Modeling and Node Generation Best Practices/Recommendations Tech Note

2013-05-31

1 Introduction

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.

2 Node Generation Overview

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.

2.1 Determining Which Members Should Be Connected By Nodes

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.

2.2 Member Analytical Locations

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)

2.3 Generating Nodes

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.

2.4 Correcting Nodes

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:

1. Members that are too far from nodes they are connected to are removed from the nodes.
2. Nodes that are no longer connected to members are removed.
3. New nodes are generated for connected members where no existing node is provided.

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.

3 ProStructures Modeling Styles & STAAD Import Options

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).

3.1 Include Member Offsets

The Include Member Offsets option in STAAD.Pro’s ISM link is so influential that it merits discussion before other modeling topics.

Using Offsets

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.

Not Using Offsets

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.

Using Offsets Consistently

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.

3.2 Centerline Modeling

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:

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.

Cutback Variation

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.

Rigid Links Acceptable

If rigid links in the STAAD.Pro model are acceptable to the engineer, the following settings are recommended for cutback situations:

Tip: For steel structures, a Connectivity Tolerance of 25 mm often works well.

Rigid Links Unacceptable

If rigid links are not acceptable in the STAAD.Pro model, the following settings are recommended for cutback situations:

Tip: For steel structures, a Connectivity Tolerance of 25 mm often works well.

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.

3.3 Top of Steel -Center Modeling

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:

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.