Seismic Design of Cast-in-Place Concrete Diaphragms, Chords and Collectors

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1 DESIGN OF TALL BUILDINGS: TRENDS AND ADVANCEMENTS FOR STRUCTURAL PEFORMANCE November 10, 2016 Pathumthani, Thailand Seismic Design of Cast-in-Place Concrete Diaphragms, Chords and Collectors Pramin Norachan Manager, Structural Engineering Unit AIT Solutions

2 Presentation Outline 1. Introduction 2. Overview of Structure 3. Analysis and Design Criteria 4. Force Scaling 5. Section Cuts 6. Forces from Section Cuts 7. Diaphragm Design

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4 Estimating the inelastic properties for a real component is not a simple task. If there is substantial inelastic behavior in an actual structure, the results of an elastic analysis may be of uncertain value for making design decisions, and may even be misleading. PERFORM-3D is an ideal tool for nonlinear performancebased analysis and design, created by Dr. Graham H. Powell, University of California at Berkeley Professor Emeritus of Civil Engineering. As a tool for obtaining information for design, even a crude inelastic model can be more useful than an elaborate elastic model. Please keep in mind that the goal is to get useful information for design, not to calculate "exact" response. Pramin Norachan 4

5 LATBSDC 2014 ACI NEHRP (NIST GCR ) Pramin Norachan 5

6 Pramin Norachan 6

7 Building structures generally comprise structural elements to support gravity and lateral loads. The seismic force-resisting system is composed of vertical elements, horizontal elements, and the foundation. The vertical elements provide a continuous load path to transmit gravity and seismic forces from the upper levels to the foundation. The horizontal elements typically consist of diaphragms, including collectors. Pramin Norachan 7

8 Diaphragms transmit inertial forces from the floor system to the vertical elements of the seismic force-resisting system. They also tie the vertical elements together to stabilize and transmit forces among these elements as may be required during earthquake shaking. Diaphragms are thus an essential part of the seismic force-resisting system and require design attention by the structural engineer to ensure the structural system performs adequately during earthquake shaking. Pramin Norachan 8

9 Diaphragm in-plane forces: Diaphragms span between, and transfer forces to, vertical elements of the lateral-force resisting system. Diaphragm transfer forces: Force transfers between vertical elements which have different properties over their height, or their planes of resistance may change from one story to another. A common location where planes of resistance change is at the grade level of a building with an enlarged subterranean plan (podium diaphragm). Pramin Norachan 9

10 Large diaphragm transfer forces should be anticipated at offsets or discontinuities of the vertical elements of the seismic-force-resisting system. (a) Setback in the building profile (b) Podium level at grade. Pramin Norachan 10

11 In general, low-rise buildings and buildings with very stiff vertical elements such as shear walls are more susceptible to floor diaphragm flexibility problems than taller structures. Pramin Norachan 11

12 Pramin Norachan 12

13 Different parts of a diaphragm include: - Diaphragm slab - Chords - Collectors (Drag struts or Distributors) - Connections to the vertical elements. These different parts can be identified by considering the load path in a simple diaphragm. We can idealize the diaphragm as a simply supported beam spanning between two supports, with reactions and shear and moment diagrams Pramin Norachan 13

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15 = 128 m. 40-Story RC Building Pramin Norachan 15

16 A B C D E F G H I J K M m m. Floor Framing Plan Pramin Norachan 16

17 Material Properties Materials Expected Strength (MPa) Modulus of Elasticity (MPa) Concrete (fc ) - Shear Walls & Columns ,642 Concrete (fc ) Girders, Coupling Beams & Slabs ,649 Reinforcement Steel (fy) ,000 Sections Properties Shear Walls Columns Girders Coupling Beams Slabs Members Dimension b x h = 400 x 700 mm b x h = 800 x 800 mm b x h = 400 x 700 mm b x h = 800 x 800 mm Thickness = 200 mm Pramin Norachan 17

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19 2014 LATBSDC ** Nonlinear fiber elements automatically account for cracking of concrete because the concrete fibers have zero tension stiffness. Stiffness modifiers for RC diaphragms commonly fall in the range of 0.15 to 0.50 when analyzing the building for design-level earthquake demands (Nakaki, 2000). Pramin Norachan 19

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22 Base Shear (KN) Comparison of Base Shear 100,000 90,000 89,604 R x = ,000 70,000 64,639 71,465 R y = ,000 50,000 40,000 38,983 30,000 20,000 y x ug () t 10,000 0 H1 LRHA H2 NLRHA Pramin Norachan 22

23 Elevation (m) Office Tower (Story Acceleration in X-dir.) NLX (g) MCEX/R (g) Story Acceleration (g) Pramin Norachan 23

24 PERFORM 3D (NLTHA) ETABS (RSA) Before carrying out design checks at MCE, the linear analysis results of ETABS were scaled to match with the nonlinear time-history analysis results (NLTHA) from PERFORM-3D. Pramin Norachan 24

25 Before scaling - load combinations for MCE level earthquakes U1 = 1.0 DL SDL LL MCEX MCEY U2 = 1.0 DL SDL LL MCEX MCEY After scaling - load combinations multiplied with scaling factors for MCE level earthquakes U1 = 1.0 DL SDL LL MCEX MCEY U2 = 1.0 DL SDL LL MCEX MCEY Pramin Norachan 25

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27 Define the response spectrum function (MCE) that will be used for analysis. Pramin Norachan 27

28 Define load cases of response spectrum (MCE) in X and Y directions. Pramin Norachan 28

29 9.81/ Scale force based on factor obtained from floor acceleration or base shear. Pramin Norachan 29

