Most Common Errors in Seismic Design under ASCE 7-10 and IBC 2012

Transcription

1 Most Common Errors in Seismic Design under ASCE 7-10 and IBC 2012 Robert B. Paullus, Jr., P.E., S.E. Paullus Structural Consultants

2 Acknowledgements Based Heavily on: The Most Common Errors in Seismic Design'And How to Properly Avoid Them NCSEA 2014 Annual Conference Proceedings By Thomas F. Heausler, PE, SE Structural Engineer Kansas City, MO 2015 OSEA Spring Conference ASCE 7-10 Seismic - Common Errors 2

3 Preface Listing of common errors was developed by Tom Heausler and Bob Paullus during review of other engineers work Based On Experience Should not be misconstrued as a consensus document of the ASCE 7 Seismic Subcommittee May be used as a checklist for Design or Review 2015 OSEA Spring Conference ASCE 7-10 Seismic - Common Errors 3

4 Standards Covered ASCE 7-10 IBC 2012 AISC ASIC ACI (including Chapter 21 and Appendix D) Other Standards 2015 OSEA Spring Conference ASCE 7-10 Seismic - Common Errors 4

5 Favorite Top Ten Errors No. 10: Neglecting Additional Requirements for Chords, Collectors, & Diaphragms No. 9: Improper Distribution Base Shear over Height No. 8: Failure to multiply E by 0.7 for ASD design No. 7: Neglect of combining orthogonal Effects No. 6: R-Value - Improper Seismic System 2015 OSEA Spring Conference ASCE 7-10 Seismic - Common Errors 5

6 Favorite Top Ten Errors No. 5: Modal Response Spectrum Analysis will Capture All Irregularities and be Conservative No. 4: Neglecting Vertical Seismic Load Effect - E v Particularly Critical on Cantilevered Elements No. 3: No or Inadequate Redundancy in System No. 2: No Continuous Load Path No. 1: I Didn t know Y all Had Earthquakes Around Here! 2015 OSEA Spring Conference ASCE 7-10 Seismic - Common Errors 6

7 Seismic Design Category (SDC) A ASCE 7-10 Section Mapped Acceleration Parameters ASCE 7-10 Section SEISMIC DESIGN CATEGORY ASCE 7-10 Section DESIGN REQUIREMENTS FOR SEISMIC DESIGN CATEGORY A Send to Section 1.4 -General Structural Integrity Freeboard Requirements for RC IV Tanks: Section OSEA Spring Conference ASCE 7-10 Seismic - Common Errors 7

8 Seismic Design Category (SDC) A ASCE 7-10 Section 1.4 -General Structural Integrity Should Not Use Any Of The Provisions Of Chapter 12 Instead, Use The General Structural Integrity Provisions Of Section 1.4 General Structural Integrity Provisions Have Some Loads Which May Often Be Erroneously Unaccounted For: The forces required include 1% dead load, 5% of dead plus live load for beam connections, and 20% of wall weight for wall connections Non-Structural Components in Seismic Design Category A are exempt from Seismic Design requirements, as stated in Section OSEA Spring Conference ASCE 7-10 Seismic - Common Errors 8

9 Importance Factor ASCE 7-10 Section ,Table 1.5-2, Table IBC 2012 Table Importance Factor Is Based Upon Risk Category And The Associated Life Safety, Hazard And Essential Nature Of The Structure 2015 OSEA Spring Conference ASCE 7-10 Seismic - Common Errors 9

10 Importance Factor Check both ASCE 7 and IBC Tables A typical building can sometimes evolve into an I e equal to 1.25 or 1.5 when occupancy or use expands. Examples: even relatively small churches (occupancy greater than 300) a building where hazardous materials are stored For Building Design I e = 1.0, 1.25, OSEA Spring Conference ASCE 7-10 Seismic - Common Errors 10

