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# Understand a dynamic load representation2

## Understand a dynamic load representation

(OP)
Hey everyone!

So I'm designing a factory that has a vibrating equipment on one floor. The load is given like this:

Do you have any thoughts what that "6 pol" might mean?

I'm not too familiar with dynamic analysis but am trying to educate myself a little in Robot Structural Analysis.
Replies continue below

### RE: Understand a dynamic load representation

I tried to enlarge the image a bit to make your queried text slightly more readable.  It didn't help very much, but got me wondering whether what you read as "pol 50Hz" might be "p at 50Hz".  But then that would merely move the problem along to what the "p" is.

—————————————————————————————————
Engineering mathematician / analyst.  See my profile for more details.

### RE: Understand a dynamic load representation

I would guess it means a 6 pole 50 Hz motor on the machine. Then there is a load cycling at 16 Hz. What is the machine?

### RE: Understand a dynamic load representation

(OP)
It's some sort of a grain grinding mill I think.

### RE: Understand a dynamic load representation

I agree that it probably is a 6 pole 50 Hz motor. The load is +/- 2445 N at 16 Hz, but what is the direction of the load. For a rotation machine I would assume a rotation load but here it might be vertical +/-. And how does it vary in time, sinus-function at 16 Hz or something else?

### RE: Understand a dynamic load representation

That looks like a layout drawing or something. You should ask for the vendor drawing, it should usually be clearer.

### RE: Understand a dynamic load representation

#### Quote:

The load is +/- 2445 N at 16 Hz, but what is the direction of the load. For a rotation machine I would assume a rotation load but here it might be vertical +/-. And how does it vary in time, sinus-function at 16 Hz or something else?

For a electric motor (at least the kind I am use to dealing with) there would be both vertical and horizontal unbalanced force (one of them having a different phase angle than the other). Of course, we don't know what the machine actually is.

### RE: Understand a dynamic load representation

#### Quote (WARose )

Of course, we don't know what the machine actually is.
I'd guess with a 90% certainty that it is a vibrating sieve for sorting out suitable sized output from unsuitable sized output. The loads size, the loads frequency and the floor penetrations are very good clues. The fact that it is a grain mill is a big give away. (The hexagonal columns of bulk storage bins on the top of the drawing are also a good give away.)

I've dealt extensively with these machines and the problems their vibrations cause. I've seen them vibrate entire building and their foundations. With vibrations being observable over 50m away through the earth.

#### Quote (nivoo_boss)

It's some sort of a grain grinding mill I think.
You really should clarify this. Is it an animal feed mill my guess. A flour mill? Or something else.

#### Quote (ThomasH)

The load is +/- 2445 N at 16 Hz, but what is the direction of the load. For a rotation machine I would assume a rotation load but here it might be vertical +/-. And how does it vary in time, sinus-function at 16 Hz or something else?
The vibration is mostly up and down with a 25% front to back longitudinally. The screens run lengthwise so you want the vibration motion in that direction.

#### Quote (nivoo_boss)

I'm not too familiar with dynamic analysis but am trying to educate myself a little in Robot Structural Analysis.
Be extremely careful especially if the structure is steel rather than concrete. 16hz is a horrible frequency for floors to deal with. If you are inexperienced then I'd be asking for assistance.

Also I'd be chasing the supplier or other suppliers for methods of dampening. That is my general approach. Why spend a fortune on the tuning the structure when you can dampen the vibration my ~97%?

### RE: Understand a dynamic load representation

#### Quote (WARose)

For a electric motor (at least the kind I am use to dealing with) there would be both vertical and horizontal unbalanced force (one of them having a different phase angle than the other).

That's what I meant with the "rotating" load .

### RE: Understand a dynamic load representation

There is a difference between a rotating dynamic load and a combination of horizontal and vertical dynamic loads with respect to the steady state response of the structure. It generally isn't important some distance away from resonance and seems to be captured in FEA properly by using sin loads with phase angles, but just something to keep in mind.

