If the Shoe Fits: 765 kV Pylon Baseplate Design


American Electric Power (AEP) recently developed base shoe and anchor bolt designs for its family of 765 kV lattice towers. Developing a design process proved difficult, as an off-the-shelf methodology could not be identified. Instead, a variety of approaches were identified from industry publications that were contradictory and yielded significantly different results. This forced AEP to develop its own approach for this unique type of baseplate.

Achieving economical construction of lattice towers requires the ability to connect the superstructure to a variety of foundation types. This led AEP to undertake the design of basic shoes for its family of 765 kV towers which could be used to transfer loads to both concrete pillars and foundations capped with micropiles. A design using either gap or double nut anchor bolts was selected. This provided the flexibility to adjust the stub setting and avoid the water ingress, corrosion and inspection issues that can arise when the base plates are sealed after construction.

The design of anchor bolted base plates for freestanding transmission towers is unique. Compared to other structures such as moment frames, tubular steel transmission poles or road signs, lattice towers produce large axial loads as opposed to large bending moments. The result is similar design loads for all anchor bolts in the group and provides less redundancy compared to anchor bolt groups with large overturning moments where only a few bolts are subject to the maximum design loads. A review of available literature from the American Society of Civil Engineers (ASCE), the American Institute of Steel Construction (AISC), and the American Association of State Highway and Transportation Officials (AASHTO) resulted in a variety of approaches to designing base plates and anchor bolts that offered little consistency.

Bending anchor bolts

An important design decision was whether the effects of bending on the anchor bolts should be considered when determining the quantity and arrangement required. Most AASHTO, ASCE, and AISC design standards and guidelines allow anchor bolt deflection to be ignored if the distance between the top of the concrete and the bottom of the base plate is less than or equal to two anchor bolt diameters.

The recommendation is based on full-scale baseplate and anchor bolt tests where the applied normal stress in the anchor bolt due to bending is less than 10% of the total axial stress. This is a common stress condition for anchor bolt groups subjected to large overturning moments and small shear loads, but not a typical stress distribution for lattice towers. For the lattice towers in this study, the bending stress was about fifty percent of the total normal stress. Given the uncertainty of anchor bolt behavior for this combined loading condition, it was considered prudent to include anchor bolt deflection in the design approach.

Only three references were identified that provided a comprehensive documented method for including anchor bolt bending: ASCE 113-08, ASCE 48-19, and an article by Cook, Pravett, and McBride for the Florida Department of Transportation. All three methods assume that the anchor bolt deforms into a double curvature. However, the assumptions regarding the anchor bolt’s bending length and the degree of fixity at each end were all different. The methods used different values ​​for the allowable bending stress of the bolt and whether the elastic or plastic section modulus was used when calculating the bending capacity.

These three methods were used to calculate the required number of anchor bolts for a tangent, running angle and dead end structure of the 765 kV truss family and compared to the requirements if deflection was ignored. The inclusion of deflection resulted in an increase in the quantity of anchor bolts by as little as 25% and even as much as 250%. The method proposed by Cook et al. was selected because it required only a moderate increase in the required number of anchor bolts and was found to provide good comparisons with experimental results.

Finite element analysis

A nonlinear finite element (FE) model was created for each baseplate to verify the design approach. Of particular interest was the impact of plate flexibility on the stress distribution throughout the span and the force distribution of the anchor bolts. The baseplates were modeled as elastic-plastic solids in Autodesk Inventor Nastran. Each model included the base plate, angle iron, stiffeners, anchor bolts and leg diagonals.

The FE results for the most heavily loaded corner and dead-end towers showed additional stresses in the assembly than originally expected, particularly around the heel toe and stiffeners. Under load, the base plate begins to deform into a bowl. The heel and stiffeners resist deformation by concentrating forces at the ends where the deflected shape change is most pronounced. The stress concentrations in these “hot spot” locations are impacted by the thickness of the base plate. The stresses decrease in amplitude with increasing plate thickness and stiffness.

