This Page Intentionally Left BlankThis Page Intentionally Left BlankThis Page Intentionally Left BlankThis Page Intentionally Left BlankThis Page Intentionally Left BlankThis Page Intentionally Left BlankThis Page Intentionally Left BlankThis Page Intentionally Left BlankThis Page Intentionally Left BlankThis Page Intentionally Left BlankThis Page Intentionally Left BlankThis Page Intentionally Left BlankThis Page Intentionally Left BlankThis Page Intentionally Left Blank Experimental Study of High Strength Concrete Filled Circular Steel Columns Y. C. Wang Manchester School of Engineering, University of Manchester, Manchester M13 9PL UK ABSTRACT In places where usable floor space is at a premium, it is desirable to use the most structurally efficient load bearing columns. In concrete filled steel tubes, the beneficial interaction between the steel casing and the concrete core gives a load carrying system that is highly efficient. When High Strength Concrete (HSC) is used, the column load bearing performance is further improved. This paper presents the results of a series of parametric experimental study on HSC filled circular steel columns under axial compression. The parameters examined in these tests are: concrete grade, steel grade, column slenderness and steel contribution factor. The objectives of these tests are threefold: 1. To experimentally investigate the performance of HSC filled steel tubular columns; 2. To assess whether the design rules for normal strength concrete (NSC) filled columns can be extrapolated to HSC filled columns, and 3. To examine the structural load bearing efficiency by changing different design parameters. From the results of this experimental study, the following main findings have been obtained: 1. It is conservative to extrapolate the design method for NSC filled steel columns to HSC filled ones; 2. The advantage of HSC in resisting compressive load can be effectively utilised in HSC filled columns, even for slender columns where HSC does not offer much improved rigidity to resist flexural buckling; 3. The improved column strength due to concrete confinement effect is noticeable only for short columns; 4. The confinement effect may be appreciably reduced by a small eccentricity, and 5. The ductility of HSC filled columns is similar to that of NSC filled columns. 401 402 Y. C. Wang 1. Introduction In places where usable floor space is at a premium, it is desirable to use the most structurally efficient load beating columns. Concrete filled hollow steel columns are more structurally efficient in resisting compressive loads than either bare steel columns or reinforced concrete columns. They also have a number of other advantages including rapid construction, enhanced concrete strength and ductility due to the confinement effect and inherent high fire resistance. Normal strength concrete (NSC) filled columns are now being increasingly used in the construction of multi-storey and high rise buildings and design recommendations for this type of construction are now firmly established [1,2]. NSC is assumed to have the maximum cube strength of about 60N/mm 2. Using high strength concrete (HSC) can further improve the structural load bearing efficiency of concrete filled columns and improve their durabilit3'. However, before HSC filled columns can be used with confidence and improved economy, their superior load carrying capacity should be confirmed and suitable design guidelines developed. HSC filled steel tubes have been investigated by a number of researchers. For example, O'Shea and Bridge [3] concentrated on local buckling of thin walled tubes filled with HSC. Cai & Gu [4] studied the confinement effect on HSC in short columns. This paper reports the results of a series of tests on HSC (C100) and NSC (C40) filled circular hollow section (CHS) steel columns. The objectives of these tests were threefold: (1) To assess whether the design rules for NSC filled columns can be extrapolated to cater for HSC filled columns; (2) To experimentally study the performance of HSC filled columns, in particular, the confinement effect on the column strength and ductility, and (3) To examine the structural load bearing efficiency by changing different design parameters. 2. Test programme 2.1 Test parameters This series of tests were carried out to examine the influence of a number of design parameters on column performance. In total, 2 pairs of 12 columns were made and tested. Table 1 gives the values of test parameters for each pair of columns. 