CSA - S136.1-95
Commentary on CSA Standard S136-94, Cold Formed Steel Structural Members
|Publication Date:||1 January 1995|
Scope and Application
The Scope establishes that CSA Standard SI 36-94 applies to the design of cold formed steel structural members and that the design is based on limit states design (LSD) methods. Such members are roll or brake formed from carbon or low alloy fat-rolled steel products. Cold formed members find application where large surfaces are needed, as in deck and cladding, or where hot-rolled shapes are inappropriate or uneconomical. Cold formed steel structural members may be broadly divided into two categories as follows:
(a) large-surface elements, eg, deck and cladding; and
(b) individual structural sections, eg, channels, zees, and hats.
Most of the experimental evidence supporting the provisions of the Standard has been obtained on specimens with material thicknesses between 0.4 and 6.5 mm, with some work on material up to 25 mm in thickness (1).
With the exception of connections and columns, the requirements of the Standard may be deemed to apply to members of any thickness up to 25 mm. In the case of connections (see Clause 7 of the Standard), certain requirements such as the resistance of welds and fasteners in bearing are noted to apply only to a specific range of thickness. Where the requirements of the Standard are being applied in conjunction with, and supplementary to, the requirements of another design Standard (eg, SI 6.1), any upper limitation on thickness will be based on the requirements of the Standard that would normally govern.
Although the requirements of the Standard are intended principally for applications in the design of buildings, the Standard may be used as a guide to the design of cold formed members in other structures, provided allowances are made for dynamic effects or other service conditions differing from those encountered in building structures
Cold formed components may also be combined with hot-rolled components in a single entity such as an open web steel joist having cold formed chords and hot-rolled web members. In this case, two aspects of the design require consideration, namely: the design of individual components within the joist and the overall design of the joist. If the Standard is used to design the cold formed column or beam-column (compression chord), then that component should be designed in complete accordance with Clause 6.6 or 6.7. With respect to the overall design, the requirements of another Standard could be used, provided the requirements are critically appraised to determine if they apply to cold formed, thin-walled members. For example, if another Standard that specifically covers steel jo sts contains simplification in design, such as neglect of joint eccentricities and neglect of transverse loads applied to a compressive member, where such simplifications are based on tests of non-cold formed, relatively thick-walled members only, these simplifications should not be applied to the design of cold formed, thin-walled compressive members without due consideration and justification.
The Standard is thus intended not only to stand on its own but also to supplement other CSA Standards for the design of steel structural members. There are two fundamental concepts that distinguish the Standard from other structural steel design Standards.
The first, the process of cold forming, induces selective strain hardening at corners that affects response to load in a manner quite different from the response of hot-rolled members, and the Standard allows this to be taken Into account in Clause 5.2.
Secondly, in contrast to individual structural sections whose prime function is to carry load, the structural strength of many cold formed members such as deck, cladding, and various members of a steel building system is only one of several desired functions. The optimum shape or profile is not necessarily the one that would be chosen on structural considerations alone and, in particular, the width-to-thickness ratio of flat elements may be well in excess of that which would be structurally economical. The Standard recognizes postbuckling strength in order to utilize the full strength of such elements. By contrast, designers utilizing hot-rolled steel sections are rarely concerned with postbuckling strength because the design standards impose width-to-thickness limits that are intended to predude element buckling prior to overall member buckling.
The formulae in the Standard are based on SI (metric) units. The specific unit for any given symbol used in a formula is given in Clause 2.2 of the Standard and the designer should refer to Clause 2.2 to ensure the correct unit is used.
When designing a structure comprising cold formed steel structural members, the design must ensure adequate stability of the individual members and the structure as a whole.
In some cases, other components of the building may provide local or overall structural stability. The structural integrity of the bracing material for the expected service life and construction loads should be taken into consideration.
The Standard also applies to the design of steel structural members covered by other Standards (such as SI 6.1) in cases where the width-to-thickness ratio of any elements of a member, whether or not cold formed, exceeds that normally allowed. In this application, the Standard is supplementing, not supplanting, the basic design Standard.
The resistance factors used in the Standard reflect a reduction in capacity consistent with the possible variation in product dimensions and mechanical properties, the variability of the test results on which the resistance equations were based, and correlation with the loads and load factors specified in the National Building Code of Canada (2).
For load cases not specifically addressed by the National Building Code of Canada, the designer should establish the load factor such that the level of reliability implied by the Standard is maintained.
When a specific situation is such that the design expressions or dimensional limitations (such as width-to-thickness ratio) given in the Standard are not applicable, then a rational design may be used. Such a design should be based on appropriate theory, tests, or analysis. The resistance factor so used should be consistent with that used in the Standard.
Testing or other forms of analysis may not be used to circumvent the Standard. The level of reliability implied by the Standard must be maintained.