Building Systems (Seismic)

1. Building Systems (Seismic)

2. Seismic Changes • Design Forces are Based on Risk • Previous codes based on 10% chance of exceedance in 50 years • IBC 2000, 2003 codes based on 2% chance of exceedance in 50 years

3. Seismic Performance Objectives • Current design - minor damage for moderate earthquakes • Accepts major damage for severe earthquakes • Collapse is prevented of severe events

4. Seismic Performance Objectives In order to achieve the design objectives, the current code approach requires details capable of undergoing large inelastic deformations for energy dissipation.

5. Seismic Design Approach • Emulation • No special requirements for low seismic risk • Chapter 21 requirements for moderate and high seismic risk

6. Earthquake Loads – Equivalent Lateral Force Method • Base Shear, V V= Cs·W Where: Cs - Seismic Response Coefficient W - Total Weight

7. Equivalent Lateral Force Method Limitations • This method may not apply to buildings with irregularities in Seismic Design Categories D, E, or F

8. Earthquake Loads – Total Weight, W • Dead Load of structure plus: • 25% of reduced floor live load in storage areas • live load in parking structures not included • Partition load if included in gravity dead • Total weight of permanent equipment • 20% of flat roof snow load, pf where pf > 1.5kN/m2.

9. Seismic Response Coefficient, Cs • Function of • Spectral response acceleration • Site soil factors • Building Period • Response modification factors • Importance factor

10. Seismic Response Coefficient, Cs • Step 1 - Determine SS and S1 • Step 2 - Determine site Soil Classification • Step 3 - Calculate Response Accelerations • Step 4 - Calculate the 5% Damped Design Spectral Response Accelerations • Step 5 - Determine the Seismic Design Category • Step 6 - Determine the Fundamental Period • Step 7 - Calculate Seismic Response Coefficient

11. Step 1 – Determine SS and S1 • From IBC Map or from local building codes

13. Step 2 – Determine Site Soil Classification • If site soils are not known use Site Class D • From soil reports

14. Step 3 – Calculate Response Accelerations • SMS = Fa·SS • SM1 = Fv·S1 Where: • Fa and Fv are site coefficients • SS spectral accelerations for short periods • S1 spectral accelerations for 1-second period • All values based on IBC 2003

16. Step 4 – Calculate the 5%-Damped Design Spectral Response Accelerations • SDS = (2/3)SMS • SD1 = (2/3)SM1

17. Step 5 – Determine the Seismic Design Category • Sometimes this restricts the type of Seismic Force Resisting System (SFRS) used

18. Where: Ct = 0.047 for moment resisting frame systems of reinforced concrete 0.049 for other concrete structural systems x = 0.9 for concrete moment resisting frames 0.75 for other concrete structural systems hn = distance from base to highest level (in m) Step 6 – (Approximate Period) Determine the Buildings Fundamental Period

19. Rayleigh’s formula Where: wi = dead load weight at Floor i δi= elastic displacement at Floor i Fi = lateral force at Floor i g = acceleration of gravity n = total number of floors Step 6 – (Exact Period) Determine the Buildings Fundamental Period

20. Lesser of Where: R = Response Modification Factor) Ι = Seismic Importance Factor Step 7 – Determine Seismic Response Coefficient, Cs

21. Minimum Value of Cs Special Cases In Seismic Design Categories E and F Step 7 – Determine Cs Cs = 0.044·SDS·Ι

22. Where: Fx = Force per floor Cvx = Vertical distribution factor V = Base shear k = 1 - buildings with a period ≤ 0.5 sec = 2 - buildings with a period > 2.5 sec hi and hx = height from base to Level i or x wi and wx = Level i or x portion of total gravity load Vertical Distribution of Lateral Force

23. Location of Force in Plane • Accidental Torsion • calculated by assuming that the center of mass is located a distance of 5% of the plan dimension perpendicular to the applied load on either side of the actual center of mass • Total torsion = sum of the actual torsion plus the accidental torsion