The Facts about Foundation Anchor Bolts

The sturdiness and strength of a metal building is only as solid as the foundation it rests on. The foundation provides stability and prevents extreme building movement to protect the building and its contents from damage or failure. For this stability, structural steel columns are supported by high-strength anchor bolts embedded in the foundation&#;s concrete.

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Correctly installed anchor bolts secure pre-engineered metal buildings to foundations

By Mark Robins

&#;Anchor bolts serve two functions,&#; says Alexander Newman, PE, a forensic structural engineer in the Boston area and author of a book titled, &#;Metal Building Systems,&#; now in its third edition. &#;They help stabilize the steel framing during erection, and they may help transfer horizontal and vertical loads from the framing to the foundation.&#;

Most people commonly think that the foundation or footer support a building. &#;They do to a certain extent, but the primary reason for large footer and pier pads is that they act as counterweights to weigh down the building and keep it on the ground,&#; says Robert J. Quarture Jr., outside sales manager, Mohawk Construction and Supply Co. Inc., McMurray, Pa. &#;Metal buildings act as large kites in high winds and the footer/ foundation is the primary reason they stay in/on the ground. The piers and anchor bolts also act to tie the entire building design together. The piers and anchor bolts work with the steel design, the panels and the roof to complete one cohesive design and system.&#;

Types of Anchor Bolts

Mike Steiber, senior product manager of mechanical anchor systems, Simpson Strong- Tie, Pleasanton, Calif., claims the most common type of concrete anchor systems are cast-in-placeJ-bolts. &#;[They] allow an anchor to be installed after the concrete has cured and allows more flexibility in the anchor location. It is difficult to determine the correct location of cast-in-place anchors. That is the benefit of post-installed anchors, the location of the attachment can be determined after the frame has been erected. Often, the location of the anchor bolt can shift while the concrete is being poured, or can be difficult to maintain its location. A good solution is using post-installed anchors.&#;

Groups of Four

Typically, anchor bolts that secure primary building frames to foundations are used in groups of four. Each anchor in the group is placed into the predrilled holes in a plywood template, which helps keep the bolts in proper alignment. For spread footings and pile caps, the template is then fastened to wood spreaders, such as two-by-fours, that span between the concrete formwork for the foundation and are firmly nailed to it. Concrete is then placed, enveloping the anchors bolts. The wood and plywood elements are later removed. For anchor bolts placed into concrete mats, more complicated solutions are used.

Alexander Newman, PE, forensic structural engineer, Boston

The Most Common Error

The most common error is to position the anchor bolts incorrectly, whether out of place, too low or too high. When that occurs, there are many various remedies approved by the AISC. Out-of- place bolts can be repositioned by drilling and adding epoxy anchors. Bolts poured too low can be extended with couplers or welded extensions, and bolts too high can be cut off and re-threaded. or cut off and new epoxy anchors added. Worst case, but sometimes necessary, would be to jackhammer out the existing concrete and re-pour with properly placed bolts. The specific fix will ultimately depend of the needs and specifics of the job.

Dane McKinnon, QA manager, Portland Bolt & Manufacturing, Portland, Ore.

Newman believes in the past, L- or J-shaped anchor bolts were the most common types but that the preferred anchor bolt today is a headed anchor, made of either fully or partially threaded steel rod with at least one heavy hex nut placed at the top and bottom. &#;Headed anchors work best for supporting primary frames in metal building systems, which typically exert substantial horizontal and uplift forces on their foundations. L- and J-bolts may still be used in less demanding applications. The building codes still allow the use of L- and J-bolts for column anchorage. However, these types of bolts have been shown to be problematic under high uplift loading, as might occur during a strong windstorm. The American Institute of Steel Construction (AISC) recommends in their manual that L- and J-bolts not be used &#;when calculated tension exists&#;.&#;

Portland, Ore.-based Portland Bolt & Manufacturing is a specialty manufacturer able to make almost any size/grade/configuration of anchor bolt. &#;The most common types are bent anchors, headed anchors, and anchor rods with a nut on both ends,&#; says Dane McKinnon, QA manager at Portland Bolt & Manufacturing. &#;For smaller diameters, the headed and bent anchors are the most common, but as the diameter increases, the feasibility of bending and heading becomes less, so thread each end rods become more popular. Additionally, the bent and headed types are more common in lower steel grades, whereas high-strength material is more commonlymanufactured as thread each end rods.&#;

Foundation and Bolts

Bolt installation is optimized in the preplanning stage to help determine foundation design. Quarture advises a thorough analysis of the building type, geographic location (to determine weather conditions and soils) and geotech surveys to determine the possible existence of rock formations.

