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Hysteresis behavior of concrete filled square steel tube column-to-beam partially restrained composite connections
Journal of Constructional Steel Research 66 (2010) 943–953
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Hysteresis behavior of concrete filled square steel tube column-to-beam partially restrained composite connections Su-Hee Park a , Sung-Mo Choi a,∗ , Yo-Suk Kim a , Young-Wook Park b , Jin-Ho Kim c a
Department of Architectural Engineering, University of Seoul, Seoul, Republic of Korea
b
Hanmi Parsons, Seoul, Republic of Korea
c
Research Institute of Industrial Science & Technology, Kyungkido, Republic of Korea
article
info
Article history: Received 22 April 2009 Accepted 11 January 2010 Keywords: Partially restrained composite connections Concrete filled square tube (CFT) Cyclic loading test Bolted seat-angle connection Welded beam flange connection Reinforcing steel
abstract This study presents the development of an improved detail for a Concrete Filled Steel Tube (CFT) square column-to-beam partially restrained composite connection (PR-CC) and the evaluation of its hysteresis behavior under cyclic loading. The detail of the connection was designed to prevent brittle failure at the bottom of the connection due to the composite effect and to simplify its fabrication process. The suggested connection is a welded type of bottom beam flange connection and the existing PR-CC is a bolted type with a seat-angle. To evaluate their hysteresis behavior, specimens were fabricated at full scale and tested under cyclic loading. Results revealed that the suggested type fractured at the welding zone without a drop in capacity due to the anchors inside the steel tube, and reached over 0.04 radian of the plastic rotational angle. The stiffness of the suggested type was about 10% greater than the existing type because the bolted connection allowed more deformation than the welded connection. Both connections were classified as Semi-rigid connections by stiffness analysis according to Eurocode3. The suggested type can be classified as a partially restrained connection overall under cyclic loading because it reached more than 0.03 rad of an inelastic rotation angle 80% capacity of the maximum moment capacity. This type was evaluated to exhibit equal or more ductility than the existing type. As a result, it is concluded that the welded bottom beam flange connection type can be used in practice for the CFT column–beam connection instead of the existing bolt connection using a seat-angle. © 2010 Elsevier Ltd. All rights reserved.
1. Introduction Today, because high-rise buildings are becoming taller and larger, structural and economic aspects should be taken into consideration in their design. Structural aspects such as developing high-strength materials and selecting an appropriate structural system are particularly important. As structural members with excellent strength and stiffness, CFT columns have been increasingly used in the construction of high-rise buildings. The purpose of this study is to develop connection details for concrete filled square steel tube column-to-beam partially restrained composite connections. High-rise buildings with an inner core structural system to reduce horizontal strain do not need a full restraint composite connection. Therefore, many studies have been carried out recently on partially restrained composite connections consisting of reinforcing steel at slabs and steel beams. Fig. 1 shows a column-to-beam partially strained composite connection suggested by Leon et al. [1–3]. In this connection, upper slab reinforcing bars are located
∗
Corresponding author. Tel.: +82 2 2210 2396; fax: +82 2 2248 0382. E-mail address: [email protected] (S.-M. Choi).
0143-974X/$ – see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.jcsr.2010.01.006
around the column and the beam is bolted to the column using a web-angle and a seat-angle. The reinforcing bars deliver most of the moment and secure a certain degree of stiffness and ductility. As shown in the figure, because the column is fabricated of wide flange structural steel, it can be bolted easily. However, throughtype bolts were used in this study because CFT columns were used instead of W-shaped columns. Through-type bolts create a complication when installing diaphragms and casting concrete in the columns. Consequently, this study suggests a partially restrained composite connection in order to improve workability. This new type of connection detail was designed to resist the moment load at beam ends by means of slab reinforcing bars and lower beam flange welding. In the design of this new connection consideration is made of the excellent welding skills available in Korea and consequently the wide use of field welding. The basic concept of a partial strength partially restrained composite connection was first introduced by Narnard [4] & Johnson, and Hope-Gill [5]. Since 1987, studies on the partially restrained composite connection have been conducted systematically by Zandonini [6], Leon & Zandonini [1], Leon [2] and Plumier & Doneaux [7]. A series of studies have proved that the partially restrained composite connection provides excellent seismic performance and is appropriate to a system resisting lateral force.
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However, rupture finally occurred at the connection due to the large asymmetrical tensile force at the lower part of the partially restrained composite connection caused by the composite effect of the concrete slab and steel beam upon positive moment. In other words, it was inferred that the ultimate behavior of a partially restrained composite connection and CFT connection upon positive moment would be determined by the details of the upper part and diaphragm, respectively. The authors of this study have developed a new type of diaphragm and connection details inside steel tube columns. A study has also been conducted on the CFT column-tobeam connection using the new connection type in order to promote the employment of the CFT structure in the field [8–12]. The purpose of this study is to develop new details for the partially restrained composite connection to be employed in CFT structures while considering workability. In this study, three full-sized partially restrained composite connection specimens were fabricated and tests were conducted with the variables of lower connection details in accordance with the ANSI/AISC SSPEC-2002 cyclic loading program [13] in order to evaluate the stiffness, load capacity, ductility and hysteresis behavior of the suggested connection details. 2. Research background and test plan 2.1. Research background The case study for this research is a high-rise building with 37 stories and 5 underground stories, constructed by the S company in Korea. The typical plan of the building is shown in Fig. 2. The building has 9 bays of 8.4 m span in the long direction and 2 bays of 14.5 m span and a RC core in the short direction. The RC core and exterior CFT frame were used as the lateral load resisting system. Most of the lateral load is resisted by the RC core and only about 10% of the total lateral load is resisted by the exterior CFT
SHEAR STUDS
REINFORCING BARS (#4 OR #5 BARS)
COLUMN
METAL DECKING
SEAT –ANGLE (BOLTED AND WELDED)
SEAT –ANGLE (BOLTED)
Fig. 1. Existing PR-CC detail.
frame. The CFT frame connection must resist more than 60% of the beam moment according to the frame analysis of the lateral load. Therefore, a practical PR-CC connection detail must be developed. According to research carried out by Leon [1–3], the rotational stiffness and moment capacity of a PR-CC can be obtained sufficiently by the reinforcing bars of the slab and the seat-angle, and the behavior of the PR-CC under the cyclic loading is governed by the performance of the seat-angle. Therefore, the concept of the PR-CC can be used in the exterior CFT frame and the detail of the PR-CC must be modified to apply the CFT column instead of the W-shape column. In other words, a new bottom detail for the PRCC was developed as an alternative to the existing PR-CC, which carries the possibility of fracture due to the large tension caused by the composite effect of the slab and beam under positive moment. 2.2. Specimen design Three specimens were designed according to the AISC-LRFD PRCC’s Design Guide 8 [14]. The sections of members in the specimens were determined with the condition that the beam spacing
[email protected] = 23m
[email protected] = 75.6m
[email protected] = 149.8m
Partially restrained (PR)
Partially Restrained (PR) 14.5m + 13m + 14.5m = 42m (a) Plan.
14.5m + 13m + 14.5m = 42m (b) Elevation. Fig. 2. Typical plan of 37 story building.
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19mm dia. Bolt –A325
PL – 600 X 125 X 20 (SM 490, Fy=330, 3MPa)
L–200X230X200X10 (SS400)
H–500X200X10X16 (SS400)
400 X 400 X 12 (SM490)
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H–500 X 200 X 10 X 16 (SS400)
400 X 400 X 12 (SM490)
400
400
400 X 400 X 12
8 8
(SM490)
55 10
19mm dia. Bolt – A325
40
50 50 50 40
400 X 400 X 12 (SM490)
45 °
L–200X230X200X10 (SS400)
200
H–500 X 200 X 10 X 16 (SS400) A'
A
A
A'
230
(a) S-1 specimen (existing type).
(b) S-2 specimen (suggested type I).
8 – D 16 Length 60mm (Fy=330 MPa) PL – 600 X 125 X 20 (SM 490, Fy=330.3MPa) 375
50
150
400 X 400 X 12 (SM490) 400
– 400 X 400 X 12 (SM490)
50 100 50 200
H–500X200X10X16 (SS400) A'
A
(c) S-3 specimen (suggested type II). Fig. 3. Bottom connection details of specimens.
