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The Global Analysis Options provides control over features of the analysis of a model which apply to the model in its entirety. These are distinct from options which apply on a member-by-member or loadcase-by-loadcase basis.
Do Not Show Out of Balance Loads as Support Reactions
Due to rounding errors in the way computers deal with numbers, the solutions of the equations defined by the structure stiffness matrix may results in difference in the forces calculated at nodes. These forces can display as a support reaction when viewing the results. Because these forces are related to nodal forces, they can occur at nodes which do not have a restraint condition applied.
Out of balance forces can occur particularly where there are very significant differences in stiffnesses of members connection at a node. In particular, in older model with the old style rigid stiff deck applied to very short members, this could happen and may indicate potential ill-conditioning of the stiffness matrix. Other notable areas where this may occur is at the junction of columns with bracing members, where small differences in the bracing forces between levels can occur.
The default option in the software is that this option is checked, so that out of balance forces do not show in the support reaction results.
This option can be useful as a diagnosis tool in models. In general, the out of balances forces would be expected to be small.
The torsional stiffness of all elements in a model will be ignored from the analysis, with the member torsional stiffness being taken as zero. This option can prevent members having a degree of bending restraint taken at their ends due to the torsional stiffness of any other members coincident with the ends of the element.
This option can be useful in grillage structures, where restraint to members due to the torsional stiffness of adjoining members is not wanted to be taken into consideration in the analysis. The ignore torsional stiffness option can give a simple and quick away to ignore this effect. However, it is possible that the stability of the structure in terms of the analysis is dependent on the torsional stiffness of members, with the result that models which will analyse without issue with the torsional stiffness taken into consideration, return analysis warnings when the torsional stiffness is ignored.
In general, it is advised that member releases are used to prevent the transfer of bending from one member into another member in torsion, rather than using the Ignore Torsional Stiffness option.
This option is, by default, not active in the software and required the user to select it in any model.
In structures with steel or concrete, in general, the deflection resulting from shear is ignored for the purposes of design. This is not the case for timber elements, where the shear deformation is significant, with the shear deflection potentially accounting for 5-20% of the deflection of a timber element. Both the British Standard and Eurocode for timber design require the shear deformation to be considered as part of the design of timber elements.
Selecting the Shear Deflection option includes the shear deflection as part of the analysis and so includes the shear deformation of the elements in a model in the deformation of both the structure and the elements in the structure.
The ratio of Elastic Modulus to Shear Modulus in steel and concrete is approximately 2. In timber, this ratio is approximately 16. It can be seen, therefore, that the effect of the shear deformation is considerably less significant in either steel or concrete, in comparison with timber, and so the inclusion of the shear deformation for steel and concrete elements will generally not be significant.
The default setting for software is that the Include Shear Deformation option is not active.
One exception to this rule is steel cellular beams, where the presence of the web openings means that the shear deformation of the beam becomes significant. In this instance, where a cellular steel beams is added to a model, the Include Shear Deformation global setting is activated in the software.
In the Finite Element Analysis, shear deformation is also significant. However, with the use of Mindlin-Reissner plate theory, the transverse shear deformation is automatically accounted for in the analysis. Thus, the shear deformammtion of the Finite Elements utilized within MasterFrame FE is not dependent on the Include Shear Deformation.
Use faster Pardiso Solver for all models.
The MasterFrame module and MasterFrame FE modules have two different analytic engines. The MasterFrame FE module uses a proprietary Intel MLK Pardiso solver. This solver uses shared-memory, multiprocessing parallelisation to allow multi threading and so enables the software to solve multiple equations at one time. This is necessary in the solution of the equations arising from the Finite Element Analysis to allow the analysis to complete in an acceptable time frame.
The use of the Pardiso solver has now been included as an Option in MasterFrame2018. The major advantage of this is that it can represent a very significant reduction of time to analyse in large MasterFrame models which do not include Finite Elements. However, the Pardiso solver is significantly more sensitive to ill-conditioning of the stiffness matrix.
Ill-conditioning can arise in matrices where there are large differences in the magnitude of adjacent stiffness terms. This can lead to a loss of accuracy, due to the floating-point arithmetic used computers. The degree of ill-conditioning can be described through the use of a condition number, with a high number representing ill-conditioning. With a large condition number, the matrix is very sensitive to small changes in the inputs. Such a matrix is effectively singular. In this case, the set of equations described by the matrix cannot be solved and the analysis cannot complete. This will result in analysis instabilities.
The standard MasterFrame analysis engine is less sensitive to ill-conditioning of the stiffness matrix and frame instabilities. Therefore, it is possible to find a model will not analyse with the Pardiso solver but will analyse without issue while using the default solver. In general, this is an issue which is related to the matrix methods utilised within the solvers, rather than an issue which would indicate an instability in the structure which the MasterFrame model represents.
Include Warping Stiffness in Curved Members
In a standard stiffness matrix, the member torsional stiffness is based on the Saint Venant torsional constant GJ/L. While this is satisfactory for Bernoulli beams, this will significantly underestimate the stiffness of a curved open section. Ticking the ‘Include Warping Stiffness in Curved Members’ will calculate a modified warping stiffness of the member to account for the warping stiffness. The analysis then utilizes a non-linear iteration to calculate the deformations of the curved members.
The ‘Include Warping Stiffness in Curved Members’ option is active by default in the software. The method only applied to curves members, so for a frame with no curved members, the standard Saint Venant torsional constant will be used for all members. This is a conservative approach.
User defined non-linear convergence settings
When a non-linear analysis, eg, P-delta, is carried out, the analysis will run a number of iterations for each load case checking each time to see if the convergence of the out of balance forces is below the set limit. When it falls below the set limit it moves on to the next load case and analyses it and so on. If the limit is not met after the max. number of iterations, then a message to that effect will appear on screen. There are default values built into the software for the max number of iterations and the convergence percentage. The options above allow the user to set their own limits if desired.
Capture non-linear iteration snap shots
During the non-linear analysis of the frame the program can capture the deflected shape of the structure for each iteration of each load case. These can be viewed after analysis has attempted to solve the load cases. Go to the Results > Non-Linear Iteration Viewer option to open up the viewer and inspect the deflected shapes.
This is especially useful if the non-linear analysis stops during the analysis process and indicates that the frame isn’t converging in a particular load case. You can then view the deflected shape in this load case. Looking at the last iteration in the case will display the frame shape just prior to the non-convergence. This should give a good indication of where the maximum deflections are occurring and so where you need to look to overcome the issue.
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