moment ratios—I. Experimental behaviour

J. Construct. Steel Res. Vol. 38, No. 2, pp. 125-164, 1996

Pll: S0143-974X(96)00015-6

Copyright © 1996 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0143-974X/96 $15.00 + 0.00

ELSEVIER

Behaviour of Flush End-plate Composite Connections with Unbalanced Moment and Variable Shear/Moment Ratios---I. Experimental Behaviour T. Q. Li, D. A. Nethercot & B. S. Choo Departmem of Civil Engineering,Universityof Nottingham, UniversityPark, Nottingham NG7 2RD, UK (l~',eceived 17 October 1994; revised version received20 October 1995; accepted 28 February 1996)

ABSTRACT A series of seven end-plate beam-to-column connection tests is reported; these include one pure steel connection and six composite connections. The main wzriables investigated are the degrees of unbalanced moment and the shear/moment ratio. Comprehensive instrumentation has been used to monitor: beam strains, column strains, rebar strains and bolt forces as well as member and connection deformations. This comprehensive monitoring permits a full understanding of the behaviour of this kind of connection. In addiition, the large number of recorded variables provides data for the detailed validation of numerical analysis methods. Copyright © 1996 Elsevier Science Ltd.

1 INTRODUCTION In an attempt to understand and explain the characteristics of composite beam to column connections, a large number of composite joints have been tested during the past two decades. 1-19 However, most of these tests were conducted on cruciform specimens with symmetrical loading and a constant and relatively low :~hear/moment ratio. Using these test results, the prediction methods developed so far for moment capacities and rotational characteristics have concentrated on the case with symmetrical load and have largely neglected shear/moment ratio effects. Furthermore, almost all the reported tests only gave details of the connection moment-rotation curves, as well as a visual indication of the failure modes. These are not always adequate for a full understanding of the connection behaviour and are insufficient for the detailed cali125

126

T. Q. Li, D. A. Nethercot, B. S. Choo

bration of numerical models. In order to investigate the effects of shear and non-symmetrical moments on composite connections a further set of experiments and analyses has been conducted. These have used the flush end-plate composite connection since it is the most commonly used connection type in the UK. A total of seven tests were conducted, in which one test was a pure steel connection and the remaining six were composite connections. The six composite joint tests are divided into two groups: one to investigate nonsymmetrical moment effects and the other to study shear/moment ratio effects. In order that a comprehensive understanding of the detailed specimen behaviour could be obtained, several parameters in addition to the basic moment rotation characteristics were monitored during the tests. These measurements provide a wide range of data for calibration of the numerical models. The purpose of this paper is to present the complete test results and to discuss the main findings. The proposed prediction method for the end-plate composite connections is presented in the companion paper, 22 in which the non-symmetrical loading and shear force effects are considered.

2 SPECIMENS AND TEST RESULTS

2.1 Test specimens and material properties As the main parameters to be investigated were the shear/moment ratio and non-symmetrical loading, all the seven specimens were designed with exactly the same steel details and the six composite specimens were designed with the same concrete slabs, reinforcement and shear connectors. All specimens were of the cruciform arrangement as shown in Fig. l(a). In order that nonsymmetrical load could be applied to the specimens, the column was fixed at both ends to the test rig. To simulate the practical situation, transverse stub steel beams with the same type of connection were provided to the minor axis of the column. Composite slabs employing metal decking were used for all the composite joint specimens; the metal decking was PMF CF 46 with an effective depth equal to 46 mm. The overall depth of concrete slab was 110 mm, with a width of 1000 mm. Four 12-mm diameter and four 10-mm diameter high yield bars were provided as longitudinal reinforcement. Sufficient transverse reinforcement in the form of 10-mm diameter high yield bars with a spacing equal to 100 m m was supplied to prevent longitudinal splitting failure of the concrete slab; this corresponds to about 1.5 times the transverse reinforcement required by BS5950. 2° Headed studs of 19-mm diameter and total post-weld height 90 m m were adopted as shear connectors. To achieve full interaction between the concrete slab and the steel beams, two shear connectors were welded in

Experimental behaviour

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~ N~

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,

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I

(a) general l a y o u t

¢/r•¢•i•

i t a u ~.2 mm

4 TI2 4TI0

.¢

T I 0 - 100

(b) c o m p o s i t e b e a m section

(e) end plate (10 mm thick)

Fig. 1. Steel and composite joint specimens.

each troug]h of the metal decking. Grade 43 steel was adopted for all the beams, columns and end-plates. The concrete of the slab was designed to be C30. All the steel coupon test results are summarised in Table 1 and the concrete cube and cylinder test results are given in Table 2. M20 grade 8.8 bolts were used in the end-plate connections. Bolts were tightened using a torque spanner to 150 N.m so as to ensure consistency. This torque level is equivalent to prestressing the bolts to an average force of 30.5 kN.

128

T. Q. Li, D. A. Nethercot, B. S. Choo

TABLE 1 Steel Strengths of the Joint Specimens Ultimate strength (N/trim2)

Yield strength (N/mm 2) Coupons

C-W C-F B-W B-F Plate Rebar T10 Rebar T12

1

2

3

Average

1

2

3

Average

Young's Modulus (N/mm 2)

356 335 440 421 284 477

354 N/A 429 411 270 N/A

353 330 436 407 280 465

354 333 435 413 278 471

498 479 553 550 448 658

502 480 540 539 443 N/A

499 479 552 540 452 647

500 479 548 543 448 653

2-02 x los 2.03 × los 2.06 x los 2.04x l0s 1.99 x los 1-99 x los

489

486

489

488

867

833

849

850

1.99 × los

Note. C-W means steel column web. C-F means steel column flange. B-W means steel beam web. B-F means steel beam flange. Plate means the end plate.

