November 14, 2012

Validating BIM measurements in real time

SOKKIA

PETER KENTER

correspondent

Construction professionals are embracing the ability of Building Information Modeling (BIM) to visualize the installation of prefabricated building modules, such as concrete beams and floors.

However, minor discrepancies can enter the calculations when supporting components, such as cast-in-place concrete beams and columns, are slightly deformed when formwork sags and twists during placement of fresh concrete.

The use of Robotic Total Station (RTS) technology is helping builders to quantify those inaccuracies and allows them to adjust prefabricated components to real-world measurements in the field.

“No matter how accurately the model is created, and no matter how many clashes are detected during the pre-construction coordination meetings, prefabrication practices can be messed up when other building components are not accurately installed in the field,” said Julian Kang, an associate professor at Texas A&M University.

Kang pointed to recent projects, such as the 30-storey Ark Hotel, built from prefabricated components in the Hunan Province of China by Broad Group in just 15 days.

If buildings are constructed on such accelerated schedules, he argued, the collection of accurate field data becomes critical as prefabricated components are introduced.

“RTS technology can collect point data from the surface of the building on the job site to see if the building is being built accurately,” he said.

“We wanted to know if we could get the model updated in time for prefabrication. That information could help project managers to make proactive decisions regarding variations caused by inaccurate field operations.”

RTS units use information supplied by BIM and CAD, then use an automatic target-locking feature to mark layout points, check actual elevations, and locate columns, walls, anchor bolts and utilities.

While RTS devices typically use a reflecting prism to accurately locate key points, a reflectorless option allows inaccessible positions to be calculated using lasers.

The university tested the effectiveness of RTS technology in field conditions, using a 107,000-square-foot building construction project at the Texas A&M University campus.

Two RTS vendors were asked to locate four corners of a beam side, a series of pre-determined points on a beam side, several 3-D embeds and 2-D columns as quickly as possible.

“The equipment took about 10 minutes to set up for both vendors,” said Kang.

“Overall, the results were amazing.”

Beam side corners were located in two and five minutes respectively.

Beam side points were each located in three minutes, embeds were located in 1.3 minutes and 1.25 minutes each, while columns were located in 22 and 25 minutes.

Text files derived from the data were quickly imported into the BIM model.

“This information could then be given to sub-contractors to make real-time adjustments,” said Kang.

“We concluded that it’s reasonable to expect that this information can be collected quickly enough for project managers to identify potential problems and make proactive decisions to circumvent them.”

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