Earthquake Loads According to IBC IBC Safety Concept

Transcription

1 Earthquake Loads According to IBC 2003 The process of determining earthquake loads according to IBC 2003 Spectral Design Method can be broken down into the following basic steps: Determination of the maimum considered earthquake and design spectral response accelerations. Determination of the seismic base shear associated with the building or the structure s fundamental period of vibration. Distribution of the seismic base shear within the building or the structure. IBC Safety Concept The IBC intends to design structures for collapse prevention in the event of an earthquake with a 2 % probability of being eceeded in 50 years 132

2 Introduction Seismic Response Spectra: - A response spectrum provides the maimum response of a SDOF system, for a given damping ratio and a range of periods, for a specific earthquake. - A design response spectrum is a smoothed spectrum used to calculate the epected seismic response of a structure Figure (1) shows si inverted, damped pendulums, each of which has a different fundamental period of vibration. To derive a point on a response spectrum, one of these pendulum structures is analytically subjected to the vibrations recorded during a particular earthquake. The largest acceleration of this pendulum structure during the entire record of a particular earthquake can be plotted as shown in Figure 1(b). Repeating this for each of the other pendulum structures shown in Figure 1(a) and plotting and connecting the peak values for each of the pendulum structures produces an acceleration response spectrum. Generally, the vertical ais of the spectrum is normalized by epressing the computed accelerations in terms of the acceleration due to gravity g. In Figure (2), displacement, velocity, and acceleration spectra for a given earthquake are shown. In this figure, structures with short periods of 0.2 to 0.5 seconds are almost rigid and are most affected by ground accelerations. Structures with medium periods ranging from 0.5 to 2.5 seconds are affected most by velocities. Structures with long periods greater than 2.5 seconds, such as tall buildings or long span bridges, are most affected by displacements. 133

3 Reference: MacGregor, J and Wight, J., "Reinforced Concrete Mechanics and Design" 4 th Edition, Prentice Hall, NJ, Viscous damping (a) Damped pendulums of varying natural frequencies 4 Acceleration Sa % Damping 2% Damping 5% Damping Natural period of vibration, T (sec) Acceleration response spectrum Figure (1): Earthquake Response Spectrum 134

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5 Analysis Procedure 1- Determination of maimum considered earthquake and design spectral response accelerations: Determine the mapped maimum considered earthquake MCE spectral response accelerations, S s for short period (0.2 sec.) and S 1 for long period (1.0 sec.) using the spectral acceleration maps in IBC Figures 1615(1) through 1615(10). Straightline interpolation is allowed for sites in between contours or the value of the higher contour shall be used. Acceleration values obtained from the maps are given in % of g, where g is the gravitational acceleration. Determine the site class, which is based on the types of soils and their engineering properties, in accordance with IBC Section Site classes A, B, C, D, E, and F, obtained from Table , are based on the average shear velocity, v s, average standard penetration resistance, N, or the average undrained shear strength, s u. These parameters represent average values for the top 30 m of soil. When the soil properties are not known in sufficient detail to determine the site class, site class D shall be used. Unless the building official determines that the site class E or F is likely to be present at the site. Determine the maimum considered earthquake spectral response accelerations adjusted for site class effects, S MS at short period and S M1 at long period in accordance with IBC

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7 where: S = F S S MS a s M 1 = Fv S 1 F a = short-period site coefficient, given in Table (1) F = long-period site coefficient, given in Table (2) v Determine the 5% damped design spectral response accelerations period and S D1 at long period in accordance with IBC S DS at short S = (2/ 3) Ds S MS D1 ( 2 / 3) SM1 S = 138

8 2- Determination of seismic use group and occupancy important factor: Each structure shall be assigned a seismic use group and a corresponding occupancy importance factor I E, in accordance with Table Seismic use group I are structures not assigned to either seismic use group II or III. Seismic use group II are structures the failure of which would result in a substantial public hazard due to occupancy or use as indicated in Table Seismic use group III are structures required for post earthquake recovery and those containing substantial quantities of hazardous substances as indicated in Table

9 3- Determination of seismic design category: All structures shall be assigned to a seismic design category based on the seismic use group and the design spectral response acceleration coefficients, S DS and S D1. Each building and structure shall be assigned to the worst severe seismic design category in accordance with Table (1) or (2), irrespective of the fundamental period of vibration of the structure. 140