2.2 The monitoring equipment In order to obtain as much information as possible from each test, many parameters were recorded during the tests e.g. connection rotation, beam strain, column strain and rebar strain etc. A general arrangement of the instrumentation for all the joint specimens is shown in Fig. 2. In this monitoring system, one-dimensional strain gauges were used to measure the strains in the steel beam flange, column web and rebars. In order to obtain the strain and hence the stress states in the steel beam and column webs, rosette strain gauges were provided in these areas. Three inclinometers were used to measure the beam end and column web in-plane rotations. Deflections were also measured along the beams at several points. Maximum crack widths on the top surface of the concrete slab were measured for all the six composite specimens during the test by using a microscope. In addition to the above measurements, strain gauged bolts were used as the top bolts in the end-plate connections. From the measured bolt strains, the bolt forces could be evaluated using the known calibration factors for each of the bolts. The strains developed in the reinforcement during hardening of the concrete slab were also monitored for two of the composite specimens in order to evaluate the initial strain in the reinforcement before testing.

Experimental behaviour •

1

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(b) position of rebar strain gauges for specimen CJS-1 sectionl section2 section3 sectm4 section5

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(c) position of rebar strain gauges for specimens CJS-2,3,4,5,6 Fig. 2. Arrangement of instrumentation for the joint specimens.

J

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Experimental behaviour

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132

T. Q. Li, D. A. Nethercot, B. S. Choo

2.3 Test set-up and data logging As mentioned earlier, all the six composite joint specimens had the same geometrical sizes and the same nominal material properties. The variables for the six composite joint specimens were the load position and load ratio; the load positions and ratios for all seven specimens are shown in Fig. 3. The load types have been divided into two groups. In group one [Fig. 3(a),(d) and (e)], balanced loads were applied to the specimens but the shear spans were changed so as to vary the shear/moment ratio. In the second group [Fig. 3(a)(c) and (f)], the shear spans were kept constant, but the load ratios between the two ends were changed so that the non-symmetrical moment effect could be investigated. It can be seen from Fig. 3 that the load arrangements (c) and (f) are exactly the same apart from the fact that specimen CJS-6 was designed without shear connectors on the transverse stub beams. This was used to investigate the contribution of these shear connectors to the transfer of the unbalanced reinforcement forces within the non-symmetrically loaded connections. The set-up for the specimen in the test rig is shown in Fig. 4. Possible horizontal movement of the test rig was prevented by providing a diagonal bracing that allowed for the non-symmetrical load to be applied. Loads were applied to the top surface of the composite beam through hydraulic jacks and the forces were measured by load cells placed between the hydraulic jacks and the spreader beam on the test specimen.

2P

1473

I

(a) SpecimensSJS-I and CJS-I

(b) SpecimenCIS-2

(c) SpecimenCJS-3

(d) SpecimenCJS-4

(e) $pecimeaC3S-5

(0 SpecimenCJS-6

Fig. 3. Load arrangementfor the joint specimens.

Experimental behaviour

133

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o

'r~ 0

0 0 0 0

u

oooo11

T. Q. Li, D. A. Nethercot, B. S. Choo

134

2.4 Test results The main test results are presented in Fig. 5 in the form of plots of the connection moment against the post-processed data which were calculated directly from the recorded data. The detailed test results for each of the individual parameters can be found in Ref. 21. The connection moments were taken as the product of the applied load and the length of the lever arm assumed to 200 175 --m--

150

sjs- I cjs-I

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average connection rotation (mrad) (a) moment rotation curves 200 175 • -

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top bolt strain (micro-strain) (b) top bolt strains Fig. 5. Joint s p e c i m e n test results.

2100

Experimental behaviour

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steel beam bottom flange strain (micro-strain) (d} beam bottom flange strains

Fig. 5. Continued. be the distance from the centre of the applied load to the outside surface of the colunua flange as illustrated in Fig. 3. The connection rotations were obtained by taking the difference of the inclinometer readings between the beam end and the column web. Only the average rebar strains at section 3 defined in Fig. 2 are presented in this paper, the strains at other sections can

T. Q. Li, D. A. Nethercot, B. S. Choo

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sjs-I

+

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b e a m w e b h o r i z o n t a l strain ( m i c o - s t r a i n )

(f) steel beam web horizontal strains Fig. 5. Continued.

be found in Ref. 21. The principal strains in the column web and beam web were calculated using the measured three direction strains obtained from the rosette strain gauges. In order to justify the strain measurement of the beam and column webs on one side only, the strains were measured on both sides of the webs for specimen CJS-1. A comparison of the measured principal strains on either side of the web is shown in Fig. 6. It demonstrates that the measured strains

Experimental behaviour

137

75

60

/

Z 45

E 30

15

mr

J

sj$-I 1

{_.-/ f 200

I00

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beam web horizontal strain (mico-strain) (g) steel beam web horizontal strains 175

_ j..-r ~ ' - - " ~ ....lr

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m

--*

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~s-4

1000

2000

3000

4000

5000

A

cjs-5

&

cjs-6

6O00

rebar average strain at section 3 (micro-strain) (h) rebar average strains at section 3

Fig. 5. Continued.

from the two sides of the webs are very close, thereby confirming that it is satisfactory to measure the beam or column web strains from only one side. The maximum crack widths on the top surface of the concrete slab were measured using a microscope and the results are presented in Fig. 7, plotted against connection moment as well as average reinforcement stress at section 3 (Fig. 2) for all the composite specimens. A typical distribution of the crack pattern on the top of the concrete slab is presented in Fig. 8 for specimen

T. Q. Li, D. A. Nethercot, B. S. Choo

138 200 175

%,

\

150

, • 125

f /

~ E

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- - @ - -

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- | 500

- 1000

-500

500

1000

1500

principal strain at c o l u m n web (micro-strain) (b) c o l u m n w e b p r i n c i p a l strains

Fig. 6. Verification of one side measurement of the web strain.