D13@100 Reinforcing Bars
D6@100 Wire Mesh (Fy=330MPa)
2,000
D19@200 Shear stud D13@100 Reinforcing Bars (Fy=420MPa)
3,500 Fig. 4. Top connection details of specimens: placement of reinforcing bars in slab.
was 14 m and the vertical load for the common office building was applied. Because only about 10% of the total lateral load was resisted by the exterior CFT frame, the connection of the CFT frame must resist more than 60% of the beam moment according to the frame analysis on the lateral load. In this study, the reinforcing bars and bottom seat-angle in the specimens were designed for the connection to have about 75% beam moment. For all specimens, the 8 reinforcing bars of 13 mm in nominal diameter (8-D13) were ar-
ranged around the column at intervals of 100 mm in each of the vertical and horizontal directions as shown in Fig. 4. The steel beam and concrete slab were designed as a fully composite T-beam and stud bolts of 19 mm in diameter were placed at 200 mm spacings in two rows (2-D19@200). The bottom connection was designed considering the possibility of brittle failure due to the imbalance of stress resulting from the composite effect of the slab and beam under cyclic loading. The S-1 specimen detail was the same as that of the existing PRCC and the bottom flange of the beam was bolted to the column through the seat-angle, as shown in Fig. 3(a). In Fig. 3(a), the seatangle and the column were connected by through-type bolts and the seat-angle and beam were connected by fillet welding adopted to safely transfer a large amount of stress. The S-2 specimen connection detail was the suggested detail as shown in Fig. 3(b) and the bottom flange of the beam was welded rather than bolted to the column because in construction bolting using a seat-angle is more difficult than welding. In Fig. 3(b), vertical flat bars and stud bolts inside the column were used to transfer the large tension of the bottom beam flange to the filled concrete inside the column which was caused by the composite effect beam and slab under cyclic loading. In other words, the tension in the bottom beam flange was resisted by the filled concrete confining the stud bolts which were welded to the vertical flat bars. As shown in Fig. 3(c), the S-3 specimen detail is the same as the S-2 specimen, except
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8-D 13 Reinforcing Bars
PL–480X125X20 (SM490) 14
40
65
(SM490)
F10T M20
120
50
60
PL – 384X85X20 (SM490)
25
100
Detail A
PL – 600X125X20
(a) Column-to-beam connection details (S-2, S-3).
25
L–200X230X200X10
S–1 specimen
(SM490)
45°
8 8
20 nm Plate
16
60
F10T M20
12
40
80
14
S–2, S–3 specimen
(b) Detail A.
1.300
Fig. 5. Connection details of specimens.
1.700
400 x 400 x 12 H–500X200X10X16
2.575
500
350
1.300
3.500
(a) Test set-up.
(b) Photograph of loading system. Fig. 6. Loading system.
that the Reduced Beam Section (RBS, hereafter) is adopted in the bottom beam flange to examine the effect of RBS in the PR-CC. The RBS was designed according to FEMA 350 [15].
3. Test results
2.3. Specimen fabrication
To confirm the mechanical properties of the steel and concrete used in the specimens, a tensile test of steel and a compressive test of concrete were performed and are summarized as average values in Tables 2 and 3, respectively. Three tensile tests for each column, beam flange and seat-angle were carried out according to KS B 0802 (ASTM 370), the Method of Tensile Test for Metallic Material in the Korean Standard. Three compressive tests for the concrete in each slab and column were carried out. The results of material tests satisfied the design strengths.
Three CFT column-to-beam specimens with the PR-CC were fabricated based on the details mentioned in Section 2.2, as shown in Fig. 5. The primary characteristics and expected failure of each specimen are given in Table 1. In the specimens, the columns and beams were of −400 × 400 × 12 square tubular section (SM490) and H-200 × 200 × 6 × 6 wide flange section (SS400), respectively. The height of the specimen and the distance between the center of the column and the loading point were 4425 mm and 3500 mm, respectively. The compressive strengths of concrete in the column and in the slab were 49 MPa (high-flexible concrete) and 24 MPa (normal concrete), respectively. According to the estimating rules of the effective flange width of a T-beam [5], the width of the slab was 2000 mm. 2.4. Testing set-up The testing set-up is shown in Fig. 6 and the load is applied to the specimen as shown in Fig. 7 according to the ANSI/AISC SSPEC2002 cyclic loading program [13]. To determine the stress transfer under the cyclic loading, the strain gauges were attached to the column and beam at the points where high stress concentrations were predicted as shown in Fig. 8.
3.1. Material test
3.2. Cyclic loading test of PR-CC specimen The tests of the three PR-CC specimens under cyclic loading were carried out and the relationship between the moment and rotational angle of each specimen are presented in Table 4. In Table 4, the yield moment (My ) and the yield rotational angle (θy ) of each specimen are determined by the General Yield Point method as shown in Fig. 9. In Fig. 9, the yield point was the intersection point of the two lines of which the slopes were the initial stiffness (K ) of the specimen and one third of the specimen (K /3). The expected and test results about the failure mode of each specimen are tabulated in Table 5 and the Moment–Rotation (M–θ ), curve and the failure photographs of each specimen are
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Table 1 Specimen list. Bottom column-to-beam connection details
S-1 S-2 S-3
Expected failure
Seat-angle (L-200 × 230 × 200 × 10)
Welded beam flange
RBS
Column reinforcement near beam connection
◦
Penetrated bolt Vertical Flat bar + Anchor Vertical Flat bar + Anchor
◦ ◦ ◦
Bottom seat-angle Bottom anchor Bottom RBS
Table 2 Tensile test results of steel (average value). Element
Type of steel
THK (mm)
Fy (MPa)
Fu (MPa)
Yield ratio Fy /Fu
Elongation (%)
Column Beam flange Seat-angle
SM490 SS400 SS400
12 16 10
360.6 263.6 340.1
506.7 392.0 450.8
0.71 0.67 0.75
26 32 28
Table 3 Compressive test results of concrete (MPa, average value). Element
Type of concrete
Design compressive strength
7-day compressive strength
28-day compressive strength
Slab Column
Normal High-flexible
24.0 49.0
19.6 41.9
25.1 49.9
Table 4 Relationship between moment and rotational angle (test results). Specimen
S-1 S-2 S-3
My (kN m)
355.1 370.8 391.0
θy (rad) 0.0051 0.0037 0.0045
Positive moment
Negative moment
+ Mmax (kN m)
+ θmax (rad)
K + (kN m/rad)
− Mmax (kN m)
− θmax (rad)
K − (kN m/rad)
706.3 757.3 743.3
0.03 0.01 0.01
45,795 62,818 62,661
486.8 428.1 435.3
0.03 0.02 0.02
52,499 56,536 55,468
Table 5 Failure characteristics (expected and test results). Specimen
Expected failure
Test results Failure mode
S-1 S-2 S-3
Cycle at failure
+1cycle at step 9 +2cycle at step 8 +2cycle at step 8
Ductile failure at the bottom seat-angle Failure at welding zone of the bottom seat-angle Failure at welding zone of the bottom flange of the beam Ductile failure at the bottom RBS Same as S-2
247.5
30
90
20 80 80 20
30
90 90
Fig. 7. ANSI/AISC SSPEC-2002 cyclic loading program.
shown in Figs. 10–12. The calculated full plastic moment (Mp ) of each specimen are marked as a dashed line in Figs. 10(a) to 12(a). (1) S-1 specimen (existing type) The S-1 specimen was bolted and welded using the built-up seat-angle between the bottom beam flange and the column as shown in Fig. 3(a). The M–Θ curve of the S-1 specimen presented all cycles at each step in Fig. 10(a) and the welding zone of the seatangle began to crack at step 8 (at point 4) and reached its ultimate state with fracture at step 9 (at point N). The test was stopped at the increase of this fracture. In the top connection, the crack on the slab developed around the column and finally the slab came apart from the column as shown in Fig. 10(b). In Fig. 10(c), there was spacing
Fig. 8. The locations of strain gauges.
between the bottom beam flange and the column, and the bolted connection of the seat-angle allowed it to deform sufficiently. The reinforcing bars of the slab as the top connection and the seatangle as the bottom connection reached their maximum moment capacity after large plastic deformation. (2) S-2 specimen (suggested type I) The S-2 specimen was welded between the bottom beam flange and the column as shown in Fig. 3(b). The M–Θ curve of the S-2 specimen presented all cycles at every step in Fig. 11(a) and the
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M
of the S-2 specimen under positive and negative moments were about 7% larger and 12% smaller, respectively, than that of the S-1 specimen shown in Fig. 13(d). The difference between positive and negative maximum moment capacities was caused by the change in the location of the neutral axis. The initial stiffnesses of the S-1, S-2 and S-3 specimens were 45,795 kN m/rad, 62,818 kN m/rad and 62,661 kN m/rad, respectively, under the positive moment and 52,499 kN m/rad, 56,536 kN m/rad and 55,468 kN m/rad under the negative moment, respectively. The initial stiffness of the S-2 specimen under positive and negative moments was 28% larger and 6% larger, respectively, than that of the S-1 specimen. The S-2 specimen had relatively smaller rotational ability than the S-1 specimen because the S2 specimen behaved similarly to a rigid connection at the initial stage while the S-1 specimen had sufficient deformation ability due to the bolted connection. The S-3 specimen had similar initial stiffness to that of the S-2 specimen because the RBS in the bottom beam flange had little effect on the capacity of the connection.
