Steel structures, Do you use a frequency criteria? 2

[DomDunk]

Dears,

There are structural engineers who usually apply a criterion of frequencies for industrial steel structures (not subject to vibrations of equipment). Some use f> = 2Hz, others f> = 3.0Hz, etc.

What is your opinion? Do you use that criteria? If yes, what frequency do you use to ensure that no movement of the structure is perceived?

Thanks,

Daniel

 

[271828]

For evaluation of vibration relative to human comfort, use the AISC Design Guide 11 if you're in the US. There are a few similar guides in Europe, such as the SCI P354 and CCIP-016. These use acceleration limits in one form or another.

 

[DomDunk]

Yes, thanks. I have read that design guide. Does it refer only to the vertical vibrations of the floors?

I mean that some engineers use arbitrary horizontal minimum frequencies. Personally I do not agree. In any case there are steel structures with f ~ 1.00 Hz in which horizontal movement is perceived.

 

[WARose]

Unless it's excitation from human activity.....I don't give it much thought. If you are worried about it drifting too much in the wind.....usually the normal drift requirements we use will cover you for the low-rise stuff.

 

[JAE]

We try to keep everything higher than 5Hz

Check out Eng-Tips Forum's Policies here:
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[ThomasH]

The frequency criteria I have seen in design usually determine if you need to perform a dynamic analysis or not. If you are above a specified limit no vibration analysis is required. Basically, you don't have to calculate any acceleration. That ofcourse implies that vibrations should not be an issue.

You mention 2-3 Hz, I think that is low for a steel structure regarding vertical vibrations. I have analyzed that type of structures but it also needs a context. What is the use?

The frequency can be seen as a stiffness criteria. Instead of a load you use the mass of the structure to evaluate the stiffness. Instead of a displacement you get a frequency as result.

Thomas

 

[DomDunk]

Thank you,

I am asking about the HORIZONTAL frequency. Of course, for vertical frequency of floors we use >=5Hz.

I mean to perceive a HORIZONTAL movement of the building. That often happens in industrial steel structures.

 

[WARose]

I am asking about the HORIZONTAL frequency. Of course, for vertical frequency of floors we use >=5Hz.

IIRC, AASHTO's rules for pedestrian bridges specifies >1.5 Hz for the first horizontal mode. (3 Hz for the first vertical.)

The AISC Design Guide you mention doesn't deal much with horizontal displacements. (Although I've used the charts they give (for human perception) for modes in all directions.)

 

[ThomasH]

Why does the building move horizontally?

If the source is people walking, that is one thing. If it is due to wind, it's another issue.

Eurocode has a limit of 2.5 Hz for horizontal modes. Below that an analysis is required. But that is for pedestrian bridges. It is probably not suitable for a building.

I would say that a vibration criteria is an acceleration, not a frequency.

Thomas

 

[DomDunk]

It is a good idea to use 1.5 Hz as for the pedestirna bridges. 2.5 Hz HORIZONTAL requires expensive structures.

I will analyze the issue of accelerations.

Thanks,

Daniel

 

[ThomasH]

I would not say that 1.5 Hz lateral is a good criteria for pedestrian bridges. If you use a reasonable load model and analyze bridges, a 1.5 Hz bridge can prove be an issue and a 1.0 Hz bridge can work just fine. I know that from experience, and 2.5 Hz does not always give you an expensive structure. It's a bit more complicated .

You can read a little about the famous Millenium Bridge i London here:

https://en.wikipedia.org/wiki/Millennium_Bridge,_London

. That bridge and the problems with it initiated a lot of research regarding lateral vibrations.

You started by asking about industrial steel structures and now it is bridges. I would not use the same acceleration criteria for them. What are you currently working with?

Thomas

 

[TLHS]

Unless I have reason to take special interest, with industrial stuff I tend to check stiffness and strength against a reasonable fraction of the vertical load and then qualitatively determine whether I'm happy with it. I'd only check natural frequencies if I had a specific vibrational goal.

I'll also provide bracing and other restraint to meet visual expectations.

Personel platforms, I'll generally throw 10% of specified occupancy load laterally to size the lateral system unless I've got otherwise governing operational loads of some sort. Equipment can be anywhere from 5% to 30% depending on what it is.

Unless you have an odd situation, things in industrial contexts should be reasonably stocky to begin with.

 

[DomDunk]

My structures have lateral frequencies between approximately 1.50 and 2.00 Hz.

The design with respect to the earthquake (which I always have in the sites of my structures) and wind.

The answer is that some engineers specify minimum 2.00 Hz requirements. As ThomasH says it can be achieved, but from experience I see that in industrial structures in several occasions the structural scheme is not "so clean" (I mean small structures with little importance), and that makes it difficult to achieve the best behavior in modal analysis.

 

[ThomasH]

You mentioned in a previous post that you will look into accelerations. Did that give you any information regarding comfort criteria? They are usually given in accelerations, not in frequencies.

You started with a question regarding if movement will be percieved, now you mention earthquakes. What is the connection between earthquakes and comfort criteria?

If you are searching for a "simple" frequency requirement to use for all your structures, I don't thing you will find it. The 2 Hz requirement that you mention, what is that based on? I have seen that type of criteria but it was based on a specific type of structure (hollow core slab) with a specific type of loading (people walking) and a specific project.

Thomas

 

[DomDunk]

I think I expressed myself badly. I do not use frequency criteria. I mentioned the frequency that I usually get through design based on earthquake and wind. I DO NOT DESIGN to get those frequencies.

The value of 2.00Hz is a value proposed by some engineers who asked me to use it. My post is precisely because I do not see the meaning of that criterion ... I do not think it has any bibliographical support. My question was to know if that criterion is spread among engineers, or if it is a mere invention of those who asked me to use it.

Some engineers in this forum proposed to be guided with the criterion of lateral frequency of pedestrian bridges ... it seemed at least interesting ... to take it into account ... In literature I saw that comfort is based mostly on accelerations. Thank you for confirming that there is no simplified criterion based on lateral frequencies