Modeling and Node Generation Best Practices/Recommendations Tech Note
Structural detailing applications like ProStructures or AECOsim Building designer or Revit detail a physical structure, while analytical applications like STAAD.Pro or Ram Elements typically analyze and design a simplified analytical structure. These two different model types can create challenges when coordinating models from the two products. Integrated Structural Modeling (ISM) coordination between physical and analytical models can be facilitated by strategic choices of modeling style and node generation options. This Tech Note gives some best practices or recommendations on making those strategic choices. In this document we will discuss a ProStructures and STAAD.pro workflow, but the recommendations also apply to other physical to analytical workflows.
ISM assists STAAD.Pro in understanding a physical model by generating a set of nodes for the model that defined the key node locations and the member connectivity at those nodes. The STAAD.Pro settings available for controlling how the nodes are generated are shown in the following dialog.
Refer to Appendix A for details on settings in this dialog.
To detect which members should be connected, ISM simplifies the member geometry into extruded rectangles. ISM then determines the distance between the 3D volumes that represent each member. If the shortest distance between the two volumes is equal to or less than the specified Connectivity Tolerance, then ISM will consider the two members as connected.
To generate the nodes, ISM must determine a desired analytical location for the members. This is a difficult task because even two experienced human engineers may not agree on the proper analytical location for a member. ISM uses a heuristic approach that works well for building-like structures.
ISM’s analytical location for beams is the Location property of the beam, which is typically at the top-center of the beam. ISM’s analytical location for all other members is the Location property of the member, shifted to the centroid of the member (this shifting considers the Placement Point and Orientation properties, and the Section shape). ISM determines which member is a “Beam” by the Use property of the member. Other values for the Use property include “Column”, “Vertical Brace”, and “Pile”.
Tip: Create your model with the beams defined using the Location where you want the nodes to appear.
Tip: Ensure the Use property is set correctly for beams and other members (this feature is expected to be available in ProStructures V8i SELECTseries 6)
ISM generates an initial set of nodes for every pair of connected members and then merges those nodes together based on connectivity and proximity. When merging nodes, ISM uses the location of the more important node instead of averaging the node locations. A detailed explanation of the node generation algorithm is beyond the scope of this Tech Note.
After the initial set of nodes is created, ISM modifies those nodes based on the particular node generation settings selected.
When the ISM model is modified by ProStructures, previously existing nodes in the model may no longer be appropriate. STAAD.Pro and ISM will correct the nodes in a repository using the following approach:
In step 1, - the specified Node Member Separation Factor is used to determine how far a node can be from a member centroid and still be acceptable. This factor is multiplied by the size of the members at the node, so nodes with larger members have larger acceptable member-node separation distances.
The approach taken in modeling the structure in ProStructures can have a great effect on the nodes and STAAD model that is generated upon ISM import.
ProStructures allows the user to choose the insertion point for steel (and concrete) beams and columns, as well as steel horizontal braces, vertical braces, stair stringers, etc. Some other structural software programs refer to this as the “cardinal point” or “placement point”. In ProStructures, the insertion point defines the ‘member line’ of the object.
ProStructures insertion points
ProStructures is designed to allow fast and accurate structural modeling and there are behaviors built-in to the program to take advantage of the insertion points. In a typical structure, columns may be placed using an insertion point of center-center (CP=5). Beams are typically placed with an insertion point of top, center (CP=8). Vertical braces may be placed according to Bottom-Left or Centroid (CP=1 or CP=10).
This approach to physical modeling allows for extremely fast and accurate modeling of steel and concrete frames. Moreover, the resulting model is set-up very well for all types of drawing production, especially where steel elevation drawings are produced in single-line mode as shown below. Please note that the W36x135 beam ‘member line’ appears at the top of the columns (because the beam was modeled with an insertion point of top-center). Also note that the columns are dimensioned to their centerlines (because the columns were modeled by an insertion point of center-center).
Elevation drawing using single-line display with widgets (the term used to describe the short portion of the shape).
The Include Member Offsets option in STAAD.Pro’s ISM link is so influential that it merits discussion before other modeling topics.
During ISM import if Include Member Offsets is used, STAAD.Pro will consider the ISM member’s Location, Placement Point, and Section to calculate offsets from the nodes so the member can be located in STAAD.Pro in its true physical position. This effectively adds a rigid link to STAAD’s analytical model. Engineers tend to prefer simplified analytical models without rigid links.
If Include Member Offsets is used during an ISM export, STAAD.Pro will consider the STAAD.Pro offset dimensions and set the ISM member’s physical location identical to the STAAD.Pro member’s physical location.
Using Include Member Offsets gives good round-trip fidelity for member locations. However, if a STAAD.Pro member’s section changes size, its centroid location is preserved faithfully, but for floor beams the top-of-steel elevation may change in STAAD.Pro and that change will be exported to ISM when this option is used.
During ISM import if Include Member Offsets is not used, STAAD.Pro will consider only the ISM node positions to determine the STAAD member location.
