Strange structure
Strange structure
(OP)
Recently I was inspecting a building and noticed that fixed baseplate connections were used even when having braced frames! I also noticed moment beam to column connections where we have braced frames! Has anyone ever seen anything like this before? What the heck.






RE: Strange structure
Fixed baseplate connection - rarely completely fixed as in assumption.
Moment frame - sway is usually small, but not completely out of the picture. Talk to the guys there, you may find reasons for using the braced moment frame.
RE: Strange structure
Of course it could be modeling error or a misreading of 13.3b of AISC 341-05.
RE: Strange structure
I have no idea about the beam/column connections. Well, I have one, but it seems kind of crazy.
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RE: Strange structure
RE: Strange structure
RE: Strange structure
I agree.... I had a project where we had a few wind only columns that were only welded on the web. They left them unbraced for the weekend during building erection (before the beams at the top were attached) and a high wind ripped the the column about 2'-0" from the base. That was when I worked for an A/E firm. Of course, erection is supposed to be the contractor's responsibility, but you will ALWAYS get the call about how to salvage the column that has been damaged.
Now I work for a D/B firm and you bet if something like that happened it would be my problem. We always weld all the way around. Many contractor's do not brace 1 story columns during erection (even if they should). Only welding 1/2 the web to the base plate would certainly lead to many of our columns failing during erection. ASIC has a document on erection bracing of columns and covers wind load and weld capacity during erection.
RE: Strange structure
For an 8" column, the smallest column, this creates 675 lbs of tension on one face. Using ASD, the capacity of 1 inch of weld 927 lbs per 16th inch of effective throat.
In other words, I only need one inch of 1/16" fillet weld to support this OSHA load. Doesn't seem like a problem to me.
RE: Strange structure
RE: Strange structure
That is only in the major axis, what about arounf the minor axis. I think you will find that harder to justify.
RE: Strange structure
I have two lines of weld on a W8x24 column, which has a 6.5 inch flange. If I use a 1/16" weld 4 inches long at each flange and consider the load as actually horizontal to the ground at 18 inches above the base plate (a conservative approach considering the load is a vertical one not a horizontal one):
AISC 13th Ed: Table 8-4 page 8-66:
e_x = 18 inches
l = 6 inches
a = 18 inches / 6 inches = 3
C_1 = 1.0 (E70xx electrodes are common practice)
D = 1 for 1/16" weld, much smaller than my actual weld
k = 0 when the load is not in plane with the welds
Omega = 2.0 for ASD
From the table, C = 0.439,
Using the equation:
l = Omega * P / (C x C_1 x D)
And solving for P,
P = l x C x C_1 x D / Omega
P = 6 x 0.439 x 1.0 x 1 / 2 = 1,317 lbs.
This doesn't include welding on the web which also adds some capacity to the weld group. I also have to use the minimum effective throat depending on the thickness of my baseplate. Again, I think I am just fine. Does anyone else see an issue that I don't?
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RE: Strange structure