30 Define load combinations for diaphragm design at MCE level. Pramin Norachan 30

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32 Floor diaphragm at Story 20 will be used as an example for diaphragm design. Pramin Norachan 32

33 The force resultants in F11 direction on the floor diaphragm at Story 20 are shown below. SC-1 F22 F11 SC-1 F11 Pramin Norachan 33

34 Before obtaining forces, section cuts need to be defined. SC-1 SC-1 Pramin Norachan 34

35 SC-1 Select elements and nodes at the cut line. Then, assign these objects in a group. SC-1 Pramin Norachan 35

36 Define the section cut (SC-1) by selecting the group (SC-1). Pramin Norachan 36

37 However, for this presentation, the section cuts of this floor diaphragm are already defined as follows: Pramin Norachan 37

38 Section cuts for diaphragm chords and shear. DP-L Moment Pramin Norachan 38

39 Locations of the section cut for collectors at the core walls. F22 CL-L20-01 F22 CL-L20-02 Pramin Norachan 39

40 Locations of the section cut for shear friction at the core walls. SF-L20-01 F22 SF-L20-02 F22 Pramin Norachan 40

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42 F22 F11 For a given load case, display any stress of shell force. Pramin Norachan 42

43 Y X Pramin Norachan 43

44 Use the option Draw Section Cut to see the force distribution. Pramin Norachan 44

45 Display the results of section cut forces. Pramin Norachan 45

46 Pramin Norachan 46

47 Select the considered load cases and force directions. V M Pramin Norachan 47

48 Rearrange all information for design. V M Pramin Norachan 48

49 L01 L20 L39 Pramin Norachan 49

50 Shear (KN) Moment (KN-m) Story L01-25,000-20,000-15,000-10,000-5, ,000 10,000 15, Distance (m) 1, , , , Pramin Norachan Distance (m) 50

51 Shear (KN) Moment (KN-m) Story L20-20, , , , , , , , Distance (m) 2, , , , , , Pramin Norachan Distance (m) 51

52 Shear (KN) Moment (KN-m) Story L39-30,000-20, , ,000 20,000 30, Distance (m) 2, , , , , , , , Pramin Norachan Distance (m)

53 Shear (KN) Moment (KN-m) -30,000-20,000-10, ,000 20,000 30, Distance (m) L01 L20 L39 2,500 2,000 1,500 1, ,000-1,500-2,000-2, Distance (m) L01 L20 L39 Pramin Norachan 53

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55 Inertia Force = m a Shear Wall Chord (Diaphragm) Diaphragm Shear Friction (Support) Shear (Diaphragm) Chord (Diaphragm) Collector (Support) Pramin Norachan 55

56 Concrete Materials Nominal Strength Expected Strength ' f c f ' c 1.3 f ' c Reinforcing Steel f y f y 1.17 f y Action Demand (D) Capacity (C) Force Controlled (Non-Critical) Force Controlled (Critical) Mu Tu Vu Cu Tension & Flexure M T V 1.5C Shear: 1.0 Compression: 1.0 Pramin Norachan 56

57 M u Analysis ( Section Cut) Pramin Norachan 57

58 Tension Chord M 16,869 KN m u d 17 m M u 16,869 Tu Cu KN d 17 T A f u s y A s 3 Tu f (1)(484) y 2,050 mm DB20 ( As ,119 mm ) Pramin Norachan 58

59 Compression chord Use perimeter beam (400x700 mm) u 3.54 MPa ( ) Allowable stress all ' 0.5 fc MPa D C u all Pramin Norachan 59

60 Pramin Norachan 60

61 Shear V 1, 485 KN V 1.5 V 1.5 1,485 2,228 KN u V u L 2, KN / 17 m V n,limit L V L V n u L A L cv ' (200 17, 000) (17 1, 000) 0.66 f (1.0) c KN / m A L cv ' (200 17, 000) (17 1, 000) 0.17 f (1.0) c KN / m Vn 131 KN / m KN / m (Okay) L Pramin Norachan 61

62 Pramin Norachan 62

63 Demand Forces T u C u 992 KN , 268 KN T A f u s y A s 3 Tu f (1)(484) y 2,050 mm DB25 ( As , 454 mm ) Pramin Norachan 63

64 Compression Demand 2t w Cu , 268 KN A 2 t t 2(800)(200) mm w slab 2 t w Allowable compression ' (1.0)(0.85)(53.8)( ) Call (0.85) fc A 14, 634 KN 1,000 D C Cu 1,268 C 14, 634 all 0.09 Pramin Norachan 64

65 Pramin Norachan 65

66 Shear Demand Vu 1.5 1,194 1, 791 KN V V A f u n vf y 1, 791 (1.0) A vf (484)(1.0) Avf 1,791 (1.0)(484)(1.0) 3,700 mm 2 A vf L 3,700 8, mm / mm 1 DB12@ 200 A s s mm / 200 mm Pramin Norachan 66

67 Allowable shear friction V all ' (1.0)(0.2)(53.8)(200 8, 000) (0.2) fcac 17, 216 1,000 min (1.0)(5.5)(200 8, 000) (5.5) Ac 8,800 KN 1,000 8,800 KN KN V 1, 791 KN V 8,800 KN u all Pramin Norachan 67

68 7-DB20 (Chords) 5-DB25 (Collectors) (2) (6) (7) (6) (2) (4) (2) (6) (7) (6) (2) (2) (6) (7) (6) (2) (2) (2) (6) (7) (6) (2) 7-DB20 (Chords) (Shear Friction) Pramin Norachan 68

69 Collectors Collector Connection to Shear Wall Pramin Norachan 69

70 A long collector with confinement reinforcement Shear Friction Rebar Pramin Norachan 70

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