ce 479: design of building components and systems fall …jliu/courses/ce479/... · 2012-10-09 ·...

CE 479: DESIGN OF BUILDING

COMPONENTS AND SYSTEMS

FALL 2012– J. LIU

Wood Structural Panels

Wood Structural Panels

Plywood

Oriented Strand Board (OSB)

Composite Panels

Generally, wood veneer and reconstituted wood-based

material bonded

Particleboard

Medium density fiberboard

Wood Structural Panels

Composite Panels

Particleboard

made from small wood particles pressed together with glue

under extreme heat and pressure to make a solid panel.

Medium density fiberboard

made the same way except the wood particles are further

refined into even smaller particles to provide a smooth edge

to the panel.

Uses

Roof, floor and wall sheathing

Horizontal and vertical (shearwall) diaphragms

Structural components

Prefab. I-joists

Sandwich panels

Gusset plates

Concrete formwork

Sheathing

Sheathing

Thickness requirements

Often determined by sheathing-type loads

(normal to surface of panel)

Nailing requirements

Determined by unit shears in horizontal or

vertical diaphragm

More later on both topics

Panel Dimensions and Installation

Standard size 4 x 8 ft

Can be available in 9, 10, 12 ft lengths

Dimensionally stable

But some changes can be expected under varying moisture conditions

Installation – recommend clearance between panel edges and ends

Typically 1/8 in to permit panel movement

Other spacing provisions may apply, depending on type of panel, moisture conditions

Installation

Typical tolerances

+0, -1/16 in., and +0, -1/8 in.

Some panel grade stamps:

“sized for spacing”

Allow for panel spacing while maintaining basic 4 ft. module

Nail popping

If installed on green beams, nails should be “set” below surface of sheathing (will appear to “pop” upwards as supports dry)

Plywood

Plywood panel made up

of a number of veneers

(thin sheets of wood)

Veneer obtained by

rotating “peeler logs” in

a lathe

Plywood

Veneer then clipped to proper size

Dried to low moisture content (2 – 5 %)

Graded according to quality

Veneer spread with glue and cross-laminated

Adjacent layers have wood grain at right angles

Odd number of layers

Glue bond obtained is stronger than wood in plies

Panel is grade-stamped

3 ply, 3 layer panel

4 ply, 3 layer panel

Extension of simple 3-ply construction

Two center plies have grain running in same direction

5 ply, 5 layer panel

Plywood Layers

Face

Outside ply (if outside plies are different veneer quality, face is the better veneer grade)

Grain oriented typically parallel to 8 ft dim.

Back

The other outside ply (grain also parallel to 8 ft. dim.)

Crossband

Inner layer(s) placed at right angles to face and back

Center

Inner layer(s) parallel with outside plies

Plywood orientation

Strong orientation

Grain of outside layers parallel to span

Weak orientation

Grain of outside layers perpendicular to span

Thus, two sets of cross-sectional properties for use

with stresses, depending on orientation (stress and

properties tables)

Also, load tables for sizing

Design Values / Load Tables

2005

ASD/LRFD

Manual

For Engineered

Wood

Construction

We’ll use this:

Load-Span Tables for APA

Structural-Use Panels

Transformed sections (used to develop load tables)

Strong Orientation

Weak Orientation

Species Groups for Plywood

Species Groups for Plywood

Group1 – highest stiffness and strength; Group 5

– lowest

Groups 1 to 4 – reference design values

Mixing of species?

Reference design values based on species group of

face and back plies

Species group of outer plies included in grade stamp

Veneer Grades

N – special order “natural finish” veneer

A – smooth, paintable surface, without knots

B – solid-surface veneer, may contain knots up to 1 in. in width if sound and tight-fitting

C-plugged – improved C-grade with knotholes less than ¼ x ½ in

C – may contain open knotholes up to 1 in. in width and some up to 1-1/2 in.

