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Code Minimum Base Shear Requirements

Code Minimum Base Shear Requirements. February 2007 Joe Maffei RUTHERFORD & CHEKENE. OUTLINE. UBC, ASCE 7-02, ASCE 7-05 Effect on design response spectrum, spectral displacement demand and required stiffness Why we should have a minimum base shear and drift limit

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Code Minimum Base Shear Requirements

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  1. Code Minimum Base Shear Requirements February 2007 Joe Maffei RUTHERFORD & CHEKENE

  2. OUTLINE • UBC, ASCE 7-02, ASCE 7-05 • Effect on design response spectrum, spectral displacement demand and required stiffness • Why we should have a minimum base shear and drift limit • Conclusions & Recommendations

  3. 1997 UBC ASCE 7-02 MINIMUM BASE SHEAR ASCE 7-05 RUTHERFORD & CHEKENE

  4. UBC EQ. 30-7 EFFECT OF EQ 30-7 ON SPECTRA SHAPE E D A T

  5. Eq. 30-7 EFFECT ON SPECTRAL DISPLACEMENT DEMAND Sd, inches CODE SPECTRUM T, sec RUTHERFORD & CHEKENE

  6. ASCE 7-05 MINIMUM BASE SHEAR

  7. Seattle SF Salt Lake DISPLACEMENT DEMAND VS. S1 T=6 T=5 Sd, inches ASCE 7-05 Soil type B T=4 S1

  8. Applying UBC eq 30-7 in effect reduces the drift limit for actual ground motion hazard. R=5, soil B Eff. DBE drift limit No 30-7 for drift 2% 30-7 for drift, T=4 1% 30-7 for drift, T=6 0.7% 30-7 for drift, T=8 0.3%

  9. From Charlie K.

  10. From Charlie K.

  11. From WJE

  12. D P P-D EFFECT ON LATERAL CAPACITY Vmin hR Vmin MR,min = Vmin hr

  13. LIMIT P-D REDUCTION IN LATERAL CAPACITY V P-D Vmin MCE drift RUTHERFORD & CHEKENE

  14. CONCLUSIONS • There should be a minimum base shear for strength, about 4% or 5% xW for a high-rise with typical flexibility. • The current limit on the drift produced by the actual ground motion hazard, 2% DBE, 3% MCE, are needed. • These two checks provide a “safety net”, without which we would be completely putting our trust in NLRH analyses.

  15. CONCLUSIONS • Eq 30-7 reflects >mean ground motions from T=2 to T=4 seconds. If applied beyond T=4 it becomes increasingly conservative. • Applying UBC Eq 30-7 to drift in effect reduces the drift limit, severely for longer periods. • Applying UBC 30-6 and 30-7 for strength only provides an adequate safety net. • ASCE 7-05 minimum base shear of 1% (applicable to S1 < 0.6g) is unconservative.

  16. PROPOSAL ON MINIMUM BASE SHEAR • Minimum base shear should depend on the building’s actual drift demand, which is only indirectly tied to seismic hazard. • A minimum based on seismic hazard (safety net to the safety net) should be based on the long period acceleration, including soil effect. i.e., SM1

  17. PROPOSAL ON MINIMUM BASE SHEAR Design for strength, but not drift, for Cmin = (dmax, mce /0.02)(0.04)  0.05SM1 Dmax,mce = MCE story drift ratio, worst story, average of records Cmin = (dmax/0.02)(0.07)  0.05SM1 Dmax = DBE story drift ratio, worst story

  18. PROPOSAL ON MINIMUM BASE SHEAR AND NEAR-FIELD DEMANDS Consider extreme actual records, unscaled. [Tabas, Lucerne, 1979 El Centro, Sylmar] NRLH with P-D shows no collapse, and no individual story drift > 4% RUTHERFORD & CHEKENE

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