2.2 Test set up All columns were cast in December 1996 and tests were carried out about six months after casting. For each column, three concrete cubes of 100 mm and two concrete prisms of 90 mm square and 300 mm in height were cast, to be tested on the column loading day. For each concrete mix, three concrete cubes of 100 mm were cast. These cubes were tested after 28 days for quality control. Four strain gauges were attached to the external surface of the steel tube at two opposing sides at each column mid-height. Two strain gauges at each side measure the horizontal and longitudinal strains in the steel respectively. For the shortest columns of 500 mm, a vibrating strain gauge was cast in the column centre to measure the concrete axial main. Study of High Strength Concrete Filled Circular Steel Columns 403 Column tests were carried out in the BRE axial test machine with the maximum capacity of 5000 kN. Each column was simply supported about one axis and rotationally restrained in the perpendicular direction using roller supports. The end support increased the column length by about 80 mm at each end. Therefore, the total column length (L) should be the column specimen length (L0) plus 160 mm. All columns were intended to be subjected to compression axial load and were checked to be so by human eyes only. Also each column had some initial imperfections and the end supports had to be adjusted for each test. Inevitably, the column could not be aligned perfectly nor in the central position. This led to a small eccentricity, and bending moment in each column. The amount of this equivalent eccentricity will be evaluated for calculating the column strength. Each column was loaded incrementally until it reached its strength when it could not sustain the applied load. The test was continued to study the column response during unloading at increasing deformation until the column eventually found a stable position. 3. High strength concrete mechanical properties For each column, three cube tests and two prism tests were carried out to determine various properties of concrete. During each prism test, the concrete strain was measured and the complete concrete stress-strain relationship up to the maximum stress was established. Results of the compressive strength, the corresponding strain and the Young's modulus are given in Table 2. The Young's modulus was obtained by using the proposed stress-strain relationship from Cla)~ton [5]. It is observed that the stiffness of HSC is only slightly higher (about 25%) than that of NSC, and also that concrete strain at prism strength is almost independent of the concrete grade. 4. Test observation and results When high strength concrete fails in compression, the failure mode is brittle, this was observed during each prism test when HSC failure was accompanied by a noisy bang. In contrast, HSC filled steel columns failed in a ductile manner, similar to NSC. This was demonstrated by the ability of HSC filled columns to deform under decreasing loads and to find a stable position after reaching the peak strength. Different failure modes were observed for different columns. For short columns (Lo/D=3), the failure mode was clearly local due to extensive concrete crushing and steel yielding. NSC filled columns exhibited very ductile behaviour, with column failure due to splitting of the cold rolled steel tube at the welding edge. HSC filled short columns also behaved in a ductile way. Confinement effect was observed by the fact that the failure strain in HSC was several times higher than the prism crush strain. Nevertheless, the extent of concrete confinement in HSC was lower than in NSC filled columns and no steel tube splitting occurred. Global buckling was the dominant failure mode for the longest columns (Lo/D=25). Due to inevitable eccentricity induced bending effect, global buckling was not very clearly demonstrated in most columns. However, for the two columns that had very little bending moment, column failure was indicated by a sudden large lateral movement. 404 Y.C. Wang All columns w~th the intermediate length (Lo/D= 15) failed in a mixed mode, both axial strain and lateral deflection increased at steady but faster rates until peak applied load. Table 3 presents results for all columns, including the column eccentricity e. To verify the accuracy of the design method and to check the effectiveness of the confinement effect, it was necessary to evaluate the column eccentricity. This value is calculated from the two axial strain readings in the steel tube using the following equation: A6.EI e - [1] N.D where Ae = the difference in longitudinal strain recorded by the two strain gauges El =composite section flexural stiffiaess N =applied load in the column D =column cross-section diameter Equation (1) is based on elastic analysis, therefore, the value of eccentricity was obtained from the average of the few earlier load increments. In table 3, all design strengths were calculated taking into account the eccentricity and by setting the partial safety factors for steel and concrete to 1.0. Also, the short term concrete modulus of elasticity in Table 3 was used for each column. 