Clarifying Frequently Misunderstood Seismic Provisions

Transcription

1 Middle Tennessee Region of the Tennessee Structural Engineers Association Seminar: Clarifying Frequently Misunderstood Seismic Provisions By Emily Guglielmo, SE Martin/Martin, Inc. December 13, 2017 Fundamentals of Earthquake Engineering, Newmark and Rosenblueth(1971): In dealing with earthquakes, we must contend with appreciable probabilities that failure will occur... Otherwise, all the wealth of the world would prove insufficient.. the most modest structures would be fortresses. We must also face uncertainty on a large scale, for it is our task to design engineering systems about whose pertinent properties we know little to resist future earthquakes whose characteristics we know even less Earthquake engineering is a cartoon... Earthquakes systematically bring out the mistakesmade in design and construction. 2 1

2 Topics R, C d, Ω o Redundancy, ρ Vertical and Horizontal Combination of Systems Bearing Wall or Building Frame? Analysis Procedures Structural Irregularities 3 Topics R, C d, Ω o Redundancy, ρ Vertical and Horizontal Combination of Systems Analysis Procedures Structural Irregularities 4 2

3 Elastic vs. Inelastic Response The red line is the force vs. displacement if the structure responded elastically. The green line is the actual force vs. displacement of the structure. The blue line is the code force per IBC/ ASCE 7. Illustrates the significance of design parameters contained in ASCE 7. Response modification coefficient, R Deflection amplification factor, C d System overstrength factor, Ω o NEHRP Recommended Seismic Provisions 5 Response Modification Coefficient, R In dealing with earthquakes, we must contend with appreciable probabilities that failure will occur in the near future. Otherwise, all the wealth of the world would prove insufficient to fill our needs: the most modest structures would be fortresses. In ASCE 7, seismic design forces are calculated by dividing the force from a linear response when subjected to the design ground motion by the response modification coefficient, R. 6 3

4 R = 1 Like wind (elastic) Used by Nuclear and Military Essential ASCE 7-16 Proposal No ductile detailing required? Permitted in all SDCs? 7 This proposal is an admission that it is too hard to design for seismic properly, so we will let lazy, uneducated engineers continue to be lazy and uneducated and design things stupidly. JUST VOTE NO! If you accept the concept that the R factor reduces elastic seismic design forces because of system ductility, then by definition using an R of 1.0 should require no ductile detailing. Logically, this concept should apply to all buildings in all regions. Good chapter headed in the right direction. Designers don t need another design approach. The profession wants ASCE 7 to simplify what is already in the standard. It is impossible for me to express all of my concerns with regard to this proposal adequately. It will introduce into seismic design category D, E and F territory the design of building structures without the appropriate detailing. This is dangerous. 8 4

5 Deflection amplification factor, C d In ASCE 7, the elastic deformations (Δ S ) calculated under reduced forces are multiplied by C d to estimate the actual inelastic deflections. 9 System Overstrength factor, Ω o The Ω o coefficient approximates the inherent overstrength and can be broken down into several components: Ω o = Ω D Ω M Ω S 10 5

6 Ω o DESIGN OVERSTRENGTH Ω D is the overstrengthprovided by the design engineer and/ or code. EXAMPLES: Load and resistance factors. Design controlled by stiffness. Architectural requirements. 11 Ω o MATERIAL OVERSTRENGTH Ω M represents material overstrength. EXAMPLES: Reinforced masonry, concrete, and steel provisions have historically used a factor of ~1.25 to account for the ratio of mean to specified strengths. A survey of WF steel: Ratios = 1.37 and 1.15 for A36 and A572 Gr

7 Ω o SYSTEM OVERSTRENGTH Ω S represents the system overstrength. EXAMPLES: Redundancy. The degree to which non-lfrs elements provide resistance after LFRS has yielded. 13 Structural System RANGEOF Ω o FOR VARIOUS SYSTEMS: Design Overstrength Ω D Material Overstrength Ω M System Overstrength Ω S Ω o =Ω D Ω M Ω S ASCE 7 Ω o Special Moment Frames (Concrete and Steel) Intermediate Moment Frames (Concrete and Steel) Ordinary Moment Frames (Concrete and Steel) Braced Frames Reinforced Bearing Wall Unreinforced Bearing Wall Dual System (Bracing and Frame)

