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Chapter 6

Modeling Principles in Revit

Creating a BIM model requires modeling in 3D.This is very different from working with abstract
2D lines in order to represent your design. To work with Revit and be able to build a BIM model,
you need to have an understanding of how objects are constructed at various scales ranging from
the building mass down to furniture assemblies. You’ll need to know how various building ele-
ments interact with each other and depend on each other, what materials they are made of, and
how are they constructed and assembled. To this end, Revit provides a set of tools that enable you
to build your model and all the elements that go into the model. Understanding the principles of
modeling in the context of Revit will be essential to your success as you move deeper into building
information modeling.
In this chapter you will learn the basic modeling principles that support the design process
in Revit:

◆

The underlying concept of Sketch Based design

◆

How Work Planes, Datums, and Reference Planes are used in modeling

◆

Using Revit’s essential form making tools (Extrusion, Sweep, Revolve and Blend)

The entire modeling concept in Revit is based on four base class modeling forms—extrusion,
revolution, sweep, and blend—and the combinations that these can produce.
Each of the four base modeling techniques can produce either a positive or negative shape that
can be combined to create more complex forms. Each form is derived from 2D sketches that are
drawn on Work Planes. We’ll explore in more detail what sketch based forms are in the next
section.
While there is clearly room for improvements when it comes to the generic modeling capabili-
ties of Revit, you will be pleasantly surprised with the variety of 3D geometry you can generate in
a short amount of time. Figure 6.1 shows an example of what’s possible.

Figure 6.1

Example of
expressive
architecture
using Revit

Sketch-Based Design

All internal modeling techniques in Revit rely on an approach called

sketch-based design,

where you
draw a shape in a special sketch mode by creating 2D lines that then generate 3D forms. When
you start modeling Revit elements, you are basically starting a sketch: Revit enters a mode in which
everything but the sketch itself is grayed out so that the focus is given to the sketch, represented in
strong magenta colored lines. Once you’ve defined the shape by creating closed loops of lines, you

tion profile of that
wall changed to the
desired shape using
simple lines, (C) wall
shape result achieved
by simple editing of
the sketch in elevation ;
(D) the wall used in a
building.

Figure 6.3

The Design bar in
sketch mode
AB
CD
Image Courtesy of E. Di Giacommo

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For example, you can either draw lines freely, or use the Pick Walls tool to generate the floor
or roof sketch lines for you by picking on walls. When using the pick walls method, you create an


An additional loop
of lines results in a
positive shape.

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MODELING WITH REVIT

157

◆

To edit a sketch you need to be in a view that is parallel to the sketch, or a 3D view. For example,
it is not logical to edit the sketch of a wall profile in a plan view as you would not be able to
see the sketch in a way that is relevant to edit it. If you attempt to do this in plan view, Revit
will alert you and propose other views in which you CAN execute the task. The same goes
for a floor—only in plan views or 3D will you be able to edit the sketch.

◆

Sketch mode cannot be activated in Perspective (camera) view. If you select a wall or any
other element and want to edit its shape but cannot find the Edit Shape button in the Options
bar, you must be in Camera view. Switch to any other view in which the sketch makes sense
to edit.

Work Planes

Reference Planes, Reference Lines, Datum Planes, Work Planes…what are they? In order to master

two crossing references that are set as Strong, they will define the insertion point of an element
when placing it in the Project environment (as shown in Figure 6.6). In Figure 6.6, the two centrally
positioned Strong references define the insertion point of the element. The Weak references serve
only as secondary snaps.
In a project, Reference Planes can be used to drive geometry (see Figure 6.7).

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Figure 6.6

A working desk

Figure 6.7

Reference Planes are
used extensively in
the family editor to
create parametric
constraints that can
drive forms and define
snapping references


visible in Section or Elevation views. Each level commonly has a corresponding floor plan view
associated with it, although it is possible to make levels with no associated floor plan. For example,
a level may define the Top Of Steel in a building section, but there is no need to have a floor plan
view of that level. For that case, you can create levels without plans by unchecking the option Make
Plan View in the Options bar when the level tool is activated. This will create a level that appears
black and white, rather than blue—indicating that there is no hyperlinked view. Note that when
you create new levels using the Copy tool, the newly created level an elevation view, the newly cre-
ated levels will appear as this black-and white version, as copying levels will not auto-generate new
floor plans. If you need to convert a level that has no view associated with it to a level with a view
associated, use the Floor Plan… tool in the View design tab in the Design bar. You will then be able
to choose to add a view to any existing Level in the project.

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When placing elements, they are automatically associated with the Level they are drawn on so
that when the level changes position, so will the elements on that level. All sketch based families
such as floors, roofs, ceilings, and stairs will be associated with at least one level. Levels can define
both the bottom and top constraints for several elements in Revit, including walls, stairs, and ramps.

Grids

These are vertical planes used as standard references in the construction industry for creating loca-

pendicular to the Grid line. In no other case will the grid line appear. This is done to avoid confusion
on construction site due to misleading graphic description.

Extending Datum Planes

If you wish to expand grids or levels to encompass a larger portion of your model, you can use the Con-
text Menu of the grids and select Maximize Extents. This will extend the datum planes you have
selected to the maximum extents of the model geometry.

