bA = support width u = length from internal edge of end support to end of sheet

Figure A.6: Test set-up for end support tests

A.3 Tests on cold formed members A.3.1 General

(1) Each test specimen should be similar in all respects to the component or structure that it represents.

(2) The supporting devices used for tests should preferably provide end conditions that closely reproduce those supplied by the connections to be used in service. Where this cannot be achieved, less favourable end conditions that decrease the load carrying capacity or increase the flexibility should be used, as relevant.

(3) The devices used to apply the test loads should reproduce the way that the loads would be applied in service. It should be ensured that they do not offer more resistance to transverse deformations of the cross-section than would be available in the event of an overload in service. It should also be ensured that they do not localize the applied forces onto the lines of greatest resistance.

(4) If the given load combination includes forces on more than one line of action, each increment of the test loading should be applied proportionately to each of these forces.

(5) At each stage of the loading, the displacements or strains should be measured at one or more principal locations on the structure. Readings of displacements or strains should not be taken until the structure has completely stabilized after a load increment.

(6) Failure of a test specimen should be considered to have occurred in any of the following cases:

- at collapse or fracture;

- if a crack begins to spread in a vital part of the specimen;

- if the displacement is excessive.

(7) The test result should be taken as; the maximum value of the loading applied to the specimen either coincident with failure or immediately prior to failure as appropriate.

(8) The accuracy of all measurements should be compatible with the magnitude of the measurement concerned and should in any case not exceed ± 1 % of the value to be determined.

(9) The measurements of the cross-sectional geometry of the test specimen should include:

- the overall dimensions (width, depth and length) to an accuracy of ± 1,0 mm;

- widths of plane elements of the cross-section to an accuracy of ± 1,0 mm;

- inclinations of plane elements to an accuracy of ± 2,0°;

- angles between flat surfaces to an accuracy of ±2,0°;

- locations and dimensions of intermediate stiffeners to an accuracy of ± 1,0 mm;

- the thickness of the material to an accuracy of ± 0,01 mm.

(10) All other relevant parameters should also be measured, such as:

- locations of components relative to each other;

- locations of fasteners;

- the values of torques etc. used to tighten fasteners.

A.3.2 Full cross-section compression tests A.3.2.1 Stub column test

(1) Stub column tests may be used to allow for the effects of local buckling in thin gauge cross-sections, by determining the value of the ratio ]SA = y4eff Mg and the location of the effective centroidal axis.

(2) If local buckling of the plane elements governs the resistance of the cross-section, the specimen should have a length of at least 3 times the width of the widest plate element.

(3) The lengths of specimens with perforated cross-sections should include at least 5 pitches of the perforations, and should be such that the specimen is cut to length midway between two perforations.

(4) In the case of a cross-section with edge or intermediate stiffeners, it should be ensured that the length of the specimen is not less than the expected buckling lengths of the stiffeners.

(5) If the overall length of the specimen exceeds 20 times the least radius of gyration of its gross cross-section /min, intermediate lateral restraints should be supplied at a spacing of not more than 20zmin.

(6) Before testing, the tolerances of the cross-sectional dimensions of the specimen should be checked to ensure that they are within the permitted deviations.

(7) The cut ends of the specimen should be flat, and should be perpendicular to its longitudinal axis.

(8) An axial compressive force should be applied to each end of the specimen through pressure pads at least 30 mm thick, that protrude at least 10 mm beyond the perimeter of the cross-section.

(9) The test specimen should be placed in the testing machine with a ball bearing at each end. There should be small drilled indentations in the pressure pads to receive the ball bearings. The ball bearings should be located in line with the centroid of the calculated effective cross-section. If the calculated location of this effective centroid proves not to be correct, it may be adjusted within the test series.

(10) In the case of open cross-sections, possible spring-back may be corrected.

(11) Stub column tests may be used to determine the compression resistance of a cross-section. In interpreting the test results, the following parameters should be treated as variables:

- the thickness;

- the ratio bplt;

- the location of the centroid of the effective cross-section;

- imperfections in the shape of the elements of the cross-section;

- the method of cold rolling (for example increasing the yield strength by introducing a deformation that is subsequently removed).

A.3.2.2 Member buckling test

(1) Member buckling tests may be used to determine the resistance of compression members with thin gauge cross-sections to overall buckling (including flexural buckling, torsional buckling and torsional-flexural buckling) and the interaction between local buckling and overall buckling.

(2) The method of carrying out the test should be generally as given for stub column tests in A.3.2.1.

(3) A series of tests on axially loaded specimens may be used to determine the appropriate buckling curve for a given type of cross-section and a given grade of steel, produced by a specific process. The values of relative slenderness A to be tested and the minimum number of tests n at each value, should be as given in table A.l.

Table A.1: Relative slenderness values and numbers of tests

Was this article helpful?

0 0

Post a comment