PCI 6th EditionBuilding Systems(Seismic)

Presentation OutlineBuilding System LoadsSeismicStructural IntegrityLFRS WallsLFRS FramesDiaphragms

Seismic ChangesBased on new changes to ASCE 7 and ACI 318Based current seismic research and observations

Seismic ChangesSome of these changes are:Recognition of jointed panel constructionRecognition of strong and ductile connections in precast framesRecognition and requirements for connections in precast walls

Seismic ChangesAdditional changes are:Modification of drift computation and limiting driftDeformation compatibility of elements Additional soil type classificationsSpecial considerations locations near seismic faultsConsideration of redundancy and reliability in strength design requirements

Seismic ChangesDesign Forces are Based on RiskPrevious codes based on 10% chance of exceedance in 50 yearsIBC 2000, 2003 codes based on 2% chance of exceedance in 50 years

Seismic RiskSoil factorsOther 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 earthquakesAccepts major damage for severe earthquakesCollapse 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 ApproachEmulationNo special requirements for low seismic riskChapter 21 requirements for moderate and high seismic riskNon-emulative designPRESSSAcceptance criteria for frames

Earthquake Loads Equivalent Lateral Force Method Base Shear, V V= CsW

Where:Cs - Seismic Response CoefficientW - Total Weight

Equivalent Lateral Force Method LimitationsThis 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 areaslive load in parking structures not includedPartition load if included in gravity deadTotal weight of permanent equipment20% of flat roof snow load, pf where pf > 30 psf

Seismic Response Coefficient, Cs Function ofSpectral response accelerationSite soil factorsBuilding PeriodResponse modification factorsImportance factor

Seismic Response Coefficient, CsStep 1 - Determine SS and S1 Step 2 - Determine site Soil ClassificationStep 3 - Calculate Response AccelerationsStep 4 - Calculate the 5% Damped Design Spectral Response AccelerationsStep 5 - Determine the Seismic Design CategoryStep 6 - Determine the Fundamental PeriodStep 7 - Calculate Seismic Response Coefficient

Step 1 Determine SS and S1 From IBC MapFrom local building codesIBC 2003 CD-ROMBased onLongitude / LatitudeZip Code

Step 2 Determine Site Soil Classification If site soils are not known use Site Class DFigure 3.10.7 (a) (page 3-111)From soil reports

Step 3 Calculate Response AccelerationsSMS = FaSSSM1 = FvS1 Where:Fa and Fv are site coefficients from Figure 3.10.7 (b) and (c) (page 3-111)SS spectral accelerations for short periodsS1 spectral accelerations for 1-second periodAll values based on IBC 2003

Step 4 Calculate the 5%-Damped Design Spectral Response AccelerationsSDS = (2/3)SMSSD1 = (2/3)SM1

Step 5 Determine the Seismic Design CategoryTable 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 PeriodWhere:Ct =0.016 for moment resisting frame systems of reinforced concrete 0.020 for other concrete structural systemsx = 0.9 for concrete moment resisting frames 0.75 for other concrete structural systemshn = distance from base to highest level (in feet)

Step 6 (Exact Period) Determine the Buildings Fundamental PeriodRayleighs formula

Where:wi = dead load weight at Floor ii = elastic displacement at Floor iFi = lateral force at Floor ig = acceleration of gravityn = total number of floors

Step 7 Determine Seismic Response Coefficient, CsLesser of

Where:R = Response Modification Factor Figure 3.10.8 (page 3-112) = Seismic Importance Factor

Step 7 Determine CsMinimum Value of Cs

Special Cases In Seismic Design Categories E and F

Cs = 0.044SDS

Vertical Distribution of Lateral ForceWhere:Fx = Force per floorCvx = Vertical distribution factorV = Base sheark= 1 - buildings with a period 0.5 sec = 2 - buildings with a period > 2.5 sechi and hx = height from base to Level i or xwi and wx = Level i or x portion of total gravity load

