Duane Phillips, MPMDirector of Project Management

Understand what structure deflection is and the impacts to design & construction

Understand the mechanisms behind what drives structure deflection and its design implications

Discuss the construction impacts of structure deflection and construction considerations

Deflection is the degree to which a structural element is displaced under a load.

Measure of the amount of movement from unloaded (resting) position

Components: pole top, arms

Some deflection is inevitable in any structure materials

Manufacturing cost savings with a reduction in steel (significant)

Shifts further away from brittle fracture point (stress & strain resulting in failure)

Reliability› Recurrence Interval/Return Period› Operational Contingencies

Safety› Construction› Operation› Maintenance

Existing Codes› 2012 NESC

Maximum un-factored loads

Should not cause damage to structure

Based upon the facility configuration & resulting loads

Maximum factored loads

Should not cause failure of structure

Based upon the ultimate strength of installed components

Among Sub-Systems› Tangent Structures› Angle Structures› Dead End Structures› Conductor

Within Sub-Systems› Structure› Foundation› Hardware

Weakest

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Strongest

Weakest

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Strongest

Types of Structures Monopole H-frame/Multi-pole Lattice

Deflection Points Structure Shaft (pole top - vertical) Arms (arm tip – vertical & transverse)

Various loads act on structure simultaneously

Very unlikely that every load will reach ultimate load at once

For ultimate loads, design specifications provide factors for load magnification

Conductor Point Loads› Wire Tension & Wind› Vertical Load Weight Span NESC Ice Load Local Utility – Extreme Ice

Pole Self-Weight Wind Acting on the Pole Hardware Construction

Weight Span Concept

Continental Winds› NESC Rule 250 C [Based on ASCE 7-05]

Not applicable if structure or attachments does not exceed 60 feet above ground

Load (lbs) = .00256*(Vmi/h)2*kz*GRF*I*Cf*A(ft2)

V → Basic Wind Speed kz → Velocity Pressure CoefficientGRF → Gust Response Factor I → Importance FactorCf → Shape Factor A → Projected Area

Sag and tension are inversely proportional.As one increases, the other decreases.

Tension = span2 x weight/foot of wireSag x 8

Transverse & Longitudinal Load due to Conductor Tension› Changes in Line Direction› Span Length Variation› Unbalanced Ice› Broken Wires› Dynamic Loads

Based on total loads applied to structure & components

Comparison to design specification limits (allowable deflection)

Considers total load vs. vibration impacts› Some deflection is fine – too much is not

Determine forces & associated stresses

Apply each load combination

Define critical demands

Max Load Demand ↔ Design Load Limits

Used to optimize design (situational)

Operational or ultimate limits may govern specific feature

Should include both strength and fatigue design loading

Clearances → additional sag › Other facilities & electrical ‘window’

Adjacent span sag/tension

Moment impacts to foundation

Constructability› Wire sag/tension specifications› Aesthetics

Safety → understanding deflection & potential energy

Wire installation → properly sag/tension› Alternate means of sag verification› Shift installation sequence› Temporary guying deadends

‘Up front’ discussion on means & methods of wire installation

Clear specification requirements

Review/approve wire installation methods

Manufacturing cost reductions (quantity of steel)

Shift in field aesthetic perspective

Changes in means & methods of wire installation

Duane Phillips, MPMDirector of Project Management