on style-Experimental research on seismic performance of K-style s

发布时间：2018-01-22 所属栏目：on style

一 : Experimental research on seismic performance of K-style s

Structures Congress 2013 ? ASCE 20132802

Experimental research on seismic performance of K-style steel outrigger truss to

concrete core tube wall joints

Jianguo Nie1 and Ran Ding1

erved.Key Laboratory of Civil Engineering Safety and Durability of China Education Ministry, Department of Civil Engineering, Tsinghua University, Beijing 100084, China; email: dingran1988@163.com ABSTRACT

Since outriggers play an important role in the structure system, the outrigger truss -

core tube joint is surely a key point which has a significant effect on the seismic

performance of tall buildings. This paper presents experimental study on the seismic

behavior of K-style steel outrigger truss to concrete core tube wall joints. Two

specimens with different details of the joint, including joint with outside steel plate and

encased steel plate, were tested under cyclic vertical loads on column ends. Several

indexes that could reflect the seismic performance of the composite joint, such as the

strength degradation, the rigidity degradation, ductility and the energy dissipation

capacity were analyzed. It was found that the composite joints could transfer the load

reliably and exhibited favorable seismic performance. It was also concluded that the

joint with outside steel plate was better than that with encased steel plate with more

construction convenience, higher buckling load and less concrete cracks.

Keywords: outrigger truss; core tube; joint details; seismic behavior; outside steel plate

1. INTRODUCTION

High-rise and super high-rise buildings are being widely used in recent years in

China (Wang and Zhou 2010). For the structural design of these buildings, the control

of top drift and base moment in the core under lateral loads are the key problems. As

shown in Fig.1, the core tube – frame – outrigger truss system is regarded as one of the

most effective ways to solve the problem and has been widely applied in tall building

structures (Wu and Li 2003).

Since outriggers play an important role in the structure system, the outrigger truss -

core tube joint is surely a key point which has a significant effect on the seismic

performance of tall buildings. Therefore, it is necessary to investigate the seismic

performance of the connection both experimentally and numerically. Park (1982),

Shahrooz (1993) and Park and Yun (2005) studied the seismic behavior of steel beam to

concrete wall joints and paid special attention to the calculation of embedded length of

the steel beams. Shahrooz et al. (2004a, b) studied the performance of outrigger beam –

core wall connections with headed studs that are typically used to connect a stud plate,

onto which the outrigger beam is connected through a shear tab, to the wall. A new

design technique was developed and the performance based on the new methodology

was found to be better.

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erved.As shown in Fig. 1, when the structure is subject to horizontal loadings, compared to the conventional outrigger truss, the upper and lower web members of the K-style truss are under tension and compression, respectively. The horizontal components are thus balanced and the shearing forces of the perimeter columns are thus vanished. With this advantage, the K-style truss has been widely used recently (Lei et al. 2011; Zhou et al. 2012). The sections of upper and lower web members in the K-style outrigger trusses which are subject to large axial force and bending moment, are usually very large. The joints of trusses and core tube wall are thus more complicated and important than those

of steel beams and core tube wall. Therefore, it is very necessary to investigate the

seismic performance and loading transfer mechanism of the joint and simplify the joint

details to improve construction convenience. Ma et al. (2009) conducted a finite element

analysis (FEA) on the seismic performance of K-style truss to core tube wall joint.

However, the key problems in the design and application of the K-style truss to core

tube wall joint mentioned above are lacking of research and no tests have been reported

in the literature up to now.

In the present study, two specimens were tested under cyclic loads with different

details, including joint with outside steel plate and encased steel plate. Several indexes

that could reflect the seismic performance of the composite joint, such as the strength

degradation, the rigidity degradation, the ductility and the energy dissipation capacity

were then analyzed. The seismic performance of these two kinds of joint was compared.

Perimeter column

(a) K-style truss(b) Conventional truss

Fig. 1 Schematic view of outrigger trusses in tall buildings

2. EXPERIMENTAL PROGRAM

2.1 Design of test specimens

Fig. 2(a) shows a schematic drawing of the structure of a high-rise building and the

strengthened story. The concrete filled steel tubular (CFST) columns around are

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erved.connected to the core tube by the K-style outrigger trusses with frame beams and belt trusses around. As shown in Fig. 2(b), the outrigger truss to core tube joint is selected to be the test specimens. The joint consists of two pieces of outrigger trusses at the corner of the core tube and the corresponding walls. Since the frame beams and belt trusses have little influence on the performance of the joint, they are not included in the test specimens. A scale factor of approximately 1/8 is chosen to establish the specimen considering the loading capacity of the test apparatus and space of the laboratory. The loading conditions of the joint are presented in Fig. 2(c). In the figure, P represents the

load caused by the horizontal earthquake or wind and N refers to the vertical load

transferred from the upper floors.

