The practical motivation for this research is based on our experience
working with a project team throughout design and construction of a pilot
plant facility for Sequus Pharmaceuticals (Staub and Fischer 2000).
We will describe the drywall subcontractor's estimating process to determine
the labor and material costs for all four walls of the room shown in Figure
1. Specifically, we will illustrate what component properties and
product features are important to the estimator, how they affect the construction
cost information, and how well current tools help the estimator to maintain
the cost estimate as the design changes (example product features and properties
are annotated in Figure 1).
Figure 1: Current estimating process using tools that link
design and construction costs
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Figure 1 shows an overview of the current estimating process using
commercial estimating software that links with 3D CAD software. The
steps to create an estimate are as follows:
1) Create Cost Assembly: First, the estimator creates a cost
assembly to aggregate all the estimating items that are needed for each
wall in the room. Each item in the assembly contains information
about the material unit costs, labor and equipment resources, and the production
rates.
2) Apply Cost Assembly: After the assembly is created, the estimator
attaches it to all four walls. It is the estimator's responsibility to
ensure that the right assembly is attached to the appropriate design object.
3) Create Estimate: After all the assemblies are attached, the
estimator orders the estimating software to create the estimate and it
automatically calculates the quantities for each item in the assembly.
4) Manually Add Costs not Captured in Assembly : Finally, the
estimator manually adds the costs for the additional production time for
framing around the door opening. This item could have been added
to the cost assembly but then the estimator would have had to make two
assemblies: one assembly for the wall without an opening and one assembly
for the wall with the opening. The estimator chose to make one assembly
and manually add the costs for the opening. |
Figure 2: Comparison of estimate generated using current tools
and estimate
generated by estimator to reflect the impact of design changes.
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Designs change often and it is important to investigate how current
tools help estimators to maintain estimates. On the Sequus Project,
the height of one wall changed from a height of 8' to 12.5' and a window
was added. Figure 2 shows that current tools reflect the change in
design by calculating a new quantity. Unfortunately, the changed
quantity is not the only impact of this design change. Changing the
wall height and adding a window affected the metal stud and gypsum wallboard
(GWB) in the following ways:
Resource composition: Rolling scaffolding was added to access the
upper portion of the wall beyond a ten-foot height.
Productivity rates: Productivity was reduced due to the hindered
access to the upper portion of the wall from the rolling scaffolding.
Quantity and cutting of GWB: The GWB are purchased as 4'x8' boards
and must be cut to fit the size of the wall. Since the wall height
was increased to 12.5', additional boards had to be purchased and cut to
fit the new height.
Quantity and productivity of metal stud installation and cutting of
GWB for Opening: The opening required additional metal stud framing
with a different productivity rate. Moreover, it required cutting
of the GWB although the quantity purchased remained the same.
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Figure 3: Comparison of current process and Feature-based Estimating
process being developed as part of thsi research.
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This example illustrates the importance of identifying product features
and properties and their effects on construction costs to support the maintenance
of cost estimates as the design changes. However, current tools do not
capture the estimator's rationale for relating product featuers and properties
and their effects on cost information forcing estimators to manually create
and maintain the cost information, as shown in the top portion of Figure
3.
To address this limitations, estimators need a framework to capture
their rationale for relating product and cost information in a formalized
and systematic way. Moreover, estimators need a computer tool that
utilizes the computer-interpretable representation of their rationale to
assist them in identifying relevant product features and component properties
in an IFC product model and their effects on construction costs.
My research addresses this problem by providing a computer-interpretable
representation of estimator's rationale that enables the computer to assist
estimators with the maintenance of cost information, as shown in the bottom
portion of Figure 3. The Feature-based estimating process will allow estimators
to enter their rationale easily in templates and the system will identify
relevant cost information when creating estimates and notify estimators
when cost information is no longer applicable in the case of design changes. |
Figure 4: IDEF view of Feature-based Estimating Process
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Figure 4 shows an IDEF view of the proposed research. My objective
is to provide a general framework for relating product and cost information
that supports estimating for multiple domains and enables the reuse of
estimating knowledge about component properties and product features and
their effects.
Specifically, the objectives of this research are:
1. To capture an estimator's rationale by formalizing product features
and their relationships to construction cost information
2 To formalize and test methods that analyze a product model to
identify product features and relevant construction cost information |
* Laitinen, J. (1998). “Model Based Construction Process Management,”
Ph. D. Thesis, Royal Institute of Technology. Stockholm, Sweden.