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Earthquake Design Using 1997 Uniform Building Code

Earthquake Design Using 1997 Uniform Building Code. Dr. Hathairat Maneetes Department of Rural Roads. The UBC was introduced in the 1927….

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Earthquake Design Using 1997 Uniform Building Code

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  1. Earthquake Design Using 1997 Uniform Building Code Dr. Hathairat Maneetes Department of Rural Roads

  2. The UBC was introduced in the 1927… • From the early 80’s, the earthquake design provisions in UBC changed rapidly and substantially in response to the lessons learned from several major earthquakes (e.g. 1971 San Fernando earthquake and 1994 Northridge earthquake) Soft story Spiral ties 1971 San Fernando earthquake Normal ties Confinement effects Another example of soft story

  3. 1997 UBC has several important modifications following the 1994 Northridge earthquake. 1994 Northridge earthquake

  4. Some of the major changes include • Strength-based (compared with allowable stress approach for the previous versions) • Removal of pre-qualified steel connection details • Requirements to consider liquefaction • Addition of near-fault factor to base shear formulation • Deformation compatibility requirements • Redundancy requirements • Stricter detailing for non-participating elements • Aligned with NEHRP’s provisions for smooth transition into International Building Code (IBC) in 2000

  5. Three model buildings codes for seismic design in the United States. Three model building codes in US: • BOCA National Building Code (NBC) by BOCA • Uniform Building Code (UBC) by ICBO • Standard Building Code (SBC) by SBCCI NEHRP SEAOC Materials code accompanying IBC ASCE 7 BOCA NBC SBC

  6. There are three methods to estimate inertia or earthquake forces. Resultant (inertia) earthquake force distribution • Response (time) history method • Linear (elastic) or nonlinear (inelastic) • Apply acceleration history directly to base of numerical model of structure • Response spectrum method • Linear (elastic) approach to calculate the modal (peak values) responses • Modal responses combined (using SRSS or CQC) to give design values • Equivalent static-force method • Linear approach (assume response dominated by 1st mode response) • Nonlinear approach used for rehabilitation (“Push-over” analysis) Ground acceleration time Earthquake acceleration Complexity

  7. Typical response spectrum of a particular ground motion. Peak response acceleration, ar,peak Idealized for design purposes Actual Short to medium period Long Period Period (T) M Period K Ground acceleration a time

  8. The equivalent static force procedure is a simplification of the dynamic response spectrum method. Consider more than one mode to get realistic results Response Spectrum Method T6 T5 T4 T3 T2 T1 Building model Multiple (e.g. six) degrees of freedom Decoupled SDOF Consider only the fundamental (1st) mode to simply analysis Equivalent Static force Method T1

  9. UBC-97 is specific about which analysis method may or must be used. Static Lateral-force Procedure limitations: Seismic Zones • All structures (regular or irregular) in Seismic Zone 1 or in Zone 2 with occupancy category 4 or 5. • Regular structures using one of the structural systems listed in Table 16-N if they are under 240 feet (7,315.2cm) in height. • Irregular structures not more than 5 stories or 65 feet (1,981.2cm) in height. Regular structure Irregular structure 1 2A and 2B (with occupancy category 4 or 5) 3,4 < 240 feet < 5 stories or 65 feet

  10. If these conditions are not satisfied, the structure shall be designed using dynamic method. • Structures with a flexible upper portion supported on a rigid lower portion if all of the following conditions are met: • When both portions are considered separately, they can both be classified as regular. • The average story stiffness of the lower portion is at least ten times the average story stiffness of the upper portion. • The period of the whole structure is no more than 1.1 times the period of the upper portion considered as a separate structure fixed at the base. So how do we define building irregularities with respect to earthquake design?

  11. There are two types of irregularities, on plan or along the building height. Vertical Irregularities Kx < 0.7Kx+1 or Kx < 0.8 (Kx+1 + Kx+2 + Kx+3)/3 Where K is the story lateral stiffness x+3 x+2 x+1 x Stiffness irregularity – soft story UBC-97 Table 16-L

  12. There are two types of irregularities, on plan or along the building height. Vertical Irregularities Wx+1 > 1.5Wx or Wx+1 > 1.5Wx+2 Where W is the story effective weight (or mass) x+2 x+1 x Weight (mass) irregularity UBC-97 Table 16-L

  13. There are two types of irregularities, on plan or along the building height. Vertical Irregularities Where bi: Horizontal dimension of lateral force-resisting system at story i b1 > 1.3b2 b2 Lateral force resisting elements b1 Vertical geometric irregularity UBC-97 Table 16-L

  14. There are two types of irregularities, on plan or along the building height. Vertical Irregularities l2: offset l1: length of lateral-load resisting elements l2 > l1 l2 l1 In-plane discontinuity in vertical lateral-force resisting element UBC-97 Table 16-L

