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EXPERIMENTS WITH COMPONENT PLATE IN BENDING AND BOLT IN TENSION

SUBJECT TO REPEATED LOAD

DALIBOR GREGOR, FRANTIŠEK WALD, IVO JÍROVSKÝ, MILOŠ DRDÁCKÝ

Key words: steel structures, connection design, repeated loading, component method, T-stub in tension, experiments.

The paper presents an experimental work directed to the beam to column connections of the structural steel joints. The component plate in bending and bolt in tension is one of the major as well as the most studied component behaviour of which is predicted by an effective T-stub in tension. Two sets of tests under repeated loading with increasing amplitudes of the imposed displacement were applied following the ECCS recommendation.

1. INTRODUCTION Structural joints are designed assuming to be exposed to forces of quasistatic loading.

In the structures loaded cyclically by live loads or thermal and seismic actions, connections are checked against fatigue separately. In many structures, however, the number of cycles depending on load spectrum may reach 8·103÷105 [Ślęczka, Kozlowski, 2002]. In the joint the local yielding is present. Therefore low-cycle fatigue needs to be taken into account within the design procedure. Information about the behaviour of joints subjected to repeated loading is appropriate for all structures, even for these where quasistatic approach have been approved in practice as a good prediction tool. The ECCS recommendation for the cyclic loading procedure [ECCS, 1986] is commonly applied to compare the results of tests of the structural joints and to analyse the results.

A component method was proved to be an effective analytical tool for determination of joint’s behaviour. The method is based on the analytical modelling of components (individual parts) of the joint separately [Zoetemeijer, 1983]. The behaviour of each component is expressed by springs. A mechanical model of the joint assembles these springs and infinitely rigid plates. The description of behaviour of each component by three basic design characteristics: an initial (elastic) stiffness, strength and a deformation capacity enables an application of the method into the practice. The resulting force-displacement relationship for the joints is thus bi-linear. The analytical description of components offers to designer a freedom of geometrical variants. Extrapolation of the method to combination of both bolted and welded parts of joists proved to be sufficiently accurate.

The cyclic overall behaviour of joints is traditionally modelled by curve-fitting models. For fitting the model behaviour onto the tests, it is important to select main parameters of the model properly. The parameters are characterised by constants which are to be defined on a basis of test results or a FEM analysis, see [Wanzek, Gebbeken, 1999]. These models may reach, within a limited range of applications, the required accuracy. Any extrapolation out of the experimentally proved geometrical set-up is not possible. In the last decade, the mathematical models for the steel beam-to-column joints were derived for prediction of frame behaviour under seismic actions [Corte at al, 1999], [Elsati, 1996], [Mazzolani, 1988] and for column bases, see [Ermopoulos at al, 2000].

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For the component model under cyclic loading, the degradation of material and the assembly of components including the change of the contact area in the given cycle and the history of yielding in the precedent cycle are taken into account, see [Ślęczka, Kozlowski, 2002] and [Bernuzzi 1997]. The degradation phenomena may be described in the models of each component, see [Rassati at al, 2000]. Due to hysteretic character of behaviour it is not possible to determine a given point of the force-displacement or moment-rotation curve without information about a load history. The step-by-step procedure is used for establishing the force-displacement or moment-rotation curves, allowing in each step for the cumulative degradation of the material and the deterioration of stiffness. The curve is often simplified by use of linear approach. The tests with components were published in [Bursi at al, 1997], and [Wald at al, 2000].

150

92

146

92

298535

3529

70

movable jaws

2xM16 (8.8)T-stub (1/2 IPE 300)

2xO18

fixed jaws

a)

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146 50

7070

140

922xO18

29 29

T-stub (1/2 IPE 300)

fixed jaws

movable jaws

2xM16 (8.8)

b)

Figure 1: Specimen geometry a) T70 b) T140

2. TEST SET UP The evaluation of the characteristics of the springs representing components is an

important part of the analysis of cyclically loaded joints. Concerned experiments were published in [Bursi at al, 1997]. The experimental program performed at the Czech Technical University in Prague in the cooperation with the Institute of Theoretical and Applied Mechanics of the Academy of Sciences in Prague was designed to investigate the basic characteristics for the component bolt in tension and the flange/endplate (called equivalent T-stub in tension).

Specimen geometry is shown in Figure 1. The specimen consists of two bolted T-stubs cut from IPE300 section. Two sets of specimens were tested: one with the length of T-stub equal to 70 mm, marked T70, and the second one with the length equal to 140 mm, marked T140. Each specimen in the set is numbered from 1 to 3. The webs of the T140 specimens were adapted for fixing into the jaws of a test device by reducing their length to 70 mm (see Figure 1b). The bolts M16 inserted into the holes of 18 mm in diameter were used. One washer was placed under the nut.

