Seismic R = 1, 1.5, 3 with Low Seismic Design Example Thomas F. Heausler, PE, SE Structural Engineer This presentation will summarize and scrutinize the use of Seismic Response Modification Coefficient, R, equal to 1, 1.5 and 3 for Industrial/Nonbuilding Structures Similar to Buildings, and for Buildings located in Low Seismic Design Categories (SDC B, C). The benefits and disadvantages will be discussed, and Design Examples will compare Low and High R systems, and assess economics, complexity, reliability, and the potential for undesirable modes of failure.
Transcript
Seismic R = 1, 1.5, 3 with Low Seismic Design Example
Thomas F. Heausler, PE, SE Structural Engineer
This presentation will summarize and scrutinize the use of Seismic Response Modification Coefficient, R, equal to 1, 1.5 and 3 for Industrial/Nonbuilding Structures Similar to Buildings, and for Buildings located in Low Seismic Design Categories (SDC B, C). The benefits and disadvantages will be discussed, and Design Examples will compare Low and High R systems, and assess economics, complexity, reliability, and the potential for undesirable modes of failure.
2019 Structural Engineering Summit – Anaheim
Indiana Structural Engineers Association2020 Spring Conference
Seismic R = 1, 1.5 and 3With Low Seismic Design Example
By:Thomas F. Heausler, PE, SEStructural EngineerKansas City
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Thomas F. Heausler, PE, SEStructural Engineer
• ASCE 7 Seismic Voting Member since 2006
• NCSEA Seismic Code Advisory Committee, Chair
• Provide Senior Review and Code Consulting to Engineering Firms
• 38 years experience
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Seismic R = 1, 1.5 and 3 – When and Where &Low Seismic Example
• What is R• Restrictions• Low Seismic, Industrial • Pros, Cons, Prudent
Table 12.2-1SCBF R = 6, Omega Ωo = 2, Cd = 5SMF R = 8, Omega Ωo = 3, Cd = 5.5Steel Systems Not Specifically Detailed for Seismic, R = 3, Omega Ωo = 3, Cd = 3
Each R value has strings attached.
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DetailingLimitations(height, permitted)
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R, Omega Ωo, Cd
• Not like Wind
• “R” is measure of effective ductility of system
• Ductility is range between yield and fracture
• R is composed of two components: • Omega Ωo : Overstrength
• Rd: inelastic behavior/energy dissipation
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R - Response Modification Coefficient
•Overstrength
•Ductility
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R - Response Modification CoefficientOverstrength - within the SFRS
• Material Strengths, φ, Utilization• Drift governed• First yield versus fully yield
The maximum strength developed along the curve is substantially higher than that at first significant yield,
and this margin is referred to as the system overstrengthcapacity.
The ratio of these strengths is denoted as Ω.
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Rd
ΩFirst Yield
Design
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R - Response Modification Coefficient
Ductility - within the SFRS
• Region after yield but before fracture
• Reliable, controlled fuse
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Ductility
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R - Response Modification CoefficientDuctility - within the SFRS
•Region after yield but before fracture
•Reliable, controlled fuse
•Moment Frame => Hinge in beam
•Braced Frame => Inelastic buckling of brace
Northridge, CA
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Ductile
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Reduced Beam Section – Moment Frame
R = 8
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Special Concentric Braced Frame - SCBF
R = 6
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Special Concentric Braced Frame - SCBF
R = 6
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Buckling Restrained Braced Frames - BRBF
R = 8
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R = 8
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BRBF Buckling Restrained Braced Frames
R = 8
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BRBF
R = 8
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Strings Attached to High R Factor
•Collectors (ASCE 7 – SDC C, D, E, F) Ωo
•AISC 341 versus AISC 360
•ACI 318-14 Chapter 17, 18
•Masonry
•Wood
Ω is within fuse, Ωo is outside of fuse
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Collector Ωo
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High R factors• AISC 341 Special Moment Frame, R = 8
• AISC 358 “prequalified” moment connections
• Develop expected strength of beam
• Demand Critical Weld, CJP welds
• Protected Zones
• AISC 341 Special Concentric Braced Frame, R = 6•Develop expected strength of brace
• Columns• Width/Thickness Ratios
• Develop expected strength of braces
• Anchor rods, Column splices
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Lower R factors
•AISC 341 Ordinary Moment Frame, R = 3.5
•AISC 341 Ordinary Concentric Braced Frame, R = 3.25
•Not very ordinary - Ωo is triggered for connections, braces, anchor rods etc.
•Ωo = 2, 3
Table 12.2-10BF R = 3.25, Omega Ωo = 2, Cd = 3.25OMF R = 3.5, Omega Ωo = 3, Cd =3Steel Systems Not Specifically Detailed for Seismic, R = 3, Omega Ωo = 3, Cd = 3
Each R value has strings attached.