D – open knotholes up to 2-1/2 in., occasional up to 3 in. across grain (not allowed in exterior applications)

Exposure Durability

Exterior

Insoluble waterproof glue

Minimum of C-grade veneers

Required when permanently exposed to weather or when MC

will exceed 18% (continuously or in repeated cycles)

Interior

Not exposed to weather or exceed 18% MC

Typically available with exterior glue, but can be with

exterior, interior, or intermediate

“Exposure 1” if exterior glue; might be used if long

construction delays (exposure) expected

Marine grade plywood

Gap and void-free in all layers

No voids even on cut edges

Water-proof glue

Water has no effect on glue when immersed

Typically constructed of harder woods such as Douglas Fir or Western Larch

http://www.segerorman.com.tr/en/uploads/urun/big/marine_grade_plywood_807514.jpg

Plywood Grade Stamps

Veneer grades of face and back

Minimum species group of the outer plies

Highest species group number

Exposure durability classification

Mill number

Inspection agency

PS-1 (U.S. Product Standard PS-1 – Construction

and Industrial Plywood)

APA – American

Plywood

Association

GRADE OF VENEER ON PANEL FACE

GRADE OF VENEER ON PANEL BACK

SPECIES GROUP

NUMBER

DESIGNATES TYPE

ON EXTERIOR OR

INTERIOR

PLYWOOD STANDARD

GOVERNING MANUFACTURE MILL NUMBER

Plywood Grade Stamp

Plywood Grades

Sheathing Grades

Normally used for roof, floor, wall sheathing

C-C (exterior type)

C-D (typically Exposure 1)

When added strength required:

STRUCTURAL 1 can be added to designation

CC-STR 1 or C-D STR 1

Plywood Grades

Span Rating (e.g. 24/16)

Number on left is maximum recommended span in inches when used as roof sheathing

Second number is maximum recommended span in inches when used a subflooring

Meant for use of panels without any structural calculations

Panel must be oriented in the strong direction

Certain edge support requirements must be satisfied

May use panels for longer spans if justified by structural calculations

Plywood

May be sanded

Improves surface condition

However, reduces thickness of outer veneers

Different cross-sectional properties are used in

strength calculations for sanded, touch-sanded, and

unsanded panels

Or, different tables, such as Table 3 for Group 1

Sanded Plywood panels (APA, 2011)

Oriented Strand Board

Non-veneer panel manufactured from reconstituted wood strands or wafers, bonded with resin

Strands directionally oriented, typically in 3 to 5 perpendicular layers, cross laminated in similar manner to plywood

Nonveneer (OSB) Grade Stamp

Adjustment Factors

NDS Table 9.3.1 Applicability of Adjustment Factors

for Wood Structural Panels

CM and Ct – obtain from “approved source” (such as

APA, NDS commentary)

New for panels

CS – Panel Size Factor

If panel width < 24”

NDS Table 9.3.4

Adjustments

ASD only

Span Adjustments:

2-span to 1-Span

3-span to 1-Span

3-span to 2-Span

Wet Locations, CM

MC 16% or more

Roof Sheathing Example

Consider roof system with sheathing span of 24

inches, dead load of 10 psf, snow load of 75 psf,

total load deflection limit of L/240. Determine the

minimum span-rated sheathing using LRFD. Use

strength axis across supports. Assume sheathing

grade, 3-span, 4-ply, normal temperatures, dry

service conditions.

1.2 D + 1.6 S = 1.2 (10 psf) + 1.6 (75 psf) = 132 psf

D + S = 10 psf + 75 psf = 85 psf

1.4 D = 1.4 (10 psf) = 14 psf

Roof Sheathing Example, cont’d.

Note: Tables are ASD!

Assumes 3 spans Assumes 2 spans

(No span adjustment needed)

Roof Sheathing Example, cont’d.

Note: Tables are ASD!

Roof Sheathing Example, cont’d.

Note: no adjustment for, or

mention of, 3-ply, 4-ply, etc. in

Load Tables

May not know # of plies (i.e.,

not on grade stamp),

therefore tabulated values

based on most conservative

construction (Table 6)

Roof Sheathing Example, cont’d.

For Deflection, no

modification;

same for ASD and LRFD 32/16 does not satisfy

deflection limit,

but

40/20 does

163 > 85 psf (D+S)

Roof Sheathing Example, cont’d.

Bending: KF (130psf) =

2.54 (130 psf) = 330 psf

Shear: KF (218 psf) =

2.88 (218 psf) = 628 psf

Roof Sheathing Example, cont’d.

