S D RC L -R STRUCTURES

6/2/2017SEISMIC DESIGN ANDDETAILING OF RCLOW-RISESTRUCTURESMike Mota, PhD, PE, F.ACI, F. ASCE, F.SEISEAoA 51st Anniversary Convention and ConferenceJune 2, 2017About the Speaker Mike Mota, PhD, PE, F.ACI, F.ASCE, F.SEI VP of Engineering at CRSI Voting Member of ACI 318, 318B and 318R Member of ASCE‐7 (2016) Member of STRUCTURE Editorial Board Formerly Regional Engineer with PCA1

6/2/2017ObjectivesGeneralGuidelines forOverall EconomyJoints of SpecialMoment FramesDiaphragmsFootings in Areasof High SeismicRiskReferences Design Guide for EconomicalReinforced ConcreteStructures, CRSI, 20162

6/2/2017References Design and Detailing ofLow‐Rise ReinforcedConcrete Buildings, CRSI,2017 Available soonwww.crsi.orgReferences Building Code Requirementsfor Structural Concrete,ACI 318‐14, 20143

6/2/2017GeneralGuidelinesfor OverallEconomy Cost SchedulingThree Elements of Structure CostFloor systemsColumns and bearingwallsLateral force‐resistingsystems4

6/2/2017Structure Cost versus Building HeightMain Component CostsFormworkConcreteReinforcing steel5

6/2/2017Main Component CostsU.S. National Average of In‐place Costs20%50%FormworkConcrete30%Reinforcing SteelCost‐Effective Design6

6/2/2017Cost‐Effective DesignElements of Economy – Formwork Select one framing system and use itthroughout the structure whereverpossible7

6/2/2017Elements of Economy – Formwork Use standard shaped formsElements of Economy – Formwork Use floor framing systems of minimumdepth with a constant elevation for thebottom surface8

6/2/2017Elements of Economy – Formwork Orient one‐way structural members tospan in the same direction throughout theentire structureElements of Economy – Formwork Arrange columns in a regular pattern Use a consistent column size9

6/2/2017Elements of Economy – Formwork Specify time when forms may be strippedfrom self‐supporting members Specify strength of concrete when formsmay be stripped from other members Use high early strength concreteElements of Economy – Formwork Use predetermined construction joints10

6/2/2017Elements of Economy –Reinforcement Use Grade 60 reinforcing bars infloor systemsElements of Economy –Reinforcement Use the largest bar size possible11

6/2/2017Elements of Economy –Reinforcement Use straight bars wherever possible Use repetitive bar sizes and lengthsElements of Economy –Reinforcement Use stock length bars12

6/2/2017Elements of Economy –Reinforcement Use ACI standard bar bend typesElements of Economy –Reinforcement Use the appropriate splice in theappropriate situation13

6/2/2017Elements of Economy –Reinforcement Use the appropriate splice in theappropriate situationElements of Economy –Reinforcement Draw details to scale to ensure thatreinforcing bars will fit within thesection14

6/2/2017Elements of Economy –Reinforcement Draw details to scale to ensure thatreinforcing bars will fit within thesectionElements of Economy Concrete Use moderate‐strength concretefor floor systems 4,000 to 5,000 psi compressivestrength15

6/2/2017Elements of Economy Concrete Specify few mix designs Limit coarse aggregate size to ¾ inchElements of Economy Concrete Use high‐strength concrete incolumns16

6/2/2017Elements of Economy Concrete Use high‐performance concretewhere required Long‐term mechanical properties Durability in severe environments High early strengthJoints ofSpecialMomentFrames Overview Preliminary Joint Size17

6/2/2017Shear Strength 20 for joints confined on all 4 faces15 for joints confined on 3 faces or 2 opposite faces12 for all other casesShear Strength Free‐body diagram ofinterior column,ℓ,/2ℓ18

6/2/2017Shear Strengthinterior joint Free‐body diagram of1.25framing into the jointShear Strengthreinforcement in beams Amount of longitudinal191.25

6/2/2017Preliminary Joint Size Conservative to assumeis equal to zero1.251.25Preliminary Joint Size Define / /1.2520

6/2/2017Preliminary Joint Size Assume 0.967.5 Grade 60 reinforcementPreliminary Joint Size Assume Normalweight concrete 4,000 psi0.05421

6/2/2017Preliminary Joint Size, Assume Width of beamcolumnwidth of 1210015802060Preliminary Joint Size Reinforcement range Minimum Maximum0.00330.0181 Tension‐controlled section22

6/2/2017Preliminary Joint Size Reinforcement range ACI 18.6.3.2/2 /2 (approx.) 0.01 Helps alleviate congestionPreliminary Joint Size23

6/2/2017Diaphragms Design DetailingDiaphragms Diaphragm in‐plane forces Diaphragm transfer forces Connection forces betweendiaphragm and verticalelements of the LFRS Forces from bracing vertical orsloped building elements Diaphragm out‐of‐plane forcesCHOOSE CONCRETE: For Life www.crsi.org24

6/2/2017Diaphragms Analysis methods (ACI12.4.2.4) Rigid diaphragm model Flexible diaphragm model Bounding analysis FEM Strut‐and‐tie modelDiaphragms Rigid diaphragm model25

