PCI th 6 Edition Building Systems (Seismic) Presentation Outline • Building System Loads – Seismic • • • • Structural Integrity LFRS – Walls LFRS – Frames Diaphragms Seismic Changes • Based on new changes to ASCE 7 and ACI 318 • Based current seismic research and observations Seismic Changes • Some of these changes are: – Recognition of jointed panel construction – Recognition of strong and ductile connections in precast frames – Recognition and requirements for connections in precast walls Seismic Changes • Additional changes are: – – – – – Modification of drift computation and limiting drift Deformation compatibility of elements Additional soil type classifications Special considerations locations near seismic faults Consideration of redundancy and reliability in strength design requirements 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 Seismic Risk • Soil factors – Other regions of high seismic risk - not just west coast anymore Practically every precast, prestressed concrete structure designed under IBC 2000 will require some consideration of seismic effects. Seismic Performance Objectives • Current design - minor damage for moderate earthquakes • Accepts major damage for severe earthquakes • Collapse is prevented of severe events 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. Seismic Design Approach • Emulation – No special requirements for low seismic risk – Chapter 21 requirements for moderate and high seismic risk • Non-emulative design – PRESSS – Acceptance criteria for frames Earthquake Loads – Equivalent Lateral Force Method • Base Shear, V V= Cs·W Where: Cs - Seismic Response Coefficient W - Total Weight Equivalent Lateral Force Method Limitations • This method may not apply to buildings with irregularities in Seismic Design Categories D, E, or F 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 > 30 psf Seismic Response Coefficient, Cs • Function of – Spectral response acceleration – Site soil factors – Building Period – Response modification factors – Importance factor 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 Step 1 – Determine SS and S1 • From IBC Map • From local building codes • IBC 2003 CD-ROM – Based on • Longitude / Latitude • Zip Code Step 2 – Determine Site Soil Classification • If site soils are not known use Site Class D • Figure 3.10.7 (a) (page 3-111) • From soil reports Step 3 – Calculate Response Accelerations • SMS = Fa·SS • SM1 = Fv·S1 Where: – Fa and Fv are site coefficients from Figure 3.10.7 (b) and (c) (page 3-111) – SS spectral accelerations for short periods – S1 spectral accelerations for 1-second period – All values based on IBC 2003 Step 4 – Calculate the 5%-Damped Design Spectral Response Accelerations • SDS = (2/3)SMS • SD1 = (2/3)SM1 Step 5 – Determine the Seismic Design Category • Table 3.2.4.1. • Sometimes this restricts the type of Seismic Force Resisting System (SFRS) used (see Figure 3.10.8) (page 3-112) Step 6 – (Approximate Period) Determine the Buildings Fundamental Period Ta C thnx Where: Ct = 0.016 for moment resisting frame systems of reinforced concrete 0.020 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 feet) Step 6 – (Exact Period) Determine the Buildings Fundamental Period Rayleigh’s formula n T 2 2 w ii i1 n g Fii i1 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 7 – Determine Seismic Response Coefficient, Cs Lesser of Cs SDS R I or C s SD1 TR I Where: R = Response Modification Factor Figure 3.10.8 (page 3-112) Ι = Seismic Importance Factor Step 7 – Determine Cs Minimum Value of Cs Cs = 0.044·SDS·Ι Special Cases In Seismic Design Categories E and F Cs 0.5 S1 R I Vertical Distribution of Lateral Force Fx C vx V C vx w x hkx n k w h i i i1 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 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 Seismic Drift Requirements • Elastic Displacement Amplification Factor, x • Stability Coefficient Limits, q • P-D Effects Drift Limits • Figure 3.10.9 (page 3-113) Drift Amplification Factor, x x C d xe I Where: δx = Amplified deflection of Level x δxe = Deflection of Level x determined from elastic analysis, includes consideration of cracking Cd = Deflection amplification factor (Figure 3.10.8) Ι = Seismic Importance Factor Stability Coefficient, θ q Px D Vx hsx C d Where: Px = Total vertical unfactored load including and above Level x ∆ = Difference of deflections between levels x and x-1 Vx = Seismic shear force acting between levels x and x-1 hsx = Story height below Level x Cd = Deflection amplification factor Stability Coefficient, θ The stability coefficient is limited to: qmax 0.5 0.25 Cd Where: β = ratio of shear demand to shear capacity between Levels x and x-1 P-D Effects • To account for P-∆ effects, the design story drift is increased by (1− θ)-1 • If θ < 0.10, P-∆ effects may be neglected Reliability