### RE: Understand a dynamic load representation

The machine has a vibrator or several on it. So it will be a heavy unbalanced load mounted on a frame that is made to vibrate at the preferred frequency. As mentioned it will be orientated with the axis of up-down the page. So the vibration will be left&right as well as in and out of the page.

#### Quote (canwesteng)

There is a difference between a rotating dynamic load and a combination of horizontal and vertical dynamic loads with respect to the steady state response of the structure. It generally isn't important some distance away from resonance
Well in this case we have machinery that has a combination of horizontal and vertical dynamic loads. It is heavy and vibrating. And I don't know where you get the notion that it isn't important some distance away from the resonance. As I've said this type of machine can shake buildings, foundations and the earth around them if you aren't careful.

### RE: Understand a dynamic load representation

What is the difference between a rotating unbalanced load and horizontal and vertical dynamic loads? Isn’t that what a rotating unbalanced mass does? Induce horizontal/vertical dynamic loads perpendicular to the axis?

### RE: Understand a dynamic load representation

Where did I say vibration was unimportant away from resonance? The fact is that the steady state response of the structure is different when the forcing function is a rotating mass vs an externally forced system.

Tomfh - an externally forced system will have force amplification go below 1 and trend to zero as the ratio of forcing frequency to natural frequency goes above 1, and a rotating mass excited system will do the opposite. Here is a pdf I found from Brown explaining it better https://www.brown.edu/Departments/Engineering/Cour...
Good to understand the fundamentals if you are going to be dealing with these types of problems, though I find the equipment manufacturers must be putting some ample safety factors on their dynamic loads.

### RE: Understand a dynamic load representation

Canwesteng it seems you have been talking at cross purposes with both Tomfh and me.

My quote was quoting you directly "It generally isn't important some distance away from resonance" (albeit with the word the inserted), though your meaning in that context may have been misinterpreted.

I'm not sure where this externally forced distinction comes into relevance here. We were always talking about a piece of equipment that was vibrating and the fact that it was rotationally induced was pretty damn implicit.

### RE: Understand a dynamic load representation

The difference between rotating mass and external forcing isn't as important away from resonance. The distinction between rotating mass and a force acting up and down is fairly important for how you model the loading, something the OP should take into account when modelling the load.

### RE: Understand a dynamic load representation

#### Quote (canwesteng)

The distinction between rotating mass and a force acting up and down is fairly important for how you model the loading.
You are continuing to muddy the waters. The FORCE IS a force that is mostly acting up and down. (unless te equipment is something different from what I've been lead to believe.)

And designing for the up and down motion is absolutely what is required.

Here is a video for you:

While the true severity of the vibration isn't completely evident, having a 3.5T machine vibrating 16 times a second with a decent and measurable amplitude (on a ruler) is no joke when it comes to structural design.

### RE: Understand a dynamic load representation

I don't think I'm the one muddying the water, you've switched from saying the force comes from a rotating mass, to now saying the force is a vertical only forcing function. In any case, the OP should get the actual info from the vendor, and make the sure the behaviour observed in their model matches theoretical for either external forcing or a rotating mass.

### RE: Understand a dynamic load representation

(OP)
OP here. Received a little more info on the machine. The "6 pol 50 Hz" does indeed mean that it is a motor with 6 poles operating at 50 Hz as many of you thought as well. It is a screening machine/sieve for separating grain, I think. It has four supports with some dampers as well (in red on the drawing). But other than that the supplier could not answer what the function for the dynamic load might be or what would be the allowable deflections for the support structure.

