Base Plate Design A Neglected Priority Mark Fairbairn, PE, M.ASCE, - - PowerPoint PPT Presentation

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Base Plate Design A Neglected Priority Mark Fairbairn, PE, M.ASCE, Grant Cleveland, PE, M.ASCE, Guy Faries, PE, M.ASCE SEPTEMBER 5 - 7, 2018 Base Plate Design A Neglected Priority The opening sentence of ASCE 48-11 Appendix VI

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SEPTEMBER 5 - 7, 2018

Base Plate Design – A Neglected Priority

Mark Fairbairn, PE, M.ASCE, Grant Cleveland, PE, M.ASCE, Guy Faries, PE, M.ASCE

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SEPTEMBER 5 - 7, 2018

Base Plate Design – A Neglected Priority

  • The opening sentence of ASCE 48-11 Appendix VI

states “Currently there are no industry standards that provide specific requirements for the analysis of base plates for tubular steel transmission pole structures”

  • Note: Appendix VI is NOT a code standard nor

requirement and should be used with caution.

  • Fabricator Methods have been Proprietary
  • Intellectual Property
  • Liability
  • Responsibility

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Base Plate Design – A Neglected Priority

Test – 1980 Flexing in plate was a contributing factor Failure mode at weld or in pole wall Solutions: Increase base plate thickness Modify design approach

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Base Plate Design – A Neglected Priority

  • Value of testing – Knowledge of Actual Behavior

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Base Plate Design – A Neglected Priority

Example – 1995 Test

  • Thin Base Plate (thickness?)-

flexure

  • Note failure (bending) in

anchor bolts and base plate

  • Measured deflection 5 ft

greater than expected (68%)

  • Base Plate deformed

1995 Retest

  • Added gussets and ring
  • Labor Intensive fix
  • Required FEA analysis
  • Gusset detailing requires care to

avoid “hot” spot or potential notch 5

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Base Plate Design – A Neglected Priority

  • Anchor Bolt Layout Options
  • Bolts in Quadrants
  • Always most efficient for bolts
  • As bolts near full (equal spacing) watch gap between quadrants
  • Bolts Equally Spaced
  • Spacing angle = 360/N
  • Can start at major axis or at (Spacing Angle)/2
  • Base Plate Square will generally be larger than quadrant equivalent
  • Special Spacing
  • Anchor Bolt Equations

BLi = Fx / (N) + [(MomentY)(Ciy)(Ab)]/ Iy + [(MomentZ)(Ciz)(Ab)]/ Iz I = (N/2)(Ab)(BC/2)2 Si = I/Ci Smin= (N/4)( Ab)(BC) where Ci = BC/2

  • N = Number of Anchor Bolts
  • Ab =Area of Anchor Bolt = 3.25 in^2
  • BC = Bolt Circle
  • Ci = Distance to Bolt i
  • The Moment of inertia of the bolts about its own axis is often ignored (small)

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Base Plate Design – A Neglected Priority

  • Base Plate Design Options

A. Bend Lines

a. Elastic b. Plastic

B. ASCE 48 – 05 (Effective Bend Line) C. ASCE 48 – 11 (“Wedge Method”) D. Design of Monopole Bases – Daniel Horn P.E. E. Telecommunications TIA / EIA F. Finite Element Analysis (FEA) G. Gussets H. Socket I. Proprietary methods

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Base Plate Design – A Neglected Priority

  • Bend Line is based on Flat Plate Bending
  • fb= 6Mb/(W*T2)
  • Mb = Base Plate Moment
  • W = Bend line width
  • T = Base plate thickness
  • S= WT2/6 (elastic design)
  • Solve for T
  • Limit stress to Fy or as specified by project (Fb)
  • May include a strength factor, such as limiting

the stresses to a % of Fy. For example 0.9Fy

T = (Mbp)(6) (W)(Fb)

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Base Plate Design – A Neglected Priority

  • Bend Lines

Possible bend line Wedge

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Base Plate Design – A Neglected Priority