Factor, ri • Required in High Seismic Design Categories D, E, and F • The Earthquake Force is increase by a Reliability Factor, ri • 1.5 Maximum Required Value ri = 1.0 for structures in Seismic Design Categories A, B and C Reliability Factor, ri For Moment Frames ri 2 - 20 rmax i A i Where, for each level: Ai = floor area rmaxi = For moment frames, the maximum of the sum of the shears in any two adjacent columns divided by the story shear. For columns common to two bays with moment-resisting connections on opposite sides, 70% of the shear in that column may be used in the column shear summary. Reliability Factor, ri For Shear Walls ri 2 - 20 rmax i A i Where, for each level: Ai = floor area rmaxi = For shear walls, the maximum value of the product of the shear in the wall and 10/lw divided by the story shear. Load Combinations • • • • • • • U = 1.4(D+F) U = 1.2(D+F+T) + 1.6(L+H) U = 1.2D +1.6(Lr or S or R) + (1.0L or 0.8W) U = 1.2D + 1.6W + 1.0L + 0.5(Lr or S or R) U = 1.2D + 1.0E + f1L + 0.2S U = 0.9D + 1.6W + 1.6H U = 0.9D + 1.0E + 1.6H f1 = 1.0 Parking garages = 1.0 Live load ≥ 100 psf on public assembly floors = 0.5 All others Modification for Vertical Acceleration • E = ρ·QE ± 0.2·SDS·D Seismic Load Combinations Become • U = (1.2 + 0.2·SDS)D + ρ·QE + f1L + 0.2S • U = (0.9 – 0.2·SDS)D + ρ·QE + 1.6H Notice Building weight increase as Ground move Up Where QE = Horizontal Seismic Force Modification for Vertical Acceleration • E = ρ·QE ± 0.2·SDS·D Seismic Load Combinations Become • U = (1.2 + 0.2·SDS)D + ρ·QE + f1L + 0.2S • U = (0.9 – 0.2·SDS)D + ρ·QE + 1.6H Notice Building weight decreases as Ground move Down Overstrength Factor, Wo • Components within the Diaphragm – Chord ties – Shear Steel – Connectors • Ωo = 2.0 - Seismic Design Categories C, D, E and F • Ωo = 1.0 - Seismic Design Categories A and B Special Load Combinations • U = 1.2D + fi·L + Em • U = 0.9D + E Where: Em = Wo·QE + 0.2·SDS·D and Wo = Overstrength Factor Overstrength Factor, Wo • Connections from Diaphragms to Seismic Force Resisting System (SFRS) – Ωo = Seismic Design Categories C and higher Figure 3.10.8 (page 3-112) Structural Integrity Requirements • All members must be connected to the Lateral Force Resisting System (LFRS) • Tension ties must be provided in all directions • The LFRS is continuous to the foundation • A diaphragm must be provided with – Connections between diaphragm elements – Tension ties around its perimeter • Perimeter ties provided – Nominal strength of at least 16 kips – Within 4 ft of the edge • Column splices and column base connections must have a nominal tensile strength not less than 200Ag in pounds Structural Integrity Requirements • Precast vertical panels connected by a minimum of two connections • Each connection is to have a nominal strength of 10 kips • Precast diaphragm connections to members being laterally supported must have a nominal tensile strength not less than 300 lbs per linear ft • Connection details allow volume change strains • Connection details that rely solely on friction caused by gravity loads are not to be used Lateral Force Resisting Systems (LFRS) • Rigid frames and shear walls exhibit different responses to lateral loads Influential Factors • The supporting soil and footings • The stiffness of the diaphragm • The stiffness LFRS elements and connections • Lateral load eccentricity with respect to center of rigidity of the shear walls or frames Shear Wall Systems • Most common lateral force resisting systems • Design typically follows principles used for cast-in-place structures International Building Code (IBC) Requirements • Two categories of shear walls – Ordinary – Special ACI 318-02 Requirements • Created an additional intermediate category, but has assigned no distinct R, Ωo and Cd ACI 318-02 Wall Definitions • Defines all shear walls as “structural walls” • Three levels of definition – Ordinary structural (shear) wall – Intermediate precast structural (shear) wall – Special precast structural (shear) wall Ordinary Structural (Shear) Wall • Wall complying with the requirements of Chapters 1 through 18 • No special seismic detailing Intermediate Precast Structural (Shear) Wall • Wall complying with all applicable requirements of Chapters 1 through 18 • Added requirements of Section 21.13 – Ductile connections with steel yielding – 1.5 factor for non-yielding elements • IBC imposes restriction that yielding be in the reinforcing Special Precast Structural (Shear) Wall • Precast wall complying with the requirements of 21.8. • Meeting the requirements for ordinary structural walls and the requirements of 21.2 – Requires precast walls to be designed and detailed like cast-in-place walls, “emulative” design – Meet the connection requirements of Section 21.13. Design Guidelines for Shear Wall Structures • • • Evaluation of building