONLIN Options module licensed, the user has two powerful but simple-to-use 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.


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 first-order elastic linear analysis must be replaced by second-order non-linear 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 'margin-of-safety' 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 Non-Linear Analysis should be undertaken.

aCR < 1 The structure must certainly be stiffened either locally or generally. The displacement plot for the lowest mode of buckling will indicate where the structure will benefit from stiffening.
aCR < 5 A substantial increase in capacity may be possible from a fairly modest additional stiffening.
aCR < 10 Do a full Non-Linear Analysis. This is a requirement under Eurocode EN1993, except for specific structural forms and loading for which there are provisions for calculating second-order behaviour using specially modified first-order analyses.


For many years, the structural design engineer’s calculation workhorse has been first-order 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 non-linear 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 Non-Linear Analysis Option is a design tool to help the engineer with this.

The WinNONLIN Non-Linear Analysis Option is an iterative second-order analysis, which with each iterative cycle works towards establishing the equilibrium position for the current stage loading. Nodal out-of 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/member-end co-ordinate 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 second-order P-D effect and if frame members with axial loading flex sideways there will be a second-order p-d effect. Both effects can be either stabilising or de-stabilising, the first in an overall way and the second in a local way. A further second-order 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 Non-Linear Analysis Option can allow for all three effects.

Use of Non-Linear Analysis Option

l The Non-Linear Analysis option is used to obtain a more accurate indication of structural response to load than is possible with a Linear Elastic Analysis.
l The Non-Linear Analysis option presents analysis results for displacements, internal loads, internal stresses and reactions in an identical format to the Linear Elastic Analysis and these can be similarly studied in tabular or plotted form.
l By setting up a series of combination loadcases at pre-processor stage, each with increasing load intensity, it is possible to track the response of a structure towards buckling collapse.
l Depending on the structural form and loading, a Snapped Through equilibrium state may be calculated and a list of Snapped Through members given.


Interfacing products:WinFRAM, WinFELA

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