How to Model Related Walls and Slabs

by GRAPHISOFT and lnagy · updated: 05.27.2011

One of the most common relationships within buildings it that of Walls and Slabs, plus their related structures.

This article wsoliill detail some of the most important methods and considerations during the creation of these elements.

See WallSlabIntersections.

Wall Height

There are several approaches to defining wall height:

1. A multi-story Wall extending the full height of the structure

2. Each wall extends from top of Slab to top of Slab of Story above

3. Each wall extends from top of Slab to bottom of Slab of Story above


In the image above, hotspots indicate wall height for each of these three cases.

Things to consider:

– In the first two cases, you must do Solid Element Operations if the bodies of Slabs and Walls intersect.

See Solid Element Operations in ArchiCAD Help.

– In the third case, no SEO is necessary, because the Slabs and Walls do not intersect.

– Multi-story walls are easier and faster to create and modify than if you are working with separate walls on each story. A vertical multi-story wall can easily be converted into a slanted multi-story wall and vice versa. (See Create a Slanted or Double-Slanted Wall in ArchiCAD Help.) Also, a Wall modeled from one piece will be more accurately displayed in 3D (no danger of lines appearing at joins of wall segments).

– If you create individual walls on each story, their creation and modification will take longer. Also, lines might appear at the joins of individual wall segments. However, individual walls can be scheduled more easily: their quantities can be better calculated, as these operations are usually done story by story and not in one large piece.

Slab Vertical Placement

Two main options are available:

1. Top of Slab is at top of planned structure (Slab represents only current structure)

2. Top of Slab is at top of planned floor level (Slab represents both the structure and the floor in one piece)


The illustration shows the two possible choices.

Things to consider:

– Placing a Slab to represent the current structure only is feasible, for example, if you wish to send model information to structural engineers. However, you will need to represent the Floor structure separately.

– Placing a Slab to represent both the structure and the floor is a faster solution. However, this level of detail will be adequate only for schematic design phases and you may need to later modify it in later phases that require a greater level of detail.

Slab Contours

Three main possibilities:

1. Edge of Slab at inner side of Wall

2. Edge of Slab at inner side of insulation around Slab

3. Edge of Slab at exterior side of Wall


The Slab at the above illustration (from bottom to top, respectively) show the three options above.

Things to consider

– If the Slab edge is at the inner side of Wall, there is no problem with handling Wall-Slab intersections, but the Section of the structure will not be correct. You will need to work on it in 2D and Section/Elevation views.

– If the Slab edge is at the inner side of the insulation, you can create a structurally accurate model. This is especially important when working on more complex buildings or when you anticipate frequent exchanges of information with structural consultants. Wall-Slab intersections must be handled. Insulations around the Slab must be modeled separately.

– If the Slab edge extends to the Wall’s exterior, modeling is faster, but Wall-Slab intersections must be handled; the intersections will be structurally correct. This modeling choice works well if the insulation is placed in front of both the Wall and Slab, and no additional insulation is needed around the Slab. It also works for quick schematic design phases. However, since the edge of the Slab is in the same plane as the exterior side of the Wall, their materials should match. Also, unwanted lines may appear at the exterior surface of the Wall.


Consider these five choices for modeling insulation:

  1. using 2D elements in Section/Elevation Views
  2. with Slabs
  3. with Beams
  4. with Walls
  5. including the insulation as part of a Complex Profiled wall (ArchiCAD 10 and above)


Things to consider:

– It is easiest to create and modify insulation and other wall details using a Complex Profile

– It is next easiest to create and modify insulations if you do so in 2D only. However, if the structural engineer requires structurally accurate data, this may not be adequate, and a different approach should be used.

– Using Slabs to model insulation is handy if the width of the insulation is not constant. However, modifying them is more time consuming.

– Using Beams to model insulation may be a good solution for two reasons: (a) most insulations are of a constant thickness, so a Beam tool is practical for the task, since it joins automatically. Also, a Beam automatically cuts its geometry out of intersecting Slabs. It will also cut its geometry out of intersecting walls if the Beam – Priority is higher than the Wall Priority. (See Beams and Other Elements in ArchiCAD Help.)

– Modeling insulation using walls provides many options, since walls have several geometry methods, shapes and floor plan representation options. However, you should watch out for possible unwanted intersections with other walls. For this reason, walls representing insulation should be placed on separate layers, and assigned a different Layer Intersection Group number, than structural walls. Also, any Wall-Wall or Wall-Slab intersections must be handled.

See Wall-Wall Intersections in ArchiCAD Help.

Floor Structure

Consider these two choices for modeling floor structure:

1. Create the Floor structure in 2D in Section/Elevation views

2. Create the Floor structure by modeling it in 3D

Things to consider:

– 2D is useful because it does not increase model complexity. (In contrast, creating all floor structures in 3D can increase the complexity of the model considerably. ) It may be good practice to draw only those floor structures in 2D which are actually visible in Section/Elevation Views. However, some floors may still need to be modeled, because they are visible in Elevations or in the Model from which Renderings are created. Also, if the Floor structure changes, their 2D representations must be modified manually, one by one.

– Modeling Floor structures in 3D is useful for generating obtain quantities of materials necessary for the construction of these Floors. They can also be easily created and modified using Floor Accessories Add-Ons available as “Goodies” for ArchiCAD. (Access “Goodies” through the link in ArchiCAD’s Help menu.) Also, floors can be modeled using Composite Structures, which makes them very easy to modify if the structure changes. However, a large number modeled floors can make the model large and complex. A good choice may be to model only those floors which are visible in your Sections/Elevations/Model Views, and draw the rest in 2D.

Note: If the wall in a more complex floor structure contains a window/door, see also Sill Height Using Subfloor Thickness in ArchiCAD Help.

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