Experimental and numerical investigation of high strength stainless steel structures

Experimental and numerical investigation of high strength stainless steel structures

Journal of Constructional Steel Research 64 (2008) 1225–1230 Contents lists available at ScienceDirect Journal of Constructional Steel Research jour...

2MB Sizes 7 Downloads 115 Views

Journal of Constructional Steel Research 64 (2008) 1225–1230

Contents lists available at ScienceDirect

Journal of Constructional Steel Research journal homepage: www.elsevier.com/locate/jcsr

Experimental and numerical investigation of high strength stainless steel structures Ben Young ∗ Department of Civil Engineering, The University of Hong Kong, Pokfulam Road, Hong Kong

article

info

Article history: Received 12 November 2007 Accepted 13 May 2008 Keywords: Cold-formed steel Experimental investigation Finite element analysis High strength steel Stainless steel structures Structural stability Tubular structures

a b s t r a c t The paper summarises research on high strength stainless steel tubular structures conducted at the University of Hong Kong, and the Hong Kong University of Science and Technology. Square and rectangular hollow sections were investigated. The test specimens were cold-rolled from high strength austenitic and duplex stainless steel sheets. The material properties of the test specimens were determined by tensile coupon tests at normal room and elevated temperatures. The initial geometric imperfection and residual stress of the specimens were measured. The experimental and numerical investigation focused on the design and behaviour of cold-formed high strength stainless steel structural members. The results were compared with design strengths calculated using the American, Australian/New Zealand and European specifications for cold-formed stainless steel structures. © 2008 Elsevier Ltd. All rights reserved.

1. Introduction Stainless steel sections have been increasingly used in architectural and structural applications because of their superior corrosion resistance, ease of maintenance and pleasing appearance. The mechanical properties of stainless steel are quite different from those of carbon steel. For carbon and low-alloy steels, the proportional limit is assumed to be at least 70% of the yield point, but for stainless steel the proportional limit ranges from approximately 36%–60% of the yield strength [1]. Therefore, the lower proportional limits would affect the buckling behaviour of stainless steel structural members. The American Society of Civil Engineers (ASCE) Specification for the design of cold-formed stainless steel structural members [2], the Australian/New Zealand Standard (AS/NZS 4673) for coldformed stainless steel structures [3], and the European Code (Eurocode 3) design of steel structures, part 1.4: supplementary rules for stainless steels [4] provide design rules of stainless steel structural members. Cold-formed square or rectangular hollow section is formed by cold-rolling a circular hollow section which is welded (causing an annealed strip of material) and then further rolled into square or rectangular hollow section. This process of forming by cold-working produces considerable enhancement to the material properties of the annealed steel. More economic design can be achieved by taking the enhancement of the material properties due to cold-working into account. In this paper, the



Tel.: +852 2859 2674; fax: +852 2559 5337. E-mail address: [email protected]

0143-974X/$ – see front matter © 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.jcsr.2008.05.004

design strengths were calculated based on the material properties obtained from the finished specimens. Stainless steel structural members are more expensive than carbon steel. Therefore, more economic design and the use of high strength stainless steel could offset some of the costs. Coldformed high strength stainless steel structural members are being increasingly used in structural applications. However, few test data are available on cold-formed high strength stainless steel. Therefore, it is important to investigate the behaviour of high strength stainless steel structures. The purpose of this paper is to briefly describe the experimental and numerical investigation of cold-formed high strength stainless steel tubular structures conducted at the University of Hong Kong, and the Hong Kong University of Science and Technology. Design recommendations for high strength stainless steel structural members have been proposed. The research findings have been published in international journals and conferences, and reference is made to these publications for further details. 2. Test specimens The tests were performed on square hollow sections (SHS) and rectangular hollow sections (RHS) of high strength austenitic and duplex stainless steel. The grade of duplex stainless steel is approximately equivalent to EN 1.4462 and UNS S31803. The test specimens were cold-rolled from flat strips. The specimens consisted of seven different sections that included four SHS and three RHS. The test specimens had nominal thicknesses (t) ranged from 1.5 to 6 mm, nominal overall depth of the webs (D) ranged