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3. INSTRUMENTATION

During the tests which were carried out on the MTS servo-hydraulic loading machine, the applied load was measured by a load cell of capacity of 250 kN (channel V0) and the actuator position by an internal inductive transducer (channel V20). Two external potentiometer transducers were fixed on the specimen providing the capture of the gap between two T-stubs on both sides (they are labelled „22“ and „23“). Transducers were screwed to the web of the upper T-stub by M5 screws (Figure 2). Short steel L-shaped cantilevers fixed to the lower T-stub webs provided the opposite supports for transducers „22“ and „23“. Another inductive transducer (labelled „24“) was fastened on the upper movable clamping jaw to measure relative displacement between the jaws. The force in the bolts was determined from the strains measured using the strain gauges Mikrotechna 6JP120A (labelled „100“ a „101“) glued in the hole drilled in the bolt head. The hole of 2 mm in diameter is drilled along the bolt axis from the head side till 5 mm into the bolt shank. The strain gauges were calibrated for the tension force applied on the bolt in the range 0 – 80 kN. The calibration proved a linear strain-force relationship without a visible hysteresis during unloading. For each bolt a strain gauge constant was determined.

Figure 2: Location of measurement devices 4. MATERIAL CHARACTERISTICS

The bolts of grade 8.8 were used. Material characteristics of the T-stubs were

established based on standard tensile coupon tests [ČSN EN 10002-1, 1994] on two sets of coupons taken from the flange in the longitudinal and transverse direction. The average values from three coupons in each set are summarized in the Table 1.

Table 1 : Results of standard tensile coupon tests of coupons taken from flanges of the T-stub

Type of coupon ReH [MPa] Rm [MPa] A [%] Z [%]

Longitudinal direction 272,4 411,9 32,7 67,0

Transverse direction 301,8 412,1 36,8 65,5

22

101

22 23

24 101

100

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5. LOADING PROCEDURE The loading procedure follows the ECCS recommendation [ECCS, 1986] which

specifies a loading sequence and the procedure to analyse the results for the force-displacement test type. The procedure is based on the evaluation of yielding displacement δy caused by the force Fy corresponding to the conventional yield stress in the tested joint or the component. [ECCS, 1986]. The conventional yield stress may be defined by the intersection of the initial stiffness and the tangent stiffness as 1/10 of the initial one, see Figure 3.

1

Et

1 Et /10

Fy

F,

y

δ,

Force

Displacementδ

Figure 3 : Definition of conventional yielding displacement δy and force Fy, [ECCS, 1986]

0

2

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8

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18

0 200 400 600 800 1000 1200

Time, s

Displacement, mm

Test T70-01

Figure 4: Loading procedure of test T70-01

The ECCS, see [ECCS, 1986], recommends a complete testing procedure and as well

as an abridged one. The value δy is determined for the tension and compression parts separately from the monotonic tests in case of the complete procedure. In the simplified procedure, the value δy is predicted by an analytical model. The cyclic test is designed as a displacement-controlled one with the increase of the amplitude of subsequent cycles of δy / 4, δy / 2, 3 δy / 4, δy, 2 δy, (2+2 n) δy, for n = 1, 2, 3, … . Four cycles under the δy are designed in the abridged procedure. The concept of the ECCS is prepared for the full cycle loading with the tensile and compressive forces and the positive and negative displacements. The negative displacement of the tested component is beyond the limit of the transducers accuracy. The minimum value of the displacement is taken zero, δy = 0. A compressive force in the next cycle returns the specimen into the zero displacement after reaching the limit of elasticity.

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The loading velocity is based on the record of the data. Reading and storing intervals of transducer measurements is 2 s. The frequency of 0,025 Hz was adopted, see Figure 4, in order to be able to read 10 sets of data at each cycle.

6 TEST RESULTS Tests consist of two sets of three specimens: tree specimens T70 and three

specimens T140. All tested specimens failed by yielding and a subsequent fracture of the T stub flange near the toe of radius between the T-stub flange and the web, see Figure 5.

a)

b) c)

Figure 5 : a) Crack initiation b) Crack development before failure c) Failure of flange

Deformation of short specimens T70 was constant along the T-stub length, see Figure

6b). In the case of long specimens T140, an effect of the third dimension was visible, see Figure 6a). The component method assumes two dimensional behaviour of a joint, the third dimension is condensed to the vertical plane. The length of the contact area of the prying force along the T-stub length was studied. The specimens T70 failed with the contact zone length equal to the total length of the T-stub, i.e. 70 mm. The specimens T140 failed with the contact length reduced to 50 mm.

Figure 6 : a) Deformation of specimen T140 b) Deformation of specimen T70

The bolts bent visibly after the displacement exceeded 50% of the ultimate value, see

Figure 7 and 9. The influence of bending moments to the bolt force was reduced by locating the strain gage into the bolt axis. The reached level of prying is expressed by the inclination to

a) b)

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the line of 45° in Figures 8 and 10. Figures 7 and 9 include the comparison of prediction models, see [Zoetemeijer, 1983], to the envelope of test results. The prediction is good.