This is a schematic for the machine:

One more thing - the supplier wants the openings between the steel beams cast in concrete under this thing, something like this is done in other factories - there is a 200 mm concrete slab between the I-beams:

I did run a time history analysis in Robot Structural Analysis for the beams that support the thing - but I really don't know how to adequately take into account the concrete between the beams so I just added it as mass to the beams and then ran the calculation - even though the concrete in reality probably adds quite a bit to the stiffness of the beams. The natural frequency of the system modelled this way is around 8 Hz. I then ran the analysis with the given dynamic load at the given frequency with the load pulsing to a sine function. The maximum displacement of the beams under this dynamic load is 0,4 mm according to the analysis. But this is achieved with some quite mean beams - IPE400 sections while the span is only about 5 m :)

### RE: Understand a dynamic load representation

And I'll confirm that I was bang on to the exact machine and brand. All my above comments apply. This is not the sort of machine you want to mess around with.

I had to deal with several of these which were shaking the building to its foundations and beyond (quite literally).

### RE: Understand a dynamic load representation

(OP)
Thanks! I read from the link you posted that elastomer dampers where eventually used to reduce the vibrations - does this mean the support beams where not stiffened at all then? And can you please specify how the dampers where detailed?

Also in the thread you posted someone's advice was to get the natural frequency of the floor to basically double to that of 16 Hz - seems pretty insane to achieve that with steel beams.

### RE: Understand a dynamic load representation

According to my calculations stiffening the floor would have made things worse. As the floor's natural frequency was below 16hz.

#### Quote (nivoo_boss)

Also in the thread you posted someone's advice was to get the natural frequency of the floor to basically double to that of 16 Hz - seems pretty insane to achieve that with steel beams.
Exactly, not practical.

With concrete there is more weight (lower natural frequency) and more natural damping so in such cases you be OK. My understanding is that in Europe it is common for such factories to be concrete floor. Where I'm located this is uncommon.

Detailing the dampers was relatively straight forward. I used these:
https://www.mecanocaucho.com/

### RE: Understand a dynamic load representation

(OP)
Many thanks!

I think I'll still go with the high IPE400 beams just in case.

### RE: Understand a dynamic load representation

The frequency for the load is 16 Hz. You say that the frequency for the system is 8 Hz, I assume that is the lowest but are there any other modes can can be affected by this?

You mention time history analysis, In my opinion, vibrations are often better to study in frequency domain. And the mentioned concrete will add mass but it can also add stiffness.

### RE: Understand a dynamic load representation

(OP)

#### Quote (ThomasH)

...but are there any other modes can can be affected by this?

At least the technical documentation for the machine I have does not mention any.

### RE: Understand a dynamic load representation

#### Quote (nivoo_boss)

At least the technical documentation for the machine I have does not mention any.

I mean that the structure will have more than one natural frequency.

### RE: Understand a dynamic load representation

You need to do some sensitivity analysis to see what happens if the concrete is composite. This kind of work usually requires several models. I'm not sure how robot works, but other FEA will need to have captured enough natural frequencies to properly get the dynamic amplification. I would try and capture either enough mass participation or at least natural frequencies up to 32 Hz

### RE: Understand a dynamic load representation

Isn't there a risk for resonance during start-up and stop of the machine? How often does that happen?

I would run an analysis for several frequencies up to 32 Hz and and check how the system responds.

### RE: Understand a dynamic load representation

#### Quote (nivoo_boss)

the supplier could not answer what the function for the dynamic load might be or what would be the allowable deflections for the support structure.

This is a red flag to me. I'm not a building structural designer but I am a machinery designer and it's essential for every part of the system to have this kind of data. Our equipment applies large varying overhung loads to the mounting surface and it's not at a regular frequency. There needs to be a crisp agreement between the machinery supplier and the mounting structure supplier. We require deflection limits for our machinery mounting surfaces and we provide loads to be applied to the structure to confirm suitability. There are other ways but the load/deflection is certainly valid and easy to communicate.
Perhaps the sizing is not yet available, or your immediate customer does not appreciate the importance of this data, but some exchange must happen. If nobody can speak to the actual loads, I would state on my drawings which assumed loads were considered in your design and your deflection limits.

