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Lesson 6 ASCE 7-10: Basic Requirements for Structural Design

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Lesson 6 ASCE 7-10: Basic Requirements for Structural Design
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Lesson 6 ASCE 7-10: Basic Requirements for Structural Design

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  1. Lesson 6ASCE 7-10: Basic Requirements for Structural Design

  2. Outline of Lesson • Basic requirements for all structures • Purpose, exclusions • MCE motion on rock, site amplification, design values • Risk categories, seismic design categories • Geological hazards, geotechnical studies

  3. Outline of Lesson • Basic requirements for building structures • Load path, strength, stiffness • System limits and design parameters • Irregularities and redundancy • Load combinations • Diaphragms, walls, foundations • Drift limits • Simplified alternate

  4. Governing Documents

  5. 2012 IBC Chapterfor Seismic Loading • Chapter 16 Structural Design • Section 1613 Earthquake Loads – Refers to ASCE 7-10, excluding Chapter 14 and Appendix 11A, with one alternative to ASCE 7 provisions.

  6. ASCE 7-10 Section 11.1.1Purpose • “…specified earthquake loads are based upon post-elastic energy dissipation in the structure, and because of this fact, the requirements for design, detailing, and construction shall be satisfied even for structures and members for which load combinations that do not contain earthquake loads indicate larger demands than combinations that include earthquake loads.…”

  7. ASCE 7-10 Section 11.1.2 Scope of Coverage • Every structure, and portion thereof, including nonstructural components • Certain nonbuilding structures, as described in ASCE 7-10 Chapter 15

  8. ASCE 7-10 Section 11.1.2 Exceptions • Detached one- and two-family dwellings located where SS < 0.4 or where the Seismic Design Category is A, B, or C. • Detached one- and two-family wood-frame dwellings with not more than two stories and that comply with the IRC. • Agricultural storage structures that are intended only for incidental human occupancy. • Structures that require special consideration of their response characteristics and environment that are not addressed in Chapter 15 and for which other regulations provide seismic criteria, such as vehicular bridges, electrical transmission towers, hydraulic structures, buried utility lines and their appurtenances, and nuclear reactors.

  9. Seismic Design Steps STEP 1: Determine mapped ground motion at building site STEP 2: Determine Site Class at the building site STEP 3: Determine design ground motion at building site STEP 4: Determine building occupancy and associated Risk Category STEP 5: Determine Seismic Design Category STEP 6:Perform analytical modeling of the structure STEP 7: Determine if horizontal and vertical structural irregularities exist

  10. Steps of Seismic Design STEP 8: Determine redundancy and structural system requirements (Lesson 7) STEP 9:Determine permitted analysis procedure and seismic force distribution in the structure (Lesson 18) STEP 10: Perform structural analysis (Not discussed) STEP 11: Use appropriate load combinations to calculate member forces (Lesson 9) STEP 12: Design structural members in accordance with the 2012 IBC, ASCE 7-10 and various material standards (Lessons 10 through 17, 20 through 24)

  11. Seismic Design Steps STEP 1: Determine mapped ground motion at building site STEP 2: Determine Site Class at the building site STEP 3: Determine design ground motion at building site STEP 4: Determine building occupancy and associated Risk Category STEP 5: Determine Seismic Design Category STEP 6:Perform analytical modeling of the structure STEP 7: Determine if horizontal and vertical structural irregularities exist

  12. STEP 1 Mapped Ground Motion As discussed in Lesson 3, ASCE 7-05: Maximum Considered Earthquake (MCE) - Ground motion that has a 2% probability of exeedance in 50 years. ASCE 7-10: Risk-Targeted Maximum Considered Earthquake (MCER)- Ground motion associated with a 1% probability of structural collapse in 50 years. UNIFORM EQ HAZARD UNIFORM RISK OF COLLAPSE

  13. STEP 1 Mapped Ground Motion Determine SS and S1 in accordance with Section 2-1.6.1 of UFC 3-301-01 for locations within the U.S • Table E-3 of UFC 3-301-01 • USGS Seismic Design Maps Web Application with the approval of the authority having jurisdiction http://geohazards.usgs.gov/designmaps/us/application.php