11 Importance Factor Non-Structural Components I p = 1.0, or 1.5 only ASCE 7-10 Section I p may not equal I e, and in some instances I p may be less than I e OSEA Spring Conference ASCE 7-10 Seismic - Common Errors 11

12 Continuous Load Path ASCE 7-10 Section Very Specific Provisions for Many Elements Collectors Connections Diaphragms Walls Others 2015 OSEA Spring Conference ASCE 7-10 Seismic - Common Errors 12

13 Continuous Load Path ASCE 7-10 Section In Addition To The Specifics, The Engineer Is Required To Provide A Continuous Load Path For All Inertial Forces From Their Origin To The Foundation Such Load Path Shall Conform To The Relative Stiffness And Strength Of The Elements Which Exist In The Structure Deformational Compatibility 2015 OSEA Spring Conference ASCE 7-10 Seismic - Common Errors 13

14 Response Modification Factor: R ASCE 7-10 Table and , 2 The Response Modification Coefficient, R, Is Part Of A Concept Where An Elastic Design May Be Performed But With Due Consideration Of The Overstrength And Ductility Inherent In The Lateral Force Resisting System OSEA Spring Conference ASCE 7-10 Seismic - Common Errors 14

15 Response Modification Factor: R To Assure Reliability In The Overstrength And Ductility, Many Requirements Are Triggered With Each R Factor R Tables List The Detailing Requirements Triggered The Strings Attached To AISC R>3 Structures And ACI Chapter 21 Structures Can Be Extremely Significant 2015 OSEA Spring Conference ASCE 7-10 Seismic - Common Errors 15

16 Overstrength Factor: Ω 0 ASCE 7-10 Table Also in ASCE 7-10 Tables , and When Diaphragms Are Flexible Allowed To Reduce Ω 0 Factor By 0.5 This Provision Is Listed In Small Print As Footnote g And Is Often Overlooked 2015 OSEA Spring Conference ASCE 7-10 Seismic - Common Errors 16

17 Modal Response Spectrum Analysis Triggers ASCE 7-10 Section 12.3 Table Horizontal Irregularities Table Vertical Irregularities Tables Describe Various Irregularities and Trigger Specific Provisions One Such Provision Is Elimination Of The Option Of The Equivalent Lateral Force Procedure ASCE 7-10 Section 12.8 and the Need To Perform a Modal Dynamic Response Spectrum Analysis 2015 OSEA Spring Conference ASCE 7-10 Seismic - Common Errors 17

18 Modal Response Spectrum Analysis Triggers ASCE 7-10 Section 12.3 Table and Table Tables Require Close Review To Interpret The Triggers. The ASCE Design Guide Seismic Loads: Guide To The Seismic Provisions Of ASCE 7-10 By Finley Charney Has A Listing Of The Triggers In A More User Friendly Format 2015 OSEA Spring Conference ASCE 7-10 Seismic - Common Errors 18

19 Overstrength Factor, Ω 0,Triggers ASCE 7-10 Section Load Combinations with Ω 0 ASCE 7-10 Section Cantilever Columns ASCE 7-10 Section Collectors Light Frame, Wood Excepted ASCE 7-10 Section Columns, Beams Supporting Discontinuous Walls ASCE 7-10 Section Pile Anchorage AISC where R>3 ACI Chapter 21, Appendix D, etc OSEA Spring Conference ASCE 7-10 Seismic - Common Errors 19

20 Overstrength Factor, Ω 0,Triggers Ω 0 Is An Amplification To The Forces In Certain Elements In The Seismic Load Path Required To Prevent A Weak Link Form Occurring Prior To The Full Energy Dissipation And Ductility Potential Of The Primary Feature Of The Lateral System 2015 OSEA Spring Conference ASCE 7-10 Seismic - Common Errors 20

