288 Section 2 Architectural Planning
are found and design revisions are in place to
correct the errors.
The ability to check for interference in
different models is one of the most important
benefi ts of BIM. If clashes are not found prior to
construction and errors occur in the fi eld, change
orders must be issued and costly delays can occur.
A BIM project coordinator is generally
responsible for managing model coordination.
Typically, coordination meetings are held weekly
to identify and resolve confl icts. To check for
interference, the different models are imported
into special software. The number of models to
be checked depends on the complexity of the
project. However, in a typical project, there may
be individual models from the architectural,
structural, mechanical, electrical, and plumbing
trades.
In a clash detection study, the software eval-
uates model geometry and highlights areas in the
building where interference exists. This process
identifi es clashes between elements such as struc-
tural members, mechanical ductwork, plumbing
lines, and electrical conduit. See Figure 12-11.
Preventing these types of clashes from happening
sheathing, stud, insulation, vapor barrier, and
gypsum board, is considered and listed indi-
vidually. This provides accurate quantities for
estimating costs and ordering materials.
Some modeling software programs provide
tools for creating a material takeoff automatically.
The material takeoff appears similar to a spread-
sheet and can be formatted to include material
counts and quantities in units such as linear
feet, square feet, and cubic feet. However, a more
common method to create a material takeoff is
to import the design model into an estimating
software program. The software is then used
by an estimator to prepare the estimate. The
estimator creates a material takeoff from the
model and adds material prices and labor costs to
calculate the estimate. The imported model fi le
is linked so that if changes are made to the design
model, information in the material takeoff
updates automatically and changes are refl ected
in the estimate. This allows estimates to be
precisely calculated. Estimating is discussed in
more depth in Chapter 36, Estimating Building
Costs.
Traditionally, estimators prepared cost esti-
mates manually by hand by working from the con-
struction documents. This is a time-consuming
process that is subject to mistakes caused by
human error. Preparing estimates directly from
information in a BIM model is much more accu-
rate and effi cient. It also provides fl exibility to the
design team in managing costs and exploring
alternatives. For example, different brands of
products, such as windows or cabinets, can be
compared in the model to determine the impact
on the overall cost.
Clash Detection
Coordination of work with different engi-
neering disciplines plays an essential role
throughout design and construction. In a typical
design project involving BIM, a number of models
are created in different disciplines. The models
are based on the same design, but contain dif-
ferent building systems related to each engi-
neering discipline. Prior to construction, the
models are checked for interference between
building components. Interference checking in
BIM software is commonly referred to as clash
detection. This process takes place before con-
struction begins. It helps ensure that confl icts
GRAPHISOFT
Figure 12-11. A clash detection study. This study
indicates that the rectangular duct requires redesign.
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