1226

B. Young / Journal of Constructional Steel Research 64 (2008) 1225–1230

Table 1 Material properties obtained from tensile coupon tests at normal room temperature Specimen D × B × t (mm)

Type

Eo (GPa)

σp (MPa)

σ0.2 (MPa)

σu (MPa)

εf (%)

40 × 40 × 2 50 × 50 × 1.5 150 × 150 × 3 150 × 150 × 6 140 × 80 × 3 160 × 80 × 3 200 × 110 × 4

Duplex Duplex HSA HSA Duplex Duplex HSA

216 200 189 194 212 208 200

164 182 155 147 199 167 150

707 622 448 497 486 536 503

827 770 699 761 736 766 961

29 37 52 52 47 40 36

Note: HSA = High Strength Austenitic.

(a) SHS.

(b) RHS. Fig. 1. Definition of symbols.

from 40 to 200 mm, and nominal flange widths (B) ranged from 40 to 150 mm, as shown in Fig. 1. The web slenderness value of the specimens ranged from approximately 16 to 50. 3. Material properties 3.1. Normal room temperature The material properties of the test specimens were determined by tensile coupon tests at normal room temperature (ambient temperature). The tensile coupon specimens were taken from the center of the face at 90◦ angle from the weld in the longitudinal direction of the stainless steel sections. The coupons were prepared and tested according to the American society for testing and materials Standard [5] and the Australian Standard AS 1391 [6] for the tensile testing of metals, using 12.5 mm wide coupons of gauge length 50 mm. The coupons were tested in a 250 kN capacity MTS displacement controlled testing machine using friction grips. Two strain gauges and a calibrated extensometer of 50 mm gauge length were used to measure the longitudinal strain. A data acquisition system was used to record the load and strain at regular intervals during the tests. The static load was obtained by pausing the applied straining for 1.5 min near the 0.2% tensile proof stress and the ultimate tensile strength. This allowed the stress relaxation associated with plastic straining to take place. The material properties of the SHS and RHS specimens obtained from the tensile coupon tests are summarised in Table 1, which includes the type of stainless steel, the measured initial Young’s modulus (Eo ), the proportional limit (σp ), the static 0.2% tensile proof stress (σ0.2 ), the static tensile strength (σu ), and the elongation after fracture (εf ) based on a gauge length of 50 mm. The tensile coupon tests are detailed in Young and Lui [7]. 3.2. Elevated temperatures Tensile coupon tests were also conducted to determine the material properties of stainless steel at elevated temperatures by Chen and Young [8]. Both steady and transient state tests were conducted at different temperatures approximately ranged from 20 to 1000 ◦ C. The elastic modulus, 0.2% tensile proof stress (yield strength) obtained at different strain levels, ultimate

Fig. 2. Comparison of reduction factor of 0.2% yield strength predicted by AS 4100 and proposed equation with test results.

strength, ultimate strain and thermal elongation versus different temperatures were plotted and compared with the predictions from the Australian, British and European standards. The test results of 0.2% yield strength of stainless steel types EN 1.4462 (duplex stainless steel) and EN 1.4301 (austenitic stainless steel type 304) are plotted in Fig. 2. The vertical axis of the graph plotted the reduction factor f0.2,T /f0.2,normal , where f0.2,T is the 0.2% yield strength at temperature T in degree Celsius (◦ C), and f0.2,normal is the 0.2% yield strength at normal room temperature. The horizontal axis of the graph plotted against different temperatures. It is shown that the test results of stainless steel types EN 1.4462 and EN 1.4301 obtained from the steady state and transient state tests are similar. The reduction factor of 0.2% yield strength obtained from the tests were compared with the Australian Standard AS 4100 [9] prediction and also compared with the test results conducted by Ala-Outinen [10], Sakumoto et al. [11] and Ala-Outinen et al. [12], as shown in Fig. 2. It is shown that the reduction factors of 0.2% yield strength predicted by the Australian Standard AS 4100 [9] are unconservative for temperatures ranged from 80 to 500 ◦ C, therefore, a new equation is needed for the prediction of reduction factor of 0.2% yield strength. The proposed unified equation for yield strength is shown in Eq. (1). f0.2,T f0.2,normal