5. Analysis of test results The test results have been analysed by a comparison against the predictions of various design methods for concrete filled columns. From this comparison, a number of conclusions may be drawn. This paper presents some of the more important ones. 5.1 Accuracy of current design rules for HSC filled columns The current design rules for concrete filled columns have been derived from test results on NSC. From the comparative results in Table 3, it may be concluded that these design rules give quite accurate predictions for NSC filled columns (TI&T2, T5&T6, T9&TI0). Furthermore, it seems that these design rules may be extended to HSC filled steel columns, as indicated by the overall accuracy in Table 3. Indeed, the current design rules give conservative results for HSC filled columns, thus they are acceptable for safety. Nevertheless, for HSC filled steel cohmms, Table 3 suggests that the accuracy of the NSC-based design rules depends on the column slenderness. While the code predictions are quite accurate for short columns, discrepancy between predicted and test results increases at higher column slenderness. Figure 1 presents the results in Table 3. It is clear that as the slenderness of HSC filled steel columns increase, both BS 5400 Part 5 [2] and Eurocode 4 Part 1.1 [2] predict lower column strength. Whilst this means that both design methods are safe to use for HSC filled steel columns, it also suggests that it is possible to use a higher cohann buckling curve for HSC filled steel columns for improved column efficiency. However, this can only be confirmed atter more extensive experimental studies. Study of High Strength Concrete Filled Circular Steel Columns 5.2 Effect of confinement on concrete 405 Strength It is now well reax~gnised that when concrete is under tri-axial compression, both its load carrying capacity and ductility are increased. Concrete confinement can be obtained through placing hoop reinforcement or using steel casing. For concrete filled columns, although increase in the concrete strength is at the expense of a reduction in the steel strength, the overall effect is a net increase in the column strength. This confinement effect diminishes for slender columns. Although BS 5400 Part 5 [1] gives a limiting length of L/D=25, realistically, the confinement effect is noticeable only for columns of m L/D not greater than 5. In Eur~xxte 4 Part 1.1 [2], the limiting column slenderness is at 2 =0.5. Nevertheless, in places where the column foot print is large, a L/D ratio of less than 5 is realistic. Thus, it is beneficial to explore the enhancement due to concrete confinement. However, the effect of confinement is greatly reduced by bending in the column. To illustrate the effect of concrete confinement, only Eurocxxle 4 Part .1.1 [2] is used in this paper. Results are given in Table 4 for L0/D=3. Without bending moment, the squash load of a column can be increased by up to 20% due to enhancement. However, with an eccentricity to diameter (e/D) ratio of only 3%, column strength increase due to the confinement effect is reduced by about 30%. For columns in simple construction, BS 5950 Part 1 [6] gives a nominal eccentricit)" of 100mm plus D/2 for beam reactions. For medium rise buildings, this end bending moment can give a significant eccentricity to the overall column axial load, which may completely remove the enhancement due to confinement. For example, for a 10 storey building with 300 mm diameter columns, the column eccentricity (e/D) to the overall axial load of the bottom floor column is about 8%. Therefore, to make use of the enhancement in design, an accurate assessment of the column ~tricity should be carried out. Duetili~ ~ One of the main concerns with using HSC is its lack of ductility and its brittle and explosive failure. However, in the author's tests, no HSC filled steel column suffered from this failure mode and all columns performed in a ductile manner. The ductilit3, of a column is rather difficult to quantify. The