See original reference (CODE) for full details

Transcription

1 Earthquake Engineering Course Notes Ahmed Elgamal Ahmed Elgamal 1997 Uniform Building Code (Seismic load brief introduction) See original reference (CODE) for full details From the section on Structural Engineering Design Provisions, Section : Earthquake Provisions are primarily to safeguard against major structural failures and loss of life, not to limit damage or maintain function. Codes only set minimum standards and are intended to help prevent catastrophic failures Code developers attempt to represent the results of more elaborate analyses in ways that are, simple, accurate, and convenient for use by practicing engineers Considerable reasoning and deliberations take place during the preparation/modification of a code, with top practitioners/researchers participating to define the most appropriate approaches. Static Force Procedure The total design Base Shear in a given direction shall be determined from V = ( C v I / R T ) W not to exceed V = ( 2.5 C a I / R ) W and not less than V = 0.11 C a I W In addition, for seismic Zone 4, V will not be less than V = ( 0.8 Z N v I / R ) W W is the total seismic dead load defined as: Total dead load and applicable portions of live load such as a min of 25% of live load in Warehouses, 10 psf (0.48 kn/m 2 ) for partition loads when applicable, snow load of 30 psf (1.44 kn/m 2 ) or less, and total weight of permanent equipment.

2 I is the Importance factor defined in a table (table 16-k, page 2-30) similar to Occupancy Category Occupancy or Function of Seismic Importance factor, I Structure 1. Essential facilities Hospitals, fire and police 1.25 stations, garages and shelters for emergency vehicles, aviation control towers, tanks containing water for fire suppression efforts,.. 2. Hazardous facilities Non-building structures 1.25 housing toxic or explosive chemicals, 3. Special occupancy School buildings housing 1.0 structures more than 300 students, all structures housing more than 5000 occupants,.. Standard Occupancy Other housing structures not 1.0 structures listed above.. Miscellaneous Structures Other structures, 1.0 Figure 16-2 Seismic Zone Map of the United States

3 R is a numerical coefficient representative of the inherent overstrength and global ductility capacity of lateral forceresisting systems, as set forth in Table 16-N (p. 2-32) or 16-P. Example of Table 16 N Basic Structural System (see section for description of basic systems) Lateral force-resisting system description R Height Limit for seismic zones 3 and 4 (feet) x for mm 1. Bearing wall system Light-framed walls with shear Panels (Wood structural panel walls for structures three stories or less. Braced frames where bracing carries gravity load a. steel b. Concrete c. Heavy timber 2. Building Frame system Steel eccentrically braced frame (EBF) Moment-resisting frame system Ordinary braced frames Steel Concrete Heavy timber Special moment resisting frame (SMRF) Steel concrete Masonary moment resisting wall frame (MMRWF) Ordinary moment resisting frame (OMRF) Steel concrete 4. Dual systems 1. Shear walls Concrete with SMRF Masonry with masonry MMRWF 2. Steel EBF with steel SMRF with steel OMRF N.L. N.L N.L. 5. Cantilevered Column Cantilever column elements building systems 6. Shear wall-frame Concrete (prohibited in seismic zones 2A, 2B, 3 and 4) 5.5 interaction systems 7. Undefined systems See sections and N.L. Example of Table 16 P (p. 2-34) R factors for non-building structures Vessels, including tanks and pressurized spheres, or braced or unbraced legs 2.2 Cast-in-place concrete silos and chimneys having walls continuous to the foundations 3.6 Cantilevered column type structures 2.2 Amusement structures and monuments 2.2

4 In the above, C v /T and 2.5 C a are derived from geotechnical site amplification studies that show that site amplification envelopes take the following form Figure Design Response Spectra (UBC 1997) C v is found from Table 16-R (p. 2-35), and depends on Seismic zone factor Z, and soil profile type S A - S F Seismic Coefficient C v (Table 16-R, p. 2-35) Soil Profile Seismic Zone Factor, Z Type Z=0.075 Z=0.015 Z=0.2 Z=0.3 Z=0.40 S A N v S B N v S C N v S D N v S E N v S F Site specific geotechnical investigation and dynamic site response analysis shall be performed to determine seismic coefficients for Soil Profile Type F Near Source Factor N v (Table 16-T, p. 2-35) Seismic Source Type Closest Distance to a known seismic source <= 2 km 5 km 10 km >= 15 km A B C Locations of faults are determined based on available approved geotechnical data (e.g, USGS maps) Closest (min) distance between the site and the area described by the vertical projection of the source on the surface (i.e., surface projection of fault plane). The surface projection need not include portions of the source at depths of 10 KM or greater. The largest value of near source Factor considering all sources shall be used.

