Figure 10.10: Determination of moment resistance — wide flange in tension
(3) Flange curling need not be taken into account in determining deflections at serviceability limit states.
(4) As a simplified alternative, the moment resistance of a liner tray with an unstiffened wide flange may be approximated by taking the same effective area for the wide flange in tension as for the two narrow flanges in compression combined.
10.3 Stressed skin design
(1)P The interaction between structural members and sheeting panels that are designed to act together as parts of a combined structural system, may be allowed for as described in this clause 10.3.
(2)P The provisions given in this clause shall be applied only to sheet diaphragms that are made of steel.
(3)P Diaphragms may be formed from profiled sheeting used as roof or wall cladding or for floors. They may also be formed from wall or roof structures based upon liner trays.
NOTE: Guidance on the verification of such diaphragms can be obtained from:
ECCS Publication No. 88 (1995): European recommendations for the application of metal sheeting acting as a diaphragm.
(1)P In stressed skin design, advantage may be taken of the contribution that diaphragms of sheeting used as roofing, flooring or wall cladding make to the overall stiffness and strength of the structural frame, by means of their stiffness and strength in shear.
(2) Roofs and floors may be treated as deep plate girders extending throughout the length of a building, resisting transverse in-plane loads and transmitting them to end gables, or to intermediate stiffened frames. The panel of sheeting may be treated as a web that resists in-plane transverse loads in shear, with the edge members acting as flanges that resist axial tension and compression forces, see figures 10.11 and 10.12.
(3) Similarly, rectangular wall panels may be treated as bracing systems that act as shear diaphragms to resist in-plane forces.
Figure 10.11: Stressed skin action in a flat-roof building 10.3.3 Necessary conditions
(1)P Methods of stressed skin design that utilize sheeting as an integral part of a structure, may be used only under the following conditions:
- the use made of the sheeting, in addition to its primary purpose, is limited to the formation of shear diaphragms to resist structural displacement in the plane of that sheeting;
- the diaphragms have longitudinal edge members to carry flange forces arising from diaphragm action;
- the diaphragm forces in the plane of a roof or floor are transmitted to the foundations by means of braced frames, further stressed-skin diaphragms, or other methods of sway resistance;
- suitable structural connections are used to transmit diaphragm forces to the main steel framework and to join the edge members acting as flanges;
- the sheeting is treated as a structural component that cannot be removed without proper consideration;
- the project specification, including the calculations and drawings, draws attention to the fact that the building is designed to utilize stressed skin action.
(2)P Stressed skin design shall be used predominantly in low-rise buildings, or in the floors and facades of high-rise buildings.
(3)P Stressed skin diaphragms shall be used predominantly to resist wind loads, snow loads and other loads that are applied through the sheeting itself. They may also be used to resist small transient loads, such as surge from light overhead cranes or hoists on runway beams, but may not be used to resist permanent external loads, such as those from plant.
Figure 10.12: Stressed skin action in a pitched roof building 10.3.4 Profiled steel sheet diaphragms roof sheeting
Figure 10.12: Stressed skin action in a pitched roof building 10.3.4 Profiled steel sheet diaphragms
(1)P In a profiled steel sheet diaphragm, see figure 10.13, both ends of the sheets shall be attached to the supporting members by means of self-tapping screws, cartridge fired pins, welding, bolts or other fasteners of a type that will not work loose in service, pull out, or fail in shear before causing tearing of the sheeting. All such fasteners shall be fixed directly through the sheeting into the supporting member, for example through the troughs of profiled sheets , unless special measures are taken to ensure that the connections effectively transmit the forces assumed in the design.
(2)P The seams between adjacent sheets shall be fastened by rivets, self-drilling screws, welds, or other fasteners of a type that will not work loose in service, pull out, or fail in shear before causing tearing of the sheeting. The spacing of such fasteners shall not exceed 500 mm.
(3)P The distances from all fasteners to the edges and ends of the sheets shall be adequate to prevent premature tearing of the sheets.
(4) Small randomly arranged openings, up to 3 % of the relevant area, may be introduced without special calculation, provided that the total number of fasteners is not reduced. Openings up to 15% of the relevant area may be introduced if justified by detailed calculations. Areas that contain larger openings should be split into smaller areas, each with full diaphragm action.
(5)P All sheeting that also forms part of a stressed-skin diaphragm shall first be designed for its primary purpose in bending. To ensure that any deterioration of the sheeting would be apparent in bending before the resistance to stressed skin action is affected, it shall then be verified that the shear stress due to diaphragm action does not exceed 0,25/yb/7M1.
(6)P The shear resistance of a stressed-skin diaphragm shall be based on the least tearing strength of the seam fasteners or the sheet-to-member fasteners parallel to the corrugations or, for diaphragms fastened only to longitudinal edge members, the end sheet-to-member fasteners. The calculated shear resistance for any other type of failure shall exceed this minimum value by at least the following:
- for failure of the sheet-to-purlin fasteners under combined shear and wind uplift, by at least 40%;
- for any other type of failure, by at least 25%.
Purlin Shear connector
Figure 10.13: Arrangement of an individual panel 10.3.5 Steel liner tray diaphragms
Purlin Shear connector
Sheet-to-shear connector fasteners
Figure 10.13: Arrangement of an individual panel 10.3.5 Steel liner tray diaphragms
(1) Liner trays used to form shear diaphragms should have stiffened wide flanges.
