the most common errors in seismic design …and how to

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