2008 special provisions seismic ... - american wood council€¦ · 2005/2008 wind & seismic...

Copyright © 2013 American Wood Council.All rights reserved.

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AWC’s 2008 Special Design Provisions for Wind and

Seismic ASD/LRFD – Part 1: Overview and Changes from

Previous Editions

Copyright © 2013 American Wood Council

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This presentation is protected by US and International Copyright laws.

Reproduction, distribution, display and use of the presentation without written

permission of AWC prohibited.

© American Wood Council 2013

Copyright Materials


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Learning Objectives

On completion of this course, participants will:

1. Be able to understand load path basics and how it applies to wood structural design.

2. Be familiar with the significant changes between the 2005 and 2008 SDPWS.

3. Be able to identify the lateral resisting systems and understand where to obtain design specifications for these systems

4. Be able to analyze format and content within the 2008 SDPWS.


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Outline

Lateral load basicsCode acceptance of Standard2005/2008 Wind & Seismic

Standard Overview2008 Wind & Seismic

StandardFrequently Asked Questions


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Load Path

Wind

Seismic (Earthquake) Motion

IBC Section 1604.4 requires complete load path

Image courtesy APA – The Engineered Wood Association 2003

Point of origin

Resisting element


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Lateral Loads

Wind Load Path

Diaphragm SUPPORTS out‐of‐plane walls

In‐plane walls SUPPORT diaphragm

Image courtesy APA – The Engineered Wood Association 2003

Windward wall Leeward wall


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Building Response Motions

Horizontal force transfer

in‐plane shear

Vmax at edges

shear wall

diaphragm

Reaction

Reaction

Diaphragm load, w


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Building Response Motions

torsion mass center stiffness center

eccentricity

stiffness centermass

center


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Lateral Structural Elements

Shear wall

Drag strut (or collector)

Diaphragm

Collector beam (or drag strut)

Vertical plane

Horizontal plane


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wood frame

Shear Wall ‐ PartsFive parts of a shear wall

wood structural panels wood frame

nails

plate anchors

hold downs

2

13

5

4


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Shear Wall ‐ Types

1. Individual Full‐Height Wall Segments

Moves hold downs to cornersWith or without force transfer around openings

Ignore all but full‐height segments

2. Perforated Shear Walls

3. Force Transfer Shear Walls


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Diaphragm ‐ Types Rigid

Diaphragm behaves as a fully rigid body

diaphragm < 2shearwalls

Inherent and accidental torsion considered in design

Flexible Diaphragm behaves as a series of simple

beams

diaphragm > 2shearwalls

No torsion

Tributary loading to vertical resisting elements (shear walls)


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Drag Struts and Collectors“collects” diaphragm load and “drags” it back to a shear wall

occur most frequently at junction of diaphragm and shear wall

Drag struts (wall plane)

Collector beams (floor plane)


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Collector Beam in Floor

For “wall continuity”

Collector beamBoundary nail (B.N.) diaphragm


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Drag Strut in WallUsually the top plate serves as collector and diaphragm chord

Might be window or door header

Drag strut elements

Shear wall panel

Diaphragm shear

Wall shear


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Code Acceptance of Standard

2006 IBC Permitted alternative to

Section 2305 References 2005 SDPWS

2009 IBC Mandatory References 2008 SDPWS

in Section 2305 for lateral design and construction


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General Overview

“The provisions of this document cover materials, design and construction of wood members, fasteners, and assemblies to resist wind and seismic forces.“

ASD and LRFD

Scope:


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General Overview

Chapter 1: Flowchart

Chapter 2: General Design Requirements

Chapter 3: Members and Connections

Chapter 4: Lateral Force Resisting Systems

Outline


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Chapter 1 – Designer Flowchart


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Chapter 2 – General Requirements

General

Terminology

Notation


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Chapter 2 – Design Methodologies


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Chapter 3 ‐Members and Connections

Framing

Sheathing

Connections

Covers out‐of‐plane wind load resistance of shear walls and diaphragms


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Framing – walls

Accounts for composite action

Extension of 1.15 repetitive member factor, Cr


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Sheathing ‐ walls


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Sheathing – roof


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Chapter 4 ‐ Lateral Force‐Resisting Systems

General

Wood Diaphragms

Wood Shear Walls

Covers in‐plane wind and seismic load resistance of shear walls and diaphragms


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Wood Diaphragms


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Wood Diaphragms


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Chapter 4 ‐ Lateral Force‐Resisting Systems• Wood Diaphragms

• Deflection example

• 2008 SDPWS Commentary


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Chapter 4 – Design Value FormatNominal design values

tabulated for diaphragms:

ASDreduction factor (2.0)

LRFD

resistance factor (0.80)


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Wood Diaphragms


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Chapter 4 – Nominal Design Value

Wind nominal unit shear capacity vw IBC allowable stress design value x 2.8

Seismic nominal unit shear capacity vs vs = vw / 1.4


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Wood Shear Walls


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Chapter 4 – Design Value Format

Nominal design values tabulated for shear walls:

ASDreduction factor (2.0)

LRFD

resistance factor (0.80)


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Wood Perforated Shear Walls


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Adhesive Attachment of Shear Wall Sheathing

SDPWS 4.3.6.3.1 Exception…


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Wood Shear Walls

2:1 3:1 3½:1

1.00 0.67 0.57

2:1 unless va = 2(bs/h)


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Panel Widths for Shear Walls

4.3.7.1…Panels shall not be less than 4′×8′, except at boundaries and changes in framing where 24″ is allowed unless framing members or blocking is provided at the edges of all panels.