Fig. 2 Schematic view of joint models in a real structure

As shown in Fig. 2(b), the main experimental parameters investigated are the details

of the joint, including joint with encased steel plate and outside steel plate. In the joint

with encased steel plate (OTJ-2), the web members of the outrigger truss are necked and

welded to the embedded steel reinforcement of the wall, which is then connected to two

steel plates embedded into the wall. On the other hand, in the joint with outside steel

plate (OTJ-1), the web members keep the same width and welded to the steel plate

outside the wall. The steel plates from two directions form a rectangular CFST column

at the corner of the wall. Fig. 3 shows the configurations of both specimens.

2.2 Material properties

Tension tests were carried out to determine the material properties of steel plates

and reinforcements. The yield strength (fy), the ultimate strength (fu) and the elongation

ratio of steel (δ) are listed in Table 1, respectively. The compressive strength of 150mm

size cubes at the age of 28 days for the concrete used to fill the steel tubes and form the

wall was tested. The cubic compressive strength (fcu) is 42.2 MPa and 44.6MPa for

specimen OTJ-1 and OTJ-2, respectively.

2.3 Loading apparatus and measurements

Fig. 4 gives a general view of the test setup. Cyclic loads (P) in vertical direction

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were applied at the ends of the two circular CFST columns synchronously. The axial compressive load N was applied on the top of the wall by prestressed tendons. In the beginning of the test, all the prestressed tendons were tensioned to the expected level. Then the vertical column loads P were applied cyclically in small increments until failure of the composite joint. The loading history consists of a force control stage followed by a displacement control stage, as shown in Fig. 5.

erved.

2.4 Measurements

Fig. 6 shows the arrangement of measuring equipment. The loads at the column ends were measured by load cells. Dial indicators were used to measure the displacement of columns and walls. Strain gauges were arranged on the steel truss, ending column, outside (encased) steel plate and concrete wall to measure the strain distributions.

1-12-2

3-3

4-4

(a) OTJ-1(b) OTJ-2

Fig. 3 Configuration of the joint specimens (unit: mm)

Table 1 Material properties of steel and reinforcement

Components

t or d fy fu (mm)

(MPa)

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erved.Web of truss members Part of ending column at the corner of wall Circular steel tube Flange of truss members Outside (encased) steel plate Longitudinal steel bar in the wall Stirrups of the concealed columns 5 300.2 57.6 3.5 6 298.7 38.7 35.7 8 35.6 72.8 3.3 12 316.9 434.6 39.9 12 316.9 434.6 39.9 367.3 549.3 - Φ 6 284.8 489.7

- Φ 8

Rigid frameHydraulic jack

Lateral bracePrestressed tendon

CFST column)y

Corner tendonsyP(

oeLc

DOutrigger trussWall

North

Base beam

444

Fig. 4 A general view of the test set-up Fig. 5

Schematic view of loading history 44

Fig. 6 Main measuring points arrangement

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3 EXPERIMENTAL RESULTS

Some of the measured experimental results are summarized in Table 2, where, Py(+)

and Py(-) are the yield load of the joint under tensile and compressive vertical load,

respectively; Pmax is the maximum load of the joint, respectively; μ is the displacement

ductility coefficient, which is defined as Δu/Δy, where Δu is defined as the maximum

displacement corresponding to the load no less than 0.85Pmax; he is the equivalent

damping ratio. erved.

Table 2 Summary of the measured results

Specimen Truss Py(kN) Pmax(kN) Δy(mm) Δu(mm) μ

segment (+) (-) (+) (-) (+) (-) (+) (-) (+) (-) he

OTJ-1 East 760.2 697.

923.6 823.0 19.7 17.8 5.0 2.9 2.7 2.0 0.225

South 70.8 892.6 900.3 1039.1 17. 19.7 5. 36. 3.13 1.85 0.23

OTJ-2 East 735.1 735.1 860.7 823.0 17.8 18.3 8. 38.7 2.71 2.11 0.198

South 709.9 885.9 848.2 936.1 20.2 18.3 44

.7 38.2 2.22 2.09 0.200

444444

1200

900900

600

600300300

-300-300-600-600-900-900-1200-60-45-30-15015304560-1200

/mm-60-45-30-15/0mm15304560

(a) OTJ-1 (east truss)(b) OTJ-1 (south truss)

12001200900900600600

30030000

-300-300

-600-600

-900-900-1200-60-45-30-15015304560-1200/mm/mm

Fig. 7 Measured P-Δ hysteretic relations

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P/k

erved.Fig. 8 P-Δ skeleton curves under different parameters

The P-Δ hysteretic curves of the trusses are presented in Fig.7. The skeleton curves

were obtained by connecting the peak points on the measured hysteretic curves together.

