Layout work is increasingly being performed by using total stations. They're fast and very accurate, and robotic total stations make it a one-person job.
Setting anchor bolts may sound like a simple task in preparing a slab or foundation for placement. But for industrial/commercial concrete contractors it can become a very complicated and important detail. Some contractors are selected specifically because they can accurately and efficiently set anchor bolts for heavy machinery and steel columns. Locating and installing anchor bolts can have a tremendous impact on the rest of the project. Here are some of the steps that professionals take.
Two silos holding fly ash will be anchored to this mat slab. The circular templates will hold the anchor bolts for securing the tanks.
When Lavy Concrete Construction, Piqua, Ohio, decided to purchase a robotic total station (a surveying instrument that combines the features of a digital theodolite and electronic distance measuring (EDM) technology) for its commercial/residential concrete foundation business, it also began working for other construction companies to lay out anchor bolts for machine pads and steel columns. Ben Lavy, the company's layout specialist, says they start by drawing out the locations with computer assisted design (CAD) software, correcting any errors on plans, and making the inevitable changes that occur in most projects. Dimensions are included for all anchor bolt locations, which CAD automatically converts to x-y coordinates. The coordinates are loaded into the data collector of the robotic total station. Then the layout person does the layout work alone on the jobsite.
Traditionally anchor bolts are located by string line and tape measures, but Lavy says accuracy, efficiency, and consistency are greatly improved with total station technology. Human error and cumulative errors are greatly reduced compared with tape measured layouts. Lavy's goal is accuracy within 1/8 inch of the true point on most projects, but he can get closer tolerances.
Positioning and securing
Anchor-bolt assemblies are securely positioned with welded template supports. The bottom rebar is set before the anchor bolts are positioned, then the anchor bolts, and finally the top rebar.
Setting anchor bolts can be as easy as cutting out a plywood jig to hold the bolts and positioning the assembly just above a slab during concrete placement. More precision requires much more elaborate methods. Brian Long, a superintendent for Lindblad Construction, Joliet, Ill., specializes in highly accurate anchor bolt placements. His company's goal is to do the job right so that remedial action isn't needed after concrete is placed.
Long says that the length of an anchor bolt is important, too. Shorter bolts are much easier to secure than longer ones. In either case, before work starts in the field, Lindblad constructs CAD shop drawings for each of the templates needed for a project. Its steel fabricator uses the CAD files to guide the machines in the production of extremely accurate templates from ¼-inch-thick steel plate. The templates hold the anchor bolts, and usually two or three template plates hold each anchor assembly in position.
For this assembly, universal strut was used to position the anchor bolts, and there were no steel templates.
On the jobsite, concrete “mud mats” are first cast under the slab. Layout on the mud mats locates the position for the templates. Long says his company hires surveyors who use theodolites to locate the centerlines in both directions for the templates. With the centerlines marked on the mud mats, workers can use tape measures to mark out the template locations.
After locations are marked, rigid steel frames are erected and bolted into the mud mats to hold the templates at the proper position and elevation. Depending on the size and complexity of a template, either angle-iron or universal strut is used to build the frame. The final step is checking. By setting lines at the top of a slab on the template center marks, workers can easily check to see that the positioning is right.
Even though anchor bolts are very secure when concrete is being placed, Long says the placing crew is told: “Don't touch the anchor bolts.” There is constant monitoring during placement. Long adds that it's helpful to have the same concrete crew for each placement.
The process starts with the installation of mud mats. Layout is done, and framework for the anchor bolts is bolted into the mud mats so that there is no movement during concrete placement.