During ISM export if Include Member Offsets is not used, STAAD.Pro will set the ISM member’s Location to the STAAD.Pro location, but will use the existing ISM repository Placement Point for the member, if one exists.
Not using Include Member Offsets yields simplified STAAD.Pro analytical models. However, if an ISM model is not centerline-aligned or centerline-top-of-steel-aligned (as explained in the following sections), some members will move upon updating the ISM repository; these changes generally need to be rejected in Structural Synchronizer.
Whichever method most suits your need, it is important to always set Include Member Offsets to the same value when synchronizing a particular model with ISM. Inconsistent settings can cause significantly member location changes when centroid modeling is not used.
Modeling the members along their centerlines and locating the members so their centerlines intersect at desired node locations would be one way to ensure smooth operation with STAAD.Pro. When the members are centerline-aligned, the physical model is identical to the desired analytical model, so ISM and STAAD.Pro can easily generate the nodes and make the conversion. However, in ProStructures, this approach is much more manual and slow than using top-center insertion points for beams.
Users should be aware that the Centerline Modeling approach will result in undesired results in ProStructures drawing output. In an elevation view with single-line display (as pictured above), the beam will show it’s member line at the centroid and not at the top of steel. Because of this, the Centerline Modeling method in ProStructures is not recommended by Bentley as it is not a viable approach for drawing production. However, there may be some instances where this approach does make sense, therefore the recommended settings are listed below.
Tip: You need to ensure that the beams are modeled along the centerline for this to work.
For this type of modeling, the following settings are recommended:
Include Member Offsets
Method for Fixing Nodes
Move nodes to analytical line and common plane
Small number such as 1mm
Node Merging Tolerance
Small number such as 25mm
Plane Merging Tolerance
Node Member Separation Factor
Tip: A larger Node Member Separation Factor will do no harm initially, but it may cause STAAD.Pro to not notice small member movements in further round-tripping of model data.
Tip: A larger Connectivity Tolerance (such as 25mm) may be better for models where most of the model is centerline connected, but some members are not.
A variation of Centerline Modeling is an approach where the members are modeled along their centerlines, but the members do not overlap and are instead cut back as they will be in actual fabrication.
If rigid links in the STAAD.Pro model are acceptable to the engineer, the following settings are recommended for cutback situations:
Move nodes to common plane
Maximum separation between connected members
Tip: For steel structures, a Connectivity Tolerance of 25 mm often works well.
If rigid links are not acceptable in the STAAD.Pro model, the following settings are recommended for cutback situations:
Note: In this option (cutbacks exist, but rigid links are not acceptable), when STAAD.Pro updates the ISM repository, it will attempt to extend the members to the node locations. These Location changes will need to be rejected.
The recommended approach to modeling in ProStructures is the Top of Steel – Center approach. In this approach, beams are modeled at top of steel – center, while columns are modeled at insertion point center-center. This way, beam member lines intersect the member lines of other members (e.g., columns)., at the same floor level elevation. This modeling approach typically results in more desirable node locations and has the advantage of keeping the floor level flat even if the floor beams have varying depths.
Tip: Modeling the columns or braces using a non-centroid member line will not harm ISM’s generation of nodes, as ISM will use an analytical line along the centroid for those member types.
Please note however, that if a structure modeled by this approach is exported to ISM, and then imported into STAAD (without offsets) – then the beams will be at locations which differ slightly (offset to the centroid) from the physical model.
A sample of the differences can be shown with a simple model:
Imported to STAAD without offsets:
Imported to STAAD with offsets:
Note: The model shown above is not strictly Top of Steel Center Modeling as the chevron braces do not intersect at the beam top of steel.
Notice the difference in the STAAD models – the last image shows the beams at the correct physical location, the middle image shows the beams at centroid locations (no offsets imported). Both of the STAAD models above are valid analytical models, the user should decide which approach is most accurate for their needs.
Tip: You need to ensure that the beams are modeled along the top centerline for this to work.
The recommended node generation settings are the same as for the Centerline Modeling case. The effect of cutbacks on ISM synchronization is also identical to the Centerline Modeling case.
However, if Include Member Offsets (rigid links) is used and STAAD.Pro changes the floor beam sections, STAAD.Pro may shift the modified beams’ elevations in the ISM repository when updating (STAAD.Pro will keep the centroid at a constant elevation). Those beam Location changes should be rejected in Structural Synchronizer.
Most structures can be modeled by the Top of Steel - Center approach except for a few members in the model. There are two approaches that can be taken for these members:
For these models, changes to the other node generation options have greater influence.
If Include Member Offsets is used, STAAD.Pro will consider the ISM member’s Location, Placement Point, Orientation and Section to calculate an offset from the node so member can be located in the STAAD.Pro model in its true physical position. The following node generation settings are recommended:
100mm (subject to model particulars)
Half of the depth of the deepest member in the floor system
Tip: In most models, the Plane Merging Tolerance can be set to a huge value (such as 10m) with no harm. You might try that value first and adjust it only if you do not like the resulting model.