It also makes sense to run resonance calcs if you have adequate data.

### RE: Understand a dynamic load representation

#### Quote (dgeesaman)

This is a red flag to me.

I had missed the comment that the supplier could not supply the load or the requirements. But I would consider that to be a problem.

I have had the problem that if a supplier is uncertain, they set the requirements so high that they are difficult or impossible to meet. The reasoning seemed to by: If things don't work out, it is not their fault .

### RE: Understand a dynamic load representation

Remember the "supplier" here is likely not the manufacturer. (which is a well known Swedish company who has an excellent reputation). I see these sieves regularly across a broad number of industries.

nivoo_boss: "the supplier wants the openings between the steel beams cast in concrete under this thing"
Reinforces the above. Sounds like the 'supplier' in this case isn't that manufacturer. It would be abnormal for this equipment manufacturer to be involved in designating the required floor openings.

#### Quote (ThomasH)

Isn't there a risk for resonance during start-up and stop of the machine? How often does that happen?
I would run an analysis for several frequencies up to 32 Hz and and check how the system responds.
Yes there is a risk but the consequences are unlikely to be problematic. I'd be focussed on the behaviour under steady state because that is where the machine spends 99.9% of the time. Regarding the start-stop patterns. That really depends on how they are used but having them running 24/7 is certainly quite common.

#### Quote (ThomasH)

I would run an analysis for several frequencies up to 32 Hz and and check how the system responds.
I've never seen the equipment not vibrate at the specified frequency except during start up and slow down where it operates at lower frequencies for around 5-20s. The motor would get up to speed in 1 or 2 seconds. The sieve takes a little longer to reach steady state.

#### Quote (dgeesaman)

This is a red flag to me. I'm not a building structural designer... There needs to be a crisp agreement between the machinery supplier and the mounting structure supplier. We require deflection limits for our machinery mounting surfaces
This isn't exactly a delicate piece of manufacturing machinery. In fact, quite the opposite. The mounting surfaces and deflection limits of the structure are not really relevant to the sieve, it will quite happily vibrate and deflect because that is what it is DESIGNED to do.

The problem is if it induces excessive resonance in the structure. (As per my last thread, I've seen situations that are so bad it shakes the entire structure a 30mx30mx30x facility and could felt over 50m away through the earth.) Though in most installations I've seen or implemented myself there is some hard to avoid local floor vibration, but it doesn't extend beyond that.

#### Quote (dgeesaman)

If nobody can speak to the actual loads
The loads are listed in the first post and in the image posted two days ago. The function is sinusoidal, though the horizontal and vertical aren't in the same phase.

### RE: Understand a dynamic load representation

#### Quote:

Isn't there a risk for resonance during start-up and stop of the machine? How often does that happen?

There is a very real possibility that you will pass through a resonance frequency (for either the equipment or the supporting structure) during start up. (Note I say "a" resonance.....since there are multiple frequencies involved for such a system.) I normally check the displacements with a variety of charts that include the short-term performance of the machinery. I typically do a time-history analysis to facilitate this.

### RE: Understand a dynamic load representation

#### Quote (nivoo_boss)

The maximum displacement of the beams under this dynamic load is 0,4 mm according to the analysis.
Whoah... Hang on a second! Have you thought about what 0.4mm means at 16hz! If I'm not mistaken, 0.4mm is quite significant. Try viewing the results in terms of acceleration rather than displacement.

Your displacement results are quite similar to my calculated displacement results in the other thread where the vibration was extremely 'severe'.

I don't want my floor or structure experiencing accelerations of 4m/s^2! These values where of the order I measured in the 'severe' case previously mentioned.

#### Quote (nivoo_boss)

But this is achieved with some quite mean beams - IPE400 sections while the span is only about 5 m :)
I haven't done the calculation but that sounds pretty stiff which might make things worse in this situation. Hence your results.