A review of several variants of FE models (above) led to the conclusion that the ratio of the distance between the anchor bolt and the bend line, c, divided by the plate thickness, t, should be a maximum of 3.25 for this family of pylons to mitigate the additional stresses caused by the deformation of the plate.

The FE models were also used to calculate the anchor bolt forces for the maximum compression load case, which controlled the design of the anchor bolts and the base plate. For the three-bolt and four-bolt designs, these forces were found to be less than five percent of the forces calculated using a simplified rigid base plate assumption. The center of gravity resulting from anchor bolt forces acts at approximately five percent of the center of gravity of the weldment.

For designs requiring more than four anchor bolts, however, the FE study did not always agree with the simplified rigid base plate approach. The rigid base plate pattern is only appropriate when the anchor bolts are placed equally in four quadrants around the welded stub connection and concentric to the load centroid. The original base shoe design for the Dead End Tower placed eight anchor bolts in a square bolt pattern, one in each corner and one in the middle of each face. In this arrangement, the anchor bolts at the corners of the base plate were not engaged. Ninety-five percent of the vertical reaction was collected by the four side bolts. The four corner bolts were essentially unloaded, indicating that the flexibility of the base plate is important.

The bolts on the sides of the base plate are closer to the reinforced central core. The relative cantilever stiffness of the base plate between the bolts and the reinforced web is higher for the side bolts compared to those at the corners. Even a marginal difference in cantilever distance results in a large difference in stiffness. By simple analogy, the stiffness of a simple cantilever beam subjected to a peak load is inversely proportional to the length of the cantilever cubed. A 10% increase in cantilever length results in a 25% decrease in stiffness.

Based on these results, the basic eight-bolt shoe was redesigned to place the eight anchor bolts equally in each quadrant and concentrically around the centroid of the weldment. An FE analysis of the fitted design revealed that the rigid base plate assumption would predict the anchor bolt axial forces by about 10%. Additionally, the location of the resultant leg forces lands about ten percent from the centroid of the welded base shoe assembly. These results indicate that all anchor bolts must be equidistant from the reinforced center of the plate to use the stiff plate assumptions when calculating the design axial forces.

For the specific eight-bolt geometry included in this study, it was also found that it is prudent to apply a load factor of approximately 10% to the calculated anchor bolt forces to account for the remaining uncertainty. .

Constructability Considerations

Past experience has shown that the best technical solution is worthless if the design turns out to be a challenge to build. For these designs, anchor bolts were limited to circular patterns to reduce the chance of misalignment in the field. In addition, the anchor bolts were located so that the center of gravity of the circle of anchor bolts coincided with the center of gravity of the weldment of the stub and stiffener, the center of the base plate and the center of the drilled shaft.

The diagonal geometry of the legs must be taken into account when arranging the bolt patterns to avoid interference with the anchor bolt. Details such as the diagonal fitting of the leg inside or outside the heel can be important. A simple plan view drawing may suffice, but 3D modeling is recommended if the bolts are located below the leg diagonals.

Conclusion of the study

This study allowed AEP to develop a generalized base shoe and anchor bolt design methodology that is consistent with current approaches and incorporates the results of finite element analysis. Anchor bolt deflection was included in the final design approach, although the safety distance between the concrete and the base plate was kept below the two diameter limit.

Finite element analysis was used to refine the design approach and revealed that the assumption of a rigid base plate to calculate anchor bolt forces is effective for designs with four anchor bolts or less. For more heavily loaded towers, the rigid base plate assumption can still be used with a few simple rules to follow for anchor bolt placement and minimum base plate thicknesses.

There is a gap in large-scale testing of base plates and anchor bolts for lattice towers. Existing test data is limited and does not account for cases where the magnitude of axial stress due to bending approaches fifty percent of the total axial stress in the bolt. Further testing is required to determine the most appropriate design for including the effects of anchor bolt deflection in base plates of this type.

Source link


Comments are closed.