Transmission And Substation Foundations - Technical Design Manual

SECTION 7: DESIGN EXAMPLES

Design Example 7

Type RS Helical Piles for Substation Lateral Support

e= Eccentricity; distance above grad to resolve load: Given e = 10 ft. L= Minimum Length of foundation based on above criteria. EQUATION 7-11

Purpose This design example is intended to assist with the design of new construction substation structures that requires a low axial load with high shear load on a single pile. This example will show how to calculate the lateral capacity of a single pile using the Broms’ Method for a 345KV medium switch support. After the loads for the structure have been determined, it is possible to design the piles. For this example the working loading is as follows: Loads

F = P/ [9 (C u ) D] = 4600 lbs/ [9 (500 psf) (10.75in/12)] = 1.141 ft M POS mAX = P (e +1.5D + 0.5F)

= 4600 lbs [10 + 1.5(10.75 in /12) + 0.5 (1.141 ft)] = 54,806 ft-lbs

M POS mAX = 2.25 D x g2 x C u 54,806 ft-lb = 2.25 (10.75 in/12) g 2 (500 psf)

g 2 = 54.38 g = 7.38 ft L = 1.5D + F + G = 1.5 (10.75 in/12) + 1.141ft + 7.38ft = 9.87 FT

• Shear: 2300 lbs • Shear applied to switch 10 feet above grade. Soil Profile: • Soil is a clay with a cohesion of 0.5 ksf. Solution

Summary The 10” Nominal Round Shaft helical pile should be at least 10’-0 long to resist the 2300 lb lateral load applied 10 feet above grade

P = Applied horizontal shear load: Use 2300 lbs. Include a Factor of Safety of 2 in the calculations, thus doubling the horizontal shear load; P = 2 x 2300 lbs = 4600 lbs C u = Cohesion of Clay: 500 psf D= Diameter of foundation: Use D = 10.75” (10” nominal pipe size)

P=fx9xCuxd

SHEAR AT DEPTH ((1.5xd)+f)=0.

P

f x 9 x Cu x d =P f = P/9 x Cu x d

P

1.5 x d

e

AREA 1

AREA 1 = (P x (e + 1.5 d)) + 1/2 x f x p = P x (e + 1.5d + 1/2 Xf)

AREA 2 = 1/2 x g x g/2 x 9 x Cu x d = (g*2/4) x 9 x Cu x d = 2.25 x d x g*2 x Cu

AREA 1-AREA 2

g/2

AREA 2

g= ((P x (e + 1.5d + .5f)) / (2.25 X d X Cu))*.5 M max = AREA 1 or AREA 2 M max = P x (e +1.5d + .5f) L = REQUIRED DEPTH INTO SOIL WITH COHESION OF “Cu L = 1.5d + f +g

L=1.5d+f+g

M max-AREA 1 M max-AREA 2

g/2

M max

9x Cu X d

SOIL RECATION (KIPS/FT)

PILE SHEAR (KIPS)

PILE MOMENT (FT. KIPS)

Energy Structures Incorporated

RECOMMENDED UNITS

d - PILE DIAMETER (FT.) Cu - SOIL COHESHION (KSF) P - LATERAL LOAD (KIPS)

TITLE BROMS, METHOD - COHESIVE SOILS

f - FT g- FT

M max - MAXIMUM PILE BENDING MOMENT (FT.KIPS) FB max - MAXIMUM PILW BENDING STRESS (KSI)

DWG. NO: 921102

Broms’ Method for Laterally Loaded Short Piles Figure 7-27

www.hubbell.com/hubbellpowersystems | 7-29