=a−

(T − b)n c

(1)

where the coefficients a, b, c and n of the equation were calibrated with the stainless steel test results, and the coefficients are detailed in Chen and Young [8]. The temperature T is in degree Celsius (◦ C). The proposed unified equation is applicable for both high strength and normal strength stainless steel. A unified equation for elastic modulus, ultimate strength and ultimate strain of stainless steel at elevated temperatures was also proposed. Furthermore, stress–strain curves at different temperatures were plotted and a stress–strain model was also proposed. The details of this study are shown in Chen and Young [8].

B. Young / Journal of Constructional Steel Research 64 (2008) 1225–1230

Fig. 3. Pure bending test.

4. Geometric imperfection and residual stress measurements The initial local geometric imperfections of the specimens were measured for the SHS and RHS specimens prior to testing. The specimens were measured using a Mitutoyo Co-ordinate Measuring Machine with an accuracy of 0.001 mm. The maximum measured local geometric imperfections were 0.113, 0.164, 0.343, 0.460 and 1.084 mm for 40 × 40 × 2, 50 × 50 × 1.5, 140 × 80 × 3, 160 × 80 × 3 and 200 × 110 × 4 specimens, respectively. The magnitudes and distributions of residual stresses for the coldformed high strength stainless steel sections were measured. The residual stress measurements were conducted on the RHS 200 × 110 × 4 specimen. The length of the specimen was 300 mm. The longitudinal residual strains were measured by the method of sectioning, and the strains were converted to residual stresses. The membrane and bending residual stresses were calculated as the average and the difference in residual stress measurements at the two surfaces, respectively. The geometric imperfection and residual stress measurements are detailed in Young and Lui [7]. 5. Beams 5.1. Pure bending A series of tests on cold-formed stainless steel square and rectangular hollow sections subjected to major axis bending has been presented by Zhou and Young [13]. The test setup of the pure bending tests is shown in Fig. 3. The test strengths were compared with the design strengths obtained using the American Specification [2] and Australian/New Zealand Standard [3] for coldformed stainless steel structures. In addition, the test strengths were compared with the theoretical elastic and plastic bending moments. It is shown that the design strengths predicted by the specifications and the theoretical bending moments are generally conservative for the tested specimens. The comparison of the test strengths with the design strengths is detailed in Zhou and Young [13]. 5.2. Web crippling Tests of cold-formed high strength stainless steel tubular sections subjected to web crippling have been presented by Zhou and Young [14]. Tensile and compression coupon tests were performed to obtain the longitudinal tension and transverse compression material properties. The web crippling tests were conducted under four loading conditions specified in the American

1227

Specification [2] and Australian/New Zealand Standard [3] for coldformed stainless steel structures, namely End-One-Flange (EOF), Interior-One-Flange (IOF), End-Two-Flange (ETF), and InteriorTwo-Flange (ITF) loading conditions. The test strengths were compared with the design strengths obtained using the American Specification [2] and Australian/New Zealand Standard [3]. In addition, the test strengths were also compared with the design strengths obtained using the unified web crippling equation as specified in the North American Specification [15] for coldformed carbon steel structural members. It is shown that the design strengths predicted by these specifications are either unconservative or very conservative. Hence, a unified web crippling equation with new coefficients for cold-formed stainless steel square and rectangular hollow sections has been proposed. Eq. (2) is the proposed unified equation with new coefficients of C , CR , CN and Ch , and the new coefficients are detailed in Zhou and Young [14]. PP = Ct fy sin θ 2

r !