CJS-1 after testing. In order to know the prestress of the reinforcement caused by concrete hardening before any connection load was applied, the development of rebar strains during concrete hardening was also monitored for specimens CJS-4 and CJS-6 and these results are given in Fig. 9, shown plotted against the time after the concrete was cast.

Experimental behaviour

139

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~

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,

>"-

--"--

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CJS. I

e' ~

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0.30

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maximum crack width (mm) (a) crack width versus connection moment 600 540

~ / ~

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----

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CIS-3 CIS4

CJS'5 CJS-6

120

0.00

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0.30

0.45

0.60

0.75

0.90

1.05

1.20

1.35

1,50

1,65

1.80

maximum crack width (mm) (b) crack width versus rebar stress

Fig. 7. Maximum crack widths of the top surface of concrete slab. 3 ANALYSIS AND DISCUSSION OF THE TEST RESULTS In the following discussion, the computed stresses and forces are used rather than the measured strains. One-dimensional stresses for the beam flange, column web and reinforcement are computed from the measured strains according to Hooke',; law and the Young's Modulus given in Table 1. Since Hooke's law is not applicable beyond the yield limit, it should be mentioned that the

T. Q. Li, D. A. Nethercot, B. S. Choo

140

Fig. 8. Typical crack pattern of the concrete slab.

160 A •~

i e~''~

140

•

.O 120 .2 E

S-I (cjs~l

O----- S-| |cj.s-61

100

*

~___i ~ '

S-21c~41

O------- S-2 Icjs-61 S-3 lois-41 S-31¢i.*1-6)

.~

2o

40

80

120

160

200

240

280

time (hour)

Fig. 9. Rebar strain during concrete hardening.

stresses shown in all the figures in this paper will not be true values once the relevant yield strength has been exceeded, von Mises stresses in the steel beam and column webs are calculated according to the measured three direction strains of the rosette strain gauges. Similar to the one dimensional stresses, the von Mises stresses shown in all the figures are also not true stresses when the values are beyond the yield limit. Bolt forces are obtained from the measured bolt strains and the conversion factors; the conversion factors were calibrated for each of the gauged bolts before testing. The limits o f the yield stresses and the yield forces are marked on the relevant figures by block lines.

Experimentalbehaviour

141

3.1 Test results with different shear/moment ratios

3.1.1 Top bolt forces The variations of bolt force with connection moment are plotted in Fig. 10. It shows that the bolt forces for the bare steel connection (SJS-1) remained constant at the beginning of the loading. When the connection moment reached about 10 kN.m, the bolt force was just starting to increase along with the connection moment (external force). This phenomenon was expected since the bolts were prestressed during the assembly of the specimens through tightening of time bolts. Because of prestresses, the bolt stress would not need to increase wlhilst the connection moment was small because the prestress in the bolts was sufficient to sustain the applied connection moment. According to this, the connection moment corresponding to the start of bolt force increase should be approximately equal to the moment that can be supported by the prestressed bolt forces. This is confirmed by the following calculations: since the initial bolt force is about 30.5 kN for each bolt, the total bolt force at the top bolt row is: F b , t ~-~ 2Fb

(1)

= 2 X 35.5 = 61 KN.

Approximating the length of the lever arm in the connection as 90% of the distance from the top bolt row to the bottom flange (Fig. 2), then the connection moment to be supported by the prestressed bolt force is:

(2)

Mb.p = F~,.t&ev~r= 61 X 0.9 X 207 = 11.4 kN.m.

200 175

"•150 0

!.

[

__

"-

50

, ~-m

-"

--

25 o 20

40

60

80

100

120

140

160

180

200

220

240

top bolt force (kN) F i g . 10. T o p b o l t f o r c e v e r s u s c o n n e c t i o n

moment.

•

sjs-I

Q

cjs-I

*

cjs-4

-, 0

cjs-5

142

T. Q. Li, D. A. Nethercot, B. S. Choo

This is very close to the connection moment when bolt forces started to increase in the test and explains the delay of bolt force increase. Since the top bolt row carded all the tensile force in the pure steel connection, the top bolts were finally yielded in the test of SJS-1 when the applied moment was close to the connection moment capacity. In contrast to the pure steel connection, the top bolt forces in the composite connections decreased at the beginning of the loading. This is because the neutral axis of the connection was higher than the top bolts since the concrete slab was uncracked. Because the top bolts were initially in the compression zone, the applied bolt force was in compression, therefore, the prestress was reduced during the early stages of loading. However, after the connection moment exceeded the moment level corresponding to cracking in the concrete, the top bolt force started to increase. This was due to the reduction of slab stiffness caused by cracking, resulting in the neutral axis moving down, thereby putting the top bolt row into the tension zone. It was at this stage that the bolt forces started to increase. By examining the moment levels at which the top bolt force started to increase, it has been found that the corresponding moments are about 38 kN.m, which is slightly higher than the moments (31 kN.m) corresponding to concrete cracking determined elastically using the test material properties. This indicates that the neutral axis of the connections still remained above the top bolt row when the concrete slab was just cracked. As cracking progressed, the neutral axis moved below the top bolt row and the bolt force started to increase. This increase in bolt forces was fairly slow during the early stages since the reinforcement sustained most of the tensile force in the connection. The rate of increase became faster as connection moment increased. However, the bolt forces did not increase significantly until the applied connection moment reached about 125 kN.m for all three specimens. This moment level corresponds to yield of the reinforcement. At this stage, any further increment of the tensile force in the connection had to be sustained by the bolts, therefore the rate of increase of the bolt force became faster. Although the increase of the bolt force was significant with increased connection moment during later stages of the test, the bolts did not yield for any of the three composite specimens. The reasons for the comparatively low bolt forces in the composite connections were: • tensile forces in the connection were principally transferred by the reinforcement since it is far from the neutral axis of the connection; • the strain hardening of the reinforcement gave an increase in its capacity after the reinforcement yielded; • deformation in the upper region of the end-plate released the bolt forces and limited the increase of the bolt force according to the deformation compatibility.

