Axial Tension

The criteria governing the capacity of tension members is based on two limit states. The limit state of yielding in the gross section is intended to prevent excessive elongation of the member. The second limit state involves fracture at the section with the minimum effective net area. The net section area may be specified by the user through the use of the parameter NSF (see Table 3B.1). STAAD calculates the tension capacity of a member based on these two limits states per Cl.13.2 of CAN/CSA-S16-01. Parameters FYLD, FU, and NSF are applicable for these calculations.

Axial Compression

The compressive resistance of columns is determined based on Clause 13.3 of the code. The equations presented in this section of the code assume that the compressive resistance is a function of the compressive strength of the gross section (Gross section Area times the Yield Strength) as well as the slenderness factor (KL/r ratios). The effective length for the calculation of compression resistance may be provided through the use of the parameters KT, KY, KZ, LT, LY, and LZ (see Table 3B.1). Some of the aspects of the axial compression capacity calculations are :

1. For frame members not subjected to any bending, and for truss members, the axial compression capacity in general column flexural buckling is calculated from Cl.13.3.1 using the slenderness ratios for the local Y-Y and Z-Z axis. The parameters KY, LY, KZ and LZ are applicable for this.

2. For single angles, which are frame members not subjected to any bending or truss members, the axial compression capacity in general column flexural buckling and local buckling of thin legs is calculated using the rules of the AISC - LRFD code, 2^nd ed., 1994. The reason for this is that the Canadian code doesn’t provide any clear guidelines for calculating this value. The parameters KY, LY, KZ, and LZ are applicable for this.

3. The axial compression capacity is also calculated by taking flexural-torsional buckling into account. The rules of Appendix D, page 1-109 of CAN/CSA-S16-01are used for this purpose. Parameters KT and LT may be used to provide the effective length factor and effective length value for flexural-torsional buckling. Flexural-torsional buckling capacity is computed for single channels, single angles, Tees and Double angles.

4. The variable “n” in Cl.13.3.1 is assumed as 2.24 for WWF shapes and 1.34 for all other shapes.

5. While computing the general column flexural buckling capacity of sections with axial compression + bending, the special provisions of 13.8.1(a), 13.8.1(b) and 13.8.1(c) are applied. For example, Lambda = 0 for 13.8.1(a), K=1 for 13.8.1(b), etc.)

For Class 4 members subjected to axial compression, factored compressive resistance should be determined by either of the following equations.

1. C_r= ϕA_e F_y (1+λ²ⁿ )^-1⁄n

   where

   n	=	1.34
   λ	=	√(Fy/Fe )
   Fe	=	(π2 E)/(KL/r)2

   Ae is calculated using reduced element widths meeting the maximum width to thickness ratio specified in Table 1.

   Effective width required for the calculation of effective area Ae, for different section shapes are as follows.

   • For flanges of I-section, T-section and channel section and legs of angle section

     b_e= 200t/√(F_y )

   • For stem of T-section

     b_e= 340t/√(F_y )

   • For flanges of HSS rectangular or Tube sections

     b_e= 670t/√((F_y )

   • For circular HSS or Pipe section

     D= 23000t/(F_y

2. C_r= ϕAF_ye (1+λ_ye ²ⁿ )^-1⁄n

   where

   n	=	1.34
   λye	=	√(Fye/F_e )
   Fe	=	(π2 E)/(KL/r)2

   With an effective yield stress, F_ye, determined from the maximum width (or diameter)-to-thickness ratio meeting the limit specified in Table 1.

   Following are the expressions for effective yield stress for different shaped section.

   • For I-section, T-section, channel section and angle section

     F_ye= 40000/(b/t)²

   • For rectangular HSS section

     F_ye= 448900/(b/t)²

   • For circular HSS section

     F_ye= 23000/(D/t)