wood fasteners bse 2294 animal structures and environment dr. susan wood gay
TRANSCRIPT
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Wood Fasteners
BSE 2294
Animal Structures and Environment
Dr. Susan Wood Gay
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A primary objective in building wood structures is to design joints that are as strong as the members to be joined.
• Many wood building failures due to inadequate joints
– Design– Assembly
• Many farm buildings still stand without a single nail in them
– Mortise and tenon– Wooden pegs
Example of mortise and tenon joinery.
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Mechanical connectors for wood structures are a relatively new technology.
• Early 1800s – cut iron nails
• 1875 – steel wire nails
• 1930 – metal timber connectors
Cut nails from Shawnee Village – an early settlement in Ohio (1805-1808).
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Mechanical connectors allow the load to be transferred from one main member to another.
• Nails
• Screws
• Bolts
• Metal plate connectors
Wood screws.
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Common wire nails are used for framing where there will be considerable lateral load.
Common
Spike
Box
Finishing
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Nail holding capacity depends on numerous factors.
• Direction of nail loading
• Orientation of nail to grain
• Depth of nail penetration
• Diameter of nail
• Species of wood
• Nail Coating
• Nail type Examples of basic nail types.
Common
Spike
Box
Finishing
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Spikes parallel the length of a number of the sizes of common nails.
• Larger in diameter than common nails
• Greater holding power than common nails
• Must be used carefully to avoid splitting
Spike nail – 0.092” - 0.283” in diameter. (10d - 60d) or 3” – 6” in length
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Box nails are used for installing sheathing or roof decks.
• Smaller in diameter than common nails
• Lesser holding power than common nails
• Reduced danger of splitting
Box nail – 0.067” – 0.162” in diameter.(2d – 40d) or 1” – 5” in length
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Casing and finishing nails are used where a neat appearance is desired.
• Smaller in diameter than common nails
• Smaller head than common nails
• Can be countersunk and covered
Finishing nail – 0.058” – 0.135” in diameter.(2d -20d) or 1” – 4” in length.
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The shape of the point affects the tendency of the wood to split when a nail is used close to an end of edge.
• Diamond point is most widely used
• A sharper point increases:
– Holding power– Ease of driving nail– Tendency to split wood
Examples of common nail points.
Diamond
Blunt - diamond
Long diamond
Conical
Duckbill
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The type of nail shank affects the resistance of the nail to withdraw from the wood.
• Nail shanks types:
– Smooth– Spiral (helical) – Ringed (annular)
• Spiral and ringed nails have greater withdrawal resistances
Examples of smooth (top), spiral (middle), and ringed nails.
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Nail finish especially affects holding power and rusting.
• Electrogalvanized – average holding power; tends to rust
• Hot-dipped galvanized – zinc-coated to reduce corrosion
• Cement-coated – dipped in resin to increase holding power
• Blued nails – free of residues and improved appearance
Galvanized nails – 1 ¼ inches in diameter.
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Nails are sized in pennyweight (d), which is related to their length.
Penny Size Length (in)
4d 1 ½
8d 2 ½
10d 3
12d 3 ¼
16d 3 ½
20d 4
60d 6
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Slant driving or toe-nailing is superior in strength to straight driving.
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Clinching nails across the grain increases withdrawal strength by 170%.
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Withdrawal load from end grain is approximately zero.
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Proper nail spacing increases the strength of nailed joint.
Nail location Spacing (in)
From edge, member in compression 1½
From edge, member in tension 2¼
Between nails, perpendicular to grain 1
Between nails, parallel to grain 2½
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Nail Load Example
Determine the load that can be supported at the midspan of a
10-ft long, southern pine (species group 2) 2 x 6 if it is nailed at
both ends to 2 x 4’s with two, 10d nails. Also determine the
quality of the lumber to carry the load.
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Nail Lateral Loads
• Lateral Load (Pn) = KD3/2
– Pn = safe load in pounds per nail (assuming that the point penetrates ½ of it’s length
into the second member for hardwoods and 2/3 of it’s length into the second member for softwoods)
– K = a constant depending on the type of wood
– D = diameter of the nail in inches
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1. Find the load per nail and the nail penetration (Table 7).
