Strength of an Interlocking FRP 

Connection 

Dat Duthinh 

Kunwar Bajpai 

U.S. DEPARTMENT OF COMMERCE 

Tech no logy Ad m i n istrat ion 

Building and Fire Research Laboratory 

National Institute of Standards 

Gaithersburg, MD 20899 

and Technology 

NlST CENTENNIALII 

National Institute of Standards 

and Technology 

Technology Administration 

U.S. Department of Commerce 

Strength of an Interlocking FRP 

Connection 

Dat Duthinh 

Kunwar Bajpai 

U.S. DEPARTMENT OF COMMERCE 

Technology Ad mini strati on 

Building and Fire Research Laboratory 

National Institute of Standards 

Gaithersburg, MD 20899 

and Technology 

November 2001 

US. DEPARTMENT OF COMMERCE 

Donald L. Evans, Secretary 

TECHNOLOGY ADMINISTRATION 

Phillip J. Bond, Under Secretary 

of Commerce for Technology 

NATIONAL INSTITUTE OF STANDARDS 

AND TECHNOLOGY 

Karen H. Brown, Acting Director 

ABSTRACT 

The strength of an interlocking fiber-reinforced polymer (FRP) connection was measured in 

bending, and in-plane and out-of-plane shear. This connection allows the rapid assembly of FRP 

panels into houses, and can be used in interloclung mode only, or interlocking and adhesively 

bonded mode. An additional mode was tested, where the connection was adhesively bonded 

only, without an interlocking toggle. The complete characterization of the behavior of the 

connection allows rational prediction of the performance of a building assembled with such 

panels. 

Keywords: adhesive, bending, building technology, composite, connection, fiber-reinforced 

polymer, interlocking, joint, panel, shear strength. 

DISCLAIMER 

Certain trade names and company products are mentioned in the text or identified in an 

illustration in order to adequately specify the experimental procedure and equipment used. In no 

case does such an identification imply recommendation or endorsement by the National Institute 

of Standards and Technology, nor does it imply that the products are necessarily the best 

available for the purpose. 

... 

111 
Contents 

1 . 

2 . 

3 . 

4 . 

5 . 

6 . 

7 . 

8 . 

9 . 

Introduction ................................................................................................ 1 

Panel Tests ................................................................................................ 1 

Flexural Tests ............................................................................................. 2 

In-Plane Shear Tests ...................................................................................... 3 

Out-of-Plane Shear Tests .................................................................................. 4 

Uncertainty ................................................................................................. 5 

Conclusions .......................................................................... ,. ...................... 5 

Acknowledgments ........................................................................................ -6 

References .................................................................................................. 5 

iv 
Strength of an 

Interlocking FRP Connection 

1. Introduction 

FRP (fiber-reinforced polymer) structural members are increasingly used in civil engineering 

applications, such as in bridge decks and girders. One promising application is in residential 

housing, where FRP panels can be assembled quickly on site, without the need for additional 

framing. The speed of erection is particularly important for emergency and military shelters. 

Further advantages are the light weight of the material, the possibility of fabricating standardized 

panels and connections in the factory, thus potentially leading to lower cost and improved quality 

control. Furthermore, the shelter being designed according to engineering principles rather than 

tradition may lead to a greater resistance against natural disasters. 

This work evaluates the strength of a particular type of interlocking joint used to assemble 

double-skinned panels (Fig. 1). The panels can be used in walls, floors and roofs of shelters and 

are assembled by driving a toggle into the jaws of the end cells (Head and Churchman, 1989). 

Foam can be injected into the cells formed by the panel skins and stiffeners for insulation. For 

permanent structures, adhesive bonding can also be used in addition to mechanical interlocking. 

If no adhesive is used, disassembly and reuse are possible. 

Several interlocking composite joints are commercially available (Duthinh, 2000a). They all are 

proprietary designs, however, and their performance is not generally known to the engineering 

community. To facilitate the use of this promising technology, NIST is undertaking a series of 

tests to measure the performance of this type of joint and to make the results available in the 

public domain. It is our hope that more companies will participate in this program, which 

enables innovation to modernize and ensure the safety of construction, in conformance with the 

Advanced Construction Technology goals of the NIST Building and Fire Research Laboratory. 