5 Seismic Source Type (Table 16-U, p. 2-35) Seismic source Type A Seismic source description Faults that are capable of producing large magnitude events and that have a high rate of seismic activity B All faults other than type A or C M >= 7.0 M < 7.0 M >= 6.5 C Faults that are not capable of producing large magnitude earthquakes and that have a relatively low rate of seismic activity Seismic source definition Max Slip rate, Moment SR(mm/yr) Mag, M M >=7.0 SR >= 5 SR < 5 SR > 2 SR < 2 M < 6.5 SR < = 2 Subduction sources shall be evaluated on a site-specific basis. Both max moment magnitude and slip rate conditions shall be satisfied concurrently when determining the seismic source type. Seismic Coefficient C a (Table 16-Q, p. 2-34) Soil Profile Seismic Zone Factor, Z Type Z=0.075 Z=0.015 Z=0.2 Z=0.3 Z=0.40 S A N a S B N a S C N a S D N a S E N a S F Site specific geotechnical investigation and dynamic site response analysis shall be performed to determine seismic coefficients for Soil Profile Type F Near Source Factor N a (Table 16-S, p. 2-35) Seismic Source Type Closest Distance to a known seismic source <= 2km 5 km >= 10 km A B C Locations of faults are determined based on available approved geotechnical data (e.g, USGS maps) Closest (min) distance between the site and the area described by the vertical projection of the source on the surface (i.e., surface projection of fault plane). The surface projection need not include portions of the source at depths of 10 KM or greater. The largest value of near source Factor considering all sources shall be used.

6 Soil Profile Types (Table 16-J, p. 2-30) Soil Profile type Soil profile name/ generic description Average soil properties for top 100 feet (30,480 mm) of soil profile Shear wave velocity V s bar feet/second (m/s) S A Hard Rock > 5000 (1,500) S B Rock 2500 to 5,000 (760 to 1,500) S C Very Dense soil 1,200 to 2,500 and Soft Rock (360 to 760) S D Stiff soil profile 600 to 1,200 Standard Penetration Test N bar [or N CH bar for cohesionless soil layers] (blows/ft) Undrained shear strength, s u bar psf (kpa) > 50 > 2,000 (100) 15 to 50 1,000 to 2,000 (50 to 100) (180 to 360) S E Soft soil profile < 600 (180) < 15 < 1,000 (50) S F Soil requiring site specific Evaluation, see section Soil profile type S E also includes any soil profile with more than 10 ft (3048 mm) of soft clay defined as a soil of plasticity index, PI >20, w mc >=40 percent and s u < 500 psf ( 24 kpa). The Plasticity Index, PI, and the moisture content, w mc, shall be determined in accordance with approved national standards. Examples of S F sites Soils vulnerable to potential failure or collapse under seismic loading such as liquefiable soils, quick and highly sensitive clays, and collapsible weakly cemented soils. Peats and/or highly organic clays [H > 10 ft (3048 mm) of peat or highly organic clay where H = thickness of soil]. Very high plasticity clays [H > 25 ft. (7620 mm) with PI > 75] Very thick soft to medium stiff clays [H > 120 ft. (36580 mm)]..... Average shear wave velocity v s bar shall be determined from v s bar = ( sum of d i from i = 1 to n ) / ( sum of d i /v si from i = 1 to n) where d i = thickness of layer i in feet (m). v si = shear wave velocity in layer i in ft./sec. (m/sec.)

7 similar expressions apply for N bar (average field standard Penetration Resistance) and N CH bar (average standard penetration resistance for cohesionless soil layers) N bar = ( sum of d i from i = 1 to n ) / ( sum of d i /N i from i = 1 to n) and N CH bar = ( sum of d s from i = 1 to n ) / ( sum of d i /N i from i = 1 to n) Where d i = thickness of layer I in feet (mm). d s = the total thickness of cohesionless soil layers in the top 100 feet (30480 mm). N i = the standard penetration resistance of soil layer in accordance with approved nationally recognized standards. and finally, s u bar (average undrained shear strength) shall be determined from s u bar = d c / ( sum of d i /s ui from i = 1 to n) where d c = the total thickness (100 d s ) of cohesive soil layers in the top 100 feet (30480 mm) s ui = the undrained shear strength in accordance with approved nationally recognized standards, not to exceed 5,0000 psf (250 kpa). Zone factor Z is found from Table 16 I, p Zone 1 2A 2B 3 4 Z The zone shall be determined from the seismic zone map in Figure 16.2 (depends on geographic location and associated seismic risk) Estimation of Structural Period T in seconds (method A presented below): T = C t (h n ) 3/4 Where C t = in ft. or ( in m) for steel moment resisting frames = (0.0731) for reinforced concrete moment-resisting frames and eccentrically braced frames = (0.0488) for all other buildings h n = height in ft. or (m) above the base

8 Vertical Distribution of Force In the absence of a more rigorous procedure, use V = F t + sum of F i from i = 1 to n The concentrated force at the top is in addition to Fn and shall be determined from F t = 0.07 T V but need not exceed 0.25 V and may be considered as zero for T = 0.7 second or less. The remaining portion of the shear shall be distributed over the height of the structure, including level n, according to the following formula F x = ( ( V F t ) w x h x ) / ( sum of w i h i from i = 1 to n) At each level designated as x, the force F x shall be applied over the area of the building in accordance with the mass distribution at that level. Structural displacements and design seismic forces shall be calculated as the effect of F x and F t applied at the appropriate level above the base. Other considerations Detailing requirements in Seismic Zones 3 and 4 (section ) Drift (section ) Story Drift Limitation (section ) Important for serviceability, and for non-str5uctural damage control (.g., windows,..door ways,..) Rigid Structures (T less than 0.06 second) V = 0.7 C a I W See Code for full details