Location of Force in PlaneAccidental 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 massTotal torsion = sum of the actual torsion plus the accidental torsion

Seismic Drift RequirementsElastic Displacement Amplification Factor, dxStability Coefficient Limits, qP-D Effects

Drift LimitsFigure 3.10.9 (page 3-113)

Drift Amplification Factor, dxWhere:x = Amplified deflection of Level xxe = Deflection of Level x determined from elastic analysis, includes consideration of crackingCd = Deflection amplification factor (Figure 3.10.8) = Seismic Importance Factor

Stability Coefficient, Where:Px = Total vertical unfactored load including and above Level x = Difference of deflections between levels x and x-1Vx = Seismic shear force acting between levels x and x-1hsx = Story height below Level xCd = Deflection amplification factor

Stability Coefficient, The stability coefficient is limited to:

Where: = ratio of shear demand to shear capacity between Levels x and x-1

P-D EffectsTo account for P- effects, the design story drift is increased by

(1 )-1 If < 0.10, P- effects may be neglected

Reliability Factor, riRequired in High Seismic Design Categories D, E, and FThe Earthquake Force is increase by a Reliability Factor, ri1.5 Maximum Required Value

ri = 1.0 for structures in Seismic Design Categories A, B and C

Reliability Factor, ri For Moment FramesWhere, for each level:Ai = floor arearmaxi = 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 WallsWhere, for each level:Ai = floor arearmaxi = For shear walls, the maximum value of the product of the shear in the wall and 10/lw divided by the story shear.

Load CombinationsU = 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 + 0.2SU = 0.9D + 1.6W + 1.6HU = 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 = QE 0.2SDSD

Seismic Load Combinations BecomeU = (1.2 + 0.2SDS)D + QE + f1L + 0.2SU = (0.9 0.2SDS)D + QE + 1.6H

WhereQE = Horizontal Seismic Force

Modification for Vertical Acceleration E = QE 0.2SDSD

Seismic Load Combinations BecomeU = (1.2 + 0.2SDS)D + QE + f1L + 0.2SU = (0.9 0.2SDS)D + QE + 1.6H

Overstrength Factor, WoComponents within the Diaphragm Chord tiesShear SteelConnectorso = 2.0 - Seismic Design Categories C, D, E and Fo = 1.0 - Seismic Design Categories A and B

Special Load CombinationsU = 1.2D + fiL + EmU = 0.9D + E Where:Em = WoQE + 0.2SDSDandWo = Overstrength Factor

Overstrength Factor, WoConnections from Diaphragms to Seismic Force Resisting System (SFRS) o = Seismic Design Categories C and higher Figure 3.10.8 (page 3-112)

Structural Integrity RequirementsAll 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 foundationA diaphragm must be provided withConnections between diaphragm elementsTension ties around its perimeter Perimeter ties providedNominal strength of at least 16 kipsWithin 4 ft of the edgeColumn splices and column base connections must have a nominal tensile strength not less than 200Ag in pounds

Structural Integrity RequirementsPrecast vertical panels connected by a minimum of two connectionsEach connection is to have a nominal strength of 10 kipsPrecast diaphragm connections to members being laterally supported must have a nominal tensile strength not less than 300 lbs per linear ftConnection details allow volume change strainsConnection 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 FactorsThe supporting soil and footingsThe stiffness of the diaphragmThe stiffness LFRS elements and connectionsLateral load eccentricity with respect to center of rigidity of the shear walls or frames

Shear Wall SystemsMost common lateral force resisting systemsDesign typically follows principles used for cast-in-place structures

International Building Code(IBC) RequirementsTwo categories of shear wallsOrdinarySpecial

ACI 318-02 RequirementsCreated an additional intermediate category, but has assigned no distinct R, o and Cd

ACI 318-02 Wall DefinitionsDefines all shear walls as structural walls Three levels of definitionOrdinary structural (shear) wallIntermediate precast structural (shear) wallSpecial precast structural (shear) wall

