Experimental analysis on strength and failure modes of wood beam-column connections

RESEARCH ARTICLE

Experimental analysis on strength and failure modes ofwood beam-column connections

Zhenhua HUANG*, Sheldon Q SHI, Liping CAI

Deportment of Engineering Technology, University of North Texas, Denton, TX 76210, USA*Corresponding author. E-mail: [email protected]

© Higher Education Press and Springer-Verlag Berlin Heidelberg 2014

ABSTRACT This research experimentally examined the strength, failure modes, and behaviors of dowel-bearing andfiber-bearing wood beam-column connections and explored the effects of cyclic loading on the strength, failure modes,and behaviors of those connections. Base on limited numbers of exploratory laboratory tests (6 preliminary tests in total),the authors observed that the typical bolted connection (dowel-bearing type wood beam-column connection with fiber-bearing surfaces) showed good behavior (large peak moment) under the monotonic loads, and the tenon joint connection(fiber-bearing wood beam-column connection) showed good behavior under cyclic loads. The cyclic property of loadingreduced the strength of the dowel-bearing type wood beam-column connections, but increased the strength of fiber-bearing type wood beam-column connections. More importantly, the authors identified a possible location of safetyconcern in current national design specifications (NDS) standards for the typical bolted connection (dowel-bearingconnection with fiber bearing surface) under cyclic loading because the tested value was smaller than the NDS calculatedvalue. But, because of the small amount of tests conducted, no final conclusion can be drawn based on those preliminaryobservations yet. A large number of repetitive laboratory tests should be conducted.

KEYWORDS wood, connection, dowel-bearing, fiber-bearing

1 Introduction

The wood beam-column frame is a commonly usedstructural system for residential buildings in the UnitedStates, and was the most popular building structure systemin ancient Asia, such as in ancient China, Korea, Japan, etc.The typical wood beam-column frame is an assembly ofwood beams and columns connected by nails, bolts, metalstraps, or proprietary connectors, categorized as dowel-bearing connections. Some wood connections do not needa connector, such as tenon and mortise connection. Thetenon joint connections are popular in furniture applicationin the US and in building construction in ancient Asia. Theconnection details for ancient Asia’s wood connections,such as types, dimensions, shapes, and functions, weresummarized by Li [1] and Ma [2]. The tenon jointconnection is categorized as wood fiber-bearing connec-tions.

A great number of studies regarding the wood beam-column frames and wood connections have been carriedout recently. Song and Lam [3] explored the stabilitycapacity and lateral bracing force of wood beam-columnframes subjected to biaxial eccentric compression loading.By taking the nonlinear parallel-to-wood-grain stress–strain relationship, size and stress distribution effects ofwood strength, shear deformation, and the P-Delta effect ofcompression load into account, a numerical analysis modelbased on the column deflection curve method wasdeveloped. The model was verified by the biaxial eccentriccompression tests of wood beam-column frames. Xinget al. [4] investigated the dynamic properties of conven-tional beam-column timber frame structure under succes-sive damage through a full-scale, three-story conventionaltimber structure. A three-dimensional finite element modelwas used for predicting its natural frequency. Both thesimulated and the experimental results showed a similartrend in most test cases. Damage sensitivity, as well as theinfluence of temperature and humidity on the naturalArticle history: Received Mar. 30, 2014; Accepted May. 18, 2014

Front. Struct. Civ. Eng. 2014, 8(3): 260–269DOI 10.1007/s11709-014-0261-y

frequency was also examined. The results from this studycould benefit the research of structural health monitoring.O’Loinsigh et al. [5] examined the performance of full-scale multi-layered timber beams with composite actionachieved with welded-through wood dowel connections.Different multi-layer beam designs, where the timberlayers were interconnected with welded wood dowelsproviding interlayer shear resistance, were tested inbending with different dowel densities. The resultsdemonstrated that the multilayered timber sections bywood dowels were structurally efficient and do not requirenon-wood based joining agents such as nails or adhesive.Using densified veneer wood (DVW) reinforcement andexpanded tube connectors, a great moment transferringcapacity of timber connections can be achieved. Leijtenand Brandon [6] proved that, when certain conditions werefulfilled, two connections in series had the same rotationalstiffness as one while the bending moment capacityincreases. The rotational stiffness of two DVW reinforcedconnections joined in series by a steel plate in a splice andcolumn-beam type of connection. The test results con-firmed the isotropic properties of this connection type.This study examines experimentally the strength, failure