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0 1 2 3 4 5 6 7 8 10 12 15 16 17 18 19 20 21 22

T70-01T70-02T70-03

Gap opening, mm

Force, kN

δ

Prediction by f u

Prediction by f y

Envelope curve

Figure 7 : Gap opening of the specimens set T70

(the average values of the transducers I22 and I23 are shown)

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0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85

T70-01T70-02T70-03

Bolt force, kN

Force, kN

FF

Fb1F

b1

b2

b2

Figure 8 : Bolt force-applied force relationship of specimens set T70

(the bolt force represents the average value of both bolts)

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-180

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T140-01T140-02T140-03

Gap, mm

Force, kN Prediction by f u

Prediction by fyEnvelope curve

δ

Figure 9 : Gap opening of the specimens set T140

(the average values of the transducers I22 and I23are shown)

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T140-01T140-02T140-03

force, kN

50 55 60 65 70 75

Fb1

Fb2

Fb1Fb2

Bolt

Force, kN

Figure 10 : Bolt force-applied force relationship of specimens set T140

(the bolt force represents the average value of both bolts)

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CONCLUSIONS

Experiments confirm a lower stiffness and a lower deformation capacity of the

component T-stub in tension under cyclic loading compared to the static ones. The reached deformation capacity from 16 to 18 mm corresponds to the rotation of 46 mrad for the IPE300 beam with an expected lever arm of internal forces equal to 350 mm. The ductile behaviour has been confirmed.

The strain gauges glued in the hole of the centre of the bolt, and calibrated in the range of 50% of the bolt resistance, worked linearly and reported the relative deformation/internal forces as well as the distribution of forces between bolts.

Acknowledgement This work has been supported by grant GAČR 103/01/0708 of the Czech Grant

Agency and by grant J01-98:210000004 of the Czech Ministry of Education.

References [Bursi at al, 1997] Bursi O.S., Ballerini M., Nemati N., Zandonini, R. (1997) Quasi-static

monotonic and low-cycle behaviour of steel isolated tee stub connections, Proc. STESSA (ed. F.M. Mazzolani and H. Akiyama), Kyoto, pp. 554-565.

[Bernuzzi 1997] Bernuzzi C., Balado L., Castiglioni C.A. (1997) Steel beem to column joints: Failure criteria and cumulative damage models. Proc. STESSA (ed. F.M. Mazzolani and H. Akiyama), Kyoto, pp. 538-545.

[Corte at al, 1999] Della Corte G., De Matteis G., Landolfo R.(1999) A mathematical model interpreting the cyclic behaviour of steel beam-to-column joints. Proc. 17. kongres C.T.A. - Settimana della construzione in acciaio, Napoli, pp. 115-126.

[ČSN EN 10002-1, 1994] ČSN EN 10002-1 (1994) Zkouška tahem, část 1, zkouška tahem za okolní teplot, ČNI Praha, p. 28.

[ECCS, 1986] ECCS TC1 TWG 1.3 (1986) Recomended testing procedure for assessing the behaviour of structural steel elements under cyclic loading, European convention for constructional steelwork, Brussels.

[Elsati, 1996] Elsati M.K., Richard R.M. (1996) Derived moment rotation curves for partially restrained connections. Structural Engeneering Review, Vol. 8 (2/3), 1996, pp. 151-158.

[Ermopoulos at al, 2000] Ermopoulos J., Stamatopoulos G., Wald F., Sokol Z. (2000) Mathematical modelling of semirigid connections in steel column-bases cyclic loading, Report of bilateral cooperation in civil engeneering research (MŠMT ČR and Greek Ministry of Idustry, Energy and Technology), Athens - Praha 2000.

[Mazzolani, 1988] Mazzolani, F.M. (1988) Mathematical model for semi-rigid joints under cyclic loads. Connections in Steel Structures: Behaviour, Strength and Design, Elsevier Applied Science Publisher, London, pp. 112-120.

[Rassati at al, 2000] Rassati G.A., Noè S., Leon R.T. (2000) PR Composite joints under cyclic and dynamic loading conditions: The component model approach. Proc. 4th AISC International Workshop on Connections in Steel Structures, Roanoke, pp. 213-222.

[Rassati at al, 1984] De Martino A., Faella C., Mazzolani F.M. (1984) Simulation of beam-to-column point behaviour under cyclic loads. Construzioni Metalliche, Vol. 6, pp. 346-356.

[Ślęczka, Kozlowski, 2002] Ślęczka L., Kozlowski, A. (2002) Low cycle fatigue as load capacity criterion of semi-rigid connections, COST C12 –WG2, Timisoara.

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[Wald at al, 2000] Wald F., Mareš J., Sokol Z., Drdácký M. (2000) Component Method for Historical Timber Joints. The Paramount Role of Joints into the Reliable Response of Structures, NATO Science Series, Series II, Vol. 4, ed. Banitopoulos C.C., Wald F., Kluver Academic Publishers, Dortrecht, pp. 417-425.

[Wanzek, Gebbeken, 1999] Wanzek T, Gebbeken N. (1999) Numerical aspects for the simulation of the endplate connections, Numerical simulation of semirigid connections by the finite element metod, ed. Virdi K.S., COST C1 paper, Brussels, pp. 13-31.

[Zoetemeijer, 1983] Zoetemeijer P. (1983) Proposal for Standardisation of Extended End Plate Connection based on Test results and Analysis, Rep. No. 6-83-23, Steven Laboratory, Delft, p. 56.