2. Panel Tests 

A previous report (Duthinh, 2000b) investigated the performance of this connection under 

tension. The present work aims at fully characterizing the performance of the connection under 

flexure, in-plane shear, and out-of-plane shear. Knowledge of the connection behavior under 

various loads is necessary to predict the strength and behavior of a structure built with such 

elements, when subjected to gravity loads and lateral loads such as wind or earthquake. 

All tests used panels and connectors of width 150 mm in three possible configurations: toggle 

only, adhesive only, and toggle and adhesive combined. For the interlocked and bonded 

specimens, we followed the manufacturer’s bonding recommendations: First, we lightly sanded 

and wiped the mating surfaces clean. Next, we applied to only one of the two pieces to be joined 

a 3 mm diameter bead of adhesive in each of two grooves on the sides of the toggle cavity. We 

then aligned and mated the two parts together and inserted the toggle, making sure that no 

adhesive was applied to the toggle or toggle cavity. The specimen was left undisturbed to cure 

for at least 16 hours. For the bonded only specimens, the procedure was similar, except that 

adhesive was applied to both mating surfaces, which were subsequently clamped together. 
3. Flexural Tests 

13 

16 

Nine panels were loaded in flexure under four-point loading (Photo 1, Fig. 2). The panel webs 

were stiffened with a cementitious grout to prevent them from buckling under the applied loads. 

LVDTs (linear variable differential transformers) measured the deflections at midspan and the 

gap between the mating flanges 12 mm from the bottom face of the flange as the connection 

opened up. In general, the connection remained engaged until failure, which occurred by 

cracking of the jaws (Photo 2). Table 1 gives the bending moments and deflections at the first 

peak and at ultimate. Figures 5 to 13 show the curves of moment versus deflection and opening. 

Very little opening until final failure. 

625 1 1.8 Sudden cracking of jaws caused failure. 

840 21 Rupture of upper junction of flange and intermediate 

860 23 web. Connection held. 

775 

130 

Results show the flexural strength per unit width of the connector joined with a toggle only to 

have a mean of 471 Nmm/mm with a standard deviation of 65 N-dmm. Specimen BTl 

behaved linearly up to the first peak, which was fairly close to the ultimate moment. The joint 

opening of BT1 was an order of magnitude smaller than that of BT2 and BT3, probably due to 

the high sensitivity of this measurement to the location of the extensometer. Also, BT2 and BT3 

behaved much more non-linearly than BT1, with major jumps in loads before ultimate. 

In the bonded configuration, and the toggle plus adhesive configuration, the connection was 

stronger than the junction between web and flange, at least in this test geometry. Momentdeflection behavior ending with failure at the junction between flange and intermediate web was 

fairly reproducible, but the joint opening varied over a factor of five (BGl and BG3), probably 

Table 1 : Flexural Tests Results 

m 

Test I M1 D1 Comments 

Significant opening at loads much less than ultimate. 

Large deflection, connection remained engaged. 

Connection opened up at failure. 

Jaws cracked and caused failure. 

10 540 I 25 

BT2 1 370 7 412 I 12 

BT3 I 320 5 

Mean I I 

St. Deviation I 65 I 

Adhesive onlv I Very little opening until final failure. 

850 20 Rupture of upper junction of flange and intermediate 

1070 26.5 web (circled region in Fig. 2). Connection held. ' 850 24 

, 923 

BGl I 680 12 ~ 

BG2 I 925 18 

BG3 I 

Mean I 

St. Deviation I 

andl 

adhesive 

BTG2 I 740 

BTG3 I 760 

MI = first peak, N-mdmm 

Mu = ultimate moment, N-mdmm 

D1= Deflection at MI, rnm 

D, = Deflection at MU, rnm 

2 
for the same reason as mentioned previously. Test BTGl (toggle and adhesive) failed by 

cracking of the jaws of the connection at an ultimate moment of 625 Nmm/mm. 

