Two Stage Analysis Procedure: Wood on Podium Design

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1 Two Stage Analysis Procedure: Wood on Podium Design Part 1: Flexible Upper Portion Jared Cope, S.E. Two Stage Analysis Procedure: Wood on Podium Design 1

2 Two Stage Analysis: ASCE 7-10 Chapter Two-Stage Analysis Procedure Permitted for structures having 1) Flexible Upper Portion 2) Rigid Lower Portion 3) Additional Criteria Two Stage Analysis: Purpose Why would a structural engineer choose Two Stage Analysis for a structural system? The Two Stage Analysis Procedure separates the vertical distribution of seismic forces of the rigid lower portion from the design of the flexible upper portion. It also allows the adjusted seismic base shear of the flexible upper portion to be applied at the top of the rigid lower portion. 2

3 Two Stage Analysis: Purpose Example: ASCE 7-10, Vertical Distribution of Seismic Forces Two Stage Analysis: Purpose Example: ASCE 7-10, Vertical Distribution of Seismic Forces 3

4 Two Stage Analysis: Purpose What types of buildings are best suited for this design procedure? Mixed-Use buildings Apartments over parking Apartments over retail Assisted living over parking Hotels Two Stage Analysis: Purpose Which factors might lead a client to choose a building suited for Two Stage Analysis? Limited real estate Mixed-use zoning Cost of construction Existing/Future phases of construction Convenience of parking and living space within the same structure 4

5 Two Stage Analysis: Common Systems Light-frame wood (sheathed with wood panels) on concrete or masonry podium Light-frame cold-formed steel (sheathed with wood panels) on concrete or masonry podium Light-frame walls (sheathed with panels of all other materials) on concrete or masonry podium Two Stage Analysis: Common Systems Light-frame wood sheathed with wood structural panels R = 6.5, Ω o = 3*, C d = 4 Structural height (h n ) limitations: 65ft for seismic design categories D, E and F Economical materials * ASCE 7-10, Table footnote g: Ω o is permitted to be reduced to 2.5 for structures with flexible diaphragms. 5

6 Two Stage Analysis: Common Systems Light-frame cold-formed steel sheathed with wood structural panels R = 6.5, Ω o = 3*, C d = 4 Structural height (h n ) limitations: 65ft for seismic design categories D, E and F Non- combustible construction * ASCE 7-10, Table footnote g: Ω o is permitted to be reduced to 2.5 for structures with flexible diaphragms. Two Stage Analysis: Common Systems Light-frame walls sheathed with panels of all other materials R = 2, Ω o = 2.5*, C d = 2 Structural height (h n ) limitations: 35ft for seismic design category D, not permitted for E and F Non- combustible construction * ASCE 7-10, Table footnote g: Ω o is permitted to be reduced to 2 for structures with flexible diaphragms. 6

7 Two Stage Analysis: ASCE Two-Stage Analysis Procedure Additional Criteria: Two Stage Analysis: Trivia 7

8 Two Stage Analysis: Trivia Two Stage Analysis: Trivia 8

9 Two Stage Analysis: Trivia Two Stage Analysis: Trivia 9

10 Two Stage Analysis: Trivia Courtesy: The Teatropolitan Times Two Stage Analysis: Trivia Courtesy: ebaumsworld.com 10

11 Special Design Provisions for Wind and Seismic (SDPWS) Seismic Force Resisting System (SFRS) Shear Walls: Wood Structural Panels Panel Types Oriented Strand Board (OSB) Plywood 11

12 Seismic Force Resisting System (SFRS) Shear Walls: Wood Structural Panels Oriented Strand Board (OSB) Seismic Force Resisting System (SFRS) Shear Walls: Wood Structural Panels Plywood 12

13 NDS 2015: Special Design Provisions for Wind and Seismic (SDPWS) Shear Wall Types Segmented (Full-Height) Force Transfer Around Openings Perforated IBC 2015: Chapter 23 - Wood Shear Wall (Braced Wall Panel) Types Alternate Braced Wall (ABW) Portal Frame with Hold-downs (PFH) Both options are limited to use in structures with a maximum of two stories and they have very specific geometric, attachment and loading constraints. As such, these shear wall alternatives are unlikely candidates for use on multi-story two-stage structures. 13

