Advances in Steel Structures - part 46 doc

The paper describes a series of static and cyclic axial compression tests on empty and concrete-filled columns made from cold-formed C450 RHS profiles.. The specimen label system used in

430 X.L Zhao et al

Most of the steel hollow sections manufactured in Australia are in Grade C350 and C450 (minimum yield stress of 350 and 450 MPa) The width-to-thickness (H/t) limits are lower for C450 RHS than those of C350 RHS (Zhao and Hancock (1991)), where H is the overall width or depth (whichever is larger) of an RHS and t is the thickness of the RHS It is the C450 RHS that will be studied in this paper The calculated (H/t) limit using AISC (1997) is 16.7 for C450 empty RHS under cyclic axial load It can be demonstrated that most of the C450 sections manufactured in Australia, which have a H/t ratio ranging from 12.5 to 75 (AISC (1992), Tubemakers (1994)), do not comply with these limits

The technique of filling tubes with concrete increases the ductility and prevents or delays local buckling

of tubular sections under cyclic loading (Nakai et al 1994, Ge & Usami 1994, 1996, Ricles 1995, Haijar and Gourley 1997, Zhao and Grzebieta 1999) Most of the tubular sections in these studies were either hot-rolled or welded box sections Little research was performed on concrete-filled cold-formed RHS columns under cyclic axial loading (Liu & Goel 1988) This paper forms part of a research project on Tubular Structures under Large Amplitude Dynamic Loading currently running at Monash University, Australia Only the case of empty and concrete-filled cold-formed RHS under cyclic axial loading is reported in this paper

The paper describes a series of static and cyclic axial compression tests on empty and concrete-filled columns made from cold-formed C450 RHS profiles The two loading protocols used, the Cyclic-Direct procedure and the Cyclic-Incremental procedure are outlined The first-cycle buckling loads noted in the tests are then compared with design loads predicted using various national standards and CIDECT formulae The paper demonstrates that concrete filling increases the post-peak-load residual strength and reduces the rate of residual strength loss per cycle for thinner RHS columns subjected to overload cyclic situations It also proposes that the AISC Seismic Provisions (1997) specify width-to-wall thickness ratio limits that may be conservative in regards to full effective sections under cyclic loading

WIDTH-TO-THICKNESS RATIOS

Width-to-thickness ratio limits are given in various codes (SSRC (1991), Rondal et al (1996)) to prevent local buckling of RHS under static compression The concept of a form factor (a ratio of effective area to gross area) is used in AS4100 to indicate the effect of local buckling Local buckling occurs if the form factor is less than 1.0 A width to thickness ratio limit is also given in AISC Seismic Provisions for Structural Steel Buildings (AISC (1997)) for RHS bracing members in concentrically braced frames A summary of width-to-thickness ratio limits for different countries is presented in Table 1 where the following four width-to-thickness (H/t) limits are given:

1 Empty RHS under static load (loaded to failure)

2 Empty RHS bracing members under cyclic load (repeated loading)

3 Concrete-filled RHS under static load (loaded to failure)

4 Concrete-filled RHS bracing members under cyclic load (repeated loading)

Table 1 shows that the width-to-thickness ratio limits increase for concrete-filled RHS

For static loaded structures, the limit for concrete-filled RHS given in Eurocode 4 is about 13% higher than that for empty RHS given in Eurocode 3 A theoretical analysis determining the width-to-thickness limit for concrete-filled RHS under static loading was carried out by Uy et al (1998) and Wright (1995) Depending on the boundary conditions, Uy et al (1998) showed that the limit can increase by either 27%

or 53% whereas Wright (1995) found it increased by 22% or 68% compared with that given in AS4100-

1998

In the case of cyclic loaded structures, the limit (18.0) for concrete-filled RHS members in the USA is about 7.8% higher than that (16.7) for empty RHS members

Concrete Filled Cold-Formed C450 R H S Columns

TABLE 1

235 WIDTH-TO-THICKNESS RATIO LIMITS ( ~ = ~ , ~y in MPa)

~Jy

431

Sections,

Loading

Empty

RHS,

Static

loading

Empty

RHS,

Cyclic

loading

Concrete-

filled

RHS,

Static

load

Concrete-

filled

RHS,

Cyclic

load

Country/

Region

Australia

Reference AS4100 (SAA (1998))