8 Where the tabulated value of the overstrength factor, Ω o, is greater than or equal to 2½, Ω 0 is permitted to be reduced by subtracting the value of 1/2 for structures with flexible diaphragms. ASCE 7 Section

9 Load Combinations with Overstrength Factor Question: When do I need to design with load combinations with overstrength factors, Ω o? Answer: IBC Buildings shall be designed to resist the load combinations with overstrength factor specified in Section of ASCE 7 where required by Section , , or SDC B-F : Cantilever Column Systems Foundations and other elements used to provide overturning resistance at the base of cantilever column elements shall have the strength to resist the load combinations with overstrength factors of Section

10 : Elements Supporting Discontinuous Walls or Frames SDC B-F Columns, beams, trusses, or slabs supporting discontinuous walls or frames shall have the strength to resist the maximum axial force that can develop in accordance with the load combinations with overstrength factors of Section MASONRY SHEAR WALL ELEMENTS SUPPORTING DISCONTINOUS WALL : Collector Elements SDC C-F Collector elements, splices, and their connections to resisting elements shall resist the load combinations of Section In a way, earthquake engineering is a cartoon... Earthquake effects on structures systematically bring out the mistakes made in design and construction, even the minutest mistakes

11 Ω o Triggers Summary 12.4 Load Combinations with Omega zero Cantilever Columns SDC B,C,D,E,F Collectors (Light Frame, Wood excepted) SDC C,D,E,F Columns, Beams Supporting Discontinuous Walls or Frames SDC B,C,D,E,F Pile Anchorage SDC D,E,F Material Specifications: SDC B,C,D,E,F AISC where R>3 ACI Chapter 21, Appendix D, Etc. 21 Topics R, C d, Ω o Redundancy, ρ Vertical and Horizontal Combination of Systems Bearing Wall or Building Frame? Analysis Procedures Structural Irregularities 22 11

12 REDUNDANCY FACTOR, ρ Damage from the 1994 Northridge earthquake was concentrated in buildings with low redundancy. The code was modified to increase redundancy for structures in Seismic Design Categories D, E and F. For structures with low inherent redundancy, the required design forces are (arbitrarily?) amplified to increase strength and resistance to damage. 23 REDUNDANCY FACTOR, ρ ASCE 7 SECTION : Conditions Where the Value of ρ is conditions : Redundancy Factor, ρ, for SDC D, E, F Either ρ = 1.0 or

13 REDUNDANCY FACTOR, ρ= Structures assigned to Seismic Design Category B or C. 2. Drift calculation and P-delta effects. 3. Design of nonstructural components (Chapter 13). Examples: Mechanical/ electrical components, ceilings, cabinets. 4. Design of non-building structures that are notsimilar to buildings (Chapter 15). Examples: Tanks, amusement structures/ monuments, signs and billboards, cooling towers. 25 REDUNDANCY FACTOR, ρ= Design of collector elements, splices and their connections for which the load combinations with overstrength factor of are used. 6. Design of members or connections where the load combinations with overstrength of are required for design. 7. Diaphragm loads determined using Eq Structures with damping systems designed in accordance with Chapter Out-of-plane wall anchorage (including connections)

14 ASCE ρ = 1.0 or 1.3 ρ = 1.3 unless ONE of the following conditions is met: Condition 1: Can an individual element be removed from the lateral force resisting system without: Causing the remaining structure to suffer a reduction in story strength > 33%, or Creating an extreme torsional irregularity? 27 Condition 1: Requires Calculations! 28 14

15 ASCE ρ = 1.0 or 1.3 ρ = 1.3 unless ONE of the following conditions is met: Condition 2: If a structure is regular in planand there are at least 2 baysof seismic force resisting perimeter framingon each sideof the structure in each orthogonal direction at each story resisting > 35% of the base shear. 29 ASCE ρ = 1.0 or 1.3 ρ = 1.3 unless ONE of the following conditions is met: Condition 2: If a structure is regular in planand there are at least 2 baysof seismic force resisting perimeter framingon each sideof the structure in each orthogonal direction at each story resisting > 35% of the base shear

