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P-Delta and Minimum Base Shear

P-Delta and Minimum Base Shear. Min. Base Shear. V = (0.8ZN v I/R)W for drift limitation? Is it for ground motion uncertainty or “modeling” uncertainty or both? Let’s say explicitly what it is for! Should be the same “enforcement” as for code-conforming structures

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P-Delta and Minimum Base Shear

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  1. P-Delta and Minimum Base Shear

  2. Min. Base Shear • V = (0.8ZNvI/R)W for drift limitation? • Is it for ground motion uncertainty or “modeling” uncertainty or both? • Let’s say explicitly what it is for! • Should be the same “enforcement” as for code-conforming structures • If for modeling uncertainties and alternative design, then it belongs to Level 3 – if for collapse safety?

  3. Collapse Safety + Drift ≡ P-Delta P-Delta is controlled by • P (large in lower stories) • Delta - but inelastic d • Collapse mechanism • Length of post-yield “plateau” • Effective post yield stiffness • Deterioration • Frame problem very diff. from wall problem

  4. Pushover Deflection Profiles, without and with P-delta -- N =18, T = 3.6 -- Frame

  5. Global Pushover Curve, without and with P-Delta -- N =18, T = 3.6 -- Frame

  6. Vy VASD Global Pushover Curve, LA-20, without and with P-D

  7. Sensitivity to Strain Hardening, Pushover, LA-20

  8. Elastic and Inelastic Stability Coefficient N =18, T = 3.6 -- Frame

  9. Effect of P-Delta on Median Collapse Capacity (Deteriorating Frame Systems)

  10. Effect of P-Delta on Median Collapse Capacity (as function of base shear yield coefficient) Frames versus Walls (8-story)

  11. So, what’s the point? • P-Delta, which is amplified by deterioration, causes collapse (not the only source) • P-Delta effect is very sensitive and not straight forward to predict • We should safeguard against prediction errors • But min. base shear does not look like the right vehicle to do so • In codes: establish a limit on Pd/(Vyh)????

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