11kV Worked Example - Seismic Design -Trf & Stayed Pole

7/14/20111 AS/NZS 7000:2010 Worked Examples Seismic Design 11kV Single Circuit Transformer and Stayed Concrete Poles AS/NZS7000:2010 Provisions and Appendix C Clause 6.2.6.1 Dynamic load effects—Seismic loads • In general, transmission/distribution lines are largely unresponsive to the dynamic forces associated with seismic activity, however, due consideration should be given to structures where the normal dynamic response is altered; e.g. ancillary devices such as pole mounted transformers, etc. Clause 7.2.4.2 Earthquakes • When overhead lines are to be constructed in seismically active regions, consideration shall be given to forces on lines due to earthquakes and/or seismic tremors. Guidance information on this subject is given in Appendix C. 7/14/2011 2 Australian Panel B2 – Overhead Lines Seminar – AS/NZS 7000 :2010 Overhead line Design Sydney 28-29 March 2011 Design Load Combinations AS/NZS7000:2010 Provisions and Appendix C Clause C4.2 • In areas near to known seismic activity designs should recognize potential for large ground movements • Particular structures types should be designed to resist earthquake loads include: – Poles supporting transformers – Poles attached to a rigid termination structure with short spans or guyed terminal – Poles in alpine area with large ice load (>50% mass at top 1/3 of structure) 7/14/2011 3 • Clause C4.3 Seismic Mass • Includes all dead load from line components and plant • Clause C4.4 Fundamental period of structure Ti (Use Rayleigh method from NZS 1170.5 ) • Clause C4.5 Ductility Factor – For timber poles μ =1.0 – For concrete poles μ=1.25 • Clause C4.6 Modelling of cables and conductors – For distribution work generally ignore • Clause C4.7 Method of analysis – OK to use equivalent static force method • Clause C4.8 Combined Effects – Need to consider simultaneous action from both directions (100% Y + 30% X) and (30%Y + 100%X) Clause C4.12 Seismic displacements For poles structures the seismic displacement at the centre of mass can be taken as follows: where ∆ = the seismic displacement at centre of mass (m) µ = ductility coefficient g = 9.81 ms -2 T 1 = the fundamental period of the structure (s) C(T), Z, R, S p are factors in NZS 1170.5 2 1 2 ( ) 4 p C T gZRS T µ π ∆ = 7/14/2011 4 NZS1170.5 Earthquake actions- New Zealand Clause 2.1.4 Earthquake limit state design performance requirements • Ultimate limit state design loading shall provide for • The avoidance of collapse of the structural system • Serviceability limit state design (If line provides critical post disaster function) • All elements are to remain in an operational state Clause 2.3.1 Ultimate limit state performance requirements • The design strength is > design actions • The ultimate limit state deflections does not cause contact with adjacent structures ( and electrical clearance or operational problems) Worked Example 11 kV and LV 11kV LV 315kVa Transformer Mass =1540kg 500 6000 Vtran Vs 800 2400 Vc Vc X Y 7/14/2011 5 Location Design Parameters Component Detail Reference Location Coastal plain North Island –near Palmerston North Soil Type S oft / Firm clays max depth <20m Subsoil Class C Cd(T) =0.66 for T = 2.0sec Table 3.2 NZS1170.5 Table 3.1 NZS 1170.5 Hazard Factor (Z) Distance to major fault zone Z = 0.38 D= 20km Table 3.3 and Figure 3.3 NZS 1170.5 Design life 50yrs Table 6.1 AS/NZS 7000 Design security level Level 1 Table 6.1 AS/NZS 7000 Return Period Factor Ru 0.35 Table 3.5 NZS 1170.5 Near Fault Factor N (T,D) for D =20km 1.0 Cl 3.1.6.2 NZS 1170.5 Ice load Nil Appendix EE AS/NZS7000 7/14/2011 6 Line Design Parameters Component Detail Reference Pole type 11.0mPrestressed concrete Conductor ‘Dog’ ACSR LV and 11kV Earthwire Nil Wind span 100m Weight span 100m Deviation angle 0 degrees Pole details Item Assumed Pole Details Pole type 11m PSC Rectangular I section Embedment Depth 1.8m Conductor attachment Ht 9.2m Transverse Base width @GL 430mm Longitudinal Base width @GL 150 mm Transverse