16 Q&A for Redundancy Question: Does the redundancy factor apply to the design of foundations? Answer: Yes. 31 Q&A for Redundancy Question: How many bays are there in shear wall buildings? Answer: b: The number of bays for a shear wall is the length of wall divided by the story height (or two times the length of shear wall divided by the story height for light-framed construction)

17 Q&A for Redundancy Question: Using Condition 1 to determine ρ for a wood-framed building: All of the shear walls are relatively long (height of each shear wall is less than its length). Can I assign ρ=1.0 because there are no shear walls with an h/l w ratio>1.0? Answer: Yes 33 Question: In Table does height-to-length ratio mean: Height-to-length ratio of a story Overall height-to-length ratio Answer: The h/l w ratio is story heightto-length ratio. Q&A for Redundancy 34 17

18 Question: Can the value of ρ be different at different levels of the same building? Answer: No. Q&A for Redundancy Question: Can the value of ρ be different in the two orthogonal directions of the same building? Answer: Yes, ρ can be different for two orthogonal directions when Condition #1 is being used (not true for Condition 2). 35 Q&A for Redundancy Question: If you have a dual system, can you assume ρ=1.0? Table doesn t seem to address dual systems? Answer: No.. As indicated in the table, braced frame, moment frame, shear wall, and cantilever column systems must conform to redundancy requirements. Dual systems also are included but, in most cases, are inherently redundant. Shear walls or wall piers with a height-to-length aspect ratio greater than 1.0 within any story have been included; however, the required design of collector elements and their connections for Ω o times the design force may address the key issues. In order to satisfy the collector force requirements, a reasonable number of shear walls usually is required. Regardless, shear wall systems are addressed in this section so that either an adequate number of wall elements is included or the proper redundancy factor is applied

19 Q&A for Redundancy Question: Does the redundancy factor need to be determined if dynamic analysis is used? Answer: Yes. The method of analysis doesn t make the building more or less redundant. 37 Topics R, C d, Ω o Redundancy, ρ Vertical and Horizontal Combination of Systems Bearing Wall or Building Frame? Analysis Procedures Structural Irregularities 38 19

20 HorizontalCombinations of Framing Systems Different Directions Different lateral systems may be used to resist seismic forces in each direction. R, C d, and Ω o coefficients shall apply to each system. Note: It is possible that one of the two systems will limit the overall system for use and height. The more restrictive of the limitation systems governs. H H Special Concrete Shear Walls R=5.0, C d =5.0, Ω o =2.5 H H Special Steel Moment Frames R=8.0, C d =5.5, Ω o = Horizontal Combinations of Framing Systems: Same Direction Where different lateral systems are used in combination to resist seismic forces in the same direction (other than dual systems) the more stringent system limitation shall apply. R=

21 VerticalCombinations of Framing Systems: Same Direction (ASCE 7-05) R=6.0 C d =5.0 Ω o =2.0 R=6.0 C d =5.0 Ω o =2.0 R=5.0 C d =5.0 Ω o =2.5 R=5.0 C d =5.0 Ω o =2.5 Special Steel Braced Frames above Special Concrete Shear Walls R: Cannot Increase As You Descend the Building! C d, Ω o : Cannot Decrease as You Descend the Building! Exception for rooftop structures, residential VerticalCombinations of Framing Systems: Same Direction (ASCE 7-05) R=5.0 C d =5.0 Ω o =2.5 R=5.0 C d =5.0 Ω o =2.5 R=5.0 C d =5.5 Ω o =3.0 R=8.0 C d =5.5 Ω o =3.0 Special Concrete Shear Walls Above Special Steel Moment Frames R: Cannot Increase As You Descend the Building! C d, Ω o : Cannot Decrease as You Descend the Building! 42 21