modes, and behaviors of the commonly used dowel-bearing and fiber-bearing wood beam-column connectionsand explores the effects of cyclic loadings. As aninternationally and widely accepted design manual forthe wood structures, national design specifications forwood construction (NDS) [7] describes procedurescalculating the reference and adjusted force/momentstrength values and predicting the failure modes of bothtypes of wood connections (dowel-bearing and fiber-bearing connections). Through limited number of explora-tory laboratory tests (6 preliminary tests in total) on thedowel-bearing and fiber-bearing wood beam-columnconnections and comparison of the preliminary test resultswith the NDS calculated values and predicted failuremodes, the researchers of this study try to either validate

the safety conservativity and/or identify possible safetyconcerns for wood connections in current standards.

2 Wood beam-column connection tests

2.1 Test specimen

Monotonic and cyclic tests on three wood beam-columnconnection configurations were conducted: 1) typicalbolted wood beam-column connections (Fig. 1), 2)simplified bolted wood beam-column connections (Fig.2), and 3) the tenon joint wood beam-column connections(Fig. 3). The configurations 1) and 2) are the dowel-bearing type connections, and the configuration 3) is afiber-bearing type connection. Two assemblies were builtfor each configuration, one for the monotonic pushover testand the other for the cyclic test. Each assembly used twoNo. 2 kiln dried 38 mm � 234 mm (2″ � 10″) southernpine lumbers as beam members and one No. 2 air-dried140 mm� 140 mm (6″� 6″) cypress timber as the columnmember.The typical bolted wood beam-column connection

(configuration 1) has been widely used in wooden deckconstruction in the United States. The proceduresdescribed in the DCS [8] were used to build the assemblies.Two 12.7 mm (1/2 in) diameter through bolts were used asthe connectors. The pair of bolts was placed at 114 mm (4-1/2 in) apart aligned with the centerline of the post. A 140mm by 234 mm (6″� 10″) notch was cut in the post so thatthe beam can sit flush with the 140 mm � 140 mm (6″ �6″) post with enough fiber-bearing strength to help supportthe load.For the simplified bolted wood beam-column connec-

tion (configuration 2), instead of notching the post, thebeam was set flush on the post, and the 38 mm � 234 mm(2″ � 10″) beams were attached to the side surfaces of the140 mm � 140 mm (6″ � 6″) post. Two 12.7 mm (1/2 in)

Fig. 1 Typical wood beam-column bolted connection (2 � 10 = 50.8 mm � 254 mm, 6 � 6 = 152.4 mm � 152.4 mm, 2.5 = 63.5 mm,1/2” = 12.7 mm)

Zhenhua HUANG et al. Experimental analysis on strength and failure modes of wood beam-column connections 261

diameter and 127 mm (5 in) long lag bolts were used oneach side. The loads from the beam were carried only bythe shear strength of the lag bolts.The tenon joint wood beam-column connection (con-

figuration 3) consisted of a male part, the tenon, in thebeam and a female part, the mortise, in the column. Thespecimen assembly for the tenon joint connection in thisstudy was a simplified version of Ma [2]. It should be notedthat the fabrication procedure of the ten on joint connectionis much more complicated and takes much longer timethan the bolted connections.