Ordinary Structural (Shear) WallWall complying with the requirements of Chapters 1 through 18No special seismic detailing

Intermediate Precast Structural(Shear) WallWall complying with all applicable requirements of Chapters 1 through 18Added requirements of Section 21.13Ductile connections with steel yielding1.5 factor for non-yielding elementsIBC imposes restriction that yielding be in the reinforcing

Special Precast Structural (Shear) WallPrecast wall complying with the requirements of 21.8.Meeting the requirements for ordinary structural walls and the requirements of 21.2Requires precast walls to be designed and detailed like cast-in-place walls, emulative designMeet the connection requirements of Section 21.13.

Design Guidelines for Shear Wall StructuresEvaluation of building function and applicable precast framePreliminary development of shear wall systemDetermination of vertical and lateral loads

Design Guidelines for Shear Wall StructuresPreliminary load analysisSelection of shear wallsFinal load analysisFinal shear wall designDiaphragm design

Moment Frame ClassificationsThree ClassificationsOrdinary Moment FrameIntermediate Moment FramesSpecial Moment FramesBased on DetailingSeismic Design Categories

Ordinary Moment FramesSeismic Performance Categories A & BACI 318 Chapters 1 to 18Response modification factor, R = 3

Intermediate Moment FramesSeismic Performance Category CACI 318 only defines intermediate as cast-in-placeResponse modification factor, R = 5

Special Moment FramesSeismic Performance Categories D, E, and FYielding will be concentrated in the beam, Strong column -weak beam behaviorSpecial Moment framesACI 318 Sections 21.2 through 21.6Response modification factor, R = 8

DiaphragmsA diaphragm is classified as rigid if it can distribute the horizontal forces to the vertical lateral load resisting elements in proportion to their relative stiffnessLong-span applications suggest that many precast diaphragms may in fact be flexible

Diaphragm DesignThe 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 ClassificationFlexible diaphragmLateral deflection twice average story driftRigid diaphragmNot flexibleImplies capability to distribute load based on relative stiffness of LFRS elements

Steps in the Design MethodStep 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 limitsStep 3 - Check attached element drift limitsStep 4 - Adjustments in vertical element stiffness and placement to limit drift

Diaphragm Design ForcesBased on Wind and Seismic Events

WindCombined windward and leeward wind pressuresAct as uniform load on building perimeterDistributed to the LFRS based on diaphragm behavior

Seismic Diaphragm Design Forces Separate calculations from the design of the LFRSDiaphragm Design force, FPSeismic Design Categories B or C

Fp = 0.2IESDSWp + Vpx

WhereVpx represents forces from above levels that must be transferred through the diaphragm due to vertical system offsets or changes in stiffness.

Seismic Diaphragm Design ForcesSeismic Design Category D

0.2IESDSwpx< Fp < 0.4IESDSwpx

Diaphragm DetailingWind and Low Seismic HazardsModerate Seismic HazardsSeismic Design Category D - Topped SystemsHigh Seismic Hazards - Untopped Systems

Wind and Low Seismic HazardSeismic Design Category AStrength requirements imposed by the applied forces, No AmplificationSeismic Design Category BRequires the design of collector elementsDoes not require forces to be increased by over strength factor, o (Revised from IBC 2000)

Moderate Seismic HazardTopped and Pretopped SystemsSeismic Design Category CConcrete wall systems have special requirements IBC 2003Diaphragm must includespecial continuous struts or ties between diaphragm chords for wall anchorage. use of Sub-Diaphragms, the aspect ratio of is limited to 2 to 1

Moderate Seismic HazardWalls classified as Intermediate Precast WallsCollector elements, their connections based on special load combinationsNeed to include overstrength factorDuctile connections with wall interfaceThe body of the connection must have sufficient strength to permit development of 1.5fy in the reinforcing steel