### RE: Understand a dynamic load representation

The main concern during ramp up would be when it runs through the natural frequency of the springs or dampers it sits on, if it has any. This can create a large force that the structure needs to resist. The actual vibrations will be short lived so are unlikely to damage the equipment or cause concern to occupants.

### RE: Understand a dynamic load representation

#### Quote:

The actual vibrations will be short lived so are unlikely to damage the equipment or cause concern to occupants.

Depends on the equipment.....and also the number of times it ramps up & down during it's use. I was brought it on one project (this is a specialty area of mine) where the equipment was having maintenance issues because of trasient motion that was not checked. This thing started up & ramped down at least several times every few hours. That adds up over a service life.

### RE: Understand a dynamic load representation

#### Quote (human909)

Yes there is a risk but the consequences are unlikely to be problematic. I'd be focussed on the behaviour under steady state because that is where the machine spends 99.9% of the time.

#### Quote (human909)

I've never seen the equipment not vibrate at the specified frequency except during start up and slow down where it operates at lower frequencies for around 5-20s.

Since the machine works at 16 Hz and from what I have seen the structure has it's first natural frequency at 8 Hz, there is a risk for resonance. Is that a problem? Probably not but I would check.

My approach would probably be something like this:

When I said that I would analyze several frequencies, I would run it in frequency domain, instead of time-steps i would use frequency steps. That means one analysis, "steady state", from 0 Hz to 32 Hz, step 0.1 Hz and also include all natural frecuencies in that range. Then I would plot displacement, velocity and/or acceleration (something) as function of frequency for the relevant positions on the floor. That would give me the level of the vibrations for the floor during start/stop and running and also an indication regarding the sensitivity since I would use the same load for all frequencies. Strong vibrations during start/stop can motivate a time-history approach since steady-state can be very conservative for that.

This is by no means the only "correct" approach but I often use this for vibration analysis. Depending on the results, it may suffice or more things may need to be investigated.

As for the displacement 0.4 mm, that is huge. I would be more comfortable with 0.4 mm/s . Are you sure about that calculation?

### RE: Understand a dynamic load representation

#### Quote (ThomasH)

As for the displacement 0.4 mm, that is huge. I would be more comfortable with 0.4 mm/s smile. Are you sure about that calculation?
I'm not sure whether you were question me or the OP here. The OP calculated values are 0.4mm as stated. My calculated values in the case of the existing installation was of similar magnitude.

I'm also sure about my measurements. In some locations the measured floor acceleration was 1.5g RMS with max observed at 3.0g. As already mentioned these vibrations could be seen, felt and measure over 50m away through the earth.

#### Quote (ThomasH)

Since the machine works at 16 Hz and from what I have seen the structure has it's first natural frequency at 8 Hz, there is a risk for resonance. Is that a problem? Probably not but I would check.
Why check for the minor problem when the major problem is still present? The major issue here is going to be during steady state when there is continuing energy being pumping into the system. 8hz isn't far enough away from 16hz for it not to be a problem in steady state.

### RE: Understand a dynamic load representation

#### Quote (human909)

I'm not sure whether you were question me or the OP here. The OP calculated values are 0.4mm as stated. My calculated values in the case of the existing installation was of similar magnitude.

I apologize, that was far from obvious . I don't question your harmonic plots. I got the same results.

But I find the results so high that I wonder if they are correct, question for OP. If they are correct I believe the design needs some modification.

#### Quote (human909)

Why check for the minor problem when the major problem is still present? The major issue here is going to be during steady state when there is continuing energy being pumping into the system. 8hz isn't far enough away from 16hz for it not to be a problem in steady state.

I would check both 8 Hz and 16 Hz, and a number of other frequencies. All in one analysis. I don't quite understand what you mean with the minor and the major problem?