21 Overstrength Factor, Ω 0,Triggers Ω 0 Is An Amplification To The Forces For Example: In a steel braced frame system, in order for the diagonal brace to yield and dissipate energy in a controlled and reliable manner, all other portions of the load path (E.G. Connections, bolts, welds, gusset plates, anchor bolts, columns and collectors) need to be stronger than the maximum anticipated strength, or force, in the brace. Therefore, omega zero amplification and load combinations are specifically triggered in the sections mentioned above and in material standards such as AISC and ACI OSEA Spring Conference ASCE 7-10 Seismic - Common Errors 21

22 Redundancy Rho (ρ) Rho Is A Factor That Penalizes Structures That Do Not Have Redundancy ASCE 7-10 Section Rho = 1.0 or 1.3 Rho = 1.0 for: SDC B, C, for Drift Calculations, F p (Non-structural Components) Forces, Collectors, Ω 0 Load Combinations, and Diaphragms 2015 OSEA Spring Conference ASCE 7-10 Seismic - Common Errors 22

23 Vertical Seismic Load Effect - E v ASCE 7-10 Section Requires That A Vertical Load Effect Equal To 0.2 S DS Be Applied To Dead Load It Is Applied As A Dead Load Factor Adjustment And May Act Downward Or Upward It Is At Strength Design Level So It May Be Multiplied By 0.7 For Allowable Stress Design (ASD). No I e, I p, or Rho Is Applied To E v 2015 OSEA Spring Conference ASCE 7-10 Seismic - Common Errors 23

24 Load Combinations and Allowable Stress Design 0.7 E For Allowable Stress Design (ASD) Load Combinations ASCE 7-10 Section Section shall be used in lieu of Sections and Earthquake Forces Are At Strength Level So For The ASD Combinations, Use 0.7 E (For LRFD Use 1.0 E) The 0.7 E Applies To F p (Non-structural Component Forces) Also 2015 OSEA Spring Conference ASCE 7-10 Seismic - Common Errors 24

25 Orthogonal Effects Earthquake Forces Shall Be Calculated For Each Of The Two Primary Orthogonal Directions To Consider The Effects Of Earthquake Forces At Some Angle Other Than Those Two Directions, Orthogonal Effects Must Be Considered 2015 OSEA Spring Conference ASCE 7-10 Seismic - Common Errors 25

26 Orthogonal Effects ASCE 7-10 Section 12.5 Requires That Irregular Buildings In SDC C, And Corner Columns In SDC D,E,F Be Considered With 100% Of Forces In One Directions And 30% In Other It Should Be Noted That IEEE 693 (Electrical Equipment) Applies Orthogonal Effects To All Conditions, Including Corner Anchor Bolts 2015 OSEA Spring Conference ASCE 7-10 Seismic - Common Errors 26

27 Effective Seismic Weight ASCE 7-10 Section defines the Effective Seismic Weight, W. Except as mentioned below, live load is not included in the inertial force, however the seismic force is later combined with dead and live loads in the load combinations ASCE 7-10 Section stipulates that W shall include the following masses: 25% of Storage live load, Partitions 10 psf 2015 OSEA Spring Conference ASCE 7-10 Seismic - Common Errors 27

28 Effective Seismic Weight ASCE 7-10 Section Industrial Operating Weight (and unbalanced conditions), 20% of snow > 30psf, and Roof Gardens OSEA Spring Conference ASCE 7-10 Seismic - Common Errors 28

29 Period T ASCE 7-10 Section has complex methods and limits on calculating the Period T. It Is Acceptable To Use T = Ta, For Simplicity Approximate Formulas Shall Not Be Used For Non-building (Industrial) Structures, ASCE 7-10 Section OSEA Spring Conference ASCE 7-10 Seismic - Common Errors 29

30 Distribute Base Shear over Height ASCE 7-10 Section Once The Base Shear, V, Is Calculated, It Must Be Distributed Over The Height Of The Structure For A One Story Building All Of The Base Shear Would Be Applied At The Roof For Multi-story Structures: The Base Shear Must Be Proportioned To Each Floor, Not Only In Proportion To Each Floors Mass, But Also In Proportion To The Distance Of` The Floor From The Base. A Triangular Distribution Of Force Results For Regular Multi-story Buildings OSEA Spring Conference ASCE 7-10 Seismic - Common Errors 30