Seismic Design Category (SDC) D Topped SystemsUntopped SystemsNot implicitly recognized in ACI 318 - 02Section 21.2.1.5permits 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-in-place systems

SDC D Topped SystemsHigh strain demand across the joints Reinforcing steel needs to be compatible with this demandUse of larger wire spacing or bars may be neededMesh in the topping must take the entire shear across the joint. Correct lapping to maintain diaphragm integrity

SDC D Topped SystemsSpecific provisions in ACI 318-02Chord steel determined from flexural analysisShear strength based entirely on reinforcement crossing the joint:Vn = Acvrnfy

WhereAcv = thickness of the topping slab n = steel ratio of the reinforcement

SDC D Topped SystemsACI 318-02 minimum spacing requirement of 10 inDiaphragm f -factor vertical element fshear -factorMay result in f = 0.6, based on ACI 318-02 Section 9.3.4

Questions?

Recognition of jointed panel construction as an alternative to emulation of monolithic construction.Achieving ductile structural behavior by using strong connections that remain elastic while nonlinear action (plastic hinging) occurs in the member away from the connection.Modification of drift computation and limiting drift.Deformation compatibility of structural elements and attached non-structural elements.Additional soil type classifications.Special considerations for building sites located near seismic faults.Special considerations for structures possessing redundancy.

Recognition of jointed panel construction as an alternative to emulation of monolithic construction.Achieving ductile structural behavior by using strong connections that remain elastic while nonlinear action (plastic hinging) occurs in the member away from the connection.Modification of drift computation and limiting drift.Deformation compatibility of structural elements and attached non-structural elements.Additional soil type classifications.Special considerations for building sites located near seismic faults.Special considerations for structures possessing redundancy.

Ct = 0.016 for moment resisting frame systems of reinforced concrete in which the frames resist 100% of the required seismic forces and are not enclosed or adjoined by more rigid components that prevent that frame from deflecting when subjected to seismic forces .For buildings with a period between 0.5 and 2.5, determine k by linear interpolation.

The Cd factor represents an approximation of the post-yield or Non-linear displacement.

f1 = 1.0 for floors in places of public assembly, for live loads in excess of 100 psf, and for parking garages, otherwise, f1 = 0.5D = Dead loadF = Pressure of fluids of known density and controlled depthsT = Effects of temperature, creep and shrinkageL = Live loadH = Soil loadLr = Roof live loadS = Snow loadR = Rain loadW = Wind loadE = Seismic load

All members must be connected to the lateral force resisting system and their supporting members. Tension ties must be provided in the transverse, longitudinal, and vertical directions and around the perimeter of the structure.The lateral force resisting system must be continuous to the foundation. A diaphragm must be provided with connections between diaphragm elements, with tension ties around its perimeter and around openings that significantly interrupt diaphragm action. Section 16.5.2.4 of ACI 318-02 requires perimeter ties to provide a nominal strength of at least 16 kips and to be within 4 ft of the edge.Column splices and column base connections must have a nominal tensile strength not less than 200Ag in lbs, where Ag is the gross area of the column in sq in. For a compression member with a larger cross section than required by consideration of loading, a reduced effective area, Ag, not less than one-half the total area, may be used.

Precast walls, other than cladding panels, must be connected across horizontal joints by a minimum of two connections per panel. Each connection is to have a nominal tensile strength of not less than 10 kips. When design forces result in no tension at the base, these connections are permitted to be anchored into an appropriately reinforced slab on grade. If panels are too narrow to accommodate two connections, a single connection is satisfactory, as long as it is connected to adjacent panels.Where precast elements form roof or floor diaphragms, the connections between the diaphragm and those members being laterally supported must have a nominal tensile strength not less than 300 lbs per linear ft.To accommodate volume change strains (temperature and shrinkage) in supported beams, tie connections are typically located at the top of the member, with elastomeric pads used at the bottom-bearing surface. Such ties can be accomplished by welding, bolting, reinforcing steel in grout joints or bonded topping, or by doweling.Connection details that rely solely on friction caused by gravity loads are not to be used. Exceptions may be permitted for heavy modular unit structures where resistance to overturning or sliding has a large factor of safety.The relative flexural and shear stiffnesses of the shear walls, and of connections.In a warehouse type structure, it is common to include the exterior walls as part of the lateral load resisting system. In parking structures, shear walls can be located at stair and elevator towers, at the perimeter or ramped bays, at selected locations on the perimeter of the structure, or any in combination of the above locations

