Question

1) Select the lightest section to support a axial load (DL=700 kips y LL= 300 kips). The
element is braced, so LX= 22 ft and LY= 12 ft. The end support conditions are:
a) (Fixed/pinned) and it is specified to use A572 gr. 60. (verify slenderness and
local buckling)
b) (Fixed/Fixed) and you must use A-36

Answer 1

For A-572 grade 60

Fy =60 ksi

Limiting KL/r = 104 (table below curve E-1 in AISC steel manual V 14)

For A-36 steel

Fy = 36 ksi

Limiting KL/r = 134 (table below curve E-1 in AISC steel manual V 14)

AISC Specification Commentary Table C-A-7.1

k= 0.7 (fixed/pinned)

K= 0.5 for (fixed/fixed)

By Lfrd

Pu = 1.2(700)+1.6(300)= 1320 kips

= 0.9

From AISC Manual Table 1-1, the geometric properties are as follows:

W14×90 Ag = 26.5 in.2

rx = 6.14 in.

ry = 3.70 in

For A)

KLx/rx = 0.7*22/6.14*12=30 (governs)

KLy/ry = 0.7*12/3.7*12=27.24

The available critical stresses may be interpolated from AISC Manual Table 4-22 or calculated directly as follows: Calculate the elastic critical buckling stress,

Fe= 317.69 ksi

Fcr =    = 113 > kl/r(30)

so,

Fcr = 55.43 ksi

Pn = Fcr * Ag = 55.43*26.5 = 1469 ksi

*Pn= 0.9*1469 = 1322 >1320 ok

Similarly part B) can be solved but Fy= 36 and k= 0.5