For steel design, STAAD compares the actual stresses with the allowable stresses as defined by ANSI/AISC N690-1994 and as amended by ANSI/AISC N690 1994(R2004)s2.

All the design steps are done as described in D1.B.1.1 Allowables per AISC Code except for allowable stress in compression for AUSTENITIC STAINLESS STEEL. Section Q1.5.9 is used to calculate allowable compressive stress for Austenitic Stainless Steel. Correction made in Supplementary s1 published in April 15, 2002 has been applied.

Note: By default, N690 code uses Stainless Steel material in the design. Care should be taken to assign the proper Stainless Steel material properties to the members for the analysis. There is a parameter – STYPE – to change material type to either Stainless Steel (STYPE=1) or Carbon Steel (STYPE=0).

## D1.K.1.1.1 Design Process

Members subjected to both axial compression and bending stresses are proportioned to satisfy equation Q1.6-1a:

$f a SFC ⋅ F a + C m y f b y SMY ⋅ F b y ( 1 − f a F ′ e y ) + C m z f b z SMZ ⋅ F b z ( 1 − f a F ′ e z ) ≤ 1.0$

and Q1.6-1b:

$f a SFC ⋅ 0.6 F y + f b y SMY ⋅ F b y + f b z SMZ ⋅ F b z ≤ 1.0$

when, fa/Fa > 0.15, as per section Q1.6.1 of the code.

Otherwise, equation Q1.6-2 must be satisfied:

$f a SFC ⋅ F a + f b y SMY ⋅ F b y + f b z SMZ ⋅ F b z ≤ 1.0$

It should be noted that during code checking or member selection, if fa/Fa exceeds unity, the program does not compute the second and third part of the formula, because this would result in a misleadingly liberal ratio. The value of the coefficient Cm is taken as 0.85 for side-sway and [0.6 - 0.4·(M1/M2)], but not less than 0.4 for no side-sway.

Members subjected to both axial tension and bending stress are proportioned to satisfy equation Q1.6-3:

$f a SFT ⋅ 0.6 F y + f b y SMY ⋅ F b y + f b z SMZ ⋅ F b z ≤ 1.0$

Where:

• SFC, SFT, SMZ, and SMY are stress limit coefficient parameters used to control the components of the interaction equations. Refer to D1.K.1.2 Design Parameters for details.