31 Distribute Base Shear over Height ASCE 7-10 Section Once The Base Shear, V, Is Calculated, It Must Be Distributed Over The Height Of The Structure For Distributed Mass Structures Like Stacks And CMU Fences, The Centroid Of The Load Should Be Applied At 2/3 Of The Height Above The Base, Not 1/2 Height Or Center Of Gravity 2015 OSEA Spring Conference ASCE 7-10 Seismic - Common Errors 31

32 Distribute Base Shear Over Height Formula ASCE 7-10 Section (Eq ) F x = C vx V C vx = w x h xk /Sum w i *h ik Spreadsheets are often used to calculate the forces at each floor level A common mistake is to have errors within the spreadsheet logic and/or calculate C vx and then multiply it by W (total Weight) or w x weight at each floor. C vx should be multiplied by the Base Shear V, and the sum of all the F x should equal 2015 OSEA Spring Conference ASCE 7-10 Seismic - Common Errors 32

33 Modal Response Spectrum Analysis ASCE 7-10 Section 12.9 The purpose of a Modal Response Spectrum Analysis is NOT to refine the magnitude of the Base Shear Purpose IS to more accurately perform the following: Distributes Base Shear Over Height Horizontal Torsional Effects Higher Mode Effects 2015 OSEA Spring Conference ASCE 7-10 Seismic - Common Errors 33

34 Modal Analysis ASCE 7-10 Section Scaling Of The Results Of Modal Response Spectrum Analysis Is Required And Allowed. Especially For Buildings, The Results Of The Response Spectrum Should Be Very Similar To The Base Shear When Scaling Results, One Should Verify That R, I, g (Gravity Conversion For Mass) Factors Are Included 2015 OSEA Spring Conference ASCE 7-10 Seismic - Common Errors 34

35 Modal Analysis ASCE 7-10 Section : Results must be scaled to within 85% of V calculated by ELF Method It is often wise to check your software results with a small regular structural model where results are predictable with hand calculations Confidence is gained to use the software on a large complex structural models OSEA Spring Conference ASCE 7-10 Seismic - Common Errors 35

36 Accidental Torsion ASCE 7-10 Section Accidental Torsion Must Be Applied In Addition To Inherent Torsion Required This Is To Prevent Weak Torsional Resisting Arrangements As Well As Account For Unexpected Distribution Of Live Load And Unexpected Stiffness Of Structural And Non-structural Elements Provision Applies To Non-building Structures As Well As Buildings 2015 OSEA Spring Conference ASCE 7-10 Seismic - Common Errors 36

37 Accidental Torsion ASCE 7-10 Section For Torsionally Irregular Buildings, Amplification Of The Accidental Torsion May Be Required ASCE 7-10 Section See Figure Torsional Amplification Factor, A x Very Small Drifts Should Not Trigger a Torsional Irregularity 2015 OSEA Spring Conference ASCE 7-10 Seismic - Common Errors 37

38 Accidental Torsion ASCE 7-10 Section ASCE 7-10 Section When Checking for Type 1a or 1b Torsional Irregularity Use Table to Determine a The Following Recommendation Is Not Currently in ASCE 7 Recommendation From Presenter, Only IFThe Story Drifts At All Locations On A Level Are Less Than 0.15 a to 0.20 a, Do Not Consider A Torsional Irregularity 2015 OSEA Spring Conference ASCE 7-10 Seismic - Common Errors 38

