Etabs Walking vibration (peak load factor) 5

[TLycan]

Guy's,

In CSI ETABS, the walking vibration form data, there is an item called Peak load Factor (default value) = 1.4

Can anyone tell me what is this factor for? and its reference?



Thanks in advance

 

[271828]

The last time I checked, they were representing a footstep with a triangular pulse. Check the documentation. If they're still doing that, then that's crazy-wrong.

See the AISC Design Guide 11 for loading functions. For resonance calcs, use a Fourier Series. For impulse calcs, use their effective impulse.

 

[JoshPlumSE]

There are TWO ways that ETABS can handle walking excitation.

1) The way I normally rely on is somewhat automatic for composite beams and shows up in the report for those composite beams. This is based entirely on AISC Design Guide 11.

2) The second way is the one the OP is referring to. It can be accessed by selecting Define - Walking Vibrations from the main ETABS menu. When done this way, the program creates a Time History Analysis based on the data entered on the form you discussed. This information, I believe, is also based on the AISC design guide. However, it is normally only used for special or unusual cases.

To learn more about this 2nd (more unusual) option, take a look at the Watch and Learn video on our YouTube channel. This is how you would do it without this helpful form:

https://youtu.be/faUI-IJVqS4

 

[271828]

The second method JoshPlumSE described is what I was talking about. A triangular pulse is a very unreliable representation of a footstep. I very strongly recommend not using this approach. For resonant responses, it is critical that the input force has a similar frequency content as a series of footsteps. Who knows what frequency content a series of triangular pulses would have. The approach could be off by many multiples, not like 10-20%.

Go into Design Guide 11 and find the Fourier Series representation of a series of footsteps. This would define the amplitudes and frequencies that could then be used to define sinusoidal load functions. These functions could be used in a time history analysis like shown in the video.

 

[JoshPlumSE]

Caveat: I should point out that I work for CSI (Computers and Structures, Inc) the company that authors the ETABS application. So, I'm not exactly an impartial observer on this subject.

I will also point out that I had never used this 2nd procedure until today when I was testing it out before I posted my response. I'm personally don't find this 2nd method as objectionable as 271828 does. If you look at figure 1-7 of the 2nd edition of that design guide, the image shows something pretty close to a triangle.... It just ignores a small amount of the negative force on either side of the peak measured force.

That being said, the AISC design guide (when doing FEM to evaluate these functions) would probably push us more towards an FRF (Frequency Response Function) type of analysis rather than a time history.

I imagine that the time history type of loading might be more useful when evaluating very sensitive equipment. I recall a colleague of mine one evaluating an MRI machine at a football stadium. I wasn't personally involved in that project, but I recall that they did a number of different time histories to see the vibration response at the MRI location. Not just walking excitation, but related to crowd activity in the stands and such.

 

[271828]

Josh,

I don't want to argue, but Figure 1-7 is a heel-drop.

Figure 2-3 is the figure to look at. If you take the FFT of the "Total," the resulting harmonic force amplitudes would be something like those given in Table 1-1. Those harmonic amplitudes are everything for resonant response calculation.

If you take the FFT of a series of triangular pulses, the amplitudes could be anything. I've done this before, with more realistic footstep forces than a triangular pulse. If the force time history is changed just a little, then the harmonic forces change in who-knows-what direction. Similarly, keep the same footstep force pulse but change the step frequency and again, the harmonic forces go crazy.

Respectfully!

271828

 

[WillisV]

Agree with 271828.

 

[JoshPlumSE]

I'll look into what sort of forcing function we're using for that 2nd method. Because I genuinely don't know. All I know for sure is that it appears to have been added in March of 2016.

It was 271828 that suggested it was a triangular pulse. I didn't say otherwise, because I'm not sure and the Watch and Learn (which predates that feature by at least two years) used a triangular function.

My belief is that the move away from a heel drop function to something more like you guys would like began with Brad Davis' research. So, sometime between 2008 (his PhD thesis) and 2016 (when the 2nd edition of the AISC Design Guide was released).

I'm going through Brad's research now to better understand how to form that forcing function when doing a time history analysis. Once I figure that out, I'm going to quiz my superiors at CSI about what we're actually doing with that particular feature. And, make sure that it's more clearly documented in our help files and such.

Though, I will emphasize (again) that for the vast majority of composite beam systems, you'll probably want to use the traditional method 1 that I referred to.

 

[TLycan]

I was very Engineerly delighted to read the whole comments.

Now let me tell you why brought this question:-

First I have a pedestrian bridge as shown in the attached that I needed to check for vibration and I have the following problem:-

1) I can't check it according to the ordinary procedures of DG11 due to the huge opening in the middle of the bridge
2) if I conservatively check it as two girders only , you need to take a look on the bridge, it will be unsatisfactory for vibration
3) I could not go for the Aashto requirement as I did not if it is based on the 1.5% or 5% frequency criteria for indoor and out door bridges. Moreover because of what listed in point 1 I could not be sure that the aashto requirement will apply

therefore I went to:-
1) the FRF, which strangely yielded a safe Vibration design
2)I went to Time history equivalent to that FRF obtained the same value of FRF (both FRF and TH are equal actually)
3)Used the procedure JoshplumSE described in the video it yielded a very different value the 1 and 2 and a more safer design
4)Used ETABs walking vibration check yielded the same values as 3
5) Changed the ETABS walking frequency to that of the Modal frequency provided a reslts a tittle bit close to 1 and 2


and now I've to read more investigate and read more

 

[WillisV]

Josh - Bingo - Brad's research. Thanks for pursuing the proper implementation of such.

 

[271828]

Agreed. Bravo Josh for taking this on!

 

[TLycan]

guys where I can find Brad Davis research?

 

[JoshPlumSE]

A lot of it is in the 2nd edition of AISC DG-11. At least as it relate to the FEM modeling for floor vibrations.

SJI has a "Technical Digest 5 - Vibration of Steel Joist - Concrete Slab Floors" has some of it in there as well.

But, what I'm reading right now is:
Davis, D.B. "Finite Element Modeling for Prediction of Low Frequency Floor Vibrations Due to Walking" PhD dissertation, Virginia Polytechnic Institute and State University

 

[WillisV]

Also a decent summary by Dr. Murray and Dr. Davis from the NASCC is here - the FE approach discussed in the latter 1/3:

http://cloud.aisc.org/nascc/2017/370322-E5A.mp4