Load per nail = 94 lb/nail
Nail penetration = 1.5 in
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2. Determine the load at the left end.
Load at left end = Load per nail x Number of nails
Load at left end = 94 lb/nail x 2 nails = 188 lb
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3. Determine the total nail load for the whole board.
Total load = 188 lb x 2 = 376 lbs
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4. Determine the bending moment of the board.
M = PL 4
M = 376 lb x 120 in = 11280 in-lb 4
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5. Determine the bending stress of the board.
Fb = 6M bh2
Fb = (6)(11280 in-lb) = 1492 psi (1.5 in)(5.5 in)2
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6. Find quality of lumber from the Southern Pine Use Guide.
Use No. 1, non-dense, Southern pine.
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Screws are unique in their particular resistance to withdrawal loads, compared to nails of equal diameter.
• Used where vibrations are common
– Floor sheathing to joists– Gypsum board to support
members
• Used to bring members into alignment
• Used for millwork and finishing rather than for structural framing
Example of double lead (top), single lead (middle), and tapping wood screws.
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Bolted joints may be used when loads on wood connections are particularly heavy.
• Greater lateral load capacities than nails
• No withdrawal load ratings
• Predrill holes from 1/32 to 1/16 larger than bolt diameter
Examples of bolts used in wood connections.
Machine
Carriage
Lag
Stove
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• Applications:
– Metal-to-metal– Wood-to-metal
• Washers between:
– Nut and wood surface– Nut and the bolt head
Machine bolts are precision made and generally applied where close tolerance is desirable.
Example of a square-headed machine bolt.
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• Applications:
– Wood-to-wood– Wood-to-metal
• Pre-bored holes
• Washers between:
– Nut and wood surface– Nut and the bolt head
Carriage bolts are not designed to be driven and are used where the bolt head is inaccessible.
Example of a round-headed carriage bolt with a square neck.
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• Applications:
– Wood-to-wood– Wood-to-metal
• Threaded along the length of the shank
• Washers between:
– Nut and wood surface– Nut and the bolt head
Stove bolts are less precisely made than machine bolts.
Example of a stove bolt with nut.
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• Applications:
– Wood-to-wood– Wood-to-metal
• Sharp points and coarse threads designed to penetrate and grip wood fiber
• Washers between:
– Head and wood surface
Lag bolts or screws are used where main member is too thick to be penetrated by machine bolts.
Example of a lag bolt or screw.
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• Used to fasten:
– Plywood sheeting– Subflooring materials
• Manufactured from 14- to 16-gauge wire
Heavy- duty staples are often used in place of nails.
Pneumatic staple gun.
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• Joists hangers
• Truss hangers
• Truss plates
Framing connectors are often used to connect wood members at critical locations.
Truss plates are an example of framing connectors.
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• Made from sheet metal:
– Light gauge– Galvanized
• Affixed to wood members with special nails
• Best method to:
– Affix joists to headers– Affix trusses to girders
Joists and truss hangers are manufactured to connect joists and trusses to supporting wood members.
Example of a single joist hanger.
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• Made from sheet metal
– Light gauge – Galvanized
• “Teeth” protrude from one side
• Connections
– Chord to chord– Chord to web
Truss plates are used to connect chords and webs to one another.
Truss plates connecting web members to chords.
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• Used for:
– Trusses– Beams– Posts
• Glue must be applied under controlled conditions
• Types
– Casein– Synthetic adhesives
Glued joints between wood members can be particularly strong and rigid
Example of wood glue.
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• Made with mold inhibitors
• Highly moisture resistant
• Fills well in imperfectly fitting joints
• Workable to 40 °F
• Best bond for naturally oily woods
Casein glue is made from the milk protein.
Source of casein.
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• Resorcinol
– Water-proof– Used for high moisture
levels– Expensive– Short working time
• Urea-formaldehyde
– Moisture resistant– Light-colored glue line– Long working time
Synthetic adhesives include resorcinol and urea-formaldehyde.
Cans of resorcinol glue.