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Behaviour of flexible end plate beam-to-column joints
J. Construct. Steel Research 16 (1990) 111-134
Behaviour of Flexible End Plate Beam-to-Column Joints A. K. Aggarwal Department of Civil Engineering, University of Technology, Private Mail Bag, LAE, Papua New Guinea (Received 5 September L989;revised version received 24 April 1990; accepted 23 May 1990)
A BSTRA CT The paper describes an experimental investigation into the structural behaviour of flexible end plate bearn-to-column connections. Eight specimens of bearn-colurnn joints were tested under gradually increasing static loads with 16 rnrn, 20 rnrn attd 24 turn diameter bolts and 12 rnrn and 16 rnm thick flexible end plates for a constant beam attd a constant column cross-section. In two specimens, the flexible plate was conventionally welded to the beam web, while in the other specimens, the plate was welded after coping the beam web. With this modification, load carrying capacity attd end restraint of joints increased considerably. Design offlexible end plate joints using a serni-rigid method of design may be a better approximation to the actual behaviour.
NOTATION Bo
Maximum permissible bolt force on a bolt due to bearing on material By M a x i m u m permissible bolt force in shear--all failure modes considered Bvn M a x i m u m permissible bolt force in shear with thread included in shear plane Bvx Maximum permissible bolt force in shear on u n t h r e a d e d shank with thread excluded from shear plane c Clearance in angle seat connection Ill J. Construct. Steel Research 0143-974X/90/$03"50(~) 1990 Elsevier Science Publishers Ltd, England. Printed in Great Britain
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li liv /7e
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Bolt diameter Specified yield stress for angle Yield stress of the plate material Yield stress of beam web Distance on a beam from outer face of flange to inner termination of root radius Distance between bottom flange of beam and flexible end plate Length of flexible end plate Effective length of weld on plate Width of shear pad or angle seat Number of bolts in one line Number of bolts in angle seat Maximum permissible longitudinal load/length of fillet weld Root radius of angle Thickness of beam web Thickness of seat angle End plate thickness Fillet weld leg length Plate strength Bolt strength Weld strength Gross strength of beam web Strength of beam web at plate end Strength of angle seat bolts Strength of seat angle Rotation of the beam Rotation of the column
1 INTRODUCTION The structural steel connections which join beams and columns are generally classified as (i) 'simple' or pin-jointed, (ii) semi-rigid and (iii) rigid or fixed. The important difference between these connections in the context of overall structural behaviour lies in their rotational stiffness, i.e. their ability to transmit the restraining effect of the beam to the column and thus to transmit moments. Figure 1 gives typical m o m e n t - r o t a t i o n curves for beam-to-column connections. Even the 'simple' connections have significant rotational stiffness and are able to transfer substantial m o m e n t s from beam to
113
Behaviour of flexible end plate beam-to-column joints
Flexible F--Rotation
of
Joint
(0
-
~)
Fig. 1. Typical moment-rotation curves for beam-column joints.
column. Research has shown that most commonly encountered beam-tocolumn connections are neither perfectly rigid nor simple but fall into an intermediate category of semi-rigid connections. Perfect rigidity and complete flexibility are idealised forms of behaviour and cannot be achieved in practical connections. Research into the behaviour of connections started as early as 1917, when Wilson & Moore I (as referred to by Baker-') conducted experiments to determine the rigidity of riveted joints in steel structures. Since then changes in the design of connections have continued and many investigations have been published offering various design models. Recently a review of all the test data available for beam-to-column connections has been done by Nethercot 3 and in his report, he has expressed the need for additional testing, to help in our understanding of the contribution of connections in the performance of frames. Flexible end plate connection is considered by designers to be 'simple', that is, members meeting at a joint do not have rotational continuity. In this connection a plate is shop welded to the beam web and then the complete assembly is bolted at site to a column flange by high strength bolts. In this study, the behaviour of a flexible end plate connection has been investigated with particular reference to its strength and moment-rotation characteristics.
112.
.4.
K. Aggarwal
2 PREVIOUS W O R K Four series of tests on flexible end plate connections have been identified with two of these conducted in Canada and the other two in Australia. A preliminary study of flexible end plate connection was first done in 1969, when Sommer a carried out an experimental investigation into the behaviour of beam-column joints with the intention to devise a rational basis for their design. At about the same time, Kennedy 5 published the results of his investigation and suggested a design procedure for this connection. He observed that the m o m e n t - r o t a t i o n behaviour of the connection consisted of two distinct phases, the transition between them occurring when the bottom flange of the beam rotates sufficiently to bear against the column flange. In Australia, a study of flexible end plate connection was first carried out by Bennetts e t a l . (' to determine the connection strength under high shear loading. In 1978, the Australian Institute of Steel Construction published a two part manual 7 on standardised connections with a design model for a flexible end plate joint derived from the design procedure suggested by Kennedy. 5 In 1982, Mansell & Pham s conducted experiments on standardised connections to check the design provisions and also to assess the margin of safety awfilable in connections designed using these criteria. They observed a relatively high factor of safety for most of the connections, thus suggesting further investigation into the design criteria.
3 EXPERIMENTAL PROGRAMME To study the behaviour of flexible end plate beam-to-column connections, eight specimens were tested under gradually increasing 'static' loads as shown in the experimental programme given in Table 1. Three different diameter ( 1 6 m m , 2 0 m m and 2 4 m m ) bolts and two different plates (12 mm and 16mm) were used in specimens with constant beam and constant column cross-sections. In two specimens, the flexible end plate was conventionally welded to the end of the beam web (type 1 connection) (Fig. 2(a)) while in the other six specimens, the plate was welded after coping the beam web (type 2 joint) (Fig. 2(b)). The depth of the cut in the beam web was made equal to the plate thickness--to ensure good contact between the beam flanges and the column flange. The beam used in all test specimens was 200 UB 25.4 kg/m and the column 200 UC 46.2 kg/m with British equivalents of 203 × 103 UB 25 and
Behavioar of flexible end plate beam-to-column joints
t15
TABLE 1 Experimental Programme
Specimen ,Vo.
Endplate thickness (ram)
Bolt diameter (ram)
Type of connection
Seat size and location
M 1
16
20
l
M2 M3 M4 M5 M6 M7
16 12 12 16 16 16
20 20 2(/ 24 L6 20
2 1 2 2 2 2
--
-----90 x 90 x 10
MS
12
20
2
90 x 90 x 10
(c) (T) C is c l e a t o n c o m p r e s s i o n f l a n g e o f b e a m . T is c l e a t o n t e n s i o n f l a n g e o f b e a m .
203 × 203 UC 46 respectively. All steel sections conformed to the Australian specification AS 12049 and had a nominal yield stress of 250 N/mm 2 and dimensional tolerances conforming to AS-1227. m The length of the flexible plate was kept constant (150 mm) for all specimens. The test connections were designed according to the Australian Institute of Steel Construction publication, Standardised Connections Manual, 7 Part B and a sketch of the beam-column connection used for testing is shown in Figs 3a and 3b. It may be noted that the Australian design criteria recommend that the length of the flexible end plate could vary from a maximum equal to the depth between flange fillets of the supported beam to a minimum of approximately half the beam depth. In the present study, the length of the plate ( 1 5 0 m m ) was just short of the maximum permissible length by 22 mm. When using this connection, some designers and detailers extend the thin flexible end plate to the bottom of the flange of the beam and weld to it. While doing this, they are concerned with the damage to the thin flexible end plate c o m p o n e n t during transportation of the beam and also expect to achieve better restraint for the column when the tension flange of the beam bears against the column. Such a modification renders the end plate connection very much stiffer than what is assumed in the analysis. In effect, the behaviour of the connection becomes more or less similar to the connection shown in Fig. 2(b) because the lower flange is touching the support right from the beginning of rotation.
ilO
.q. K. Aggarv~al
-i: A Fig. 2(a). Typical flexible end plate joint (type 1).
i"
fBB BPJfFF
JPlJJJh
h~h
h i
11
Fig. 2(b). Modified flexible end plate joint (type 2).