H/t limits 3+40.2e

Canada CAN/CSA-S16.1-M89 (CSA (1989)) 3+37.6e

Europe Eurocode 3 Part 1.1 (EC3 (1992)) 3+42.0e

USA

Europe

Research

Australia

Research

Australia

Research

UK

AISC Seismic Provisions (AISC (1997))

Eurocode 4 (1992), Rondal et al

(1996)

Uy et al (1998) assuming Simply- Supported boundary condition in Finite Strip Analysis

Uy et al (1998) assuming Fixed boundary condition in Finite Strip Analysis

Wright (1995) assuming Simply- Supported boundary condition Wright (1995) assuming Fixed boundary condition

AISC Seismic Provisions (1997)

Research in

UK

USA

3+18.9e

52e

50 for

~y : 300 MPa

60 for Cyy = 300 MPa

50 for Cyy - 275 MPa

69 for

(Yy 275 MPa 3+20.8e

H/t limit (C450RHS) 32.1 32.1 30.2 37.5 33.5 33.4 32.5 16.7

37.6 40.8

49.0

39.1

53.9

18.0

When comparing cyclic loaded members to static loaded members Table 1 shows that the limits decrease for the cyclic loading case The limit (16.7) for empty RHS bracing members under cyclic load is about half of that (30.2 to 37.5) for empty RHS under static load The limit (18.0) for concrete-filled RHS bracing members under cyclic load is about 33% to 48% of that (37.6 to 53.9) for static load

EXPERIMENTAL INVESTIGATION

Materials

Cold-formed RHS were supplied by BHP Steel- Structural Pipeline and Products (Newcastle, NSW) The nominal dimensions, width-to-thickness ratio and measured material properties are shown in Table 2 where a cross-section number (S1 or $2) is given Three tensile coupons were extracted from the flat surfaces of each tube size The tensile coupon tests were performed according to the Australian Standard

432 X.L Zhao et al

AS1391 (SAA (1991)) The 0.2% proof stress was adopted as the yield stress for the cold-formed steel tubes Six concrete cylinders with a diameter of 100 mm and a height of 200 mm were tested to determine the compression strength of the concrete filler The average compression strength was found to

be 44.4 MPa

TABLE 2 CROSS-SECTION SIZES AND MATERIAL PROPERTIES Cross- RHS Size Ratio Ratio

section H x B x t (H-2t)/t H/t

number (mm)

S 1 100 x 50 x 2 48 50

Form factor (SAA (1998))

Yield stress (MPa)

Ultimate tensile strength (MPa)

Comply with AISC (1997)

Test Specimens

Twelve tests were performed as listed in Table 3 and Table 4 The specimen label system used in this paper is: the first two letters (S 1 or $2) refers to the cross-section number defined in Table 2, the second part of the label (H or CF) refers to the filler material (Hollow or Concrete Filled), the third part of the label (SC, CD or CI) refers to the loading scheme (Static Compression, Cyclic-Direct or Cyclic- Incremental) as defined in the section on "Test Procedures"

TABLE 3 FIRST CYCLE PEAK LOADS (IN KN) FOR EMPTY RHS Specimen

Label

S 1HSC

Measured First cycle

peak load P1 (kN)

158

COV

Pke=0.5 Pke=0.65 Pke=0.7

-_

Pk~.5]Pl Pke~.65/Pl Pke~.V/Pl

1.0 0.829 0.766 1.0 0.829 0.766

1.003 0.755 0.675 1.093 0.823 0.736

0.058 0.053 0.065 TABLE 4

FIRST CYCLE PEAK LOADS (IN KN) FOR CONCRETE-FILLED RHS Specimen

Label

S 1CFSC

S 1CFCD

S 1CFCI

S2CFSC

S2CFCD

S2CFCI

Mean

COV

Tested AISC Eurocode 4

P l (kN) (1993)

