ProtaStructure : Model Options

Model Options

Options to be used in analysis can be specified in “Model Options” page of “Building Analysis” form. “Model Options” comprises of four tab pages namely, Model, Shear Wall Model, Slab Model, and Settings.
Any modifications made in this page can be undone by pressing “Default Settings”.

Model

Parameters related with the analytical model of the building can be adjusted on the “Model” tab.

Material and Section Effective Stiffness Factors 

  1. “Elasticity Modulus”, "Axial Area", "Bending Stiffness", “Shear Area” and “Torsional Constant” of member groups can be scaled by using the coefficients.
  2. Values less than “1” will reduce the related parameter whereas values greater than “1” will increase its value.
  3. Values in grey cannot be modified, there are auto-calculated. 
For example, if you want to minimize the lateral load sharing of columns in a Wall-Frame structure, you can use a coefficient of 5% for column moment of inertia. This operation will globally multiply all column moment of inertias by 0.05 during the analysis data preparation and analysis will be conducted accordingly.
If any modification is made in this field, analysis must be repeated.
Please note that for most of template, the “Torsional Stiffness Factor” of Beams defaults to 0.01 (1%) to minimize torsional forces on main beams generated by secondary beams. This is because of feedback from many users who does not wish to see secondary beam generating torsion on the primary beam (which they claim are not so in-line with traditional assumption & expectation since it may require torsional links in the primary beam). 
If there are situation such as curve beams that user want to utilize more of torsional stiffness, this setting can be increased. Section Properties can also be changed for individual members by selecting the member > right-click > Edit Section/Material.

Storey Degrees of Freedom

There is a list for defining the degrees of freedom for each storey. Storey degrees of freedom options in ProtaStructure are:

  1. X/Y and Torsion Permitted: Translations in X and Y directions and floor torsion about Z-axis are permitted.
  2. X/Y Permitted, Torsion Prevented: Translations in X and Y directions are permitted. But torsion about Z-axis is prevented.
  3. Only X Permitted: Only translation in X direction is permitted. Translation in Y direction and rotation about Z-axis are prevented.
  4. Only Y Permitted: Only translation in Y direction is permitted. Translation in X direction and rotation about Z-axis are prevented.

The last two degrees of freedom options are useful for the analysis of a 2d-frame system defined along the unrestrained direction.

You can select the  “Only X permitted” or the ”Only Y permitted” options to define the degrees of freedom of a single frame system

Storey Degrees of Freedom

Rigid Zones (for concrete members)

Rigid zones formed at the “Column-beam” intersections are taken into consideration automatically by the program in the analysis model. Rigid zones are important in determining the effective lengths of concrete members. Hence, for steel structures, always set no rigid zone (none). 

In order to consider concrete cracks that can be formed in the intersections, rigid zones may be reduced by a certain amount to calculate more realistic effects. ProtaStructure Analysis Model has the following rigid zone options:

  1. “Maximum” 
  2. "Reduce by 50%"
  3. “Reduce by 25%” 
  4. “None” 
For a detail analytical comparison on the rigid zones options, refer to Rigid Zone Interpretations

Beam Section

Using the radio buttons located in this section, flanges of the beam can be taken into account in the building model.

Soil-Structure Interaction

This feature enables foundations to be integrated into building analysis model for more accurate assessment of building behaviour.  For example, effects of how the settlement of foundation will influence the forces in the superstructure members can be investigated.

This feature can be accessed by selecting Merged Foundation Model under Soil-Structure Interaction.

The following are the steps to use Merged Foundation Model :

  1. Model the superstructure and the foundation in ST00
  2. Run Building Analysis
  3. Go to ST00 (Foundation Level)
  4. Run FE Raft Foundation Analysis
  5. Re-run Building Analysis
  6. Examine the behavior of the structures in Analytical Model and Result Display.
Soil spring stiffness/Soil Subgrade Coefficient can be adjusted locally in the properties of each raft foundation panel or globally in the FE Raft Foundation Analysis Parameters.

ShearWall Model

ProtaStructure can model the walls in the building in two different ways. One of them is “Mid-pier Model” and the other is “Finite Element Shell Model”. In “Wall Model” field, you can determine the wall model to be used in the building globally (By default). When a "Wall Model" of a wall set to “Mid-pier Model” uses a single column at the mid-point of the wall panel to model the wall. Interaction with neighbouring elements such as columns, beams or other walls are established by rigid beams extending to two sides.