r 

 1 − CR

ri t

1 + CN

N t

r ! 1 − Ch

h t

(2)

where Pp is the proposed web crippling strength, C is the coefficient, CR is the inside corner radius coefficient, CN is the bearing length coefficient, Ch is the web slenderness coefficient, t is the thickness of the web, fy is the yield stress (σ0.2 proof stress), θ is the angle between the plane of the web and the plane of the bearing surface, ri is the inside corner radius of the section, N is the length of the bearing and h is the depth of the flat portion of the web measured along the plane of the web. The four loading conditions (EOF, IOF, ETF and ITF) specified in the American Specification [2] and Australian/New Zealand Standard [3] do not directly simulate floor joist members seated on a solid foundation subjected to concentrated bearing load. Hence, experimental and numerical investigation of coldformed stainless steel square and rectangular hollow sections subjected to concentrated bearing load have been studied by Zhou and Young [16]. The tests were carried out under end and interior loading conditions. A non-linear finite element model was developed and verified against the test results. Geometric and material non-linearities were included in the finite element model. The test and finite element analysis of a stainless steel specimen under interior loading condition are shown in Fig. 4. It is shown that the finite element model closely predicted the web crippling strengths and failure modes of the tested specimens. The test results and the web crippling strengths predicted by the finite element analysis were compared with the design strengths obtained using the American, Australian/New Zealand and European specifications for stainless steel structures. The comparison is detailed in Zhou and Young [16]. The yield line mechanism analysis on web crippling of cold-formed stainless steel tubular sections has been investigated by Zhou and Young [17]. Furthermore, tests of cold-formed high strength stainless steel tubular members subjected to combined bending and web crippling are also detailed in Zhou and Young [18]. 6. Columns Experimental investigation of cold-formed high strength stainless steel columns has been presented by Young and Lui [19]. The SHS and RHS columns were compressed between fixed ends. The fixed-ended columns were tested at different column lengths, and column curves were obtained for each test series. The test setup of a typical fixed-ended column is shown in Fig. 5. The test strengths were compared with the design strengths predicted using the American, Australian/New Zealand and European specifications for cold-formed stainless steel structures. It is shown that the design

1228

B. Young / Journal of Constructional Steel Research 64 (2008) 1225–1230

(a) Experimental.

(a) Local buckling.

(b) Overall buckling.

Fig. 6. Buckling modes of a RHS column obtained from finite element analysis.

(b) Finite element analysis. Fig. 4. Comparison of experimental and finite element analysis failure mode for interior loading condition.

has been developed. The initial local and overall geometric imperfections, residual stresses, non-linear material properties of flat and corner portions of the cold-formed stainless steel columns have been included in the finite element model. Fig. 6 shows the local and overall buckling modes of a RHS column. It is shown that the finite element model accurately predicted the failure modes and column strengths of the tests. The comparison between the numerical results and the experimental results showed good agreement in predicting the column behaviour and strengths. The column strengths, load-shortening behaviour and failure modes have been predicted using the finite element model. A parametric study was performed using the developed finite element model. The results of the parametric study showed that the design rules specified in the American Specification [2], Australian/New Zealand Standard [3] and European Code [4] are generally conservative for the cold-formed high strength stainless steel square and rectangular hollow section columns, except for some of the short columns. The numerical investigation is detailed in Ellobody and Young [20]. 7. Concrete-filled columns

Fig. 5. Typical fixed-ended column test.

strengths predicted by the three specifications are generally conservative for the cold-formed high strength stainless steel square and rectangular hollow section columns. Therefore, the current column design rules specified in the three specifications are applicable to high strength material, despite the fact that the specifications were mainly based on the investigation of normal strength material. Numerical investigation of cold-formed high strength stainless steel columns has been presented by Ellobody and Young [20]. A finite element model for cold-formed stainless steel columns