  14. STEP 1 Mapped Ground Motion UFC 3-310-01

  15. STEP 1 Mapped Ground Motion Select Building Code: 2012 IBC/ASCE 7-10 Select Site Classification

  16. STEP 1 Mapped Ground Motion Type in the Latitude and Longitude of the building site OR Enter Street Address

  17. STEP 1 Mapped Ground Motion Output is displayed in a separate window. Make sure to disable any pop-up blocker on your web browser View Detailed Report Input Information SS, S1, SMS, SM1, SDS, SD1 Values MCER and Design Response Spectra

  18. STEP 1 Mapped Ground Motion Determine SS and S1 in accordance with Section 2-1.6.2 of UFC 3-301-01 for locations outside of the U.S • Table F-3 of UFC 3-301-01 • For locations not shown, use best available information with the approval of the authority having jurisdiction or use Appendix G of UFC 3-301-01https://geohazards.usgs.gov/secure/designmaps/ww/application.php

  19. STEP 1 Mapped Ground Motion

  20. STEP 1 Mapped Ground Motion UFC 3-310-04 Section 2-1613.7 requires a site-specific response analysis for structures on sites classified as Site Class F (see ASCE 7-10 Section 20.3.1), unless: SS≤ 0.25, and S1 ≤ 0.10 as determined in accordance with UFC 3-301-01.

  21. Seismic Design Steps STEP 1: Determine mapped ground motion at building site STEP 2: Determine Site Class at the building site STEP 3: Determine design ground motion at building site STEP 4: Determine building occupancy and associated Risk Category STEP 5: Determine Seismic Design Category STEP 6:Perform analytical modeling of the structure STEP 7: Determine if horizontal and vertical structural irregularities exist

  22. STEP 2 Site Classification Use ASCE 7-10 Chapter 20 to classify the site as below AHard Rock B Rock C Very Dense Soil or Soft Rock D Stiff Soil E Soft clay soil F Soils requiring site response analysis in accordance with Section 21.1

  23. STEP 2 Site Classification Table 20.3-1 Site Classification VU N or Nch SU

  24. STEP 2 Site Classification General Site Classification

  25. Site Class F, Section 20.3.1 STEP 2 Site Classification

  26. STEP 2 Site Classification ASCE 7-10 Section 20.1 Site Class D must be used when the soil properties are not known in sufficient detail, unless the building official determines that Site Class E or F is likely to be present at the site

  27. STEP 2 Site Amplification See Lesson 4

  28. STEP 2 Fa: Site Coefficient for SS ASCE 7-10 Table 11.4-1

  29. STEP 2 Fv: Site Coefficient for S1 ASCE 7-10 Table 11.4-2

  30. STEP 2 Site-Adjusted Ground Motion • Maps all drawn for one reference site condition: rock (Site Class B) • Determine the MCERmotion at a specific site by adjusting for the Site Class at the site: SMS = Fa SS SM1 = Fv S1

  31. STEP 2 MCER Response Spectra Modified for Site Class D SMS=FASS=1.2(0.75)=0.9g Basic SM1 =FVS1=1.8(0.30) = 0.54g Spectral Acceleration, g SiteAmplified Period, sec

  32. Seismic Design Steps STEP 1: Determine mapped ground motion at building site STEP 2: Determine Site Class at the building site STEP 3: Determine design ground motion at building site STEP 4: Determine building occupancy and associated Risk Category STEP 5: Determine Seismic Design Category STEP 6:Perform analytical modeling of the structure STEP 7: Determine if horizontal and vertical structural irregularities exist

  33. STEP 3 Design Ground Motion • Structures are designed for a Collapse Prevention performance criterion under MCER • Design spectral acceleration values account for expected reserve strength in a structure SDS= 2/3FaSS SD1= 2/3FvS1