M max My
Fig. 9. General yield point method.
4.2. Failure of specimen S-2 specimen reached its ultimate state at step 8 (at point N) by the fracture of the welding zone between the bottom beam flange and the column. At step 5, the anchor inside the column began to yield (at point 4) and the bottom connection of the column swelled out. The moment capacity and stiffness of the S-2 specimen then decreased due to the yielding and micro-crack found in both ends of the beam flange at the welding zone. The concrete slab cracked around the connection at step 5 due to the tension derived from the bending moment and the crack developed considerably at step 8 as shown in Fig. 11(b). Although the slip occurred due to the crack of the concrete, the S-2 specimen exhibited overall stable hysteresis behavior and the test ended at step 9. The M–Θ curve of the S-2 specimen in Fig. 11(a) showed different hysteresis behavior from that of the S-1 specimen shown in Fig. 10(a) because the S-2 specimen reached its ultimate state at a smaller deformation than that of the S-1 specimen due to the welding of the bottom beam flange and the column. (3) S-3 specimen (suggested type II) The M–Θ curve of the S-3 specimen presented all cycles at each step in Fig. 12(a) and the S-3 specimen reached its ultimate state at step 8 (at point N) with the fracture of the welding zone between the bottom beam flange and the column. Although the mill scales on the RBS in the bottom beam flange fell off at step 6, the moment capacity of the S-3 specimen exhibited almost the same behavior as that of the S-2 specimen. The test was ended at step 9 due to the increase of fracture similar to that of the S-2 specimen. 4. Analysis of test results 4.1. Maximum moment capacity and initial stiffness To evaluate the moment capacity and the stiffness of each specimen, the M–Θ hysteresis curves were converted to the equivalent M–Θ monotonic curves. This M–Θ monotonic curve of each specimen was compared with one from its theoretical value in Fig. 13. The M–Θ curves of the S-1 and S-2 specimens were close to each theoretical value in Fig. 13(a) and (b), respectively. However, the M–Θ curve of the S-3 specimen differed from its theoretical value in Fig. 13(c) because it failed due to fracture of the welding zone not because of expected ductile failure of the RBS. The maximum moment capacities of the S-1, S-2, and S3 specimens were 706.3 kN m, 757.3 kN m and 743.3 kN m respectively under positive moment (tension in bottom) and 486.8 kN m, 428.1 kN m and 435.3 kN m under negative moment (tension in top), respectively. The maximum moment capacities
In the S-1 specimen, the ductile failure of the bottom seatangle was expected. As expected, the seat-angle underwent large deformation through the tensile resultant force resisted by the bolts and was also subjected to prying action. Finally, the S-1 specimen fractured at the welding zone of the horizontal and vertical plates in the built-up seat-angle. In the S-2 specimen, the fracture failure at the welding zone between the bottom beam flange and the steel tube column occurred as expected. In the S-3 specimen, although the ductile failure at the RBS in the beam was expected, the same failure as that of the S-2 specimen occurred. This implied that adopting the RBS in the beam would not improve the performance of the connection suggested in this study. In the S2 and S-3 specimens, the anchors inside the column yielded in turn while the fracture developed at the bottom of the connection, and this gave sufficient deformation ability to the connection. Failure of the S-2 specimen at the welding zone occurred without any significant drop in capacity. The S-2 specimen exhibited stable hysteresis behavior even while the load continued to be applied after failure. On the other hand, the S-1 specimen fractured abruptly at the welding zone of the seat-angle with an abrupt decrease in capacity. 4.3. Performance evaluation of PR-CC The performance of each specimen was assessed based on the performance evaluation specification of the connection in Eurocode3 [16] in order to verify whether the suggested connection belonged to the partially restrained connection. Eurocode3 classifies each connection according to the capacity and stiffness of the connection, as illustrated in Fig. 14. The requirements of the performance of the connection depend on whether it is supported laterally or not by braced and unbraced frames. In Fig. 14, the X axis represents the non-dimensional rotational angle of the connection and the Y -axis represents the non-dimensional moment of the connection. To convert the dimensionless moment–rotational curve, the rotational angle and the moment were divided by the plastic rotational angle and the plastic bending moment of the beam, respectively. The connection is classified as Pin, Semi-rigid and Rigid according to its stiffness and is classified as Partial strength and Full strength according to its capacity. (1) Evaluation of stiffness To apply the exterior CFT frame of the building constructed by the S company in Korea, the connection developed in this study was required to have a stiffness of more than 60% of the beam
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(a) M–Θ curve.
(b) Photograph of slab failure.
(c) Photograph of connection failure: welding zone of the seat-angle. Fig. 10. Test results of S-1 specimen.
connected to the column. Therefore, the connections of specimens were designed to have 75% of the beam under negative moment and this was examined through testing. The moment–rotation curves of each specimen under negative and positive moments were converted to non-dimensional curves in Fig. 15 and applied to the performance evaluation of the connection in the Eurocode3 specification. The stiffnesses of the S-1, S-2 and S-3 specimens were 82%, 85% and 80% respectively of the beam under negative moment and 76%, 86% and 81% respectively of the beam under positive moment. According to Eurocode3, the connections of all specimens under negative and positive moments were classified as Semi-rigid according to the stiffness, as shown in Fig. 15(a) and (b). In Fig. 15(a), the S-1 and S-2 specimens had similar stiffnesses under negative moment due to the use of the same reinforcing bars in the slab. This indicates that this connection can obtain sufficient stiffness from the reinforcing bars in the slab only, without the connection between the top flange of the beam and the column. In Fig. 15(b), the S-2 specimen under positive moment showed about 10% higher stiffness than the S-1 specimen, because the deformation of the S-2 specimen was restrained more by the welding between the bottom flange of the beam and the column. (2) Evaluation of moment capacity To be classified as a partially restrained connection according to Eurocode3, the moment capacities of connections must lie between 25% and 100% of MFp , where MFp is the full plastic moment capacity of the beam. The maximum moment capacities of the S1, S-2 and S-3 specimens under the negative moment were about 80%, 80% and 85% of MFp , respectively. Therefore, all specimens could be classified as Partial strength connections according to the capacity under the negative moment. On the other hand, the maximum moment capacities of the S-1, S-2 and S-3 specimens
under positive moment were about 90%, 150% and 150% of MFp , respectively. Therefore, the S-1 specimen could be classified as Partial strength but the S-2 and S-3 specimens were classified as Full strength connections under positive moment. This is because the S-2 and S-3 specimens were reinforced with vertical flat bar and anchors at the bottom of the connection. The classification results of the specimens through the stiffness and the moment capacity according to Eurocode3 are as follows. The S-1 specimen was classified as Semi-rigid Partial strength under both negative and positive moments. The S-2 specimen was classified as Semi-rigid Partial strength under negative moment and as Semi-rigid Full strength under positive moment. Considering its overall behavior, the S-2 specimen can be classified as a partially restrained connection. 4.4. Strain distribution according to the variation of neutral axis The failure of all specimens under cyclic loading occurred at the bottom of the connection as shown in Table 5. This resulted from the composite effect of the concrete slab and steel beam, as mentioned above. The concrete slab served as the top flange of the beam under positive moment when the slab was subjected to compression. Therefore, the neutral axis of the composite beam in the PR-CC moved upward due to the large compression of the slab. Also, a very large stress at the bottom flange of the beam occurred and finally resulted in failure at the bottom of the connection. Based on the strain in the concrete slab and the bottom flange of the beam, the variation in the location of the neutral axis under the positive moment for the S-2 and S-3 specimens is presented in Fig. 16(a). In Fig. 16(a), the location of the neutral axis for
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(a) M–Θ curve.
