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Investigation of Through-Tenon Keys on the Tensile Strength of Mortise and
Tenon Joints
Daniel Hindman, Lance Shields
• Introduction to Keyed Joints
• Significance
• Goal and Objectives
• Measurement of Joint Strength/Stiffness
• Conclusions
• Future Work
Through Tenon
Keys
Mortise Member
“Keyed through-tenon joints used notoriously as anchor beams between posts in Dutch barns to resist large tensile forces.” -Goldstein 1999
Anchor Beam Through-Tenon
Keyed Joint
King Post
Collar Tie
•Through-Tenon Keyed joints can be used to carry great tensile forces (Tension Joints often Adjacent to Compressed Braces)
• “No research on the behavior of wedged joints in timber frames is available." -TFEC 1-10 (Timber Frame Design Standard)
Compressed Knee Brace acting as Fulcrum
Adjacent Joint under Tension
Goal Examine behavior of keyed through-tenon joints and predict
the strength of joints for comparison to experimental values
Objectives 1) Measure Strength, Stiffness, and Behavior of Joints 2) Determine Effects of Species and Details on Joint Strength
and Stiffness
Cross-Head
Hold-Downs
LVDTs
Block Supports
Joint
Species
Tenon
Length
Number
of Keys
Number of
Specimens
Whi
te O
ak K
eys
White
Oak
4" 1 5
2 5
11" 1 5
2 5
Douglas-
fir
4" 1 5
2 5
11" 1 5
2 5
Ipe
Keys
White
Oak 11"
1 3
2 1
Douglas-
fir 11"
1 1
2 1
Total 46
Joint Species
Tenon Length
Number of Keys
Average Proportional
Limit Strength, lbs (COV,%)
Average 5% Offset Yield
Strength, lbs (COV,%)
Average Ultimate Strength,
lbs (COV,%)
Average Stiffness,
lbs/in (COV,%)
Whi
te O
ak K
eys White
Oak
4” 1 2,300 (21.1) N/A 4,700 (22.4) 76,100 (27.0) 2 5,820 (8.1) 12,500 (4.9) 12,100 (13.6) 163,000 (13.1)
11” 1 2,960 (11.6) 6,480 (8.5) 7,810 (10.1) 99,200 (11.6) 2 6,200 (26.2) 13,300 (12.0) 15,400 (6.4) 191,000 (7.5)
Douglas-fir
4” 1 2,670 (25.3) N/A 3,340 (27.4) 77,900 (28.5) 2 4,740 (16.9) N/A 7,370 (29.7) 127,000 (10.9)
11” 1 3,440 (20.6) 6,780 (1.6) 7,160 (8.9) 81,000 (11.4) 2 6,020 (30.0) 13,900 (12.3) 15,600 (8.0) 165,000 (13.5)
Ipe
Keys
White Oak 11”
1 6,000 (12.0) 14,900 (2.7) 15,300 (2.1) 150,000 (5.7) 2 9,300 19,600 20,800 189,000
Douglas-fir 11”
1 5,100 12,400 12,500 188,000 2 9,200 19,700 21,100 84,600
Brittle Failures: Occurred in Tenons of all Joints with 4” Tenons
Tenon Split Plane Shearing 'A' Relish 'B'
Full Relish
Key Bending and Crushing
(White Oak Keys)
Key Bending and Slight Crushing
(Ipe Keys )
Ductile Failures: Occurred in Keys of all, but four, Joints with 11” Tenons
Failure: decrease in joint strength by 20% with little to no sign of recovery or greatest load prior to a post-failure event Ultimate Load: Maximum Load prior to post-failure event, such as Key Wedging
Key Wedging: Post-failure event where both halves of a failed key become lodged in mortise and tenon shear plane interfaces, often causing a load-increase
Key Wedging Severed Key Mortise Split
Key Wedging (Post-failure Event)
White Oak > Douglas-fir • Moisture Content of Douglas-fir Joints were less
than Fiber Saturation Point, while White Oak Joints were greater than Fiber Saturation Point – This may explain the more brittle behavior of Douglas-fir Joints compared to White Oak Joints
• White Oak had greater parallel-to-grain shear strength than Douglas-fir
• White Oak Mortise Members were denser (greater SG) than Douglas-fir Mortise Members
11” Tenons > 4” Tenons • Joints with 11” Tenons permitted key strength utilization,
where joints with 4” Tenons failed prior to key strength utilization
2 Keys > 1 Key (Joint Strength and Stiffness Responses Normalized to Key Width)
• More Shear Planes and Greater Total Key Width
• After Normalizing Strength and Stiffness responses to Key Width, Synergy seemed to exist between Joint Strength/Stiffness and Number of Keys - May be explained by denser White Oak Keys in Joints with Two Keys
1) Tenon keyed joints show high strength –up to 15,000 lbs ultimate load – depending on configuration
2) Joint Comparisons 1) Higher SG increased strength and stiffness 2) Longer tenons changed failure mode 3) More shear planes increase strength and stiffness
3) Higher key SG (in the Ipe Keys) increased strength and
stiffness
4) Change in failures (4” Brittle vs. 11” Ductile) indicates that a ‘balanced’ failure is between these tenon lengths
1
• Models for strength prediction of Mortise and Tenon Members and Key Bearing used sections from the NDS
• Key Bending and Horizontal Shear were developed by the researcher using engineering mechanics and the TR-12
Joint Strength Concluded when: full key horizontal shear strength was matched by that created by transverse shear forces from bearing of mortise and tenon members
Joint Strength Concluded when: key bending strength, at center tenon thickness, was matched by key bending moment created by bearing forces of mortise and tenon members
Thanks to Bob Shortlidge and Dreaming Creek Timber Framers for providing materials and
advice
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