  • Goldilocks Dilemma
  • Too thin - flexure
  • Too thick in relation to pole may create heat related

and cracking problems

  • Galvanizing ~840 degrees
  • Thermal stress differentials
  • What is the proper ratio of Tb/Tp?
  • Tb = Base plate thickness
  • Tp = Pole plate thickness
  • Different ideal ratios for weathering or galvanized finishes?
  • There is currently not a standard definition of the “proper

ratio” for Tb/Tp

  • Tb ≤ 6Tp has been mentioned and in some cases specified

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Base Plate Design – A Neglected Priority

11 Diameter (ft-ft) t (in.) w/t Fa (ksi) Stress (ksi) Bolts B.C (in.) Axial (kips) Shear (kips) Moment (ft-kips) 62.07 0.5 30.61 64.17 63.94 24 70 100 100 8140 29.11 0.25 28.54 65.00 64.71 4 or 8 36 10 10 911

Stresses - (Deflection exaggerated for visual purposes) 3.5” Base Plate 2.0” Base Plate

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Base Plate Design – A Neglected Priority

Deflection – Example 1 Factor of 100

3.5” Base Plate Actual ~0.026” 2.0” Base Plate Actual ~0.07”

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Base Plate Design – A Neglected Priority

Example 1 Bottom of Base Plate

2.0” Base Plate Stress Increase 2.93 times 3.5” Base Plate

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Base Plate Design – A Neglected Priority

4.0” Base Plate 2.5” Base Plate 3.0” Base Plate 5.0” Base Plate

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Base Plate Design – A Neglected Priority

Small Pole 2.0” Base Plate Small Pole 2.5” Base Plate Small Pole 2.5” Base Plate Equally Spaced Large Pole 3.5” Base Plate Equally Spaced

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  • Welding
  • Welding Procedure Specifications (WPS)
  • Procedure Qualification Records (PQR)
  • Proper Pre-heating
  • Galvanizing
  • Thermal Differences
  • Hydrogen embrittlement
  • Weathering Material
  • Inspection
  • AWS D1.1 Section 6, Inspection, Part C (per ASCE 48-11 Section 10.3.6)
  • Complete Penetration welds inspected by UT(Ultrasonic) or

RT(Radiographic) methods

  • Post-galvanizing
  • Need to consider time to wait prior to Inspection - 48 hours?

Base Plate Design – A Neglected Priority

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  • Toe Cracks
  • Galvanized - difficult to prevent 100% of the time, must always perform

post-galvanizing inspection

  • Not an issue with weathering, metalized, or painted poles
  • Material Considerations
  • Tensile Strength
  • Ratio of pole/base thicknesses

Base Plate Design – A Neglected Priority

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SEPTEMBER 5 - 7, 2018

  • Base Plate Material
  • Material Specification
  • Yield Strength Fy
  • Tensile Strength Fu (minimum and maximum)
  • Chemistry
  • Notch Toughness
  • Ratio of Base Thickness to Pole Thickness
  • Maintenance
  • Inspection
  • Frequency
  • Type
  • Periodic Tightening of Anchor Bolts

Base Plate Design – A Neglected Priority

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In Conclusion

  • Currently there are no industry standards that provide specific

requirements for the analysis and design of base plates for tubular steel transmission pole structures.

  • Appendix VI is NOT a code standard and should be used with

caution and not specified!

  • Further industry research is needed into the behavior of these

connections.

  • May need to consider what is the proper tb/tp ratio range for

galvanized structures?

  • Should special thickness restrictions, material requirements or

strength factors be applied to galvanized steel to reduce toe cracks?

  • Should base plate material Fu be limited?

Base Plate Design – A Neglected Priority

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Thank You

Mark Fairbairn, PE, M.ASCE, mark.fairbairn@trin.net Grant Cleveland, PE, M.ASCE, grant.cleveland@trin.net Guy Faries, PE, M.ASCE guy.faries@trin.net