Duane Phillips, MPM Director of Project Management
Transcript of Duane Phillips, MPM Director of Project Management
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