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
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
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
LATBSDC 2014 ACI 318-14 NEHRP (NIST GCR 10-917-4) Pramin Norachan 5
Pramin Norachan 6
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
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
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
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
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
Pramin Norachan 12
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
40@3.2 = 128 m. 40-Story RC Building Pramin Norachan 15
A B C D E F G H I J K M 8.0 8.0 8.0 8.0 8.0 8.0 8.0 8.0 8.0 8.0 8.0 4 3 2 8.0 3.0 19 m. 8.0 1 88.0 m. Floor Framing Plan Pramin Norachan 16
Material Properties Materials Expected Strength (MPa) Modulus of Elasticity (MPa) Concrete (fc ) - Shear Walls & Columns 80.6 38,642 Concrete (fc ) Girders, Coupling Beams & Slabs 53.8 30,649 Reinforcement Steel (fy) 484 200,000 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
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
Pramin Norachan 20
Pramin Norachan 21
Base Shear (KN) Comparison of Base Shear 100,000 90,000 89,604 R x = 1.4 80,000 70,000 64,639 71,465 R y = 1.8 60,000 50,000 40,000 38,983 30,000 20,000 y x ug () t 10,000 0 H1 LRHA H2 NLRHA Pramin Norachan 22
Elevation (m) Office Tower (Story Acceleration in X-dir.) 100 90 80 70 60 50 40 30 NLX (g) MCEX/R (g) 20 10 0 0 1 2 3 Story Acceleration (g) Pramin Norachan 23
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
Before scaling - load combinations for MCE level earthquakes U1 = 1.0 DL + 1.0 SDL + 0.25 LL + 1.0 MCEX + 0.3 MCEY U2 = 1.0 DL + 1.0 SDL + 0.25 LL + 0.3 MCEX + 1.0 MCEY After scaling - load combinations multiplied with scaling factors for MCE level earthquakes U1 = 1.0 DL + 1.0 SDL + 0.25 LL + 0.71 MCEX + 0.21 MCEY U2 = 1.0 DL + 1.0 SDL + 0.25 LL + 0.17 MCEX + 0.57 MCEY Pramin Norachan 25
Define the response spectrum function (MCE) that will be used for analysis. Pramin Norachan 27
Define load cases of response spectrum (MCE) in X and Y directions. Pramin Norachan 28
9.81/ 2 4.905 Scale force based on factor obtained from floor acceleration or base shear. Pramin Norachan 29
Define load combinations for diaphragm design at MCE level. Pramin Norachan 30
Floor diaphragm at Story 20 will be used as an example for diaphragm design. Pramin Norachan 32
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
Before obtaining forces, section cuts need to be defined. SC-1 SC-1 Pramin Norachan 34
SC-1 Select elements and nodes at the cut line. Then, assign these objects in a group. SC-1 Pramin Norachan 35
Define the section cut (SC-1) by selecting the group (SC-1). Pramin Norachan 36
However, for this presentation, the section cuts of this floor diaphragm are already defined as follows: Pramin Norachan 37
Section cuts for diaphragm chords and shear. DP-L20-01 02 03 04 05 06 07 08 09 10 11 Moment Pramin Norachan 38
Locations of the section cut for collectors at the core walls. F22 CL-L20-01 F22 CL-L20-02 Pramin Norachan 39
Locations of the section cut for shear friction at the core walls. SF-L20-01 F22 SF-L20-02 F22 Pramin Norachan 40
F22 F11 For a given load case, display any stress of shell force. Pramin Norachan 42
Y X Pramin Norachan 43
Use the option Draw Section Cut to see the force distribution. Pramin Norachan 44
Display the results of section cut forces. Pramin Norachan 45
Pramin Norachan 46
Select the considered load cases and force directions. V M Pramin Norachan 47