Four bolts conforming to the Australian specifications AS-12521~ (high strength friction grip bolts equivalent to BS 4395 (Part 1)) ~2 were used to connect the beam/plate assembly to the column flange. In specimens M7 and M8, a seat angle (90 × 90 × 10) was also used in addition to the plate. The cleat was shop welded to the beam flange and bolted to the column by 2-M20 bolts. All bolts were tightened using a torque wrench which was earlier calibrated for torque versus axial tension for each of the three bolt diameters used in test specimens. In each test joint, two strain gauged bolts coupled to a strain indicator were used--to measure the initial tension and to monitor subsequent changes under load.
Behaviour of flexible end plate beam-to-column joints 800
117
_J !50
mirror moun:ing holes
50
/ •. . . . . . .
I "
J
1 I
200UB25.4
mirror mounzlng holes lOthk stiffeners
t
Load
L9 ('4
col stiffeners h zdraulic jack
~--
I'
l. . . . .
--~-~
' uaso ~rame ,,,. ............
---n
I .,,,,,,J
.......
.v,
I
Fig. 3(a). Test specimen and loading arrangement.
4 C A L C U L A T E D B E H A V I O U R OF TEST SPECIMENS For specimens M1-M6, tested without a seat angle, the maximum allowable load for each connection was calculated based on the model recommended 7 for flexible end plate joints (Table 2). In the model, the end plate is assumed to be a simple extension of the beam with no bending included in any part and having a maximum permissible shear stress of 0-30 Fy. In the design criteria, the strength of the bolts is derived by considering only vertical shear and calculating the effective number of bolts in each line. The strength of the welds is also derived using only vertical shear on the welds. The strength of the beam web at the plate end is calculated by assuming
I I~'<
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:o[umn f[~n~ 200UC~6.2
/
6ram we '.d
/ strain bo[~
~auged _
I 50
mirror
holes
mounting
203. i
----"
t
_ strain
gauged
bo1~
20OUB25.~
_i -I
Fig. 3(b). D e t a i l s of flexible e n d plate joint.
a shear stress distribution similar to that in an I section at the end plate web interface (see Fig. 4) and the average shear stress over the plate length (l~) is permitted to be 0.37 Fv,,. The allowable maximum load for a connection was taken as the lowest load nominally necessary to cause failure of the plate, the bolts, the welds and the beam web. For specimens tested with a seat angle, the maximum allowable load for each connection was taken as the sum of the loads required to cause failure of the flexible end plate and angle seat connections. Sample calculations for the figures given in Table 2 are shown in the Appendix. The change in the load carrying capacity of the joint arising from the modification shown in Fig. 2(b) was not considered because the design criteria do not make any provision. It is to be noted that according to the Australian design criteria, components of connections are sized and evaluated independently and the interactive effect of different components is ignored--thus introducing an empirical aspect to the design. It is observed from Table 2 that the allowable load for specimens M I-M6 is governed by the strength of the beam web at the plate end, while for specimens M7-M8, the combined effect of seat angle bolts and the strength of beam web at the plate end governed the strength.
220
220 2211 2211 316 140 2211 220
360
360 27(I 2711 360 3611 360 270
MI
M2 M3 M4 M5 M6 M7 M8
(kN)
Bolt strength (kN)
Plate strength
Specimen No.
TABLE 2
157.3 157.3 157.3 157.3 157.3 157-3 157.3
157.3
Wehl strength (kN)
113 113 l 13 113 113 I 13 113
113
Gross (kN)
81.(} 81.0 8141 81.0 81-0 81 .(I 81.0
81.0
A t plate (kN)
Beam web shear strength
Calculated Behaviour of Test Specimens
-----I I0 I10
--
Seat angle bolts (kN)
Seal
-----78.4 78.4
--
angle (kN)
8 I- 0 8 I-0 81 .I} 81 .ll 81.0 159.4 159.4
81.11
7btal shear capacity (kN)
~a
,.k 9
~--.
;1
ta.
4, "".aa
e~
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A. K. Aggarwal A
rx
_
1i
jIl!
_
i B
A
A
.
~ i
-A
B
-
sur.:ed
g
Fig. 4. Shear stress distribution at end plate-web interface.
5 L O A D I N G OF T E S T S P E C I M E N S All specimens were loaded to failure in discrete load increments. The connections were assumed to have failed when large displacements/slips took place and it became difficult to record any further observations on account of the large rotation of the beam. At each load increment, two complete cycles of loading and unloading were carried out to see the effect of a second cycle of loading on the behaviour of the joints. Details of the loading arrangement and the technique used for the measurement of rotations have been discussed by the author e l s e w h e r e . ~ ~
6 OBSERVED BEHAVIOUR The eight specimens tested as a part of the experimental programme are compared in Table 3 for their strength-ductility behaviour. Specimen M1 tested with a 16 mm thick plate failed at a load much lower than the calculated shear capacity of the joint. For this specimen, large out-of-plane rotation was observed and the failure was caused by lateral bending of the beam. To keep the out-of-plane bending to a minimum, the position of the loading jack had to be adjusted continuously. Specimen M3, with a 12 mm thick end plate, also failed at 32 kN shear force against the calculated value of 81.0 kN--in the presence of bending moment. The failure of this specimen was caused by excessive deformation of the end plate and lateral bending of the beam. It should be noted that both the major and minor axes of the column were unrestrained during tests and out-of-plane rotation of the joints was difficult to eliminate.
Behaviour of flexible end plate beam-to-column joints
!~1
TABLE 3 Strength-Ductility Characteristics of Connections
Specimen No.
Max, shear force (kN)
Max. moment (kN m)
Total cumulative rotation x I0 -3 radians (in the plane of joint)
M1 M2 M3 M4 M5 M6 M7 M8
32.0 70-0 32.0 60-0 60.0 56-0 84.0 100-0
21.44 46- 90 21.44 40-20 40.20 37.52 56-28 67-00
32.77 99.74 91.49 145.60 44.94 119.67 93-44 93.75
The low failure load and large out-of-plane rotations of the first two specimens led to the consideration of an alternative design for the connection. It was decided to cope the beam web equal in size to the length (150 ram) and thickness of the flexible plate and to weld the plate in the slot (type 2 joint). With this modification, the stiffness of the joints was expected to increase and the tendency for lateral bending to reduce. Specimen M2, which was otherwise similar in construction to specimen M 1, failed at much higher load, with buckling of the compression flange of the beam, near the face of the column, Fig. 5. The specimen failed at 70 kN shear force in addition to a bending m o m e n t of 46-90 kN m. Specimen M4, with a 12 mm thick plate, failed at 60 kN with excessive deformation of the plate in the tension zone. Inspection of the components after testing revealed very little deformation of the column flange but the end plate and bolt holes close to the tension flange of the beam had deformed considerably. Specimen M5 with 24 mm diameter bolts and 16 mm thick end plate revealed an extremely stiff behaviour with the joint having a total cumulative rotation of 44.94 × 10 -3 radians. At failure large deformation of the column flange and column stiffeners was observed. The compression flange of the beam bearing against the column flange also buckled under high compressive stress. Specimen M6 also failed in a similar manner but with more pronounced deformation of the column flange. Specimens M7 and M8, tested with an additional seat angle, failed at 84 kN and 100 kN respectively against the calculated value of 159-4 kN. For test joint M7, excessive bending of the seat angle and buckling of the beam compression flange caused the failure. The buckling of the flange was observed about 120 mm from the face of the column because near the
!22
A. K. Aggarwui
Fig. 5. Failure of test specimen M2.
Fig. 6, Failure of test specimen M8.