  15. There are two types of irregularities, on plan or along the building height. Vertical Irregularities Sx+1/Sx < 0.8 Where S: Total strength of lateral force resisting elements x+1 x Discontinuity in capacity – weak story UBC-97 Table 16-L

  16. There are two types of irregularities, on plan or along the building height. Plan Irregularities Torsional irregularity – to be considered when diaphragms are not flexible UBC-97 Table 16-M

  17. There are two types of irregularities, on plan or along the building height. Deformation incompatibility leading to stress concentration Less stiff; more deformation Stiffer; less deformation Plan Irregularities Stress concentrations Re-entrant corners UBC-97 Table 16-M

  18. There are two types of irregularities, on plan or along the building height. Plan Irregularities Aopening > 0.5Agross Agross Open Aopening Diaphragm discontinuity UBC-97 Table 16-M

  19. There are two types of irregularities, on plan or along the building height. Vertical lateral force resisting elements offset out-of-plane Plan Irregularities Out-of-plane offsets UBC-97 Table 16-M

  20. There are two types of irregularities, on plan or along the building height. Plan Irregularities These lateral force resisting elements are not parallel to major axes These lateral force resisting elements are not parallel and symmetric to major axes Nonparallel systems UBC-97 Table 16-M

  21. The UBC-97 governing equations are … Spectral Acceleration Estimation of Total Base Shear 2.5Ca Equation 30-4 But need not be greater than Equation 30-5 Ca But need to be at least Equation 30-6 T0 Ts Period (T) UBC-97 Design Spectra Equation 30-7 (for Seismic zone 4)

  22. Inertial forces is developed from Newton’s Second Law. Damping ar = f(M, K, a, c) Lumped mass (M) ar = a ar = a F = M ar F = M.a F = M.a Flexible with stiffness K Infinitely rigid a a a Rigid box of mass M fixed to the ground Ground acceleration time NON-RIGID BODY RIGID BODIES

  23. UBC-97 has broadly zoned US territories into six seismic zones. Seismic Zones 0 1 2A 2B 3 4 Increasing seismic risk UBC-97 Figure 16-2: Seismic Zone Map of the United States

  24. Each seismic zone is assigned a factor that corresponds to the maximum ground acceleration. UBC-97 Table 16-I

  25. The “effective” ground acceleration imparted to the structure is affected by the soil conditions. UBC-97 Table 16-J a = ag a  ag a a Reference soil type ag Default soil profile Ground acceleration based on SB soil profile (i.e. rock). ag Other soil profiles tend to amplify the ground acceleration impart to the structure base a < ag (Hard rock, rock) a > ag (All other soil profiles)

  26. Seismic coefficients represent the seismicity of the region and the characteristics of the soil. Seismic Coefficient Cv Seismic Coefficient Ca ag UBC-97 Table 16-Q UBC-97 Table 16-R a “Short” to “medium” period “Long” period

  27. Response Modification Factor to account for nonlinear building response. • Need to consider the inherent ability of the structure to reduce the earthquake forces through overstrength, ductility and damping. • A response reduction factor or R-factor is introduced to account for the beneficial effects of nonlinear building behavior. • R-value greater than 1, inelastic response is assumed and earthquake forces is reduced. Base Shear Elastic response V Reduction in earthquake forces arising from nonlinear building response V/R “Actual” Inelastic response Higher ductility Displacement s M = (0.7R)s

  28. R-value is a convenient method to describe the nonlinear response of the structural system. Total Base Shear V System 1 V/R1 Increase in inelastic response System 2 V/R2 System 3 V/R3 R3 > R2 > R3 Displacement

  29. UBC-97 categories 7 basic structural systems with R-values varying from 2.2 to 8.5 • These are maximum values for each structural system type; lower value can be used if required. • Great care must be exercised in selecting the R-value! UBC-97 Table 16-N

  30. What are the common structural systems? Lateral Gravity Dual system Bearing wall Building frame Moment-resisting frame • Supports all gravity and lateral loads • Lack redundancy • R-value varies from 2.8 to 5.5 • Frame carries gravity (i.e. gravity frame • Shear walls or braced frames carry lateral load • Need to consider deformation compatibility • R-value varies from 5.5 to 7.0 • Specially detailed frame to support both gravity and lateral loads • High level of ductility and redundancy • R-value varies from 3.5 to 8.5 • Similar to building frame system except the gravity frame also provide secondary lateral force resistance. • R-value varies from 4.2 to 8.5

  31. Examples of structural systems Building frames Column Beam Concentric braced frames (CBF) Example of moment resisting connection Moment frame Steel eccentric braced frame (EBF) Example of simple shear connection Special truss moment frame

  32. For essential or hazardous buildings, the margin of safety in seismic design needs to be higher • The importance factor is used to increase the earthquake force • Depends on the occupancy category. • In UBC-97, I is 1.25 for essential facilities and hazards facilities; no enhancement for other facilities. UBC-97 Table 16-K