= 0.8 for 1.2 D + 1.6 S

b = 0.85 and v = 0.75

Bending: 330 psf (0.8)(0.85) = 224 psf

Shear: 628 psf (0.8)(0.75)= 377 psf

Both > 132 psf OK! Use 40/20 sheathing

Roof Sheathing

Span ratings assume panel edge support

Support intended to limit differential movement between adjacent panels

Unsupported edge – may require thicker panel or closer joist spacing

Lumber blocking – cut and fitted between roof joists

Tongue and groove edges

Panel clips – metal H-shaped clips placed between plywood edges

Floor Sheathing

Floor systems may use two layers of panels or a

single layer

Subfloor – bottom layer in a two-layer system (the

basic structural sheathing material)

Underlayment – the top layer in a two-layer system

Combined subfloor-underlayment – a single layer

system (usually with nontextile flooring)

A finish floor such as tile, hardwood, carpeting

usually provided

Underlayment

Purpose is to provide solid surface for application

of floor finish

Grade: Underlayment

Typical thickness ¼ in. for remodeling and use over

panel subfloor and 3/8 to ½ in. for use over

lumber subfloor or new construction

When finish floor has some structural capacity (e.g.

wood strip flooring, lightweight concrete)

underlayment is not required

Subfloor

Panels must be used in strong direction and continuous over two or more spans

Differential movement at panel edges must be limited by:

T&G edges

Blocking

¼ in. underlayment with panel edges offset over subfloor

Finish floor of ¾ in. wood strips

1-1/2 in. of lightweight concrete

Single-Layer Floor Systems

Performance rated panels known as Sturd-I-Floor

Include plywood, composite, and non-veneer panels

Floor Sheathing Example

APA-rated Plywood Sturd-I-Floor is considered for

an office building subfloor. The floor utilizes a two-

layer floor system with a separate subfloor and

underlayment. A ¼ in. plywood UNDERLAYMENT-

grade panel is used over the subfloor; its joints are

staggered with respect to the joints in the subfloor

(no special edge support is required). Assume

strength axis perpendicular to supports, 3-span, 4-

ply, normal temperature, but MC at 16% or more.

Floor Sheathing Example, cont’d.

Dead load 12 psf, distributed (moveable)partition

load of 15 psf, floor live load of 50 psf. (L from

occupancy)

Floor framing members spaced at 16 in. o.c.

Floor system must satisfy live load deflection limit of

L/360 and total deflection limit of L/240.

1.2 D + 1.6 L = 1.2 (12 psf) + 1.6 (65 psf) = 118 psf

D + L = 12 psf + 65 psf = 77 psf

Floor Sheathing Example, cont’d.

Floor Sheathing

Example, cont’d.

Floor Sheathing Example, cont’d.

For Deflection, modify by

CM only

270(0.85)=230 > 65 LL

And

405(0.85)=344 > 77 TL

OK!

Bending: KF (213 psf) = 2.54 (213 psf) = 541 psf

Shear: KF (338 psf) = 2.88 (338 psf) = 973 psf

Floor Sheathing Example, cont’d.

= 0.8 for 1.2 D + 1.6 L (occupancy)

b = 0.85, v = 0.75, CM = 0.75

Bending: 541 psf (0.8)(0.85)(0.75) = 276 psf

Shear: 973 psf (0.8)(0.75)(0.75) = 438 psf

Both > 118 psf

OK! USE 16 oc APA-rated Sturd-I-Floor

Floor Example

Bending: 541 psf (0.8)(0.85)(0.75)(0.8) = 221 psf

Shear: 973 psf (0.8)(0.75)(0.75)(1.2) = 527 psf

For Deflection,

270(0.85)(0.53) =122 > 65 LL

405(0.85)(0.53)=182 > 77 TL

OK!

What if we have a 1-span condition?

> 118 psf

OK! USE 16 oc APA-rated Sturd-I-Floor

Wall Sheathing

Two basic uses:

Structural only: as sheathing to distribute normal

wind forces to the studs or function as basic shear-

resisting elements (shearwalls)

Combined sheathing-siding

Wall Sheathing

Sizing for sheathing-type loads (wind loads normal

to panels) – same procedure as for floor and roof

sheathing

Design as shear walls – later topic!