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Introduction

Load Path Continuity (not new!)

1982 UBC Section 2303:

shall result in a system that provides acomplete load path capable of transferringall loads and forces from their point of

origin to the load-resisting elements.

Load Path Design & Detailing

Most Important!

Not Always Provided by EngineerNot Always Enforced by Jurisdiction

Not understood by all players


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2

Quotes from the

EERI White Paper Report:There is a lack of conceptual understanding of building

performance in an earthquake.The engineering profession is placing insufficient

emphasison developing education materials for andproviding training to trades workers and inspectors

Encourage providers and producers of educationmaterials that primarily target engineers to cooperatewith existing providers serving the trades andinspectors.

The goal is to improve the presentation oftechnical information to nontechnicalaudiences.


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Sharing Common Goals

Building Owner

Architect/Engineer

Contractor/Trades Worker

Specialty Inspector

Building Department Staff (BO/PCE/BI)

Insurance Company

Our Common Goal:Protect the Public Who Occupy Buildings


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Pre-Quiz1. What element provides support for the top of a wall which is subjected to out-of-plane wind or seismic

loading?

A[ ] The shear walls

B[ ] The foundation

C[ ] The horizontal diaphragm

D[ ] The drag strut

2. What element carries and distributes the horizontal diaphragm shear to the shear walls?

A[ ] The collector or drag strut

B[ ] The shear walls

C[ ] The subdiaphragmD[ ] The diaphragm chord

3. What element usually serves as the horizontal diaphragm chord in a typical single family dwelling with

plywood shear walls?

A[ ] The holdown device

B[ ] The double top plates

C[ ] The eave blocking

D[ ] The sill anchor bolts

4. What elements transfer the in-plane shear (sliding) force of the shear wall to the shear wall footing?

A[ ] The holdown device

B[ ] The drag strut

C[ ] The diaphragm chord

D[ ] The sill anchor bolts


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Earthquake Basics

Pacific/N. American Plate

San Andreas Fault - Plate Boundary160 KnownActive Faults

Ground Motion

Acceleration/Inertial Forces

EQ Strength

1. Distance to Focus (hypocntr)

Shallow Stronger Shaking

2. Magnitude Energy

Higher Magnitude Stronger

3. Site Soil

Soft Soil Stronger Shaking


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California really has Earthquakes

Date Fault Location Magnitude

1836 Hayward 7.0*

1838 San Andreas 7.0*

1852 Big Pine ---

1857 San Andreas 8.0*

1861 Calveras ---

1861 San Andreas ---

1868 Hayward 7.0*

1872 Owens Valley 8.3*

1890 San Andreas ---

1899 San Jacinto 6.6*

1906 San Andreas 8.3


1925 Mesa/Santa Ynez 6.3

1927 Santa Ynez 7.5

1933 Newport-Inglewood 6.3


1934 San Jacinto 7.1

Date Fault Location Magnitude

1940 Imperial 7.1

1947 Manix 6.4

1950 Fort Sage 5.6

1951 Superstition Hills 5.6

1952 White Wolf 7.7

1956 San Miguel 6.8

1966 Imperial 3.6


1968 Coyote Creek 6.4

1971 San Fernando 6.6

1979 Imperial 6.6

1987 Whittier 6.1

1989 Loma Prieta 7.1

1990 Upland 5.5

1992 Yucca Valley 7.4

1992 Cape Mendocino 7.0

1994 Northridge 6.6


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Building Response

EQ Force on Depends:

Type of Structural SystemDynamic Properties: Building Period

Stiffness / Mass / Height

Design Response< Ground AccelerationDamping / Ductility / Overstrength

TBLDG ~ TEQ

Resonance Amplifies ForcesStiff Systems

Shorter Periods Higher Forces

Stiffer Systems Attract More Force


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Building Response

Building Shape & Configuration

Regular BuildingsUniform Force Distribution

More Predictable Response

Irregular BuildingsForce Concentrations

Less Predictable Response

Building Offsets

Vertical & Horizontal

Stiffness & Strength Variations

Weak Story

Soft Story


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Key Elements in the LFRS

Roof (horiz. diaphragm)

Floors (horiz. diaphragm)Shear Walls (vert. diaphragm)

Foundation

Load Distributing Elements:

Get the load FROM the point of origin TO theresisting element: Complete Load Path

Details, details, details

Connections, connections, connections


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Wood Frame Buildings

Horizontal Diaphragm:

A large thin deep beam loaded in itsplane which spans between anddistributesloads tothe supportingshear walls. The horizontaldiaphragm supportsthe out-of-planewalls, and distributesloads to thesupporting shear walls.


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Wood Frame Buildings

Factors affecting the allowable load of wood

structural panels horizontal diaphragms:Blocked or unblocked

Sheet thickness

Nail size and spacing

Sheet layout patternFraming member size

Orientation of load

New Terminology:Wood Structural Panel defined in Section 2302

and means all structural panel products (UBC Std. 23-2 or 23-3)includes plywood, OSB, waferboardetcbut NOT particleboard


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Horizontal Diaphragm Boundaries

Boundaries

Boundaries

Interior shear

wall

Boundary

Boundaries

Diaphragm boundaries may not just

occur at the perimeter of the

diaphragm. Interior shear walls and

drag members create diaphragmboundaries.