39 Accidental Torsion Example: Torsion w/ Very Small Drift Assume Story Height = 10 ft. (120 inches) Assume a =0.007h sx (Worst Case In Table ) 0.2*0.007(120 ) = (120 ) = If the Maximum Story Drift (δ x max )at Any Point On The Story Being Considered Is Less Than : There Really Isn t A Torsional Irregularity Issue Certain West Coast Building Officials May Argue This Point, Since It Is Not In ASCE 7 Don t Waste Time With Values to Small to Be Significant Remember, δ= C d *(Calculated Story Drift from Point Considered) I e (δ x = C d δ ex / I e ) 2015 OSEA Spring Conference ASCE 7-10 Seismic - Common Errors 39

40 Drift Check ASCE 7-10 Section Section Drift Determination ASCE 7-10 Table Allowable Drift Limits Results from the elastic analysis must be amplified by C d to render actual expected deflections. C d is a very large value, typically a factor of about 3 to OSEA Spring Conference ASCE 7-10 Seismic - Common Errors 40

41 Drift Check ASCE 7-10 Table Allowable Drift Limits The drift is then divided by I e, because the allowable drifts are organized into a table which considers Risk Category Be careful when using Allowable Stress Design load combinations DO NOT apply the 0.7E to drift calculations 2015 OSEA Spring Conference ASCE 7-10 Seismic - Common Errors 41

42 Diaphragm Forces ASCE 7-10 Section Forces At Lower Floor Diaphragms May Be Higher Than Those Used For The Lateral Force Resisting System Due to Diaphragms Transferring Loads from One Vertical Lateral Resisting to Another May Also Be Due to Localized Diaphragm Discontinuities, Such As Holes in Floors 2015 OSEA Spring Conference ASCE 7-10 Seismic - Common Errors 42

43 Diaphragm Forces ASCE 7-10 Section (Eq ) F x =C vx V This Is Due To Higher Mode Effects (I.E. Modes Higher Than The First Mode) Where The Lower Floors May Be Accelerating Higher Than Calculated In (Eq ) The Diaphragm Force Equation ASCE 7-10 Section (Eq ) Is Similar To (Eq ), However F px Minimums Of (Eq ) Often Govern For The Lower Floors 2015 OSEA Spring Conference ASCE 7-10 Seismic - Common Errors 43

44 F p Non-structural Components Chapter 13 ASCE 7-10 (Eq ) Non-structural components may also experience higher local accelerations due to higher mode effects as well as amplification of the force within the nonstructural element itself Industrial structures often feature very large F p forces 2015 OSEA Spring Conference ASCE 7-10 Seismic - Common Errors 44

45 F p Non-structural Components Chapter 13 ASCE 7-10 (Eq ) It Is Unlikely That The F p Forces On Two Different Floors Would Occur At The Same Point In Time One Method Of Accounting For The F p Forces In A Computer Model Is To Run Two Conditions 2015 OSEA Spring Conference ASCE 7-10 Seismic - Common Errors 45

46 F p Non-structural Components Chapter 13 To Run Two Conditions: Run V Load Combinations With The Weight Of The Equipment Included In The Seismic Weight Of The Floor And Base Shear Distributed Over The Height Per (Eq ) Run F p On Piece Of Equipment Only To Verify An Adequate Load Path To The Vertical System And/Or Foundation 2015 OSEA Spring Conference ASCE 7-10 Seismic - Common Errors 46

47 F p Non-structural Components Chapter 13 ASCE 7-10 Section Non-Building Structures Supported By Other Structures When the Weight of the Supported Non- Building Structure is Less than 25% of the Total Structure Mass Use Chapter 13 When Non-structural Components Get Very Large, I.E. 25% or More Of Total Structure Mass Use Section OSEA Spring Conference ASCE 7-10 Seismic - Common Errors 47

48 Large F p Non-structural Components Chapter 15 When Non-structural Components Exceed 25% Of Total Structure Mass For These Heavy Components, A Computer Model Capturing The Stiffness And Design Coefficients Of Both The Component And The Primary Structure Must Be Considered Together Must Apply E v To F p Combinations Rho (Redundancy) = 1.0 Ω 0 Does Not Apply To F p Load Combinations Except, Ω 0 Does Apply In ACI Appendix D Calculations 2015 OSEA Spring Conference ASCE 7-10 Seismic - Common Errors 48

