Base Plate Design – A Neglected Priority Mark Fairbairn... · 2018. 9. 20. · SEPTEMBER 5 - 7,...

SEPTEMBER 5 - 7, 2018

Base Plate Design – A Neglected PriorityMark Fairbairn, PE, M.ASCE,

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


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 PriorityTest – 1980Flexing in plate was a contributing factor

Failure mode at weld or in pole wallSolutions: 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 PriorityExample – 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

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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)]/ IzI = (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 Linesa. 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. GussetsH. SocketI. 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

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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 PriorityDeflection – Example 1

Factor of 100

3.5” Base PlateActual ~0.026”

2.0” Base PlateActual ~0.07”

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Base Plate Design – A Neglected PriorityExample 1 Bottom of Base Plate

2.0” Base PlateStress Increase

2.93 times

3.5” Base Plate

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4.0” Base Plate2.5” Base Plate

3.0” Base Plate 5.0” Base Plate

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Small Pole2.0” Base Plate

Small Pole2.5” Base Plate

Small Pole2.5” Base Plate Equally Spaced

Large Pole3.5” Base Plate Equally Spaced


• 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?


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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


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• 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


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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?


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Thank You

Mark Fairbairn, PE, M.ASCE,[email protected]

Grant Cleveland, PE, M.ASCE,[email protected]

Guy Faries, PE, [email protected]

Base Plate Design – A Neglected Priority Mark Fairbairn... · 2018. 9. 20. · SEPTEMBER 5 - 7,...

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