Building Elements

Building elements are used to give the geometry certain qualities which means the polygons can be described very specifically as building components. A building element is a construction type, and this means building elements allow you to draw walls, floors, ceilings, windows, and so on. Without building elements the geometry would be input as nondescript "lines" instead of "walls", and the individual polygons would just be "sides" with a "top" and "bottom". Giving the polygons different attributes which relate to construction types means that the lines and faces become walls, ground floors, and roofs.

Building data isn't actually added until after the geometry has been exported to the Building Simulator, given the 3D Modeller is purely for geometry input. This means that a building element doesn't inherit any thermophysical properties until a construction has been assigned to it. They don't have any thermal mass of their own, and they have any affect on the thermal behaviour of any zones. This information will come about through construction assignment in the TBD later on.

Default building elements are available for you to use, in addition to these you can create your own. You can use the <default> and <null> options to start drawing in walls and dividing up the floor plate, e.g. to create lighting zones. Floor and ceiling surfaces will be added in automatically when a closed path is formed. You can set floor and ceiling surfaces to <null> if you need to remove the surfaces from the model. A null surface will act as an aperture, allowing air to flow freely between the adjacent spaces. The software will also create other building elements when it generates the analysis model and these will be visible on the Building Elements dialog.

The default building elements will now be described:

External Wall

These are walls that are exposed to the outside. When you draw in <default> walls which are exposed to the outside, you don't have to manually set them to "External Wall", as the software will do this for you automatically. Do note, however, when you select a <default> wall in a 2D view, and then click on "Properties" on the right-click menu (you can open the context menu by right-clicking on the wall), although the building element will be shown on the screen as <default>, it will be exported to TBD as an external wall.

So the software will be able to recognise the wall as being an external one without you needing to tell it. You can see this in the TBD file easily enough. On the Surfaces tab for the adjacent zone, the Type column will list the surfaces by their type. If the surface type is shown on the screen as "Exposed" and the wall is <default> in the 3D Modeller, then the software will set the building element to "External Wall" by itself.

On the Building Elements dialog, the "Used" box next to "External Wall" will not be ticked unless you have used this element to actually draw something. The box will denote which elements have been assigned, not necessarily how they're going to be exported.

If the outside wall is going to be shaded by external shades then you will have to draw in some <null> lines around the wall (there is a video demonstrating how to set up external shades). In the 3D view, adding in these walls will appear to change a wall type to an internal one. You don't have to manually set the wall back to an external wall because when the area outside the wall is given an external zone, the wall will be reset to external for you.

Internal Wall

These are walls that are inside the building, not exposed to the outside. Internal walls that are drawn using the <default> element will be exported to the Building Simulator as "Internal Wall". The surface type, as denoted in the TBD, will be described as "Internal" when the adjacent room has the same zone name, or it will be described as "-> <zone name>" if the adjacent rooms have been given different zones. It will be called a "Null Link" when a zoned space is adjacent to an unzoned space. A null link will assume that both spaces are at the same environmental condition so the interface between the two spaces will set up an adiabatic link.

Here's an example of a situation where the software would create the "<element 1>-exposed" building element when setting up the 3D analysis model:

• Geometry
• Internal Wall-exposed

In this example, two spaces are separated by a <default> wall where the ceiling height of one side has been raised by some arbitrary amount. One space has a ceiling height of 6.0m, while the other space is only 3m high. Where two adjacent spaces have a different ceiling height the intervening wall will be internal between the ground and the lower of the two ceiling heights, and (in this example) external between 3 and 6 metres. The exposed section of wall will be coloured green and will be set by the program to "Internal Wall-exposed" but only after the analysis model has been generated. The element width is 0.1m, which is the default width for "Internal Wall". You can edit the width of this element on the Building Elements dialog. This new element will be exported to the Building Simulator and to it you will be able to assign a suitable construction.

If the intervening wall was drawn as a <null> line instead of using the <default> option, then the software would have created an element called "null-exposed", and its width would be 0.0m. The width value can be edited on the Building Elements dialog. The "null-exposed" element is a physical construction type, not a <null> line (and therefore not an aperture).

Ground Floor

An element will be in contact with the ground when the "Ground" box on the Building Elements table is ticked, which it will be by default for this one element. Provided a zone does not then link to the adjacent zone when the zone floor is <default>, the "Ground Floor" building element will automatically be set and the surface type will be set to "Ground" in the TBD. Solar gain cannot fall on the outside of a ground surface so if the "Ground" flag is on for a transparent object then solar gain will not be absorbed.

Internal Floor

As you draw in walls, various surface polygons will be formed. Floor and ceiling surfaces will automatically enclose the spaces, and by default the floor polygon will be set to "Internal Floor".

The element which is exported to the Building Simulator will be a concatenation of internal floor and internal ceiling. When you display the geometry in a 3D view and set the analysis mode (either on the toolbar or through the View menu) the software will merge adjacent surfaces and adopt the convention "<element 1> / <element 2>", creating a simple concatenation of the two names.

With the exception of the ground floor, the internal floor will either be exposed or adjoined to a room on a lower level. If it is exposed then the software will assign the "Exposed Floor" building element. If it sits directly above a space which is on a lower level then the software will merge the floor surface with the ceiling of the space beneath, and it will create a new building element called "Internal Floor/Internal Ceiling".