6/2/2017Rigid Diaphragm Model Reactions in walls A, B, andC known from analysis Determineequilibriumandfrom Includes eccentricityRigid Diaphragm Model2ℓ2ℓℓℓℓℓℓ3ℓℓℓℓℓ26

6/2/2017Rigid Diaphragm Model Chord forces, 0.95 It is inherently assumed thatdiaphragm behavior followsclassical flexural theoryRigid Diaphragm Model Large openings Seismic forces Loads on top and bottomdiaphragm segmentsproportional to area Wind forces Loads based on lengths (in‐plane stiffness)27

6/2/2017Rigid Diaphragm Model Large openings Diaphragm segments areidealized as beams thatare fixed at each endRigid Diaphragm Model Large openings Chord forces For other than openingscentered in the diaphragm, it isconservative to use a totaltensile force equal to,0.95,0.9528

6/2/2017Rigid Diaphragm Model Chord reinforcement0.90Rigid Diaphragm Model Shear transfer reinforcement Transfer unit shear forces fromdiaphragm to vertical elements of the LFRS any collectors29

6/2/2017Rigid Diaphragm Model Shear transfer reinforcement Wall B/0.75Rigid Diaphragm Model Shear transfer reinforcement Wall A Shear transfer depends on widthof collector Collector same width as wall Collector wider than wall30

6/2/2017Rigid Diaphragm Model Shear transfer reinforcement Wall A Collector same width as wall/0.75Rigid Diaphragm Model Shear transfer reinforcement Wall A Collector wider than wall Uniform shear along wall plus aportion of the total collectorforce31

6/2/2017Rigid Diaphragm Model Shear transfer reinforcement Dowel bars must also bedesigned for any out‐of‐planewind and seismic forcesCollectors Portion of slab Beam32

6/2/2017CollectorsCollectors33

6/2/2017Footings Traditional Design Methods Proposed Design MethodTraditional DesignMethods Base area of footing Service load combinations Allowable soil bearingcapacity34

6/2/2017Traditional DesignMethods Base area of footing Service load combinations Allowable soil bearingcapacityTraditional DesignMethods Strength design loadcombinations Flexural strength Shear strength35

6/2/2017ASCE/SEI 7‐16 Section 12.13 Strength‐level design(12.13.5) Service‐level design(12.13.6)ASCE/SEI 7‐16 1.4 Strength‐level design Base area of footing Strength‐design loadcombinations ofASCE/SEI 2.3 1.21.60.5or or 1.21.6or or 1.21.0 1.21.0 0.91.0 0.91.00.5or 0.5or or0.236

6/2/2017ASCE/SEI 7‐16 Strength‐level design Design soil bearing strength resistance factor perASCE/SEI Table 12.13‐1Direction and Type ofResistanceResistance Factors,Vertical ResistanceCompression (bearing)0.45Pile friction0.45Lateral ResistanceLateral bearing pressure0.50Sliding (friction or cohesion)0.85ASCE/SEI 7‐16 Strength‐level design Nominal soil bearingstrength Presumptive load‐bearing values Geotechnical site investigations In‐situ testing of prototype foundations37

6/2/2017ASCE/SEI 7‐16 Strength‐level design Overturning effects permittedto be reduced by 25%, ELFP is used Not inverted pendulum orcantilevered column typestructureASCE/SEI 7‐16 Strength‐level design Elastic soil response38

6/2/2017ASCE/SEI 7‐16 Strength‐level design Elastic soil responseASCE/SEI 7‐16 Strength‐level design Inelastic soil response 39

6/2/2017ASCE/SEI 7‐16 Earthquake effects areless than those that wouldbe expected during adesign‐basis earthquakeASCE/SEI 7‐16 Some inelastic behavior isallowed in the footingregardless if strength‐levelor service‐level loadcombinations are used40

6/2/2017ASCE/SEI 7‐16 Foundations designed inthis way may possibly be damaged during aseismic event may not perform as intended duringsubsequent seismic eventsASCE/SEI 7‐16 Furthermore inspecting foundations after anearthquake can be very expensive orimpossible repairing foundations can also becostly and may not be feasible41

6/2/2017Proposed Method 1.4 Buildings assigned to SDC D, E,or F 1.21.60.5 1.21.6or or Determine factored loadeffects using strength designload combinations in ASCE/SEI2.3 with seismic load effectsincluding overstrength inASCE/SEI 12.4.3 1.21.0 (1.2 0.9 (0.90.2or or0.5Ωor 0.5or or0.21.00.2ΩProposed Method Buildings assigned to SDC D,E, or F Determine base area offooting using ,and Elastic soil response Inelastic soil response42

6/2/2017Proposed Method Lateral LoadsΩ 0.85 (ASCE/SEI Table12.13.1)Proposed Method 1.4 Flexural strength 1.21.60.5or or Shear strength 1.21.6 1.21.0 Interface strength (1.2 0.9 (0.90.2or or0.5Ωor 0.5or or0.21.00.2Ω43

6/2/2017 3 References Design and Detailing of Low‐Rise Reinforced Concrete Buildings, CRSI, 2017 Available soon www.crsi.org References Building Code Requirements for Structural Concrete, ACI 318‐14, 2014