Experimental behaviour

143

The defiarmations of the top of the end-plate were visible for all three specimens and a typical result from specimen CJS-5 is shown in Fig. 11. From a comparison of the test results among the connections with different shear/moment ratios, it is clear that the top bolt forces are larger when the shear/moment ratio is higher. This may be caused by the higher shear stresses in the bolts causing the longitudinal strain gauge within the gauged bolts to exhibit a higher reading during the test and thus to indicate a higher bolt force. Although the trend did exist for the bolt forces to increase with the connection shear/moment ratio, it was not particularly significant and more evidence is required before any firm conclusions may be drawn. Figure 10 also shows that the bolt force for specimen CJS-5 exhibited a sudden increase at a moment of about 127 kN.m; this was caused by the sudden shear slip between the slab and the steel beam. A loud sound was heard during the test when this slip happened. It will be shown later that this slip also reduced the rate of increase of rebar stress for several load increments. 3.1.2 Steel beam stresses The steel beam flange stresses are shown plotted against connection moments in Fig. 12. It indicates that the stress in the top flange of the steel beam for the pure steel connection SJS-1 increased only during the initial stage of loading. After the connection moment exceeded about 60% of its capacity, the top flange stress began to decrease. The initial increase of the top flange stress

Fig. 11. The separation of end-plate from column flange.

144

T. Q. Li, D. A. Nethercot, B. S. Choo 200

~

O..-O

175 150 Z

•

125

,

SjS-I cjs-I

J

o O------- cjs-5

25 0

-30

0

30

60

90

120

150

180

210

240

steel beam top flange stress ( N / r a m 2 )

(a) top flange stresses 200 175

\

150 Z "~ 125

~

•

s~-I ¢~-I

z00

--4~-.--

cJ$.4 cjs-5

~

5o 25 0 .500

.450

4o0

-350

-300

-250

.200

450

-I00

-so

o

steel beam bottom flange stresses (N/mm2)

(b) bottom flange stresses Fig. 12. Measured steel b e a m stresses.

was expected since the top flange was in the tension zone of the connection. At this stage, the tensile force in the beam is mainly sustained by the top flange with this force being transferred to the top of the end plate through the welds. Through the bending of the end-plate, the force from the top flange was transferred to the top bolts and eventually into the column. However, once the end-plate yielded and large deformations were present as shown in Fig. 13, this load path was no longer effective and a new load path was initiated. Now that the top flange force cannot be effectively sustained by the

Experimental behaviour

145

175 150 Z ~,~ 125

50

"f

25

"

~-1

-----0

ejs-I

-------o

cj#-$

]k _.,-"

0 *180

-150

-120

-90

-60

-30

0

beam web horizmtal s m ~

30

60

90

120

(N/nun2)

(c) beam w e b horizontal stress 200 175

150

J

j

s'//

i

0

,,,

•

sjs-I

u,

~-I

i

cj~l~

75

25 0 0

50

100

150

200

250

300

350

400

450

500

steel beam web Von Mises stress (N/ram2) (d) beam web Von Mises stress

Fig. 12. Continued.

end-plate, the tensile force in the top flange had to be redistributed to the upper part of the beam web since this part was closer to the top bolts and was able to transfer the force. In this new load transfer mechanism, the tensile force in the beam was mainly sustained by the upper part of the beam web. The tensile force was then transferred to the end-plate but to the location around the top bolts rather than to the top of the end-plate. Through horizontal bending of the end-plate (T-stub action), the force from the beam web was transferred to the top bolts and eventually into the column. In this new load

146

T. Q. Li, D. A. Nethercot, B. S. Choo

Fig. 13. The failure of specimen SJS-1.

transfer mechanism, the top flange is no longer very effective in transferring the internal force, resulting in the decrease of the top flange stress during the later stage of the test. Because the length of the internal lever arm of the connection in the new load transfer mechanism was reduced, it was expected that both the tension and compression forces would be larger for the same connection moment. This is demonstrated in Fig. 12(b) by a fast rate of increase in the bottom flange stress during the later stages of the test. In contrast to the pure steel connection, the top flange stresses for the composite connections were initially compressive and gradually changed to tensile. The relationship between the moment and the top flange stress was similar to that between the moment and the force in the top bolt row. In the same way as for the bolt forces, the top flange was initially in the compression zone and moved to the tension zone after the concrete slab was cracked. Figure 12(a) shows that the top flange stresses were relatively small, especially for the connections with the higher shear/moment ratios. The top flange stresses for the three specimens remained below the yield strength. These low top flange stresses are due to: • the tensile forces in the connection being mainly transferred by the reinforcement since it is positioned far from the neutral axis of the connection; • the deformation of the end-plate preventing the increase of the top flange stress as explained earlier for the pure steel connection.