2.2 Test setup

All the 6 preliminary laboratory experiments wereconducted on a 2.5 m (8′2″) � 2.5 m (8′2″) adaptablesteel testing frame. Four W16 � 67 hot-rolled steelmembers (406 mm high and 100 kg/m weight) were usedfor the frame beams and columns. The testing frame wasequipped with a 155.7 kN (35 kips) hydraulic actuator witha �127 mm (5 in) stroke. An 89.0 kN (20 kips) universalcompression-tension load cell was attached to the actuatorfor the force measurement. A steel rod at the bottom of the

load cell was used to prevent the out-of-plane displacementof the test sample, and a pin connection at the top of thesteel rod released the bending moment from the arm of thehydraulic actuator.Figure 4 shows the test setups for (a) a monotonic test

and (b) a cyclic test. In Fig. 4(a), a supporting bar made bya 25.4 � 25.4 � 2.85 mm (1 � 1 � 1/8 in) hollowed steeltube was bolted to the testing frame to help control thehorizontal and vertical displacements of the column. Four12.7 mm (1/2 in) diameter anchor bolts with standard cutwashers were used to attach the test sample to the testingframe and the supporting bar. In Fig. 4(b), a supportingtruss instead of the supporting bar and two SimpsonStrong-Tie S/HD10S hold-downs instead of four boltswere used to hold the column in position, controlling theuplifting displacement during the cyclic tests. The loadingsof the tests were applied vertically on the top of the beams.Two 32 mm (1-1/4 in) inner diameter washers were used

at each loading contact point to better distribute the forceon the surface of the wood beam, and a 229 mm (9 in)position transducer was employed to measure the verticaldisplacement of the loading point on the beam (Fig. 5). Thetransducer was mounted on the steel testing frame. The

Fig. 2 Simplified wood beam-column bolted connection (2� 10 = 50.8 mm� 254 mm, 6� 6 = 152.4 mm� 152.4 mm, 2.5 = 63.5 mm,1/2″ = 12.7 mm, 5″ = 127 mm, 2″ = 50.8 mm)

Fig. 3 Wood tenon joint connection (2� 10 = 50.8 mm� 254 mm, 6� 6 = 152.4 cm� 152.4 mm, 4.5 = 114.3 mm, 4.75 = 120.65 mm,2.25 = 57.15 mm)

262 Front. Struct. Civ. Eng. 2014, 8(3): 260–269

applied force and the vertical displacement were measuredand recorded simultaneously during the test.

2.3 Test protocol

Both the 3 monotonic and the 3 cyclic tests were conductedunder the displacement control procedure. The loading ratefor the monotonic pushover tests was 6.35 mm/min (2.5 in/min) until failure. The CUREE (Consortium of Univer-sities for Research in Earthquake Engineering) protocol, in

accordance with the Method C in ASTM E2126 [9], waschosen for the cyclic tests. A constant cycling frequency of0.2 Hz in the CUREE loading history was used for all thecyclic tests in this study. The CUREE basic loading historyincluding 40 cycles is shown in Fig. 6. Because of thevertical position offset in setting up the test samples, threedifferent specified displacement amplitudes were used inthis study: 75 mm (2.95 in) for the typical boltedconnection, 76 mm (3 in) for the simplified boltedconnection, and 66 mm (2.60 in) for the tenon jointconnection. Table 1 summarizes the test matrix.

3 Dowel bearing and fiber bearingconnections in NDS

The adjusted strength values and failure modes of thetested wood beam-column connections were estimatedusing the equations in NDS [7]. The yield limit (Z)equations were used to calculate the reference bearingforce values (by setting the reduction term Rd equal to 1.0)of each fastener for the dowel-bearing connections(configurations 1 and 2 in this study). Four yieldingmodes are defined in NDS [7]. Mode I is that the fastenercrushes the framing members (main or the side member)when the dowel-bearing stress passes the yield limit of theframing members. Mode I includes Im for crushing in mainmember and Is for crushing in side member. Mode II is that

Fig. 4 Test setups. (a) Monotonic test; (b) cyclic test (thesupporting truss was used because a support bar actually failedunder the counterclockwise moment during a cyclic test)

