New Czechoslovak Standard for Cold-formed Steel Structures

Transcription

1 Missouri University of Science and Technology Scholars' Mine International Specialty Conference on Cold- Formed Steel Structures (1990) - 10th International Specialty Conference on Cold-Formed Steel Structures Oct 23rd New Czechoslovak Standard for Cold-formed Steel Structures Jiri Studnicka Follow this and additional works at: Part of the Structural Engineering Commons Recommended Citation Studnicka, Jiri, "New Czechoslovak Standard for Cold-formed Steel Structures" (1990). International Specialty Conference on Cold- Formed Steel Structures This Article - Conference proceedings is brought to you for free and open access by Scholars' Mine. It has been accepted for inclusion in International Specialty Conference on Cold-Formed Steel Structures by an authorized administrator of Scholars' Mine. This work is protected by U. S. Copyright Law. Unauthorized use including reproduction for redistribution requires the permission of the copyright holder. For more information, please contact scholarsmine@mst.edu.

2 Tenth International Specialty Conference on Cold-formed Steel Structures St. Louis, Missouri, U.S.A., October 23-24, 1990 NEW CZECHOSLOVAK STANDARD FOR COLD-FORMED STEEL STRUC'l'ORES By Jiri Studnicka, PhD Professor of Steel Structures, Czech Technical University Prague, Czechoslovakia ABSTRACT This paper presents a short review of the new Czechoslovak Standard for cold-formed steel structures. The Standard is based on the AISI Specification but a different approach is used for members under compression and/or bending. In spite of this difference a good correlation exists between both codes. 1. INTRODUCTION Czechoslovakia is a big producer of steel with 15 million tons per year production (it corresponds to 1 t/person/year!). In this huge production the cold rolled profiles represent about 0.5 mil. ton. For many years after the war, cold-formed profiles were designed either on a semi-experimental basis or according to principles set out in the AISI specification!l) The original Czechoslovak Standard was formed in 1977 and is based entirely on limit states principles. The second revised edition (1988) is briefly described in this paper. 2. SCOPE AND APPLICATION The Standard applies to the design of structural members cold-formed to shape from carbon or low alloy steel sheet, strip or plate up to 8 I11III in thickness and intended for load carrying purposes in buildings. The Standard does not cover special cases of behaviour under dynamic loading and fatigue. 3. DESIGN PHILOSOPHY The East Europe's modification of limit state method is used. The basic condition for ultimate limit state, which must be fulfilled during extreme loading, is YfL<! R - ym (1) 677

3 678 where L is the load effect on construction (member), R is the resistance of construction (member), Y f is the partial coefficient for loading, and Y m is the partial coefficient for material properties. When necessary, the load combination factor (W) and importance factor (Y u ) can be used in the same way as for example in the Canadian Code~2) For the serviceability limit state, which corresponds mainly to unacceptable deformation of the member, Y f = 1.0. In the case of the ultimate limit state, the magnitude of this coefficient oscillates between 1.1 and 1.4 for different loadings. The value of the coefficient for material properties is 1.15 for carbon steel with Fy ~ 300 MPa and 1.25 for Fy > 300 MPa. exploited. The strength increase from cold work of forming may be However, so far only for members in tension. 4 LOCAL BUCKLING In wide compression elements, the universal effective width equation for both stiffened and unstiffened elements is adopted, following the work of Desmond, Pekoz and Winter~3) This equation is 0.95 t vi a ke [ t vi a ke (2) max max where k is the plate buckling coefficient which depends upon support conditions, band b ef are the flat width and effective width respectively, t is the thickness, E is Young's modulus, and a max is the stress in the compression element computed on the basis of the effective width. of the coefficient k can be taken from tables which are contained for The value typical cross sections in the Standard, or can be calculated for example according to Kalyanaraman(4) or, for Czech readers, according to Brezina!5) For simplicity, k may be taken 4.0 for a stiffened member, and 0.5 for an unstiffened member. For compressed elements with an intermediate or edge stiffener the coefficient k may be computed similar to the approach in the AISI Specification or in Eurocode 3~6) It means that inadequate (weak, flexible) stiffeners may be used as well as adequate (strong, nonflexible) stiffeners. The conditions for moment of inertia of an adequate stiffener are compared