If “Finite Elements Shell Model” is selected, wall members will be modelled by quadrilateral shell elements (as shown below)


Maximum height and width of shell elements can be entered into “Shell Width” and “Shell Height” fields. Default value of 500 mm by 500 mm is adequate for most cases. Analysis time increases as the width and height values decrease.  

The following are example of cases where FE Shell model maybe preferred:

  1. If the building includes narrowing walls, or wide basement walls, it is recommended to use "Finite Element Shell Model".
  2. If there are walls that are discontinuous (not extended to foundation) and supported by beams or slabs, FE Shell model will be more accurate as the variation in forces along the length of the wall can be captured.
You are encouraged to use the Finite element (FE) shell if you are modelling the following cases (but not limited to):
  1. Beam intersecting the wall other than wall ends and midpoint
  2. Beam with delZ
  3. Wall with delZ
  4. Wall is supported by beam
  5. Wall with wall span load
  6. Inclined (inter-storey) wall

Walls modelled as “Mid-pier Model” require less analysis time than the walls modelled as “Finite Elements Shell Model”.

Each individual wall member in the system can be defined to use a different wall model in the analysis.

  1. This can be done via wall properties or wall table > "Wall Model Type". There are 3 options:
    1. Default: Use the global Wall Model Setting in Pre-analysis
    2. Mid-Pier: Use Mid-Pier Model for this wall only
    3. FE Shell: Use Finite Element Model for this wall only
    4. Basement Wall (Meshed): Use basement wall (Meshed) will consider for seismic analysis and design
  2. For example, a wall can be modelled using "Mid-Pier Model" in first storey and "Finite Element Shell Model" in the second storey.
  3. This is especially useful for specific discontinuous walls where FE Shell may be required only for the transfer level while the rest of the walls can use "Mid-Pier"

CoreWall Model

Multiple shear wall panels can be selected and merged into a single core wall by multiple selecting shearwalls > Right click > Merge Vertical Members.  This allows the shear wall to be analyzed & designed as single integrated entity, increasing efficiency & productivity. The merged core walls can be analyzed as separate panels or as single mid-pier :

  1. Go to Building Analysis > Model Options > ShearWall Model > CoreWall Model
  2. Pick Model Using Corewall Panels   

Analysis will create separate mid-pier for each wall panel. The mid-piers will still act as integrated core-wall as they are connected by their rigid arms. The core-wall will still be designed as a combined entity.

  1. OR pick Single Mid-Pier Model

The analysis will combine all walls as single mid-pier so analysis forces can be viewed as single entity. The core-wall will be designed as a combined entity. Resultant design forces are automatically calculated with respect to geometric center of the merged core wall.

During design, the rebar sizes are automatically selected with the pre-defined numbers & position (in polyline column editor). In short, rebar size will change; but rebar numbers is fixed.

We suggest you always use "Model Using Corewall Panels" as analytically, it is the simplest, easiest to verify & results are generally in-line with general 3D analysis program.
"Single Mid-Pier Model"  is solely to be used in existing building assessment using either push-over or time-history methods. The reason is that is the only way to add lumped plasticity to corewalls since multiple mid-piers do not perform properly and it is not possible to define lumped plasticity for shell walls. 

Slab Model

Parameters related to slab model consideration such as rigid diaphragm and meshing can be changed in Slab Model tab.

Storey Diaphragm Model

Under the excitation of lateral loads, it must be specified whether the slabs in each storey level will behave infinitely rigid in their plane (i.e. rigid diaphragm action) or not. This behaviour is generally modelled as “Rigid Diaphragm”.

By assuming rigid diaphragm action in a storey level, degrees of freedom are reduced to translation X and Y directions and a torsion about Z axis normal to the plane of X and Y. There will be no axial deformation in the beams residing in rigid diaphragm.

If “Slabs To Define Rigid Diaphragm” option is selected, rigid diaphragms will be created by examining the continuity and neighbourhood of slabs both in horizontal and vertical directions. If two or more slab groups do not touch each other directly or by means of other intermediate slabs, each of these groups will form a separate rigid diaphragm. There may be nodes that do not touch any of the slabs. These nodes, then, are defined as free nodes and they do not belong to any of the diaphragms.