AU CIDECT

294 292

294 292

294 292

437 428

437 428

437 428

PAISC/ PEC4/

Pl 0.939 1.062 1.066 0.977 1.024 1.086 1.026 0.056

PAIJ

Pl /el 0.878 0.945 0.993 1.069 0.996 1.073 0.964 0.991 1.010 1.038 1.071 1.101 0.985 1.036 0.064 0.056

PCIDECT /PI 0.939 1.062 1.066 0.971 1.017 1.078 1.022 0.056

Concrete Filled Cold-Formed C450 R H S Columns 433 The end-to-end length of all specimens was 2500 mm Each end was welded to a 20 mm thick steel plate which in turn was bolted to end supports The column end conditions were treated as fixed-fixed The corresponding effective length factor (ke) is 0.5 in an idealised condition, 0.65 according to Galambos (1998) and 0.7 as defined in AS4100-1998 The modified slenderness ratio (~,n) is about 96 for columns with S 1 RHS and 116 for columns with $2 RHS based on AS4100-1998 The maximum measured initial mid-span lateral deflection was within L/5000 where L is the column length

Test Set Up

A test rig was built on the N W Murray strong floor in the Civil Engineering Laboratory, Monash University, as shown in Figure 1 A 1000 kN capacity Instron Performance Reckoner actuator was used together with an Instron 8500 controller to apply the cyclic axial loading Two spring pots were used to measure the axial deformation and the mid-span lateral deformation

Figure 1 Test Set Up

Test Procedures

Different researchers used different loading histories For example, a loading history was defined by Liu and Goel (1998) in terms of a yield deformation Ay, where Ay was the axial deformation in tension of the specimen corresponding to the nominal yield stress Five cycles were applied at each axial deflection increment of 5Ay, 10Ay and 15Ay A similar loading history was defined by Walpole (1995), where one cycle was applied at each axial deflection increment of 2Ay, 5Ay, 10Ay, 15Ay, 20Ay and 25Ay In the tests

by Sherman and Sully (1994), the applied axial displacement was between 5 mm in compression and 5

mm in tension, or 10 mm in compression and 5 mm in tension with the number of cycles varying from 18

to 40

Constant axial displacement ranges were used in the current test program Ten cycles were applied for each axial displacement range Two types of loading schemes were used One is called Cyclic-Direct as shown in Figure 2 (a) where the cyclic load was directly applied when the axial displacement reached 10

mm The other loading scheme is called Cyclic-Incremental as shown in Figure 2(b) where the cyclic load was applied at several accumulating axial displacement increments

Test Results

Inward folding mechanism with cracks was observed for the empty S 1 RHS under cyclic loading as shown in Figure 3 (a), whereas an outward folding mechanism was observed for the filled S 1 RHS as shown in Figure 3 (b) Cracks at tube corners were observed in S 1CFCI but not in S 1CFCD No folding

434 X.L Zhao et al

mechanism was observed for the empty and filled $2 RHS as shown in Figure 3 (c) The peak load obtained in each test is listed in Tables 3 and 4 The peak load for the S 1 RHS increased by about 58% due to the concrete filing The increase in peak load for the $2 RHS was about 23% The load versus axial deflection curves are compared in Figure 4 for hollow and filled sections, where both upper bound (peak load in first cycle of each increment) and lower bound (peak load in last cycle of each increment) curves are given It seems that the concrete filling increases the residual load capacity of RHS to cyclic load especially for thinner sections No local failure was observed for $2 RHS in spite of the fact that their H/t ratio was larger than the limit specified in AISC (1997), ie the code may be too conservative

Figure 2 Loading Schemes

Figure 3 Failure modes

Figure 4 Load versus Axial Deflection Curves

Concrete Filled Cold-Formed C450 R H S Columns

FIRST CYCLE B U C K L I N G LOAD AND RESIDUAL STRENGTHS

435

The first cycle buckling load (ie the peak load Pl) for empty sections is presented in Table 3 The nominal column capacity predicted by AS4100-1998 using different values of effective length are compared with test values It seems that the use of an effective length factor of 0.5 gives the best agreement whereas using the recommended factor of 0.7 provides a conservative value