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Chapter 4 – Nominal Design ValueWind nominal unit shear capacity vw IBC allowable stress design value x 2.8

Seismic nominal unit shear capacity vs vs = vw / 1.4


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Chapter 4 – Nominal Unit Shear CapacitiesASD Reduction factor = 2.0

Wind vw / 2.0

Seismic vs / 2.0

LRFD = 0.8 Wind vw x 0.8

Seismic vs x 0.8 = vw x 0.8 / 1.4

= vw x 0.57

LRFD has up to 12% strength benefit for seismic design for shear when using the following IBC load factors:

LRFD: 1.0E or 1.6W

ASD: 0.7E or 1.0W


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Summing Shear Capacities 4.3.3.2 Summing Shear Capacities… For shear

walls sheathed with dissimilar materials on opposite sides, the combined nominal unit shear capacity, (νsc) or (νwc), shall be either two times the smaller nominal unit shear capacity or the larger nominal unit shear capacity, whichever is greater.


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Appendix ANominal Shear Capacity Tables

Wood‐Frame Plywood BlockedWood Structural Panel Diaphragms

Wood‐Frame Plywood UnblockedWood Structural Panel Diaphragms

Wood‐Frame Plywood Shear Walls


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2008 Wind & Seismic Standard

Top Ten Changes! 1. High load diaphragms2. WSP ‐ combined uplift and shear3. Unblocked shear walls4. Shear wall anchorage provisions - 3x square5. WSP over gypsum shear walls6. Shear walls sheathed on 2 sides7. Fiberboard shear wall aspect ratio adjustment8. Increased strength limit for PSW9. PSW shear strength equation10. Appendix: Standard Nail Sizes and Cut Washers


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Consistent with IBC 2006

Includes apparent shear stiffness (Ga)

High Load Diaphragms (4.2.7.1.2)

Table 4.2B Nominal Unit Shear Capacities for Blocked Wood-Frame DiaphragmsUtilizing Multiple Rows of Fasteners (High Load Diaphragms)


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4.2.7.1.2 rules for building them

Need 3" or greater nominal members

High Load Diaphragms


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Boundary and panel edge nailing details

High Load Diaphragms


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Combined Wind Uplift & Shear ‐WSP

Wood structural panels (WSP) Resist combined wind uplift and shear Resist tension only from wind uplift Alternate to metal straps


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Table 4.4.1 Shear & Uplift

amount of available uplift capacity beyond shear capacity



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Photo: APA


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Minimum panel thickness = 7/16“Vertical sheathingMinimum spacing of fasteners in a row = 3“Horizontal blockingAll shear wall types Individual full‐height Perforated Force‐transfer

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Combined Wind Uplift & Shear ‐WSPCritical details

Note minimum edge distance is 1/2"


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Sheathing tension splice vs. shear blocking

(deals with tension perp)

(for shear)


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Combined Wind Uplift & Shear ‐WSPSheathing Splices per 4.4.1.7

Number of Stories

No Horizontal Sheathing Joint over Studs

Horizontal Sheathing Joint over Studs

Sheathing Tension Splice over Studs

One‐Story No splice required •Blocking to resist shear (4.4.1.3)•Stud nailing to resist uplift (4.4.1.7(2))

Resists both shear and uplift (4.4.1.7 Exception)

Multi‐Story Nail spacing at common horizontal framing > 3″ single row or > 6″ double row (4.4.1.7 (1) and Fig. 4H)

•Blocking to resist shear (4.4.1.3)•Stud nailing to resist uplift (4.4.1.7(2))

Resists both shear and uplift (4.4.1.7 Exception)


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3″ o.c. minimum6″ o.c. minimum


Multi‐story – sheets splice at band joist

Nail spacing limited to address tension perp

Uplift

Shear

Combined


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Multi‐story – sheets splice at stud mid‐height

Uplift

Shear

Combined


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Example: Splice over studs 110 MPH Exposure B

Uplift to resist: 277 plf• Use (2.0 x 277 = 554 plf) and Table 4.4.1 below



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Example: Splice over studs Uplift too high to allow multi‐story

splice to occur over band joist, so design splice over studs



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Double row 8d nails @ 4″ o.c.

4 x 12

4 x 8

Example: Splice over studs



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Single row 8d nails @ 4″ o.c.