Experimental behaviour

147

Figure 12(a) also shows that an increase of shear/moment ratio tended to reduce the top flange stresses, but more test evidence is required for this to be confirmed. In contrast to the top flange, very high bottom flange stresses were developed in the tests, with values for all three composite specimens exceeding the yield strength. This was expected since most of the compression force in the beam had to be carded by the bottom flange and then transferred to the column. It is clear from Fig. 12(b) that the shear/moment ratio significantly affected the stress level in the bottom flange when connection moments were similar. The higher the shear/moment ratio, the lower the bottom flange stress. This phenomenon may be explained as follows: if the applied loads are closer to the column (high shear/moment ratio), some of the applied vertical load on the beam could be directly transferred to the column by a diagonal path through the steel beam web rather than through the bending path with the bottom flaJage in compression. By this separation of the applied load, the actual connection moment was smaller than the nominal connection moment obtained as the product of the applied vertical load and the lever arm. Therefore, the bottom flange stress was smaller for the connections with high shear/moment ratios, since this stress is more closely related to the actual connection moment rather than the nominal connection moment. The test horizontal stresses at the centreline of the steel beam web are given in Fig. 12(c), which shows these stresses to be tensile for the pure steel connection and compressive for all the composite connections. Thus, from the sign of this stress it follows that the neutral axis for the pure steel connection was lower than the centreline of the steel beam web and the neutral axis for the composite connections was higher than the centreline of the steel beam web. The shear/moment ratio effect on this stress was not significant and can only be seen in the later stages of loading. As the steel beam web was under the action of a two-dimensional stress system, the yield of the steel beam web should be judged by a relevant yield criterion. According to the measured three direction strains of the rosette, von Mises stress for the beam web at the measuring point was calculated and is given in Fig. 12(d). It can be seen that the von Mises stresses are larger for the connections with the higher shear/moment ratios. This was expected since the high shear force will produce a high beam web shear stress. In the elastic stage the yon Mises stress of the beam web was almost proportional to the connection shear/moment ratio for the same connection moment. These results demonstrable that the steel beam web will yield earlier when the connection shear/moment ratio is higher. Therefore, if the steel beam web falls into the compression zone and its resistance is considered in the connection moment calculation, its horizontal design strength should be reduced according to the beam web shear stress (connection shear force). This will be discussed in the companion paper 22 on the prediction of the connection moment capacity.

T. Q. Li, D. A. Nethercot, B. S. Choo

148

3.1.3 Column web stresses The variation of column web stresses against connection moments is shown in Fig. 14. Figure 14(a) indicates that all the column web horizontal stresses at the strain measuring point shown in Fig. 2 eventually exceeded the yield strength. As the internal lever arm of the connection was smaller for the pure steel connection, the column web horizontal stress was accordingly higher for the pure steel connection at the same connection moment level. It is shown in Fig. 14(a) that the shear/moment ratio had no significant effect on the column web horizontal stress in the initial stages of loading and no consistent 140 120

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influence iLn the stage immediately prior to and after yield. The column web von Mises stress (for location see Fig. 2) is shown in Fig. 14(b). There is no consistent relationship between the connection shear/moment ratio and the column web von Mises stress. 3.1.4 Reirforcement stresses Reinforcement strains were measured at either three or five sections as shown in Fig. 2; the results from section 3 are presented herein. The average stresses of the reinforcement at section 3 are presented against the connection moment in Fig. 15. It indicates that rebar stresses are not affected by the connection shear/moment ratio as all three curves are similar. The rebar stresses for all the three specimens exceeded the yield strength before the ultimate connection moments were reached. The effects of concrete cracking on the rebar stress can be seen at a connection moment of about 30 kN.m corresponding to the point that ~he rate of the rebar stress increase became faster. Once the concrete slab was cracked, the force sustained previously by the concrete slab had to be redistributed to the reinforcement. This caused the reinforcement stress to increase faster. It can also be seen that the increasing rate of rebar stress for specimen CJS-5 at a moment of about 130 kN.m became slower for several increments; this is due to the shear slip between the slab and the steel beam as mentioned before in the discussion of bolt forces. 3.1.5 Moment-rotation curves and connection moment capacity Connection rotations are obtained by taking the difference between the readings of the beam end inclinometer and the column web inclinometer. It is 175 9"

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T. Q. Li, D. A. Nethercot, B. S. Choo

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clear from the moment-rotation curves in Fig. 16 that composite connections with exactly the same steel detail have a considerably higher moment capacity than the pure steel connection as demonstrated by all previous test r e s u l t s . 1-19 This further confirmed that the benefit of high moment capacity can readily be achieved when designing the steel connection as composite. In addition to this, the rotation capacities of the composite connections as compared with the steel connection were not significantly reduced. This showed the advantage and feasibility of utilising properly designed composite connections since they can achieve a high moment capacity without losing ductility. From the comparison of the moment-rotation curves between specimen CJS-1 and CJS-4, it can be seen that connection moment capacity and stiffness are slightly lower for the high shear/moment ratio specimen CJS-4. However, this observation is not supported by the comparison between specimen CJS4 and specimen CJS-5. In fact, the moment capacity for the high shear/moment ratio specimen CJS-5 is actually higher than that for the low shear/moment ratio specimen CJS-4. This may be caused by the formation of a diagonal loading path in specimen CJS-5 as discussed earlier. Owing to this division of the applied load, the actual connection moment was smaller than the nominal connection moment calculated by the vertical load times the span length. The formation of this diagonal load path is confirmed by the smaller stresses in the bottom flange of the steel beams for the higher shear/moment ratios shown in Fig. 12(b). According to this load transfer mechanism, the nominal connection moment capacity (the product of vertical load and shear span) could be higher for the connections with a very short shear span and higher beam web shear strength than the same connections with a long shear span. Figure 17 shows that the failure of specimen CJS-5 is a typical shear failure, it can be 200 1"/5 150 --1