14 NDS 2015: SDPWS Shear Walls Shear Wall Aspect Ratio To Be Continued NDS 2015: SDPWS Shear Walls Segmented (Full-Height) Individual shear wall segments extending from bottom plate to top plate of wall. Segments are permitted to be unblocked at abutting horizontal seams, but incur shear capacity penalties. 14

15 NDS 2015: SDPWS Shear Walls Force Transfer Around Openings NDS 2015: SDPWS Shear Walls Perforated Full-height shear wall segments are required at each end of perforated shear wall. Multiple restrictions for aspect ratio, maximum shear capacity, dimensional constraints, etc. 15

16 NDS 2015: SDPWS Shear Walls Perforated (Continued) Translation: Large shear capacity = Large anchorage requirement Perforated shear walls are unlikely candidates for use on multi-story two-stage structures. NDS 2015: SDPWS Shear Walls Outcome The most common shear wall types used in the upper portion of multi-story two-stage structures are: Full-height shear wall segments Force Transfer Around Openings 16

17 NDS 2015: SDPWS Shear Walls Shear Wall Aspect Ratio (Continued) Stiffness Penalty NDS 2015: SDPWS Shear Walls Shear Wall Aspect Ratio (Continued) Strength Penalty 17

18 NDS 2015: SDPWS Shear Walls Shear Wall Aspect Ratio (Continued) Important points: Stiffness and strength penalties are NOT additive Both penalties apply to both seismic and wind lateral forces (this was not the case in previous NDS versions). NDS 2015: SDPWS Shear Walls Hold-Down Types Steel straps Conventional Continuous Rods 18

19 NDS 2015: SDPWS Shear Walls Continuous Hold-Down Rods NDS 2015: SDPWS Shear Walls Continuous Hold-Down Rods 19

20 NDS 2015: SDPWS Shear Walls Continuous Hold-Down Rods NDS 2015: SDPWS Shear Walls Continuous Hold-Down Rods 20

21 NDS 2015: SDPWS Shear Walls Continuous Hold-Down Rods Commonly restrained at each floor level Most jurisdictions restrict rod elongations to 1/8 per floor and total system elongation to 0.2 per floor. NDS 2015: SDPWS Shear Walls 21

22 Two Stage Analysis: ASCE : Story Drift Determination NDS 2015: SDPWS Oriented Strand Board (OSB) Sheathing 22

23 NDS 2015: SDPWS Plywood Sheathing Two Stage Analysis: ASCE 7-10 Expansion Joints: 23

24 IBC 2015: Wood Shrinkage Shrinkage. Consideration shall be given in design to the possible effect of cross-grain dimensional changes considered vertically which may occur in lumber fabricated in a green condition. IBC 2015: Wood Shrinkage Shrinkage. Wood walls and bearing partitions shall not support more than two floors and a roof unless an analysis satisfactory to the building official shows that shrinkage of the wood framing will not have adverse effects on the structure or any plumbing, electrical or mechanical systems or other equipment installed therein due to excessive shrinkage or differential improvements caused by shrinkage. The analysis shall also show that the roof drainage system and the foregoing systems or equipment will not be adversely affected or, as an alternate, such systems shall be designed to accommodate the differential shrinkage or movements. 24

25 IBC 2015: Wood Shrinkage IBC 2015: Wood Shrinkage Wood is anisotropic Typical moisture content of 19% or less at time of milling 6% - 10% Equilibrium moisture content in dry climates Average shrinkage constant of 0.002/1% change in moisture content 25

26 IBC 2015: Chapter 5 2-Hour Fire Wall (Exterior) for 5 Stories of Light-Frame Wood (Type III) Construction Balloon-Framed walls Two Layers of gypboard (drywall) Fire-treated lumber and sheathing Fire Wall Hangers IBC 2015: Chapter 5 Fire Wall Hangers 26

27 IBC 2015: Chapter 5 Fire Wall Hangers Additional Topics Diaphragms Structural Irregularities Alternative Lateral Systems Six stories 27

28 Common Mistakes 28

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35 Questions 35