(b) Photograph of slab failure.
(c) Photograph of connection failure: welding zone of the bottom beam flange. Fig. 11. Test results of S-2 specimen.
(a) M–Θ curve.
(b) Photograph of slab failure.
(c) Photograph of connection failure: welding zone of the bottom beam flange. Fig. 12. Test results of S-3 specimen.
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(a) S-1 specimen.
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(b) S-2 specimen.
(c) S-3 specimen.
(d) S-1 vs. S-2. Fig. 13. Monotonic curve of M–Θ .
Fig. 14. Performance evaluation of connection in Eurocode3 specification.
(a) Under negative moment.
(b) Under positive moment. Fig. 15. Evaluation of stiffness.
both specimens is inside the concrete slab. To confirm this effect experimentally, the strain distribution according to the variation in the neutral axis under cyclic loading was verified and is presented in Fig. 16(b). In Fig. 16(b), the strain on the top and bottom of the connection distributed unequally and the strain on the bottom was larger than the strain on the top.
4.5. Plastic deformation ability To absorb large energy under an earthquake, plastic deformation ability is essential to the column-to-beam connection of the structure and the ductility of the connection is a very important factor. Generally, the connection is classified as either ductile or
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(a) Variation of neutral axis.
(b) Strain distribution. Fig. 16. Strain distribution according to the variation of neutral axis.
(a) S-1 specimen.
(b) S-2 specimen.
(c) S-3 specimen.
(d) Comparison between S-1 and S-2 specimen. Fig. 17. M–Θ curve after step 7 (δ = 105 mm).
brittle based on its ductility at 80% maximum moment capacity. The rotational angle of the ductile connection is more than 0.03 rad, and the rotational angle of the brittle connection is less than 0.03 rad. To evaluate the deformation ability of the specimens, the M–Θ curves after step 7 (δ = 105 mm) were compared as shown in Fig. 17. In Fig. 17, M ∗ is 80% of Mmax , where Mmax is the maximum moment capacity of the connection in each specimen. Under negative moment (tension on top), the plastic rotational angles were 0.033 rad at 0.73Mmax (≈ M ∗ ) in the S-2 specimen and 0.034 rad at M ∗ in the S-1 and S-3 specimens. Also, all specimens exerted a close capacity to M ∗ and showed stable hysteresis curves without dropping off in capacity even at 0.04 rad. Under positive moment (tension on bottom), the S-1 specimen began to crack at the connection of the bottom seat-angle at about 0.04 rad and then fractured with an increase of the crack. The S-1 specimen reached the plastic rotation angle of 0.037 rad at M ∗ and its capacity decreased abruptly after 0.04 rad. On the other hand, the S-2 and S-3 specimens failed at the welding zone and no abrupt
decrease in capacity appeared until the end of the test. S-2 and S-3 reached plastic rotational angles of 0.046 rad and 0.047 rad at M ∗ , respectively. Only the S-2 specimen under negative moment had 73% of Mmax at over 0.03 rad, while the other specimens satisfied 0.03 rad at M ∗ as the required plastic rotational capacity for the ductile connection. However, failures of all specimens occurred at the bottom of the connection under positive moment and plastic rotational angles of all specimens at M ∗ under positive moment had over 0.03 rad. Therefore, all specimens can be classified as having a ductile connection. 5. Conclusion In this study, a welded bottom beam flange connection in the CFT column-H steel beam connection was suggested as a new bottom detail of a partially restrained composite connection which has sufficient capacity and ductility. This connection was designed to improve the existing seat-angle connection because using through-type bolts between the CFT column and the seat-
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angle is more difficult than welding in construction. Therefore, full scale existing and suggested specimens were tested under cycle loading to compare mode of failure, stiffness and moment capacity of connections. The composite effect of the concrete slab was considered in the design of the connection and the suggested type was designed to have 75% stiffness of the steel beam. The test results are summarized below. (1) The failure mode of specimens was fracture. The suggested type fractured at the welding zone without a drop in capacity due to the anchors inside the steel tube, and reached over 0.04 rad of the plastic rotational angle. The anchors were very effective in reinforcing the bottom of the connection to carry the large tension caused by the composite effect of the beam and slab. The suggested type obtained more than 0.03 rad of the plastic rotational angle at 80% of the maximum moment capacity and exhibited a stable hysteresis curve until the end of the test. On the other hand, the existing type fractured abruptly at the seat-angle with a dropoff in capacity after the plastic rotational angle reached 0.04 rad. Therefore, the suggested type showed equal or better deformation ability than the existing type. (2) Under the negative moment, the stiffness of the suggested and the existing types were 85% and 82% of the beam, respectively. This indicates that stiffness of the connection can be obtained sufficiently from only the reinforcing bars in the slab. Under the positive moment, the stiffness of the suggested and the existing types were 86% and 76% of the beam, respectively. The stiffness of the suggested type was about 10% greater than the existing type because the bolted connection allowed more deformation than the welded connection. Both connections were classified as Semi-rigid connections by stiffness analysis according to Eurocode3. (3) Under the negative moment, the moment capacity of the suggested type was 80% plastic moment of the steel beam (MFp ) to be classified as a Partial strength connection. However, the moment capacity of the connection under the positive moment was 1.5 times MFp to be classified as a Full strength connection. Therefore, the suggested type behaved as a partially restrained connection (PR) overall under cyclic loading and can be classified as PR. As a result, it is concluded that the welded bottom beam flange connection type can be used at the CFT column–beam connection in practice to replace the existing bolt connection using a seatangle.
953
Acknowledgements This study was conducted with the support of the National Research Laboratory Project (R0A-2007-000-10047-0) funded by the Korea Science and Engineering Foundation (KOSEF) and Brain Korea 21 Stage II project (C6A2204) of the Korea Research Foundation. References [1] Leon RT, Zandonini R. Composite connections. In: Bjorhovde R, Dowling P, editors. Steel design: An international guide. Elsevier; 1992. p. 501–22. [2] Leon RT. Composite connections. Progress in Structural Engineering and Materials 1998;1(2):159–69. [3] The ASCE task committee on design criteria for composite structures in steel and concrete, design guide for partially restrained composite connections. Journal of Structural Engineering 1998;124(10):1099–114. [4] Barnard RP. Innovations in composite floor systems. In: Canadian engineering conference. Canadian Steel Industries Construction Council. 1970. [5] Johnson RP, Hope-Gill MC. Semi-rigid joints in composite frames. In: Preliminary report of the ninth congress of IABSE. 1972. p. 133–44. [6] Zandonini R. Semi-rigid composite joints. In: Narayanan R, editor. Structural connections: Stability and strength. London: Elsevier; 1989. p. 63–120. [7] Plumier A, Doneux C. Seismic behavior and design of composite steel concrete structures. Rep. 4, ECOST2/ICON. LNEC, Lisbon. 2001. [8] Choi SM, Hong SD, Kim YS, Kim JH. Simple tension testing for CFT column-tobeam connections at tension side with new diaphragm. In: Proceedings of the second international symposium on steel structures. pp. 405–16. [9] Choi SM, Yoon YS, Kim YS, Kim JH. Cyclic testing for CFT column-to-beam connections with new diaphragm. In: Proceedings of the second international symposium on steel structures. 440–51. [10] Choi SM, Yun YS, Kim JH. Experimental study on seismic performance of concrete filled tubular square column-to-beam connections with combined cross diaphragm. Journal of Steel and Composite Structures 2006;6(4):303–18. [11] Choi SM, Jung DS, Kim DJ, Kim JH. An evaluation equation of load capacities for CFT square column-to-beam connections with combined diaphragm. Journal of Steel and Composite Structures 2007;7(4):303–20. [12] Choi SM, Lee SH, Hong SD, Kim JH. Structural capacities of tension side for CFT square column-to-beam connections with combined-cross diaphragm. Journal of Advances in Structural Engineering 2008;11(2):209–27. [13] American Institute of Steel Construction. Seismic provisions for structural steel buildings: Part 2. Chicago: 2002. [14] Leon RT, Hoffman JJ, Staeger T. Steel design guide series 8: Partially restrained composite connections. American Institute of Steel Construction. 2001. [15] SAC Joint Venture. Recommended seismic design criteria for new steel moment frame buildings. Rep. no. FEMA-350, Federal Emergency Management Agency. Washington (DC): 2000. [16] Eurocode 3: Design of steel structures – Part 1.1: General rules and rules for buildings, ENV 1993-1-1, British Standards Institution: UK; 2005.