Behaviour of flexible end plate beam-to-column joints
123
joint the seat angle tends to increase the stiffness of the flange. For specimen MS, the leg of the seat angle connected to the column flange had large deformation with centre of rotation of the ioint occurring about the seat angle bolts (Fig. 6).
7 TEST RESULTS AND DISCUSSION From the experimental data, moment-rotation curves for flexible end plate connections were obtained. An assessment of end restraint (connection stiffness) provided by the joints was also made using these curves. The end restraint of a connection has been defined as a proportionate complement of/3 where/3 is the slope of the moment-rotation curve at zero rotation and is represented by: /3 = t a n - L ( 0 -
¢b)/(M)
It was expected that the end restraint provided by type 2 joints would be greater than type 1, but the magnitude of the increase had to be assessed. As an indication, plots of moment versus cumulative rotation of the joint were obtained for specimens M1 and M2 (Fig. 7a) and for specimens M3 and M4 (Fig. 7b). It is observed from the plots that the end restraint shown by specimens M1 and M3 (type l) joints is 66-6% and 67-8% respectively while specimens M2 and M4 (type 2) have 80-3% and 75-0% respectively. The increase in end restraint for type 2 connections resulted from the beam flange bearing against the column flange as soon as the joint begins to rotate. In the case of specimen M2, the increase in end restraint is higher because of the stiffer end plate. The average end restraint shown by type 1 connections (67.2%) is certainly much greater than is normally assumed for this type of connection. Therefore while designing flexible end plate joints, it would be desirable to take the magnitude of end restraint into account. An obvious advantage of designing these joints using the semi-rigid method (rather than assuming pinned connections) lies in the reduction of beam moments and thus leading to lighter beam sections. Another possible source of economy lies in the design of the column, where a better understanding of actual end restraint conditions would lead to a more rational and less conservative method of column design. At present, almost all codes of practice for the design of a column section rely heavily on the concept of effective length, which is dependent upon the degree of end restraint present. The type of moment-rotation curves obtained from this study differ
.4. K Aggarwal
[24
-Z
--7 I
z
3
•
= ,9
r-n
1 /ztl I I
! I
MI
o
--
i
I
I
I t
o 3
I I I
o.oo~,
40.09
Ro[a[ion
80.00
of Joint
(0--~) x
P20.OO
16O.OO
fO -3 rad~ans
Fig. 7(a). Variation of total cumulative rotation with moment for type l joints.
from those obtained by Sommer 4 (as shown by Nethercot3). Different curves were expected for type 2 connections because of the design modification, but type 1 joints were supposed to have curves similar to those obtained by Sommer. Even for specimens M I and M3, a sharp increase in the stiffness of the joints was not observed because the test joints failed by large out-of-plane rotation before the beam flange could come in contact with the column flange. For both these specimens, the moment-rotation behaviour was essentially bi-linear. To study the effect of plate thickness on the moment-rotation characteristics of the connections, plots of moment versus total cumulative rotation were obtained for specimens with 16 mm and 12 mm thick plate (Fig. 8a) type 1 joints and (Fig. 8b) for type 2 joints. It is observed from Fig. 8a that both specimens reveal identical behaviour up to 13.4 kN m and later joint M3 had better ductility because of the thinner plate. However, for the other set, joint M4 with a 12 mm thick plate revealed better ductility over the complete range of loading.
Behaviour of flexible end plate beam-to-column joints
125
-Z
i),~]O
17 !),)
~,O,()q)
"~0.00
20
. IJ ( )
150
.
;
Fig. 7(b). Variation of total cumulative rotation with moment for type 2 joints.
The difference in the magnitude of rotations of these two specimens increased as the m o m e n t on the joints increased. It should be noted that the Australian design criteria do not r e c o m m e n d any specific value for the flexible end plate thickness as is done in the British ~5 and the American design criteria t6--according to the beam size. However, the Australian design criteria v do incorporate the thickness of the end plate in an expression suggested for the maximum permissible rotation of the joint. The limitation as specified in the criteria is l~/t~ <- 33 (see Fig. 9). According to the British criteria, the maximum plate thickness for this connection should be 8 mm, but the thickness adopted in this study was much higher and was considered suitable for providing torsional restraint to the column. In the preliminary stages of the work on this connection, a specimen was tested with an 8-mm-thick end plate, but large out-of-plane rotation of the beam at extremely low loads forced the use of a thicker plate in further testing. The effect of different bolt diameters on the rotational capacity of joints was also considered. As an indication, m o m e n t - r o t a t i o n curves were
126
A. K. Aggarwal
~C
-0.00
80.00
I
20.30
k , ~ z a z i o n ,.'( doi. nz (Q--',~) :': 10
-3
160.00
radLans
Fig. 8(a). E f f e c t o f p l a t e t h i c k n e s s o n m o m e n t - r o t a t i o n c u r v e s for type I joints.
obtained for specimens M6, M2 and M5 having 16 mm, 20 mm and 24 mm diameter bolts respectively (Fig. 10). It is observed from the plot that maximum rigidity is provided by specimen M2 and the least by the joint M6. The behaviour of joint M5 is intermediate between M2 and M6. Maximum rigidity was expected from the joint M5 because of the bigger clamping force and reduced clear distance between the bolts but the results indicated otherwise. However it was realised that the restraint provided by a joint is not only a function of the bolt diameter but also depends on flexible plate thickness in relation to column flange thickness. Under static loads, the second cycle of loading reduced the rotational ability of the joints considerably because of strain hardening resulting from the first loading cycle. A typical plot obtained for specimen M4 showing the behaviour of the specimen on reloading is given in Fig. 11. To satisfy the serviceability and limit states design requirements, it is desirable to formulate design criteria which are either based on the initial
Behaviour of flexible end plate beam-to-column joints
127
H2 "-2I Z
3
0. O0
~*0. O0
Ror. a ~ L o n
of
8 0 . CO
Joi. nc
(¢9--,~
120. O0
:< I0 - }
160. '30
rad[ans
Fig. 8(b). Effect of plate thickness on moment-rotation curves for type 2 joints.
~~ion
t
i
Fig. 9. Connection geometry and rotational behaviour.
cf
join
A. K. Aggarwal
128
M2
~2_ = i
z
~
/ ¢_
¢
3
!
.00
40.00
Rotation
80.00
of Joint
]20.00
(--G-O) x 10
-3
16C.07
radians
Fig. 10. E f f e c t o f bolt d i a m e t e r o n m o m e n t - r o t a t i o n
curves.
loading conditions or take into account the complete load history of the joints. Not unexpectedly, the effect of providing a seat angle resulted in a further increase in the end restraint for specimens M7 and M8. Comparing the behaviour of specimens M2 and M7 (Fig. 12), it is observed that a cleat on the compression flange of the beam, increased the load carrying capacity of the joint by 20% and the end restraint by 7.1%, i.e. the joint had a restraint of 87-4%. However, due to the increased stiffness of the joint M7, overall rotational capacity decreased. The effect of providing a seat angle on the tension flange of the beam was also explored. It is observed from Fig. 13, that specimen M8 has consistently lower rotations over the complete range of loading thus showing reduced ductility. Restraining the tension flange of the beam, increased the end restraint by 11.4% and the load carrying capacity by 66.7%.
Behaviour of flexible end plate beam-to-column joints
5~ec~mea
2nd
":'i
129
Y-
cycle
Is:
C '~" .: ] . e
2
z v
7_
y
o . oo
- o . oo
RoEa:~on
of
s o . oo
Ja~_nt.
(G--~)
120. oo
10 . 3
I 6 r. . oo
rad~ans
Fig. 11. Effect of Ist and 2nd cycle of load on moment-rotation curves for specimen M4.