function and applicable precast frame Preliminary development of shear wall system Determination of vertical and lateral loads Design Guidelines for Shear Wall Structures • • • • • Preliminary load analysis Selection of shear walls Final load analysis Final shear wall design Diaphragm design Moment Frame Classifications • Three Classifications – Ordinary Moment Frame – Intermediate Moment Frames – Special Moment Frames • Based on Detailing • Seismic Design Categories Ordinary Moment Frames • Seismic Performance Categories A & B • ACI 318 Chapters 1 to 18 • Response modification factor, R = 3 Intermediate Moment Frames • • • Seismic Performance Category C ACI 318 only defines intermediate as cast-in-place Response modification factor, R = 5 Special Moment Frames • Seismic Performance Categories D, E, and F • Yielding will be concentrated in the beam, Strong column -weak beam behavior • Special Moment frames – ACI 318 Sections 21.2 through 21.6 • Response modification factor, R = 8 Diaphragms • A diaphragm is classified as rigid if it can distribute the horizontal forces to the vertical lateral load resisting elements in proportion to their relative stiffness • Long-span applications suggest that many precast diaphragms may in fact be flexible Diaphragm Design • The distinction between rigid and flexible diaphragms is important not just for diaphragm design, but also for the design of the entire lateral force resisting system. Diaphragm Classification • Flexible diaphragm – Lateral deflection twice average story drift • Rigid diaphragm – Not flexible – Implies capability to distribute load based on relative stiffness of LFRS elements Steps in the Design Method Step 1 - Calculate and compare distribution and diaphragm forces Based on rigid diaphragm action Based on flexible diaphragm action Step 2 - Check of diaphragm deformation with respect to drift limits Step 3 - Check attached element drift limits Step 4 - Adjustments in vertical element stiffness and placement to limit drift Diaphragm Design Forces • Based on Wind and Seismic Events • Wind – Combined windward and leeward wind pressures – Act as uniform load on building perimeter – Distributed to the LFRS based on diaphragm behavior Seismic Diaphragm Design Forces • Separate calculations from the design of the LFRS • Diaphragm Design force, FP • Seismic Design Categories B or C Fp = 0.2·IE·SDS·Wp + Vpx Where Vpx – represents forces from above levels that must be transferred through the diaphragm due to vertical system offsets or changes in stiffness. Seismic Diaphragm Design Forces • Seismic Design Category D n Fpx F ix n i w i x wpx i 0.2·IE·SDS·wpx< Fp < 0.4·IE·SDS·wpx Diaphragm Detailing • Wind and Low Seismic Hazards • Moderate Seismic Hazards • Seismic Design Category D Topped Systems • High Seismic Hazards - Untopped Systems Wind and Low Seismic Hazard • Seismic Design Category A – Strength requirements imposed by the applied forces, No Amplification • Seismic Design Category B – Requires the design of collector elements – Does not require forces to be increased by over strength factor, Ωo (Revised from IBC 2000) Moderate Seismic Hazard • Topped and Pretopped Systems • Seismic Design Category C • Concrete wall systems have special requirements IBC 2003 • Diaphragm must include – special continuous struts or ties between diaphragm chords for wall anchorage. – use of Sub-Diaphragms, the aspect ratio of is limited to 2½ to 1 Moderate Seismic Hazard • Walls classified as Intermediate Precast Walls – Collector elements, their connections based on special load combinations – Need to include overstrength factor – Ductile connections with wall interface – The body of the connection must have sufficient strength to permit development of 1.5fy in the reinforcing steel Seismic Design Category (SDC) D • Topped Systems • Untopped Systems – Not implicitly recognized in ACI 318 - 02 – Section 21.2.1.5 • permits a system to be used if it is shown by experimental evidence and analysis to be equivalent in strength and toughness to comparable monolithic cast-inplace systems SDC D – Topped Systems • High strain demand across the joints • Reinforcing steel needs to be compatible with this demand • Use of larger wire spacing or bars may be needed • Mesh in the topping must take the entire shear across the joint. • Correct lapping to maintain diaphragm integrity SDC D – Topped Systems • Specific provisions in ACI 318-02 • Chord steel determined from flexural analysis • Shear strength based entirely on reinforcement crossing the joint: Vn = Acv·rn·fy Where Acv = thickness of the topping slab ρn = steel ratio of the reinforcement SDC D – Topped Systems • ACI 318-02 – minimum spacing requirement of 10 in – Diaphragm f -factor ≤ vertical element fshear -factor – May result in f = 0.6, based on ACI 318-02 Section 9.3.4 Questions?