1 INTRODUCTION

During the 90s three strategies have been devel-oped to overcome the problems provided by the brit-tle behavior of welded connections (SAC, 2000c; SAC, 2000b) which, after Northridge and Kobe earthquakes, have been demonstrated to be unrelia-ble. The first one, namely the strengthening ap-proach, provides the enhancement of the welding techniques, imposing the adoption of properly certi-fied electrodes and specifying, through seismic codes, the welding techniques to be used in dissipa-tive MRFs (SAC, 2000a). The second one, namely the weakening approach, is based on the idea of promoting the plastic engagement at beam ends by cutting a portion of the beam flanges (Reduced Beam Section) in a zone close to the span end, but sufficiently far from joints (Moore et al., 1999). In this way, stress concentrations in welds are avoided and dissipation is provided by cyclic flexural behav-ior of H-shaped sections. The third approach is con-stituted by the use of partial strength connections, so that dissipative zones are shifted from the beam ends to the connecting elements, which are designed to be weaker than girders. According to this design phi-losophy, the dissipation capacity is relied on the ability of end-plates and/or panel zones (Faella et al., 2000; Iannone et al., 2011) to withstand plastic de-formations. Therefore, welds are designed to be ade-quately over-resistant compared to the weakest joint component.

Regarding seismic provisions, both in (AISC, 2005) and in (CEN, 2005c), MRFs can be designed either according to the full-strength criterion and the partial-strength criterion. The first one is based on the possibility of dissipating the seismic input ener-gy at the beam ends, the second one concentrates damage in connection elements. In the former case, aiming to promote the yielding of the beam ends, the beam-to-column joint has to be designed to possess an adequate overstrength with respect to the con-nected beam to account for strain-hardening and random material variability effects. In the latter case, the beam yielding is prevented as the joints are de-signed to develop a bending resistance less than the beam plastic moment. In addition, as a consequence, regarding the column design, the hierarchy criterion has to be applied by making reference to the maxi-mum moment that connections are able to transmit. This design philosophy, as demonstrated by Faella et al. (1998), is particularly cost/effective in cases where the beam size is mainly governed by vertical loads rather than lateral loads, i.e. low-rise/long span MRFs.

The traditional design of MRFs, based on the use of full-strength beam-to-column joints, requires only the prediction of the monotonic response of connec-tions (CEN, 2005b; CEN, 2005a). In particular, to characterize joints behavior under Serviceability Limit State and Ultimate Limit State, only the pre-diction of the initial stiffness and of the plastic re-sistance is needed. Conversely, as the energy dissi-

Friction T-stub Joints under Cyclic Loads: Experimental Behavior

M.Latour, V.Piluso, G.Rizzano Department of Civil Engineering, University of Salerno, Fisciano (SA), Italy

ABSTRACT: Dealing with the seismic behavior of steel MRFs, in last decade, the use of dissipative partial strength beam-to-column joints has gained the due attention as an effective alternative to the classic design approach which, aiming to dissipate the seismic input energy at beam ends, is based on the design of full strength beam-to-column joints with adequate overstrength. On the base of past experimental results, the use of dissipative Double Split Tee (DST) connections can be considered an interesting solution from the techno-logical standpoint, because they can be easily replaced after destructive seismic events. Nevertheless their dis-sipation supply under cyclic loads has been demonstrated to be characterized by significant pinching and strength degradation. For these reasons, in this paper, an innovative approach aimed to enhance the response of DST connections is proposed and the performances of the new system are investigated within an experi-mental program currently in progress at Salerno University.

pation supply of semi-continuous MRFs relies on the ability of connections to sustain a number of excur-sions in plastic range without losing their capacity to carry vertical loads, it is clear that, in order to suc-cessfully apply partial-strength joints, it is necessary to properly characterize and predict the response of connections under cyclic loading conditions (Latour et al., 2011; Piluso & Rizzano, 2008; Jaspart, 1991; Astaneh-Asl, 1987; Bernuzzi et al., 1996). For this reason, the use of partial-strength joints is allowed, both in AISC and Eurocode 8 provided that a con-formance demonstration of the cyclic behavior of connections adopted in the seismic load resisting system is shown by the designer. As a result, joints have to be pre-qualified accordingly with the ductili-ty class of MRFs. It is clear that this requirement is often out of the possibility of common designers. Therefore, aiming to provide engineers with the tools needed to predict the cyclic behavior of joints, new efforts for the developments of analytical ap-proaches are necessary.