8 CONCLUSIONS Moment-rotation characteristics of flexible end plate beam-to-column connections have been obtained experimentally. The improved understanding of the behaviour of the joint provides additional information for design and more accurate prediction of moment distribution between beam and column in a joint. The moment-rotation curves indicate non-linearity over the complete range of loading. The rotation of the connections result from yielding of the flexible end plate and deformation around bolt holes in the column flange. The end restraint shown by the conventionally welded end plate joints is approximately 67.2% but with minor design modifications, as suggested for type 2 connection, the end restraint increased to 80.3%. A flange cleat in the compression zone increased the end restraint of the
A. K. Aggarwal
130
t'!7
7!2
=
.-,q
=
0.00
40.00
Rotation
80.00
of Joint
120.00
160.00
(O-qb) x 10 -3 rad[ans
Fig. 12. Effect of seat angle on m o m e n t - r o t a t i o n curves (seat on compression flange of beam).
joint to 87.4% and the load carrying capacity by 20%. However, a flange cleat in the tension zone increased the end restraint to 86.4% and the load carrying capacity of the joint by 66.7%. The magnitude of end restraint shown by these two joints is considered far too large to allow for the redistribution of bending moments assumed in the analysis. In the presence of moment, the connections were observed to fail at loads much lower than their maximum calculated values. The strengthductility characteristics of the joints indicate sufficient rotational capacity before failure. The Australian design criteria separate the behaviour of flexible end plate joints into two distinct phases, viz. (1) unhindered rotation of the connection, (2) beam flange bearing against support. In this experimental programme, the second phase could not be obtained because of large out-of-plane rotation of the joints occurring at low loads.
Behaviour of flexible end plate beam-to-column
131
joints
{10
~.18
z
i ......~M4
-j "3
cD o
0.00
A0.00
Rotation
of Joint
80.00
(0-~)
x
120.00
160.00
10 -3 radians
Fig. 13. Effect of seat angle on moment-rotation curves (seat on tension flange of beam).
Reloading of test specimens reduces their rotational ability because of strain hardening resulting from the first cycle of loading. Therefore design criteria should either be based on the initial cycle of loading or should take into account the complete load history of the specimen. Finally, the conclusions drawn herein are from a small number of specimens and must be regarded as suggestive rather than definitive.
9 ACKNOWLEDGEMENTS The financial support given by the Papua New Guinea University of Technology for this project is gratefully acknowledged. The authors would like to thank members of the staff of the Central Engineering Workshop, University of Technology, for fabricating the test specimens.
1_,_
A. K. Aggarwal REFERENCES
l. Wilson, W. M. & Moore, H. F., Tests to determine the rigidity of riveted joints of steel structures. Bull. IlL Engng Exp. Sta., No. 104 (1917). 2. Baker, J. F., The Steel Skeleton. Elastic Beha~'iour a~td Desigtz, Vol. 1. Cambridge University Press, Cambridge, 1960. 3. Nethercot, D. A., Steel beam to column connections--a review of test data. Construction Industry Research and Information Association Project No. 338, Report, London, 1985. 4. Sommer, W. H., Behaviour of welded header plate connections. Master of Applied Science Thesis, University of Toronto, 1969. 5. Kennedy, D. J. L., Moment rotation characteristics of shear connections. American Institute of Steel Construction Engineering Journal (Oct. 1969) 105-15. 6. Bennetts, I. D., Thomas, I. R. & Grundy, P., Shear connections for beams to columns. Metal Structures Conference, Institution of Engineers, Australia, Perth, Nov. 1978, pp. 70-5. 7. Hogan, T. J. & Thomas, I. R., Design o/" Structttral Connections (Standardised Cotmections Manual--Part B), 2nd edn (first published 1978). Australian Institute of Steel Construction, 1981. 8. Mansell, D. S. & Pham, L., Testing of standardised connections--AWRA contract 76, Australian Welding Research, Dec. 1982. 9. Standards Association of Australia. AS 1204, Weldable structural steels-ordinary weldable grades, 1980. 10. Standards Association of Australia, AS 1227, General requirements for the supply of hot-rolled steel plates, sections, piling and bars for structural purposes, 1980. 11. Standards Association of Australia, AS 1252, General grade high strength steel bolts with associated nuts and washers for structural engineering, 1973. 12. British Standards Institution, BS 4395, High-strength friction-grip bolts and associated nuts and washers for structural engineering: Part 1, general grade. London, 1969. 13. Aggarwal, A. K. & Coates, R. C., Structural damping in bolted beamcolumn connections. Proceedings of the Metal Structures Confere:.we, Melbourne, Australia, 1985, pp. 20--5. 14. Aggarwal, A. K. & Coates, R. C., Moment-rotation characteristics of bolted beam-column connections. Journal of Constructional Steel Research, 6(4) (1986) 303--18. 15. Pask, J. W., Manual on Connections for Beam and Column Construction. Publication No. 9/82, British Constructional Steelworks Association, London. 16. American Institute of Steel Construction, Manual of Steel Construction, 8th edn (Part 4, Connections), 1980.
Behaviour of flexible end plate beam-to-column joints SAMPLE CALCULATIONS G I V E N IN T A B L E
APPENDIX:
133
FOR THE VALUES 2
(a) Plate s t r e n g t h , V~ Va = 2 X 0"30
Fyiliti
= 2 x 0-30 × 250 x 150 x ti
J'where li is the length l
= 22.5 q kN
[ o f plate = 150 mm
,i
J
V~, = 270 kN for 12 mm thick plate = 360 kN for 16 m m thick plate (b) B o l t s t r e n g t h , Vb Vb = 2 x nc By = 2x2Bv w h e r e Bv is the m a x i m u m p e r m i s s i b l e bolt f o r c e in s h e a r , i.e. m i n i m u m o f (Bvo, & , , Bo) w h e r e Bo = 1.35 FyitiD = 1.35 x 250 x tiDi
12 mm
16 rnm
79 - - ) (55 76 81) (79 1 1 0 - - )
( 3 5 79 86.4) (55 76 108 ) (79 110 129.6)
Plate thickness Bolt dia. ( m m ) ~ . . ~ 16d~
(35
204, 244,
Vb = 4 x 35 = 140 kN for 16~b bolts = 4 x 55 = 220 kN for 20~b bolts = 4 x 79 = 316 kN for 244, bolts (c) W e l d s t r e n g t h , Vc Vc = 2 x liv x P1
liv ----- li -- 2 X tw
= 2x 138x0"57
=
150--2X
= 157-32 kN
=
138 mm
6
A. K. Aggarwal
i34
(d) G r o s s s t r e n g t h o f b e a m w e b , V d V d as o b t a i n e d f r o m A I S C m a n u a l f o r 200 U B 2 5 . 4 b e a m = 113 k N (e) S t r e n g t h o f b e a m w e b at p l a t e e n d , V e V~ = 0.37 Fywtli = 0-37 x 250 x 5.84 x 150 = 81.03 kN
Calculations for angle seat connections Bolted seat--seat flange
a n g l e b o l t e d to c o l u m n f l a n g e a n d w e l d e d to b e a m
B o l t s t r e n g t h , Vf
Vf=N.
Bv
= 2xBv Vf = 2 x 55 = 110 k N f o r 20 m m dia. b o l t s
S t r e n g t h o f s e a t a n g l e , V~,p V~p = 0.375 Fywt. Z + V'y. lsFyw, t+ (0.375 FywtZ) 2 where y = 0-1875 Fy~,. t~~= 0-1875 x 250 x 9.5'- = 4230.47 Z
----- k b
c o t 3 0 ° + 2(t~ + rl) - 2c
Z = I 5 . 6 c o t 3 0 ° + 2(9-5 + 8 ) - 2 x 0.0 = 62.02 Vc~,p = 0-375 x 250 x 5.84 x 62-02 + %/-~-30.47 x 133.4 x 250 x 5.84 + (0-375 x 250 x 5.84 x 62.02) 2 = 78 418-8 N = 78-42
Behaviour of Flexible End Plate Beam-to-Column Joints A. K. Aggarwal Department of Civil Engineering, University of Technology, Private Mail Bag, LAE, Papua New Guinea (Received 5 September L989;revised version received 24 April 1990; accepted 23 May 1990)
A BSTRA CT The paper describes an experimental investigation into the structural behaviour of flexible end plate bearn-to-column connections. Eight specimens of bearn-colurnn joints were tested under gradually increasing static loads with 16 rnrn, 20 rnrn attd 24 turn diameter bolts and 12 rnrn and 16 rnm thick flexible end plates for a constant beam attd a constant column cross-section. In two specimens, the flexible plate was conventionally welded to the beam web, while in the other specimens, the plate was welded after coping the beam web. With this modification, load carrying capacity attd end restraint of joints increased considerably. Design offlexible end plate joints using a serni-rigid method of design may be a better approximation to the actual behaviour.
