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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