The skeleton curves so determined with different truss segments and joint details are

shown in Fig. 8

. OTJ-2

(a) Cracking pattern(d) Spalling of concrete at joint region(OTJ-2)

(b) Truss buckling of OTJ-1(e) Plate deformation at joint necking region(OTJ-2)

(OTJ-1)

(OTJ-2)

Fig. 9 Failure phenomena of joint specimens

(1) Specimen OTJ-1

Two visible inclined cracks were observed on the wall between the two outside steel

plates at the load of P=+0.75Py.The largest width of the concrete diagonal cracks was

approximately 0.2mm at 1.5Δy. Fig. 9(a) shows the distribution of cracks for specimen

OTJ-1. At the load of P = Py, the outrigger truss section experienced a yielding at this

moment while the joint zone was far from yielding. At the loading stages of 3.5Δy, the

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lower member suffered a severe overall buckling with local buckling of flanges as

shown in Fig. 9(b). One of the flanges of lower member were completely fractured at

the second cycle of 4.5Δy and the web nearby were partly fractured, as shown in Fig.

9(c).

(2) Specimen OTJ-2

erved.As shown in Fig. 9(a), more cracks which appeared earlier were observed for specimen OTJ-2 than specimen OTJ-1. The directions were similar to those observed

for specimen OTJ-1 while the largest width of the concrete diagonal cracks was larger

with approximately 0.3mm at 1.5Δy. The spalling of large amount of concrete on the

wall below the lower joint plate was observed and the spalling region occupied nearly

all over the wall below the lower joint plate, as shown in Fig. 9(d). At the load of P = Py

and -Py, the outrigger truss section experienced a yielding while the joint zone was far

from yielding, which is similar to specimen OTJ-1.Slight overall buckling was observed

on the lower member at -2.5Δy, which was earlier than that of specimen OTJ-1. Obvious

lateral distortion of the plate at the necking region was observed, as shown in Fig. 9(e).

At the loading stages of 4Δy and -4Δy, the load reached 85% of the ultimate bearing

capacity and severe damage of truss members were shown in Fig. 9(f).

4 ANALYSIS OF EXPERIMENTAL RESULTS

4.1 Load versus displacement relations

Fig. 7 plots the load-displacement hysteretic relations at the columns ends for both

specimens. It can be seen that the P-Δ curves for the two joints and different column

ends are similar in the overall outline, which look ‘plump’ indicating that they have

good energy dissipation capacity.

As shown in Fig. 8, the load-displacement skeleton curves looks like a letter ‘S’,

indicating that the joint exhibits three different stages under the cyclic loads which

include elastic, plastic and ultimate failure. The P-Δ skeleton curves for the two

different column ends are almost the same when subjected to tensile force and

moderately different when subjected to compressive force. This could be explained by

the different initial imperfections measured for the lower truss members. The initial

imperfections for the east lower truss members are larger than the south ones so the

buckling capacity is lower.

4.2 Strength degradation

The strength degradation coefficient (λj) versus displacement (Δ/Δy) for both

specimens is shown in Fig. 10. λj is used to illustrate the degradation, which is defined

by Han and Li (2010). It can be seen from the figure that the coefficient λj is generally

smaller than 1 meaning that the strength of the joints is degrading with the loading

repeated. The strength degradation is found to be decreasing with the increase of

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displacement level generally. As shown in Fig. 10(a), λj decreases rapidly for the south

truss after the fracture of the lower member.

4.3 Rigidity degradation

The index of cyclic rigidity (Kl) is used to describe the degradation of the rigidity,

which is defined by Nie et al. (2008a). Fig.11 shows the cyclic rigidity Kl versus

displacement relationship of the joints. It can be found from the figure that Kl for the

south truss is generally larger than those for the east truss when subjected to erved.

compressive force while they are almost the same when subjected to tensile force.

Similar to the illustration of bearing capacity, this could be explained by the different

initial imperfections for the lower truss members. The initial imperfections for the east

lower truss members are larger than the south ones so the rigidity is lower. It can also be

seen that Kl of the truss under compression were smaller and exhibited a quicker

decrease than those of the same one under tension. This is due to the fact that the truss

under compression suffered a more severe and earlier buckling than those under tension.