NOTATION Bo
Maximum permissible bolt force on a bolt due to bearing on material By M a x i m u m permissible bolt force in shear--all failure modes considered Bvn M a x i m u m permissible bolt force in shear with thread included in shear plane Bvx Maximum permissible bolt force in shear on u n t h r e a d e d shank with thread excluded from shear plane c Clearance in angle seat connection Ill J. Construct. Steel Research 0143-974X/90/$03"50(~) 1990 Elsevier Science Publishers Ltd, England. Printed in Great Britain
l l2
D F yLI Fyi
Fyw kh Ii
li liv /7e
N PI rl t ta /'i tw Va
Vb
vc Vd Vr Vcap
0
4,
,4, K. Aggarwal
Bolt diameter Specified yield stress for angle Yield stress of the plate material Yield stress of beam web Distance on a beam from outer face of flange to inner termination of root radius Distance between bottom flange of beam and flexible end plate Length of flexible end plate Effective length of weld on plate Width of shear pad or angle seat Number of bolts in one line Number of bolts in angle seat Maximum permissible longitudinal load/length of fillet weld Root radius of angle Thickness of beam web Thickness of seat angle End plate thickness Fillet weld leg length Plate strength Bolt strength Weld strength Gross strength of beam web Strength of beam web at plate end Strength of angle seat bolts Strength of seat angle Rotation of the beam Rotation of the column
1 INTRODUCTION The structural steel connections which join beams and columns are generally classified as (i) 'simple' or pin-jointed, (ii) semi-rigid and (iii) rigid or fixed. The important difference between these connections in the context of overall structural behaviour lies in their rotational stiffness, i.e. their ability to transmit the restraining effect of the beam to the column and thus to transmit moments. Figure 1 gives typical m o m e n t - r o t a t i o n curves for beam-to-column connections. Even the 'simple' connections have significant rotational stiffness and are able to transfer substantial m o m e n t s from beam to
113
Behaviour of flexible end plate beam-to-column joints
Flexible F--Rotation
of
Joint
(0
-
~)
Fig. 1. Typical moment-rotation curves for beam-column joints.
column. Research has shown that most commonly encountered beam-tocolumn connections are neither perfectly rigid nor simple but fall into an intermediate category of semi-rigid connections. Perfect rigidity and complete flexibility are idealised forms of behaviour and cannot be achieved in practical connections. Research into the behaviour of connections started as early as 1917, when Wilson & Moore I (as referred to by Baker-') conducted experiments to determine the rigidity of riveted joints in steel structures. Since then changes in the design of connections have continued and many investigations have been published offering various design models. Recently a review of all the test data available for beam-to-column connections has been done by Nethercot 3 and in his report, he has expressed the need for additional testing, to help in our understanding of the contribution of connections in the performance of frames. Flexible end plate connection is considered by designers to be 'simple', that is, members meeting at a joint do not have rotational continuity. In this connection a plate is shop welded to the beam web and then the complete assembly is bolted at site to a column flange by high strength bolts. In this study, the behaviour of a flexible end plate connection has been investigated with particular reference to its strength and moment-rotation characteristics.
112.
.4.
K. Aggarwal
2 PREVIOUS W O R K Four series of tests on flexible end plate connections have been identified with two of these conducted in Canada and the other two in Australia. A preliminary study of flexible end plate connection was first done in 1969, when Sommer a carried out an experimental investigation into the behaviour of beam-column joints with the intention to devise a rational basis for their design. At about the same time, Kennedy 5 published the results of his investigation and suggested a design procedure for this connection. He observed that the m o m e n t - r o t a t i o n behaviour of the connection consisted of two distinct phases, the transition between them occurring when the bottom flange of the beam rotates sufficiently to bear against the column flange. In Australia, a study of flexible end plate connection was first carried out by Bennetts e t a l . (' to determine the connection strength under high shear loading. In 1978, the Australian Institute of Steel Construction published a two part manual 7 on standardised connections with a design model for a flexible end plate joint derived from the design procedure suggested by Kennedy. 5 In 1982, Mansell & Pham s conducted experiments on standardised connections to check the design provisions and also to assess the margin of safety awfilable in connections designed using these criteria. They observed a relatively high factor of safety for most of the connections, thus suggesting further investigation into the design criteria.
3 EXPERIMENTAL PROGRAMME To study the behaviour of flexible end plate beam-to-column connections, eight specimens were tested under gradually increasing 'static' loads as shown in the experimental programme given in Table 1. Three different diameter ( 1 6 m m , 2 0 m m and 2 4 m m ) bolts and two different plates (12 mm and 16mm) were used in specimens with constant beam and constant column cross-sections. In two specimens, the flexible end plate was conventionally welded to the end of the beam web (type 1 connection) (Fig. 2(a)) while in the other six specimens, the plate was welded after coping the beam web (type 2 joint) (Fig. 2(b)). The depth of the cut in the beam web was made equal to the plate thickness--to ensure good contact between the beam flanges and the column flange. The beam used in all test specimens was 200 UB 25.4 kg/m and the column 200 UC 46.2 kg/m with British equivalents of 203 × 103 UB 25 and
Behavioar of flexible end plate beam-to-column joints
t15
TABLE 1 Experimental Programme
Specimen ,Vo.
Endplate thickness (ram)
Bolt diameter (ram)
Type of connection
Seat size and location
M 1
16
20
l
M2 M3 M4 M5 M6 M7
16 12 12 16 16 16
20 20 2(/ 24 L6 20
2 1 2 2 2 2
--
-----90 x 90 x 10
MS
12
20
2
90 x 90 x 10
(c) (T) C is c l e a t o n c o m p r e s s i o n f l a n g e o f b e a m . T is c l e a t o n t e n s i o n f l a n g e o f b e a m .
203 × 203 UC 46 respectively. All steel sections conformed to the Australian specification AS 12049 and had a nominal yield stress of 250 N/mm 2 and dimensional tolerances conforming to AS-1227. m The length of the flexible plate was kept constant (150 mm) for all specimens. The test connections were designed according to the Australian Institute of Steel Construction publication, Standardised Connections Manual, 7 Part B and a sketch of the beam-column connection used for testing is shown in Figs 3a and 3b. It may be noted that the Australian design criteria recommend that the length of the flexible end plate could vary from a maximum equal to the depth between flange fillets of the supported beam to a minimum of approximately half the beam depth. In the present study, the length of the plate ( 1 5 0 m m ) was just short of the maximum permissible length by 22 mm. When using this connection, some designers and detailers extend the thin flexible end plate to the bottom of the flange of the beam and weld to it. While doing this, they are concerned with the damage to the thin flexible end plate c o m p o n e n t during transportation of the beam and also expect to achieve better restraint for the column when the tension flange of the beam bears against the column. Such a modification renders the end plate connection very much stiffer than what is assumed in the analysis. In effect, the behaviour of the connection becomes more or less similar to the connection shown in Fig. 2(b) because the lower flange is touching the support right from the beginning of rotation.
ilO
.q. K. Aggarv~al
-i: A Fig. 2(a). Typical flexible end plate joint (type 1).
i"
fBB BPJfFF
JPlJJJh
h~h
h i
11
Fig. 2(b). Modified flexible end plate joint (type 2).
