Transmission Line Design Structures & Foundations …web02.gonzaga.edu/orgl/TADP549FINAL/549W6P3V2013/ARTIC/549W6… · Reference Documents RUS Bulletin 1724E-214, Guide Specification

Transmission & Distribution Program

Transmission Line Design Structures & Foundations

TADP 549

Steel Poles - Direct

Embedment Foundations -

Point of Fixity

Presentation 6.3

Dr. Prasad Yenumula

Reference Documents

RUS Bulletin 1724E-214, Guide Specification

for Standard Class Steel Transmission Poles

Poulos & Davis (1980), Pile Foundation

Analysis & Design

What is Point of Fixity?

Per RUS Bulletin 1724E-214,

“The point where the maximum moment

occurs. The actual location of this point is

dependent on the characteristics of soils

around the embedded portion of the pole”

Why it is important?

Location of Maximum Moment

For direct embedment poles, Maximum

moment occurs

– at the groundline or

– below the groundline

Depends on the soil/backfill conditions

At Maximum moment location, the shear

force is zero

Why is it Important?

Steel pole needs to be designed for

bending moments acting on steel pole

Steel pole section at point of fixity should

be sufficient to withstand the applied

bending moment

Theoretical Models

Let us consider Broms (1964) theory

– Cohesive soils (Clayey soils)

– Granular soils (Sandy soils)

Broms Theory (Cohesive Soils) Cont.

Source: Poulos & Davis, 1980


Depth of point of fixity below the

groundline =

– 1.5d + f

– d=diameter of pole and f is as shown in figure

At depth of 1.5d + f below groundline, the

maximum bending moment occurs

Source: Poulos & Davis


Equation for ‘f’

f = Hu / (9 cu d)

– Hu = Ultimate lateral capacity of soil

– d = Diameter of pole

– cu = Undrained shear strength of cohesive soil



Equation for Maximum Moment (Mmax)

Mmax = Hu (e + 1.5d + 0.5f)

Note-maximum moment at groundline = Hue

– e = eccentricity of load


Illustrated Example - 1

A transmission pole structure of length 80

feet is to be installed in homogeneous

clayey deposit with undrained shear

strength of 3ksf using direct embedment

foundation

Determine the depth of embedment, point

of fixity and maximum bending moment

using Broms’ method. Water table is about

40 feet below the ground level

Illustrated Example - 1 (Cont.)

The thickness of the backfill annulus = 0.5

feet and the crushed rock backfill unit weight

is 140 pcf and friction angle 45 degrees.

The ultimate horizontal load at groundline

=40 kips and ultimate moment load at ground

line =2411 kip-ft

The average diameter of pole below

groundline is 3.7ft

Illustrated Example - 1 (cont.)

Solution

Because the hole is too narrow with relatively

stronger backfill, to make a conservative

estimate, it can be assumed that the failure

occurs in the surrounding in-situ cohesive

soil

The pole diameter should be considered in

the calculation. Now the problem is

simplified to rigid pile under lateral load



f = Hu / (9 cu d) = 40/ (9x3x3.7) = 0.4 ft

Hu = Ultimate lateral capacity of soil=

equated to ultimate horizontal load

d = Diameter of pole

cu = Undrained shear strength of cohesive

soil


Equations for Maximum Moment (Mmax)

Mmax = Hu (e + 1.5d + 0.5f)

Mmax = 2.25 cu d g2

Solved for g =10.283 ft by substituting

e = eccentricity of load = (M/H) = 2411/40 =

60.275ft

H = 40kips

f = 0.40 ft


Total Foundation Depth = L = 1.5d + f + g

L = 1.5x3.7 + 0.4 + 10.283 =16.23 ft (minimum)

Depth of point of fixity below the groundline

=1.5d + f = 1.5*3.7 + 0.4 = 5.95ft

Maximum Moment (Mmax)

Mmax = Hu (e + 1.5d + 0.5f) = 40 (60.275 +

1.5*3.7 + 0.5*0.4) = 2641 kip-ft


Ultimate moment load at ground line =2411

kip-ft

Maximum Moment (Mmax) = 2611 kip-ft

Pole section has to provide enough

resistance for the moment load at different

locations

Broms Theory (Granular Soils)


Broms’ Theory (Granular Soils) Cont.


f = 0.82 [Hu / (Kp d γ)]0.5

Hu = Ultimate lateral capacity of soil

γ = unit weight of the soil

Kp = Rankine’s earth pressure coefficient

Tan2(45+φ/2)]

d = diameter of pole

φ = angle of internal friction of the soil



Equation for Maximum Moment (Mmax)

Mmax = Hu [e + (2f /3)]

– e = eccentricity of load



•Hu = Ultimate lateral capacity of soil

•γ = unit weight of the soil

•L = embedded length of the pile

•Kp = Rankine’s earth pressure coefficient =Tan2(45+φ/2)

•φ = angle of internal friction of the soil

•e = eccentricity of horizontal load

•d = diameter of pole (‘B’ or ‘d’ are used for diameter)

L) + (e

d K L 0.5 = H

p

u

g 3

RUS Method

Simplified method

For RUS Standard class steel poles (RUS

Bulletin 1724E-214)

“For this specification it will be assumed

to be equal to 7 percent of the pole length

from pole butt”

RUS Method (Cont.)

* Foundation embedment depth = 10% of Pole Length + 2ft

**RUS Point of Fixity depth from Ground Line = Standard Foundation

Embedment depth - RUS Point of Fixity from Pole Butt

***Ratio of Depth = (RUS Point of Fixity depth from Ground Line) /

(Standard Foundation Embedment depth)

Let us Talk about Practice

Utilities may/may not provide point of fixity

information to pole manufacturer when they use

direct embedment poles

Manufacturer do not know site specific soil

information & foundation depth to determine this

point

Typically, Engineers who use pre-engineered steel

poles check pole section only at the groundline (and

above the groundline using design software such as

PLSPOLE)

Let us Talk about Practice (Cont.)

Because pole sections are tapered

– the extra section of pole below groundline

(compared to section of pole at groundline)

may likely compensate the extra moment load

at point of fixity - but needs to be checked!

Let us Talk about Practice (Cont.)

In fact, there may be an opportunity to

convert tapered section to straight section

from point of fixity

– Because moment loads decreasing after point

of fixity

– May be an economical consideration for deep

embedment pole sections

Cost of weld worth the transition from tapered to

straight section ?