49 Wall Design Wall Panels Made Of Concrete And Concrete Masonry, CMU Have Performed Poorly In Past Earthquakes ASCE 7-10 Section Wall Panels ASCE 7-10 Section Wall Connections Should Be Implemented, As Well As ACI Appendix D For Anchorage 2015 OSEA Spring Conference ASCE 7-10 Seismic - Common Errors 49

50 Foundation Ties ACI Chapter 21: To Assure That The Foundation System Acts As An Integral Unit Prevents One Column Or Wall From Moving Appreciably with Respect to Another Applies To Pile Caps In SDC C, As Well As D,E,F, and Spread Footings for SDC E and F Can Possibly Be Integral With Slab For Lightly Load Buildings (Engineered Metal Buildings) 2015 OSEA Spring Conference ASCE 7-10 Seismic - Common Errors 50

51 Reduction of Foundation Overturning ASCE 7-10 Section Allows for a reduction of the bearing pressures at the Soil-Foundation Interface Forces may be reduced by 25% in recognition that the first mode triangular force distribution will likely not occur without higher mode effects occurring and negating the direction of the first mode Results in reduced overall overturning moments 2015 OSEA Spring Conference ASCE 7-10 Seismic - Common Errors 51

52 Errata ASCE 7 Website and IBC 2012 Website (ICC) have the latest errata. Significant Entries Due To Typographical Or Unintended Consequences Of Changes Are Corrected In The Errata 2015 OSEA Spring Conference ASCE 7-10 Seismic - Common Errors 52

53 IBC 2012 Overrides IBC 2012 Section 1613 IBC 2012 Section Amendments to ASCE 7-10 IBC 2012 Materials -Chapters 18 through 23 contain amendments to the ASCE 7-10 document 2015 OSEA Spring Conference ASCE 7-10 Seismic - Common Errors 53

54 IBC 2012 Overrides ASCE 7 is on a six year update cycle and IBC is on a three year Also technical changes to IBC often have to be approved well before the issue date Inevitably, Coordination Between The Two Suffers Material Standards Issues Must Be Coordinated, Often Through Errata, Supplements Or IBC Published Amendments. It Is Essential To Periodically Check For These Changes See Standards Such As: AISC, ACI, AISI, AWC, Etc OSEA Spring Conference ASCE 7-10 Seismic - Common Errors 54

55 ASCE 7-10 THIRD PRINTING It Is Recommended That The User Make Use Of The ASCE 7 Expanded Seismic Commentary 135 Pages Of Valuable Background Information Incorporated In The Third Printing Of ASCE 7 Only 2015 OSEA Spring Conference ASCE 7-10 Seismic - Common Errors 55

56 ASCE 7-10 THIRD PRINTING For Those That Own A First And Second Printing, You May Download A (_.pdf) File Of The Commentary For Free From The ASCE Website This Commentary Was Developed By The NEHRP/BSSC Provisions Update Committee and Describes The Reasons For The Individual Provisions Of The ASCE 7 Standard 2015 OSEA Spring Conference ASCE 7-10 Seismic - Common Errors 56

57 References ASCE 7-10, Minimum Design Loads for Buildings and Other Structures, Third Printing, American Society of Civil Engineers, Reston, VA IBC 2012, International Building Code 2012, International Code Council, Country Club Hills, IL 2015 OSEA Spring Conference ASCE 7-10 Seismic - Common Errors 57

58 References ACI , American Concrete Association, Farmington Hills, MI ASCE Charney Ph.D., P.E., Finley A.: Seismic Loads: Guide to the Seismic Provisions of ASCE 7-10, American Society of Civil Engineers, Reston, VA 2015 OSEA Spring Conference ASCE 7-10 Seismic - Common Errors 58

59 Questions? 2015 OSEA Spring Conference ASCE 7-10 Seismic - Common Errors 59