Contents lists available at ScienceDirect
Journal of Constructional Steel Research journal homepage: www.elsevier.com/locate/jcsr
Hysteresis behavior of concrete filled square steel tube column-to-beam partially restrained composite connections Su-Hee Park a , Sung-Mo Choi a,∗ , Yo-Suk Kim a , Young-Wook Park b , Jin-Ho Kim c a
Department of Architectural Engineering, University of Seoul, Seoul, Republic of Korea
b
Hanmi Parsons, Seoul, Republic of Korea
c
Research Institute of Industrial Science & Technology, Kyungkido, Republic of Korea
article
info
Article history: Received 22 April 2009 Accepted 11 January 2010 Keywords: Partially restrained composite connections Concrete filled square tube (CFT) Cyclic loading test Bolted seat-angle connection Welded beam flange connection Reinforcing steel
abstract This study presents the development of an improved detail for a Concrete Filled Steel Tube (CFT) square column-to-beam partially restrained composite connection (PR-CC) and the evaluation of its hysteresis behavior under cyclic loading. The detail of the connection was designed to prevent brittle failure at the bottom of the connection due to the composite effect and to simplify its fabrication process. The suggested connection is a welded type of bottom beam flange connection and the existing PR-CC is a bolted type with a seat-angle. To evaluate their hysteresis behavior, specimens were fabricated at full scale and tested under cyclic loading. Results revealed that the suggested type fractured at the welding zone without a drop in capacity due to the anchors inside the steel tube, and reached over 0.04 radian of the plastic rotational angle. The stiffness of the suggested type was about 10% greater than the existing type because the bolted connection allowed more deformation than the welded connection. Both connections were classified as Semi-rigid connections by stiffness analysis according to Eurocode3. The suggested type can be classified as a partially restrained connection overall under cyclic loading because it reached more than 0.03 rad of an inelastic rotation angle 80% capacity of the maximum moment capacity. This type was evaluated to exhibit equal or more ductility than the existing type. As a result, it is concluded that the welded bottom beam flange connection type can be used in practice for the CFT column–beam connection instead of the existing bolt connection using a seat-angle. © 2010 Elsevier Ltd. All rights reserved.
1. Introduction Today, because high-rise buildings are becoming taller and larger, structural and economic aspects should be taken into consideration in their design. Structural aspects such as developing high-strength materials and selecting an appropriate structural system are particularly important. As structural members with excellent strength and stiffness, CFT columns have been increasingly used in the construction of high-rise buildings. The purpose of this study is to develop connection details for concrete filled square steel tube column-to-beam partially restrained composite connections. High-rise buildings with an inner core structural system to reduce horizontal strain do not need a full restraint composite connection. Therefore, many studies have been carried out recently on partially restrained composite connections consisting of reinforcing steel at slabs and steel beams. Fig. 1 shows a column-to-beam partially strained composite connection suggested by Leon et al. [1–3]. In this connection, upper slab reinforcing bars are located
∗
Corresponding author. Tel.: +82 2 2210 2396; fax: +82 2 2248 0382. E-mail address: [email protected] (S.-M. Choi).
0143-974X/$ – see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.jcsr.2010.01.006
around the column and the beam is bolted to the column using a web-angle and a seat-angle. The reinforcing bars deliver most of the moment and secure a certain degree of stiffness and ductility. As shown in the figure, because the column is fabricated of wide flange structural steel, it can be bolted easily. However, throughtype bolts were used in this study because CFT columns were used instead of W-shaped columns. Through-type bolts create a complication when installing diaphragms and casting concrete in the columns. Consequently, this study suggests a partially restrained composite connection in order to improve workability. This new type of connection detail was designed to resist the moment load at beam ends by means of slab reinforcing bars and lower beam flange welding. In the design of this new connection consideration is made of the excellent welding skills available in Korea and consequently the wide use of field welding. The basic concept of a partial strength partially restrained composite connection was first introduced by Narnard [4] & Johnson, and Hope-Gill [5]. Since 1987, studies on the partially restrained composite connection have been conducted systematically by Zandonini [6], Leon & Zandonini [1], Leon [2] and Plumier & Doneaux [7]. A series of studies have proved that the partially restrained composite connection provides excellent seismic performance and is appropriate to a system resisting lateral force.
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However, rupture finally occurred at the connection due to the large asymmetrical tensile force at the lower part of the partially restrained composite connection caused by the composite effect of the concrete slab and steel beam upon positive moment. In other words, it was inferred that the ultimate behavior of a partially restrained composite connection and CFT connection upon positive moment would be determined by the details of the upper part and diaphragm, respectively. The authors of this study have developed a new type of diaphragm and connection details inside steel tube columns. A study has also been conducted on the CFT column-tobeam connection using the new connection type in order to promote the employment of the CFT structure in the field [8–12]. The purpose of this study is to develop new details for the partially restrained composite connection to be employed in CFT structures while considering workability. In this study, three full-sized partially restrained composite connection specimens were fabricated and tests were conducted with the variables of lower connection details in accordance with the ANSI/AISC SSPEC-2002 cyclic loading program [13] in order to evaluate the stiffness, load capacity, ductility and hysteresis behavior of the suggested connection details. 2. Research background and test plan 2.1. Research background The case study for this research is a high-rise building with 37 stories and 5 underground stories, constructed by the S company in Korea. The typical plan of the building is shown in Fig. 2. The building has 9 bays of 8.4 m span in the long direction and 2 bays of 14.5 m span and a RC core in the short direction. The RC core and exterior CFT frame were used as the lateral load resisting system. Most of the lateral load is resisted by the RC core and only about 10% of the total lateral load is resisted by the exterior CFT
SHEAR STUDS
REINFORCING BARS (#4 OR #5 BARS)
COLUMN
METAL DECKING
SEAT –ANGLE (BOLTED AND WELDED)
SEAT –ANGLE (BOLTED)
Fig. 1. Existing PR-CC detail.
frame. The CFT frame connection must resist more than 60% of the beam moment according to the frame analysis of the lateral load. Therefore, a practical PR-CC connection detail must be developed. According to research carried out by Leon [1–3], the rotational stiffness and moment capacity of a PR-CC can be obtained sufficiently by the reinforcing bars of the slab and the seat-angle, and the behavior of the PR-CC under the cyclic loading is governed by the performance of the seat-angle. Therefore, the concept of the PR-CC can be used in the exterior CFT frame and the detail of the PR-CC must be modified to apply the CFT column instead of the W-shape column. In other words, a new bottom detail for the PRCC was developed as an alternative to the existing PR-CC, which carries the possibility of fracture due to the large tension caused by the composite effect of the slab and beam under positive moment. 2.2. Specimen design Three specimens were designed according to the AISC-LRFD PRCC’s Design Guide 8 [14]. The sections of members in the specimens were determined with the condition that the beam spacing
[email protected] = 23m
[email protected] = 75.6m
[email protected] = 149.8m
Partially restrained (PR)
Partially Restrained (PR) 14.5m + 13m + 14.5m = 42m (a) Plan.
14.5m + 13m + 14.5m = 42m (b) Elevation. Fig. 2. Typical plan of 37 story building.
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19mm dia. Bolt –A325
PL – 600 X 125 X 20 (SM 490, Fy=330, 3MPa)
L–200X230X200X10 (SS400)
H–500X200X10X16 (SS400)
400 X 400 X 12 (SM490)
945
H–500 X 200 X 10 X 16 (SS400)
400 X 400 X 12 (SM490)
400
400
400 X 400 X 12
8 8
(SM490)
55 10
19mm dia. Bolt – A325
40
50 50 50 40
400 X 400 X 12 (SM490)
45 °
L–200X230X200X10 (SS400)
200
H–500 X 200 X 10 X 16 (SS400) A'
A
A
A'
230
(a) S-1 specimen (existing type).
(b) S-2 specimen (suggested type I).
8 – D 16 Length 60mm (Fy=330 MPa) PL – 600 X 125 X 20 (SM 490, Fy=330.3MPa) 375
50
150
400 X 400 X 12 (SM490) 400
– 400 X 400 X 12 (SM490)
50 100 50 200
H–500X200X10X16 (SS400) A'
A
(c) S-3 specimen (suggested type II). Fig. 3. Bottom connection details of specimens.