4 679 with AISI's and CAN's in the Fig. 1. A rather good conformity can be observed. Effective width of webs and stiffened elements with stress gradient may also be determined from Eq. 2. However, a different coefficient k must be used, and the distribution of effective width follows from Fig MEMBER RESISTANCE 5.1. Tension Tensile resistance of a concentrically loaded member is T = A n F d (3) where An is net cross sectional area, and Fd is the tensile (design) strength, i.e. yield strength divided by the partial coefficient for material, F!Ym 5.2. Compression Compression resistance of a concentrically loaded member is (4) where Aef is the effective cross sectional area (determined for 0max = ~Fd)' Fd is the design strength (see above), and ~ the buckling coefficient for the slenderness ratio (5) for which ~ is the member length, r the radius of gyration, and A the gross sectional area. The buckling coefficient follows from the relationship rp =1. [ 1 + X + (~/210 )2]_~ {l + X + (~ M )2}2 _ 2 Aef Fd 4 Aef Fd _ (~ ;'210)2]0.5 Aef Fd C6) where X = 0.17 for hollow section X = 0.26 for other sections.

5 680 It can be seen that a disadvantage of the above approach is the necessity of an iterative procedure. Fairly good correlation between theory and experiment is shown in the Fig. 3 where some results of the experimental study performed by Studnicka(7) with short, middle and long struts are presented. The study was conducted with a total of SO press braked U-shaped columns, one half were annealed to supress the influence of residual stresses. Good correlation between annealed and non-annealed struts was observed. It shows that the problem of residual stresses is not as severe for cold-formed profiles as for hot rolled or welded profiles. Comparison between the Czech approach and that of AISI's for an axially loaded pin ended strut of changeable length is shown in Fig. 4. Fairly satisfactory agreement may be observed again. Another comparison was done with Rondal's approach(s) which is used in Eurocode 3. Again, very good conformation was achieved, see Studnicka!9) 5 3. Bending The moment resistance of a member in bending shall be the lesser of M =!Plat ~f Fd (7) (S) c t where Wef and Wef are the compressive and tensile section modulus of the effective cross sectional area respectively, see Fig. S. Fd is the design strength, and!plat' lateral buckling coefficient which follows fram the equation An~- A~!Plat = O.S ( 93 I 210)2 + [{O.S (93 I n1i )2}2_ - 0.6] '5 for A < 104 (9) and!p = ( 93 /210 ) 2 for A.? 104 lat A Fd (10) The slenderness ratio for lateral buckling is

6 681 Ql Yt-' r c ( 11) where l is the unbraced length of member, rc the radius of gyration of the compressed chord of the beam, il the coefficient of equivalent uniform bending, and y a coefficient which contains the influence of: - shape of cross section - support conditions of beam - torsional characteristic of cross section - position of loading Because the Czech engineers are familiar with Eq which are used in common steel structures, only finding of W ef is the peculiarity needed for cold-formed profiles. Shear resistance of the web shall be determined by v (12) where hw,ef is the effective width of the web in shear. For a web without a stiffener h < 100 t /210 Fd (13) for a web with one longitudinal stiffener (Fig. 6) + h <70t / t /210 hwl,ef w2,ef - Fd Fd (14) The conditions for the moment of inertia of longitudinal stiffeners in Czechoslovak Standard are the same as given by Nguyen and YuPO) To avoid crippling of an unreinforced web of a member in bending the concentrated loads or reactions shall not exceed the values given in formulae very similar to the AISI equations. To take advantage of inelastic reserve capacity in bending is not allowed in the standard Torsion For the pure and/or warping torsion the gross area of the cross-section is used in computations.

7 Combination of loading The generalized formula for simultaneous action of normal forces, bending moment and torsion is given for normal stresses (15) while for tangential stresses is i16) where B is the bimoment, Iw the warping constant for the gross area, It the st. Venant torsion constant for the gross area, Tw the torque moment of warping torsion, Tt the torque moment of pure torsion, Sw the sectorial moment of area, and w the sectorial area 6. CONNECTIONS Only clauses for statically loaded connections are contained in the Standard. Fully covered by this Standard are: (1) welded connections for fillet welds, butt welds, arc spot welds, and resistance spot welds; and (2) connections made by bolts, rivets, self drilled screws, and special fasteners. 7. TESTING lind FABRICATION Two special Standards one for testing of members and constructions, and the other for fabrication of thin-walled cold-formed structures, are available in Czechoslovakia. 8. CONCLUSIONS This paper discusses the new Czechoslovak standard for cold-formed, thin-walled structures. Good correlation of this Standard and the codes of other countries was observed.