Fig. 5 Loading and position transducer details


the fastener rotates and crushes the outside surface of eachmember when the dowel-bearing capacity of both framingmembers (beam and column) goes beyond their yield limit.Mode III is that a plastic hinge forms in the fastener closeto the shear plane. The formation of the plastic hinge isfollowed by crushing in a frame member. Mode III alsoincludes IIIm for crushing in main member and IIIs forcrushing in side member. Mode IV is that two plastichinges form near the shear plane of the connection. Theyield limit equations for the yielding modes I through IVare listed as Eq. 11.3-3 through 11.3-6 in NDS [7].The adjusted bearing force value of the dowel-type

fastener is equal to the reference bearing force value (byyield limit equations) of the fastener multiplied byapplicable adjustment factors, such as the wet servicefactor, size factors, etc. Based on the adjusted bearing forcevalue of one single dowel-bearing fastener, equilibriumequations were then built using the free body diagrams inFig. 7(a) and (b) to calculate the adjusted bending momentvalue that the connection can carry. In Fig. 7(a) and (b), theforces P1 and P2 represent the adjusted bearing forces ofthe single dowel-type fastener calculated by the yield limitequations. When P1 and P2 for the typical US connection(Fig. 7a) were calculated, in order to obtain a more accurateestimation, the origin point was assumed at the maximum

wood bearing point. This assumption was based on theobservation on the tested specimen.For the tenon joint connection, forces P1 and P2 in

Fig. 7(c) were calculated differently because no fastenerwas used. Since fiber yielding on the bearing surfaces wasthe failure mechanism for this type of connection, thereference compression (perpendicular to grain) designvalue equation (NDS 2012 [7] Tables 4A through 4F) wereused in the calculation. The bearing area of 33.87 cm2

(based on the observation on the tested specimen)and theeffect of loading direction (the angle between the force andthe grain) were taken into account in this analysis.

4 Results and discussion

M-θ curves, the bending moment (M) vs. the angulardeformation (rotation) (θ) between the beam and columnmember, were used to show the test results of themonotonic and the cyclic pushover tests. The M-θrelationship has been widely utilized in commercialsoftware, such as in SAP2000, RISA, and DRAIN, todepict the properties of a connection. The bendingmoment, M, was calculated as the tested vertical loadmultiplied by the horizontal distance between the loadingpoint and the surface of the 140 mm � 140 mm (6″ � 6″)timber post. The vertical load applied on the top of thebeam was measured by the load cell attached to thehydraulic actuator. The rotation, θ, was defined as the ratioof the vertical deflection of the loading point over thehorizontal distance between the loading point and the postsurface. The vertical deflection at the loading point wasmeasured by the position transducer.Figure 8 presents the resulting M-θ curves for (a)

monotonic pushover tests and (b) cyclic pushover tests.Monotonic tests have commonly been used to simulate thecases of static gravity loading, such as dead loads, liveloads, snow, rain, and construction loads, etc., or tosimulate the uni-directional lateral loading cases, such aswind loads. The cyclic pushover tests were used tosimulate bi-directional dynamic loading cases, such asseismic load cases, and the bi-directional wind loads.The tested peak moments (shown in Fig. 8) and the

Table 1 Test matrix for wood connections

test label protocol connection configuration fastener type fastener spacing hold -down

1 monotonic typical 1/2″ � 7″ bolts w/washer 4 1/2″ none

2 monotonic simplified 1/2″ � 5″ lag bolts w/owasher 4 1/2″ none

3 monotonic tenon joint none - none

4 cyclic typical 1/2″ � 7″ bolts w/washer 4 1/2″ S/HD10S

5 cyclic simplified 1/2″ � 5″ lag bolts w/owasher 4 1/2″ S/HD10S

6 cyclic tenon joint none - S/HD10S

Note: 1/2″ = 12.7 mm, 4 1/2″ = 114.3 mm, 5″ = 127 mm, 7″ = 177.8 mm

Fig. 6 CUREE basic loading history (0.2 Hz)


adjusted bending moment design value (by NDS 2012 [7])are compared in Tables 2 through 5.