A series of tests on concrete-filled cold-formed high strength stainless steel tube columns has been presented by Young and Ellobody [21]. The tests on concrete-filled high strength stainless steel square and rectangular hollow section columns were concentrically loaded. The overall depth-to-plate thickness ratio of the tube sections varied from 25.7 of compact sections to 55.8 of relatively slender sections. Different concrete cylinder strengths varied from 40 to 80 MPa were investigated. A failed specimen of concrete-filled stainless steel RHS column is shown in Fig. 7(a). The column strengths, load-axial strain relationships and failure modes of the columns have been reported by Young and Ellobody [21]. The test strengths were compared with the design strengths predicted using the American and Australian/New Zealand specifications for cold-formed stainless steel and concrete structures. The material properties of the high strength stainless steel tube specimens obtained from tensile coupon tests and stub column tests were used to calculate the design strengths. The test strengths of

B. Young / Journal of Constructional Steel Research 64 (2008) 1225–1230

(a) Experimental.

1229

concrete strengths showed good agreement in predicting the strength of the columns. Fig. 7 shows the comparison of the finite element results and the test results. The column strengths and deformed shapes of the columns have been predicted using the finite element model and compared well with the experimental results. A parametric study was performed using the finite element model to investigate the concrete-filled cold-formed stainless steel tube columns of square and rectangular hollow sections having the depth of the flat portion-to-plate thickness (h/t) ratio ranged from 16 to 96. Different concrete cylinder strengths ranged from 20 to 100 MPa were also investigated. The column strengths obtained from the finite element analysis were compared with the design strengths predicted using the American and Australian/New Zealand specifications for coldformed stainless steel and concrete structures. The material properties of the high strength stainless steel tube specimens obtained from tensile coupon tests of flat and corner portions were used to calculate the deign strengths. It is shown that the design strengths calculated using the American and Australian/New Zealand specifications are generally conservative for concrete-filled cold-formed stainless steel square and rectangular hollow section columns having h/t ratio less than 60, whereas the design strengths were more conservative for columns having h/t ratio greater than 60. Hence, modification to the design rules specified in the American and Australian/New Zealand specifications was proposed for concrete-filled cold-formed stainless steel square and rectangular hollow section columns having h/t ratio greater than 60. The design strengths predicted using the proposed modified equation are more accurate compared with the design strengths calculated using the American and Australian/New Zealand specifications. The proposed modified equation is detailed in Ellobody and Young [22]. 8. Conclusions

(b) Finite element analysis. Fig. 7. Composite columns.

the high strength stainless steel tubes without concrete infilled were also used to calculate the design strengths. It is shown that the design strengths calculated using the material properties of stainless steel obtained from the tensile coupon tests as well as the test strengths of the high strength stainless steel tubes without concrete infilled are generally unconservative for the tested concrete-filled stainless steel tube columns. It is demonstrated that the design strengths calculated using the material properties of stainless steel obtained from the stub column tests are generally conservative for both compact and slender sections with different concrete strengths. Therefore, it is recommended that the design rules in the American and Australian/New Zealand specifications for cold-formed stainless steel and concrete structures can be used for the design of concrete-filled cold-formed high strength stainless steel tube columns provided that the design strengths are calculated using the material properties of stainless steel obtained from stub column tests. A non-linear finite element model for the analysis of concretefilled cold-formed stainless steel tube columns has been presented by Ellobody and Young [22]. The material nonlinearities of high strength stainless steel tubes and confined concrete have been accurately introduced in the finite element model. The comparison between the finite element results and the experimental results of the columns with different cross-section geometries and different