Tip width 160 mm Longitudinal tip width 150 mm Pole tip load longitudinal capacity 8.0kN Pole tip load transverse capacity Pole Mass 22.0kN 1290kg =12.65kN 7/14/2011 7 Frequency and Modal Response of PSC Poles • Based on full scale load testing we can assume PSC poles have a first order single mode of response when acting as a clamped base cantilever and any sudden release of tip load is dampened within about 2 seconds. • The fundamental frequency is assumed to be 0.5 Hz with an equivalent Ti = 2.0sec • With PSC poles this damping characteristic is quite pronounced and the poles are characteristically flexible and ductile. • In addition pole footings also demonstrate ability to absorb any overload with soil deformation Design Loads Conductor vertical loads Maximum vertical load from conductor V c = Span x weight per m = 100 x 0.00388 x 1.3 (Load Factor 1.3 from Table 7.3 AS/NZS 7000) = 0.504 kN /conductor Vc total = 3x 0.504 = 1.513kN Structure vertical loads Maximum vertical load from structure V s = Mass of Pole + crossarms + fittings = (1.290 + .30 +.45)x 9.806 x 1.1 (Load Factor 1.1 from Table 7.3 AS/NZS 7000) = 22.00 kN Transformer vertical loads Maximum vertical load from transformer V tran = Mass of transformer + mounting brackets = (1.54 + .20)x 9.806 x 1.3 (Load Factor 1.3 from Table 7.3 AS/NZS 7000) = 22.18 kN 7/14/2011 8 Using the equivalent static method (Cl 6.2 of NZS 1170.5): Horizontal seismic shear = V = Cd.(T 1 ) Wt Now Cd.(T 1 ) = 0.66 Wt = (1.513 x 2) + 22.0 + 22.18 = 47.20kN Then V = 0.66 x 47.20 = 31.15kN Equivalent static force Fi: Ft = 0.08 x V = 2.49kN Fi = Ft +(0.92V x1) (assuming a single level) = 2.49 + (0.92x31.15) = 31.14kN Assuming normal I section PSC poles this force is to be resisted at the level of the transformer mounting bracket with 100% in X axis and 30% simultaneously applied in the transverse Y axis; AND 100% in Y axis and 30% simultaneously applied in the X axis. (Cl 5.3.1.2 NZS 1170.5) Calculate Equivalent simultaneous tip loads: Equivalent Pole tip capacity X axis = 6.0/9.2 x 31.14 = 20.30kN Equivalent pole tip capacity Y axis = 0.30 x 20.30 = 6.0kN Assumed section properties X axis tip load rated capacity = 8kN Assumed section properties Y axis tip load rated capacity = 22kN This would indicate : For this site a single pole would be OK if rotated 90 degrees for one load direction combination but grossly inadequate in the other simultaneous load combination. Solution here would be to use twin poles with shear bolts to provide a composite pole. Note: If twin poles are used the pole is still ductile but becomes a very rigid element and not as flexible. Ie some superficial pole top damage could be expected. In other regions of lower seismic activity, it may be advisable to have single but more ‘ductile’ poles to assist in dissipation of seismic actions 7/14/2011 9 Check seismic displacements: Clause C4.12 Seismic displacements For poles structures the seismic displacement at the centre of mass can be taken as follows: where ∆ = the seismic displacement at centre of mass (m) µ = ductility coefficient g = 9.81 ms -2 T 1 = the fundamental period of the structure (s) C(T), Z, R, S p are factors in NZS 1170.5 2 1 2 ( ) 4 p C T gZRS T µ π ∆ = Now Δ = the seismic displacement at centre of mass (m) µ = ductility coefficient = 1.25 G = 9.81 ms -2 T 1 = the fundamental period of the structure (s) = 2.0sec C(T) = 0.66 Z = 0.38 R = 0.35 for 50 yr Return period S p = 1.3 – 0.3 µ = 1.65 (NZS 1170.5 cl 4.4.2) 2 1 2 ( ) 4 p C T gZRS T µ π ∆ = = (1.25x 0.66 x 9.806 x 0.38x 0.35x1.65 x 4)/ (4x 3.1416x3.1416 ) = 0.179m (179mm) This horizontal displacement would seem OK 7/14/2011 10 Single circuit stayed pole Line PULL DIRECTION END POLE Ground Line 9.2m 1.8m Single Circuit Stay Stay θ T C V s T H T S T V V C Seismic actions Need to consider the vertical component of stay load as a virtual load at top of pole 7/14/2011 11 Questions? Australian Panel B2 – Overhead Lines Seminar – AS/NZS Overhead Line Design Sydney 28 -29 March 2011