To this scope, in the last two decades a number of experimental programs dealing with the characteri-zation of the cyclic behavior of beam-to-column connections have been carried out. In a work of the same authors, the behavior of four bolted joints, de-signed to possess the same strength, but detailed to involve in plastic range different components, has been characterized pointing out the hysteretic behav-iors of the tested connections (Iannone et al., 2011). Due to the significant advantages which are able to provide, DST connections have been recognized as an interesting solution to be applied in dissipative semi-continuous MRFs. In fact, DST connections can be easily repaired after destructive seismic events and allow to govern the design process by simply calibrating three geometrical parameters: the width and the thickness of the T-stub flange plate and the distance between the bolts and the plastic hinge arising at the stem-to-flange connection. On the other hand, joints involving in plastic range bolt-ed components provide also several disadvantages. First of all, even though experimental studies demonstrate that bolted components are able to dis-sipate significant amounts of energy, it has to be recognized that their hysteretic behavior is less dis-sipative compared to other joint typologies or to the cyclic response of steel H-shaped sections. This is mainly due to contact and pinching phenomena which usually lead to the quick deterioration of strength and stiffness of the tee elements.

For this reason, on one hand, the use of hourglass shaped T-stub flanges has been proposed (Latour & Rizzano, 2011). In particular, the dissipative capaci-ty of classical tee elements has been improved by applying to T-stubs the concepts usually developed to design hysteretic metallic dampers, such as ADAS devices (Aiken et al., 1993a; Whittaker et al., 1989; Soong & Spencer Jr, 2002).

On the other hand, an innovative approach aimed to enhance the dissipation capacity of classical rec-tangular T-stubs by using friction pads has also been proposed (Piluso et al., 2001).

This last approach, which can be considered an innovative application of the seismic protection strategy based on supplementary energy dissipation, is herein presented. The main scope of the work is to point out a strategy to improve the dissipative capac-ities of DST connections by exploiting the cyclic behavior of friction materials. In particular, as shown in the following, two innovative DST joints are detailed aiming to dissipate the seismic input en-ergy by means of the slippage of the stems of the tees on a layer of friction material, which is inter-posed between the tee stems and the beam flanges. In this way, under cyclic loading conditions, struc-tural elements do not undergo to any damage, but energy dissipation is provided by the alternate movement of the tee stems on the friction pads, which are preloaded by means of high strength bolts. Therefore, in the present paper a new type of dissi-pative beam-to-column joint, namely dissipative DST connections with friction pads, to be employed in semi-continuous MRFs is proposed and its behav-ior is investigated by means of experimental tests under displacement control in cyclic loading condi-tions.

2 EXPERIMENTAL TESTS ON COMPONENTS

As mentioned above, friction dampers dissipate the seismic input energy by means of the slippage of two surfaces clamped by an adjustable normal ac-tion. Obtained hysteresis cycles are similar to those of elastic-plastic dampers with high initial stiffness. This is a great advantage of friction devices, because they can be designed aiming to work as a fixed re-straint under serviceability limit states (SLS) and to slip when energy dissipation is needed, i.e. under ul-timate limit states (ULS). In this way, under service-ability conditions, friction devices can be used to work as displacement reducers because of their rigid behaviour, whilst, under destructive seismic events, they can be effectively used to dissipate the seismic input energy. The classical theory of dry friction is based on the following three postulates:

the total frictional force is independent of the ap-parent surface area of contact;

the total frictional force that can be developed is proportional to the normal applied action;

in case of slow sliding velocities, the total fric-tional force is independent on the sliding velocity.

During slippage, the classical relationship to

compute the tangential force acting at the sliding in-

terface in the direction opposed to the motion is the

Coulomb friction equation:

NT (1)

where T is the sliding force, N is the normal action

and is the friction coefficient. The force of friction is always exerted in a direction opposed to the

movement (in case of kinetic friction) or potential

movement (in case of static friction).