The research on cold-formed high strength stainless steel tubular structures conducted at the University of Hong Kong, and the Hong Kong University of Science and Technology has been summarised in this paper. The material properties of the stainless steel specimens at normal room and elevated temperatures were obtained from tensile coupon tests. The initial local geometric imperfections of the test specimens were measured. The membrane and bending residual stresses were also measured using the method of sectioning. A series of tests was conducted on structural members subjected to pure bending, web crippling, combined bending and web crippling, and axial compression. Experimental and numerical investigation of cold-formed high strength stainless steel columns and concrete-filled columns have been investigated. Design recommendations for cold-formed stainless steel tubular structures have been proposed. The details of the investigation can be found in the publications as referred in this paper. Acknowledgment The test specimens provided by STALA Tube Finland are gratefully acknowledged. References [1] Yu WW. Cold-formed steel design. 3rd ed. New York: John Wiley and Sons, Inc.; 2000. [2] ASCE. Specification for the design of cold-formed stainless steel structural members. SEI/ASCE-8-02. Reston (Virginia): American Society of Civil Engineers; 2002. [3] AS/NZS. Cold-formed stainless steel structures. Australian/New Zealand Standard. AS/NZS 4673:2001. Sydney (Australia): Standards Australia; 2001. [4] EC3. Eurocode 3: Design of steel structures – Part 1.4: General rules – Supplementary rules for stainless steels. ENV 1993-1-4, CEN, Brussels: European Committee for Standardization; 1996.

1230

B. Young / Journal of Constructional Steel Research 64 (2008) 1225–1230

[5] American Society for Testing and Materials. Standard test methods for tension testing of metallic materials. E 8M-97. West Conshohocken; 1997. [6] Methods for tensile testing of metals. Australian Standard. AS 1391. Sydney (Australia): Standards Association of Australia; 1991. [7] Young B, Lui WM. Behavior of cold-formed high strength stainless steel sections. J Struct Eng ASCE 2005;131(11):1738–45. [8] Chen J, Young B. Stress-strain curves for stainless steel at elevated temperatures. Eng Struct 2006;28(2):229–39. [9] Steel Structures. Standards Australia, AS 4100:1998, Sydney, Australia; 1998. [10] Ala-Outinen T. Fire resistance of austenitic stainless steels polarit 725 (EN 1.4301) and polarit 761 (EN 1.4571). VTT Research Notes. ESPOO (Finland): Technical Research Center of Finland; 1996. [11] Sakumoto Y, Nakazato T, Matsuzaki A. High-temperature properties of stainless steel for building structures. J Struct Eng ASCE 1996;122(4):399–406. [12] Ala-Outinen T, Oksanen T. Stainless steel compression members exposed to fire. VTT Research Notes. ESPOO (Finland): Technical Research Center of Finland; 1997. [13] Zhou F, Young B. Tests of cold-formed stainless steel tubular flexural members. Thin-Walled Struct 2005;43(9):1325–37. [14] Zhou F, Young B. Cold-formed high-strength stainless steel tubular sections subjected to web crippling. J Struct Eng ASCE 2007;133(3):368–77.

[15] NAS. North American Specification for the design of cold-formed steel structural members. North American Cold-formed Steel Specification. North American Specification. Washington (DC): American Iron and Steel Institute; 2001. [16] Zhou F, Young B. Experimental and numerical investigations of cold-formed stainless steel tubular sections subjected to concentrated bearing load. J Construct Steel Res 2007;63(11):1452–66. [17] Zhou F, Young B. Yield line mechanism analysis on web crippling of coldformed stainless steel tubular sections under two-flange loading. Eng Struct 2006;28(6):880–92. [18] Zhou F, Young B. Experimental investigation of cold-formed high-strength stainless steel tubular members subjected to combined bending and web crippling. J Struct Eng ASCE 2007;133(7):1027–34. [19] Young B, Lui WM. Tests of cold-formed high strength stainless steel compression members. Thin-Walled Struct 2006;44(2):224–34. [20] Ellobody E, Young B. Structural performance of cold-formed high strength stainless steel columns. J Construct Steel Res 2005;61(12):1631–49. [21] Young B, Ellobody E. Experimental investigation of concrete-filled coldformed high strength stainless steel tube columns. J Construct Steel Res 2006; 62(5):484–92. [22] Ellobody E, Young B. Design and behaviour of concrete-filled cold-formed stainless steel tube columns. Eng Struct 2006;28(5):716–28.