### RE: Understand a dynamic load representation

#### Quote (ThomasH)

I apologize, that was far from obvious. I don't question your harmonic plots. I got the same results.
And I apologize for if my responses have come off as rude in anyway. I suppose I'm a little forthright in my responses because I've dealt extensively with this EXACT machine as well as many of their variants. (The smaller 24hz machines are far more forgiving for most structures.) I've been involved in the installation of dozens of them and also been involved in rectifications in cases where others haven't suitably addressed these issues.

#### Quote (ThomasH)

But I find the results so high that I wonder if they are correct, question for OP. If they are correct I believe the design needs some modification.
I agree that the OP should recheck his results to ensure he has accounted for all the relevant masses suitable stiffness. But I'm not shocked by his results. These 16hz machines are particularly bad.

#### Quote (ThomasH)

I would check both 8 Hz and 16 Hz, and a number of other frequencies. All in one analysis. I don't quite understand what you mean with the minor and the major problem?
I'm referring startup/shutdown being the minor problem. I see the major problem as being the steady state as there is enough time and energy for the local vibration problem (the floor around the sieve) to become a global problem (engaging other significant masses in the structure) and causing serviceability issues or worse.

All that said I must say I still have plenty to learn from yourself and others and their approach to this type of analysis. I'm a long way from being an expert in structural dynamics.

### RE: Understand a dynamic load representation

#### Quote (human909)

And I apologize for if my responses have come off as rude in anyway. I suppose I'm a little forthright in my responses because I've dealt extensively with this EXACT machine as well as many of their variants.

No problem . I haven't perceived you as rude and I don't mind fortright. I haven't dealt with this exact machine before. But I was involved in a project several years ago with something similar but smaller. I don't remenber any significant issues, but it was smaller.

I have worked with machine induced vibrations but more with comfort vibrations from people walking on floors or slender pedestrian bridges. In that context these vibrations are very high. For the machines I have dealt with the acceptable vibrations can also be very small.

#### Quote (human909)

I'm referring startup/shutdown being the minor problem. I see the major problem as being the steady state as there is enough time and energy for the local vibration problem (the floor around the sieve) to become a global problem (engaging other significant masses in the structure) and causing serviceability issues or worse.

I agree that startup/shutdown is probably the minor problem and steady state the major problem. But if I assume that the result 0.4 mm is correct, that is a huge value for a non-resonant vibration. What will happen during startup when resonance occurs? And when we start to change the system, is it best to change the mass or the stiffness. And that will change the natural frequencie(s) but how will it change the vibrations, startup and steady state? That is the reason why I like include several frequencies in the analysis. And I would like to know what vibration level is acceptable for the machine to function properly. Maybe the governing criteria actually is an office floor 40 m from the machine .

Regarding learning, I think there is always room for learning more. And I think discussions in this forum is one way of doing that .

### RE: Understand a dynamic load representation

#### Quote (ThomasH)

I have worked with machine induced vibrations but more with comfort vibrations from people walking on floors or slender pedestrian bridges. In that context these vibrations are very high. For the machines I have dealt with the acceptable vibrations can also be very small.
In that aspect I've never dealt in any depth with walking induced vibrations. The only depth I've gone too is "basic rules of thumb" based on deflection limits and a few experiences where I've directly felt the discomfort of excessive deflection in a couple of my own designs and learnt from that. (One advantage of design and construct experience IMO, is you can readily experience and if necessary fix any design shortcomings regarding serviceability.)

#### Quote (ThomasH)

What will happen during startup when resonance occurs?
Not much in my experience. This experience isn't calculative, simply experience standing next to the damn things. The these get get up to speed very quickly so the time crossing the resonance frequency is almost negligible. During shut down, it takes a little longer and from memory local vibrations do increase somewhat briefly (1-5seconds). But no more energy is being added so I'd argue it is a non issue. (though inherent in that argument is a certain amount of local inertia)

#### Quote (ThomasH)

And when we start to change the system, is it best to change the mass or the stiffness.
In most installations this is likely to be a difficult issue to fix by adding stiffness. Removing stiffness could affect other serviceability/strength criteria. Adding mass is certainly a solution and one that I have considered in rectification works. I suspect, but haven't done calculations to confirm that if you have concrete flooring you stand a much better chance of acceptable behaviour due to the extra dead load. I'm used to steel construction with 5mm floor plate.