Figure 1. Device used to define materials friction coefficients

In order to design dissipative DST connections

with friction pads, a preliminary analysis on a sub-

assemblage constituted by two inner plates with slot-

ted holes, two outer plates with normal holes and

two interposed layers of friction material fastened by

means of 16 preloaded bolts, has been developed at

Materials and Structures Laboratory of Salerno Uni-

versity (Fig.1). The tests have been carried out by

means of a universal testing machine Schenck Hy-

dropuls S56. The testing apparatus is constituted by

a hydraulic piston with loading capacity equal to +/-

630 kN, maximum stroke equal to +/- 125 mm and a

self-balanced steel frame used to counteract the axial

loadings. The machine works both under displace-

ment and load control. In order to measure the axial

displacements the testing device is equipped with an

LVDT, while the tension/compression loads are con-

trolled by means of a load cell. The cyclic tests have

been carried out for different displacement ampli-

tudes at a frequency equal to 0.25 Hz.

The experimental analysis has been carried out by

varying the preloading level of the bolts and the

characteristics of the friction material. In particular

two kinds of friction material (in the following the

two materials will be identified with the tags M1 and

M2) have been tested and the bolts have been tight-

ened with bolt torques contained in the range be-

tween 200 and 550 Nm. The experimental results

have pointed out that the hysteretic behaviour of the

device, with both friction materials, can be consid-

ered sufficiently stable with slight strength degrada-

tion due to the progressive loose of the bolt preload-

ing as far as the friction material wears out.

-150

-100

-50

0

50

100

150

-20 -15 -10 -5 0 5 10 15 20F [

kN

]

d [mm]

Material M1 - Bolts Torque = 200 Nm

-200

-150

-100

-50

0

50

100

150

200

-20 -15 -10 -5 0 5 10 15 20F [

kN]

d [mm]

Material M2 Bolts Torque= 200 Nm

Figure 2. Test results on materials M1 and M2

As an example, in Fig.2, typical cyclic force-

displacement curves for a bolt preloading equal to

200 Nm are shown for both materials M1 and M2.

Furthermore, in Table 1 the friction coefficients ob-

tained by applying the relationship given in Euro-

code 3 are reported. In particular, Eurocode 3 pro-

pose to determine the sliding force by means of the

following equation:

bbsss FnnkF (2)

where ks is a coefficient accounting for the geometry

of the holes, ns is the number of contact surfaces, nb

is the number of bolts and Fb is the force exerted by

one bolt. In case of long slotted holes with axis par-

allel to the applied force, Eurocode 3 proposes the

use of a value equal to ks=0.63, for the coefficient

accounting for hole geometry.

Table 1. Friction coefficient for materials M1-M2 derived on the base of Eq. (2) Material M1 Material M2 __________________________________________ Ts Fs Ts Fs ____________________ ______________________ Nm kN Nm kN __________________________________________ 200 108 0.21 200 152 0.30 300 210 0.28 300 180 0.24 400 292 0.29 400 310 0.31 550 388 0.28

Average 0.27 Average 0.28 C.V. 0.13 C.V. 0.13

As the values delivered in Table 1 have been evalu-

ated starting from the knowledge of the friction re-

sistance obtained from experimental tests under cy-

clic loading conditions, they have to be interpreted

as kinetic friction coefficients.

3 EXPERIMENTAL TESTS ON DST CONNECTIONS WITH FRICTION PADS

Starting from the behaviour of the plate sub-

assemblage with friction layers, the design of dissi-

pative DST connections with friction pads, i.e. with

interposed layers of friction material between the

beam flanges and the stems of the tee elements, has

been performed. The cyclic behavior of the pro-

posed innovative DST connections with friction

pads can also be compared with the energy dissipa-

tion capacity of a traditional double split tee connec-

tion tested in a previous work (Iannone et al., 2011),

namely TS-CYC 04. Experimental tests have been

carried out at Materials and Structures Laboratory of

Salerno University. The testing equipment is that al-

ready adopted to test traditional beam-to-column

connections (Iannone et al., 2011).