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151

152

T. Q. Li, D. A. Nethercot, B. S. Choo

seen that the diagonal cracks were formed on the concrete slab, and the vertical yield lines were visible on the steel beam web. In addition, the effect of shear connector slip on the moment-rotation curve can also be seen for specimen CJS-5 when a sudden increase in the rotation occurred at a moment of about 127 kN.m. This corresponds to the sudden increase of the bolt force and the sudden decrease of the stress increment in the reinforcement discussed above. This also indicates that slip between the slab and the steel beam will reduce the connection stiffness and increase the connection rotation.

3.2 Test results with different moment ratios Including specimen CJS-1, a total of three moment ratios between left and fight hand side connections of 1:1, 1:05 and 1:0 were adopted in the tests. In the following discussion, the results from the larger moment side of the connection are used for connections CJS-2, CJS-3 and CJS-6 which have nonsymmetrical moments; the average results are used for connection CJS-1 which has symmetrical moments. Two specimens CJS-3 and CJS-6 were designed with the same moment ratio 1:0 but different shear connector arrangements on the transverse stub steel beams. Specimen CJS-3 had one shear connector on each side of the stub beams, and specimen CJS-6 had no shear connectors on the stub beams. This arrangement was designed to investigate the contribution of these shear connectors to the transfer of the unbalanced rebar force in the non-symmetrical moment connections. It is known that the rebar forces are different between fight and left hand side connections if the connections are loaded non-symmetrically and that the unbalanced rebar force has to be sustained by the column. There are two possible paths for transferring this force into the column; these are: (i) contact of the concrete slab with the column on the smaller moment side of the connection, for which detailed discussion is presented in the companion paper, 22 (ii) shear connectors on the transverse steel beams which is discussed below. 3.2.1 Top bolt forces The test results for the variation of the top row bolt forces versus connection moments are presented in Fig. 18. The top bolt forces also decreased in the early stages of loading, due to the neutral axis being raised by the contribution of the concrete slab before it was cracked as explained above for the symmetrically loaded connections. It is interesting to note that the bolt forces for the two specimens CJS-3 and CJS-6 reduced for a second time after the bolt forces were restored to the prestress levels. The second reduction was probably caused by yield of the column web in shear, which would raise the neutral

153

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axis and &;crease the distance between the bolts and the neutral axis, thereby leading to a reduction of the bolt forces since the bolts are now closer to the neutral axis. After the bolt forces reduced to the previous lowest level, the bolt forces started to increase again until the connection ultimate moment was reached. However, the bolt forces remained at a relatively low level even at the ultimate moment level. In addition to the reasons given above for the low bolt forces, the higher neutral axis caused by early yield of the column web in shear contributes to these low bolt forces as well. The second increase of the bolt force was the result of the column web stiffening since this provides some support to the bolts and permits them to attract more load. Comparing the bolt forces for different non-symmetrical moment ratios shows that an increase in the moment ratio will cause a reduction in the bolt forces. 3.2.2 Steel beam stresses The stresses in the steel beams are shown in Fig. 19, from which it can be seen that the top flange stresses of the steel beams were also compressive during the initial stage of loading and altered to be in tension later. This was also caused by the movement of the neutral axis as influenced by cracking of the concrete slab. Consistent with the bolt forces, a higher non-symmetrical moment ratio also tended to produce lower top flange stresses in the steel beam. The bottom flange stresses were very high for all the specimens as shown in Fig. 19(b), with values exceeding the yield strength before the ultimate connection moment was attained. The non-symmetrical moment tended to decrease the bottom flange stress as shown by comparison between specimen

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T. Q. Li, D. A. Nethercot, B. S. Choo

156

the fact that the column webs were completely yielded in shear for the nonsymmetrical moment connections, thus limiting the beam stresses. 3.2.3 Column web stresses

The variation of column web stresses against connection moments is shown in Fig. 20. It can be seen that these stresses exceeded the yield strength for all the specimens. The general indication from comparing the test results is that non-symmetrical load tended to reduce the horizontal stress on the column web. The reason is that this stress is generated by the compression of the 200

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beam bottom flanges, since the force from the bottom flange on one side is smaller than that on the other side, this compression action is not so effective. The result of the lower column web horizontal stresses for the connections with high non-symmetrical moment ratios is consistent with that obtained for beam bottom flange stresses as shown in Fig. 19(b). As expected, the column web von Mises stresses shown in Fig. 20(b) were very high for the connections with non-symmetrical load even at moment levels well below the ultimate moment capacities of the connections. This is because the,, column webs were subjected to high shear and the shear stresses make a greater contribution to the von Mises stress than the normal stresses. It shows that the column webs were all yielded at a relatively low moment level for the connections with large non-symmetrical moment ratios. Even though the column webs contained high stresses, local buckling of the column web was not seen for any of the seven specimens. This was attributed to the stiffening effect provided by the end-plate on the transverse stub beam. Because of this restriction, large deformations and strain hardening of the column web could be achieved. This allowed the load capacity of the column panel zone to further increase and resulted in the connection moment further increasing even though the column web was yielded. 3.2.4 Reinforcement stresses The variation in average reinforcement stresses at section 3 against connection moment is shown in Fig. 21, from which it is clear that the non-symmetrical moment has no significant influence on the rebar stresses since these stresses are similar for all the specimens. Similar to the symmetrical moment connections, the rebar stresses for all specimens exceeded the yield strength. The 175 150 125