D13@100 Reinforcing Bars
D6@100 Wire Mesh (Fy=330MPa)
2,000
D19@200 Shear stud D13@100 Reinforcing Bars (Fy=420MPa)
3,500 Fig. 4. Top connection details of specimens: placement of reinforcing bars in slab.
was 14 m and the vertical load for the common office building was applied. Because only about 10% of the total lateral load was resisted by the exterior CFT frame, the connection of the CFT frame must resist more than 60% of the beam moment according to the frame analysis on the lateral load. In this study, the reinforcing bars and bottom seat-angle in the specimens were designed for the connection to have about 75% beam moment. For all specimens, the 8 reinforcing bars of 13 mm in nominal diameter (8-D13) were ar-
ranged around the column at intervals of 100 mm in each of the vertical and horizontal directions as shown in Fig. 4. The steel beam and concrete slab were designed as a fully composite T-beam and stud bolts of 19 mm in diameter were placed at 200 mm spacings in two rows (2-D19@200). The bottom connection was designed considering the possibility of brittle failure due to the imbalance of stress resulting from the composite effect of the slab and beam under cyclic loading. The S-1 specimen detail was the same as that of the existing PRCC and the bottom flange of the beam was bolted to the column through the seat-angle, as shown in Fig. 3(a). In Fig. 3(a), the seatangle and the column were connected by through-type bolts and the seat-angle and beam were connected by fillet welding adopted to safely transfer a large amount of stress. The S-2 specimen connection detail was the suggested detail as shown in Fig. 3(b) and the bottom flange of the beam was welded rather than bolted to the column because in construction bolting using a seat-angle is more difficult than welding. In Fig. 3(b), vertical flat bars and stud bolts inside the column were used to transfer the large tension of the bottom beam flange to the filled concrete inside the column which was caused by the composite effect beam and slab under cyclic loading. In other words, the tension in the bottom beam flange was resisted by the filled concrete confining the stud bolts which were welded to the vertical flat bars. As shown in Fig. 3(c), the S-3 specimen detail is the same as the S-2 specimen, except
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8-D 13 Reinforcing Bars
PL–480X125X20 (SM490) 14
40
65
(SM490)
F10T M20
120
50
60
PL – 384X85X20 (SM490)
25
100
Detail A
PL – 600X125X20
(a) Column-to-beam connection details (S-2, S-3).
25
L–200X230X200X10
S–1 specimen
(SM490)
45°
8 8
20 nm Plate
16
60
F10T M20
12
40
80
14
S–2, S–3 specimen
(b) Detail A.
1.300
Fig. 5. Connection details of specimens.
1.700
400 x 400 x 12 H–500X200X10X16
2.575
500
350
1.300
3.500
(a) Test set-up.
(b) Photograph of loading system. Fig. 6. Loading system.
that the Reduced Beam Section (RBS, hereafter) is adopted in the bottom beam flange to examine the effect of RBS in the PR-CC. The RBS was designed according to FEMA 350 [15].
3. Test results
2.3. Specimen fabrication
To confirm the mechanical properties of the steel and concrete used in the specimens, a tensile test of steel and a compressive test of concrete were performed and are summarized as average values in Tables 2 and 3, respectively. Three tensile tests for each column, beam flange and seat-angle were carried out according to KS B 0802 (ASTM 370), the Method of Tensile Test for Metallic Material in the Korean Standard. Three compressive tests for the concrete in each slab and column were carried out. The results of material tests satisfied the design strengths.
Three CFT column-to-beam specimens with the PR-CC were fabricated based on the details mentioned in Section 2.2, as shown in Fig. 5. The primary characteristics and expected failure of each specimen are given in Table 1. In the specimens, the columns and beams were of −400 × 400 × 12 square tubular section (SM490) and H-200 × 200 × 6 × 6 wide flange section (SS400), respectively. The height of the specimen and the distance between the center of the column and the loading point were 4425 mm and 3500 mm, respectively. The compressive strengths of concrete in the column and in the slab were 49 MPa (high-flexible concrete) and 24 MPa (normal concrete), respectively. According to the estimating rules of the effective flange width of a T-beam [5], the width of the slab was 2000 mm. 2.4. Testing set-up The testing set-up is shown in Fig. 6 and the load is applied to the specimen as shown in Fig. 7 according to the ANSI/AISC SSPEC2002 cyclic loading program [13]. To determine the stress transfer under the cyclic loading, the strain gauges were attached to the column and beam at the points where high stress concentrations were predicted as shown in Fig. 8.
3.1. Material test
3.2. Cyclic loading test of PR-CC specimen The tests of the three PR-CC specimens under cyclic loading were carried out and the relationship between the moment and rotational angle of each specimen are presented in Table 4. In Table 4, the yield moment (My ) and the yield rotational angle (θy ) of each specimen are determined by the General Yield Point method as shown in Fig. 9. In Fig. 9, the yield point was the intersection point of the two lines of which the slopes were the initial stiffness (K ) of the specimen and one third of the specimen (K /3). The expected and test results about the failure mode of each specimen are tabulated in Table 5 and the Moment–Rotation (M–θ ), curve and the failure photographs of each specimen are
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Table 1 Specimen list. Bottom column-to-beam connection details
S-1 S-2 S-3
Expected failure
Seat-angle (L-200 × 230 × 200 × 10)
Welded beam flange
RBS
Column reinforcement near beam connection
◦
Penetrated bolt Vertical Flat bar + Anchor Vertical Flat bar + Anchor
◦ ◦ ◦
Bottom seat-angle Bottom anchor Bottom RBS
Table 2 Tensile test results of steel (average value). Element
Type of steel
THK (mm)
Fy (MPa)
Fu (MPa)
Yield ratio Fy /Fu
Elongation (%)
Column Beam flange Seat-angle
SM490 SS400 SS400
12 16 10
360.6 263.6 340.1
506.7 392.0 450.8
0.71 0.67 0.75
26 32 28
Table 3 Compressive test results of concrete (MPa, average value). Element
Type of concrete
Design compressive strength
7-day compressive strength
28-day compressive strength
Slab Column
Normal High-flexible
24.0 49.0
19.6 41.9
25.1 49.9
Table 4 Relationship between moment and rotational angle (test results). Specimen
S-1 S-2 S-3
My (kN m)
355.1 370.8 391.0
θy (rad) 0.0051 0.0037 0.0045
Positive moment
Negative moment
+ Mmax (kN m)
+ θmax (rad)
K + (kN m/rad)
− Mmax (kN m)
− θmax (rad)
K − (kN m/rad)
706.3 757.3 743.3
0.03 0.01 0.01
45,795 62,818 62,661
486.8 428.1 435.3
0.03 0.02 0.02
52,499 56,536 55,468
Table 5 Failure characteristics (expected and test results). Specimen
Expected failure
Test results Failure mode
S-1 S-2 S-3
Cycle at failure
+1cycle at step 9 +2cycle at step 8 +2cycle at step 8
Ductile failure at the bottom seat-angle Failure at welding zone of the bottom seat-angle Failure at welding zone of the bottom flange of the beam Ductile failure at the bottom RBS Same as S-2
247.5
30
90
20 80 80 20
30
90 90
Fig. 7. ANSI/AISC SSPEC-2002 cyclic loading program.
shown in Figs. 10–12. The calculated full plastic moment (Mp ) of each specimen are marked as a dashed line in Figs. 10(a) to 12(a). (1) S-1 specimen (existing type) The S-1 specimen was bolted and welded using the built-up seat-angle between the bottom beam flange and the column as shown in Fig. 3(a). The M–Θ curve of the S-1 specimen presented all cycles at each step in Fig. 10(a) and the welding zone of the seatangle began to crack at step 8 (at point 4) and reached its ultimate state with fracture at step 9 (at point N). The test was stopped at the increase of this fracture. In the top connection, the crack on the slab developed around the column and finally the slab came apart from the column as shown in Fig. 10(b). In Fig. 10(c), there was spacing
Fig. 8. The locations of strain gauges.
between the bottom beam flange and the column, and the bolted connection of the seat-angle allowed it to deform sufficiently. The reinforcing bars of the slab as the top connection and the seatangle as the bottom connection reached their maximum moment capacity after large plastic deformation. (2) S-2 specimen (suggested type I) The S-2 specimen was welded between the bottom beam flange and the column as shown in Fig. 3(b). The M–Θ curve of the S-2 specimen presented all cycles at every step in Fig. 11(a) and the
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M
of the S-2 specimen under positive and negative moments were about 7% larger and 12% smaller, respectively, than that of the S-1 specimen shown in Fig. 13(d). The difference between positive and negative maximum moment capacities was caused by the change in the location of the neutral axis. The initial stiffnesses of the S-1, S-2 and S-3 specimens were 45,795 kN m/rad, 62,818 kN m/rad and 62,661 kN m/rad, respectively, under the positive moment and 52,499 kN m/rad, 56,536 kN m/rad and 55,468 kN m/rad under the negative moment, respectively. The initial stiffness of the S-2 specimen under positive and negative moments was 28% larger and 6% larger, respectively, than that of the S-1 specimen. The S-2 specimen had relatively smaller rotational ability than the S-1 specimen because the S2 specimen behaved similarly to a rigid connection at the initial stage while the S-1 specimen had sufficient deformation ability due to the bolted connection. The S-3 specimen had similar initial stiffness to that of the S-2 specimen because the RBS in the bottom beam flange had little effect on the capacity of the connection.