22 R=6.0 C d =5.0 Ω o =2.0 R=5.0 C d =5.0 Ω o =2.5 R=5.0 C d =5.5 Ω o = VerticalCombinations of Framing Systems: Same Direction (ASCE 7-05) R=6.0 C d =5.0 Ω o =2.0 R=5.0 C d =5.0 Ω o =2.5 R=8.0 C d =5.5 Ω o =3.0 Special Steel Braced Frames Above Special Concrete Shear Walls Above Special Steel Moment Frames R: Cannot Increase As You Descend the Building! C d, Ω o : Cannot Decrease as You Descend the Building! 43 R=6.0 C d =5.0 Ω o =2.0 R=5.0 C d =5.0 Ω o =2.5 R=5.0 C d =5.0 Ω o = VerticalCombinations of Framing Systems: Same Direction (ASCE 7-10) R=6.0 C d =5.0 Ω o =2.0 R=5.0 C d =5.0 Ω o =2.5 R=8.0 C d =5.5 Ω o =3.0 Special Steel Braced Frames Above Special Concrete Shear Walls Above Special Steel Moment Frames R: Cannot Increase As You Descend the Building! C d, Ω o : Correspond to R! 44 22

23 Topics R, C d, Ω o Redundancy, ρ Vertical and Horizontal Combination of Systems Bearing Wall or Building Frame? Analysis Procedures Structural Irregularities 45 Bearing Wall v. Building Frame 46 23

24 Bearing Wall v. Building Frame WALL SYSTEM, BEARING: A structural system with bearing walls providing support for all or major portions of the vertical loads. Shear walls or braced frames provide seismic force resistance. BUILDING FRAME SYSTEM: A structural system with an essentially complete space frame providing support for vertical loads. Seismic force resistance is provided by shear walls or braced frames. 47 Bearing Wall v. Building Frame WALL SYSTEM, BEARING: A structural system with bearing walls providing support for all or major portions of the vertical loads. Shear walls or braced frames provide seismic force resistance. BUILDING FRAME SYSTEM: A structural system with an essentially complete space frame providing support for vertical loads. Seismic force resistance is provided by shear walls or braced frames. What about moment frames? 48 24

25 Bearing Wall v. Building Frame Question: If some of the gravity loads are resisted by shear walls, is it possible to classify the system as a building frame system? 49 Bearing Wall v. Building Frame SEAOC: Assume all portions of the walls not reinforced as columns or beams are removed, but the self-weight of the wall is still present. If wall can support gravity loads and conforms to detailing requirements for gravity frame members Building Frame System 50 25

26 Bearing Wall v. Building Frame Question: Are walls required to be physically separate from the building frame system? Answer: No Building frame columns can be integral with/ boundary elements. 51 Bearing Wall v. Building Frame NEHRP: A building frame system is when gravity loads are carried primarilyby a frame supported on columns rather than by bearing walls. Some minor portions of the gravity load may be carried on bearing walls, but the amount.. should not represent more than a few percent of the building area

27 Topics R, C d, Ω o Redundancy, ρ Vertical and Horizontal Combination of Systems Bearing Wall or Building Frame? Analysis Procedures Structural Irregularities

28 Permitted Analysis Procedures 55 Height Threshold for Dynamic Analysis UBC Dynamic for Ht > stories IBC 2003 T>3.5 T s T s =S D1 /S DS 1/2 sec T>3.5*1/2=1.75s stories IBC 2012 T>3.5T s AND Ht > stories 56 28

29 Permitted Analysis Procedures Question:Can I use static (equivalent lateral force procedure) analysis for the following building: SDC F Type 1a (torsional) horizontal irregularities 5-story hotel ballroom (Occupancy III) Load bearing metal studs 57 Permitted Analysis Procedures 58 29

30 Permitted Analysis Procedures Question:Can I use static (equivalent lateral force procedure) analysis for the following building: SDC F Type 1a (torsional) horizontal irregularities 5-story hotel ballroom (Occupancy III) Load bearing metal studs Answer: ELF Procedure is acceptable. Dynamic analysis is not required! 59 Permitted Analysis Procedures Question:Can I use static (equivalent lateral force procedure) analysis for the following building: SDC E Type 2 (reentrant corner) vertical irregularities 2-story office building (Occupancy II) Concrete shear walls with steel floor/ roof framing 60 30

31 Permitted Analysis Procedures 61 Permitted Analysis Procedures Question:Can I use static (equivalent lateral force procedure) analysis for the following building: SDC E Type 2 (reentrant corner) vertical irregularities 2-story office building (Occupancy II) Concrete shear walls with steel floor/ roof framing Answer:ELF Procedure is acceptable. Dynamic analysis is not required! 62 31

