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Perimeter Column Bracing? 1

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SteelPE

SteelPE

Structural
Mar 9, 2006
2,759
For some reason, every few years I question my design procedures... I don't know if this is something that is good, or if I am just getting old.

I am in the process of designing a large boring warehouse. The building is single story with an eave height of 40'-0" designed in accordance with IBC 2015. The perimeter facade is 3" insulated metal panel on a horizontal girt cold formed girt system with girts spaced at 5'-0" to 6'-0" o.c. Column spaces vary, but are either 26'-3", 25'-0", 21'-0" or 15'-9" depending on where you are. Girts are anticipated as being 8Cx3.5x12ga.

When it comes to the design of the perimeter columns, I am questioning as to whether or not the girts will brace the columns in the weak direction against axial buckling. Historically, we have assumed an unbraced length of 15'-0" for perimeter columns with this style of perimeter facade. This 15'-0" dimension is also the location where we place flange braces on the columns to resist LTB.

Would you normally assume the columns are braced by the girts in similar instances?

Before we get too far, I know there are bracing requirements in the appendix of the AISC. I am just wondering what other designers do with this style of construction?
 
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I generally consider girts provide lateral support for columns. The 15'9 dimension seems small, unless for a OH door, or something.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik
 
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Yes, the 15'-9" is quite small. The interior bay spaces are 42'-0" to allow for optimum racking layout (with what the owner requested at the beginning), so we have open-web steel joists spaced at 5'-3" o.c. The architect placed the columns such that they align with a tie joist and we can get doors in-between the columns. I think he could have done a slightly better job, but I think he was handcuffed by the owners door spacing requirements amongst other thing.
 
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It's nice to have tie joists, but sometimes you can't practically have them... I then use rolled angle bracing from adjacent joists to the column.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik
 
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I tried telling the architect that having tie joists wasn't necessary but this is what we ended up with..... my wind loads are not small, so I am a bit happy that we have them.... but the building is going to look funny without a consistent door spacing. Not my problem at this point. I am 3/4 of the way done with the preliminary design, so I will not be happy if they make any changes.
 
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1) With respect to any form of torsional/LTB buckling, I would consider the girts to brace the column for most of the common girt to column connections. Obviously, if there's not the ability within the connection to transfer girt flexure into column torsion then the roll beam bracing scheme breaks down.

2) With respect to straight up, weak axis column buckling, I would normally consider the girts to brace the column so long as:

a) The line of girts themselves was braced longitudinally by a braced bay, shear wall etc and;

b) The line of girts was not connected to the columns by a gaggle of LSL slotted, non-slip critical connections.
 
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I would use the girts as a brace against LTB buckling for the outer flange for bending resistance, but only as bracing under torsional compression buckling if there is a knee brace back from the girt to the column (although this still normally won't govern). I see PEMB add these braces fairly often on walls, so I figure it's at least backed up by something. Otherwise I'd do as kootk proposes.
 
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Kootk,

With regards to 1a...... this is where my brain cramp comes into play. Insulated metal panel typically only has fasteners at the panel joints...... and those fasteners are only through one panel as the other panel just hooks into the male end of the panel with the attachment into the girt system. So there is no really diaphragm strength developed by this type of system. Back fastening the panel to the girt system is out of the questions unless I want to face a firing squad from the owner.

So, in your opinion, since the insulated metal panel can not develop shear strength then the girts do no work to brace the column in the weak direction against buckling?
 
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I think he means that somewhere along the framing line there is a lateral force resisting system that would take the braced load down to the foundation at some point. Or at least that's how I read his note.
 
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Jayrod,

In this particular instance, we have lateral frames, but the braces do not bisect the columns, they run from the column base up to the roof in a single shot.
 
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Wow, I have a project with similar eave height in design right now, I broke my bracing up into two levels just because I felt the diagonal distance is too long for reasonable shipping pieces. I've got 25 foot bays, so that would mean bracing members that are almost 50 feet.
 