a. Provide at least three non-collinear walls to ensure torsional as well as direct lateral resistance.b. Overturning will often be the governing criterion. Thus, the first choice is to use shear walls that also function as bearing walls.c. Arrange shear walls so that they minimize restraint due to volume changes. d. Consider whether the shear walls could be individual full height walls (vertical joints only). e. Consider the practicality of shipping and erection when selecting the size of wall panels. f. Balance the design requirements of the shear walls with the design requirements of the associated diaphragms.

Determine the vertical gravity loads that are applicable to each of the shear walls. Use the applicable seismic design criteria to determine the magnitude of lateral load at each floor, and compare with wind loading. Choose the critical condition for design.

Determine the overturning moment, the lateral in plane shear and the axial load at the base of each of the shear walls.

a. Review the preliminary choice of shear wall size and location. b. Modify the number, location, and dimensions of shear walls as necessary to satisfy the requirements at the base of each. It is economically preferable thatfoundations not be subject to uplift.

Based on the final location and dimensions of shear walls, perform the final lateral load and vertical load analysis to determine the design load for each of the shear walls. Consider shear stiffness as well as flexural stiffness when distributing lateral loads to the shear walls.

Design the shear wall reinforcement and the connections to the associated diaphragms. Where there is insufficient length of shear wall available to accommodate the necessary number of shear connectors, consider using an element in the plane of the diaphragm (drag strut) as an extension of the shear wall to pick up additional connectors to the diaphragm. Consider the additional requirements necessary to satisfy the structural integrity provisions of the code (see Section 3.3).

a. Design the diaphragms to respond elastically to applied lateral loads in order to prevent formation of plastic regions in any diaphragm. See Section 3.8 for amore detailed discussion of diaphragm design. b. Design the diaphragms as beams, provide the necessary tensile reinforcement for each chord, and choose shear connectors using design procedures of Chapter 6, or shear reinforcement using shear-friction methods. c. Consider the additional requirements necessary to satisfy the structural integrity provisions of the code (see Section 3.3). The same frame with different rebar details can be Ordinary, Intermediate or Special.Vpx represents forces from above levels that must be transferred through the diaphragm due to vertical system offsets or changes in stiffness.

Vpx represents forces from above levels that must be transferred through the diaphragm due to vertical system offsets or changes in stiffness.

the interface with the wall must be with ductile connections including the reinforcing steel that would yield prior to crushing of the concrete. The body or mechanical parts of the connection (e.g., plates, welds, etc.) must have sufficient strength to permit development of 1.5fy in the reinforcing steel.

The provisions for precast diaphragms that are included in ACI 318-02 are for topped composite and topped non-composite diaphragms, so untopped diaphragms are implicitly not recognized. Section 21.2.1.5 of ACI 318-02,

These provisions include systems with the topping composite with the precast components and systems with the topping noncomposite part acting alone.The composite system requirements recognize that connections may be part of the design proportioned and detailed to transfer forces. They require that the interface be clean, free of laitance, and intentionally roughened.

These provisions include systems with the topping composite with the precast components and systems with the topping noncomposite part acting alone.The composite system requirements recognize that connections may be part of the design proportioned and detailed to transfer forces. They require that the interface be clean, free of laitance, and intentionally roughened.

Diaphragm phi-factor must be no greater than that used in the shear design of the supporting vertical elements (columns or walls).