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June 2, 2017
Mike Mota, PhD, PE, F.ACI, F. ASCE, F.SEISEAoA 51st Anniversary Convention and Conference
SEISMIC DESIGN ANDDETAILING OF RC
LOW-RISESTRUCTURES
About 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 PCA
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Objectives
General Guidelines for
Overall EconomyJoints of Special Moment Frames Diaphragms
Footings in Areas of High Seismic
Risk
References
•Design Guide for Economical Reinforced Concrete Structures, CRSI, 2016
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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
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General Guidelines for Overall Economy
•Cost• Scheduling
Three Elements of Structure Cost
Floor systems Columns and bearing wallsLateral force‐resisting
systems
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Structure Cost versus Building Height
Main Component Costs
Formwork Concrete Reinforcing steel
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Main Component Costs
50%
30%
20%
U.S. National Average of In‐place Costs
Formwork
Concrete
Reinforcing Steel
Cost‐Effective Design
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Cost‐Effective Design
Elements of Economy – Formwork
• Select one framing system and use it throughout the structure wherever possible
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Elements of Economy – Formwork
• Use standard shaped forms
Elements of Economy – Formwork
• Use floor framing systems of minimum depth with a constant elevation for the bottom surface
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Elements of Economy – Formwork
• Orient one‐way structural members to span in the same direction throughout the entire structure
Elements of Economy – Formwork
• Arrange columns in a regular pattern• Use a consistent column size
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Elements of Economy – Formwork
• Specify time when forms may be stripped from self‐supporting members
• Specify strength of concrete when forms may be stripped from other members
• Use high early strength concrete
Elements of Economy – Formwork
• Use predetermined construction joints
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Elements of Economy –Reinforcement
• Use Grade 60 reinforcing bars in floor systems
Elements of Economy –Reinforcement
• Use the largest bar size possible
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Elements of Economy –Reinforcement
• Use straight bars wherever possible• Use repetitive bar sizes and lengths
Elements of Economy –Reinforcement
• Use stock length bars
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Elements of Economy –Reinforcement
• Use ACI standard bar bend types
Elements of Economy –Reinforcement
• Use the appropriate splice in the appropriate situation
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Elements of Economy –Reinforcement
• Use the appropriate splice in the appropriate situation
Elements of Economy –Reinforcement
• Draw details to scale to ensure that reinforcing bars will fit within the section
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Elements of Economy –Reinforcement
• Draw details to scale to ensure that reinforcing bars will fit within the section
Elements of Economy -Concrete• Use moderate‐strength concrete for floor systems
• 4,000 to 5,000 psi compressive strength
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Elements of Economy -Concrete• Specify few mix designs
• Limit coarse aggregate size to ¾ inch
Elements of Economy -Concrete• Use high‐strength concrete in columns
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Elements of Economy -Concrete• Use high‐performance concrete where required
• Long‐term mechanical properties
• Durability in severe environments
• High early strength
Joints of Special
Moment Frames
•Overview•Preliminary Joint Size
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Shear Strength
•
20forjointsconfinedonall4faces15forjointsconfinedon3facesor2oppositefaces12forallothercases
Shear Strength
• Free‐body diagram of interior column
ℓ, , /2
ℓ
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Shear Strength
• Free‐body diagram of interior joint
1.25
1.25
Shear Strength
•Amount of longitudinal reinforcement in beams framing into the joint has a direct impact on the magnitude of
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Preliminary Joint Size
•Conservative to assume is equal to zero
1.25 1.25
Preliminary Joint Size
•Define• /• /
1.25
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Preliminary Joint Size
•Assume• ≅ 0.9• Grade 60 reinforcement
67.5
Preliminary Joint Size
•Assume• Normalweight concrete• 4,000psi
0.054
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Preliminary Joint Size
•Assume•Width of beam width of column •
,
⁄12 10015 8020 60
Preliminary Joint Size
•Reinforcement range• Minimum 0.0033• Maximum 0.0181
• Tension‐controlled section
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Preliminary Joint Size
•Reinforcement range• ACI 18.6.3.2
• /2• /2 (approx.)
