Strange structure

[GalileoG]

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.

 

[kslee1000]

Just some thinkings:

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.

 

[dougantholz]

Was the fixity of the column base connections a fixed base plate to column, base plate to foundation, or both? Looking at 8.5c of AISC 341-05 it has some flexural strength requirements of column bases. I seem to recall that there is some notes about beam to column connections in SCBF's that aren't at gusset plates having some rotation restraint.
Of course it could be modeling error or a misreading of 13.3b of AISC 341-05.

 

[Lion06]

How do you know the column bases were fixed? All columns get fillet welds all the way around and 4 anchor bolts. Without knowing the forces and anchor rod embedments, how did you determine that the bases were fixed?
I have no idea about the beam/column connections. Well, I have one, but it seems kind of crazy.

 

[valleyboy]

Is it possible the vertical bracing was retro-fitted for some reason after construction?

 

[dougantholz]

Structural EIT: fillet welds all around on gravity columns is a waste of money. We typically show one side of a flange and one side of the web with a 1/4" fillet or so, depending on the plate size. This works well on multistory buildings quite well and can save hundreds of inches of fillet welds - and can be done without repositioning the column in the shop.

 

[Lion06]

one side of one flange and one side of the web? That seems a little weak. Will that even work for the minimum column moment during erection per OSHA? I think it is a 300# guy 18" from the column.

 

[knelli]

Structural EIT,
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.

 

[dougantholz]

300 lbs at 18" of eccentricity creates a 5,400 lb-inch moment.
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.

 

[Lion06]

Seems reasonable, but there are still plenty of people that do put fillet welds all the way around columns without designing the base as fixed so I don't think the fact that a column has a weld all the way around the base makes it automatic that it was designed as fixed.

 

[csd72]

douganholtz,

That is only in the major axis, what about arounf the minor axis. I think you will find that harder to justify.

 

[dougantholz]

okay, weak axis about column.
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?

 

[dougantholz]

Oops, I used 6" in my equation, but 4" was what i used in my problem statement, my capacity (P) becomes around 800 lbs with a 4 inch weld, still good. (I don't have the equation for C handy when it is off the chart and with a 4" weld, a becomes 4.5 which isn't in the tables).

 

[LowLax]

I agree with doug. AISC Design Guide 1 section 2.4 recommends 5/16 fillet weld one side of each flange for gravity only columns. No web welds at all.