Tensile Capacity of Timber-Frame Mortise and Tenon Connections

Tensile Capacity of Timber-Frame Mortise and Tenon Connections
Carson R. Walker
Research Assistant
Department of Civil and Environmental Engineering, Brigham Young University
Provo, Utah, U.S.A.
Fernando S. Fonseca, Ph.D., P.E.
Associate Professor
Department of Civil and Environmental Engineering, Brigham Young University
Provo, Utah, U.S.A.
Johnn P. Judd, P.E., and Paul R. Thorley, P.E.
Engineer, and Principal
Acute Engineering
Orem, Utah, U.S.A.
Summary
This paper discusses the results of full-scale tensile testing of varied-angled mortise and tenon
connections. Twelve full-size mortise and tenon connection specimens were tested: four specimens
used a 90_ connection, four specimens used a 67.5_ connection, and four specimens used a 45_
connection. Each connection consisted of an 184×292 mm “blind” tenon inserted 102 mm into an
184×184 mm timber mortise, connected with two 25 mm diameter wood peg dowels.
1. Introduction
Timber-frame mortise and tenon connections are common in both traditional timber-frame
structures and in modern structures that incorporate timber frames (Fig. 1). Timber frames use
heavy timber beams and columns joined together using carpenter-style connections, such as a
mortise and tenon connection. In a mortise and tenon connection, a projecting member (tenon) is
slid into a slotted hole in the receiving member (mortise) and wood pegs are inserted through both
the mortise and the tenon. A “blind” tenon connection is created when the tenon does not penetrate
the opposite face of the mortise. Although nailed and bolted connections have replaced mortise and
tenon connections in ordinary construction, structures are increasingly incorporating timber frames
for energy efficiency, aesthetic, and other architectural reasons.
Peg
Tenon
Mortise
Fig. 1 Timber frame mortise and tenon connection.
Previous research of timber-frame mortise and tenon connections has been relatively limited
compared to efforts in other areas of wood engineering. Church and Tew [1] studied the effects of
various parameters, such as peg diameter and wood grain orientation, on the bearing strength of
mortise and tenon connections. In another study, Bulleit et al. [2] and Sandberg et al. [3]
demonstrated the importance of a tightly-fit joint on connection capacity.
Schmidt and Mackay [4] reported that full-scale tensile testing of mortise and tenon connections
exhibited mortise splitting, tenon plug shear, and peg bending and shear. Later, Schmidt and
Daniels [5] determined the yield, dowel bearing, and shear strengths of wood pegs and also tested
full-size mortise and tenon connections. They concluded that a peg failure mode was more ductile
compared to other failure modes. In a related study, Schmidt and Scholl [6] determined the effects
of seasoning and the applicability of load duration factors. They indicated that tensile capacity of
the mortise and tenon connection did not decrease over time. Burnett et al. [7] tested full-scale
mortise and tenon connections to determine the effect of end distances. Their study concluded that
although end distance did not significantly effect connection stiffness, end distance may
significantly reduce connection capacity and ductility.
Current research has focused on providing a basis for code-recognized design provisions [8].
Schmidt and Miller [9] tested full-size mortise and tenon connections, developed a finite element
model, and proposed an equation for the design of connections in tensile loading. This study and
previous research have culminated in the Standard for Design of Timber Frame Structures,
published by the Timber Frame Engineering Council [10].
This study is part of a research effort to improve timber frame engineering in conjunction with
industry and design professionals [11]. The objective of this study is to determine the tensile
capacity of mortise and tenon connections with varied angled connections and a blind tenon. To
accomplish the objective, twelve full-size mortise and tenon connection specimens (four 90_
specimens, four 67.5_ specimens, and four 45_ specimens) were tested. Connections used an
184×292 mm blind tenon inserted 102 mm into an 184×184 mm timber mortise, connected with
two 25 mm diameter wood peg dowels. A full description of the study is available [12].
2. Experimental Approach
Full-size mortise and tenon connection specimens (Fig. 2) were constructed by a local timber-frame
manufacturer using a Hundegger K1 milling machine. Each specimen used an 184×292 mm blind
tenon and an 184×184 mortise of Douglas-Fir lumber, with two 25 mm diameter pegs of White
Oak hardwood. The tenon measured 38.1 mm wide by 102 mm deep. For manufacturing
purposes, the tenon length varied according to the connection angle. The distance between the pegs
was 165 mm for 90_ and 45_ connections, and 140 mm for 67.5_ connections.
45_ connections
67.5_ connections
90_ connections
Fig. 2 Mortise and tenon connection specimens
Tenon
Pegs
Mortise
Fig. 3 Test set-up: 45_ connection specimen
Tensile capacity of specimens was determined using a monotonic load test set-up (Fig. 3). The
tenon remained vertical (aligned with the direction of applied tensile load) while the mortise varied
from horizontal to sloped, depending on the connection angle. A 9.53 mm thick steel cap was
bolted to the top of the tenon and secured with six 19.1 mm diameter bolts. A 25.4 mm diameter
tension rod was welded to the top of the cap and threaded through the top cross brace of the
Baldwin testing machine; two tension rods secured the mortise ends to a HSS section bolted to the
bottom cross brace (the distance between the bottom rods was 914 mm). The displacement-control
loading rate was 2.54 mm/min. until failure.
Displacement of the connection was measured by the Baldwin testing machine. Mortise splitting
(gap) displacement was measured using two wire rope actuated linear position and linear velocity
transducers (string pots) located on each side of the mortise between the pegs. One string pot was
attached at the bottom of the mortise and the other was attached above the pegs. In addition, two
string pots were attached to angled connection specimens to measure rotational displacement.