10 4- Determination of the Seismic Base Shear: 4-1 Simplified Analysis: A simplified analysis, in accordance with Section , shall be determined to be used for any structure in Seismic Use Group I, subject to the following limitations, or a more rigorous analysis shall be made: 1- Building of light-framed construction not eceeding three stories in height, ecluding basement. 141

11 2- Building of any construction other than the light-framed construction, not eceeding two stories in height, ecluding basement, with fleible diaphragm at every level. Since the above limitations rule out the use of this method for concrete buildings, it will not be covered here. 4-2 Inde Force Analysis: Structures assigned to Seismic Design Category A need only comply with the requirements of Section through , summarized below: Structures shall be provided with a complete lateral force resisting system designed to resist the minimum lateral force, F, applied simultaneously at each floor level according to the following equation: F = w Where: F = The design lateral force applied at level w = The portion of the total gravity load of the structure, W, located or assigned to level W = The total dead load and other loads listed below: 1- In areas used for storage, a minimum of 25 % of the reduced floor live load. 2- Where an allowance for partition load is reduced in the floor load design, the 2 actual partition weight or 50 kg / m of the floor area, whichever is greater. 3- The total weight of permanent equipment % of flat roof snow load where flat roof snow load eceeds 145 kg / m. The direction of application of seismic forces used in design shall be such that which will produce the most critical load effect in each component. The design seismic forces are permitted to be applied separately in each of the two orthogonal directions. Load combinations as per Section 9.2 of ACI Code. 4-3 Equivalent Lateral Force Analysis: Section of ASCE 7-02 ** shall be used. ** ASCE, ASCE Standard Minimum Design Loads for Buildings and Other Structures, ASCE 7-02, American Society of Civil Engineers, Reston, VA,

12 V = C The seismic base shear V in a given direction is determined in accordance with the following equation: s W where: C s = Seismic response coefficient = S DS S D1 ( R/ I ) ( R/ I )T E E S DS R = Response modification coefficient, given in Table I E = Seismic occupancy importance factor T = Fundamental period of vibration An approimate value of Ta may be obtained from: 0.75 n T a = CT h where: C T = Building period coefficient = for moment frames resisting 100% of the required seismic force = for all other buildings h n = Height of the building above the base in meters The calculated fundamental period, T, cannot eceed the product of the coefficient, C u, in the following table times the approimate fundamental period, T a. The base shear V is to be based on a fundamental period, T, in seconds, of 1.2 times the coefficient for the upper limit on the calculated values, C u, taken from the following table, times the approimate fundamental period, Ta 143

13 Vertical Structural Irregularities Irregularity Type and Description 1a- Stiffness Irregularity- Soft Story A soft story is one in which the lateral stiffness is less than 70 percent of that in the story above or less than 80 percent of the average stiffness of the three stories above. 1b- Stiffness Irregularity- Etreme Soft Story An etreme soft story is one in which the lateral stiffness is less than 60 percent of that in the story above or less than 70 percent of the average stiffness of the three stories above. 2- Weight (Mass) Irregularity Mass irregularity shall be considered to eist where the effective mass of any story is more than 150 percent of the effective mass of an adjacent story. A roof that is lighter than the floor below need not be considered. 3- Vertical Geometric Irregularity Vertical geometric irregularity shall be considered to eist where the horizontal dimension of the lateral force-resisting system in any story is more than 130 percent of that in an adjacent story. 4- In-Plane Discontinuity in Vertical Lateral Force-Resisting Elements An in plane offset of the lateral load-resisting elements greater than the length of these elements or a reduction in stiffness of the resisting element in the story below. 5- Discontinuity in Capacity-Weak Story A weak story is one in which the story strength is less than 80 percent of that in the story above. The story strength is the total strength the story above or less than 80 percent of that in the story above. The story strength is the total strength of all seismic-resisting elements sharing the story shear for the direction under consideration. 144