  34. STEP 3 Design Response Spectra Site Amplified SDS= (2/3)(0.90) = 0.60g Basic SD1 = (2/3)(0.54) = 0.36g Spectral Acceleration, g Scaled Period, sec

  35. STEP 3 Design Response Spectrum Sa = SD1 / T Sa = SDS(0.4 + 0.6 T/T0) Drawn for SS = 1.0, Fa = 1.0 S1 = 0.4, Fv = 1.5 TL= 4 Spectral Acceleration, g 0.4SDS Sa= SD1TL / T2 T0 TS Period, sec

  36. STEP 3 TL Map of Contiguous USA(ASCE 7-10 Figure 22.12)

  37. Seismic Design Steps STEP 1: Determine mapped ground motion at building site STEP 2: Determine Site Class at the building site STEP 3: Determine design ground motion at building site STEP 4: Determine building occupancy and associated Risk Category STEP 5: Determine Seismic Design Category STEP 6:Perform analytical modeling of the structure STEP 7: Determine if horizontal and vertical structural irregularities exist

  38. STEP 4 Building Occupancy and Risk Category • Occupancy of a building determines its required performance level • Buildings are assigned Risk Categories based on their occupancies • Higher Risk Category requires more stringent design requirements

  39. STEP 4 Building Occupancy and Risk Category UFC 3-310-04 Section 2-202 [Replacement] RISK CATEGORY. A categorization of buildings and other structures for determination of flood, wind, snow, ice, and earthquake loads based on the risk associated with unacceptable performance as prescribed in UFC 3-301-01 Table 2-2.

  40. STEP 4 Building Occupancy and Risk Category See UFC 3-301-01 Table 2-2 for detailed descriptions

  41. STEP 4 Seismic Importance Factor, Ie(ASCE 7-10 Table 1.5-2) Buildings assigned to a higher risk category are designed for a higher level of seismic force by using Importance Factor larger than one.

  42. STEP 4 Seismic Importance Factor, Ie(ASCE 7-10 Table 1.5-2) Systems with Ie=1.5 have lower ductility demands than systems with Ie =1.0, and will likely have less damage. Base Shear Elastic Ie=1.5 Ie=1.25 Ie=1.0 Roof Displacement

  43. STEP 4 Performance Basis Collapse Prevention Building Performance Immediate Occupancy Operational Life Safe Frequent Ordinary Buildings Ie= 1.0 High Occupancy Ie= 1.25 Design Ground Motion Emergency Response Ie= 1.5 Maximum Considered

  44. Seismic Design Steps STEP 1: Determine mapped ground motion at building site STEP 2: Determine Site Class at the building site STEP 3: Determine design ground motion at building site STEP 4: Determine building occupancy and associated Risk Category STEP 5: Determine Seismic Design Category STEP 6:Perform analytical modeling of the structure STEP 7: Determine if horizontal and vertical structural irregularities exist

  45. STEP 5 Seismic Design Category(ASCE 7-10 Section 11.6) • Seismic Design Category (SDC) is a function of the seismic hazard at the location of a structure, the occupancy or the Risk Category of the structure, and the Site Class at the site of the structure. • Most seismic requirements are based on the Seismic Design Category of a structure.

  46. STEP 5 SDC Based on SDS(Adapted from ASCE 7-10 Table 11.6-1)

  47. STEP 5 SDC Based on SD1(Adapted from ASCE 7-10 Table 11.6-2)

  48. STEP 5 Seismic Design Category ASCE 7-05 Section 11.6 SDC is to be determined from ASCE 7-10 Tables 11.6-1 (based on SDS) and 11.6-2 (based on SD1), and the more severe one governs.

  49. STEP 5 Seismic Design Category ASCE 7-05 Section 11.6 SDC can be based on SDS alone, provided • S1< 0.75g • Ta< 0.8Ts •T used to calculate story drift < Ts • Upper-bound design base shear is used in design • Diaphragms are rigid, or for diaphragms that are flexible, vertical elements of seismic-force-resisting system are spaced at < 40 ft