Four bolts conforming to the Australian specifications AS-12521~ (high strength friction grip bolts equivalent to BS 4395 (Part 1)) ~2 were used to connect the beam/plate assembly to the column flange. In specimens M7 and M8, a seat angle (90 × 90 × 10) was also used in addition to the plate. The cleat was shop welded to the beam flange and bolted to the column by 2-M20 bolts. All bolts were tightened using a torque wrench which was earlier calibrated for torque versus axial tension for each of the three bolt diameters used in test specimens. In each test joint, two strain gauged bolts coupled to a strain indicator were used--to measure the initial tension and to monitor subsequent changes under load.
Behaviour of flexible end plate beam-to-column joints 800
117
_J !50
mirror moun:ing holes
50
/ •. . . . . . .
I "
J
1 I
200UB25.4
mirror mounzlng holes lOthk stiffeners
t
Load
L9 ('4
col stiffeners h zdraulic jack
~--
I'
l. . . . .
--~-~
' uaso ~rame ,,,. ............
---n
I .,,,,,,J
.......
.v,
I
Fig. 3(a). Test specimen and loading arrangement.
4 C A L C U L A T E D B E H A V I O U R OF TEST SPECIMENS For specimens M1-M6, tested without a seat angle, the maximum allowable load for each connection was calculated based on the model recommended 7 for flexible end plate joints (Table 2). In the model, the end plate is assumed to be a simple extension of the beam with no bending included in any part and having a maximum permissible shear stress of 0-30 Fy. In the design criteria, the strength of the bolts is derived by considering only vertical shear and calculating the effective number of bolts in each line. The strength of the welds is also derived using only vertical shear on the welds. The strength of the beam web at the plate end is calculated by assuming
I I~'<
,4.
K..4ggarwal
:o[umn f[~n~ 200UC~6.2
/
6ram we '.d
/ strain bo[~
~auged _
I 50
mirror
holes
mounting
203. i
----"
t
_ strain
gauged
bo1~
20OUB25.~
_i -I
Fig. 3(b). D e t a i l s of flexible e n d plate joint.
a shear stress distribution similar to that in an I section at the end plate web interface (see Fig. 4) and the average shear stress over the plate length (l~) is permitted to be 0.37 Fv,,. The allowable maximum load for a connection was taken as the lowest load nominally necessary to cause failure of the plate, the bolts, the welds and the beam web. For specimens tested with a seat angle, the maximum allowable load for each connection was taken as the sum of the loads required to cause failure of the flexible end plate and angle seat connections. Sample calculations for the figures given in Table 2 are shown in the Appendix. The change in the load carrying capacity of the joint arising from the modification shown in Fig. 2(b) was not considered because the design criteria do not make any provision. It is to be noted that according to the Australian design criteria, components of connections are sized and evaluated independently and the interactive effect of different components is ignored--thus introducing an empirical aspect to the design. It is observed from Table 2 that the allowable load for specimens M I-M6 is governed by the strength of the beam web at the plate end, while for specimens M7-M8, the combined effect of seat angle bolts and the strength of beam web at the plate end governed the strength.
220
220 2211 2211 316 140 2211 220
360
360 27(I 2711 360 3611 360 270
MI
M2 M3 M4 M5 M6 M7 M8
(kN)
Bolt strength (kN)
Plate strength
Specimen No.
TABLE 2
157.3 157.3 157.3 157.3 157.3 157-3 157.3
157.3
Wehl strength (kN)
113 113 l 13 113 113 I 13 113
113
Gross (kN)
81.(} 81.0 8141 81.0 81-0 81 .(I 81.0
81.0
A t plate (kN)
Beam web shear strength
Calculated Behaviour of Test Specimens
-----I I0 I10
--
Seat angle bolts (kN)
Seal
-----78.4 78.4
--
angle (kN)
8 I- 0 8 I-0 81 .I} 81 .ll 81.0 159.4 159.4
81.11
7btal shear capacity (kN)
~a
,.k 9
~--.
;1
ta.
4, "".aa
e~
120
A. K. Aggarwal A
rx
_
1i
jIl!
_
i B
A
A
.
~ i
-A
B
-
sur.:ed
g
Fig. 4. Shear stress distribution at end plate-web interface.
5 L O A D I N G OF T E S T S P E C I M E N S All specimens were loaded to failure in discrete load increments. The connections were assumed to have failed when large displacements/slips took place and it became difficult to record any further observations on account of the large rotation of the beam. At each load increment, two complete cycles of loading and unloading were carried out to see the effect of a second cycle of loading on the behaviour of the joints. Details of the loading arrangement and the technique used for the measurement of rotations have been discussed by the author e l s e w h e r e . ~ ~
6 OBSERVED BEHAVIOUR The eight specimens tested as a part of the experimental programme are compared in Table 3 for their strength-ductility behaviour. Specimen M1 tested with a 16 mm thick plate failed at a load much lower than the calculated shear capacity of the joint. For this specimen, large out-of-plane rotation was observed and the failure was caused by lateral bending of the beam. To keep the out-of-plane bending to a minimum, the position of the loading jack had to be adjusted continuously. Specimen M3, with a 12 mm thick end plate, also failed at 32 kN shear force against the calculated value of 81.0 kN--in the presence of bending moment. The failure of this specimen was caused by excessive deformation of the end plate and lateral bending of the beam. It should be noted that both the major and minor axes of the column were unrestrained during tests and out-of-plane rotation of the joints was difficult to eliminate.
Behaviour of flexible end plate beam-to-column joints
!~1
TABLE 3 Strength-Ductility Characteristics of Connections
Specimen No.
Max, shear force (kN)
Max. moment (kN m)
Total cumulative rotation x I0 -3 radians (in the plane of joint)
M1 M2 M3 M4 M5 M6 M7 M8
32.0 70-0 32.0 60-0 60.0 56-0 84.0 100-0
21.44 46- 90 21.44 40-20 40.20 37.52 56-28 67-00
32.77 99.74 91.49 145.60 44.94 119.67 93-44 93.75
The low failure load and large out-of-plane rotations of the first two specimens led to the consideration of an alternative design for the connection. It was decided to cope the beam web equal in size to the length (150 ram) and thickness of the flexible plate and to weld the plate in the slot (type 2 joint). With this modification, the stiffness of the joints was expected to increase and the tendency for lateral bending to reduce. Specimen M2, which was otherwise similar in construction to specimen M 1, failed at much higher load, with buckling of the compression flange of the beam, near the face of the column, Fig. 5. The specimen failed at 70 kN shear force in addition to a bending m o m e n t of 46-90 kN m. Specimen M4, with a 12 mm thick plate, failed at 60 kN with excessive deformation of the plate in the tension zone. Inspection of the components after testing revealed very little deformation of the column flange but the end plate and bolt holes close to the tension flange of the beam had deformed considerably. Specimen M5 with 24 mm diameter bolts and 16 mm thick end plate revealed an extremely stiff behaviour with the joint having a total cumulative rotation of 44.94 × 10 -3 radians. At failure large deformation of the column flange and column stiffeners was observed. The compression flange of the beam bearing against the column flange also buckled under high compressive stress. Specimen M6 also failed in a similar manner but with more pronounced deformation of the column flange. Specimens M7 and M8, tested with an additional seat angle, failed at 84 kN and 100 kN respectively against the calculated value of 159-4 kN. For test joint M7, excessive bending of the seat angle and buckling of the beam compression flange caused the failure. The buckling of the flange was observed about 120 mm from the face of the column because near the
!22
A. K. Aggarwui
Fig. 5. Failure of test specimen M2.
Fig. 6, Failure of test specimen M8.