&#;In the field, the layout of the foundation and anchor bolts begins with a rough outline to over excavate the foundation and pier/pad locations,&#; he adds. &#;A more precise layout takes place once the footer and foundations are below the frost line in the northern regions. This prevents cold and frost in the ground from heaving&#;swelling the ground under the concrete and causing issues with the concrete and the building itself. The design of larger buildings may require additional footing attachments implementing the use of caissons. These are usually drilled and poured prior to the excavation, so that the drill rig can access the exact location of the caisson.&#;

Once the caissons and excavation are complete along with the layout, the forming of the footer and piers is done. &#;The precise locations of the anchor bolts are now laid out to within an 1/8-inch tolerances,&#; Quarture says. &#;Working closely with the building manufacturer is critical to getting this layout correct. The concrete is then poured, allowed to set and cure. The forms are stripped and the frost wall or gradebeam can be completed now, if they were not done with the pier/footer pour. The concrete is usually required to cure at least seven days but the breaks of the samples from the concrete pours sometime show the concrete has cured to its design strength.&#;

Sources at Peikko USA Inc., Lebanon, Pa., The most common error is to position the anchor bolts incorrectly, whether out of place, too low or too high. When that occurs, there are many various remedies approved by the AISC. Out-of- place bolts can be repositioned by drilling and adding epoxy anchors. Bolts poured too low can be extended with couplers or welded extensions, and bolts too high can be cut off and re-threaded. or cut off and new epoxy anchors added. Worst case, but sometimes necessary, would be to jackhammer out the existing concrete and re-pour with properly placed bolts. The specific fix will ultimately depend of the needs and specifics of the job.

Dane McKinnon, QA manager, Portland Bolt & Manufacturing, Portland, Ore. advises that in the United States, a template is recommended for anchor bolt installation to foundation. &#;When the steel structure is made, it has an accurate bolt pattern in the bottom plates, which anchor bolts have to meet in installation phase. To ensure proper fit use a template that has the same bolt pattern than the bottom plates of the steel structure. Check the location of the bolt group before fixing it to foundation formworks, after the templates have been fixed, and right after the concrete is cast. Things get much more complicated if concrete is hardened before displacement of the bolt group is noticed. The template can be made either of steel plate or plywood. Peikko uses a 4-mm steel plate which is plasma cut, and equipped with nailing holes as well as location marks.&#;

Accuracy and Errors

Because accuracy is so important with foundation bolt installation, a registered land surveyor should be retained to either supervise bolt placement or to actually place the anchor bolts. &#;A good surveyor is worth every dime they are paid,&#; Quarture says.

Newman says an AISC Code of Standard Practice requires that a field survey of as installed anchor-bolt locations be performed before the steel is delivered. &#;If that&#;s not possible for some reason, the task of setting anchor bolts should be assigned to the most technically adept person on the construction site: the superintendent or the field engineer. Leaving this work to laborers often results in misalignment and expensive remedial work.&#;

Peikko USA sources stress to ensure accuracy and the correct bolt pattern, use the template. &#;Check the location of the bolt group before fixing it to foundation formworks, after the templates have been fixed, and right after the concrete is cast. If a contractor is using two-by- fours as a template, and nailing the bolts to approximate positions, then walking over the bolts during casting, problems will occur for sure.&#;

Newman cautions that while a properly made template will ensure the proper position of the anchors in relation to one another, the entire group of anchor bolts could still be placed incorrectly. &#;There are certain tolerances for their placement, and if they are exceeded, the columns will not fit on the bolts. The best prevention is to use the services of technically competent personnel for setting the anchors. Another common error is setting the anchors too low, which also necessitates remedial work.&#;

Good Baseline Benchmarks

Ensuring successful bolt foundation involves the bolt manufacturer, the International Building Code (IBC), the erector and foundation contractor. &#;As with all construction, it starts with good baseline benchmarks that everyone works from,&#; Quarture says. &#;The IBC governs the requirements. The communication needs to be crystal clear between the foundation installer and the building manufacturer. The bolt size strength and depth are all part of the upfront analysis and design. This is where the heavy lifting takes place. If the design is coordinated and followed, it makes for an easy job.&#;

Newman contends there is an anchor bolt setting plan that the metal building system manufacturer typically provides for review and approval by the structural engineer of record (SER), assuming one exists. &#;Sometimes the design locations of the anchor bolts are indicated on the contract drawings prepared by the SER. Assuming the metal building system manufacturer is working with those contract drawings, the locations of anchor bolts should be the same. Problems tend to arise when the prospective building owner does not engage the services of a structural engineer and deals directly with the contractor. In this case, there is no SER.&#;

McKinnon believes clear and concise communication, as well as complete information will help to make sure all parties get what they want, when they want it. &#;[Also], listen to your engineer,&#; he adds. &#;I can&#;t tell you how many times we are contacted by someone who doesn&#;t want to pay for or wait for the proper product, and substitutes something cheaper or faster in its place. Then they get upset when the product gets rejected. There is a ton of liability at stake if you make an unauthorized substitution. A lot of users just assume that stronger is better, but that&#;s not the case. To get more strength you usually give up ductility, and that isn&#;t a trade you want to make without doing the design calculations.&#;