Table 2 shows a comparison of the tested peak momentsbetween the bolted connections and the tenon joint

Fig. 7 Free body diagrams for equilibrium equations. (a) Typical US; (b) simplified US; (c) tenon (18″ = 457.2 mm, 2″ = 50.8 mm,3.625″ = 92.075 mm)

Fig. 8 M-θ curves of wood beam-column connections. (a) Monotonic pushover tests; (b) cyclic pushover tests (1 k-ft = 1.36 kN-m)

Table 2 Comparison of bolted connection with tenon connection (unit: k-ft)

test label tenon connectionbolted connection % difference

typical simplified typical simplified

monotonic test 2.019 2.563 1.517 26.9% – 24.9%

cyclic test (+M) 2.427 2.205 1.373 – 9.1% – 43.4%

cyclic test (-M) 2.256 1.221 1.373 – 45.9% – 39.1%

Note: 1 k-ft = 1.36 kN-m

Table 3 Comparison of tested peak moments

connection monotonic test (k-ft)cyclic test (k-ft) % difference

+M -M +M -M

typical 2.563 2.205 1.221 – 14.0% – 52.4%

simplified 1.517 1.373 1.373 – 9.5% – 9.5%

tenon joint 2.019 2.427 2.256 + 20.2% + 11.7%



connection for both monotonic and cyclic tests. Thepositive moment (M+) in the table represents the clock-wise direction moment. The percent differences in the tablewere calculated as bolted connection over the tenon joint

connection (%Diff ¼ bolted – tenon

tenon). It can be seen from

Table 2 that under the monotonic loading, the typicalbolted connection showed a higher peak moment than thatof the tenon joint connection, while the simplified boltedconnection showed a lower peak moment compared to thetenon joint connection. The possible reason is that thetypical bolted connection has a wood fiber bearing inaddition to the connection through the bolts. During themonotonic loading, because both fiber-bearing (betweenthe beam and the column) and dowel-bearing (between themembers and bolts) affect the strength of connection, thepeak moment was increased compared to the tenon jointconnection. Since the simplified bolted connection onlyrelied on the dowel-bearing strength between the bolts andthe wood members, its peak moment was lower comparedto the tenon joint connection. For the cyclic loading, thetenon joint connections showed larger peak moments thanthose of the bolted connections in both clockwise andcounterclockwise directions. The reason is that the tenonjoint connection relied on the fiber-bearing between thewood components, while the bolted connections dependedon the dowel-bearing between the wood components andthe metal bolts. The yielding on the fiber-bearing contactsurface was recoverable during the cyclic loading, but notfor the dowel-bearing contact surfaces.Table 3 shows a comparison of tested peak moments

between the cyclic tests and the monotonic tests. Same asin Table 2, the positive moment (M+) in Table 3represents the clockwise direction moment, and thepercent differences were calculated by %Diff ¼cyclic –monotonic

monotonic. As shown in Table 3, the bolted

connections (both typical and simplified) under cyclicloading gave lower peak moments compared to themonotonic loading with a reductionofaround10% formost cases (9.5% for simplified bolted connection ofboth positive and negative moments, 14% for the typicalbolted connection of the positive moment), except for thetypical bolted connection in the negative direction (about52%). However, for the tenon joint connection, the resultswere opposite. The cyclic loading showed higher peakmoments (in a range of 11.7%–20.2%) than that of themonotonic loading, because of the recoverable yielding onthe fiber-bearing contact surfaces as well. This indicatesthat tenon joint connections (fiber-bearing type connec-tions) perform better under bi-directional wind or earth-quake loading.Tables 4 and 5 show the comparisons between the tested

peak moments and adjusted bending moment values(calculated through NDS 2012 [7]). Tables 4 and 5compare the monotonic and cyclic test results, respec-

tively. The ratios (test

NDS) are also presented in Tables 4 and

5, which indicate “safety margins” for the connections. Forthe negative moment of the typical US connection in Table5, the ratio was smaller than 1.0, indicating that theconnection could not reach the NDS provided adjustedstrength value. This preliminary test result points out apossible safety concern here. But, conclusion cannot bedrawn based on a single test result. A large number ofrepetitive laboratory tests should be conducted to validatethe finding. This will be a future research topic for theresearch team.Figures 9–11show the comparisons of the tested

yielding modes with the NDS [7], predicted yieldingmodes for the bolted connections and the tenon jointconnections, respectively. It can be seen that the yieldingmodes predicted by NDS [7] were comparable with thetested results.