unloading slope of the column may give some indication. Figure 2 plots the load-axial strain relationship for tests T5-T8, two of which used HSC and the other two NSC. In this figure, the applied load is norrnalised with regard to the column test strength. The unloading slope seems to be comparable between NSC and HSC filled columns. However, while the two NSC filled column curves are almost identical, there is a great variability in the behaviour of the two HSC filled columns. On the other hand, if the column ductility is measured by the maximum concrete strain reached at the peak column strength, the enhanced strain due to the conflnernent effect may be predicted using the equation obtained by Mander et al [7]: 8cc I Cr cc |1 - 1 + 5 - [2] s \ 0"~ 406 Y. C. Wang Table 5 gives a comparison between test results and predictions using equation (4). Since the confinement effect is negligible for slender columns, the comparison was carried out for short columns (I.o/D=3) only. In addition, the theoretical value of the concrete strength enhancement factor (t~/(rck) has been calculated using recommendations in Eurocode 4 Part 1.1 [2]. Table 5 only indicates a broad agreement between the predictions of equation (4) and test results. Nevertheless, it suggests that the confinement effect can significantly increase the concrete ductility and that equation (4) gives conservative results. Table 5: Test ID T9 T10 TII T12 T17 T18 T27 T28 Increased concrete strain due to confmement effect t~cc/cck according to EC4 test 1.774 (1.544) 15.8 1.796 (1.56) 13.2 1.230 (1.162) 2.03 1.227 (1.213) 2.36 1.60(1.458) 4.0 1.571 (1.469) 6.12 1.442 (1.18) 5.07 1.412 (1.288) 4.5 2 0.17 0.17 0.21 0.21 0.20 0.21 0.18 0.19 NB: Values in brackets include the effect of eccentricity. model [7] 4.87 (3.72) 4.98 (3.8) 2.15(1.81) 2.14 (2.07) 4.0 (3.29) 3.86 (3.35) 3.21 (1.9) 3.06(2.44) 5.3 Effect of high strength steel One of the original objectives of this series of tests was to examine the effectiveness of using high strength materials, including both high strength steel and high strength concrete. The effect of using HSC has already been discussed in 5.1. It seems that despite only a modest increase in HSC modulus of elasticity., column test strength increases in line with increase in the column squash load regardless of the column slenderness. However, unless the column is short, using high strength steel only gives a small increase in the column strength. Tests T13-TI8 are directly comparable to Test T23-T28, the only difference being that $355 steel was used in the former and S275 steel was used in the latter. Table 6 gives increases in the column strength due to high strength steel. Clearly, the benefit of using high strength steel diminishes at higher column slenderness. Table 6: Comparison between results for different grades of steel L/D=25 L/D=15 L/D=5 1.075 1.386 1.368 6. Conclusion This paper has presented the results of a series of compression tests on NSC and HSC filled circular steel columns. From an analysis of the test results, the following conclusions may be drawn: Study of High Strength Concrete Filled Circular Steel Columns 407 (1) Using HSC can significantly increase the strength of concrete filled columns. This conclusion applies to a wide range of tested column slenderness (2, = 0.2-1.4 ). (2) Since the modulus of elasticit3' of HSC is only slightly higher than that of NSC, it follows that a higher column buckling curve may be used in design calculations for HSC filled steel columns. However, a large number of tests should be carried out for confirmation. In the meantime, the design rules for NSC filled steel columns may conservatively be used for HSC filled columns. (3) Using high strength steel is far less effective tlwu using HSC in increasing the column strength. (4) The benefits of concrete confinement in increasing the concrete strength and ductility are realised for short columns only. Furthermore, the increase in concrete strength can be reduced by a small ~tricity. Therefore, in order to reliably use the beneficial effect of confining concrete, accurate assessment of the column eccentricity should be made in design calculations. Acknowledgments The tests reported in this paper were carried out by the author at the Building Research Establishment and he acknowledges the technical support of various BRE staff members. He also thanks Mr. Nigel Clayton of BRE for the concrete prism tests. References 1. Design of composite bridges: use of BS 5400: Part 5:1979 for Department of Transport structures, Department of