HE200B

IPE270

Hydraulic Actuator

Concrete floorSleigh base

Left hinge

Right hinge

Vertical frame

Horizontal frame

JOINT

IPE270L=170cm

L=200cm

max load: 250 kNmax disp.: 500mm+/-

+/-

Hydraulic Actuatormax load: 1000 kNmax disp.: 125mm+/-

+/-

Figure 3. Experimental Setup for tests on joints

Two steel hinges, designed to resist shear actions

up to 2000 kN and bolted to the base sleigh have

been employed to connect the specimens to the re-

acting system. The specimen is assembled with the

column (HEB 200) in horizontal position, connected

to the hinges, and the beam (IPE 270) in vertical po-

sition (Fig.3). The loads have been applied by means

of two different hydraulic actuators. The first one is

a MTS 243.60 actuator with a load capacity equal to

1000 kN in compression and 650 kN in tension with

a piston stroke equal to +/- 125 mm which has been

used to apply, under force control, the axial load in

the column equal to 630 kN. The second actuator is

a MTS 243.35 with load capacity equal to 250 kN

both in tension and in compression and a piston

stroke equal to +/- 500 mm which has been used to

apply, under displacement control, the desired dis-

placement history at the beam end. The loading his-

tory has been defined according to AISC (2005).

During tests many parameters have been monitored

and acquired, in order to get the test machine history

imposed by the top actuator and the displacements

of the different joint components. Aiming at the

evaluation of the beam end displacements due to the

beam-to-column joint rotation only, the displace-

ments measured by means of the LVDT equipping

MTS 243.35 have been corrected by subtracting the

elastic contribution due to the beam and column

flexural deformability according to the following re-

lationship:

aL

a

aL

L

EI

LFL

EI

FL

cc

c

c

bc

b

bTj

2

6

2123

223

3

(3)

where Ib and Ic are the beam and column inertia

moments, Lc is the column length, Lb is the beam

length and a is the length of the rigid parts, due to

the steel hinges. The experimental tests carried out

up to now concern two specimens:

IPE 270

HEB 200

Bolts M20 class 10.9

Slotted plate (t=15 mm)

t=30 mm

Bolts M27 class 10.9

400

t=10 mm

45

173

45

116

15

132

263

458145

171

45

173

45

116

15

132

263

458145

171

Friction Material

Figure 4. Geometrical detail of joints TSJ-F1-CYC08 and TSJ-F1-CYC09 specimens

TSJ-F1-CYC08 and TSJ-F2-CYC09, which are two double split tee connections with layers of

friction material interposed between the T-stub

stems and the beam flanges. Friction pads are

clamped by means of eight M20 class 10.9 bolts

tightened with a torque equal to 450 Nm. In order

to allow the relative movement between the stems

of the T-stubs and beam flanges, two slotted holes

have been realized on the tee stems. The tested

joints are characterized by different types of fric-

tion materials (Material M1 and M2). The flanges

of the T-stubs are fastened to the column flanges

by means of eight M27 bolts, class 10.9. The de-

tail of the tested specimens is reported in Fig.4.

4 CYCLIC BEHAVIOR OF SPECIMENS

As already stated, the main goal of the present work

is to identify a strategy to improve the seismic be-

haviour of DST connections.

Figure 5. Arrangement of DST connections with friction pads

The approach proposed in this paper is aimed to

concentrate the energy dissipation in two layers of

friction material interposed between the tee stems

and the beam flanges. Hysteretic Curve M-q TS-CYC 04

-250

-200

-150

-100

-50

0

50

100

150

200

250

-0,100 -0,075 -0,050 -0,025 0,000 0,025 0,050 0,075 0,100

Joint Rotation [rad]

Mo

me

nt

[kN

m]

Envelope

Mmax = 186.3 kNm

Mmin = -197.5 kNm

-150

-100

-50

0

50

100

150

-0,060 -0,035 -0,010 0,015 0,040

Mo

me

nt

[kN

m]

Joint Rotation [rad]

Hysteretic Curve M-q TS-M1-460-CYC 08

Mmax = 116.6 kNmMmin = -132.5 kNm

-150

-100

-50

0

50

100

150

-0,060 -0,035 -0,010 0,015 0,040

Mo

me

nt

[kN

m]

Joint Rotation [rad]

Hysteretic Curve M-q TS-M2-460-CYC 09

Mmax = 116.6 kNm

Mmin = -124.6 kNm

Figure 6. TS-CYC04 vs TS-M1 and TS-M2

In order to reach this scope, the design of the

specimens has been carried out by establishing hier-

archy criteria at the level of joint components.