Exposed Floor

If the floor surface does not link to an adjacent space, then the software assigns the "Exposed Floor" building element to the polygon. This means that the U-value of the construction must be suitable for a surface which is exposed to the external climate. Suitable constructions will be added later in the Building Simulator.

Internal Ceiling

As walls are drawn in, various surface polygons are formed which represent the different rooms and spaces within the building. Floor and ceiling surfaces will enclose the spaces and by default the ceiling polygon will be assigned "Internal Ceiling". Much like the "Internal Floor" building element, the name will be exported to the Building Simulator as a combination of internal ceiling and internal floor. When you view the geometry in a 3D window and open analysis mode, the software will merge the two surfaces and adopt the convention "<element 1> / <element 2>". With the exception of the ground floor, the internal ceiling is either exposed or it is adjoined to a room on a higher level. If it is exposed, then the software assigns the "Roof" building element. If it occupies a space directly below a room on a higher level, then the software merges the ceiling surface with the floor surface above it and creates a new building element called "Internal Floor/Internal Ceiling".

Roof

This building element represents a ceiling surface which is exposed to the outside environment. As walls are drawn in, the polygons begin to form closed spaces. Floor and ceiling surfaces are assumed to enclose the spaces, and by default an exposed ceiling surface is assigned the "Roof" building element.

Areas and volumes

Every building element will have a width value, which is usually non-zero. Building element widths do not represent insulative properties, and they don't actually need to be the same as the real building materials. The width of the <null> element is always zero, as it is used as a dividing line. By changing the width of a building element, floor areas and volumes will be affected.

Floor and ceiling polygons occupy a proportion of the adjacent space, where the volume occupied by these surfaces will increase with building element width. The overall room volume reduces simply because the building elements that are applied to the floor and ceiling will "grow" into the room when their widths are made bigger.

Zone floor areas and volumes are displayed in the TBD file, after having exported the geometry data to the Building Simulator. In addition to zone floor areas, the individual surface areas are also displayed. Individual surface areas are calculated by averaging the internal and external areas, which themselves are based on the building element widths. In other words the Building Simulator uses the resulting areas during thermal simulations. Therefore, the zone floor area will not necessarily be the same as the floor surface area. Resulting areas are used in thermal calculations as a means of modelling thermal bridging.

Imagine having drawn a cube with the <null> building element, where the floor and ceiling surfaces are also <null>. The building element width on all sides is zero. Let the floor to ceiling height be "h". Given that the cube is drawn entirely with <null> lines, it doesn't matter whether the "floor to ceiling" height is specified as finished or unfinished, since both heights will be defined as "h".

Given that all the sides are of equal length "h",

• The unfinished floor-to-ceiling height is h.
• The finished floor-to-ceiling height is also h.
• The zone floor area is h^2.
• The zone volume is h^3.
• All surface areas, including the floor, ceiling, and walls, are h^2.

The following screenshots of the geometry and building data illustrate the situation where the cube has sides where "h" = 3m: 3D Modeller; Building Simulator.

Suppose that the floor and ceiling element widths are now non-zero. Let their widths be denoted by "f" and "c", respectively. The finished floor-to-ceiling height becomes h-f-c, where the default wall height, as determined by the Floors dialog box, is "h". In other words, the actual height of the room within which an occupant could stand upright is reduced by the sum of the floor and ceiling widths, since we know that these elements will grow into the room.

• The unfinished floor-to-ceiling height (default wall height)  is "h".
• The finished floor-to-ceiling height is (h-f-c).
• The zone floor area is h^2 (given that the wall widths are zero).
• The zone volume is (h-f-c)*h^2, which is the finished floor-to-ceiling height multiplied by the zone length, "h", to obtain the internal wall area, and then multiplied by zone depth, which is also "h".
• The floor surface area is also h^2.
• The wall surface areas are 0.5*[(h-f-c)*h+h^2], which is the average of the external and internal walls areas, where the internal wall area is the default wall height minus the floor and ceiling widths, multiplied by the zone length, "h". The external wall area is simply h^2. The average is the product of these areas, divided by 2.

Now assume that "h" = 3.0m and "f" = "c"= 0.2m: 3D Modeller; Building Simulator.

Finally, suppose that all the walls are now drawn using non-<null> building elements, each with a building element width of "w". Assume the walls are drawn with 'snap to centre line' enabled,

• The unfinished floor-to-ceiling height (default wall height) is "h".
• The finished floor-to-ceiling height is (h-2w).
• The zone floor area is [h-(w/2)-(w/2)]^2, which simplifies to (h-w)^2.
• The zone volume is (h-2w)*(h-w)^2.
• The floor surface area is 0.5*[(h-w)^2]+[(h+w)^2], the average of the external and internal floor areas.
• The ceiling surface area is the same as the surface area of the floor area.
• The wall surface areas are 0.5*[((h-w)*(h-2w))+((h+w)*h)], where (h-w)*(h-2w) is the internal wall area, and (h+w)*h is the external wall area.

In this example, "h" = 3.0m and "w" = 0.2m: 3D Modeller; Building Simulator.

On the Building menu, click Building Elements.

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