Behaviour of flexible end plate beam-to-column joints
123
joint the seat angle tends to increase the stiffness of the flange. For specimen MS, the leg of the seat angle connected to the column flange had large deformation with centre of rotation of the ioint occurring about the seat angle bolts (Fig. 6).
7 TEST RESULTS AND DISCUSSION From the experimental data, moment-rotation curves for flexible end plate connections were obtained. An assessment of end restraint (connection stiffness) provided by the joints was also made using these curves. The end restraint of a connection has been defined as a proportionate complement of/3 where/3 is the slope of the moment-rotation curve at zero rotation and is represented by: /3 = t a n - L ( 0 -
¢b)/(M)
It was expected that the end restraint provided by type 2 joints would be greater than type 1, but the magnitude of the increase had to be assessed. As an indication, plots of moment versus cumulative rotation of the joint were obtained for specimens M1 and M2 (Fig. 7a) and for specimens M3 and M4 (Fig. 7b). It is observed from the plots that the end restraint shown by specimens M1 and M3 (type l) joints is 66-6% and 67-8% respectively while specimens M2 and M4 (type 2) have 80-3% and 75-0% respectively. The increase in end restraint for type 2 connections resulted from the beam flange bearing against the column flange as soon as the joint begins to rotate. In the case of specimen M2, the increase in end restraint is higher because of the stiffer end plate. The average end restraint shown by type 1 connections (67.2%) is certainly much greater than is normally assumed for this type of connection. Therefore while designing flexible end plate joints, it would be desirable to take the magnitude of end restraint into account. An obvious advantage of designing these joints using the semi-rigid method (rather than assuming pinned connections) lies in the reduction of beam moments and thus leading to lighter beam sections. Another possible source of economy lies in the design of the column, where a better understanding of actual end restraint conditions would lead to a more rational and less conservative method of column design. At present, almost all codes of practice for the design of a column section rely heavily on the concept of effective length, which is dependent upon the degree of end restraint present. The type of moment-rotation curves obtained from this study differ
.4. K Aggarwal
[24
-Z
--7 I
z
3
•
= ,9
r-n
1 /ztl I I
! I
MI
o
--
i
I
I
I t
o 3
I I I
o.oo~,
40.09
Ro[a[ion
80.00
of Joint
(0--~) x
P20.OO
16O.OO
fO -3 rad~ans
Fig. 7(a). Variation of total cumulative rotation with moment for type l joints.
from those obtained by Sommer 4 (as shown by Nethercot3). Different curves were expected for type 2 connections because of the design modification, but type 1 joints were supposed to have curves similar to those obtained by Sommer. Even for specimens M I and M3, a sharp increase in the stiffness of the joints was not observed because the test joints failed by large out-of-plane rotation before the beam flange could come in contact with the column flange. For both these specimens, the moment-rotation behaviour was essentially bi-linear. To study the effect of plate thickness on the moment-rotation characteristics of the connections, plots of moment versus total cumulative rotation were obtained for specimens with 16 mm and 12 mm thick plate (Fig. 8a) type 1 joints and (Fig. 8b) for type 2 joints. It is observed from Fig. 8a that both specimens reveal identical behaviour up to 13.4 kN m and later joint M3 had better ductility because of the thinner plate. However, for the other set, joint M4 with a 12 mm thick plate revealed better ductility over the complete range of loading.
Behaviour of flexible end plate beam-to-column joints
125
-Z
i),~]O
17 !),)
~,O,()q)
"~0.00
20
. IJ ( )
150
.
;
Fig. 7(b). Variation of total cumulative rotation with moment for type 2 joints.
The difference in the magnitude of rotations of these two specimens increased as the m o m e n t on the joints increased. It should be noted that the Australian design criteria do not r e c o m m e n d any specific value for the flexible end plate thickness as is done in the British ~5 and the American design criteria t6--according to the beam size. However, the Australian design criteria v do incorporate the thickness of the end plate in an expression suggested for the maximum permissible rotation of the joint. The limitation as specified in the criteria is l~/t~ <- 33 (see Fig. 9). According to the British criteria, the maximum plate thickness for this connection should be 8 mm, but the thickness adopted in this study was much higher and was considered suitable for providing torsional restraint to the column. In the preliminary stages of the work on this connection, a specimen was tested with an 8-mm-thick end plate, but large out-of-plane rotation of the beam at extremely low loads forced the use of a thicker plate in further testing. The effect of different bolt diameters on the rotational capacity of joints was also considered. As an indication, m o m e n t - r o t a t i o n curves were
126
A. K. Aggarwal
~C
-0.00
80.00
I
20.30
k , ~ z a z i o n ,.'( doi. nz (Q--',~) :': 10
-3
160.00
radLans
Fig. 8(a). E f f e c t o f p l a t e t h i c k n e s s o n m o m e n t - r o t a t i o n c u r v e s for type I joints.
obtained for specimens M6, M2 and M5 having 16 mm, 20 mm and 24 mm diameter bolts respectively (Fig. 10). It is observed from the plot that maximum rigidity is provided by specimen M2 and the least by the joint M6. The behaviour of joint M5 is intermediate between M2 and M6. Maximum rigidity was expected from the joint M5 because of the bigger clamping force and reduced clear distance between the bolts but the results indicated otherwise. However it was realised that the restraint provided by a joint is not only a function of the bolt diameter but also depends on flexible plate thickness in relation to column flange thickness. Under static loads, the second cycle of loading reduced the rotational ability of the joints considerably because of strain hardening resulting from the first loading cycle. A typical plot obtained for specimen M4 showing the behaviour of the specimen on reloading is given in Fig. 11. To satisfy the serviceability and limit states design requirements, it is desirable to formulate design criteria which are either based on the initial
Behaviour of flexible end plate beam-to-column joints
127
H2 "-2I Z
3
0. O0
~*0. O0
Ror. a ~ L o n
of
8 0 . CO
Joi. nc
(¢9--,~
120. O0
:< I0 - }
160. '30
rad[ans
Fig. 8(b). Effect of plate thickness on moment-rotation curves for type 2 joints.
~~ion
t
i
Fig. 9. Connection geometry and rotational behaviour.
cf
join
A. K. Aggarwal
128
M2
~2_ = i
z
~
/ ¢_
¢
3
!
.00
40.00
Rotation
80.00
of Joint
]20.00
(--G-O) x 10
-3
16C.07
radians
Fig. 10. E f f e c t o f bolt d i a m e t e r o n m o m e n t - r o t a t i o n
curves.
loading conditions or take into account the complete load history of the joints. Not unexpectedly, the effect of providing a seat angle resulted in a further increase in the end restraint for specimens M7 and M8. Comparing the behaviour of specimens M2 and M7 (Fig. 12), it is observed that a cleat on the compression flange of the beam, increased the load carrying capacity of the joint by 20% and the end restraint by 7.1%, i.e. the joint had a restraint of 87-4%. However, due to the increased stiffness of the joint M7, overall rotational capacity decreased. The effect of providing a seat angle on the tension flange of the beam was also explored. It is observed from Fig. 13, that specimen M8 has consistently lower rotations over the complete range of loading thus showing reduced ductility. Restraining the tension flange of the beam, increased the end restraint by 11.4% and the load carrying capacity by 66.7%.
Behaviour of flexible end plate beam-to-column joints
5~ec~mea
2nd
":'i
129
Y-
cycle
Is:
C '~" .: ] . e
2
z v
7_
y
o . oo
- o . oo
RoEa:~on
of
s o . oo
Ja~_nt.
(G--~)
120. oo
10 . 3
I 6 r. . oo
rad~ans
Fig. 11. Effect of Ist and 2nd cycle of load on moment-rotation curves for specimen M4.