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OMF R = 3.5 • Develop expected strength of beam• Demand Critical Weld, CJP welds
0BF R = 3.25• Ωo overstrength required• K-Brace prohibited• Beam Support DL+LL for inverted V bracing
Columns – (for both OMF and OBF)• Width/Thickness Ratios• Ωo overstrength required• Anchor rods, Column splices
“Ordinary” = not very ordinary!
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R = 3 for Steel Structures
• Not specifically detailed for seismic – AISC 360 only
• Not permitted in SDC D, E, F
• Ωo only required by ASCE 7 (e.g. collectors) and ACI Concrete anchorage.
• Rationale: Low ductility demand, mostly overstrength, accelerations are low.
• Frequently and economical to use where allowed.
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R = 3 Braced Frame, Moment Frame
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R = 3 Braced Frame, Moment Frame
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R = 3 Braced Frame
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Industrial Structures
•Difficult or impossible to meet prescriptive requirements of high R factor.
R = 1.0 for Steel Moment Frames
R = 1.5 for Steel Braced Frames
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ASCE 7 Chapter 15Nonbuilding Structures Similar to Buildings
BUILDING: Any structure whose intended use includes shelter of human occupants.
NONBUILDING STRUCTURE: A structure, other than a building, constructed of a type included in Chapter 15 and within the limits of Section 15.1.1. (applicability).
11.1.3 Applicability. Structures and their nonstructural components shall be designed and constructed in accordance with the requirement of the following chapters based on the type of structure or component:
a. Buildings: Chapter 12;
b. Nonbuilding Structures: Chapter 15;
Buildings whose purpose is to enclose equipment or machinery and whose occupants are engaged in maintenance or monitoring of that equipment, machinery, or their associated processes shall be permitted to be classified as nonbuilding structures designed and detailed in accordance with Section 15.5 of this standard.
Same as Ch 12, Buildings
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Pipe Rack – Power Plant
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Prescriptive requirements for High R Systems
• Brace Slenderness
• Develop full strength of brace
• Strong Column – Weak Beam
• AISC 358 Prequalified
• Weak axis bending, tube columns, discontinuous systems
• Protected Zones BF, MF
• Field Welding, CJP welds
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Pipe Rack – Power Plant
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Chimney and Stacks – R = 2…ASCE 7 15.6.2, ACI 307
• Low ductility• Industry Standard
Nonbuilding not similar to buildings
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Bin on braced legs R=3
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Bin on Unbraced Legs R = 2
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R – Tanks, SCR, HRSG, Soundwalls
Tanks on Grade, R = 3HRSG R = 8, 3.5, 3, 1.0Cantilevered fence, Wall >6’ R = 1.25
Negatives:• Need to substantiate R values for all types of structures• Ignores the most basic tenets of seismic performance: ductility,
continuity and capacity based design (acknowledging overstrength).
• Our codes and standards have never allowed unrestricted trade-off between strength and inelastic deformabilty or ductility in seismic design, which starts with what is now SDC B. There has always been a minimum set of detailing requirements specified, dependent solely on the SDC.
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SIMPLIFICATION OF SEISMIC CODE
PROVISIONS
A WHITE PAPER Prepared under the BSSC Simplified Seismic Design Procedures
Development Program
William T. Holmes
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Reduce Material Detailing Requirements (to Achieve Ductility) with Use of Lower R Factors
The detailing requirements for systems with high R factors generally involve design checks to avoid
brittle limit states (e.g., tensile fracture of structural steel or compressive crushing of concrete) and to
avoid focusing the inelastic demand in a small portion of the system (e.g., the “strong-column, weak-
beam” rule for special moment frames). Many of these detailing rules are rooted in the concept of
“capacity design,” in which the structure is constrained by design to perform in certain desirable
manners. These detailing requirements can be very time consuming in engineering practice.
The NEHRP Provisions and ASCE/SEI 7-10 are written to exclude the most brittle systems, generally
those with the lower R factors, from use on the higher seismic design categories, generally in locations
with the potential for very large seismic ground motions. In some cases, the restrictions apply primarily
to tall structures whereas in others, the restrictions apply to all heights. Suggestions to relax these
restrictions by requiring higher loading (smaller R factors) have been made in the past. Since the design
procedures include a reduction in the MCE ground motion as a part of the basic equation for an
equivalent design force, even an R of 1.0 implies acceptable structural performance beyond the design
loading assumptions. Some individuals cognizant of this fact suggest a value of R equal to 2/3 as a safe
alternative. Given that the ground motions to be considered exhibit a significant variability in key
parameters and that the capacity of a structure is not known with certainty, a probabilistic approach is
probably needed when considering these marginal cases. The methodology of FEMA P-695 would
provide guidance in this regard, but the amount of work required to perform these analyses
systematically for a wide category of structural systems would be overwhelming. Therefore, this option
was not selected for consideration in this study.
