Cross-Laminated Timber (CLT) in California: Guidelines, Testing and Recommendations

Transcription

1 Cross-Laminated Timber (CLT) in California: Guidelines, Testing and Recommendations Presented by Scott Breneman, PhD, PE, SE Senior Technical Director 1

2 The Wood Products Council is a Registered Provider with The American Institute of Architects Continuing Education Systems (AIA/CES), Provider #G516. Credit(s) earned on completion of this course will be reported to AIA CES for AIA members. Certificates of Completion for both AIA members and non-aia members are available upon request. This course is registered with AIA CES for continuing professional education. As such, it does not include content that may be deemed or construed to be an approval or endorsement by the AIA of any material of construction or any method or manner of handling, using, distributing, or dealing in any material or product. Questions related to specific materials, methods, and services will be addressed at the conclusion of this presentation. 2

3 Course Description The 2016 California Building Code prescriptively recognizes crosslaminated timber (CLT) as an acceptable building material in construction types III, IV and V. However, its use as part of a seismic force resisting system either as a diaphragm or shear wall is not yet codified. This panel session will focus on three topics key to the design, review and approval of CLT buildings. First, it will raise awareness regarding a process guideline document developed jointly by the California Building Officials (CALBO) and the Structural Engineers Association of California (SEAOC) in an effort to streamline mass timber project review and approval. Next it will discuss shake table testing conducted at the University of California at San Diego to investigate several resilient CLT shear wall configurations, including a post-tensioned rocking wall system and a repairable rocking wall system. Finally, it will discuss design processes and resources specific to the use of CLT in diaphragms. 3

4 Learning Objectives 1. Develop an understanding of the process recommendations in the California Mass Timber Guide developed jointly by CALBO and SEAOC to achieve a consistent and streamlined approach to mass timber project review and approval. 2. Describe a recently completed full-scale two-story mass timber building shake table test and understand the potential impacts these tests can have on building approvals. 3. Discuss how mass timber buildings with a resilient rocking wall system can minimize owner loss and repair time even after major earthquakes. 4. Examine the use of CLT in diaphragm applications and review design options and recommendations for seismic resistance. 4

5 CLT Seismic Design CLT Seismic Force Resisting Systems Not addressed In ASCE/SEI 7-10 or 7/16 SDPWS

6 FLATWISE Panel Loading Span in MAJOR Strength Direction Parallel Direction Reference & Source: ANSI/APA PRG Span in MINOR Strength Direction Perpendicular Direction 6

7 EDGEWISE Panel Loading Span in MAJOR Strength Direction Span in MINOR Strength Direction Reference & Source: ANSI/APA PRG

8 CLT in Lateral Force Resisting Systems CLT Panels have a significant in-plane shear strength. Source: ICC-ES/APA Joint Evaluation Report ESR to 290 PSI Allowable Edgewise Shear = 1.7 to 3.5 kips/ft/incd = 1.6 for short term loading = 2.8 to 5.6 kips/ft length (ASD) per Inch of Thickness. Source: APA Product Report PR-L306 8

9 CLT Diaphragms Strength of CLT rarely governs. Strength of Connections covered by NDS and Proprietary Fastener Evaluation Reports 9

10 2 30 ft ft = 80 ft Example Mass Timber Floor System 4 30 ft = 120 ft Columns Girders Joists CLT Panels 10

11 Example CLT Diaphragm Design Lateral Load, w Chord Diaphragm Shear, v c d b a Collector Shear Transfer Details: a panel to panel b panel to panel over beam c panel to wall / collector d panel to chord 11

12 Suggestions for CLT Diaphragm Design Until CLT diaphragms are formally defined through a consensus standardization, following are suggestions when considering diaphragms with CLT through an alternative means and methods process 12

13 CLT Diaphragm Design Suggestions The basics: Diaphragms shall be designed for both the shear and bending stresses resulting from design forces of ASCE CLT diaphragms shall be designed in accordance with the principles of mechanics using fastener and member strength in accordance with the provisions of the NDS. (or proprietary connectors using 3 rd party verified equivalence) 13