M max My
Fig. 9. General yield point method.
4.2. Failure of specimen S-2 specimen reached its ultimate state at step 8 (at point N) by the fracture of the welding zone between the bottom beam flange and the column. At step 5, the anchor inside the column began to yield (at point 4) and the bottom connection of the column swelled out. The moment capacity and stiffness of the S-2 specimen then decreased due to the yielding and micro-crack found in both ends of the beam flange at the welding zone. The concrete slab cracked around the connection at step 5 due to the tension derived from the bending moment and the crack developed considerably at step 8 as shown in Fig. 11(b). Although the slip occurred due to the crack of the concrete, the S-2 specimen exhibited overall stable hysteresis behavior and the test ended at step 9. The M–Θ curve of the S-2 specimen in Fig. 11(a) showed different hysteresis behavior from that of the S-1 specimen shown in Fig. 10(a) because the S-2 specimen reached its ultimate state at a smaller deformation than that of the S-1 specimen due to the welding of the bottom beam flange and the column. (3) S-3 specimen (suggested type II) The M–Θ curve of the S-3 specimen presented all cycles at each step in Fig. 12(a) and the S-3 specimen reached its ultimate state at step 8 (at point N) with the fracture of the welding zone between the bottom beam flange and the column. Although the mill scales on the RBS in the bottom beam flange fell off at step 6, the moment capacity of the S-3 specimen exhibited almost the same behavior as that of the S-2 specimen. The test was ended at step 9 due to the increase of fracture similar to that of the S-2 specimen. 4. Analysis of test results 4.1. Maximum moment capacity and initial stiffness To evaluate the moment capacity and the stiffness of each specimen, the M–Θ hysteresis curves were converted to the equivalent M–Θ monotonic curves. This M–Θ monotonic curve of each specimen was compared with one from its theoretical value in Fig. 13. The M–Θ curves of the S-1 and S-2 specimens were close to each theoretical value in Fig. 13(a) and (b), respectively. However, the M–Θ curve of the S-3 specimen differed from its theoretical value in Fig. 13(c) because it failed due to fracture of the welding zone not because of expected ductile failure of the RBS. The maximum moment capacities of the S-1, S-2, and S3 specimens were 706.3 kN m, 757.3 kN m and 743.3 kN m respectively under positive moment (tension in bottom) and 486.8 kN m, 428.1 kN m and 435.3 kN m under negative moment (tension in top), respectively. The maximum moment capacities
In the S-1 specimen, the ductile failure of the bottom seatangle was expected. As expected, the seat-angle underwent large deformation through the tensile resultant force resisted by the bolts and was also subjected to prying action. Finally, the S-1 specimen fractured at the welding zone of the horizontal and vertical plates in the built-up seat-angle. In the S-2 specimen, the fracture failure at the welding zone between the bottom beam flange and the steel tube column occurred as expected. In the S-3 specimen, although the ductile failure at the RBS in the beam was expected, the same failure as that of the S-2 specimen occurred. This implied that adopting the RBS in the beam would not improve the performance of the connection suggested in this study. In the S2 and S-3 specimens, the anchors inside the column yielded in turn while the fracture developed at the bottom of the connection, and this gave sufficient deformation ability to the connection. Failure of the S-2 specimen at the welding zone occurred without any significant drop in capacity. The S-2 specimen exhibited stable hysteresis behavior even while the load continued to be applied after failure. On the other hand, the S-1 specimen fractured abruptly at the welding zone of the seat-angle with an abrupt decrease in capacity. 4.3. Performance evaluation of PR-CC The performance of each specimen was assessed based on the performance evaluation specification of the connection in Eurocode3 [16] in order to verify whether the suggested connection belonged to the partially restrained connection. Eurocode3 classifies each connection according to the capacity and stiffness of the connection, as illustrated in Fig. 14. The requirements of the performance of the connection depend on whether it is supported laterally or not by braced and unbraced frames. In Fig. 14, the X axis represents the non-dimensional rotational angle of the connection and the Y -axis represents the non-dimensional moment of the connection. To convert the dimensionless moment–rotational curve, the rotational angle and the moment were divided by the plastic rotational angle and the plastic bending moment of the beam, respectively. The connection is classified as Pin, Semi-rigid and Rigid according to its stiffness and is classified as Partial strength and Full strength according to its capacity. (1) Evaluation of stiffness To apply the exterior CFT frame of the building constructed by the S company in Korea, the connection developed in this study was required to have a stiffness of more than 60% of the beam
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(a) M–Θ curve.
(b) Photograph of slab failure.
(c) Photograph of connection failure: welding zone of the seat-angle. Fig. 10. Test results of S-1 specimen.
connected to the column. Therefore, the connections of specimens were designed to have 75% of the beam under negative moment and this was examined through testing. The moment–rotation curves of each specimen under negative and positive moments were converted to non-dimensional curves in Fig. 15 and applied to the performance evaluation of the connection in the Eurocode3 specification. The stiffnesses of the S-1, S-2 and S-3 specimens were 82%, 85% and 80% respectively of the beam under negative moment and 76%, 86% and 81% respectively of the beam under positive moment. According to Eurocode3, the connections of all specimens under negative and positive moments were classified as Semi-rigid according to the stiffness, as shown in Fig. 15(a) and (b). In Fig. 15(a), the S-1 and S-2 specimens had similar stiffnesses under negative moment due to the use of the same reinforcing bars in the slab. This indicates that this connection can obtain sufficient stiffness from the reinforcing bars in the slab only, without the connection between the top flange of the beam and the column. In Fig. 15(b), the S-2 specimen under positive moment showed about 10% higher stiffness than the S-1 specimen, because the deformation of the S-2 specimen was restrained more by the welding between the bottom flange of the beam and the column. (2) Evaluation of moment capacity To be classified as a partially restrained connection according to Eurocode3, the moment capacities of connections must lie between 25% and 100% of MFp , where MFp is the full plastic moment capacity of the beam. The maximum moment capacities of the S1, S-2 and S-3 specimens under the negative moment were about 80%, 80% and 85% of MFp , respectively. Therefore, all specimens could be classified as Partial strength connections according to the capacity under the negative moment. On the other hand, the maximum moment capacities of the S-1, S-2 and S-3 specimens
under positive moment were about 90%, 150% and 150% of MFp , respectively. Therefore, the S-1 specimen could be classified as Partial strength but the S-2 and S-3 specimens were classified as Full strength connections under positive moment. This is because the S-2 and S-3 specimens were reinforced with vertical flat bar and anchors at the bottom of the connection. The classification results of the specimens through the stiffness and the moment capacity according to Eurocode3 are as follows. The S-1 specimen was classified as Semi-rigid Partial strength under both negative and positive moments. The S-2 specimen was classified as Semi-rigid Partial strength under negative moment and as Semi-rigid Full strength under positive moment. Considering its overall behavior, the S-2 specimen can be classified as a partially restrained connection. 4.4. Strain distribution according to the variation of neutral axis The failure of all specimens under cyclic loading occurred at the bottom of the connection as shown in Table 5. This resulted from the composite effect of the concrete slab and steel beam, as mentioned above. The concrete slab served as the top flange of the beam under positive moment when the slab was subjected to compression. Therefore, the neutral axis of the composite beam in the PR-CC moved upward due to the large compression of the slab. Also, a very large stress at the bottom flange of the beam occurred and finally resulted in failure at the bottom of the connection. Based on the strain in the concrete slab and the bottom flange of the beam, the variation in the location of the neutral axis under the positive moment for the S-2 and S-3 specimens is presented in Fig. 16(a). In Fig. 16(a), the location of the neutral axis for
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(a) M–Θ curve.