8 683 APPENDIX - REFERENCES (1) Specification for the Design of Cold-Formed Steel Structural Members, AISI, 1986 ed. (2) Cold Formed Steel Structural Members, CAN3 - S136 - M84 (3) Desmond, T.P., Pekoz, T. and Winter, G., "Intermediate Stiffeners for Thin-Walled Members", Journal of Structural Division ASCE, Vol. 107 No ST4, April 1981 (4) Kalyanaraman, V., "Local Buckling of cold-formed Steel Members", Journal of Structural Division ASCE, Vol. 105, No 5, May 1979 (5) Brezina, V., "Stabilita tenkych sten", SNTL Prague 1963 (in Czech) (6) Eurocode 3 "Design of Steel Structures", Annex A - Cold-Formed Thin Gauge Members and Sheeting, Draft 1989 (7) Studnicka, J., "Ultimate Load of Axially Compressed Thin-Walled Steel Bars", Stavebnicky casopis, Vol. 34, No 6, June 1986 (8) Randal, J. and Batista, E., "Stability Problem of Thin-Walled Cold Formed Steel Columns", Stavebnicky casopis, Vol. 36, No 7, July 1988 (9) Studnicka, J., "Ultimate Loads of Steel Columns made of Thin-Walled Profiles", Stavebnicky casopis, Vol. 37, No 10, October 1989 (10) Nguyen, R.P. and Yu, W.W., "Longitudinally reinforced Cold Formed Steel Beam Webs", Journal of Structural Division ASCE, Vol. 108, No 11, November 1982

9 684 APPENDIX - NOTATION A A n Aef B C E Fd Fy I Iw It L M N R Sw T Tw T t V W ef b b ef gross cross sectional area net cross sectional area effective cross sectional area bimoment compression resistance Young modulus design strength yield strength moment of inertia warping constant st. Venant torsional constant load effect bending moment normal force resistance sectorial moment of area tensile resistance torque moment of warping torsion torque moment of pure torsion vertical shear sectional effective modulus flat width effective width h clear distance between the flats of flanges hw,ef effective width of web k t r r c plate buckling coefficient length radius of gyration of the cross sectional area radius of gyration of the compressed chord of beam

10 685 t X p Y Y f Y m A Aef thickness coefficient for geometrical imperfections coefficient of equivalent uniform bending coefficient for lateral buckling partial coefficient for loading partial coefficient for material slenderness ratio effective slenderness ratio ~ buckling coefficient ~lat 0max lateral buckling coefficient maximal stress in compressive element 00 sectorial area

11 O'l 00 O'l 200 li t 4 a) 1000 IIt4 b) ~.. b,.. b '1 '1 ",t r 500 r-- ~~l 50 Fy:Fd =210 MPa Fy: Fd = 210MPa 9 c==,< bit bit ~- 200 Fig. 1 Minimal moment of inertia according to different standards a) edge stiffener b) intermediate stiffener

12 " b "" b r 1 '1 'I ) ~mqx Fig. 2 Distribution of effective width Fig. 5 Effective cross sectional area a> OJ -.J COMPRESSION ~ bet bet if Fig. 6 Web with one stiffener

13 C/:l C/:l a) C (kn) C (kn) C (kn) 30 ~.L= 2 b 20 ~ r:: 10 ~ 30 L=75 b ' 30 ;< 20 10/,, 20 '..., /... l -: 15 b b1 I {.. b1 I.,.2L ,4 0,6 b ,4 0,6 b 0 0,2 0,4 0,6 b b) C(kN) C (kn) C (kn) 1-= 2 L=75 L= 15 b 30 t b ' 30 t b 30~ f1l~ cross section m 20 ~ ~. 20 ~ / 20,, b,10! 10. t _.Q.t QL 0 0,2 0.4 O,G b 0 0,2 0,4 0,6 b 0 0,2 0,4 O,G b 10 ~ ~ b Fig. 3 Comparisson of proposed design method for struts with channel section a) nonannealed, b) annealed (dashed line - old version of Czechoslovak Standard)

14 689 C(kNl CROSS-SECTION 10 L---'---'-_"'---"""'"----'-_"'---'--... _"""'--'-_l(m) Fig. 4 Load capacity of concentrically loaded thin-walled strut with hollow cross section according to AISI and Bzechoslovak Standards

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