NOTE: Version 14.04.00.00 of RAM Structural System included changes that resulted in a significant reduction in the incidences of the polygon intersection error. The release notes for this version can be found here:

Version 14.04.00 Release Notes,

Error in Polygon Intersection

The message, “Error in polygon intersection” is fairly general, and could be indicative of several different conditions related to the geometry of a RAM SS model. Several polygons exist on any floor plan such as:

• surface loads
• snow loads
• areas of deck

The program also defines some internal polygons when calculating tributary areas of individual members. We call these

• “beam loops”

A polygon error may occur at various stages of the modeling and design, specifically during any of the following events: 

• data check in the Modeler 
• building of the “Framing Tables” at the start of any design module 
• building the “Frame Data” in RAM Frame 
• analyzing Wind loads in RAM Frame.

Modeler - Data Check

When a polygon error occurs while running the data check, the modeler will terminate. This should not cause you to lose any recent changes, however.

One possible cause for such an error is the presence of zero-length beams or walls in the model. These members cannot be seen, but the numbers can be displayed. If you see beam numbers with no associated beams or overlapping beam numbers, this will help in locating the problem. You can then delete these beams with a fence. The beam numbers should then disappear as confirmation.

Under normal circumstances, zero-length beams cannot be modeled, but they can be imported from DXF or Revit, or they can sometimes result from modifications to the grids after the beams have been modeled.

Beam or Column Design - Framing Tables

When any design module is initiated, the program starts by “Building the Framing Tables”. During the process, the program goes through the beam loops one at a time to determine the tributary loads. If a polygon error occurs while the framing tables are being built, the user can identify the beam number and coordinates together with the floor type where the program stopped processing information. This information can be helpful in locating the source of the error.

The polygon error may be followed by a second error message saying, “SetLineEq() passed two identical points at 0.00 0.00”.

This usually indicates a problem with the shape of the beam loop. For example, if there is only one beam connected to an interior column within the deck area, a polygon error will arise for one of the members in that immediate vicinity:

A second type of message saying, “Clipping a degenerate polygon” or “Too many intersection points” indicates a different type of polygon error.

The error in this case may be due to internal numerical accuracy, especially when the beams or polygons in that area are ever so slightly skewed in plan.

This type of error is often caused from having multiple, overlapping surface loads or decks. The program only counts the top-most layer of surface loads when analyzing a structure, but it must make a determination about what load is "on top" by comparing the areas of the various overlapping surface load polygons as they were applied. When multiple layers of loading are used, this calculation becomes more complex.

The program may give an error when only a tiny sliver of one load overlaps another, or if the edge of a surface load polygon is very slightly off of a beam line. In this case, it is necessary to delete and re-apply that surface load in a different area. The more complicated the beam loops, the more likely a polygon error will occur.

In the example below, the end coordinates of beam 113 did not match the end of beam 108 or the column in that location creating a tiny polygon for the program to deal with. Furthermore, the surface load also angles just slightly away from beam 98.

To solve the problem in this case, beam 113 was adjusted to match the column coordinates and the surface loads were recreated.

Tip: It may be necessary to save the file under a test name and systematically eliminate load and deck polygons from the file until it works, then return to the original file and rework the problem area only. When laying out deck or surface load polygons, first turn off all the unnecessary snap points. Typically the only snap point needed are Columns, Beam Ends, and Wall Ends:

RAM Frame – Building Frame Data

After the framing tables are finished, the RAM Frame program goes through a second step of constructing the Frame Data. During this phase the program determines the diaphragm masses and the gravity for notional loads of each diaphragm. This process is similar to the load calculations done during the framing tables, but slightly more complicated because the program is working with the larger polygon defined by the entire slab edge.

Regretfully, the error in RAM Frame does not isolate the level type with the problem. In order to isolate the problem area the user should systematically remove stories from the story data until it works, then go back and examine the loads or decks on the problem level.

RAM Frame – Analysis / Wind Exposure data

If a polygon error occur near the end of the analysis in RAM Frame the issue is a little different. At this point the program is creating the polygons associated with the wind exposure. These shapes determine the total wind load tributary to any diaphragm and can be reported using Reports – Exposure boundaries.

Typically the situation that causes this error is a complex slab edge that overlaps itself. Consider the geometry pictured below. If the inlet is 1’ wide and the program will function perfectly well when the slab edge overhang is less than 6” on each side, but once the overhangs are increased such that they meet or overlap problems can occur:

RAM Concrete – Meshing Diaphragm

Below is a polygon intersection that was produced when the diaphragms are meshed during the RAM Concrete Analysis. This error is most likely associated with too many overlapping deck polygons. The problem floor level will be identified in the Analysis Status window. The coordinates, defined in inches, can be used to find the offending polygon.

See Also

RAM SS Common Framing Table Errors [TN]

Structural Product TechNotes And FAQs