14 CLT Diaphragm Design Suggestions Diaphragm Shear Connections Connections in CLT diaphragms transferring seismic shear forces into and out of CLT panels should be detailed using regularly spaced dowel-type fasteners in shear governed by ductile yield Mode III or Mode IV of NDS Design capacity of connection (ductile mode governing) φ Z Z D E h Applied Seismic Forces Z D 0.7 E h 14

15 Force Conceptual Fastener Behavior Adhesive Screw in Tension Screw or Nail in Shear Smooth Nail in Tension Displacement 15

16 Force Conceptual Fastener Behavior Well behaved seismic systems have ductile failure modes. Adhesive Screw in Tension Screw or Nail in Shear Smooth Nail in Tension Displacement 16

17 Panel to Panel Connection Styles Surface Spline Half Lap Butt Joint Load Sharing not load bearing 17

18 An Efficient Panel to Panel Connection Self-Tapping Screws as erection bolts ~18 24 o.c 5 ½ to 6 wide plywood or LVL Nails at spacing required for shear transfer Graphics: ASPECT Structural Engineers 18

19 Panel to Beam Connection Styles 19

20 Fastener Vendor Design Support 20

21 CLT Diaphragm Design Suggestions Diaphragm Shear Connections The remainder of the diaphragm shear connection, including wood limit states and the CLT panel edgewise shear strength, should provide a design capacity great enough* to develop yield capacity of the connection. *See the U.S. CLT Handbook Section 4-2 for one definition 21

22 CLT Diaphragm Design Suggestions Design strength for the non-ductile limit states Modified Nominal strength of the ductile yield mode of the connection LRFD Design: φ R R ND Z D,mod R ND 1 ASD Design: φ Z = 0.65 φ Z Z D,mod 1.54 Overstrength 22

23 CLT Diaphragm Design Suggestions Diaphragm Chords Designed to a capacity great enough* to develop yielding in the shear connections Tension splices detailed with dowel-type fasteners governed by ductile yield Mode III or Mode IV of NDS OR Designed to Seismic Overstrength 23

24 Special Diaphragm Conditions? Cantilevers past 35 wood structural panel diaphragm limit Elevated seismic performance (low damage objective) Suggest: ASCE 7-16 Section Alternative Diaphragm Loading Rs = 1.0 ~essentially elastic response to DBE Rs = 0.7 ~essentially elastic response to MCE 24

25 UCSD Two-Story Shake Table Test Maximum floor acceleration from MCE ground motions Courtesy Shilling Pei Diaphragm went through 22 DBE or greater ground motions with no repairs! Courtesy Andre Barbosa ASCE 7-16 Alt. Diaphragm 25 Design R = 4, Rs = 1.0

26 Is a CLT Diaphragm Rigid or Flexible? Calculated Diaphragm Rigidity OR Enveloped Diaphragm Design 26

27 Flexible by Calculation ASCE

28 IBC1604.4: A diaphragm is rigid for the purpose of distribution of story shear and torsional moment when the lateral deformation of the diaphragm is less than or equal to two times the average story drift. Rigid by Calculation IBC

29 CLT Diaphragm Design Example Paper Available from structurlam.com 29

30 CLT Diaphragm Design Example Paper Design example following NDS 2015, US CLT Handbook Includes approximate deflection equation: Modified 4-term wood panel sheathed diaphragm equation in SDWPS 15 δ dia = 5vL3 8EAW + vl + CLe 4G v t n + σ xδ c v 2W Chord Flexure Panel Shear Connector Slip Chord Slip C = P L P W P L is panel length P W is panel width e n is connector slip at diaphragm edge 30

31 WoodWorks Solutions Paper on CLT Modeling 31

32 Shear Walls What R value can I use? Photo: KLH Photo: FPI 32

33 Code Recognized Vertical Seismic System Photos: WoodWorks 33

34 Range of Shear Wall Systems Rocking Wall Systems (R = ~6?) Ordinary Shear Walls (R = ~2?) 34

35 Range of CLT Shear Wall Systems Seismically Detailed Walls (R = 4?) (R = ~3?) Ongoing FEMA P-695 study by John van de Lindt 35

36 QUESTIONS? This concludes The American Institute of Architects Continuing Education Systems Course Scott Breneman WoodWorks Wood Products Council 36