How to retrofit floor to reduce vibrations

[civilguy55]

An elevated floor slab vibrates substantially from activity as simple as people walking across it. It's a mezzanine in a factory. The floor is 175-ft (7 bays at 25-ft) by 50-ft (2 bays at 25-ft). The slab consists of 2" normal weight concrete over 1-3/8" form deck (non-composite) supported by bar joists 16H6 at 2'-6" on center. There are no partitions or suspended ceiling either above or below the floor, and there are no significant mechanical/electrical items suspended below the floor. The floor is very lightly loaded and used mainly for office functions. The workers sit at desks and computer workstations and are bothered by the excessive vibrations. I'm looking for ways to stiffen the floor to dampen the vibrations. I'm ruling out installing partitions below the floor and thickening the floor slab. Does anyone have a successful way to stiffen the floor joists?

 

[JAE]

I just quickly ran SJI's Vibrate program based on your information (I assumed W16x40 beams) and it indicated "Distinct Perceptibility" - so it adds up.

It is very difficult to stabilize a floor without adding partitions or mass. According to the program, you'd have to add 2 1/2" to 3" of concrete to get the vibration down to "slight perceptibility" and that would probably kill your 16H6's.

According to most published info, adding X bridging supposedly doesn't do any good but I have often found that X braced joists are much less "shaky" than horiz. braced joists.

Dynamic dampers can be purchased but they tend to be expensive and require expert design/installation to do any good.

The cheapest/most effective solution is partitions.

 

[JedClampett]

If these were normal wide flange shapes, I would suggest retrofitting Nelson studs attached to the top flange of the section. If there is a clean way to get a good attachment to the top member of the joist and you can get enough information on the joist properties, this might work. You could cut holes in the concrete topping and replace it around the studs with grout. This has the advantage of not requiring scaffolding.

 

[dougantholz]

JedClampett,
According to Dr. Murry's Steel Design Guide 11, because vibration is an impulse load, all members with a concrete slabs act as composite beams. So more attatchment to the slab is not the answer.
civilguy, you have I think 3 options.
First - a fifty foot span is relatively long, so you migh try adding a column and girder at midspan and attatch to the underside of the joist top chord (keep the joist continuous). THis should drastically change the floor naturual frequency.
Second - you need to add mass, but since you don't like addiding it above the floor, how about a pipe running parrallel to the joists that is filled with water. What about hanging concrete blocks from the joist panel points.
Third - you need to add section to the joists, to change their I. Unfortunately this is likely the hardest and most expensive option. Try adding a tube or channel to the bottom flange would be the way to go for my interests.
A less attractive fourth option would be to make the girders continuous with moment connections. This can change your ap/g by a couple of hundredths, but you likely need to change by a couple of tenths, and it would really change your lateral system.
Hope this helps.
Doug

 

[emceditor]

Hi Civil Guy,

I had a project years back with bar joists spanning approximately 15 ft or so with vibrations that the owner considerd excessive. I specified to apply 3/4" plywood sheathing with powder-actuated nails to the bottom flange of the joists. This reduced the vibrations to an acceptable limit. It appears that this could be done more economically then the other suggestions above. Considering the large area of your project, I would recommend to do this in one or two bays to verify it will work for this projects constraints.

 

[SteveBausch]

I think there is a distinct difference between wooden x-bracing and welded x-bracing.

A nail cannot compare to a weld. In tension, screws are (easily) twice as strong as nails.