The first cycle buckling load for the concrete-filled RHS is presented in Table 4 Nominal capacities determined using AISC (1993), Eurocode 4, AIJ (Matsui et al 1997) and CIDECT (Bergmann et al 1995) formulae are compared with the experimental values All the design formulae give good predictions The residual strength (P/PI) for the CD (Cyclic-Ddirect loading) test series is plotted in Figure 5 (a) against the number of cycles It can be seen that the residual strength reduces more rapidly for thinner empty S 1 sections than for thicker $2 sections Furthermore, concrete filling induces more increase in residual strength for thinner sections than for the thicker sections

The residual strength for the CI (Cyclic-Incremental loading) test series was also plotted against the number of cycles A typical graph is shown in Figure 5 (b) for specimen S1CHCI where A1 is the first cycle axial deflection when the load peaks at Pl and A2 to A6 are the axial displacement at different increments It can be seen that the rate of reduction of residual load per cycle is about the same at different axial deflection increments This can also be seen in Figure 2 (b)

Figure 5 Residual Strength versus Number of Cycles

CONCLUSIONS

1 The effective length factor of 0.7 recommended in AS4100-1998 for a fully clamped empty RHS columns subjected to axial compression may be too conservative An effective length factor of 0.5 is more appropriate

2 Concrete filling reduces the rate of reduction in residual load (per cycle) for RHS subjected to cyclic loading especially for thinner sections Concrete filling induces more increase in residual strength for thinner sections than for thicker sections

3 No local failure mechanism was observed for all the $2 RHS profiles tested, which have a H/t ratio larger than the limit specified in AISC (1997), ie the core may be too conservative

4 The predicted first cycle buckling load for concrete-filled RHS members using AISC, Eurocode 4, AIJ and CIDECT formulae is in good agreement with tested values

436

REFERENCES

X.L Zhao et al

AIJ (1990) Standard for Limit State Design of Steel Structures Architectural Inst of Japan, Tokyo AISC (1992) Design Capacity Tables for Structural Steel Hollow Sections AISC, Sydney

AISC (1993) LRFD Specification for Structural Steel Buildings AISC, Chicago, Illinois

AISC (1997) Seismic Provisions for Structural Steel Buildings AISC, Chicago, Illinois

Bergmann, R et al (1995) Design Guide for Concrete-Filled Hollow Section Columns under Static and Seismic Loading, CIDECT, TOV- Verlag GmbH, K61n

BSI (1990) Structural use of Steelwork in Building, BS5950, Part 1, British Standards Inst., London CSA (1989) Steel Structures for Buildings CAN/CSA-S 16.1-M89, Rexdale, Ontario

Eurocode 3 (1992) Design of Steel Structures, Part 1.1 ENV 1993-1-1

Eurocode 4 (1992) Design of Composite Steel and Concrete Structures, Part 1.1 ENV 1994-1-1 Galambos, T.V (1998) Guide to Stability Design Criteria for Metal Structures, John Wiley & Sons, NY

Ge, H.B and Usami, T (1994) Strength Analysis of Concrete-Filled Thin-Walled Steel Box Columns, J

Construct Steel Research, 30, 259-281

Ge, H.B and Usami, T (1996) Cyclic Tests of Concrete Filled Steel Box Columns." J Struct Engrg.,

ASCE, 122(10), 1169-1177

Grzebieta, R.H., Zhao, X.L and F Purza (1997) Multiple Low Cycle Fatigue of SHS Tubes subjected to Gross Pure Bending, Proc., SDSS'97, Nagoya, Japan, 847-854

Haijar, J.F and Gourley, B.C (1997) A Cyclic Nonlinear Model for Concrete-Filled Tubes - I: Formulation J Struct Engrg., ASCE, 123(6), 736-744

Jain, A.K., Subhash, C., Goel, M and Hanson, R.D (1980) Hysteretic Cycles of Axially Loaded Steel Members J Struct Engrg., ASCE, 106(ST8), 1777-1795

Liu, Z.Y and Goel, S (1988) Cyclic Load Behaviour of Concrete-Filled Tubular Braces J Struct Engrg., ASCE, 114(7), 1488-1506

Matsui, C., Mitani, I., Kawano, A and Tsuda, K (1997) AIJ Design Method for Concrete Filled Steel Tubular Structures ASCCS Seminar, September, Innsbruck, 93-116

