Strength Report

The Strength report is created when you select Strength from the Compute menu (only available for sections). If ASD is selected on the Compute menu, this report includes strengths for the Allowable Stress Design method. If LRFD is selected on the Compute menu, this report includes design strengths for the Load and Resistance Factor Design method. The strength values in this report are for the fully braced case. In other words, axial and bending strengths are based on the initiation of yielding.

This report includes strengths for compression, tension, shear, bending, and torsional bimoment. A positive moment about the X axis has compression above the X axis. A positive moment about the Y axis has compression to the right of the Y axis. For a C section with a vertical web, a positive bimoment causes tension in the upper-right and lower-left quadrants of the section, and causes compression in the upper-left and lower-right quadrants of the section.

The compression and bending strengths are accompanied by the effective section properties used in determining the nominal strength of the section, where the maximum stress is the yield stress. The effective widths of elements are determined in accordance with the AISI Specification for carbon steel, and the ASCE Specification for stainless steel (see Element Behavior).

If the section has any holes, the effective properties are computed based on the net section. This is done because fully braced failure will occur at the weakest point in the member.

The yield stress used for each strength type is increased to the appropriate calculated value if you choose to apply strength increase from cold work of forming in the Section Inputs window, and the section meets the other specification criteria necessary to use the increase.

Since CFS allows you to specify radii of different sizes, it uses a generalized form of the AISI calculation for the strength increase: Fya = (SAcFyc + SAfFy)/(SAc+SAf), where Ac and Fyc are the area and the tensile yield point of each corner, and Af is the area of each flat, disregarding any holes. For axial loads, the summations apply to all elements. For bending loads, the summations apply to flats at the extreme fiber of the full section parallel to the axis of bending, and the adjacent corners. If there are no flats along the extreme fiber, no strength increase is used. For carbon steel, any bend with R/t>7 or angle of bend (exterior angle) <60 does not get a strength increase (Fyc=Fy). The details of the cold work of forming increase can be obtained using the Trace option.

Since the strength increase applies only when the compression elements are compact enough, each strength type (tension, compression, +Mx, -Mx, +My, and -My) is independently checked to determine whether the strength increase is allowed. Since fully braced strengths are computed using the net section, the strength increase will only apply if both the full section and the net section are fully effective with the extreme fiber at Fya.

The strength increase from inelastic reserve cannot use the strength increase from cold work of forming. If both options are selected, CFS will compute both and report the higher strength. The ASCE specification for stainless steel also contains inelastic reserve provisions for axial tension and compression, however CFS does not utilize these provisions.

There is an unusual case for the lateral buckling strength calculation (not computed in this report), where the moment causing initial yield at the extreme compression fiber of the full section is calculated. This could result in a tension stress greater than Fy and a compression stress greater than otherwise calculated. Since this situation might not be fully effective, whereas it might have been with the extreme tension fiber at Fy, this special case requires the additional check for a fully effective full section.

It is important to note that since CFS allows the definition of any general shape, the assumptions made by CFS for determining these strengths may not always be appropriate. It is the responsibility of the engineer to evaluate these assumptions and determine the validity of the results.