the most common errors in seismic design …and how to
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The Most Common Errors in Seismic Design
…And How to Properly Avoid Them
By Thomas F. Heausler, PE, SEStructural EngineerKansas City
Thomas F. Heausler, PE, SEExpert in Errrors
• Tulane• San Francisco• Midwest
• East of the Rockies Representation• ASCE 7• NCSEA SCAC
Challenges
• Interpretation of Seismic Code • Standard of Care across Multiple States• Inexperience of Engineers for Seismic
• ERRRORS
Background of Audience
• Users of ASCE 7 – Seismic West Coast East of the Rockies
20,000+
Checklist Format
• Checklist for your self‐knowledge and per project.
• Basis is ASCE 7‐10, IBC 2012• [ASCE 7 Section Number] in brackets
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
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,
Essential, Hazardous
Continuous Load Path
• [12.1.3]• Strength and in proportion to stiffness• In addition to all other specific
provisions
R Factor
• [Table 12.2‐1 and 15.4‐1, 2]• Strings attached!
• R > 3• Cd, Omega o, Detailing Provisions • AISC 341, ACI Chap 21, etc.
provisions triggered• R = 3: AISC 360 allowed
Table 12.2‐1SCBF R = 6, Omega = 2, Cd = 5, AISC 341 detailingSMF R = 8, Omega = 3, Cd = 5.5, AISC 341 Detailing
R = 1 (?)
• Like Wind• Used by Nuclear and Military Essential• ASCE 7 Proposal• Limitations• 5 pages instead of 70 pages
Omega o ‐ Ωo
• [Table 12.2‐1]• Minus 0.5 for flexible diaphragms• Footnote g
Modal Analysis Triggers
• [12.3] [Table 12.3‐1 Horizontal Irregularities] [Table 12.3‐2 Vertical Irregularities]
• Tables reference Sections • ASCE Guide Seismic Loads…by Finley
Charney
Omega o Triggers
• [12.4 Load Combinations with Omega zero] • [12.2.5.2 Cantilever Columns] SDC B,C,D,E,F• [12.10.2.1 Collectors – Light Frame, Wood
excepted] SDC C,D,E,F• [12.3.3.3 Columns, Beams Supporting
Discontinuous Walls] SDC B,C,D,E,F• [12.13.6.5 Pile Anchorage] SDC D,E,F• [AISC where R>3, ACI Chapter 21, Appendix D,
Etc.] SDC B,C,D,E,F
Redundancy ‐ Rho• [12.3.4] • Rho = 1.0 or 1.3• Rho = 1.0 for:
SDC B, C, Drift,Fp (non‐structural Components),Collectors, Omega Zero Load Combinations, Diaphragms.
Vertical Seismic Load Effect ‐ Ev
• [12.4.2.2] • Ev = 0.2 Sds• Applied as Dead Load Factor adjustment• x 0.7 for ASD• No Ie, Ip• No Rho• Applies to Fp calcs also!
Load Combinationsand
Allowable Stress Design – 0.7 E
• [12.4.2.3] Section 12.4.2 shall be used in lieu of [2.3.2] and [2.4.1]
• For ASD use 0.7 E• For LRFD use 1.0 E• 0.7 E applies to Fp also
Orthogonal Effects
• [12.5] • SDC C for irregular• SDC D,E,F for corner columns• IEEE 693 Equipment applies Orthogonal
Effects to all Conditions, Corner anchor bolts.
Effective Seismic Weight ‐W
• [12.7.2] • No Live Load except:o 25% of Storageo Partitions 10 psf [4.3.2]o Industrial Operating Weight ‐ Unbalancedo 20% of snow > 30psfo Roof Gardens
Period T
• [12.8.2.1]• Ok to use T = Ta• Except approximate formulas shall not be
used for Non‐building (industrial) Structures [15.4.4]
Distribute Base Shear over Height• [12.8.3] • Triangular distribution (First Mode
Effect – Conservative envelope)• Including a Cantilevered Stack,
Fence/wall• Centroid of seismic lateral load at 2/3
height
Triangular Distribution over Height
Exponent k =1 = triangleExponent k=2.0 accounts for higher mode effects
Force Distributed over Height
Triangular Force Distributionover Height
Distribution of Base Shear over Height
• [12.8.3] [Eqn 12.8‐12]• F=Cvx V• Cvx = wx hxk /Sum wi hik• Note: Cvx V, not W
Modal Response Spectrum Analysis
• [12.9]• Purpose => More accurately:1. Distributes Base Shear over height2. Horizontal Torsional Effects3. Higher Mode Effects
Modal Analysis• [12.9.4.1]• Not intended to change Base Shear• Scale to V, 85% V• Should not be far off of V• Scale by R, I, g(gravity conversion for
mass)• Check with miniature/simple model
Accidental Torsion
• [12.8.4.2]• In addition to Inherent Torsion• Non‐Building Structures also• Amplify if triggered [12.8.4.3]
Drift Check
• [12.12] [12.8.6 Drift Determination] [Table 12.12‐1 Allowable Values]
• Multiply by Cd• Divide by Ie – compare to allowable
(which has Ie embedded)• No 0.7 even if using ASD
Diaphragm Forces
• [12.10.1.1]• Fpx, minimums govern at lower stories• Locally higher due to higher mode
effects
Higher Mode effects• Multi‐story buildings are MDOF• Lower floors may be out of Phase with upper floors
• Diaphragms and non‐structural elements may be locally subjected to higher accelerations than the triangular distribution of first mode.
Diaphragms
Not simultaneous, not additive
Fp ‐ Nonstructural Components
Amplification – high ap for parapet
Nonstructural Component Forces• Masses further away from ground experience higher accelerations
• Higher mode effects cause higher accelerations than first mode effects at lower floors
• Forces may be 1.5 to 2.5 times higher at roof than at grade
Fp Non‐structural Components Chapter 13
1. Run V load combinations 2. Run Fp load combination to foundation
• [15.3 Non‐building Structure] • 25% weight rule – Combine stiffness of
equipment in model
Wall Design
• Concrete, CMU• [12.11.1 Wall panel]• [12.11.2.1 Wall connectors]
Foundation Ties
• [12.13.5.2] [12.13.6.2]• Pile Caps SDC C,D,E,F• Spread Footings SDC E, F
Reduction of Foundation Overturning
• [12.13.4]• At Soil‐Foundation Interface• Reduce by .75 factor
Errata’s
• [ASCE 7 website]• [IBC website]
IBC Override’s
• [IBC 1613] • [1613.5 Amendments to ASCE 7] • [IBC Materials ‐ Chapters 18 through 23]
ASCE 7‐10 THIRD PRINTING
• 126 page commentary• 3/8” thick
The Most Common Errors in Seismic Design
…And How to Properly Avoid Them
By Thomas F. Heausler, PE, SEStructural EngineerKansas City