ce 479: design of building components and systems fall …jliu/courses/ce479/... · 2012-10-09 ·...
TRANSCRIPT
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CE 479: DESIGN OF BUILDING
COMPONENTS AND SYSTEMS
FALL 2012– J. LIU
Wood Structural Panels
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Wood Structural Panels
Plywood
Oriented Strand Board (OSB)
Composite Panels
Generally, wood veneer and reconstituted wood-based
material bonded
Particleboard
Medium density fiberboard
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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.
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Uses
Roof, floor and wall sheathing
Horizontal and vertical (shearwall) diaphragms
Structural components
Prefab. I-joists
Sandwich panels
Gusset plates
Concrete formwork
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Sheathing
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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
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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
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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)
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Plywood
Plywood panel made up
of a number of veneers
(thin sheets of wood)
Veneer obtained by
rotating “peeler logs” in
a lathe
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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
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3 ply, 3 layer panel
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4 ply, 3 layer panel
Extension of simple 3-ply construction
Two center plies have grain running in same direction
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5 ply, 5 layer panel
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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
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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
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Design Values / Load Tables
2005
ASD/LRFD
Manual
For Engineered
Wood
Construction
We’ll use this:
Load-Span Tables for APA
Structural-Use Panels
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Transformed sections (used to develop load tables)
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Strong Orientation
Weak Orientation
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Species Groups for Plywood
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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
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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)
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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
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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
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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)
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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
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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
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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
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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)
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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
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Nonveneer (OSB) Grade Stamp
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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
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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
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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
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Roof Sheathing Example, cont’d.
Note: Tables are ASD!
Assumes 3 spans Assumes 2 spans
(No span adjustment needed)
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Roof Sheathing Example, cont’d.
Note: Tables are ASD!
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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)
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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)
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Roof Sheathing Example, cont’d.
Bending: KF (130psf) =
2.54 (130 psf) = 330 psf
Shear: KF (218 psf) =
2.88 (218 psf) = 628 psf
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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
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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
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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
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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
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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
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Single-Layer Floor Systems
Performance rated panels known as Sturd-I-Floor
Include plywood, composite, and non-veneer panels
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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.
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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
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Floor Sheathing Example, cont’d.
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Floor Sheathing
Example, cont’d.
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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
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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
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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
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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
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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!