Shear Strength 1 N/mm 

4. In-Plane Shear Tests 

Comments 

Photo 3 and Fig. 3 show the double-shear test configuration designed to test the in-plane shear 

strength of the connection. An LVDT measured the relative slip of the panels at each connection. 

The load bearing blocks were slotted to allow the toggles to clear. Since the tests with toggles 

only, resulted in no permanent damage, the toggles were sometimes reinserted and the test 

repeated (a and b). It was difficult to achieve the same slip at both connections of each test, due 

to manufacturing tolerance (some toggles were easier to insert than others) and the inevitable 

slight eccentricity of the load. 

Toggle only 

IPST 1 a 

Table 2 gives the in-plane shear strengths for the three joint configurations. Figures 14 to 27 are 

plots of shear force per unit width versus the relative slip across the joint. For the tests with 

adhesive only, or toggle and adhesive, the loads achieved were much higher than for toggle only. 

For these tests, it was difficult to achieve pure shear loading. If the loading blocks at the top and 

bottom were too far apart, then significant bending occurred. If, on the other hand, the loading 

blocks at the top and bottom were too close together, then the load path did not go through the 

Friction and slip, but reusable. 

1.97 

Table 2: In-Plane Shear Test Results 

IPST2a 

IPST2b 

I Tests 

2.00 

1.92 

IPST3 

IPST4 

2.97 

2.25 

I IPSTlb I 1.80 I I 

IPST5a* 

IPST5b 

0.66* 

1.85 

Test stopped because loads appeared too unbalanced. 

Adhesive only 

IPSGl 

IPSG2 

320 

160 Debond failure of connection. 

Shear failure at comer, between web and flange 

(Photo 4). 

I Mean (without 5a) I 2.1 1 I I 

IPSG3 

Toggle and adhesive 

IPSTG1 

IPSTG2 

IPSTG3 

Mean 

Standard Deviation 

1 St.Dev.(w/o 5a) I 0.4 1 I I 

310 Bearing failure. 

160 

150 

220 

177 

38 

Sudden failure along bond line. 

3 
connection and bearing failure occurred by longitudinal crushing of the cross section. As well, 

failure can occur by shearing of the junction between web and flange, and not of the connection 

(Photo 4). For these reasons, only the result shown in Table 2 for IPSG2 (160 N/mm) is 

indicative of the joint in-plane shear strength for the adhesive only condition. For the toggle 

only configuration, the mean shear strength is only 2.1 1 N/mm, with a standard deviation of 

0.41 N/mm; and for the toggle plus adhesive configuration, the mean shear strength is 

177 N/mm, with a standard deviation of 38 N/mm. 

Shear Strength 1 N/mm 

5. Out-of-Plane Shear Tests 

Comments 

Photo 5 and Fig. 4 show the test configuration. The test procedure is similar to the in-plane tests, 

except that the test specimen is rotated 90"so that the shear force is perpendicular to the toggle. 

Since the comers of the panel were curved, a fast setting cement (Hydrocal) was used to ensure 

good bearing over the entire comer. The difficulties mentioned previously about obtaining pure 

shear loading applied here as well. 

Toggle only 

OPST 1 a 

For test OPST3b, we reused parts from previous tests that exhibited no visible damage. (Usually, 

only one of two connectors would be visibly damaged in any one test.) This test produced the 

highest load and the only instance of shearing failure of the toggle (Photo 6). Since specimens 

155 One side of connection disengaged. Reused in OPSTlb. 

Table 3: Out-of-Plane Shear Tests Results 

OPST 1 b 

OPST2a 

OPST2b 

OPST3a 

1 Tests 

110 

113 

114 

120 (as in Photo 4). 

Same side opened up and disengaged. 

No sign of damage at small load drop after peak. 

Failure of upper junction between web and flange 

OPST3b 

Mean w/o 2b and 3a 

St. Dev. w/o 2b. 3a 

162 

135 

27 

Used parts from earlier tests. 

Failure by shearing of toggle! (Photo 6) 

Adhesive only 

OPSGl 

OPSG2 

75 

59 

Failure by cracking of jaws. (Photo 7) 

~ ~~ ~ ~ 

Toggle and adhesive 

OPSTG1 148 

OPSTG2 180 (as in Photo 4). 

