Vibration Mitigation 1

[Boiler106]

I've been tasked with creating design drawings from a vibration mitigation study performed by a specialty engineer on a composite framed steel floor with slab on metal deck for sensitive equipment. The specialty engineer used finite element modelling to determine the repairs which consist of adding posts and beams. Project is in the USA.

Beams (in red) and posts are to be added to the floor framing.

The beams parallel to the deck have us concerned. Although I'm told the FEA model took into account some orthotropic deck properties, it's strange to me that these are effective in a one way slab system. Ignoring all of this (because its not my design) we know that loads associated with vibrations are very small, such that even bolting on reinforcing is not sufficient per the literature. Reinforcing needs to be welded.

I would prefer not to discuss the effectiveness of this reinforcing layout

For those with experience, is it enough to place the beams tight to the underside of the low flutes for these to be effective? Or would they need to be engaged with the deck with a cored slab/headed stud connection?

 

[271828]

The AISC Design Guide 11 talks about this. The floor needs to be jacked up, the new components installed as tightly as possible, and then jacks released. Otherwise, the floor will probably behave as if the new components aren't even there.

I'd recommend getting construction advice from the specialty engineer who did the design.

 

[271828]

No comments on the scheme itself because the first post said "prefer not to discuss."

 

[Boiler106]

Thanks, i mustve missed that portion. I know it discusses the columns.

We've discussed it with the specialty engineer. They wanted the beams tight to the deck with no special requirements. They were not familiar with jacking the posts, which made me question their direction.

 

[271828]

Is this for mitigating walking vibrations or something else? I would only expect it to help with walking.

For mechanical, the typical approach is to attack the problem at the source. Higher frequency vibrations probably wouldn't be helped by adding some beams.

 

[271828]

The construction stuff is in Chapter 8

 

[Boiler106]

It's for walking excitation

 

[canwesteng]

For mechanical vibration, if you unloaded the equipment before installing the beams I can see it working. I've certainly reinforced vibrating floors without any jacking and they have worked, although with welded reinforcement.

 

[Boiler106]

Yes, agreed. Although Im most concerned with the beams that are on the right-hand side, parallel to the deck.

 

[271828]

What's the concern about the beams on the right-hand side? That they're parallel to the deck?

This type of analysis isn't like tracking tributary widths with one-way slab behavior. The mode shape might have significant two-way bending in the right-hand bay.

That said, without seeing the mode shapes, walking paths, etc., it's hard to see what those five beams do. If the framing is of typical proportions, and if these five beams aren't huge, it seems like the system would behave about the same whether those five beams are there or not.

 

[Boiler106]

What's the concern about the beams on the right-hand side? That they're parallel to the deck?

Yes, and to engage them so that the reinforcing is compatible with the model.

If the framing is of typical proportions, and if these five beams aren't huge, it seems like the system would behave about the same whether those five beams are there or not.

...Yes, agreed. Which is what I've alluded to in my original post.

 

[IRstuff]

If nothing else, assuming the 5 beams are properly installed, they could change the transmission modes of the slabs that rest on them. One might also guess that the 5 beams are supposed to mimic the 4 long beams around the equipment. Of interest is that the 5 beams are roughly one deck span away from the equipment and likewise the 4 columns.

The ultimate objective is to vibrationally isolate the equipment. Were this on the ground floor, vibration sensitive equipment might be put on a much thicker and isolated concrete pad.

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[Boiler106]

Thanks, but we know, based on DG 11, that reinforcement that is not properly attached to the floor framing, such as through welding or jacking of posts, is largely ineffective when it comes to vibration mitigation.

The ultimate objective is to vibrationally isolate the equipment. Were this on the ground floor, vibration sensitive equipment might be put on a much thicker and isolated concrete pad.

I am familiar with walking excitation and sensitive equipment. Isolation is not possible and the requirements for a VC-C floor are the driver here.

 

[Lomarandil]

No comment on this scenario, but it reminds me of another job --

Evaluating a bridge with known, significant weld defects for the passage of an overweight vehicle. I ran some lower bound fracture analysis based on the known parameters of the flaw, but I convinced my manager that it was more than we should be putting on his stamp, and that the client should engage a fracture specialist to verify.

Client hired another bridge engineer (from a "famous" firm) who subtracted the flaw from the flange area and approved the vehicle.

Bridge held, because of load factors and redundancy.

 

[GregLocock]

I disagree that jacking is necessary IF you are only talking about stiffness and are in the linear range. So long as the beams bend with the existing floor panels then they will stiffen them.

Cheers

Greg Locock


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[Boiler106]

I disagree that jacking is necessary IF you are only talking about stiffness and are in the linear range. So long as the beams bend with the existing floor panels then they will stiffen them.

Thanks but that contradicts AISC DG 11

Jacking is required to introduce strain into the added elements so that the retrofitted system is “tight” and able to respond to the very small loads introduced by human activity

 

[GregLocock]

Yes, practically that is a good way of helping to ensure that the beams bend with the existing floor panels. But it isn't the only way. Glue or frequent fasteners spring to mind.

Cheers

Greg Locock


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[271828]

I disagree that jacking is necessary IF you are only talking about stiffness and are in the linear range. So long as the beams bend with the existing floor panels then they will stiffen them.

Experience has shown that the fix in a building will be iffy unless some strain is induced in the new components.

One reason is the magnitude of vibration is extremely small, and there are imperfections in the floor system. Here's an example:

Imagine you have a floor with bar joists and need to stiffen it for vibration due to walking. Add a couple of angles below the bottom chord to make the truss deeper. The calcs think vertical deflection in the floor causes axial stress in the new angles. However, those angles will have some amount of sweep. When someone walks on the floor, the magnitude of the sweep will oscillate instead of the axial stresses oscillating. To make the system behave like the calcs, the new angles need some axial load.

This reminds me of lab tests with applied load versus displacement. The lower parts of the load-displacement curve are almost always trash until the load frames, specimen, etc. get some load. After that, the curve looks more like the theoretical.

 

[JoshPlumSE]

That said, without seeing the mode shapes, walking paths, etc., it's hard to see what those five beams do. If the framing is of typical proportions, and if these five beams aren't huge, it seems like the system would behave about the same whether those five beams are there or not.

I'm going to take a guess and suggest that the beam on the right are related to the "walking path". Maybe a hallway in a medical building where the equipment (an MRI machine?) might be installed.

A company I worked for did something similar where we added new steel beams that we needed to be composite with the deck. And, we jacked out a portion of the slab (making sure not to damage any reinforcing) so that we could add studs to the top of the beams, then repaired the concrete. I think we were more worried about strength and not vibration for that job though.

 

[canwesteng]

That's true for a spot welded bottom chord of a truss, or any bolted bracing, and especially true with posts, but continuously welded reinforcement doesn't have any slack to give up.