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R = 1 Status:
• Limited Scope Proposal nearly passed ASCE 7-16• SDC B, C; Regular, 3 story max
• The proposal is currently shelved• Could be used as a design guide – meets Code• Grass roots demand could bring it forward
• Needs justification of R values
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Conclusions:
• R = 3, 1.5 and 1 are allowed in certain areas and with heavy restrictions.
• SDC B, C => R = 3 for steel is the norm.
• Nonbuilding Industrial structures benefit from R = 1 MF, 1.5 BF.
• Use judgement for each configuration and proportioning; and consider vulnerability to an overload.• Collapse, redundancy, robustness/utilization
Introduction to Seismic Design Low Seismic
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byThomas F. Heausler, PE, SEStructural Engineer
Agenda
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• Complete seismic load analysis of a simple building – Small Building
• Low Seismic Risk - Not optimized for Seismic
• Calculations as Flowchart
• Present the Underlying Theory
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Given: ASCE 7-10, AISC 340, ACI 318 [Note: ASCE 7-10 maps and Provisions are used, Accidental Torsion of ASCE 7-16 is described.]Location: Hardeeville, South CarolinaUse: Storage and office without partitions. See Section 4.3.2 and 12.7.2(2) for partition load requirements), Risk Category IIRoof dead load = 70 psfRoof Live Load = 20 psf, Ground and minimum Roof Snow Load = 20 psfSeismic Force Resisting System: Steel Braced FrameSoil Allowable Net Bearing Pressure = 2,000 psfMaterials: Concrete f’c = 4,000psi; Steel Shapes and Plates Grade 50; Welding E70 electrode, Bolts A325N, Snug tight.
Figure 1 Simple Building Perspective
Calculations:
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• W10 cols with double clip angles
– prying action
Seismic Design :Determine SS and S1:Site Class D default as per 11.4.2From ASCE Hazards Tool or USGS Website SS = 0.31, S1 = 0.130, SDS = 0.356, SD1 = 0.198 Verify that exemptions do not apply (11.1.2) and (11.4.1).
Exceptions: Low seismic , one and two family dwellingsSeismic Design Category A
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Calculations:
Seismic Design Category A
• [11.4.1] [11.7] [1.4]• Don’t Use Chapter 12• [1.4] General Structural Integrity • 1% W, 5% beam connections, 20% wall
connections• Non-Structural Components Exempt
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F = ma
Base Shear - V
F = maSDS, SD1 ?
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Ground Motion
SMS = FaSS (11.4-1)SM1 = FvS1 (11.4-2)
SDS = 2/3 SMS (11.4-3)
SD1= 2/3 SM1 (11.4-4)
SDS = 2/3 FaSS
SD1 = 2/3 FvS1
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F = ma• Rock accelerates during an earthquake
• Soft soil above rock amplifies the acceleration
• Building amplifies that acceleration from its base to roof
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Ground Acceleration
Ground Acceleration measured during an earthquake
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Site Class, Fa, Fv
• Rock displaces during earthquake
• A peak acceleration may be measured
• Soft soil above rock amplifies acceleration – Fa, Fv
• Resulting in the ground motion at your site
- at base of building
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Amplification Due to Soil
SMS = FaSS (11.4-1)
SM1 = FvS1 (11.4-2)
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Amplified Forces - Clay and soft soil
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Amplified Forces - Clay and soft soil
Like runny whip cream on jelloon ceramic plate
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Liquefaction
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Building’s Response
to
Acceleration at base.
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Period of Vibration T
Ruler with Erasers
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Period T
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Period of Vibration T
Swing setPartitions dampen
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Building Response to Ground Motion
• Ground motion is amplified through the structure
• This is called “Response”
• Up to 2.5 times acceleration at ground
• Response is a function of mass and stiffness => Period, T
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Period T
How to calculate T
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Rayleigh Equation
Or Software Analysis – T…with limits
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e.g. Braced Frame, Wood Shearwall, etc.
Period T
Moment Frames
1 story T = 0.1 second5 story T = 0.5 seconds20 story T = 2 seconds
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22 Story Building – T=2 seconds
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Response of Building – ASCE 7 Maps
• 0.2 second Map
• 1.0 second Map
• Calibrate a Response Spectrum for your site
• Apply Period T to determine Response acceleration
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Map: ASCE 7-10 Chapter 22
Response Accelerations
San Francisco, CA
Ss = 1.50S1 = 0.67
PGA = 0.6
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Black line in margin indicates change from previous edition.