32 Permitted Analysis Procedures Question:Can I use static (equivalent lateral force procedure) analysis for the following building: SDC D 175 ft. tall No irregularities T<3.5T s 63 Permitted Analysis Procedures 64 32

33 Permitted Analysis Procedures Question:Can I use static (equivalent lateral force procedure) analysis for the following building: SDC D 175 ft. tall No irregularities T<3.5T s Answer:ELF Procedure is acceptable. Dynamic analysis is not required! 65 ELF: Equivalent Lateral Force (Simplified Design Procedure) 12.8 (12.14) MRS: Modal Response Spectrum 12.9 LTH: Linear Time History 16.1 NTH: Nonlinear Dynamic Time History

34 Topics R, C d, Ω o Redundancy, ρ Vertical and Horizontal Combination of Systems Bearing Wall or Building Frame? Analysis Procedures Structural Irregularities 67 STRUCTURAL IRREGULARITIES Definition of horizontal and vertical irregularities in ASCE 7. Provisions of ASCE 7 triggered by irregularities. Present changes from ASCE 7-05 to FAQ/ Q&A on structural irregularities. Photo credit: Eve Fraser-Corp/ Foter.com/ CC BY-NC 68 34

35 History of Codes on Irregular Structures: Code provisions were developed for buildings with regular configurations. Earthquakes have repeatedly shown that irregular configurations lead to greater damage. Code regulations regarding irregularities were first introduced in 1988 UBC. 69 Can you name some irregularities as defined by ASCE 7? -Soft story -Big hole in the diaphragm -Torsion 70 35

36 Horizontal Irregularities 71 Vertical Irregularities 72 36

37 Horizontal Irregularities: Type 1a and 1b 73 Horizontal Irregularities: Type 1a and 1b Torsional Irregularity δ avg δ max Torsional Irregularity: Maximum story drift (including accidental torsion) at one end of the structure is more than 1.2 (1.4) times the average of the story drift. Torsional irregularity requirements apply only to structures in which the diaphragms are rigid or semirigid

38 Horizontal Irregularities: Type 1a and 1b Torsional Irregularity δ 1 =1in. =1.5 in. =δ 2 =2in. 1.2xδ avg =1.2x1.5=1.8 in. 1.4xδ avg =1.4x1.5=2.1in. 1.8<2.0< Horizontal Irregularities: Type 1a Torsional Irregularity ASCE Section Penalty SDC % increase in seismic forces in connections in diaphragms and collectors Table Permitted Analytical Procedure D, E, F D, E, F D structural model required B, C, D, E, F Amplification of accidental torsion C, D, E, F Design story drift based on largest difference in deflection C, D, E, F D structural model required in non-linear response history procedure B, C, D, E, F 76 38

39 Horizontal Irregularities: Type 1a Torsional Irregularity ASCE Section Penalty SDC % increase in seismic forces in connections in diaphragms and collectors (unless Ω o already applied) D, E, F clarified in ASCE 7-10 but no substantial changes. 77 Horizontal Irregularities: Type 1a Torsional Irregularity ASCE Section Penalty SDC % increase in seismic forces in connections in diaphragms and collectors Table Permitted Analytical Procedure D, E, F D, E, F D structural model required B, C, D, E, F Amplification of accidental torsion C, D, E, F Design story drift based on largest difference in deflection C, D, E, F D structural model required in non-linear response history procedure B, C, D, E, F 78 39

40 Horizontal Irregularities: Type 1a Torsional Irregularity ASCE Section Penalty Table Permitted Analytical Procedure SDC D, E, F 79 Horizontal Irregularities: Type 1a Torsional Irregularity ASCE Section Penalty SDC % increase in seismic forces in connections in diaphragms and collectors Table Permitted Analytical Procedure D, E, F D, E, F D structural model required B, C, D, E, F Amplification of accidental torsion C, D, E, F Design story drift based on largest difference in deflection C, D, E, F D structural model required in non-linear response history procedure B, C, D, E, F 80 40

41 Horizontal Irregularities: Type 1a Torsional Irregularity ASCE Section Penalty SDC D structural model required B, C, D, E, F 81 Horizontal Irregularities: Type 1a Torsional Irregularity ASCE Section Penalty SDC % increase in seismic forces in connections in diaphragms and collectors Table Permitted Analytical Procedure D, E, F D, E, F D structural model required B, C, D, E, F Amplification of accidental torsion C, D, E, F Design story drift based on largest difference in deflection C, D, E, F D structural model required in non-linear response history procedure B, C, D, E, F 82 41

