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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

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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+

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Checklist Format

• Checklist for your self‐knowledge and perproject.

• 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

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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,

Essential, Hazardous

Continuous Load Path

• [12.1.3]• Strength and in proportion to stiffness• In addition to all other specific

provisions

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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

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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

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Modal Analysis Triggers

• [12.3] [Table 12.3‐1 HorizontalIrregularities] [Table 12.3‐2 VerticalIrregularities]

• Tables reference Sections• ASCE Guide Seismic Loads…by Finley

Charney

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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.

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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 lieuof [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

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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

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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

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Triangular Distribution over Height

Exponent k =1 = triangleExponent k=2.0 accounts for higher mode effects

Force Distributed over Height

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Triangular Force Distributionover Height

Distribution of Base Shear over Height

• [12.8.3] [Eqn 12.8‐12]• F=Cvx V• Cvx = wx hx

k /Sum wi hik

• Note: Cvx V, not W

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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

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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

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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 upperfloors

• Diaphragms and non‐structural elements maybe locally subjected to higher accelerationsthan the triangular distribution of first mode.

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Diaphragms

Not simultaneous, not additive

Fp ‐ Nonstructural Components

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Amplification – high ap for parapet

Nonstructural Component Forces

• Masses further away from ground experiencehigher accelerations

• Higher mode effects cause higheraccelerations than first mode effects at lowerfloors

• Forces may be 1.5 to 2.5 times higher at roofthan at grade

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Fp Non‐structural Components Chapter 13

1. Run V load combinations2. Run Fp load combination to foundation

• [15.3 Non‐building Structure]• 25% weight rule – Combine stiffness of

equipment in model

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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

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Reduction of Foundation Overturning

• [12.13.4]• At Soil‐Foundation Interface• Reduce by .75 factor

Errata’s

• [ASCE 7 website]• [IBC website]

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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

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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