With increased demand for greater output from wind energy projects, the industry is increasingly looking to maximize output from each wind turbine. Over the years, wind power has become competitive with traditional forms of non-renewable energy because advancements in generator technology allow larger and heavier generators to produce a higher-megawatt output than ever before. These newer, advanced turbines are also carried on towers with higher hub heights to capture higher wind speeds. With this comes the requirement for tower anchor bolts to carry higher tensile load capacities.
Design and installation
To safely carry larger turbines at higher hub heights, careful consideration must be given to the foundation design and system components. A proper geotechnical evaluation of the soils must occur after all of the structural loads carrying the tower and nacelle components are evaluated. With this information, the foundation designer is able to determine the width and depth of the concrete foundation.[adleft zone=’190′]
A vital component to the foundation system is the tower anchor bolt. These anchor bolts are responsible for keeping the tower and nacelle in equilibrium. The anchor bolts are installed during the foundation forming process in a large ring pattern with a matching inner and outer circle of anchor bolts symmetrically around the foundation.
Typically, between 140 and 200 anchor bolts are in each foundation design. A thick steel embedment ring containing holes for the tower anchor bolts is placed near the bottom of the foundation pour, and a template ring – ostensibly, a thinner steel ring with matching holes – is placed at the top of the foundation pour. Each anchor bolt is fitted with PVC sleeves running between the steel rings so they are flush against each ring surface in order to keep the anchor bolts de-bonded during the foundation pouring operation.
A heavy pattern hex nut and washer are underneath the embedment ring and similarly placed on the tower base plate. (Prior to placing the tower base plate, the temporary template ring is removed after the concrete pour.) It should also be noted the tower base section is shimmed into position above the top of the pedestal foundation pour, and the voided area between is filled with a high-strength epoxy grout to complete the grout pad.
Once the grout pad is cured, all of the tower anchor bolts are pre-tensioned to a load specified by the foundation designer and locked off by torquing the top hex nuts to remove the stretch created. The pre-tensioning process is typically achieved by using small-diameter, compact, high-psi capacity calibrated tensioners. This elastic stretch created by tensioning under load is permanently transferred to the anchor bolt by torquing the nut prior to removing pressure from the tensioning jack.
After pre-tensioning the tower anchor bolts in a predetermined selected pattern across the foundation, the ring pattern of the anchor bolts is placed into compression. Therefore, the equilibrium is maintained as varying load cycles are continuously placed on the foundation. Designers specify an anchor bolt lock-off pre-tensioning load to be at a level where the maximum external design load that is placed on the foundation is never reached.
Pre-tensioning prevents the anchor bolts from stretching and relaxing, which can lead to long-term fatigue, and also mitigates spalling or cracking of the concrete from tensile stresses.
The normal practice is for the foundation designer to specify a lock-off load that takes into consideration in-service design loads and, in addition, losses such as natural relaxation loss in the steel (generally averages 2% max) and slight creep losses in the foundation itself under load. Creep refers to the slight movement over time to the concrete – and, to a lesser extent, the steel tower base as a result of being under the pre-tension load. Movement resulting from the so-called creep results in a loss of direct pre-tension load.
Anchor bolt properties/quality control
Tower anchor bolts contain a rugged, course-thread pitch – a popular option because it is more forgiving to dings and residue lodged between the threads than the finer thread anchor bolts. In most cases, the diameters specified are 1-1/4” and 1-3/8”. The grade 75 and grade 90 anchor bolts are produced from a hot roll melt at the mill and do not contain secondary mill processing. This makes them a very economical choice.
Long-term assurances that the tower anchor bolts are going to maintain their pre-tensioned load are vitally important to the integrity of the wind turbine and to the foundation itself. One of the leading causes of foundation problems is load loss of the tower anchor bolts. When the pre-tensioned lock-off load is diminished, tower anchor bolts are subjected to stretching and relaxing. This action causes the grout pad and concrete to crack or spall.
Corrosion inside the protective bolt cap covers housing the hex nut, washer and tip of the anchor bolt protruding above the tower base plate is sometimes extensive enough to cause loss of material section, resulting in load loss. As the thickness of the nut and washer is diminished due to corrosion, it has the same effect as de-tensioning the nut. For this reason, it is important that only a quality protective bolt cap cover be specified that has a gasket seal and that grease be applied for corrosion resistance to the nut, washer and protruding anchor bolt prior to installing the bolt cap covers.
Bolt cap covers are usually required to be removed and inspected inside for signs of corrosion. During this time, load verification testing is conducted and recorded to ensure no load loss has occurred. If load loss is detected, the anchor bolts are locked off at the designed pre-tensioned level. This testing is usually done on a 100% basis after six months to a year and then on a randomly selected basis if no problems are detected. [adright zone=’190′]
Another quality control measure typically seen is the requirement of mill heat qualifications of tower anchor bolts. Certified mill test reports are always required for each heat of steel used for the anchor bolts supplied. Found on the mill certs are physical properties and chemistry elements, with proportions making up each heat.
In most cases, full section tensile load tests to support the mill certifications are required to be conducted for each heat of anchor bolt material. This testing ensures the yield and ultimate tensile strengths meet the minimums as specified by the foundation designer.
Rock and soil
In the right geotechnical conditions, grout-bonded ground anchors consisting of fully course-threaded, large-diameter, high-strength bars or helical earth anchors can be used as an alternate design to traditional deep excavated spread footings. The result is a foundation system requiring less excavation and more of a savings in the amount of poured concrete and steel reinforcing than is normally found in a traditional spread footing design. In a ground-anchored foundation, the number of tower anchor bolts is the same as in a traditional design, but the length of each anchor bolt is considerably decreased because the foundation is not as deep. This type of design is particularly effective in hard rock conditions, in which excavating for a traditional spread footing foundation would be challenging.
John A. White is vice president of Williams Form Engineering. He can be reached at email@example.com.