• 0.01• Helps alleviate congestion
Preliminary Joint Size
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Diaphragms •Design•Detailing
CHOOSE CONCRETE: For Life • www.crs i .org
Diaphragms
• Diaphragm in‐plane forces
• Diaphragm transfer forces
• Connection forces between diaphragm and vertical elements of the LFRS
• Forces from bracing vertical or sloped building elements
• Diaphragm out‐of‐plane forces
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Diaphragms
• Analysis methods (ACI 12.4.2.4)• Rigid diaphragm model• Flexible diaphragm model• Bounding analysis• FEM• Strut‐and‐tie model
Diaphragms
•Rigid diaphragm model
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Rigid Diaphragm Model
• Reactions in walls A, B, and C known from analysis• Determine and from equilibrium• Includes eccentricity
Rigid Diaphragm Model
ℓ ℓ ℓ2
2 ℓ ℓ ℓ3 ℓ
ℓ ℓ ℓ ℓ ℓ
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Rigid Diaphragm Model
•Chord forces• ,
• ≅ 0.95• It is inherently assumed that diaphragm behavior follows classical flexural theory
Rigid Diaphragm Model
• Large openings• Seismic forces
• Loads on top and bottom diaphragm segments proportional to area
• Wind forces• Loads based on lengths (in‐plane stiffness)
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Rigid Diaphragm Model
• Large openings• Diaphragm segments are idealized as beams that are fixed at each end
Rigid Diaphragm Model
• Large openings• Chord forces
• For other than openings centered in the diaphragm, it is conservative to use a total tensile force equal to
,0.95
,0.95
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Rigid Diaphragm Model
•Chord reinforcement
0.90
Rigid Diaphragm Model
• Shear transfer reinforcement• Transfer unit shear forces from diaphragm to• vertical elements of the LFRS• any collectors
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Rigid Diaphragm Model
• Shear transfer reinforcement• Wall B
/
0.75
Rigid Diaphragm Model
• Shear transfer reinforcement• Wall A
• Shear transfer depends on width of collector• Collector same width as wall
• Collector wider than wall
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Rigid Diaphragm Model
• Shear transfer reinforcement• Wall A
• Collector same width as wall /
0.75
Rigid Diaphragm Model
• Shear transfer reinforcement• Wall A
• Collector wider than wall• Uniform shear along wall plus a portion of the total collector force
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Rigid Diaphragm Model
• Shear transfer reinforcement• Dowel bars must also be designed for any out‐of‐plane wind and seismic forces
Collectors
• Portion of slab• Beam
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Collectors
Collectors
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Footings • Traditional Design Methods•Proposed Design Method
Traditional Design Methods
•Base area of footing• Service load combinations• Allowable soil bearing capacity
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Traditional Design Methods
•Base area of footing• Service load combinations• Allowable soil bearing capacity
Traditional Design Methods
• Strength design load combinations• Flexural strength• Shear strength
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ASCE/SEI 7‐16
• Section 12.13• Strength‐level design (12.13.5)• Service‐level design (12.13.6)
ASCE/SEI 7‐16
• Strength‐level design• Base area of footing• Strength‐design load combinations of ASCE/SEI 2.3
• 1.4• 1.2 1.6 0.5 or or• 1.2 1.6 or or or0.5• 1.2 1.0 0.5 or or• 1.2 1.0 0.2• 0.9 1.0• 0.9 1.0
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ASCE/SEI 7‐16
• Strength‐level design• Design soil bearing strength
• resistance factor per ASCE/SEI Table 12.13‐1
Direction and Type of Resistance Resistance Factors,
Vertical ResistanceCompression (bearing) 0.45Pile friction 0.45Lateral ResistanceLateral bearing pressure 0.50Sliding (friction or cohesion) 0.85
ASCE/SEI 7‐16
• Strength‐level design• Nominal soil bearing strength
• Presumptive load‐bearing values• Geotechnical site investigations• In‐situ testing of prototype foundations
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ASCE/SEI 7‐16
• Strength‐level design• Overturning effects permitted to be reduced by 25%• ELFP is used• Not inverted pendulum or cantilevered column type structure
,
ASCE/SEI 7‐16
• Strength‐level design• Elastic soil response
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ASCE/SEI 7‐16
• Strength‐level design• Elastic soil response
ASCE/SEI 7‐16
• Strength‐level design• Inelastic soil response
⁄
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ASCE/SEI 7‐16
• Earthquake effects are less than those that would be expected during a design‐basis earthquake
ASCE/SEI 7‐16
• Some inelastic behavior is allowed in the footing regardless if strength‐level or service‐level load combinations are used
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ASCE/SEI 7‐16
• Foundations designed in this way… …may possibly be damaged during a
seismic event…may not perform as intended during subsequent seismic events
ASCE/SEI 7‐16
• Furthermore… …inspecting foundations after an earthquake can be very expensive or impossible
…repairing foundations can also be costly and may not be feasible
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Proposed Method
• Buildings assigned to SDC D, E, or F• Determine factored load effects using strength design load combinations in ASCE/SEI 2.3 with seismic load effects including overstrength in ASCE/SEI 12.4.3
• 1.4• 1.2 1.6 0.5 or or• 1.2 1.6 or or or0.5• 1.2 1.0 0.5 or or• (1.2 0.2 Ω 0.2• 0.9 1.0• (0.9 0.2 Ω
Proposed Method
•Buildings assigned to SDC D, E, or F• Determine base area of footing using , and
• Elastic soil response• Inelastic soil response
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Proposed Method
• Lateral Loads• Ω••
• 0.85(ASCE/SEI Table 12.13.1)
Proposed Method
• Flexural strength• Shear strength• Interface strength
• 1.4• 1.2 1.6 0.5 or or• 1.2 1.6 or or or0.5• 1.2 1.0 0.5 or or• (1.2 0.2 Ω 0.2• 0.9 1.0• (0.9 0.2 Ω