14 Plan Structural Irregularities Irregularity Type and Description 1a- Torsional Irregularity to be considered when diaphragms are not fleible Torsional irregularity shall be considered to eist when the maimum story drift, computed including accidental torsion, at one end of the structure transverse to an ais is more than 1.2 times the average of the story drifts at the two ends of the structure. 1b- Etreme Torsional Irregularity to be considered when diaphragms are not fleible Etreme torsional irregularity shall be considered to eist when the maimum story drift, computed including accidental torsion, at one end of the structure transverse to an ais is more than 1.4 times the average of the story drifts at the two ends of the structure. 2- Re-entrant Corners Plan configurations of a structure and its lateral force-resisting system contain re-entrant corners, where both projections of the structure beyond a reentrant corner are greater than 15 % of the plan dimension of the structure in the given direction. 3- Diaphragm Discontinuity Diaphragms with abrupt discontinuities or variations in stiffness, including those having cutout or open areas greater than 50 % of the gross enclosed area of the diaphragm, or changes in effective diaphragm stiffness of more than 50 % from one story to the net. 4- Out-of-plane Offsets Discontinuities in a lateral force path, such as out-of-plane offsets of the vertical elements. 5- Nonparallel Systems The vertical lateral load-resisting elements are not parallel to or symmetric about the major orthogonal aes of the lateral force-resisting system. 145

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19 Coefficient for Upper Limit on Calculated Period Design Spectral Response, S D1 Coefficient C u In cases where moment resisting frames do not eceed twelve stories in height and having a minimum story height of 3 m, an approimate period T a in seconds in the following form can be used: T a = 0. 1N where N = number of stories

20 5- Vertical Distribution of Forces: The vertical distribution of seismic forces is determined from: and F C = C v w i= 1 V v = n h w i k h k i F F F w n w w 1 h h h where: F = Lateral force at level C v = Vertical distribution factor V = total design lateral force or shear at the base of the building w and w i = the portions of Wassigned to levels and i h and h i = heights to levels and i k = a distribution eponent related to the building period as follows: k = 1 for buildings with T less than or equal to 0.5 seconds k = 2 for buildings with T more than or equal to 2.5 seconds Interpolate between k = 1 and k = 2 for buildings with T between 0.5 and Horizontal Distribution of Forces and Torsion: Horizontally distribute the shear V V F i = i= 1 where: F = Portion of the seismic base shear, V, introduced at level i i Accidental Torsion, M ta M ta = V ( B) Total Torsion, M = M + M M T T t ta 151

21 7- Overturning Moments: The overturning moment M is given by the following equation: where: M n = τ Fi i= ( h h ) i F i = Portion of the seismic base shear, V, introduced at level i τ = Overturning moment reduction factor = 1.0 for the top 10 stories = 0.8 from the 20 th story from the top and below = Values between 1.0 and 0.8 determined by a straight linear interpolation for stories between the 20 th and 10 th stories below the top 8- Story Drift: The story drift,, is defined as the difference between the deflection of the center of mass at the top and bottom of the story being considered. Cd δ e δ = I E Where: C d = Deflection amplification factor, given in Table δ = Deflection determined by elastic analysis e The allowable story drifts,, are shown in Table P-delta Effect: The P-delta effects can be ignored if the stability coefficient, θ, from the following epression is equal to or less than P 0.5 θ = 0.25 V hs Cd Cd β Where: P = Total unfactored vertical design load at and above level 152

22 V = Seismic shear force acting between level and 1 h s = Story height below level = Design story drift occurring simultaneously with V β = Ratio of shear demand to shear capacity for the story between level and 1. Where the ratio β is not calculated, a value of β = 1.0 shall be used. When the stability coefficient, θ, is greater than 0.10 but less than or equal to θ ma, P- delta effects are to be considered. To obtain the story drift for including the P-delta effects, the design story drift shall be multiplied by 1.0/(1 θ). When θ is greater than redesigned. θ ma 10- Combination of Load Effects:, the structure is potentially unstable and has to be The value of seismic load E for use in ACI load combinations is defined by the following equations for load combinations in which the effects of gravity loads and seismic loads are additive: E = ρ Q E SDS E SDS D E = Ω o Q D (Need not apply to SDC A) where: E = the effect of horizontal and vertical earthquake-induced forces S DS = the design spectral response acceleration at short period D = the effect of dead load ρ = the reliability factor related to the etent of structural redundancy of the lateral force resisting system Q E = the effect of horizontal seismic forces Ω = the system over strength factor given in Table o The value of seismic load E for use in ACI load combinations is defined by the following equations for load combinations in which the effects of gravity loads and seismic loads are counteractive: E = ρ Q E 0. 2 S DS D E = Ω o Q 0. S D (Need not apply to SDC A) E 2 DS 153

