Bracing 1

[L17Aurora]

Can you tell me when you would consider using bracing on say a platform supported on 4 legs. (no horizontal loads)
If the legs are short I suppose they are not needed and even if they were long and wouldn't buckle (according to cals) you wouldn't need braces ??
Is there a rule for fitting them
Thanks
Mick

 

[burtonSTR]

Slenderness ratio of legs. If >200, add bracing is a simple solution.

 

[Lion06]

Even if there is no lateral loading, you still have to have some kind of bracing against lateral forces. It will not be built perfectly plumb and level, it will not always have balanced loading on it, people walking on it create minimal lateral forces, unless the columns are all cantilevered (or moment connected to the beams) then they will be inherently unstable.

You may not need intermediate bracing for the columns, but the tops will want to be braced. Look at ASIC 360-05 for bracing requirements. Bracing requirements are in App. 6 of the spec (which is available for free download at

http://www.ASIC.org).

 

[DaveAtkins]

I agree with StructuralEIT. If you can justify the columns as cantilevers with fixed bases, then no bracing is required. Otherwise, bracing is ALWAYS required--even when you think there will be no lateral loads. It is a matter of stability.

DaveAtkins

 

[L17Aurora]

StructuralEIT, I can't see anyway of getting it free from that site. Can you give me a clue?
Thanks

 

[Lion06]

Try this.

http://www.aisc.org/WorkArea/showcontent.aspx?id=17640

 

[JrStructuralEng]

Thanks for the link StructEIT

 

[kslee1000]

L17:

For short, small (no. of persons is limited by space), stocky platforms around machinery may be got away without braces, provides the legs and platform level support members are continuously welded, or with brackets.

However, the above is a judgement call. Other than that, you have to factor in human reaction with regarding to potential vibrations (induced by human & machines), which most designer chose to get away with by simply provide bracings, since it involves difficult & tedious analysis.

 

[Lion06]

kslee-

The brackets or continuous welds you mention essentially make a moment frame which acts as a lateral system to brace the columns.

I've never heard of bracing columns to help with vibrations.

 

[kslee1000]

Vibration induced lateral movement. The brace can minimize the amplitude from the effect of shaking.

 

[Lion06]

If it's not braced to something that is laterally stable, it won't matter.

Additionally, this is typically only done for mid-rise and/or high-rise buildings. It would likely be a mis-use of time and energy to do something like this for a 8'-10' high platform. Provide a real lateral system and it will be fine.

Finally, I'm not sure I believe that bracing columns (in between diaphragms, where the masses are lumped) will help with lateral "vibrations" anyway. The frequency of the structure is based on the stiffness of the lateral system and the mass (assuming the masses are lumped at each floor level) and is not appreciably affected by relative braces between floor. The modal shapes will follow the lumped masses, not intermediate braces between the masses (it may play a part in some of the higher mode shapes, but not the one(s) that we are typically concerned with).

 

[kslee1000]

What is the characteristics of cantilever column subjects to vibration? Will the tip move, or the base? How a brace is usually connected - base to tip, base to base, tip to tip?

We do not afraid of vibration (vibratory massager is quite enjoyable) but the movement induced by it, vertically & horizontally. With amber allowrance for compression, and no what-so-ever possibility for lateral load (except run-over by a vehicle), brace has no significant value but for human perceptions.

 

[Lion06]

I'm not sure I'm following your last post. Only a nodal brace could help with lower mode shapes, I don't believe a relative brace will. Columns are rarely braced at locations other than floors.

I'm going back to your first post and I think there is a disconnect here. Your first post talked about getting away with no bracing if the platform is stocky. I think that is not correct on its face because it is a stability problem (there is no lateral stability). You then talked about welding the frame or providing brackets (ostensibly to make a moment frame out of it). That is creating a a lateral system and essentrially bracing the columns (though you would need a k-factor if not using the DAM).
Once you have that lateral system in place there would be no need for intermediate column braces, just size them appropriately. Given this, and the fact that lateral motions are typically only considered for mid to high-rise construction, there would be no need to check this platform for lateral vibrations.

Am I missing something?

 

[kslee1000]

For short, small (no. of persons is limited by space), stocky platforms around machinery may be got away without braces,...

Please do not pick and chose my words. Read the whole sentence. With the conditions have been met, the welds and brackets further stabilize the platform that render the braces un-necessary.

A four leg platform essentially acting as a vertical cantilever with load (platform floor & workers) on tip if the top framing and the columns are simply connected. Once it is excited, it vibrates and the tip moves, the amplitude of the movement depends on the mode. A (diagonal) brace from lower ground to the tip will diminish the magnitude of the movement, but not the vibration, which would still be felt if it is in higher mode. However, human are not so afraid of the vibration without associated lateral displacement.

My previous conclusion, as it still stands now:
(analysis on human induced vibration), which most designer chose to get away with by simply provide bracings, since it (interactive vibration)involves difficult & tedious analysis.

 

[kslee1000]

oops, the "BOLD" didn't work.

 

[kslee1000]

Forgot to mention, "short" meant "low", as opposed to "long" & "tall".

 

[Lion06]

I did read the whole sentence. The presence of machinery doesn't matter and neither does the degree of "stockiness". At the end of the day the platform must be laterally stable. This is not necessarily the same at column bracing, but can be. The welds and brackets won't "further" stabilize the platform, they will provide the only stability for the platform. Without bracing the structure back to something that is laterally stable (assuming it isn't welded or doesn't have brackets) it doesn't matter how short and stocky it is - no lateral resistance mean no lateral resistance.

Now (and in your 3rd post) you make mention of cantilevered columns. This is providing a lateral system, so I'm not sure of the distinction you are drawing between this and "bracing" the column. Just design the column for k=2 and be done with it. Additionally, if you brace the cantilevered column back to the base of an adjacent column you are essentially creating a braced frame which is infinitely more stiff than a cantilevered column and, as a result, will have a much higher frequency (and smaller amplitude).

A lateral vibration analysis of a cantilevered column is not that tedious. fn=((3EI/(ml^3))^0.5 where m is the mass at the top of the cant. I don't have time to run through the acceleration right now, but I'm sure won't take all day.

 

[kslee1000]

"Additionally, if you brace the cantilevered column back to the base of an adjacent column you are essentially creating a braced frame which is infinitely more stiff than a cantilevered column and, as a result, will have a much higher frequency (and smaller amplitude)."

Wasn't that my intention - minimize the AMPLITUDE, should the vibration occues.

 

[Lion06]

But it's not a necessary part of the analysis or the question. Just provide a lateral system and be done with it.

 

[beton1]

Guys, I respect your replies and based on previous posts you are both very knowledgeable. Do not allow this back and forth to lose sight of the question or become personal. Perhaps the originator could have given more info or loads, but I would tend to say that bracing is usually an inexpensive insurance policy in a case like this.