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General

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External references

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EDMmodelServer referances

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Using the Area Calculator

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Preconditions

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Base for calculation are IfcSpace geometry and not IfcWall or similar objects playing role as physical walls.
For outer walls the "Storey BTA polygon" for BTA of storey is used as limitation of "outer walls" instead of actual IfcWall instances. The BTA polygon is based on a "virtual BTA Space" that is supplied in the ArK data (CAD system). If the CAD system does not supply this space, it must be approximated by an AC component, this will impact accuracy significantly.
The produced result is not area values, but a set of polygons that defines different types of areas. The area values are calculated out of the polygons when needed. For each space and storey, the total area is composed of a set of polygons, each polygon composed of "outer" polygons representing positive contributions to the area, and a set of "inner" polygons intended for adjusting the area This is very common model and is usually called Simple Concave polygon with Holes (simplicity here means no self-intersection of edges).
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Figure 1 Area polygons for a space
In this example UA is composed of two parts: PO1 og PO2, both of which has an "inner" polygon – a hole. The rest of the room is covered by an opening, represented by polygon O3.
The generated partial polygons are combined into composite polygons representing areas like UA, NTA etc. The rules for composing these polygons can relatively easy be adapted to national regulations and customer demands.
Generated composite polygons for UA can be edited using Polygon Editor as described in <TODO>.
The generated partial polygon types are listed in the following table:

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Storing AC results

Calculation results are attached to the IfcSpace objects.
If space properties are used in lease contracts or similar, they cannot be changed. Hence, we must to a certain extent store both USED and CALCULATED values.
If a space is missing the BTAspace/NTAspace properties, the calculated properties can be copied directly . This is the usual case with a "new" BIM. For other situations, the transfer of calculated properties into used properties must be a managed process.
All properties are stored in the FM_Areas property set:

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• Name "FM_Areas_zzz_outer_xxx" identifies an outer polygon, where xxx is a number starting from 1 (without any particular ordering) and zzz is a polygon class (text label as in 3.1).
• Name "FM_Areas_zzz_inner_xxx_yyy" identifies an outer polygon, where yyy is a number starting from 1 (without any particular ordering) and zzz is a polygon class (text label as in 3.1) and xxx is a number of outer polygon to which this inner polygon is bound.

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Running Area Calculator

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There may exist spaces in the model that covers an entire wing, a zone or similar. Such aggregated spaces containing a lot of other spaces can be excluded from the AC process by marking them as "virtual". This can be done via report or with zoning tool. As mentioned in 3.3.1, such areas are excluded from calculations anyway, but 'virtual' mark will also remove them from result set.

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Rules for calculation

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Figure 1 - Storey BTA polygon

Figure 2 – Walls defined by Space and BTA polygons

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BTA for building storey

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In general, columns inside a space are not influencing the area calculation.
However, as some CAD models can create "holes" in the space geometry around columns, they can be taken into account indirectly. It is advised to make sure that no such holes are present.

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Staircases and similar

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Text step is to add the area according to the 60cnm-rule in [NS3940-V]. Since the «cut line» C1-C2 is a straight line this is fairly straight forward:

Figur 6 – Kompensering for 60cm -regel

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Area of stair itself

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Stairs with irregular cut lines

Innhold
It is not as trivial as it sounds to identify "area below 1.90" under a stair. An important concept here is the "cut line" which is the common line between projected area (footprint) below 1.90 and projected area (footprint) above 1.90. For the simplest case this is a straight line. However, if the stair geometry is more complex the cut line will also be more complex. The reason for this could be as simple as a stir built as two beams bearing the foundation for the steps:
For the simplest case this is a straight line. However, if the stair geometry is more complex the cut line will also be more complex:

Figur 5 - Approximating the cut line
This approximation will probably only work if railings, fences etc is not taken into account. Until further notice, it is believed to be possible to filter these out by type.
If the stair includes railings and/or it is an open stair, the footprint becomes more complex:

Figur 6 - open stair with railings
The footprint of stair below 1.90 is lower left in figure. In this case it even contains "holes"; if these introduce problems it should be possibly to apply an algorithm to remove holes. Imposing the footprint for entire stair introduces several possibilities for points C1 and C2 and start/(en d of "cut line":

Figur 7 - footprints for symmetrical stair
As long as at least one of the C points can be found the previous algorithm should work. To ensure symmetry we can run algorithm twice, in CW and CCW directions, and pick first point found in both cases.
If it turns out to be problematic finding C points, we could apply an algorithm for making polygon convex on the stair-below-90 footprint:

Figure 8 – Apply convex footprint enclosing line
If the stair is "asymmetric" around its midpoint line, for example included railings but on only one side, the approach will not work the same way:

Figure 9 – Asymmetrical case
The line between identified points C1-1 and C2-1 are no longer a proper "cut line". TODO: Currently, a simple solution for this case is not identified. We must rely on stairs not including railings, or being symmetrical, and use C1C2 as cut line.
To enable polygon edit, and simplify the further calculation (60-cm rule), we should always perform this "cut line approximation" step.
When the "cut line" is approximated as a straight line the "60 cm rule" is rather easy to implement:

Figur 12 - Adding 60cm compensation
An acceptable and simple approximation is to create a rectangle with the cut line E1E2 as one of the sides, a parallel F1F1 as opposite side, the two other sides being 60cm in length.
There is one exception, for small curved/spiral staircases, where the compensation polygon will be a triangle version of the above: E1E2F1.

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We might come across "free form stairs", a general approach is to "cut" the stair by bottom and top (next storey floor) and by again by 1.90 height and top, creating footprints, and analyze these in 2D. See figure below:

Figur 13 - Free form stair

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Staircases and openings

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Staircase spanning multiple storeys in an opening

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• In bottom storey U1 there is a floor under the stair, so rules for "staircase starting in this storey" apply.
• In the middle storeys 01 and 02 the footprint of the stair going DOWN from this storey is included in the BTA, while rest of the opening is NOT included.
  • BTA = BTA0 – Op + S0p
• In top storey 03 likewise, the footprint of the stair going DOWN from this storey is included in the BTA, while rest of the opening is NOT included.
  • BTA = BTA0 – Op + S0p

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Staircase spanning multiple storeys with floor under stair

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