42 Horizontal Irregularities: Type 1a Torsional Irregularity ASCE Section Penalty SDC Amplification of accidental torsion C, D, E, F ALL SDC SDC B Torsional Effects Include inherent and accidental torsion Ignore torsional amplification SDC C, D, E, F Includetorsional amplification with Type 1a or Type 1b irregularities 83 Change in ASCE 7-10: Exemption for light-framed construction discontinued. δ avg δ max 84 42

43 Why Amplify Accidental Torsion? 85 Horizontal Irregularities: Type 1a Torsional Irregularity ASCE Section Penalty SDC % increase in seismic forces in connections in diaphragms and collectors Table Permitted Analytical Procedure D, E, F D, E, F D structural model required B, C, D, E, F Amplification of accidental torsion C, D, E, F Design story drift based on largest difference in deflection C, D, E, F D structural model required in non-linear response history procedure B, C, D, E, F 86 43

44 Horizontal Irregularities: Type 1a Torsional Irregularity ASCE Section Penalty SDC Design story drift based on largest difference in deflection C, D, E, F 87 Horizontal Irregularities: Type 1a Torsional Irregularity ASCE Section Penalty SDC % increase in seismic forces in connections in diaphragms and collectors Table Permitted Analytical Procedure D, E, F D, E, F D structural model required B, C, D, E, F Amplification of accidental torsion C, D, E, F Design story drift based on largest difference in deflection C, D, E, F D structural model required in non-linear response history procedure B, C, D, E, F 88 44

45 Horizontal Irregularities: Type 1a Torsional Irregularity ASCE Section Penalty D structural model required in non-linear response history procedure SDC B, C, D, E, F 89 Horizontal Irregularities: Type 1b Extreme Torsional Irregularity ASCE Section Penalty SDC Prohibited E, F % increase in seismic forces in connections in diaphragms and collectors Table Permitted Analytical Procedure D structural model required B, C, D Amplification of accidental torsion C, D Design story drift based on largest difference in deflection C, D D structural model required in non-linear response history procedure D D B, C, D 90 45

46 Horizontal Irregularities: Type 2 Reentrant Corner Irregularity Horizontal Irregularities: Type 2 Reentrant Corner Irregularity Reentrant Corner Irregularity is defined to exist where bothplan projections of the structure beyond a reentrant corner are greater than 15% of the plan dimension of the structure in the given direction

47 Horizontal Irregularities: Type 2 Reentrant Corner Irregularity Question: Do I have a Horizontal Type 2 Irregularity? Answer: Regular Structure! 93 Horizontal Irregularities: Type 2 Reentrant Corner Irregularity ASCE Section Penalty SDC % increase in seismic forces in connections in diaphragms and collectors Table Permitted Analytical Procedure D, E, F D, E, F 94 47

48 Horizontal Irregularities: Type 3 Diaphragm Discontinuity Irregularity 95 Horizontal Irregularities: Type 3 Diaphragm Discontinuity Irregularity Diaphragm Discontinuity Irregularity: diaphragms with abrupt discontinuities stiffness, including cutout or open areas greater than 50% of the gross area, or changes in effective diaphragm stiffness of more than 50% from one story to the next

49 Horizontal Irregularities: Type 3 Diaphragm Discontinuity Irregularity ASCE Section Penalty SDC % increase in seismic forces in connections in diaphragms and collectors Table Permitted Analytical Procedure D, E, F D, E, F 97 From ICC s 2006 IBC Q&A Manual: Question:If the roof diaphragm has an opening in it which results in the stiffness of the 2 nd floor diaphragm being 50% stiffer than the roof, does that make it irregular? The plan irregularity definition says story-to-story. Answer:Yes, it would be considered irregular doesn t matter if floor or roof

50 Horizontal Irregularities: Type 4 Out-of-Plane Offsets Irregularity Horizontal Irregularities: Type 4 Out-of-Plane Offsets Irregularity Out-of-Plane Offsets Irregularity is where there are discontinuities in a lateral force-resistance path, such as out-of-plane offsets of the vertical elements. Image from FEMA Educational Material