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jayrod12 said:
I think he means that somewhere along the framing line there is a lateral force resisting system that would take the braced load down to the foundation at some point. Or at least that's how I read his note.
Click to expand...

Yup.

canwesteng said:
I would use the girts as a brace against LTB buckling for the outer flange for bending resistance, but only as bracing under torsional compression buckling if there is a knee brace back from the girt to the column (although this still normally won't govern).
Click to expand...

Agreed. I was originally thinking of a hot rolled channel or something for the girt. I see now that it's CFM and, therefore, less likely to have a convincing moment connection to the column without the fly brace.

SteeelPE said:
In this particular instance, we have lateral frames, but the braces do not bisect the columns, they run from the column base up to the roof in a single shot.
Click to expand...

It would seem that you'd either need to change that or live with the columns probably being unbraced at the girt elevation.

SteelPE said:
So, in your opinion, since the insulated metal panel can not develop shear strength then the girts do no work to brace the column in the weak direction against buckling?
Click to expand...

Seems like it, yeah. Any chance that your lowest panel gets fastened to both the foundation and the girt? For this purpose, I don't feel that you need shear capacity in the panels continuously from foundation to roof. This still seems a bit sketchy to me either way.

You could also install some very light, partial height bracing explicitly for the purpose of bracing the columns if you want it badly enough.

c01_e1k4ve.png
 
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...and I've often seen these removed in plants, because they were in the way...[ponder]

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik
 
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Kootk,

Nope, I can not configure the brace as shown as the architect is placing OH doors under the bracing. If I were to lower the work points of the brace then he would not be able to squeeze his doors in. Even wit this configuration, the unbraced length would be 20'-0".

jayrod12,

In regards to brace/shipping length... I calculate the brace at 47'-2" long. The will need to lop off a foot or so at each end because the work point of the brace is not in the gusset but the theoretical intersection of the beam/column/brace. So, it will be around 45' when done. With most trailers being 48' long I don't see a reason why it would be able to be shipped. Now the size is a different story (HSS10x10x3/8) but if it works, then what's the difference?

FWIW, I spoke to a colleague here who is in agreement with the girts bracing the column without the need to reconfigure the perimeter bracing..... but I am just trying to get an idea of what everyone would do.
 
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SteelPE said:
Nope, I can not configure the brace as shown as the architect is placing OH doors under the bracing. If I were to lower the work points of the brace then he would not be able to squeeze his doors in. Even wit this configuration, the unbraced length would be 20'-0".
Click to expand...

I'm not sure that you're picking up what I'm laying down SteelPE. I was not suggesting that you configure your VLFRS bracing per my sketch but, rather, that you add some new bracing like that in any bay where it can be accommodated. Moreover, if every bay has an overhead door in it, you can still use the scheme because it works with the bracing only above the girt as well.

SteelPE said:
FWIW, I spoke to a colleague here who is in agreement with the girts bracing the column without the need to reconfigure the perimeter bracing.....
Click to expand...

Well, that begs the question of what your colleague thinks braces the girts longitudinally then. What's their opinion on that?
 
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I suspect the pre-eng world would consider it braced, but the cladding system is not superb. How many pounds of steel diff with v without brace?
 
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SteelPE said:
For some reason, every few years I question my design procedures... I don't know if this is something that is good, or if I am just getting old.
Click to expand...
Well you are lucky. I question my design procedures every few weeks! Maybe I'm just too 'young'**.

**From what I gather I do have vastly less time under my belt than most regular members on this forum.
 
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Some engineers I've encountered have 1 year of experience x 50 times... while others have 50 years...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik
 
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So, after giving it a bit of thought, and looking at the architectural layout of the building, I believe I can squeeze in a mid height brace along each side of the building where we have the insulated metal panel system.

We were/are looking at using 14" WF section as columns at the perimeter. If we were to assume the unbraced length is the full 40' then we are looking at a material increase of 40 lbs/ft to keep them as W14s..... and 20#/ft if we were to switch to W12's.
 
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