Boundaries

Boundaries


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Horizontal Diaphragm Nailing


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Sub Diaphragms

subdiaphragms shown dashed

main

diaphragm

Maximum width-to-depth ratio of

subdiaphragms is limited to 2 1/2:1

Subdiaphragm- a smaller diaphragm within the main horizontal diaphragmused to transmit anchorage forces (from out-of-plane wall loads) to themain diaphragm cross-ties (Section 1633.2.9).

Sub-Chord- the boundary member which serves as the sub-diaphragmchord member. Sub-ties carry wall anchorage forces to sub-chord.

Subdiaphragmsare primarily used in buildings with masonry or concretewalls with plywood roof or floors to minimize the number of continuous tiesbetween diaphragm chords required by Section 1633.2.9.

main cross ties


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Roof Diaphragm Nailing Diagram


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Diaphragm Chord / Beam Analogy

Simple Beam Moment Resistance:

Compression in the Top FiberTension in the Bottom Fiber

Horizontal Diaphragm Moment

Resistance:Tension Chord

Compression Chord


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Diaphragm Chord / Beam Analogy

Tensile Stress

Compressive Stress

Load

SupportSupport

Load

shearwall

shearwall Compression

Tension

W d F Sh W ll


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Wood Frame Shear Walls

Shear Wall:

A cantilevered vertical diaphragm whichsupportsthe horizontal diaphragm anddistributeslateral loads to the foundation.Bearing Walls are not necessarily shear wallsShear Walls are not necessarily bearing walls

Shear Walls are:

Engineered Designed & DetailedShear Walls are NOT:

Braced Wall Panels or Alternate BWPs

W d F Sh W ll


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Important Shear Wall Properties

StrengthResists Lateral Forces

Stiffness!

Resists Deflection

Limits Building Drift which Limits Damage

Shear Wall Deflection

Height-to-Width Ratio

Sheathing ThicknessFastener Slip

W d F Sh W ll


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Some specific requirements for wood

structural panel shear wall diaphragms:Must be fully blocked. No unblocked edge

allowed in wood structural panel shear walls

The holdown device (if required)must beconnected to the edge (chord) members ofthe shear wall.

The shear wall sheathing must be edge

nailed to the edge member that is connectedto the holdown device.

The shear wall sheathing must be edgenailed to the top and bottom (perimeter)members of the shear wall.

W d F Sh W ll


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Forces Acting on Shear Walls:

Sliding Force in Plane of Wall (Shear)Resisted by Anchorage to Sill or Foundation Plate

1997 UBC

3X foundation sill & 3X framing at abutting paneljoints where edge nail spacing less than 6 o.c.

Overturning Forces (Moment)

Resisted by Holdown Anchor Devices

Resisted by dead load (weight) of footing

Sh W ll H ld


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Shear Wall Holdowns

Not all shear walls require holdowns

Small lateral loads & large dead loads(stable)

Long shear walls - long resisting moment arm

Proper Holdown InstallationProper size:

Stud size, Holdown, Stud anchor, anchor bolt

Properly Located

NO countersunk stud bolts

NO shims at holdown

F d ti 1997 UBC Ch t 18


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Foundations - 1997 UBC Chapter 18

Anchor Bolts (Section 1806.6)

Seismic Zone 3 requires 1/2 @ 6 o.c.Seismic Zone 4 requires 5/8 @ 6 o.c.

7 embedment into concrete

7 bolt diameters from end of sillSquare Plate Washer (Section 1806.6.1)

2 x 2 x 3/16 square plate washer required

Reinforcing (Section 1806.7)Seismic Zone 3 & 4:

1-#4 Continuous top and bottom

Monolithic slab on grade may have 1-#5

P i ibl Di h A t R ti


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Permissible Diaphragm Aspect Ratios

Wood structural panels and particleboard, nailed

all edges:Horizontal Diaphragms: 4:1

Particleboard Not Permitted as Horizontal Diaphragm

Vertical Diaphragms:

3.5:1 in Zone 32:1 in Zone 4

Wood structural panels and particleboard,blocking omittedat intermediate joints:

Horizontal Diaphragms: 4:1Particleboard Not Permitted as Horizontal Diaphragm

*Vertical Diaphragms: Unblocked Not Permitted

Di h A t R ti T bl 23 II G


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Diaphragm Aspect Ratios - Table 23-II-G

8 9 10

2.28 (Zone 3)

4.00 (Zone 4)

Footnote 1 of Tables 23-II-I-1 & 23-II-I-2 requires that allpanel edges be backed with 2x or wider framing.

Section. 2315.5.3 requires that, Framing members or blocking

shall be provided at the edges of all sheets in shear walls.

2.57 (Zone 3)

4.50 (Zone 4)

2.86 (Zone 3)

5.00 (Zone 4)

A F il M h i


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A Failure Mechanism

Question: What if only oneshear transfer mechanismwere omitted? What is the result?

Suppose only oneshear wall has an incomplete loadpath. What could happen under design loading?

The share of its load never arrives at the disconnectedshear wall.

Since that shear wall is not supporting its share of load,its load is distributed to the remaining shear walls.

The remaining shear walls are subjected to more loadthan they were designed for

The remaining shear walls are then overloaded so theyin turn fail.

The Result: The entire lateral force resisting system failsbecause of incomplete load-path to only oneresistingelement.

Complete Load Path


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Complete Load Path

Complete load path

means

Complete shear transfer details

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