“Single Diaphragm per Floor Level” option forces the whole storey level to behave like a rigid diaphragm even if there is no slabs defined.

If “No Rigid Diaphragm Floor Levels” option is selected, rigid diaphragm action is not utilised in the analysis. All nodes in the storey levels will be accepted as free nodes. If there are big openings in the floor (approximately greater than 1/3 of total floor area), it will be difficult to expect a rigid diaphragm action. Then this option should be used in the analysis.

By default All Storeys are checked to assume the slabs to define rigid diaphragm. If All Storeys is unchecked, then you have the choice to pick which storeys to consider diaphragm. Storeys which are unchecked will assume No Rigid Diaphragm.

Include Slabs in Model - Finite Elements Mesh

Checking Include Slabs in Building Model enables meshed slabs to be considered together with the building analysis, i.e. the stiffness of the slab can be considered acting together with the frame elements (beams, columns and walls). This is a useful feature if you are modelling transfer plates where walls and columns may sit directly on a slab or you wish to look more closely at the interaction of the slab together with the frame. The meshing will also allow slopes and drops (or more unusual geometry) to be considered.

This option can be found in a the tab “Slab Model” in the Building Analysis tab :

The following is the explanation of the Slab Model Tab :

  1. Storey Diaphragm Model options work as previous section. If No Rigid Diaphragm is chosen, then semi-rigid diaphragm will be considered based on the In-plane (Membrane) Stiffness value entered under Slab Stiffness Coefficients.
  2. If Include Slabs in Building Model is checked,  one can specify which storeys to include by clicking on Storeys to be Meshed button.
  3. The mesh uniformity can be controlled by specifying different Min. And Max. Shell Size.
  4. “Yield Line” or “FE Load Decomposition” can be completely ignored by unchecked Use Decomposed Slab Loads
    1. In this case, only self weight, beam wall loads and additional correction loads are applied on beams.
    2. Slab self weight, live load and additional dead loads are applied on shells as “Shell Pressure”
    3. Patch Load, Line Load and Point Loads are auto-calculated and applied on the mesh.
    4.  Include Column Sections in FE Model allows rigid perimeter of the column to be considered. The slab  will be meshed to the perimeter of the column rather than the centreline. 
      1. This option is specifically for flat slab system only. If the model has beam slab layout, please uncheck this option.
      2. This option introduces meshing complexity. If there are meshing errors, kindly uncheck this option & re-run analysis. 
Use Decomposed Slab Loads and Ribbed Slab Loads in Meshed Storeys should be unchecked in analyzing the model which consist of flat slab system, transfer slab system and irregular slab system. 
For
  1. Slab Stiffness Coefficients > In-plane (Membrane) for slab shells can be changed :
    1. This option can simulate semi-rigid floor diaphragms. To activate it, “No Rigid Diaphragm Floor Levels” should be selected.
    2. If “Storey Diaphragm Model” is considered (i.e. set to either “Slabs to Define Rigid Diaphragm” or “Single Diaphragm per Floor Level”), the membrane stiffnesses will be ignored since the shells will be infinitely rigid in-plane.
    3. If a value of “0” is entered for membrane stiffness, “No Rigid Diaphragm” option should not be selected. Otherwise, there will be instability in the analysis.
  2. Slab Stiffness Coefficients > Bending Stiffness (out of plane) for slab shells can be changed :
    1. Minimum value for Bending Stiffness multiplier is “0.01”. In this case, the bending contribution is practically ignored.
    2. This stiffness is applied only applied to uncracked load cases, not cracked load cases. 
    3. If cracked load cases are used, then please modify the stiffness in Material and Section Effective Stiffness Factors. 
    4. For more explanation on stiffness modifiers refer this article: Effective Stiffness Modifiers 
When you decrease the slab in plane and bending stiffness, the deflection of slab will be larger as the slab is more flexible or less stiff.
BENDING STIFFNESS: If you set the bending stiffness to i.e. 0.01, the supporting beam design moment will be also larger as the restraint from the slab is smaller. In this case, the beam design moment shall be very close to the floor without meshing (with the slab load decomposed using the “Load decomposition by FE” method).
IN-PLANE STIFFNESS: If you set the in-plane membrane stiffness lower, the lateral stiffness of the slab will also be lower, i.e. the slab is more flexible in horizontal direction. In this case, you will expect higher lateral deflection.
  1. Ribbed and Waffle slabs can be included. Only beams are considered; the plates are included as UDL. 
The result of the analysis can be viewed in the Analysis tab> Analytical Model . You can use various icons in to view displacement, member or shell forces graphically. 