8 CONCLUSIONS Moment-rotation characteristics of flexible end plate beam-to-column connections have been obtained experimentally. The improved understanding of the behaviour of the joint provides additional information for design and more accurate prediction of moment distribution between beam and column in a joint. The moment-rotation curves indicate non-linearity over the complete range of loading. The rotation of the connections result from yielding of the flexible end plate and deformation around bolt holes in the column flange. The end restraint shown by the conventionally welded end plate joints is approximately 67.2% but with minor design modifications, as suggested for type 2 connection, the end restraint increased to 80.3%. A flange cleat in the compression zone increased the end restraint of the
A. K. Aggarwal
130
t'!7
7!2
=
.-,q
=
0.00
40.00
Rotation
80.00
of Joint
120.00
160.00
(O-qb) x 10 -3 rad[ans
Fig. 12. Effect of seat angle on m o m e n t - r o t a t i o n curves (seat on compression flange of beam).
joint to 87.4% and the load carrying capacity by 20%. However, a flange cleat in the tension zone increased the end restraint to 86.4% and the load carrying capacity of the joint by 66.7%. The magnitude of end restraint shown by these two joints is considered far too large to allow for the redistribution of bending moments assumed in the analysis. In the presence of moment, the connections were observed to fail at loads much lower than their maximum calculated values. The strengthductility characteristics of the joints indicate sufficient rotational capacity before failure. The Australian design criteria separate the behaviour of flexible end plate joints into two distinct phases, viz. (1) unhindered rotation of the connection, (2) beam flange bearing against support. In this experimental programme, the second phase could not be obtained because of large out-of-plane rotation of the joints occurring at low loads.
Behaviour of flexible end plate beam-to-column
131
joints
{10
~.18
z
i ......~M4
-j "3
cD o
0.00
A0.00
Rotation
of Joint
80.00
(0-~)
x
120.00
160.00
10 -3 radians
Fig. 13. Effect of seat angle on moment-rotation curves (seat on tension flange of beam).
Reloading of test specimens reduces their rotational ability because of strain hardening resulting from the first cycle of loading. Therefore design criteria should either be based on the initial cycle of loading or should take into account the complete load history of the specimen. Finally, the conclusions drawn herein are from a small number of specimens and must be regarded as suggestive rather than definitive.
9 ACKNOWLEDGEMENTS The financial support given by the Papua New Guinea University of Technology for this project is gratefully acknowledged. The authors would like to thank members of the staff of the Central Engineering Workshop, University of Technology, for fabricating the test specimens.
1_,_
A. K. Aggarwal REFERENCES
l. Wilson, W. M. & Moore, H. F., Tests to determine the rigidity of riveted joints of steel structures. Bull. IlL Engng Exp. Sta., No. 104 (1917). 2. Baker, J. F., The Steel Skeleton. Elastic Beha~'iour a~td Desigtz, Vol. 1. Cambridge University Press, Cambridge, 1960. 3. Nethercot, D. A., Steel beam to column connections--a review of test data. Construction Industry Research and Information Association Project No. 338, Report, London, 1985. 4. Sommer, W. H., Behaviour of welded header plate connections. Master of Applied Science Thesis, University of Toronto, 1969. 5. Kennedy, D. J. L., Moment rotation characteristics of shear connections. American Institute of Steel Construction Engineering Journal (Oct. 1969) 105-15. 6. Bennetts, I. D., Thomas, I. R. & Grundy, P., Shear connections for beams to columns. Metal Structures Conference, Institution of Engineers, Australia, Perth, Nov. 1978, pp. 70-5. 7. Hogan, T. J. & Thomas, I. R., Design o/" Structttral Connections (Standardised Cotmections Manual--Part B), 2nd edn (first published 1978). Australian Institute of Steel Construction, 1981. 8. Mansell, D. S. & Pham, L., Testing of standardised connections--AWRA contract 76, Australian Welding Research, Dec. 1982. 9. Standards Association of Australia. AS 1204, Weldable structural steels-ordinary weldable grades, 1980. 10. Standards Association of Australia, AS 1227, General requirements for the supply of hot-rolled steel plates, sections, piling and bars for structural purposes, 1980. 11. Standards Association of Australia, AS 1252, General grade high strength steel bolts with associated nuts and washers for structural engineering, 1973. 12. British Standards Institution, BS 4395, High-strength friction-grip bolts and associated nuts and washers for structural engineering: Part 1, general grade. London, 1969. 13. Aggarwal, A. K. & Coates, R. C., Structural damping in bolted beamcolumn connections. Proceedings of the Metal Structures Confere:.we, Melbourne, Australia, 1985, pp. 20--5. 14. Aggarwal, A. K. & Coates, R. C., Moment-rotation characteristics of bolted beam-column connections. Journal of Constructional Steel Research, 6(4) (1986) 303--18. 15. Pask, J. W., Manual on Connections for Beam and Column Construction. Publication No. 9/82, British Constructional Steelworks Association, London. 16. American Institute of Steel Construction, Manual of Steel Construction, 8th edn (Part 4, Connections), 1980.
Behaviour of flexible end plate beam-to-column joints SAMPLE CALCULATIONS G I V E N IN T A B L E
APPENDIX:
133
FOR THE VALUES 2
(a) Plate s t r e n g t h , V~ Va = 2 X 0"30
Fyiliti
= 2 x 0-30 × 250 x 150 x ti
J'where li is the length l
= 22.5 q kN
[ o f plate = 150 mm
,i
J
V~, = 270 kN for 12 mm thick plate = 360 kN for 16 m m thick plate (b) B o l t s t r e n g t h , Vb Vb = 2 x nc By = 2x2Bv w h e r e Bv is the m a x i m u m p e r m i s s i b l e bolt f o r c e in s h e a r , i.e. m i n i m u m o f (Bvo, & , , Bo) w h e r e Bo = 1.35 FyitiD = 1.35 x 250 x tiDi
12 mm
16 rnm
79 - - ) (55 76 81) (79 1 1 0 - - )
( 3 5 79 86.4) (55 76 108 ) (79 110 129.6)
Plate thickness Bolt dia. ( m m ) ~ . . ~ 16d~
(35
204, 244,
Vb = 4 x 35 = 140 kN for 16~b bolts = 4 x 55 = 220 kN for 20~b bolts = 4 x 79 = 316 kN for 244, bolts (c) W e l d s t r e n g t h , Vc Vc = 2 x liv x P1
liv ----- li -- 2 X tw
= 2x 138x0"57
=
150--2X
= 157-32 kN
=
138 mm
6
A. K. Aggarwal
i34
(d) G r o s s s t r e n g t h o f b e a m w e b , V d V d as o b t a i n e d f r o m A I S C m a n u a l f o r 200 U B 2 5 . 4 b e a m = 113 k N (e) S t r e n g t h o f b e a m w e b at p l a t e e n d , V e V~ = 0.37 Fywtli = 0-37 x 250 x 5.84 x 150 = 81.03 kN
Calculations for angle seat connections Bolted seat--seat flange
a n g l e b o l t e d to c o l u m n f l a n g e a n d w e l d e d to b e a m
B o l t s t r e n g t h , Vf
Vf=N.
Bv
= 2xBv Vf = 2 x 55 = 110 k N f o r 20 m m dia. b o l t s
S t r e n g t h o f s e a t a n g l e , V~,p V~p = 0.375 Fywt. Z + V'y. lsFyw, t+ (0.375 FywtZ) 2 where y = 0-1875 Fy~,. t~~= 0-1875 x 250 x 9.5'- = 4230.47 Z
----- k b
c o t 3 0 ° + 2(t~ + rl) - 2c
Z = I 5 . 6 c o t 3 0 ° + 2(9-5 + 8 ) - 2 x 0.0 = 62.02 Vc~,p = 0-375 x 250 x 5.84 x 62-02 + %/-~-30.47 x 133.4 x 250 x 5.84 + (0-375 x 250 x 5.84 x 62.02) 2 = 78 418-8 N = 78-42