If section sizes change in STAAD.Pro, STAAD.Pro will update the ISM model such that the ISM members’ centroid locations match the STAAD member locations. This may or may not be what is desired. If this results in undesirable Location changes, these changes can be rejected in Structural Synchronizer, or fixed in ProStructures.
If Include Member Offsets is not used, the STAAD.Pro member will be placed between its node locations, with no offsets. The following settings are recommended:
Tip – In most models, the Plane Merging Tolerance can be set to a huge value (such as 10m) with no harm. You might try that value first and adjust it only if you do not like the resulting model.
When Include Member Offsets is not used and the model is not centerline or centerline-top-of-steel, STAAD.Pro will attempt to move some members when updating the ISM repository. These movement changes can be rejected in Structural Synchronizer or fixed in ProStructures.
If the structure is modeled in ProStructures “by eye” or not using snapping tools appropriately to create an accurate model, ISM may have greater difficulty in creating an appropriate set of nodes. Nodes that are slightly out of alignment may cause STAAD.Pro to break members into pieces, and may result in location changes when updating the ISM model from STAAD.
ISM must respect small intentional non-alignments (such as a roof that is sloped at 1%), and this prevents ISM from straightening accidental alignments of similar magnitude. We recommend a modeling accuracy of 1:1000. For example, two parallel beams should not be out of parallel by more than 1mm in a meter length.
When less accurate modeling is used, tolerances used in node generation may need to be increased, which risks unintended side effects.
Structural Synchronizer contains many features that facilitate efficient management of changes. For an overview of change management features, refer to Section 5 “Using The Structural Synchronizer” in the IsmQuickStart_en.pdf document that installs with the program. This Tech Note provides additional details and examples.
ISM files organize the contents into “levels” (levels are the MicroStation term analogous to “layers” in AutoCAD™). The Levels control can be used to choose what is displayed graphically, which makes selecting items easier.
Levels Control in Structural Synchronizer
In this scenario, a model has brace locations (in the physical structure that were not aligned by one of the centroid modeling options (refer to Section 3.2 and Section 3.3). When updating the ISM repository from the STAAD.Pro model, you might find unintended changes to the brace locations you want to reject. However, there may be other changes you may want to keep (e.g., material, section, etc.).
Use the following procedure to simplify rejecting changes:
Note: If the Location property is not editable (e.g., it is grayed-out and inactive), you will need to unselect any brace in the Objects pane that does not have the Location property changed before Step 4.
For the same problem described in Section 4.2, if the braces all share the same section size, the following approach could be used:
Option 1 - Sort using the Objects pane (by Tag or Status)
And use the Clear All button and the checkboxes to select the appropriate members.
Option 2 – Click on the Isolate tool and use the Element Selection tool to remove an undesired items.
5. Reject the Location change.
Note: If the Location property is not editable (i.e., it is grayed-out and inactive), return to step 4.
This section lists common issues which can arise when using STAAD.Pro in conjunction with an ISM repository. The solutions paired with each problem present notes on how to address each. Please refer to previous sections of this document for additional details.
Problem: A member does not move in STAAD.Pro, even though it has moved in the ISM repository.
Solution: Set the Node Member Separation Factor to a smaller value.
A detailed description of each of the node generation options is given below
For import from ISM into STAAD.Pro, the Include Member Offsets setting causes STAAD.Pro to consider the Location, Placement Point and Section of the ISM member and convert those values into appropriate offsets from the node locations to place the STAAD.Pro member at its true physical location. If the setting is not used, STAAD.Pro places the member directly between its nodes.
For export from STAAD.Pro to ISM, the Include Member Offsets setting causes STAAD.Pro to consider the offsets to calculate the ISM Location for the member. If the setting is not used, STAAD.Pro sets the ISM Location for the member to be between the STAAD.Pro nodes.
There are four options for fixing (correcting) the nodes generated by ISM.
None – This option does not perform any correction.
Move Nodes to Analytical Lines – This option aligns the nodes in each member such that all the nodes are located on a line between the first and last node.
Move Nodes to Common Planes – This option investigates the planes defined by the cross section axes for each member. Where connected members have parallel planes the members, their nodes and their planes are considered simultaneously. Once a collection of parallel planes and associated nodes is found, the best plane for fitting the nodes is determined, and all the nodes are moved to that plane.
Move Nodes to Analytical Lines and Common Planes – The option combines the previous two options, with the Common Plane adjustment occurring before the Analytical Lines adjustment.
This distance is the maximum separation between two member’s simplified (extruded rectangle) volumes that will trigger a node to be generated connecting the two members.
This distance is the maximum separation between two nodes that will always be merged together into a single node.
This distance is the maximum distance a node will be moved when performing the Move Nodes to Common Planes adjustment.
This factor is used to determine how far a node can be from a member and still be “reasonable” for the member. This reasonable test is used to determine if a member should be disconnected from a node (which may in turn cause the node to be deleted). The factor is multiplied by a distance based on the member’s largest cross section dimension, and the largest cross section dimension of any other member connected to the same node. The distances are measured from the centroid line of the members.
Capabilities and Limitations