Table 4 Comparison of monotonic tested peak moments with NDS adjusted bending moment design value

connection NDS calculation (k-ft) monotonic test (k-ft) ratio

typical 1.768 2.563 1.45

simplified 0.963 1.517 1.58

tenon joint 0.896 2.019 2.25


Table 5 Comparison of cyclic tested peak moments with NDS adjusted bending moment design value

test labelNDS calculation (k-ft) cyclic test (k-ft) ratio

+M -M +M -M +M -M

typical 1.768 1.354 2.205 1.221 1.25 0.90

simplified 0.963 0.963 1.373 1.373 1.43 1.43

tenon joint 0.896 0.896 2.427 2.256 2.71 2.52



Fig. 9 Failure modes of typical bolted connections

Fig. 10 Failure modes of simplified bolted connections


5 Conclusions

Six full-scale preliminary monotonic and cyclic pushovertests were conducted on three commonly used wood beam-column connections: typical bolted wood connection,simplified bolted wood connection, and tenon jointconnection. The tested results were compared withadjusted strength values and predicted yielding modesfrom NDS [7]. The following preliminary observationswere drawn based on the limited test and analysis data:1) Among the three tested beam column connection

configurations, the typical bolted connection (dowel-bearing type connection with fiber bearing surfaces)behaved best (largest tested peak moment) under mono-tonic loads, while the tenon joint connection (fiber-bearingtype connection) behaved best under cyclic loads.2) The dowel-bearing type connections had lower tested

peak moment under cyclic loadings than that undermonotonic loading, and the fiber-bearing type connectionhad higher tested peak moment under cyclic loadings thanthat under monotonic loading. This observation indicatedthat the fiber-bearing type connection performed better forthe structures under the bi-directional wind or earthquakeattack.3) Most of the tested peak moments were larger than the

adjusted moment strength values calculated by NDS [7]with an exception of the typical bolted connection (dowel-bearing connection with fiber bearing surface) under cyclicloading. This exception pointed out a possible location ofsafety concern in current standards. Large numbers of tests

are needed to validate the safety concern and propose asolution.4) NDS [7] accurately predicted the failure (yielding)

modes for both the dowel bearing and wood fiber bearingtype of connections,All the above preliminary observations cannot result in

any proposable conclusion yet because of the smallnumber of tests. A large number of repetitive laboratorytests should be conducted to validate the findings.

References

1. Li J. Ying Zao Fa Shi. Royal Press, Kaifeng, China (Song Dynasty),

1100 (in Chinese)

2. Ma B J. Construction Technology of Chinese Ancient Timber

Architectures. 2nd ed. Beijing: Science Press, 2003 (in Chinese)

3. Song X, Lam F. Laterally braced wood beam-columns subjected to

biaxial eccentric loading. Computers & Structures, 2009, 87(17–18):

1058–1066

4. Xing H, Xue S, Zong G. Dynamic properties of conventional beam-

column timber structure under successive damage. Journal of Asian

Architecture and Building Engineering, 2011, 202(1): 195–202

5. O’Loinsigh C, Oudjene M, Ait-Aider H, Fanning P, Pizzi A, Shotton

E, Meghlat E M. Experimental study of timber-to-timber composite

beam using welded-through wood dowels. Construction & Building

Materials, 2012, 36: 245–250

6. Leijten A J M, Brandon D. Advances in moment transferring dvw

reinforced timber connections- Analysis and experimental verifica-

tion, Part 1. Construction & Building Materials, 2013, 43: 614–

Fig. 11 Failure modes of tenon joint connections


622

7. NDS. National Design Specification for Wood Construction ASD/

LRFD. American Forest & Paper Association, Washington, DC, 2012

8. DCS. Deck Construction Specifications of the Building Department

of the City of Lancaster, Lancaster, OH, 2004

9. ASTM E2126. Standard Test Methods for Cyclic (Reversed) Load

Test for Shear Resistance of Walls for Buildings, American Society

for Testing and Materials, West Conshohocken, PA, 2007


Experimental analysis on strength and failure modes of wood beam-column connections

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