Transport, London, December 1987 2. Eurocode 4: Design of composite steel and concrete structure, Part 1.1: General rules and rules for buildings, British Standards Institution, London, 1994 3. O'Shea M D and Bridge R Q, "Circular thin walled concrete filled steel tubes", Proceedings of the 4 th Pacific Structural Steel Conference, Vol. 3: Steel-concrete composite structures, pp. 53- 60, 1995 4. Cai, S H and Gu W P, "Behaviour and ultimate strength of steel tube confined high strength concrete columns", Proceedings of 4 th International S3~posium on Utilization of high strength/high performance concrete, pp. 827-833, Paris 1996 5. BS 5950: Structural use of steelwork in buildings, Part 1: Code of practice for design in simple and continuous construction: hot rolled sections, British Standards Institution, London, 1990 6. Clayton N, "High grade concrete - stress-strain behaviour", BRE Client Report CR44/97, Building Research Establishment, 1997 7. Mander J B, Priestley M J N and Park R, "Theoretical stress-strain model for confined concrete", Journal of Structural Engineering, Vol. 114, No. 8, pp. 1804-1826, American Society of Civil Engineering, 1988 Y.C. Wang TI,T2 168.3 5.0 T3,T4 168.3 5.0 T5,T6 168.3 5.0 T7,T8 168.3 5.0 T9,TI0 168.3 5.0 TI1,T12 168.3 5.0 T13,T14 168.3 10.0 T15,T16 168.3 10.0 T17,T18 168.3 10.0 T23,T24 168.3 10.0 T25,T26 168.3 10.0 T27,T28 168.3 10.0 Table 2: Measured Material Lo (mm) Steel $355 8rade Test ID 4200 4200 S355 C100 2500 $355 C40 2500 $355 C100 500 $355 C40 500 S355 CI00 4200 $355 C100 2500 $355 C100 500 $355 C100 4200 S275 CI00 2500 S275 C100 500 $275 C100 Concrete 8rade C40 TI 438 Steel yield stress (Nlmm 2) 52 Properties Cube strength (Nlmrn 2) 40.8 T2 438 51.8 T3 438 123.5 T4 438 121.2 T5 438 47.5 T6 438 47.8 T7 438 116.0 T8 438 115.3 T9 438 46 T10 438 44.7 Tll 438 115.3 T12 438 113.8 TI3 i480 120.7 Cylinder strength (N/mm 2) 41.3 106.3 91.0 39.0 39.0 97.8 408 Table 1: Test parameters Test ID D(mm) t(mm) Young's modulus (N/mm 2) 41000 6max (mm/m) 2.3 45000 , 2.4 52500 12.83 53000 '2.05 39500 t2.2 42000 2.05 50500 2.45 101.0 54000 2.8 37.3 41000 2.05 36.5 41500 2.15 53500 2.48 52500 2.78 49000 2.4 50500 2.25 49500 2.48 99.5 100.0 92.7 T14 480 119.5 82.3 TI5 480 113.8 93.3 T16 480 114.2 90.5 52500 2.20 TI 7 480 126.0 87.8 52000 2.95 TI8 480 120.0 90.8 49000 2.23 50000 2.78 T23 330.5 116.8 , 98.8 T24 330.5 118.7 98.5 50500 2.78 T25 330.5 113.6 89.3 53500 2.23 T26 330.5 116.3 95.8 52500 I 2.48 T27 330.5 116.0 91.8 152000 ! 2.38 T28 330.5 114.2 97.0 ~ ' i 50500 2.58 Study of High Strength Concrete Filled Circular Steel Columns 409 Table 3: Comparison between design strength and test results Test lD ~ e(mm) Test comparison between design calculations and test results load BS 5400 Part 5 [ 1] Euroexxte 4 Part 1.1 t21 (kN) (kN) _pred/test (kN~.__pred/test T1 1.16 3 900 , 964 1.071 961 1.068 ,_ _ __._._ T2 1.14 5 950 ! 963 0.993 932 0.981 T3 1 '~43 2.5 1550 ~ 0.754 1153 0.744 T4 1.35 5 1400 ~ 1124 0.803 ! 1053 0.752 T5 0.7 4 1300 i 1382 1.063 ~ 1448 1.114 T6 0.7 2 1445 1465 1.014 1513 1.047 T7 0.85 5 2330 1858 0.791 2007 0.854 T8 0.85 2.5 2450 2004 0.818 2197 0.897 T9 0.17 5 2360 2002 0.848 1891 0.801 T10 0.17 5 2360 1988 0.842 1879 0.796 Tll 0.21 5 3250 i 2784 0.857 2944 0.906 T12 0.21 1 3250 ~ 0.950 32/2 1.007 v~ T13 1.36 4.5 1900 ~ 0.827 1481 0.779 T14 1.33 1.5 2400 1661 0.692 1623 0.676 T15 0.83 2 3350 2855 0.852 2957 0.883 T16 ~ 0.2 3650 3032 0.831 3076 0.843 T17 ~ 4 4550 4326 0.951 3943 0.867 T18 0.21 3 4550 4386 0.964 4099 0.901 T23 1.25 6 1800 1429 0.794 ~ 0.738 T24 1.24 0.5 2200 1605 0.729 11627 ! 0.739 !T25 0.74 3 2600 2333 0.897 "2465 ! 0.948 L T26 0.75 3.5 2450 2314 0.945 : 1.021 IT, 0.1 lO 2,0 2 60 10. 64 , T28 0.19 5 3400 3206 0.943 3-~ 10.952 Table 4: Effect of column squash load increase due to confinement effect Test iD With bending moment Without bending moment T9 1.145 1.207 T10 1.148 1.210 T1 i 1.066 1.094 TI2 1.087 1.092 ,, TI7 1.109 1.142 T18 1.113 1.137 T27 1.056 1.137 T28 1.091 1.130 . the two axial strain readings in the steel tube using the following equation: A6.EI e - [1] N.D where Ae = the difference in longitudinal strain recorded by the two strain gauges El =composite. and ductility are increased. Concrete confinement can be obtained through placing hoop reinforcement or using steel casing. For concrete filled columns, although increase in the concrete strength. filled columns. (3) Using high strength steel is far less effective tlwu using HSC in increasing the column strength. (4) The benefits of concrete confinement in increasing the concrete strength