Settings

Additional options related to the analysis can be specified on the “Settings” tab.

Issue Warnings For Cantilever Beams Not Marked

If free end of the beams are not specified in Graphic Editor, then program will display a warning to mark this free ends. If this option is unchecked, no warning will be displayed.

If individual cantilever beams marking have been manually done (via “Mark Free End of Cantilerver Beam”), it’s recommended to uncheck this option, otherwise this warning will always appear when Building Analysis is run.

Issue Warnings For Unsupported Columns Before Analysis

If columns or walls in the system are left unsupported, warning messages will be displayed to alert the user. Column may not be sitting on another member or may not be assigned a support at the lower joint. In case of such a warning, you must go back to Graphic Editor and correct the model. This feature can be disabled by unchecking this option.

Print Column, Wall and Beam Section Properties in Post-Analysis Report

Column, Wall and Beam section properties can be included in the Post-Analysis Report by checking this option.

Use ‘Sparse Solver’ for Building Analysis

The purpose of the sparse solver is to reduce the time required for analysis. For certain model types a dramatic reduction in the analysis time can be achieved, (e.g. models utilizing the FE shell model for walls or very big models); for other models it may be less significant.


However, the sparse solver is more stringent and less forgiving on modelling errors such as unsupported members and instability. If the model does not run to completion, try switching this option off.
Kindly refer the video in How to Detect Modeling Errors with Sparse Solver? | ProtaStructure for detailed guidance to resolve "Factorization of Positive Definite Sparse Matrix Failed" warning.

Total/Relative Horizontal Drift Limits

Building horizontal drift checking is carried out and included in the Post-Analysis Report. The limit values for the "Total Horizontal Drift" and "Relative Horizontal Drift" values defined here will be used in this report.


Total Horizontal Drift is the ratio of floor horizontal displacement to the height to the level of the floor, and Relative Horizontal Drift is the ratio of relative floor horizontal displacement to the height of that particular storey. 

It is entirely the user's decision what limit values to input based on his / her design philosophy and choice of reference. If there are failures, refer to the "Post Analysis Checks Report" under "Reports" tab for detail storey by storey calculation.  Common reason of failure for relative horizontal drift check is a change in lateral stiffness between 2 consecutive storeys, example  :
  1. Change in number of columns or walls 
  2. Change in layout or material, example if there steel structure on the roof of a concrete building
  3. Break in continuity of column or walls, example, a transfer storey with discontinuous columns or walls will have relatively higher lateral deflection than the "non-transfer" storey below it. 
  4. Lack of lateral stiffness (example : too slender, lack of strong walls or columns, or indiscriminate hinging of columns)
  5. Unstable & unsupported elements (example, pinned roof truss members with no proper restrain or bracing)
It is recommended that user access the "Analytical model" and turn on the displacement plot (or even "Animate" the displacement) under a lateral load case,  to visualize & identify the source of the relatively higher displacement in that particular storey.
It is users' decision or responsibility to verify these failures, after which the decision maybe to relax the limits or ignore the failure or to take corrective action.  Corrective action maybe to change structural framing and sizing to increase the lateral (sway) stiffness & to negate any of the causes of change in lateral stiffness of the storeys as mentioned above. 

Axial Load Comparison Tolerance

ProtaStructure compares vertical loads on the building before transferred onto the beams and after decomposed onto the beams before building analysis. A third check is performed by summing the column axial loads after the analysis. These three group of values must be similar. A 5% tolerance in difference is adequate as a default value. If there are problems related with slab yield lines or load transferring in the model, this difference may exceed 5%. A warning message is issued after the analysis if 5% tolerance is exceeded.  Examine the Axial Load Comparison Report for more details.

Storey Weight and Center of Gravity Calculations

After the vertical loads are defined on the system, they are decomposed onto the beams and reaction values are calculated at the nodes. These reactions are also regarded as masses at the joints.  If “Use Decomposed Loads” option is selected, then masses in the joints are used in center of gravity and weight calculations. If there are no beams in the system (like in Flat Slabs), then “Use Undecomposed Loads” option should be used.



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