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T. Q. Li, D. A. Nethercot, B. S. Choo

effects of concrete cracking on rebar stress can also be seen from the change in slope of the curves at a moment of about 30 kN.m when the concrete slab first cracked. 3.2.5 M o m e n t - r o t a t i o n curves a n d connection m o m e n t capacity

The connection moment-rotation curves are shown in Fig. 22. As mentioned earlier, the connection rotations were obtained by taking the difference between the readings of the beam end inclinometer and the column web inclinometer. By this approach, the column panel zone shear deformations have not been included in the connection rotation. Therefore, the connection rotations are smaller for the non-symmetrical load connections as shown in Fig. 22. Except for specimen CJS-3, the moment capacities for all specimens are similar, but the initiation of yield does vary as indicated in Table 3. The order of yield of the connecting components for a symmetrically loaded connection CJS-1 starts from the column web at the bottom flange level of the steel beam and proceeds into the reinforcement. The first yield location for non-symmetrically loaded connections is in the column panel zone, demonstrated by the von Mises stress exceeding the yield strengths and then the reinforcement or column web. The reason for the smaller moment capacity of specimen CJS-3 was that excessive column panel deformation prevented further loading because of the stroke limit of the loading equipment. It was expected that if the loading had been continued for specimen CJS-3, a slightly higher connection moment capacity would have been achieved.

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45

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Experimental behaviour

159

TABLE 3 Yield Locations of the Joint Specimens

Yield order Specimens

1st

2nd

3rd

4th

5th

M, (kN.m)

SJS-1

End-plate* (54%)

Bolt (95%)

C-H (100%)

C-M (100%)

CJS- 1

C-H (61%)

Rebar (77%)

C-M (92%)

B-B-F (92%)

CJS-2

C-M (77%)

Rebar (77%)

B-B-F (87%)

176.0

CJS-3

C-M (53%)

C-H (84%)

Rebar (86%)

148.5

CJS-4

C-H (45%)

Rebar (66%)

C-M (82%)

B-B-F (87%)

CJS-5

C-H (58%)

Rebar (68%)

C-M (74%)

B-M (88%)

CJS-6

C-M (42%)

Rebar (84%)

B-B-F (88%)

C-H (92%)

62-8

B-M (100%)

181.5

177.5

B-B-F (99%)

197.2

174.0

Note. *End-plate strain was not measured, this is obtained according to the joint deformation. C-H means column web horizontai yield at steel beam bottom flange level. C-M means column web von Mises stress reaches yield strength. B-B-F means steel beam bottom flange yield. B-M means steel beam web von Mises stress reaches yield strength. The figures in parentheses are the relative load level when the yield occurred. Mu is the connection ultimate moment capacity. No rebars were fractured in all the tests. All tests were stopped because of excessive deformation.

3.2.6 Contribution of the shear connectors on the transverse steel beams to the transfer of the unbalanced reinforcement force It w a s menttioned a b o v e that the s p e c i m e n s C J S - 3 and C J S - 6 w e r e d e s i g n e d to i n v e s t i g a t e the contribution o f the shear c o n n e c t o r s on the t r a n s v e r s e steel b e a m s to the transfer o f the u n b a l a n c e d r e i n f o r c e m e n t force within the c o n n e c tions. This c o n t r i b u t i o n c a n n o t b e seen f r o m either the m o m e n t - r o t a t i o n c u r v e s or the reba:r forces. T h i s w a s p r o b a b l y b e c a u s e the l o w stiffness o f the shear c o n n e c t o r s c o m p a r e d with the contact o f the c o n c r e t e slab with the c o l u m n ,

160

T. Q. Li, D. A. Nethercot, B. S. Choo

prevented any significant contribution to the transfer of unbalanced reinforcement force within the connections. The effectiveness of load transfer depends on the stiffness of the relevant elements that can sustain the unbalanced rebar force. Since the stiffness in compression between the concrete slab and the column flange is relatively high, most of the unbalanced reinforcement force was sustained by this contact force rather than by the shear connectors on the transverse beams. It can be envisaged that if the concrete strength is lower, then a high contact force between the concrete slab and the column flange may not develop because of concrete local compression failure. This failure has been observed in a semi-rigidly connected composite frame test. 23 Although this type of failure did not actually occur in any of the present tests, signs of concrete local compression failure outside the column flange were visible as shown in Fig. 23(b). If the column web of the specimen had been stronger or the concrete strength lower, then concrete local compression failure could have occurred for specimens CJS-3 and CJS-6. It may be concluded that the contact strength of the concrete slab with the steel column is one of the factors that controls the connection moment capacity if the connections are subjected to extremely unbalanced moments.