Nakai, H et al (1994) Experimental Study on Ultimate Strength and Ductility of Concrete-Filled Thin- Walled Steel Box Columns under Seismic Load J Struct Engrg., JSCE, 40A, 1401-1412

NZS 3404 (1992) Steel Structures Standard Standards Association of New Zealand, Wellington Ricles, J.M (1995) Seismic Performance of CFT Columns-to-WF Beam Moment Connections Proc.,

3rd International Workshop on Connections in Steel Structures, Trento, Italy

Rondal, J., Wurker, K.G., Dutta, D., Wardenier, J and Yeomans, N (1996) Structural Stability of Hollow Sections, Verlag TUV Rheinland GmbH, Ktiln, Germany

SAA (1991) Methods for Tensile Testing of Metals AS 139 l, Standards Assoc Australia, Sydney SAA (1998) Steel Structures AS4100-1998, Standards Association of Australia, Sydney

Sherman, D and Sully, R M.(1994) Tubular Bracing Member Under Cyclic Loading Proc., 4th Pacific Structural Steel Conference, Singapore

SSRC (1991) Stability of Metal Structures - A World View Structural Stability Research Council Tubemakers (1994) Design Capacity Tables for DuraGal Steel Hollow Sections Structural Products Division of Tubemakers of Australia Limited, Newcastle, NSW, Australia

Uy, B., Wright, H.D and Diedricks, A.A (1998) Local Buckling of Cold-Formed Steel Sections Filled with Concrete Proc., 2 no Int Conf Thin-Walled Structures, Singapore, 367-374

Walpole, W.P (1995) Behaviour of Cold-Formed Steel RHS Members Under Cyclic Loading Proc., Technical Conf National Society for Earthquake Engrg., Waikanae, New Zealand, 44-50

Wright, H D (1995) Local Stability of Filled and Encased Steel Sections", J Struct Engrg., ASCE,

121(10), 1382-1388

Zhao, X.L and Hancock, G.J (1991) Tests to Determine Plate Slenderness Limits for Cold-Formed Rectangular Hollow Sections of Grade C450." Steel Construction, AISC, 25(4), 2-16

Zhao, X.L and Grzebieta, R.H (1999) Void-Filled SHS Beams subjected to Large Deformation Cyclic Bending J Struct Engrg., ASCE, 125(9)

R E S E A R C H ON THE HYSTERETIC B E H A V I O R OF H I G H

S T R E N G T H C O N C R E T E F I L L E D S T E E L T U B U L A R

M E M B E R S U N D E R C O M P R E S S I O N A N D B E N D I N G

Zhan Wang ~ and Yonghui Zhen 2 Department of Civil Engineering, Shantou University, Daxue Road, Shantou City, Guangdong

Province, P.R.China

2 Department of Civil Engineering, Harbin University of Civil Engineering and Architecture(HUAE),

Haihe Road, Harbin City, Heilongjiang Province, P.R.China

ABSTRACT

In this paper, force-displacement hysteretic loops of high strength concrete filled steel tubular members under compression and bending are calculated by using finite element method according to the steel constitutive model which is suitable for multiaxial cyclic loading and modified bounding surface model which is suitable for multiaxial cyclic compression of concrete The test for hysteretic behavior

of high strength concrete filled steel tubular members have been done On the basis of theoretical analysis and experiment study, the force-displacement hysteretic loop characteristic of high strength concrete filled steel tubular members under lateral loading are discussed

KEYWORDS

Concrete filled steel tube, high strength concrete, hysteretic loops, aseismatic behavior

1.1NTRODUCTION

Recently, high strength concrete (HSC) is being spread and applied in engineering areas The trend of increment will be obvious year by year The weakness of HSC is its high brittlement Its failure, especially in complex stress state, will be controlled by brittlement ,and its reliability will be lowered The members composed of steel tube and HSC (HCFST), as an ideal high strength and high ductility members, is the best way of the application of HSC to reality On the basis of the research of HCFST force-displacement hysteretic loop under compression and bending and determining the model of hysteretic loop, we can analyze the HCFST elasto-plastic earthquake reaction by using shear building model For the research of mechanism of HCFST member under compression and bending, the aseismatic behavior and the determination of force-displacement hysteretic loop model, the theoretical calculation of force-displacement hysteretic loop is very important In this paper, by making use of finite element method and some necessary tests, hysteretic behavior of HCFST under compression and