  33. 1.2D + 0.5L + Ev 1.2D + 0.5L + Ev  Eh 1.2D + 0.5L + Ev UBC-97 Load Combinations Strength level U = 1.2D + f1L + 1.0E U = 0.9D + 1.0E where E = Eh + Ev where Ev = 0.5CaID Working stress level F = D + (W or E/1.4) F = 0.9D + (E/1.4) F = D + 0.75 [L + (Lr or S) + (W or E/1.4)] OR F = 4/3[D + L + (W or E/1.4)] F = 4/3[D + L + (E/1.4)] f1 = 1.0 for public assembly LL>100 psf(4.9kN/m2) = 1.0 for garage LL = 0.5 for other LL Vertical component

  34. Numerical Example – Static lateral-force procedure Non-bearing shear wall • Determine the design seismic forces for the three-story reinforced concrete shear wall shown using UBC-97 static lateral-force procedure. The building is located in Southeastern California on rock with a shear wave velocity of 3,000 ft/sec. The story dead loads are 2,200 kips, 2,000 kips and 1,700 kips for the 1st, 2nd and roof level, respectively. The shear walls do not carry significant vertical loads. (Adapted from Naeim (2001)) 11 ft 11 ft 13 ft Building is assumed to be located here IMPORTANT: Always check the applicability of the method • Building of regular construction • No plan or height irregularity • Total height = 35 feet < 240 feet  UBC-97 static lateral-force method is applicable. UBC-97 Figure 16-2: Seismic Zone Map of the United States

  35. Numerical Example – Static lateral-force procedure UBC-97 Table 16-I • Seismic Importance Factor, I = 1.0 (Assumed non-essential facility) • Location is in Seismic Zone 3  Seismic Zone Factor, Z = 0.3 • Shear velocity = 3,000 ft/sec.  Soil Profile Type is SB i.e., rock UBC-97 Table 16-J Note corrections • Seismic Coefficients: CV = 0.3, Ca= 0.3 UBC-97 Table 16-Q UBC-97 Table 16-R

  36. Numerical Example – Static lateral-force procedure UBC-97 Table 16-N Response modification factor: • Non-load bearing shear wall, recommended highest R-value is 5.5 • Height limit is 240 ft > 35 ft (OK) Note corrections

  37. Numerical Example – Static lateral-force procedure V (kips) Estimation of Total Base Shear 1,109.7 804.5 194.7 But need not be greater than 0.29 Period (sec) But need to be at least  design base shear = 804.5 kips

  38. Numerical Example – Static lateral-force procedure Vertical Distribution of the Earthquake Forces. Use load combination! 351.7 kips Level 3 283.7 kips U = 1.2D + 0.5L + 1.0E Level 2 U = 0.9D + 1.0E 169.1 kips Level 1

  39. Numerical Example – Static lateral-force procedure Determine story drift limits Maximum inelastic response displacement: M = 0.7Rs Rearranging, we have s =  M/0.7R Where M< 0.025h for T < 0.7sec (UBC-97, Section 1630.10) 1st story: s< (0.025)(13)/0.7(5.5) = 1.01 in Other stories: s< (0.025)(11)/0.7(5.5) = 0.858 in

  40. Other important considerations • Orthogonal effects : 100% in one direction + 30% in the orthogonal effects (UBC-97 Section 1633.1) • Multiple lateral force resisting systems; requirements of more restrictive one governs (UBC-97 Section 1633.2.2) • Seismic design connections must be clearly detailed in drawings (UBC-97 Section 1633.2.3) • Deformation compatibility (UBC-97 Section 1633.2.4) • Familiarity with accompanying material codes , etc.

  41. Time History Analysis... Oakland, CA

  42. The natural frequencies fell within the dominating frequency range of the ground motions. GILROY EL CENTRO HOLLISTER

  43. The 3 OR 7 pairs of recorded ground motions were scaled to match the design spectrum. SRSS of GILROY (N-S and E-W) 0.2T 1.5T x scale factor1 PGA = 0.367 g Average response spectrum SRSS of EL CENTRO (N-S and E-W) x scale factor2 PGA = 0.371 g 1.4 x Design Spectrum SRSS of HOLLISTER (N-S and E-W) x scale factor3 PGA = 0.177 g If 3 analyses performed, use the maximum response. If 7 analyses performed, use the average response.

  44. An Innovative Design – Structural Control

  45. What’s an innovative design??? Conventional design • ductility-based approach • nonlinear behavior of the structure • Some damage may occur Energy-based design • ‘protective approach’ • ‘structural control’ • classified into 3 groups: passive, active and semi-active, hybrid controls

  46. INTRODUCTORY -Passive Control • Incorporating passive devices to control the structural motion and to modify its dynamic parameters (stiffness and damping). Seismic (base) isolation Passive EDS Mass damper

  47. INTRODUCTORY -Passive Control Source-Sink Analogies [Popov et al., 1993]

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