You wouldn't know it just by driving by any of the jobsites, but these days Southern Florida is a hotbed of laser scanning for high-rise construction. Baker Concrete Construction crews now are scanning almost every slab they're placing on metal decking in two 40-story steel-frame buildings in Miami. By capturing the profile before and after concrete placement, the company will be able to compare the structural system's field performance to the design.
Although that will be excellent information for the ACI Tolerances Committee's use in coming up with more realistic tolerances based on field measurements, that isn't the only reason for gathering the data.
The floors are being placed at a constant thickness, per the job specifications, and rely on the steel deflection to achieve a flat floor. Project executive Michael Hernandez says Baker uses a “pinning” technique to achieve the constant thickness. The steel joists typically have a camber of as much as 1 inch and are supported during the pour. After the concrete has been placed, the supports are removed and the steel deflects, at the same time flattening the floor.
What's the tolerance on deflection? Actually, that's something that engineers deal with in the design phase. But in the field, a structural system behaves according to the mechanics of materials and the laws of nature.
Because these buildings' designs also include a live load of 100 pounds per square foot, Hernandez realized the weight of the concrete alone won't be enough to make the floors turn out as flat as some might be expecting. That's one reason he sees the before-and-after laser-scanned deck profiles as a valuable tool. “It gives us a very accurate as-built at the time of construction,” he says.
The technique isn't new. The company frequently does similar checks when constructing condominium balconies, for example, scanning just prior to stripping the forms or just before stressing the post-tensioning tendons. However, on these two high-rises the company begins its scanning as the finishers are walking off the slab and also immediately takes flatness readings.
The idea is simple: the scan data provides a relatively inexpensive insurance policy. When one of the subcontractors raises issues with the concrete slab profile, Hernandez will be prepared with solid information to show that Baker's slabs have done exactly what they were supposed to do.
Field engineer Quinn Booker says comparing the scan data also is useful in detecting any variation that occurs in the slabs as they are built. “We overlay the two images, then dot match from one scanned surface to the other,” Booker says, “which shows exactly what's happening in the system.” That information is used to help plan the next stages of construction and identify any problems that require correction.
A recent graduate of San Diego State University, Booker was part of Colin Milberg's as-built data collection project his entire senior year and is among the most experienced operators of laser scanning equipment in this application.
Now with the distraction of class work behind him and in the field on a daily basis, Booker says it's surprising how quickly the clear space disappears, which adds significant challenges to laser scanning. He says it's fairly easy to scan one of the two Miami buildings he now is surveying regularly. Although the construction is moving fast, they're at the stage of going straight up so if his timing is right he can get the data with minimal interference. The other site is very congested and has multiple crews working on different aspects of the construction all the time, making it more difficult to get good scans.
Booker also says that because the profile of the metal decking has a much greater surface area, scanning the underside of these slabs is taking longer than the bottom of a stripped flat slab. As a side effect of that, the company recently purchased a more powerful workstation simply to handle the increased volume of data coming in from the field.