Transmission And Substation Foundations - Technical Design Manual

SECTION 7: DESIGN EXAMPLES

Design Example 2

Lattice Tower Design with Fixed Head Grillage

Purpose This design example is intended to assist with the design of Lattice Structure foundations using Chance® RS3500.300 Helical Piles and Can Grillages where reveal is required. The basic principles used in this example can be used for any Self Support Structure. In this example, each leg of the Lattice Structure will be supported by a grillage on helical piles. This type of structure will generally have tension, compression, and shear loads that will need to be calculated or provided by the tower manufacturer. Generally all moment loads are transferred to the structure by Tension/Compression force couples (One side has a tension load while the other side is compressed). After the loads for the structure have been determined, it is possible to design the piles. Loads For This Example • Compression: 130 kip • Tension: 100 kip • Shear: • Transverse: 11 kip • Longitudinal: 10 kip Soils • Layers • 0-15ft: 500 psf clay (500 psf is the cohesion of the clay) • 15-30ft: 1000 psf clay • >30ft: 2000 psf clay • Water Table: 10 ft below surface • Required Reveal Height: 1 ft.

Helicap Printout Figure 7-6

Pile Capacity: The first step in the design is to calculate the estimated axial capacity of the pile. This value is used for the initial T-Z curve in the group model. In this example with a 10/12/14/14/14/14, a 90 kip pile can be attained. This is done by putting the boring into HeliCAP, selecting the product line you wish to use (RS3500.300 for CAN style grillage.) At this point, the data is input into Group®. Inputs include: the soils, the loads (including different load cases for tension/compression as well as different directions the loads can act), T-Z curve, and the pile configuration/properties (Pile configuration for grillages can be found in the grillage section under the products tab and pile properties/capacities can be found in the same section under the section for the desired pile. Generally the loads put into GROUP® are working loads. Because GROUP® is estimating lateral deflection; the best way to get a factor of safety is to apply it to the Group results. Please consult the Group® Manual for any questions about how to use Group®. Here are the results of this analysis for these piles.

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