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MODELING WITH REVIT

161

Scope Boxes

These are used when you have multiple floor plans where the gridlines aren’t the same for all levels.
This tool limits the range in which data elements (grid lines, levels, and reference lines) appear. In
our example of a building with a base and tower, two separate scope boxes are created for each
major volume of the building. The grids in each part of the building are then assigned to appropri-
ate scope boxes. Figure 6.10 shows how scope boxes work in a 3D view.
Scope boxes are visible in 3D views (although not in camera views), and you can easily manip-
ulate their extent directly using the grip controls. Assigning gridlines or other datums to a scope
box is easy: Select the gridlines, Select the gridlines and in the Element Properties choose Scope Box
and select the scope box where the datums should belong. Figure 6.10b shows the Element Proper-
ties dialog box for the gridlines and the associated scope box.

Figure 6.10

B

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active Work Plane. Figure 6.11 graphically explains the Work Plane. There are a few simple rules
when using or understanding a Work Plane:

Figure 6.11

Concept diagram be-
hind Work Planes

◆

The Work Plane is set by selecting a Reference Plane/Reference Line; a Datum Plane (like a
Level, Grid) or a planar face of a Model element. Often it is pre-set and you do not need to
explicitly set it.

◆

Many elements in Revit use the active Work Plane to know where to sketch or place objects
(some objects are limited to only horizontal or vertical Work Planes).

Active Work Plane
(based on Model Face
or Datum Plane)

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MODELING WITH REVIT

163

Pick a plane

This option allows you to select a plane interactively in the model. The selection is
graphical/visual and you need to click on a Reference Plane, wall face, faces of other elements—
native or linked in the project, any level or grid that you want to become the active Work Plane
for the task at hand. Figure 6.13 shows the effect of selecting different planes on which to sketch
and create a void extrusion.

Figure 6.12

The “Work Plane”
dialog is where you
can set the active
Work Plane for a view.

Figure 6.13

The same mass and
the same 2D shape


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Work Plane Visibility

You have the option of displaying the Work Plane visually in your view. When visible in 3D, the
Work Plane is shown as a semi-transparent grid face (Figure 6.15). By activating the Work Plane
Visibility, you’ll see where you will be adding new elements. You can see the Work Plane change
location by using the Pick Plane option in the Work Plane dialog. Select on model faces and you’ll
see the active Work Plane change.

Figure 6.15

Making the Work
Plane visible helps
you understand the
active Work Plane in
a 3D view.

If you are not sure where your active Work Plane is, click the Work Plane Visibility tool in the
tool bar:

Other Work Plane Operations

Revit will let you rotate a Work Plane, by clicking on its grid and selecting Rotate from the toolbar.

ciate button. The element will then be free to move anywhere in the model. Note that this option
will only be available for Work Plane based families.

Figure 6.17

The Work Plane dialog
allows you to set Work
Plane of element, or
disassociate it from its
Work Plane.

Once you have dissociated an element from its Work Plane, you will notice that the value for the
Work Plane instance parameter will be set to <not associated>. Most importantly, you will notice
that the element will be free from constraints and is free to be moved in any three dimensional
direction.
To Rehost elements from one host (wall, floor, or roof for example) to another, you can use the
Rehost button in the Options Bar when an element is selected. This allows you to select a new host
to place the element. With this tool, you can choose any new host—it does not have to be in same
plane as the original. For example, you can select a window in one wall, click the Rehost button,
then choose a new wall for the window.
Figure 6.18 shows an extruded shape originally created on a horizontal Work Plane. Using the
Rehost tool, the form can be placed on any surface in the model.

Figure 6.18

(A) original condition,
(B) desired reposition-
ing of the curved ele-
ment, (C) the element
is rehosted and a rela-


1.

Draw four walls in plan view in the shape of a square.

2.

From the Basic design bar, select the Reference Plane tool and draw a Reference Plane parallel to the
south wall:

3.

Pick the Reference Plane and go to element properties. Give it a Name, “Front Roof Plane”.

4.

From the Modeling design tab, Select the Roof tool and select Roof By Extrusion.
A
B

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MODELING WITH REVIT

167

5.

You will be prompted to select a Work Plane:


9.

To make the roof end at the edge of the exterior walls, select the roof in plan view and select the
option

Cut Plan Profile

in the Option bar. This allows you to cut off the extruded roof exactly at the
edge of the building, regardless of its shape. In this example, draw two rectangles that cut the roof
up to the face of the walls.
Wall
Roof
Cut Plan Profile
Cut Plan Profile

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MODELING WITH REVIT

169

After using Cut Plan Profile, the final result for the first roof looks like this:

Creating a Roof using a 45 degree oriented Work Plane as a reference

To obtain the second roof (B), repeat all previous steps, but draw the Reference Plane at a 45 degree angle
to the building as show below. Remember to give the Reference Plane a different name in the Element
Properties dialog.

all created in the Family Editor using a combination of simple geometric forms that can be associ-
ated with parametrically driven dimensions. For example, a chair can be created with a combina-
tion of sweeps, blends, extrusions, and revolves. The same applies to a lighting fixture, a sitting
bench, a plumbing fixture—you name it. Figure 6.19 shows samples of standard “loadable” family
types.

The finished roof looks like this:

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