OPSTG3 186 

Failure by shearing of junction between web and flange 

I OPSG3 I 76 I 

Mean 

Standard Deviation 

I Mean I 70 I 

171 

20 

I Standard Deviation I 10 I I 

4 
OPST 2b and 3a failed by shearing of the upper junction between web and flange, in a manner 

similar to what is shown on Photo 4, they do not represent the strength of the connection. These 

two test results were not included in the calculation of the mean out-of-plane shear strength for 

the toggle only configuration (1 35 N/mm) and the standard deviation (27 N/mm). 

The specimens that were bonded (with adhesive only) all failed in the same manner, by cracking 

of the jaws of the connection, and produced a mean strength of 70 N/mm with a standard 

deviation of 10 N/mm (Table 3). For the toggle plus adhesive configuration, all three specimens 

failed outside the connection. The mean transverse shear strength of the web-flange junction is 

171 N/mm, with a standard deviation of 20 N/mm. Figures 27 to 39 show plots of load versus 

deflection, and Photos 5 to 7 show the experimental set-up and two of the failed connectors. 

6. Uncertainty 

According to the current calibration sheet of the testing machine, for the relevant range of loads, 

the load displayed by the testing machine is less than 0.3 % lower than the calibrating device. 

Loads and displacements were stored directly by the testing machine, and also recorded at a rate 

of 10 Hz by a data acquisition system. The peak loads varied from 650 N for in-plane shear tests 

with toggle only, to 90 kN for in-plane shear tests with adhesive only. The maximum load 

indicated by the data acquisition system was less than 18 N lower than the machine load at the 

higher range. At the lower range, the noise of the data acquisition system is noticeable, but the 

smoothed out loads acquired by the system agree with the test machine reading within 5 %. The 

widths of the specimens were 150 mm f 1 mm. 

7. Conclusion 

The goal of this work was to fully characterize the mechanical behavior and strength of the 

connection under static loads. The measurements can be used in designing structures and 

predicting their performance. This goal was largely achieved, but not completely, because some 

of the test specimens failed outside of the connection. That in itself is significant, because it 

shows that the connection is stronger than the rest of those specimens. It would also have been 

desirable to conduct more tests to obtain greater confidence in the mean strength and standard 

deviation. 

This work is a step in developing the technical basis for standards for this new type of 

construction. The continued research and testing from industry, academia and government will 

lead to standards that can potentially have an impact on the construction industry. 

8. References 

Duthinh, D. (2000a), “Connections of Fiber-Reinforced Polymer (FRP) Structural Members: a 

Review of the State of the Art,” NISTIR 6532, NIST, Gaithersburg, MD. 

Duthinh, D. (2000b), “Tensile strength of an Interlocking Composite Connection,” NISTIR 

6565, NIST, Gaithersburg, MD. 

5 
Head, P.R. and Churchman, Q.E. (1989), “Design, Specification and Manufacture of a Pultruded 

Composite Construction System.” Proc., BPF Symposium on Mass Production Composites, 

Imperial College, London, Sep., pp. 117-162. 

9. Acknowledgments 

This study was made possible by a grant from the Advanced Technology Program of NIST 

(Felix Wu, Program Manager, Composite Materials). Strongwell (Bristol, VA) provided test 

samples, Maunsell (Beckenham, UK) technical data and Ashland Chemicals (Columbus, OH) the 

adhesive and applicator. Frank Davis and Mark Kile assisted with the experiments. To all, the 

authors are grateful. 

Photo 1: Bending Test 

Photo 2: Bending Test, Toggle only. 

Failure of jaws controlled bending strength. 

6 
Photo 3: In-Plane Shear Test 

Photo 4: In-Plane Shear Test (IPSG1). 

Failure at flange-web junction, probably 

caused by placement of loading block. 

7 
_I - 

Photo 5: Out-of-Plane shear Test 

Photo 6: Out-of-Plane Shear Test, Toggle only (OPST3b) 

Photo 7: Out-of-Plane Shear Test, adhesive only (OPSG1) 

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