208 597

2-ty

Fn - (0 ■ 6581c )Fy - (0 ■ 6580■438 ) 40 - 36■ 914 ksi

3. Effective area, Ae. Because the given square tube is composed of four stiffened elements, the effective width of stiffened elements subjected to uniform compression can be computed from Equations 6.6 through 6.9 by using k = 4.0:

= 1.052/^4(70 . 619)^/36. 914/29,500 = 1. 314 Since 1 > 0.673, from Equation 6.7

b = pw where p =(1 — 0 . 22/1)/! =(1 — 0 .22/1. 314)/1. 314 = 0 .634

The effective area is

Ae = 3.273 — 4(7.415 — 4.701)(0.105) = 2 .133 in. 2 4. Nominal and allowable loads. Using Equation 6.75, the nominal load is Pn = AeFn = (2 .133)(36. 914) = 78 .738 kip

The allowable load is

LRFD method

In the ASD method, the nominal axial load, Pn, was computed to be 78.738 kip. The design axial load for the LRFD method is fcPn = 0 .85(78 . 738) = 66.93 kip

Based on the load combination of dead and live loads, the required axial load is

Pu = 1. 2Pd + 1. 6Pl = 1. 2Pd + 1. 6(5Pd) = 9 . 2Pd where PD is the axial load due to dead load and PL is the axial load due to live load. By using Pu = f cPn, the values of PD and PL are computed as follows:

Therefore, the allowable axial load is

It can be seen that the allowable axial loads determined by the ASD and LRFD methods are practically the same.

6.6.6 Combined Axial Load and Bending

The AISI Specification provides interaction equations for combined axial load and bending. 6.6.6.1 Combined Tensile Axial Load and Bending

For combined tensile axial load and bending, the required strengths should satisfy the interaction equations presented in Table 6.5. These equations are to prevent yielding of the tension flange and to prevent failure of the compression flange of the member.

6.6.6.2 Combined Compressive Axial Load and Bending

Cold-formed steel members under combined compressive axial load and bending are usually referred to as beam-columns. Such members are often found in framed structures, trusses, and exterior wall studs. For the design of these members, the required strengths should satisfy the AISI interaction equations presented in Table 6.6.

TABLE 6.5 Interaction Equations for Combined Tensile Axial Load and Bending

ASD LRFD and LSD

Checktension flange —^^ + + < 1.0 (6.79) -—— + -—y— + --< 1.0 (6.80)

Check compression flange —^^H--y--— < 1.0 (6.81) -—— + -—y-----< 1.0 (6.82)

Note: Mnx and Mny are the nominal flexural strengths about the centroidal x- and y-axes, respectively, Mnxt, Mnyt = SftFy, Mx and My are the required flexural strengths with respect to the centroidal axes (Mux and Muy for LRFD, Mfx and Mfy for LSD), Mx and My are the required moments with respect to the centroidal axes of the section, f is the section modulus of the full-section for the extreme tension fiber about the appropriate axis, T is the required tensile axial load, Tn is the nominal tensile axial strength, T is the required tensile axial strength (Tu for LRFD and Tf for LSD), fb is the resistance factor for bending, ft is the resistance factor for tension (0.95 for LRFD and 0.90 for LSD), Ob is the safety factor for bending, and Ot is the safety factor for tension.

TABLE 6.6 Interaction Equations for Combined Compressive Axial Load and Bending

0 0

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