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WEEK 06: TRANSMISSION: OVERHEAD AC Sections: Voltage | Conductor | Environment | Line | Mechanical | Support | Accessories | Foundation Lattice Tower foundation loads consists of vertical tension (uplift) or compression forces and horizontal shear forces. For tangent and small line angle towers, the vertical loads on a foundation maybe either uplift or compression. For terminal and line angle towers, the foundations on one side may always be loaded in uplift while the other side may always be loaded in compression. The distribution of horizontal forces between the foundations of a lattice tower vary with the bracing of the structure. When the foundations of a tower displace and the geometric relationship of the tower to its foundations remains the same, any increase in load due to its displacement will have a minimal effect on the tower and its foundation. Single-Shaft foundation loads or single pole structures have one foundation so that differential foundation movement is precluded. The foundation reactions consist of a large overturning moment and usually relatively small horizontal, vertical or torsional loads. For single-shaft structures, the foundation movement of concern is the angular rotation of the shaft in the vertical plane and horizontal displacement of the top of the foundation. When these displacements have been determined, the displacement of the conductors can be computed. Under high wind loading, a corresponding deflection of the conductors perpendicular to the transmission line can be permitted. Due to foundation rotation, the clearance between the conductors and the structure would only be decrease for structures with single string insulators. The midspan ground clearance and the change in line angle would also decrease a negligible amount. In establishing displacement criteria for a single-shaft-structure foundations, consideration should be given to how much total, as well as permanent, displacement can be permitted. Framed Structures foundation loads are exhibited by structures dependent in part for their stability on one or more of their joints resisting moment. The foundation reactions are dependent on which joints can resist moment and the relative stiffness of the members. Typical loads include foundations for four-legged and two-legged (e.g., H-frame) framed structures. Significant foundation movement will redistribute the frame and foundation loads. Externally Guyed Structures are of three general types. For all types, the guys produce uplift loads on the guy foundation and compression loads on the structure foundation. The guys are generally adjustable in length to permit plumbing of the structure during construction and to account for creep in the guy and movement of the uplift anchor. The first type, the shaft or shafts of the structures usually have a ball-and-socket base connection to the foundation to permit free rotation without transmitting moment to the foundation. This will produce compression loading with a small shear load. This guyed structure can generally tolerate large foundation movements but may require retensioning of the guys. The second type consists of the guyed single shaft. Often used as a terminal and large line angle structure and is quite flexible, allowing most of the load to be resisted by tension in the guys and compression in the main shaft. This type can generally tolerate significant foundation movement as far as its own structural integrity is concerned. If excessive guy anchor slippage occurs, however, conductor-to-ground clearance, security of adjacent structures, and the stringing and sagging of conductors can become a problem. The third type is a guyed conventional lattice tower to reduce its leg loads and foundation reactions, and it is often used to upgrade existing towers. The flexibility of the guy, together with the flexibility of the tower, are needed to compute the foundation reactions and anchor loads. The maximum amount of anchor slippage can be selected and the tower and anchors designed accordingly. The initial and final modulus of elasticity of the guys together with the creep of the guys should be considered. The amount of pretension in the guys should be specified. The leg foundations are required to resist only horizontal shear forces and vertical compression or uplift loads. As in the case of self-standing lattice towers, the load distribution among the members of the structure is sensitive to the foundation performance. Differential displacements of the tower legs will result in load distribution and may affect the integrity of the tower. Foundation Types include the following: steel grillages, rock foundations, concrete spread foundations, drilled shafts and direct embedment. The last three are normally used for framed and single-shaft structures. Except for direct embedment, obviously, all foundation types are applicable to lattice towers. Steel Grillages are of three types: the pyramid arrangement in which the leg stub is connected to four smaller stubs which in turn are connected to the grillage at the base; the shear-type, where a grillage foundation which has a single leg stub carried directly to the grillage base; and the reinforced-type, which also has a single leg stub carried directly to the grillage base, but an added leg reinforcer increases the area for mobilizing passive soil pressure as well as increasing the leg strength. Rock Foundations are employed in areas where the bedrock is either exposed at the ground surface or covered with a thin mantle of soil. Relatively simple, economical and efficient rock foundations maybe installed where terrain permits. Concrete Spread Foundations consist of a base mat and a square or round pier constructed of reinforced concrete. There are several variations: the stub angle can be bent and the pier and mat centered; the mat can be located so that the projection from the stub angle intersects the centroid of the mat; the pier itself can be battered to the tower leg slope; or anchor bolts are used instead of the direct embedment stub angle. Drilled Concrete Shaft is the most common type of foundation presently used to support lattice towers. framed structures, and single shafts. They are constructed by power augering a circular excavation, placing the reinforcing steel and pouring concrete to form a shaft foundation. The soil condition is a determining factor in this type. Direct Embedment refers to wood, steel or concrete pole foundations constructed by power augering a circular excavation in the ground, inserting the pole directly into the excavation, and backfilling the void between the pole and the sides of excavation. Thus, the pole acts as its own foundation by transferring loads to the in-situ soil via the backfill. The quality of backfill, method of placement, and degree of compaction strongly influence the stiffness and strength of the foundation. Sometimes, a push brace, instead of an anchor guy, is provided to counteract the direction of the pull force. Cribbing is necessary in marshy land when the pole must resist an unbalanced load. It is also used in crowded or residential areas when exposed guys would be dangerous, unsightly or impractical. It involves placing logs, stones or other supports at the bottom of the pole on one side and near the surface of the earth on the other side. These objects counteract the strain being put on the pole from one direction. Modern cribbing practices, however, employ the use of a pole key, which attaches to the bottom of the pole and creates a foothold on the surrounding earth on both sides. |