I've found the easiest way is through dampers. I've used dampers with a natural frequency of ~4hz with great success. I now specify these for all these 16hz installations. It isn't worth the engineering time or potential headaches if dampers aren't used.

### RE: Understand a dynamic load representation

Many times "rules of thumb" works, especially for floors. But I have done some larger pedestrian bridges and that requires analysis. But that is not the current subject .

You say that startup ususlly isn't a problem so I leave that for now.

A few thoughts:
I gree that reducing stiffness can cause seviceability problems or other problems so that is probably not a good idea. At least not without more information. Increasing the stiffness is also questionable because it will move the natural frequency (8 Hz) closer to the machines frequency (16 Hz). I would not start with that approach.
Reducing the mass would also not be something i prefer, but increasing can be worth to check.

I don't have a lot of experience with dampers. But the damping effect from a "real" damper compared to the effect from natural damping, in my experience the real damper is much better. So that would definetly be an option. But I am curious, why du you use 4 Hz?

One concern is that I have so far only seem one natural frequency, 8 Hz. There has to be more but I don't know it they are any reason for concern.

But it seems like the OP has left the discussion.

### RE: Understand a dynamic load representation

#### Quote (ThomasH)

Many times "rules of thumb" works, especially for floors. But I have done some larger pedestrian bridges and that requires analysis.
I don't doubt it. Mu comment about rules of thumb was mean to be self deprecating regarding my own expertise in that area (aka, none). I presumed you were talking about large pedestrian bridges and I certainly recognise that something more than a rule of thumb is necessary.

#### Quote (ThomasH)

A few thoughts:...
Those are the exact thought I had here. I linked this earlier but I presume you may not have seen it. I thought things would need stiffening but the calcs suggested otherwise.

#### Quote (ThomasH)

I don't have a lot of experience with dampers.
I hadn't either until dealing with the problem in the last thread

#### Quote (ThomasH)

But I am curious, why du you use 4 Hz?
Because the product was what was available, economic and would attenuate the effects by ~93%. I could have gone an even lower frequency but that would be a more expensive product.

I did do a deep dive into plenty of the theory. But this software and product supplier does make things awfully easy to buy their stuff with their helpful calculator.
https://www.mecanocaucho.com/en-GB/vibration-isola...

### RE: Understand a dynamic load representation

#### Quote (human909)

I don't doubt it. Mu comment about rules of thumb was mean to be self deprecating regarding my own expertise in that area (aka, none). I presumed you were talking about large pedestrian bridges and I certainly recognise that something more than a rule of thumb is necessary. thumbsup2

I think you probably have more experience regaring the topic for this thread than I have.
But regarding pedestrian bridges, since I mentioned "larger", the largest span I have worked with so far was 120 m, cable stayed bridge. It was a challenge from several perspectives.

#### Quote (human909)

Those are the exact thought I had here. I linked this earlier but I presume you may not have seen it. I thought things would need stiffening but the calcs suggested otherwise.

I had missed that link. But I think my thoughts are fairly general regarding what usually happens when you play with mass and stiffness.
But I can also give a simple and slightly fun example of how things can go wrong. I was involved in a project many years ago when somebody had designed a small dance floor. The approach was that if you get a hign enough natural frequency all will be good, "rule of thumb". So the floor was designed as lightweight as possible to maximize the frequenzy. That did not work out, but since the floor was small it was fixable with a reasonable approach.

When i comes to dampers I can say that for a few of the larger pedestrian bridges we have in the design stage sometimes calculated vibrations that were "on the limit". For a dynamic analysis the dynamic load is usually based on persons/area. For a large bridge that can mean a unreasonable number of persons on the bridge. In that case we can prepare the bridge for a TMD damper but we test the real bridge before we install the damper.