3. Results
The tensile testing results of full-size mortise and tenon connection specimens is summarized in
Table 1. Three primary failure modes (column 2) were observed during testing: mortise splitting,
tenon plug shear, and peg failure in bending and shear. Mortise splitting occurred as cracks formed,
or checks widened, at the peg holes (Fig. 4a). Theoretically, mortise splitting may be described as
a “cross-grain tension” failure occurring when mortise annual rings are orthogonal to the applied
tensile load.
Table 1. Test Results
Failure Modes
(specimens)
Connection
K
(kN/mm)
Fmax
(kN)
max
(mm)
fail
(mm)
(fail / max)
90_
Mortise splitting (2)
Tenon plug shear (2)
0.38
1.22
8.58
46.0
5.4
67.5_
Mortise splitting (3)
Peg failure (1)
0.27
1.07
8.37
44.5
5.3
45_
Mortise splitting (1)
Tenon plug shear (3)
Peg failure (3)
0.16
1.55
20.7
43.1
2.1
Tenon (after testing)
Splitting
Mortise (after testing)
a) Mortise splitting
b) Tenon plug shear
c) Peg failure (shear, bending)
Fig. 4 Observed failure modes
Tenon plug shear (shear failure of the tenon material behind the peg) occurred as one or both pegs
tore through the tenon (Fig. 4b). Theoretically, tenon plug shear may be described as row-tear out.
Peg failure occurred as one (or both) pegs sheared, bent, or a combination thereof (Fig. 4c). Peg
failure in shear is theoretically referred to as yield mode V in the Standard for Design of Timber
Frame Structures. Peg failure in bending may be described theoretically as a variant of National
Design Specification (NDS) for Wood Construction 2005 Edition [13] yield mode IIIs, where a
single plastic hinge is formed in the peg accompanied by crushing in the mortise.
Consistent with previous studies, the NDS yield modes Is (crushing of the mortise), Im (crushing of
the tenon), and IV (where two plastic hinges are formed in the peg, with crushing in the tenon and
mortise) were not observed.
Idealized load-displacement curve parameters are provided in columns 3 through 7, and a typical
load-displacement curve measured during testing is shown in Fig. 5. Initially, the response is linear
elastic, where a linear increase in displacement corresponds to a linear increase in load. The
connection stiffness, K (column 3) was determined using a linear fit of the load-displacement curve
up to 2 mm. A nonlinear load-displacement curve develops due to mortise, tenon, or peg failure.
The maximum load, Fmax (column 4), and corresponding displacement, max (column 5), correlate
with the primary failure mode, for the purpose of this study, and do not necessarily equate to the
ultimate load attained during testing or during subsequent failure modes.
Fmax , max
1.4
fail
Tensile load (kN)
1.2
1.0
0.8
0.6
K
0.4
0.2
0
0
10
20
30
40
50
60
70
80
Displacement (mm)
Fig. 5 Typical load-displacement curve (90_ connection specimen)
For example, mortise splitting was the primary failure mode of the specimen response shown in
Fig. 4. The primary failure was followed by a tenon plug shear failure with one peg, then a failure
with the other peg.
The deflection at failure, fail (column 6), represents the displacement reached when the specimen
no longer sustained significant load, or the displacement corresponded to a 30% decrease in load
capacity. The ductility of the connection is suggested using a displacement ductility, (column 7),
defined as the ratio of failure displacement to maximum displacement.
The most common primary failure mode was splitting of the mortise, caused by a “cross-grain
tension” failure occurring when mortise annual rings are orthogonal to applied load. The 90_
connections, for example, exhibited mortise splitting in the two specimens with orthogonal mortise
annual rings, and tenon or peg failure in the two specimens with parallel mortise annual rings.
Similarly, for the 67.5_ connections (with orthogonal mortise annual rings) the primary failure
mode was mortise splitting in three specimens, and a combination of mortise splitting and peg shear
in the fourth specimen. For the 45_ connections, two specimens had orthogonal mortise annual
rings: one exhibited mortise splitting, the other exhibited tenon and peg failure.
Connection stiffness decreased with smaller connection angles and did not appear to be a function
of the mortise annual ring orientation. The 90_ connections had an initial stiffness within a 6%
range, for example, even though two specimens had mortise grain perpendicular to the load and
two specimens had mortise grain parallel to the load.
Interestingly, compared to 90_ connections, the maximum load and displacement for the connection
decreased for 67.5_ connections and increased for 45_ connections. There was no significant
correlation between the mortise annual ring orientation and the maximum load. The failure
displacement generally decreased with smaller connection angles. Connection ductility decreased
with smaller connection angles and was related to the initial failure mode. During testing of the
90_ and 67.5_ connections, for example, where mortise splitting was frequently observed the
displacement ductility was approximately 5. By contrast, during testing of the 45_ connections,
where peg failure was usually observed, displacement ductility was reduced by more than 50%.
4. Conclusions
The tensile capacity of mortise and tenon connections was higher for 45_ connections and lower for
67.5_ connections, compared to 90_ connections. Mortise and tenon connections exhibited three
primary modes of failure: mortise splitting, tenon plug shear, and peg bending and shear. Splitting
of the mortise was most frequently observed, and occurred when mortise annual rings are
orthogonal to applied load. The primary failure mode did not significantly effect the tensile
capacity. Ductility, however, was reduced by more than 50% in mortise and tenon connections with
primary failure modes in peg bending and shear, compared to mortise splitting.
Acknowledgments
Euclid Timber Frames (Heber City, Utah) generously provided and manufactured the mortise and
tenon connections for this study. Additional support was provided by Acute Engineering, and the
Department of Civil and Environmental Engineering at Brigham Young University.
References
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[2]
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