23 Redundancy: Seismic Design Categories A, B, and C: For structures in seismic design categories A, B and C, the value of ρ may be taken as 1.0. Seismic Design Category D: For structures in seismic design category D, ρ shall be taken as the largest of the values of ρ calculated at each story of the structure in accordance with this equation 6.10 ρ = 2 rma A where: A = the floor area in square meters of the diaphragm level immediately above the story. r ma = the ratio of the design story shear resisted by the single element carrying the most shear force in the story to the total story shear for a given direction of loading. For moment frames, r ma shall be taken as the maimum of the sum of the shears in any two adjacent columns in the plane of a moment frame divided by the story shear. For columns common to two bays with moment resisting connections on opposite sides at the level under consideration, 70 percent of the shear in that column may be used in the column shear summation. For shear walls, r ma shall be taken equal to the maimum ratio, r i, calculated as the shear in each wall or wall pier multiplied by 3.3/ l w, where l w is the wall or wall pier length in meters divided by the story shear and where the ratio 3.3/ l w need not be taken greater than 1.0 for buildings of light frame construction. For dual systems, r ma shall be taken as the maimum value as defined above considering all lateral-load-resisting elements in the story. The lateral loads shall be distributed to elements based on their relative rigidities considering the interaction of the dual system. For dual systems, the value of ρ need not eceed 80 percent of the value calculated above. The value of ρ need not eceed 1.5, which is permitted to be used for any structure. The value of ρ shall not be taken as less than Diaphragm Forces: Diaphragms are designed to resist design seismic forces determined in accordance with the following equation: 154

24 F n i i= p = w n p ranges from ( ) SDS I E wp i= F w i Where: F i = The design force applied to level i F = The diaphragm design force p w i = The weight tributary to level i w p = The weight tributary to the diaphragm at level 12- Seismic Detailing Requirements Level of detailing required depends on the level of seismic risk: - Low Seismic Risk: SDC * A, B - Medium Seismic Risk: SDC C - High Seismic Risk: SDC D, E, F * SDC= Seismic Design Category 155

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26 Eample (7): For the building shown in Eample (1) and using IBC-03 evaluate the forces at the floor levels perpendicular to aes 1-1, 2-2, 3-3 and 4-4. Note that site class is D, S s = 0.25g and S 1 = 0.10g. Solution: Using Tables (1) and (2), short-period site coefficient F a = and long-period site coefficient F v = Maimum considered earthquake spectral response accelerations adjusted for site class effects are evaluated. SMS = Fa Ss = 1.60( 0.25g) = 0.4g and SM 1 = Fv S1 = 2.40( 0.10g) = 0.24g The 5% damped design spectral response accelerations S DS at short period and S D1 at long period in accordance are evaluated. 2 2 SDS = SMS = ( 0.40g) = 0.267g SD 1 = SM1 = ( 0.24g) = 0.16g 3 3 Occupancy importance factor, I E = 1. 0 as evaluated from Table From Table (1) and for S DS = 0.267g, Seismic Design Category (SDC) is B. For S D 1 = 0.16g and using Table (2), SDC is C. Therefore, seismic design category (SDC) is C. For ordinary shear walls and using Table , response modification coefficient R = The seismic base shear V in a given direction is determined in accordance with the following equation: V = Cs W Cs = SDS SD1 ( R / I ) ( R / I ) T E E S DS 0.75 Approimate period Ta = 0.049( 21) = 0.48sec. C u T a = 1.408( 0.48) = 0.676sec. T = 1.2( 0.48) = 0.576sec. < 0.676sec. O. K 157

27 C s = = < = (5.0) ( ) > 0.044( 0.267) O.K i.e., C s = The seismic base shear V = = ( ) tons Vertical distribution of forces: k w h F = Cv V and C v = n k wi h i i= 1 K = (from linear interpolation). Shear forces V = F i i= 1 n M = τ Fi hi h i= where τ =1. 0 Overturning moment ( ), Vertical Distribution of Forces: Level w i h ( ) 038 C v F