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SDS = 0.355SD1 = 0.198
PGA = 0.19
San Francisco, CASDS = 1.00SD1 = 0.67
PGA = 0.4
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Site CoefficientsFa
Fv
Seismic Design Category “C”
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New ASCE 7 Hazard ToolASCE 7-10, ASCE 7-16
Spectral Response Acceleration
SDS = 2/3 SMS (11.4-3)
SD1 = 2/3 SM1 (11.4-4)
2/3?
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2/3
• “2/3”
converts from
• Maximum Considered Earthquake (Collapse Prevention)
To
• Design Earthquake (Life Safety)
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Gro
un
d M
oti
on
The Big One
Minor EQ
Expected Performance
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Ground Motion
SMS = FaSS (11.4-1)SM1 = FvS1 (11.4-2)
SDS = 2/3 SMS (11.4-3)
SD1= 2/3 SM1 (11.4-4)
SDS = 2/3 FaSS
SD1 = 2/3 FvS1
SDS and SM1 are now calibrated for site Now use Period T, to determine Base Shear for our Building
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Base Shear - V
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Figure 11.4-1 Design Response Spectrum
SDS = 1.0
PGA = 0.4
San Francisco
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What is a Response Spectrum?
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Figure 11.4-1 Design Response Spectrum
1-Story 5-Story 10-Story 20-StorySolidBlock of Concrete
80-Story
PGA = 0.4
SDS = 1.0
San Francisco
• Spectrum• Response• Design Level
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Figure 11.4-1 Design Response Spectrum
SDS = 1.0
PGA = 0.4
San Francisco
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Create a Response Spectrum
Ground Acceleration measured during an earthquake
Sometime during the duration, a building will have a maximum response
could be here
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Creating a Response Spectrum
• One SDOF system is subjected to time history of earthquake ground acceleration record.
• The SDOF model responds to ground acceleration.
• Sometime during the earthquake a peak acceleration response is measured for that SDOF
• Not necessarily at same time as peak ground acceleration
• Analysis is repeated for various different SDOF models completing a spectrum of results
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Response Spectrum
Period
(seconds)1 Story CMU
1 Story Moment Frame
2 Story Building20 story Building
10 Story Building
Res
po
nse
Acc
eler
atio
n
Resonance to Rhythmic Forcing Function
SDS
SD1/T
Earthquake 1
Earthquake 2
Earthquake 3
Ruler with Erasers:• Short Period• Resonance• Long Period
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Response Spectrum
Calibrate this graph to your specific site
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F = ma
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Base Shear - V
We now know SDS and SD1
and T.
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Exception!
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Base Shear - V
What is R and Ie ?
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R – Response Modification Factor
Now we know the building’s response due to:
• Acceleration of Rock at our site
• Soil amplification
• Building Period, T (i.e. m,k) – Amplification/Response
• Now we modify it further – Ductility - R
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Table 12.2-1SCBF R = 6, Omega Ωo = 2, Cd = 5SMF R = 8, Omega Ωo = 3, Cd = 5.5Steel Systems Not Specifically Detailed for Seismic, R = 3, Omega Ωo = 3, Cd = 3
Each R value has strings attached.
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R, Omega Ωo, Cd
• Not like Wind
• “R” is measure of effective ductility of system
• Ductility is range between yield and fracture
• R is composed of two components: • Omega Ωo : Overstrength
• Rd: inelastic behavior/energy dissipation
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Wind
Seismic
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R, Omega Ωo, Cd
• Overstrength Omega Ωo : • Material overstrength
• Phi
• Conservative overdesign, min ratios, drift driven
•Rd, Inelastic Behavior: • Level of inelastic response capability
• Bend not break
• Period lengthens, energy dissipation/damping
• Observed System Performance
• Possibility of vertical load system failure
• Redundancy (rho) and Backup frames
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R, Omega Ωo, Cd
• See NEHRP FEMA 450-2/2003 pages 36 – 41
• Special detailing required to insure inelastic performance
• Omega is overstrength factor (e.g. collectors, connections, columns)
• Cd converts elastic analysis deflection to inelastic/actual
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Overstrength
Ductility
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Further Modifiers
Ieoffice
SDCC
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Further Modifiers
Ie
SDCC
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Importance Factor
• [11.5.1] [Table 1.5-2] [Table 1.5-1] and
• [IBC Table 1604.5] • Risk Category • Hazard, Essential, • e.g. 300 people, storage Ie= 1.0, 1.25, 1.5 Ip = 1.0, 1.5 [13.1.3] Life Safety,
