Combined Compression/Bending in a Column
Combined Compression/Bending in a Column
(OP)
I'm looking at analysis performed by an engineer no longer with the company and I'm concerned that it's incorrect but I'm not confident enough to push the issue. I'd appreciate any input.
Structure consists of two 20-ft sections of 8-in pipe bolted together through 1-in flanges. Top section is sch 40, bottom is sch 80.
Loading is from an FM antenna array and windloads (approx 2 kip compression/30 ft-kip moment at base).
He disregarded compression and treated the whole thing as a simple Mc/I bending problem saying that it's OK because ó < Sy.
However, when I follow AISC Manual of Steel Construction approach for combined column loading, looking just at the lower sch 80 section, I find the eqns in sec'n H are > 1 which says the design is insufficient.
The reason for my lack of confidence is that the axial stress is only about 0.2 ksi which seems to say this thing is basically a vertical cantilever beam in bending so Mc/I approach is right.
I'm losing sleep over this and any help would be greatly appreciated.
Thanks!
Structure consists of two 20-ft sections of 8-in pipe bolted together through 1-in flanges. Top section is sch 40, bottom is sch 80.
Loading is from an FM antenna array and windloads (approx 2 kip compression/30 ft-kip moment at base).
He disregarded compression and treated the whole thing as a simple Mc/I bending problem saying that it's OK because ó < Sy.
However, when I follow AISC Manual of Steel Construction approach for combined column loading, looking just at the lower sch 80 section, I find the eqns in sec'n H are > 1 which says the design is insufficient.
The reason for my lack of confidence is that the axial stress is only about 0.2 ksi which seems to say this thing is basically a vertical cantilever beam in bending so Mc/I approach is right.
I'm losing sleep over this and any help would be greatly appreciated.
Thanks!






RE: Combined Compression/Bending in a Column
By the way, what value did you use for your effective length factor (K)? If it's a straight cantilever K should be 2.1 per Table C-C2.1 in the Commentary.
RE: Combined Compression/Bending in a Column
RE: Combined Compression/Bending in a Column
Top 20-ft section:
L = 240 inches
W(incl pole) = 930 lb, Flateral = 840 lb at middle of pole.
Bottom 20-ft section:
L = 240 in A=12.75 in2, I=105.7 in4 r=sqrt(I/A)=2.88 in
W (incl pole) = 1060 lb, Flat = 730 lb at 180 inches from base of pole.
Therefore, looking at bottom pole:
Faxial = Wtop + Wbottom - Wbottom pole = 930+1060-870 = 1,120 lb
and fa = F/A = 1.1 kip/12.75 in2 = 86 psi
M = Flat top * (.5Ltop+Lbottom) + Flat bottom * 180
= 840 lb * (.5*240 in +240 in) + (730 lb * 180 in) = 434 in-kip
and fb = Mc/I = (434 in-kip)(8.63/2 in)/(105.7 in4) = 17.7 ksi
Obviously very little compressive stress, mostly bending stress.
K=2.1 so KL/r = 175; Cc = sqrt(2*pi2*E/Fy =
sqrt(2*3.142*29000/36) = 126
Since KL/r > Cc, Fa = 4.88 ksi
Fb = .66Fy = 23.7 ksi
Plugging these numbers into eqns H1-1 and H1-2 gives 1.57 and 1.52 respectively.
Even setting fa = 0 only lowers them to 1.49 which still indicates failure.
I guess my concern is this:
Like I said, the guy who did the original analysis just treated it from an Mc/I < Sy perspective - no consideration of "pushing a rope" - and that's not OK. But, before I make an issue out of it I'd like to make sure my analysis is right.
Thanks for any and all comments.
RE: Combined Compression/Bending in a Column
RE: Combined Compression/Bending in a Column
In this case, H1-3 doesn't show an appreciable difference.
I think I found a flaw in my analysis, though. For the bending stresses, the equations have both an x and a y component. Because I have a uniform cross-section, Fbx = Fby. I erroneously also set fbx = fby. As it turns out, they are definitely not equal. I haven't run the analysis yet but I'm guessing that I'll be much more comfortable with the result at that point.
scofie - Thanks for the There's something wrong here comment. That helped me persist in looking for the problem.
RE: Combined Compression/Bending in a Column
RE: Combined Compression/Bending in a Column
The straight answer is because the person who provides the money that keeps my family able to buy food, see a doctor, and from losing our home asked me to.