See 4.4.1.7 (1)

4 x 12

4 x 8

Example: Splice over studsCombined Wind Uplift & Shear ‐WSP


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Additional nails (n) required above and below joint in studs to resist uplift capacity (4.4.1.7(2)):

For 8d @ 4” & 12” ASD Capacity = 324 plf > 277 plf OK

n = [324 x 16/12]/Z x C_D

= [324 x 16/12]/[67 x 1.6]

= 4.03 use 4 nails per stud4 x 12

4 x 8




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Check field nailing requirements for stud supporting 4 x 12 panel below the rim joist:

Min. stud length on 4 x 12 panel below rim = 31.875”#nails/stud = 31.875”/12” + 1 = 3.65 nails = 4 nailsTotal # nails required/stud = 4 + 4 = 8 nails

Spacing: 31.875”/(8‐1) = 4.55”

Specify 4” o.c. in field 4 x 12

4 x 8




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Combined Wind Uplift & Shear ‐WSPUplift only case

Single or double row of fasteners

Tension not shear

Test verified


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Default anchorage (4.4.1.6) Anchor bolts at 16" o.c. with 3" x 3" x 0.229" steel plate washer

Plate washer within ½" of sheathing face that provides uplift

Resist cross grain bending of bottom plate


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New shear wall anchorage provisions at foundation – Section 4.3.6.4.3 3" x 3" x 0.229" steel

slotted hole permitted

placed within ½" of sheathing material

automatically satisfied for 2x4 plate

Shear Wall Anchorage – 3" x 3" Default


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New shear wall anchorage provisions at foundation–Section 4.3.6.4.3 Exception: round washers permitted

• provided hold down appropriately sized for overturning

• Only applies to Individual Full‐Height Wall Segments

• Limited shear wall capacities

Shear Wall Anchorage – 3" x 3" Default


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Nails 6" panel edge spacing

Up to 2:1 aspect ratio

16' height limit

Based on cyclic testing

Shear capacity reduction

Unblocked Shear Walls


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Deflection (4.3.2.2) Less stiffness Deflection component amplified by:

ubC

v

Unblocked Shear Walls


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Section 4.3.7.2 and Table 4.3B

Consistent with IBC 2006

Shear wall with fire resistance

WSP over Gypsum Shear Walls


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WSP over Gypsum Shear Walls


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IBC 2006 provisions for shear walls sheathed on two sides ‐ Table 4.3A Footnote 6

Shear Walls Sheathed on 2 Sides

6. Where panels are applied on both faces of a shear wall and nail spacing is less than 6" on center on either side, panel joints shall be offset to fall on different framing members. Alternatively, the width of the nailed face of framing members shall be 3" nominal or greater at adjoining panel edges and nails at all panel edges shall be staggered.


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Adjoining Panel Edge Details

Shear Walls Sheathed on 2 Sides


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Aspect ratio adjustment for structural fiberboard shear walls

Table 4.3.4 Footnote 3

Now permitted up to 3.5:1 based on cyclic testing

• used to be 1.5:1

Structural Fiberboard Shear Walls


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Aspect ratio adjustment If < 1:1 reduce design unit shear

Wind factor tied to strength• at 3.5:1 wind factor = 0.78

Seismic factor tied to equivalent stiffness for same seismic drift level• at 3.5:1 seismic factor = 0.36

- Structural



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Structural fiberboard shear walls – Table 4.3A Nail size recommendation

• 8d common is dropped

4.3.7.4 increased minimum distance from panel edge at top and bottom plates to 3/4"



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Increased strength limit for perforated shear walls ‐ Section 4.3.5.3 (3) 1740 plf nominal seismic shear capacity

(980 plf nominal in 2005 SDPWS)

2435 plf nominal wind shear capacity

(1370 plf nominal in 2005 SDPWS)

Tests with10d nails @ 2" o.c. edge

PSW Increased Strength Limit


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PSW Shear Strength Equation


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Section 4.3.3.5



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Alternative to tabulated values – Section 4.3.3.5 Allows more efficient designs

• Actual area of openings

Table requires maximum opening size• Example: 1 door & 2 windows• Table assumes windows are same height as door • Takes away panel shear area

Table



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Appendix A

Standard Nails and Cut Washers


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Wind & Seismic Standards

More details on changes

Wood Design Focus papers 2005 Special Design Provisions for Wind and Seismic

(SDPWS)

2008 Special Design Provisions for Wind and Seismic

Use of Wood Structural Panels to Resist Combined Shear and Uplift from Wind

Download free at

www.awc.org


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Frequently Asked Questions

Q: What about pneumatic nails or staples?

A: Design capacities are provided in a National Evaluation Report held by International Staple and Nail Tool Association:

www.isanta.org


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Questions?

www.awc.org Online eCourses

FAQ’s

Calculators


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2008 special provisions seismic ... - american wood council€¦ · 2005/2008 wind & seismic...

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