(b) Photograph of slab failure.
(c) Photograph of connection failure: welding zone of the bottom beam flange. Fig. 11. Test results of S-2 specimen.
(a) M–Θ curve.
(b) Photograph of slab failure.
(c) Photograph of connection failure: welding zone of the bottom beam flange. Fig. 12. Test results of S-3 specimen.
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(a) S-1 specimen.
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(b) S-2 specimen.
(c) S-3 specimen.
(d) S-1 vs. S-2. Fig. 13. Monotonic curve of M–Θ .
Fig. 14. Performance evaluation of connection in Eurocode3 specification.
(a) Under negative moment.
(b) Under positive moment. Fig. 15. Evaluation of stiffness.
both specimens is inside the concrete slab. To confirm this effect experimentally, the strain distribution according to the variation in the neutral axis under cyclic loading was verified and is presented in Fig. 16(b). In Fig. 16(b), the strain on the top and bottom of the connection distributed unequally and the strain on the bottom was larger than the strain on the top.
4.5. Plastic deformation ability To absorb large energy under an earthquake, plastic deformation ability is essential to the column-to-beam connection of the structure and the ductility of the connection is a very important factor. Generally, the connection is classified as either ductile or
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(a) Variation of neutral axis.
(b) Strain distribution. Fig. 16. Strain distribution according to the variation of neutral axis.
(a) S-1 specimen.
(b) S-2 specimen.
(c) S-3 specimen.
(d) Comparison between S-1 and S-2 specimen. Fig. 17. M–Θ curve after step 7 (δ = 105 mm).
brittle based on its ductility at 80% maximum moment capacity. The rotational angle of the ductile connection is more than 0.03 rad, and the rotational angle of the brittle connection is less than 0.03 rad. To evaluate the deformation ability of the specimens, the M–Θ curves after step 7 (δ = 105 mm) were compared as shown in Fig. 17. In Fig. 17, M ∗ is 80% of Mmax , where Mmax is the maximum moment capacity of the connection in each specimen. Under negative moment (tension on top), the plastic rotational angles were 0.033 rad at 0.73Mmax (≈ M ∗ ) in the S-2 specimen and 0.034 rad at M ∗ in the S-1 and S-3 specimens. Also, all specimens exerted a close capacity to M ∗ and showed stable hysteresis curves without dropping off in capacity even at 0.04 rad. Under positive moment (tension on bottom), the S-1 specimen began to crack at the connection of the bottom seat-angle at about 0.04 rad and then fractured with an increase of the crack. The S-1 specimen reached the plastic rotation angle of 0.037 rad at M ∗ and its capacity decreased abruptly after 0.04 rad. On the other hand, the S-2 and S-3 specimens failed at the welding zone and no abrupt
decrease in capacity appeared until the end of the test. S-2 and S-3 reached plastic rotational angles of 0.046 rad and 0.047 rad at M ∗ , respectively. Only the S-2 specimen under negative moment had 73% of Mmax at over 0.03 rad, while the other specimens satisfied 0.03 rad at M ∗ as the required plastic rotational capacity for the ductile connection. However, failures of all specimens occurred at the bottom of the connection under positive moment and plastic rotational angles of all specimens at M ∗ under positive moment had over 0.03 rad. Therefore, all specimens can be classified as having a ductile connection. 5. Conclusion In this study, a welded bottom beam flange connection in the CFT column-H steel beam connection was suggested as a new bottom detail of a partially restrained composite connection which has sufficient capacity and ductility. This connection was designed to improve the existing seat-angle connection because using through-type bolts between the CFT column and the seat-
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angle is more difficult than welding in construction. Therefore, full scale existing and suggested specimens were tested under cycle loading to compare mode of failure, stiffness and moment capacity of connections. The composite effect of the concrete slab was considered in the design of the connection and the suggested type was designed to have 75% stiffness of the steel beam. The test results are summarized below. (1) The failure mode of specimens was fracture. The suggested type fractured at the welding zone without a drop in capacity due to the anchors inside the steel tube, and reached over 0.04 rad of the plastic rotational angle. The anchors were very effective in reinforcing the bottom of the connection to carry the large tension caused by the composite effect of the beam and slab. The suggested type obtained more than 0.03 rad of the plastic rotational angle at 80% of the maximum moment capacity and exhibited a stable hysteresis curve until the end of the test. On the other hand, the existing type fractured abruptly at the seat-angle with a dropoff in capacity after the plastic rotational angle reached 0.04 rad. Therefore, the suggested type showed equal or better deformation ability than the existing type. (2) Under the negative moment, the stiffness of the suggested and the existing types were 85% and 82% of the beam, respectively. This indicates that stiffness of the connection can be obtained sufficiently from only the reinforcing bars in the slab. Under the positive moment, the stiffness of the suggested and the existing types were 86% and 76% of the beam, respectively. The stiffness of the suggested type was about 10% greater than the existing type because the bolted connection allowed more deformation than the welded connection. Both connections were classified as Semi-rigid connections by stiffness analysis according to Eurocode3. (3) Under the negative moment, the moment capacity of the suggested type was 80% plastic moment of the steel beam (MFp ) to be classified as a Partial strength connection. However, the moment capacity of the connection under the positive moment was 1.5 times MFp to be classified as a Full strength connection. Therefore, the suggested type behaved as a partially restrained connection (PR) overall under cyclic loading and can be classified as PR. As a result, it is concluded that the welded bottom beam flange connection type can be used at the CFT column–beam connection in practice to replace the existing bolt connection using a seatangle.
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Acknowledgements This study was conducted with the support of the National Research Laboratory Project (R0A-2007-000-10047-0) funded by the Korea Science and Engineering Foundation (KOSEF) and Brain Korea 21 Stage II project (C6A2204) of the Korea Research Foundation. References [1] Leon RT, Zandonini R. Composite connections. In: Bjorhovde R, Dowling P, editors. Steel design: An international guide. Elsevier; 1992. p. 501–22. [2] Leon RT. Composite connections. Progress in Structural Engineering and Materials 1998;1(2):159–69. [3] The ASCE task committee on design criteria for composite structures in steel and concrete, design guide for partially restrained composite connections. Journal of Structural Engineering 1998;124(10):1099–114. [4] Barnard RP. Innovations in composite floor systems. In: Canadian engineering conference. Canadian Steel Industries Construction Council. 1970. [5] Johnson RP, Hope-Gill MC. Semi-rigid joints in composite frames. In: Preliminary report of the ninth congress of IABSE. 1972. p. 133–44. [6] Zandonini R. Semi-rigid composite joints. In: Narayanan R, editor. Structural connections: Stability and strength. London: Elsevier; 1989. p. 63–120. [7] Plumier A, Doneux C. Seismic behavior and design of composite steel concrete structures. Rep. 4, ECOST2/ICON. LNEC, Lisbon. 2001. [8] Choi SM, Hong SD, Kim YS, Kim JH. Simple tension testing for CFT column-tobeam connections at tension side with new diaphragm. In: Proceedings of the second international symposium on steel structures. pp. 405–16. [9] Choi SM, Yoon YS, Kim YS, Kim JH. Cyclic testing for CFT column-to-beam connections with new diaphragm. In: Proceedings of the second international symposium on steel structures. 440–51. [10] Choi SM, Yun YS, Kim JH. Experimental study on seismic performance of concrete filled tubular square column-to-beam connections with combined cross diaphragm. Journal of Steel and Composite Structures 2006;6(4):303–18. [11] Choi SM, Jung DS, Kim DJ, Kim JH. An evaluation equation of load capacities for CFT square column-to-beam connections with combined diaphragm. Journal of Steel and Composite Structures 2007;7(4):303–20. [12] Choi SM, Lee SH, Hong SD, Kim JH. Structural capacities of tension side for CFT square column-to-beam connections with combined-cross diaphragm. Journal of Advances in Structural Engineering 2008;11(2):209–27. [13] American Institute of Steel Construction. Seismic provisions for structural steel buildings: Part 2. Chicago: 2002. [14] Leon RT, Hoffman JJ, Staeger T. Steel design guide series 8: Partially restrained composite connections. American Institute of Steel Construction. 2001. [15] SAC Joint Venture. Recommended seismic design criteria for new steel moment frame buildings. Rep. no. FEMA-350, Federal Emergency Management Agency. Washington (DC): 2000. [16] Eurocode 3: Design of steel structures – Part 1.1: General rules and rules for buildings, ENV 1993-1-1, British Standards Institution: UK; 2005.