Rearrange all information for design. V M Pramin Norachan 48
L01 L20 L39 Pramin Norachan 49
Shear (KN) Moment (KN-m) Story L01-25,000-20,000-15,000-10,000-5,000 0 5,000 10,000 15,000-21749 -6295-7593 -7628-5038 -4446-4518 -4957-6423 -1284-1339 1318 1373 6200 4624 3415 3487 4544 7388 7423 6328 0 10 20 30 40 12719 50 60 70 80 90 Distance (m) 1,500 932 933 1,000 753 603 694 681 420 181 194 258 292 500 0-500 -163-194 -405-276 -307-1,000-572 -734-725 -700-865 -1,500-1000 0 10 20 30 40 50 60 70 80 90 Pramin Norachan Distance (m) 50
Shear (KN) Moment (KN-m) Story L20-20,000-15627 -15660-12924 -13252-13250 -13008-15,000-8142 -8145-10,000-6165 -2797-2923 -5,000 0 5,000 2911 3036 10,000 8612 15,000 10423 10426 13634 15022 15020 13718 20,000 16853 16887 0 10 20 30 40 50 60 70 80 90 Distance (m) 2,000 1486 1312 1282 1263 1,500 915 1,000 691 698 611 224 343 500 177 0-500 -292-177 -275-1,000-548 -753-771 -842-1,500-1369 -1354-1206 -1414-2,000 0 10 20 30 40 50 60 70 80 90 Pramin Norachan Distance (m) 51
Shear (KN) Moment (KN-m) Story L39-30,000-20,000-18565 -25412-27429 -26998-27242 -26998-27440 -25452-18639 -10,000 0 10,000 20,000 30,000-2852 2498 14598 18327 17048 12188 10448 12189 17059 18366 0 10 20 30 40 50 60 70 80 90 Distance (m) 2,500 2092 2,000 1655 1280 1437 1569 1,500 958 748 715 726 1,000 196 285 500 0-500 -354-196 -1,000-571 -680-1,500-896 -1267-1103 -1110-2,000-1713 -2,500-1825 -1948 0 10 20 30 40 50 60 70 80 90 Pramin Norachan Distance (m) 52 14672-2958 2604
Shear (KN) Moment (KN-m) -30,000-20,000-10,000 0 10,000 20,000 30,000 0 10 20 30 40 50 60 70 80 90 Distance (m) L01 L20 L39 2,500 2,000 1,500 1,000 500 0-500 -1,000-1,500-2,000-2,500 0 10 20 30 40 50 60 70 80 90 Distance (m) L01 L20 L39 Pramin Norachan 53
Inertia Force = m a Shear Wall Chord (Diaphragm) Diaphragm Shear Friction (Support) Shear (Diaphragm) Chord (Diaphragm) Collector (Support) Pramin Norachan 55
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 1.0 1.5V 1.5C Shear: 1.0 Compression: 1.0 Pramin Norachan 56
M u Analysis ( Section Cut) Pramin Norachan 57
Tension Chord M 16,869 KN m u d 17 m M u 16,869 Tu Cu 992.29 KN d 17 T A f u s y A s 3 Tu 992.29 10 f (1)(484) y 2,050 mm 2 4 2 2 7 DB20 ( As 7 2.0 2,119 mm ) Pramin Norachan 58
Compression chord Use perimeter beam (400x700 mm) 3 992.29 10 u 3.54 MPa (400 700) Allowable stress all ' 0.5 fc 0.5 53.8 10.76 MPa D C u 3.54 0.34 10.76 all Pramin Norachan 59
Pramin Norachan 60
Shear V 1, 485 KN V 1.5 V 1.5 1,485 2,228 KN u V u L 2,228 131 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) 0.66 53.8 c 968.2 KN / m A L cv ' (200 17, 000) (17 1, 000) 0.17 f (1.0) 0.17 53.8 c 249.4 KN / m Vn 131 KN / m 249.4 KN / m (Okay) L Pramin Norachan 61
Pramin Norachan 62
Demand Forces T u C u 992 KN 1.5 845 1, 268 KN T A f u s y A s 3 Tu 992 10 f (1)(484) y 2,050 mm 2 4 2 2 5 DB25 ( As 5 25 2, 454 mm ) Pramin Norachan 63
Compression Demand 2t w Cu 1.5 845 1, 268 KN A 2 t t 2(800)(200) mm w slab 2 t w Allowable compression ' (1.0)(0.85)(53.8)(2 200 800) Call (0.85) fc A 14, 634 KN 1,000 D C Cu 1,268 C 14, 634 all 0.09 Pramin Norachan 64
Pramin Norachan 65
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 8,000 2 0.4625 mm / mm 1 DB12@ 200 A s s 2 12 4 1 0.565 2 mm / 200 mm Pramin Norachan 66
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
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) 1-DB12@200 (Shear Friction) Pramin Norachan 68
Collectors Collector Connection to Shear Wall Pramin Norachan 69
A long collector with confinement reinforcement Shear Friction Rebar Pramin Norachan 70