51 Horizontal Irregularities: Type 4 Out-of-Plane Offsets Irregularity ASCE Section Penalty SDC Axial force using load combinations with overstrength for discontinuous elements % increase in seismic forces in connections in diaphragms and collectors Table Permitted Analytical Procedure B, C, D, E, F D, E, F D, E, F D structural model required B, C, D, E, F D structural model required in non-linear response history procedure B, C, D, E, F 101 Horizontal Irregularities: Type 4 Out-of-Plane Offsets Irregularity ASCE Section Penalty Axial force using load combinations with overstrength for discontinuous elements. SDC B, C, D, E, F MASONRY SHEAR WALL ELEMENTS SUPPORTING DISCONTINOUS WALL

52 Horizontal Irregularities: Type 5 Non-Parallel Systems Irregularity Horizontal Irregularities: Type 5 Non-Parallel Systems Irregularity Nonparallel Systems Irregularity Nonparallel Systems-Irregularity is defined to exist where the vertical lateral force-resisting elements are not parallel to or symmetric about the major orthogonal axes of the seismic force resisting system

53 Horizontal Irregularities: Type 5 Non-Parallel Systems Irregularity ASCE Section Penalty SDC Orthogonal load combinations C, D, E, F Table Permitted Analytical Procedure D, E, F D structural model required B, C, D, E, F D structural model required in non-linear response history procedure B, C, D, E, F 105 Horizontal Irregularities: Type 5 Non-Parallel Systems Irregularity ASCE Section Penalty SDC Orthogonal load combinations C, D, E, F : SDC B seismic forces are permitted to be applied in each orthogonal directions and interaction effects are permitted to be neglected : Two procedures permitted: 1) Orthogonal combination procedure with loading applied independently in orthogonal directions: 100% x-effects + 30% y-effects or 30% x-effects and 100% y-effects 2) Simultaneous application of orthogonal ground motion

54 Horizontal Irregularities: Type 5 Non-Parallel Systems Irregularity Nonparallel Systems Irregularity Does the plan below have a horizontal irregularity Type 5? Nonparallel Systems-Irregularity is defined to exist where the vertical lateral forceresisting elements are not parallel to or symmetric about the major orthogonal axes of the seismic force resisting system. ASCE 7-05: Irregular! ASCE 7-10:?? 107 Horizontal Irregularities: Type 5 Non-Parallel Systems Irregularity Nonparallel Systems Irregularity The ASCE 7-05 text of parallel to or symmetric about was sometimes misread to require that the system be both parallel to andsymmetric about the major orthogonal axes. The revised definition of nonparallel systems irregularity clarifies that it only applies where the vertical elements are not parallel to the major orthogonal axes. ASCE 7-05: Irregular! ASCE 7-10: Regular!

55 Vertical Irregularities: Type 1a and 1b Stiffness- Soft Story Irregularity 109 Vertical Irregularities: Type 1a and 1b Stiffness- Soft Story Irregularity Stiffness (Soft Story) Irregularity Stiffness-Soft Story Irregularity is where there is a story where the lateral stiffness is less than 70 (60)% of that in the story above or less than 80 (70)% of the average stiffness of the three stories above. Image from FEMA Educational Material

56 Question: Why might a soft story exist? Architectural constraints/ parking garage at base. Increased story height. Change in lateral-force-resisting system. Connection to base/ foundation. Openings in a wall. Change in size/ shape of an element. 111 Vertical Irregularities: Type 1a Stiffness- Soft Story Irregularity ASCE Section Penalty SDC Table Permitted Analytical Procedure D, E, F

57 Vertical Irregularities: Type 1b Stiffness- Extreme Soft Story Irregularity ASCE Section Penalty SDC Prohibited E, F Table Permitted Analytical Procedure D, E, F 113 Vertical Irregularities: Type 2 Weight (Mass) Irregularity

58 Vertical Irregularities: Type 2 Weight (Mass) Irregularity Weight (Mass) Irregularity is when the mass of any story is more than 150% of the mass of an adjacent story. A roof that is lighter than the floor below need not be considered. Image from FEMA Educational Material 115 Vertical Irregularities: Type 2 Weight (Mass) Irregularity ASCE Section Penalty SDC Table Permitted Analytical Procedure D, E, F