437

438 Z Wang and Y Zhen

bending is studied It not only has important practical value on HCFST aseismatic design, but also has theoretical significance on further research of HCFST Illt21

2.THE CALCULATION OF FORCE-DISPLACEMENT HYSTERETIC LOOP

2.1 Calculating hypothesises and test method

HCFST under compression and bending is belonged to three dimension problem Three dimension finite element should be used to resolve it In this paper author resort to three dimension twenty nodes iso-parametric element which has so high precision in each element that it has being widely used on resolving three dimension problems

There are some hypothesises on HCFST analysis by using finite element method, t31

a) Horizontal section hypothesis

b) Constitutive relationship of steel is two linear random strengthen model

c) Constitutive relationship of core concrete is modified bounding surface model

There are two loading ways in calculation One is loading with force, which is used for vertical load The other is loading with displacement, which is used for horizontal load It mainly accords to the following

a) If loading with force, we will have great trouble near peak value and find no ways to calculate decent part

b) With respect to horizontal section hypothesis, we know the resultant force of imposed load and that section is still plane, but we don't know how the imposed forces distributed In this case, lateral load P and axial load N are not able to turned into equivalent joint force on member joint It

is hardly carried out even loading with force in elastic part on this question

2.Z Calculating method

The target of discussed in this paper can be regarded as a part of a frame column between inflection point and fixed end with lateral deflection, which stands for real work conditions of the column The member under compression and bending is only discussed here, that is, a constant axial force is applied

on a cantilever column first, lateral force is increased continuously later In this case, we research the relationship between lateral force and lateral displacement

There are two methods on calculating force-displacement hysteretic loop, i.e model column and data analysis method As there is bigger error in model column method, data analysis is used here

3.TEST RESEARCH

3.1 General features of test

In order to make a further research on behavior of HCFST under compression and bending and check the accuracy of theory, we carried out some experiments of force-displacement hysteretic loop There are basements which is twenty millimeters made of steel on up and bottom of test specimens The column is meld with stiffening rib of twenty-five millimeter So the rigidity of each side of column is big enough Loading sensors, displacement sensors and strain gauges are connected through the IMP(Isolated Measurement Pods) to the computer Test data are able to gathered automaticly The

interval time are 1500 milliseconds with continuous gathering control According to supervising data

curve, P - 6 hysteretic loops are drawn

Research on the Hysteretic Behavior of High Strength Concrete 439

3.Z Experiment result

Figure 1 is the analysis between experiment and calculating result in this paper These two are basely identical The difference is mainly raised by the following,

a) The rigidity of theoretical curve is higher because bending deflection is considered only except for shear deflection on theoretical calculation

b) The rigidity of real curve is lower because of crack and frictional force between each link of loading equipment

In addition to this, experiment result from others are gathered here Figure 2 and figure 3 show this compare The parameters of member in figure 2 are as following: 9 133• seamless steel tube which is 1260 millimeters long, the yield strength of steel is347.7 N/mm 2 , the cubic strength of

concrete is 70.2 N / m m 2 The subjected axial loads are 385 KN and 865 KN The parameters of

member in figure 3 are as following: 9 108 • 5 seamless steel tube which is 1100 millimeters long, the yield strength of steel is 327.8 N/mm 2 , the cubic strength of concrete is 33.8 N/mm 2 The subjected

axial loads are 20 KN and 270 KN

In the figure 1-~3, the left is experiment result and the right is calculating result

TABLE 1 FEATURES OF TEST SPECIMENS

Number of specimen D x t x L f~ f~,, r tll ~(~') Loading way

Z1-30 108 x 4.5 x 1250 312.4 77.1 1.07 300 Cyclic

Z2-20 114 x 6.0 x 1250 319.3 77.1 1.04 200 Cyclic

Z2-30 114 x 6.0 x 1250 319.3 77.1 1.04 300 Cyclic

Note[l]- r = 2Cry [11

rcfc,