What I mean by that is that for dynamics there are a lot of parameters. Some we can control and to a large extent also analyze. Others are more difficult to control. In those cases it can be nice to have a damper as a method the modify the system.

#### Quote (human909)

That was an interesting link. Thanks.

### RE: Understand a dynamic load representation

Good link - minor nitpick is that you are actually isolating vibration rather than damping it in that case. But dampers are better for that application because they will transmit much less load during ramp-up/down and also should increase the numbers of cycles before you get to steady state vibrations. I've generally seen springs on equipment that is quickly turned on/off (like screens) vs dampers on centrifuges or other equipment that has a more pronounced ramp up.

### RE: Understand a dynamic load representation

#### Quote (ThomasH)

What I mean by that is that for dynamics there are a lot of parameters. Some we can control and to a large extent also analyze. Others are more difficult to control.
I have a dead blow sledge hammer for this reason. So I can measure the fundamental frequencies of a structure rather than calculate it.

#### Quote (canwesteng)

Good link - minor nitpick is that you are actually isolating vibration rather than damping it in that case.
True, true. I've been a bit slack in my terminology here. I agree it is important to recognise the different behaviours and functions of a damper vs an isolator.

About half a decade ago this equipment manufacturer used to offer an add on kit that that was springs/weights and shock absorbers. (SHOWN BELOW) But now I believe their current kit offering is just weights and spring isolators. For the reasons you mention it was likely recognised that shock absorbers weren't really needed.

Anyway.... That is probably me done here. I hope the OP's installation goes well.

### RE: Understand a dynamic load representation

#### Quote (human909)

I have a dead blow sledge hammer for this reason. So I can measure the fundamental frequencies of a structure rather than calculate it.

When working with modifications of existing structures measured data is always valable. But most of the work I do is not with existing structures.

It has been an interesting discussion. But I think that to have any more ideas regarding the original question the OP must provide some input.

### RE: Understand a dynamic load representation

Those of you that are measuring displacements and accelerations, what software/hardware are you using? Or are you using 3rd party to gather data?

### RE: Understand a dynamic load representation

#### Quote:

Those of you that are measuring displacements and accelerations, what software/hardware are you using? Or are you using 3rd party to gather data?

Measure, as in the field? I have a device (made by LANDTEK) that is pretty good at measuring vibrations (as long as the forcing frequency is 10 Hz or more). The info it is going to get is kind of limited (without more equipment). If i need a lot of (highly detailed) info.....I'll hire a third party.

### RE: Understand a dynamic load representation

#### Quote (WARose)

Measure, as in the field?

Yes, field measurements. The LANDTEK device sounds useful. I'll check them out.

I've got a couple accelerometers and a Dewesoft data collector that I've been trying to learn but the user interface and the online help is not great.

### RE: Understand a dynamic load representation

#### Quote:

The LANDTEK device sounds useful. I'll check them out.

You can get some good bang for your buck with them. (I.e. a device for around \$300.) Vibration equipment can get pretty outrageous in terms of cost.

### RE: Understand a dynamic load representation

So does the unit you have measure acceleration, velocity, and displacement? If you have a model number that would be great. I see several versions online. And I often deal with crushers and feeders that operate below 10Hz but this tool sounds useful even if only for the cases with machines operating above that.

### RE: Understand a dynamic load representation

#### Quote:

So does the unit you have measure acceleration, velocity, and displacement?

Yes. (All three.)

#### Quote:

If you have a model number that would be great.

VM-6370

VM-6370

Thanks!

### RE: Understand a dynamic load representation

A standard mobile phone is also quite effective. Though generally they have a sampling rate of 100-200hz so you are effectively limited to 100hz or less. Though if you are doing this sort of measurement frequently or are really after the best quality data then a dedicated device is a no brainer.

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