3.3 Supplementary test results The measured maximum crack widths on the top surface of the slab against connection moment as well as the average reinforcement stress at section 3 have been plotted in Fig. 7(a) and (b), respectively. These show that except for specimen CJS-2 the crack widths are quite consistent with the connection moment until the crack width exceeds 0.3 mm. After this, the crack widths vary for the same connection moment. Quantitatively speaking, the connection moment can reach 70-100 kN.m before the maximum crack width reaches 0.3 mm; this moment level was about 40-60% of the connection ultimate moment. If the maximum crack width is limited to 0.3 mm in the serviceability limit state, then about 50% of the test ultimate connection moment capacity can be utilised for connections similar to the test specimens. It is also clear from Fig. 7(b) that the same 0.3 mm crack width limit can allow the rebar stress to reach at least 150 N/mm 2 and possibly as high as 300 N/mm 2. So it can also be concluded that if the rebar stress is limited to be 150 N/mm 2, the maximum crack width can accordingly be limited to be 0.3 mm for connections similar to those tested. It is shown in Fig. 8 that a compressive strain of about 130/xE was generated in the rebar before the connection was loaded owing to the contraction of the concrete during its hardening. This strain level corresponds to a stress of about 27 N/mm 2. Even though this stress if not very high, it demonstrates

Experimental behaviour

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162

T. Q. Li, D. A. Nethercot, B. S. Choo

that the composite connections were prestressed with the rebars in compression and the concrete in tension before the actual loads were applied. This maximum prestress corresponds to about 10% of the material yield strength; these findings can also be used in the numerical modelling of the composite connections when comparing against test results.

4 CONCLUSIONS

(i) All the connection tests were terminated because of excessive deformation that prevented further load from being applied. This indicates that properly designed composite end-plate connections have large rotation capacities. The order of yielding of the connecting components is summarised in Table 3, from which the weaker parts of the connections can be identified. (ii) It has been found from the experimental studies that variations in shear force have little effect on the moment capacities of flush end-plate composite connections. An influence is present only when the shear force is relatively high compared with the steel beam web shear capacity. (iii) The effect of non-symmetrical moments on the connection moment capacity is significant only when the unbalanced rebar force is higher than the column web shear resistance or the contact strength between the concrete slab and the column face. (iv) It is further confirmed from this series of tests that the reinforcement provides a major contribution to the enhanced performance of composite connections. (v) The concrete crack width will not exceed 0.3 m m until the connection moment reaches about 50% of the test connection moment capacity. Corresponding to this crack width, the rebar stress was at least 150 N/mm 2 for all the specimens. So by controlling the rebar stress to be less than 150 N/ram 2 or the connection moment to be smaller than 50% of its ultimate moment capacity, the concrete crack width can be controlled to below 0.3 m m for connections similar to the specimens tested herein. (vi) Because of contraction of concrete during its hardening, it is inevitable that a prestress with concrete in tension and reinforcement in compression will be built up in the connections. The reinforcement stress caused by this action can reach 10% of its yield strength for moderate reinforcement ratios.

Experimental behaviour

163

ACKNOWLEDGEMENTS The work reported in the paper was supported by a grant from EPSRC (formerly SERC). The tests were conducted in the Structural Laboratory of Nottingham University. Mr M. Bettison provided considerable assistance during the conduct of the tests. All steel for the testing was supplied as free issue by British Steel.

REFERENCES 1. JohnsolqL, R. P. & Hope-Gill, M., Semi-rigid Joints in Composite Frames. IABSE Ninth Congress, Preliminary Report, Amsterdam, 1972, pp. 133-144. 2. Benussi, F., Puhali, R. & Zandonini, R., Semi-rigid joints in steel-concrete composite frames. Construzioni Metalliche, 5 (1989) 1-28. 3. Anderson, D. & Najafl, A., Semi-continuous composite frames in Eurocode 4. Constructions in Steel Structures II: Behaviour, Strength and Design. Proc. Second Int. Workshop. Pittsburgh, PA, 10-12 April 1991, pp. 142-151. 4. Xiao, R., Choo, B. S. & Nethercot, D. A., Composite connections in steel and concrete. I. Experimental behaviour of composite beam-column connections. J. Construct. Steel Res., 31 (1994) 3-30. 5. Anderson, D. & Najafi, A. A., Performance of composite connections: major axis end plate joints. J. Construct. Steel Res., 31 (1994) 135-144. 6. Puhali, R., Smotlak, I. & Zandonini, R., Semi-rigid composite action: experimental analysis and a suitable model. J. Construct. Steel Res., 15 (1990) 121-151. 7. Davisort, J. B., Lam, D. & Nethercot, D. A., Semi-rigid action of composite joints. The Structural Engr, 68 (1990) 489--499. 8. ZandonJLni, R., Semi-rigid composite joints. In Stability and Strength of Connections, ed. R. Narayanan. Elsevier Applied Science, Barking, 1989, pp. 63-122. 9. Nethercot, D. A. & Zandonini, R., Recent studies of the behaviour of semirigid composite connections. American Society of Civil Engineer Structural Conf. Atlanta, April 1994. 10. Bamard, P. R., Innovations in composite floor system. Paper presented at the Canadian Structural Engineering Conf. Canadian Steel Industries Construction Council, 1970, p. 13. 11. Echeta, C. B. Semi-rigid Connection for Between Concrete Filled Steel Columns and Composite Beams. Ph.D. Thesis, University of London, 1982. 12. Echeta, C. B. & Owens, G. W., A semi-rigid connection for composite frames: initial test results. Proc. Int. Conf. on Joints in Structural Steelwork, ed. J. H. Howlett et al. Pentech Press, London, 1981, pp. 3.20-3.38. 13. Xiao, R., Behaviour of Composite Connections in Steel and Concrete. Ph.D. Thesis, Department of Civil Engineering, University of Nottingham, February 1994. 14. Leon, R. T., Semi-rigid composite construction. J. Construct. Steel Res., 15 (1990) 99-119. 15. Leon, R. T. & Ammerman, D. J., Semi-rigid composite steel frames. AISC Engng J., Fourth Quarter (1987) 147-155. 16. Van Dalen, K. & Godoy, H., Strength and rotational behaviour of composite beam-to-column connections. Can. J. Civil Engng, 9 (1982) 313-322.

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