(2) Liner trays in shear diaphragms should be inter-connected by seam fasteners through the web at a spacing es of not more than 300 mm by seam fasteners (normally blind rivets) located at a distance eu from the wide flange of not more than 30 mm,, all as shown in figure 10.14.
(3) An accurate evaluation of deflections due to fasteners may be made using a similar procedure to that for trapezoidal profiled sheeting.
(4) The shear flow Tw Sd due to ultimate limit states design loads should not exceed Ty Rd given by:
is the second moment of area of the wide flange; is the overall width of the wide flange.
(5) The shear flow Tv ser due to serviceability design loads should not exceed Tv Cd given by: Tv,Cd = / 375 ...(10.23)
Sv is the shear stiffness of the diaphragm, per unit length of the span of the liner trays.
(6) The shear stiffness Sv per unit length may be obtained from:
L is the overall length of the shear diaphragm (in the direction of the span of the liner trays);
b is the overall width of the shear diaphragm (b = E bu );
a is the stiffness factor.
(7) The stiffness factor a may be derived from tests in accordance with Section 9. Alternatively, in the absence of test results, a may conservatively be taken as equal to 2000 N/mm.
(1) Perforated sheeting may be designed by calculation, provided that the rules for non-perforated sheeting are modified by introducing the effective thicknesses given below.
NOTE: These calculation rules tend to give rather conservative values. More economical solutions might be obtained from design assisted by testing, see Section 9.
(2) Provided that 0,2 < d/a < 0,8 gross section properties may be calculated using 3.3.2, but replacing t by fa eff obtained from:
d is the diameter of the perforations;
a is the spacing between the centres of the perforations.
(3) Provided that 0,2 < d/a < 1,0 effective section properties may be calculated using Section 4, but replacing t by rb eff obtained from:
(4) The resistance of a single unstiffened web to local transverse forces may be calculated using 5.9, but replacing t by rc eff obtained from:
s?eT is the slant height of the perforated portion of the web;
is the total slant height of the web.
Annex A [informative] Testing procedures A.1 General
(1) This annex A gives appropriate standardized testing and evaluation procedures for a number of tests that are commonly required in practice, as a basis for harmonization of future testing.
(2) It is, however, recognized that most existing test data have been obtained on the basis of tests that differ to some extent from these procedures .
(3) So that the existing data can continue to be used, and to allow sufficient time for transition to harmonized procedures after adequate trial implementation, these testing procedures are presented as an informative annex, covering:
- tests on profiled sheets and liner trays, see A.2;
- tests on cold formed members, see A.3;
- tests on structures and portions of structures, see A.4;
- tests on beams torsionally restrained by sheeting, see A.5;
- evaluation of test results to determine design values, see A.6.
A.2 Tests on profiled sheets and liner trays A.2.1 General
(1) Although these test procedures are presented in terms of profiled sheets, similar test procedures based on the same principles may also be used for liner trays.
(2) Loading may be applied through air bags or in a vacuum chamber or by steel or timber cross beams arranged to simulate uniformly distributed loading.
(3) To prevent spreading of corrugations, transverse ties or other appropriate test accessories such as timber blocks may be applied to the test specimen. Some examples are given in figure A.l.
Rivet or screw
Rivet or screw
Figure A.1: Examples of appropriate test accessories
(4) For uplift tests, the test set-up should realistically simulate the behaviour of the sheeting under practical conditions. The type of connections between the sheet and the supports should be the same as in the connections to be used in practice.
(5) To give the results a wide range of applicability, hinged and roller supports should preferably be used, to avoid any influence of torsional restraint at the supports on the test results,
(6) It should be ensured that the direction of the loading remains perpendicular to the initial plane of the sheet throughout the test procedure.
(7) To eliminate the deformations of the supports, the deflections at both ends of the test specimen should also be measured.
(8) 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.
(1) A test set-up equivalent to that shown in figure A.2 may be used to determine the midspan moment resistance (in the absence of shear force) and the effective flexural stiffness.
(2) The span should be chosen such that the test results represent the moment resistance of the sheet.
(3) The moment resistance should be determined from the test result.
(4) The flexural stiffness should be determined from a plot of the load-deflection behaviour. A.2.3 Double span test
(1) The test set-up shown in figure A.3 may be used to determine the resistance of a sheet that is continuous over two or more spans to combinations of moment and shear at internal supports, and its resistance to combined moment and support reaction for a given support width.
(2) The loading should preferably be uniformly distributed (applied using an air bag or a vacuum chamber, for example).
(3) Alternatively any number of line loads (transverse to the span) may be used, arranged to produce internal moments and forces that are appropriate to represent the effects of uniformly distributed loading. Some examples of suitable arrangements are shown in figure A.4.
(1) As an alternative to A.2.3, the test set-up shown in figure A.5 may be used to determine the resistance of a sheet that is continuous over two or more spans to combinations of moment and shear at internal supports, and its resistance to combined moment and support reaction for a given support width.
(3) The test span s used to represent the portion of the sheet between the points of contraflexure each side of the internal support, in a sheet continuous over two equal spans L may be obtained from:
(3) If plastic redistribution of the support moment is expected, the test span s should be reduced to represent the appropriate ratio of support moment to shear force.
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