59 Vertical Irregularities: Type 1a, 1b, and 2 Soft-Story and Weight Irregularities Exceptions: Vertical irregularities Type 1a, 1b, 2 do not apply where: No story drift ratio is greater than 130% of the drift ratio of the next story. 1-story buildings in any SDC or 2-story buildings in SDC B, C, D. 117 Question: Does this building have a soft story (Type 1a or 1b)? h typ =12-0 h 1 =

60 Stiffness ratio, 1 st story to 2 nd story: = 0.42 < 0.60 Extreme soft story! Answer: Yes, type 1b, extreme soft story?!? Exception Exception:No story drift ratio is greater than 130% of the drift ratio of the next story. δe 1 δe2 δe < 1.3 h h = < 1.3 = x12 12x12 Answer: Novertical irregularity (soft/ extreme soft). Table from SK Ghosh

61 Vertical Irregularities: Type 3 Vertical Geometric Irregularity 121 Vertical Irregularities: Type 3 Vertical Geometric Irregularity Vertical (Geometric) Irregularity Vertical Geometric Irregularity is when the horizontal dimension of the seismic force resisting system is more than 130% of that in an adjacent story. Image from FEMA Educational Material

62 Vertical Irregularities: Type 3 Vertical Geometric Irregularity ASCE Section Penalty SDC Table Permitted Analytical Procedure D, E, F 123 Vertical Irregularities: Type 4 In-Plane Discontinuity in Vertical Lateral Force- Resisting Element Irregularity

63 Vertical Irregularities: Type 4 In-Plane Discontinuity in Vertical Lateral Force- Resisting Element Irregularity In-Plane Discontinuity in Vertical Lateral Force-Resisting Element Irregularity is when an inplane offset of the lateral force-resisting elements is greater than the length of those elements or there exists a reduction in stiffness of the resisting element in the story below. 125 Image from FEMA Educational Material Vertical Irregularities: Type 4 In-Plane Discontinuity in Vertical Lateral Force- Resisting Element Irregularity ASCE Section Penalty SDC Axial force using load combinations with overstrength for discontinuous elements % increase in seismic forces in connections in diaphragms and collectors Table Permitted Analytical Procedure B, C, D, E, F D, E, F D, E, F

64 Vertical Irregularities: Type 4 In-Plane Discontinuity in Vertical Lateral Force- Resisting Element Irregularity Does not qualify as a Vertical Irregularity 4 in ASCE 7-05! 127 Vertical Irregularities: Type 5a and 5b Discontinuity in Lateral Strength-Weak Story Irregularity

65 Vertical Irregularities: Type 5a and 5b Discontinuity in Lateral Strength-Weak Story Irregularity Discontinuity in Lateral Strength Weak Story Irregularity is when the story lateral strength is less than 80 (65)% of that in the story above. The story lateral strength is the total lateral strength of all seismic-resisting elements sharing the story shear for the direction under consideration. Image from FEMA Educational Material 129 Vertical Irregularities: Type 5a Discontinuity in Lateral Strength-Weak Story Irregularity ASCE Section Penalty SDC Prohibited E, F Table Permitted Analytical Procedure D

66 Vertical Irregularities: Type 5b Discontinuity in Lateral Strength-Extreme Weak Story Irregularity ASCE Section Penalty SDC Prohibited D, E, F Cannot exceed 2 stories or 30 feet (see exception) B, C Exception : The limit does not apply where the weak story is capable of resisting a total seismic force equal to Ω o times the design force. 131 Question: What s the difference between a soft story (1a/1b) and a weak story (5a/ 5b)? Answer: Soft Stiffness Weak Strength A soft story will: A weak story will: Question: Is a soft story is always a weak story or vice versa? Answer:??? Vertical Irregularities: Soft V. Weak Story drift more than the adjacent stories. fail under less force than the adjacent stories

67 Summary: 1) Understand the methodology built into the code values for R, C d, Ω o. 2) The code attempts to steer engineers into redundant, ductile designs with a linear load path. 3) Irregular structures routinely perform worse in seismic events, even when properly detailed. Emily Guglielmo eguglielmo@martinmartin.com