Fig. 10

Strength degradation of specimens

mmm

Fig. 11 Rigidity degradation of specimens

4.4 Energy dissipation

The energy dissipation capacity of the joints could be evaluated by the equivalent

damping coefficient (he) presented by Nie et al. (2008b). The calculated he for the cycle

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where the peak load appeared are listed in Table 2. It could be seen that he for the south

truss is slightly higher than those of the east truss for the same specimen. This is due to

the smaller initial imperfections of the south truss and thus plumper hysteretic response.

The total dissipated energy Etotal could also be used to describe the energy absorbing

capacity of the joints, which is obtained by accumulating the energy from each half

cycle. As shown in Fig. 12, for specimen OTJ-2, Etotal of the south truss is slightly

higher than that of the east truss; for specimen OTJ-1, Etotal of the south truss is

obviously higher than that of the east truss in the beginning and became lower finally. erved.

This contributes to the fact that after a severe buckling occurred, the displacement

loading level was reduced for the south truss.

NNk/k/

lla

tatoottEE

Fig. 12 Energy consumed versus number of half cycles

4.5 Analysis of strain and cracks

Fig. 13 presents the strain response on the encased steel plate. In the figure, strain

gauge 1 lies nearest to the joint where the height of steel plate is larger; Strain gauge 2

lies on the coupled beam where holes exist above and below; Strain gauge 3 lies furthest

from the joint. It can be seen that the steel plate is under compression with a positive

load and under tension with a negative load. The compressive strain is obviously

smaller than the tensile strain, which contributes to the fact that concrete has a good

compressive capacity and a poor tensile capacity. The strain gauge 2 exhibits a lager

strain than the other two gauges because of the larger height of steel plate for gauge 1

and larger distance from the joint for gauge 3.

Fig. 9(a) shows the cracking pattern of the wall for both specimens. The diagonal

cracks are caused because the wall is under shear due to the reverse tensile force and

compressive force of the upper and lower steel plate in the wall. The angles between the

diagonal cracks and horizontal direction are a little larger than 45° due to the change of

the principle tensile stress direction of the concrete subjected to the axial load. It is thus

concluded that the concrete wall between the upper and lower steel plate acts like a

diagonal strut.

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erved.P/k

Fig. 13 Load-strain curves of encased steel plate (specimen OTJ-2)

4.6 Influence of different joint details

It was observed that the joint detail has a moderate effect on the P-Δ curve. The

stiffness and maximum load of specimen OTJ-1 are slightly higher than those of

specimen OTJ-2 while the displacement corresponding to the maximum load of

specimen OTJ-1 is obviously larger. The ductility coefficient μ and equivalent damping

coefficient he for OTJ-1 are larger than those for OTJ-2. This is due to the fact that the

section of the truss members near the wall became narrow for specimen OTJ-2, then the

buckling capacity and stiffness were reduced and buckling occurred earlier. It could also

be seen that the cracking of the wall is more severe for specimen OTJ-2, such as the

spalling of large amount of concrete on the wall below the lower joint plate and

horizontal cracks on the wall where encased steel plate exists. This contributes to the

fact that the steel plate of specimen OTJ-1 is placed outside the wall and the cracks are

thus invisible. It could be observed from the reinforcement details and the preparation of

both specimens that OTJ-1 is much more convenient to construct than OTJ-2 since the

steel plate is outside the wall, the number of steel bars is thus reduced and concrete are

cast more easily.

5 CONCLUSIONS

The following conclusions can be drawn based on the experimental evaluations

within the limitations of the research presented in this paper:

(1). Generally, the failure mode of the joints is the elasto-plastic buckling of the

truss members. It was observed that the tested specimens behaved in a relatively ductile

manner. The experimental results presented in this paper show that these two joints

have a prospect to be applied in seismic region.

(2). It was found from the experimental results that the seismic performance of

specimen OTJ-1 with outside steel plate is better than those of specimen OTJ-2 with

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encased steel plate. The stiffness and maximum load of specimen OTJ-1 is slightly

larger than that of OTJ-2 while the displacement corresponding to the maximum load of

specimen OTJ-1 is obviously larger. It could also be seen that the cracking of the wall is

more severe for specimen OTJ-2.

(3). It could be observed from the reinforcement details and the preparation of both

erved.specimens that specimen OTJ-1 is much more convenient to construct than specimen OTJ-2 since the steel plate is outside the wall, the number of steel bars is thus reduced and concrete are cast more easily.

ACKNOWLEDGMENTS

The writers gratefully acknowledge the financial support provided by "Twelfth

Five-Year" plan major projects supported by National Science and Technology (Grant

No. 2011BAJ09B01) and Tsinghua University Initiative Scientific Research Program

(Grant No. 2010Z03078).

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二 : Experimental research on seismic performance of K-style steel outr