Therefore, the tightening torque of the bolts fas-

tening the tee stems to the beam flanges, which pro-

vides to the friction material the desired clamping

force, has been determined to obtain a design mo-

ment resistance equal to 100 kNm. Successively, the

nodal components, i.e. the T-stubs, the bolts, the

panels in tension/compression and the shear panel,

have been designed for the actions corresponding to

the maximum moment that friction pads are able to

transmit (equal to 100 kNm). In addition, as previ-

ously underlined, in order to allow the relative

movement between the friction pads and T-stub

stem, the tee stems have been slotted. The length of

the slots has been calibrated according to the desired

maximum joint rotation which ahs been assumed

equal to 0.07 rad. Therefore, according to the design

criteria, experimental tests have been carried out

without leading to any involvement of the joint

components in plastic range and, as a consequence,

without any joint damage. This is the most important

result; in fact after the two tests the only element

subjected to damage was the friction pad when made

of material type 1 (Fig. 6). Therefore, this connec-

tion typology can be subjected to repeated cyclic ro-

tation histories, i.e. to repeated earthquakes, by only

substituting the friction pads and by tightening again

the bolts to reach the desired preloading level. In ad-

dition, the rotation capacity can be easily calibrated

by simply governing the length of the slots.

-250

-200

-150

-100

-50

0

50

100

150

200

250

-0,100 -0,075 -0,050 -0,025 0,000 0,025 0,050 0,075 0,100

Mo

me

nt

[kN

m]

Joint Rotation [rad]

Hysteretic Curve M-q

Envelope TS-CYC04

TS-M1-460-CYC 08

TS-M2-460-CYC 09

0

20

40

60

80

100

120

140

160

180

200

1 6 11 16 21 26 31 36 41 46

En

erg

y [

kN

m]

n cycles

Energy dissipation

TS-M2-460-CYC 09

TS-CYC 04

TS-M1-CYC 08

Figure 7. Cyclic Envelopes and Energy Dissipation of Tested Joints

In case of friction material M2, a stable cyclic re-

sponse with a slightly hardening behaviour due to

the increase of local stresses caused by the beam ro-

tation and by the rotational stiffness due to the bend-

ing of the tee stems has been pointed Fig.6. In order

to compare the cyclic behavior of DST connections

with friction pads with that a traditional one (TS-

CYC 04) the envelopes of the cyclic moment-

rotation curves are reported in Fig. 7. It can be ob-

served that the bending moment corresponding to

the knee of the curves, corresponding to the design

value of the joint resistance, is similar for the three

tests, but the post-elastic behaviors are quite differ-

ent. In fact, compared to the case of joint TS-CYC

04, friction DST joints do not exhibit significant

hardening behavior. With reference to TS-M2-

CYC09 test, it is worth to note that its hysteresis cy-

cles are wide and stable with no pinching. This is the

reason why this joint, despite of less hardening be-

havior, is able to dissipate, more energy than con-

nection TS-CYC04.

5 CONCLUSIONS

In this paper, dealing with the seismic behavior of dissipative connections, an innovative solution to improve the response under cyclic loads of double split tee connections has been pointed out. The relia-bility of the proposed approach has been verified by means of experimental tests and the response in terms of energy dissipation and shape of the hystere-sis loops has been compared to that of a traditional DST joint tested in a recent experimental program. The obtained results are encouraging about the pos-sibility to fully validate the proposed innovative connections.

Acknowledgements: This work has been partially supported

with research grant DPC-RELUIS 2010-2013

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