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 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 2 SEPTEMBER 5 - 7, 2018
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 3 SEPTEMBER 5 - 7, 2018
Base Plate Design – A Neglected Priority • Value of testing – Knowledge of Actual Behavior 4 SEPTEMBER 5 - 7, 2018
Base Plate Design – A Neglected Priority Example – 1995 Test 1995 Retest • Thin Base Plate (thickness?)- • Added gussets and ring flexure • Labor Intensive fix Note failure (bending) in Required FEA analysis • • anchor bolts and base plate • Gusset detailing requires care to Measured deflection 5 ft avoid “hot” spot or potential • greater than expected (68%) notch • Base Plate deformed 5 SEPTEMBER 5 - 7, 2018
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 • BL i = F x / (N) + [(Moment Y )(C iy )(A b )]/ I y + [(Moment Z )(C iz )(A b )]/ I z I = (N/2)(A b )(BC/2) 2 S i = I/C i S min = (N/4)( A b )(BC) where C i = BC/2 • N = Number of Anchor Bolts A b =Area of Anchor Bolt = 3.25 in^2 • • BC = Bolt Circle C i = Distance to Bolt i • • The Moment of inertia of the bolts about its own axis is often ignored (small) 6 SEPTEMBER 5 - 7, 2018
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 7 SEPTEMBER 5 - 7, 2018
Base Plate Design – A Neglected Priority • Bend Line is based on Flat Plate Bending • f b = 6M b /(W*T 2 ) (M bp )(6) T = • M b = Base Plate Moment (W)(F b ) • W = Bend line width • T = Base plate thickness • S= WT 2 /6 (elastic design) • Solve for T • Limit stress to F y or as specified by project (F b ) • May include a strength factor, such as limiting the stresses to a % of F y . For example 0.9F y 8 SEPTEMBER 5 - 7, 2018
Base Plate Design – A Neglected Priority • Bend Lines Possible bend line Wedge 9 SEPTEMBER 5 - 7, 2018
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 T b /T p ? • T b = Base plate thickness • T p = Pole plate thickness • Different ideal ratios for weathering or galvanized finishes? • There is currently not a standard definition of the “proper ratio” for T b /T p • T b ≤ 6T p has been mentioned and in some cases specified 10 SEPTEMBER 5 - 7, 2018
Base Plate Design – A Neglected Priority Diameter t w/t Fa Stress Bolts B.C Axial Shear Moment (ft-ft) (in.) (ksi) (ksi) (in.) (kips) (kips) (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) 2.0” Base Plate 3.5” Base Plate 11 SEPTEMBER 5 - 7, 2018
Base Plate Design – A Neglected Priority Deflection – Example 1 Factor of 100 3.5” Base Plate 2.0” Base Plate Actual ~0.026” Actual ~0.07” 12 SEPTEMBER 5 - 7, 2018
Base Plate Design – A Neglected Priority Example 1 Bottom of Base Plate 3.5” Base Plate 2.0” Base Plate Stress Increase 2.93 times 13 SEPTEMBER 5 - 7, 2018
Base Plate Design – A Neglected Priority 2.5” Base Plate 4.0” Base Plate 5.0” Base Plate 3.0” Base Plate 14 SEPTEMBER 5 - 7, 2018
Base Plate Design – A Neglected Priority Large Pole Small Pole 3.5” Base Plate 2.0” Base Plate Equally Spaced Small Pole Small Pole 2.5” Base Plate 2.5” Base Plate Equally Spaced 15 SEPTEMBER 5 - 7, 2018
Base Plate Design – A Neglected Priority 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? SEPTEMBER 5 - 7, 2018 16
Base Plate Design – A Neglected Priority 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 SEPTEMBER 5 - 7, 2018 17
Base Plate Design – A Neglected Priority Base Plate Material • • Material Specification • Yield Strength F y • Tensile Strength F u (minimum and maximum) • Chemistry • Notch Toughness • Ratio of Base Thickness to Pole Thickness • Maintenance • Inspection • Frequency • Type • Periodic Tightening of Anchor Bolts SEPTEMBER 5 - 7, 2018 18
Base Plate Design – A Neglected Priority 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 t b /t p 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 F u be limited? SEPTEMBER 5 - 7, 2018 19
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 SEPTEMBER 5 - 7, 2018
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