# Ceiling joist deflection



## PerfectCntng (Jan 13, 2015)

Appreciate your insight on this situation we have. I also stated the Architects opinion as well as Engineers opinion. This should be very interesting. 

Subject property is a four story attached brick residential building (brownstone). Building width interior to interior brick foundation wall 18'-0". The joist are 3 x 8 @ 16 OC and span across entire width of building. every fourth joist is embedded into the brick wall. The others are attached to a header that are also supported by the joist that are embedded. The foundation walls are 8" thick. 4" for each of the adjoined buildings ( attached buildings) we removed a wall (non bearing) that was three feet away from party walls. There is vibration felt in the floor in bedroom directly above the work area. 

The Architect believed the wall was a bearing wall before that wall was removed and called for full blocking and sister every other joist (14 joists). The wall turned out to be non bearing and joists are supported at both ends. 

Customer feels slight vibration after that wall was removed. We retained an Engineer to visit the site and he recommended running a new steel beam below the joist with two columns at ends. This cannot be done as they want a "open floor plan" (area is a high end kitchen) and the finished height by code is 8 feet. So we cannot install a beam lower than the joist height. He then suggested cross bracing or blocking with 3 x8 at three locations. Provided the blocking are tight and installed with liquid nails and 16d nails. 
I would love to get it done once and get it right! I have to be certain that the flooring above stops vibrating. (I can't touch or alter the floor or subfloor above such as nailing (which may solve the issue) I appreciate any advice. And have a happy holidays!! 

Ron @ Perfect


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## griz (Nov 26, 2009)

What is causing the vibration that your clients are feeling?

Is there a measurable deflection in the floor joists?

IMO, feeling a vibration from something in a specific area and deflection in a joist are two different things.

Do you feel there is an over span situation?


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## GettingBy (Aug 19, 2010)

PerfectCntng said:


> There is vibration felt in the floor in bedroom directly above the work area.


More clues needed before committing to some fix.

With a helper watching in the room below, have the same HO map out the vibration by walking directly overhead each joist for each length, and then crosswise in several places.

You can check for the L/360 ratio or whatever using that same HO but the procedure is kind of tedious.


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## muskoka guy (Nov 16, 2013)

Would it be possible to strap the underside of the joists to stiffen the floor. 1x or even 2x on 12 or 16 inch centers. I have done this in the past where the span is close to not passing code and taller joist were not possible.


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## loneframer (Feb 13, 2009)

I hate to be that guy digging up old threads, but I think you'll enjoy this. Aside from all the confusion, I essentially wanted to give an example of how bridging unifies and stiffens a floor assembly. The end result should help take some of the vibration and flex out of the floor by forcing the joists to share the load applied from above. 

http://www.contractortalk.com/f14/fun-wood-x-bridging-68111/


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## Morning Wood (Jan 12, 2008)

Strapping won't help with deflection. Blocking will though. Sounds like you are trying to stiffen the floor, not actually deal with deflection. Also, if the joists are already deflected I would sister all joists so you can get a nice straight ceiling. You could also just do what archi and engineer have suggested and then strap the ceiling flat. Are the 3x8's mortised into the header or brick? The joists might be notched which will weaken them.


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## hdavis (Feb 14, 2012)

Sounds like a resonant frequency issue (structural should have been spotted right away). To get rid of it, you have to increase the resonant frequency of the floor system. Stiffen it (like Loneframer's link) and then screw and glue plywood panels to the bottoms of the joists. They'll be acting as a structural diaphram for the floor system.


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## superseal (Feb 4, 2009)

My own house is 3 x 8's 16oc and 12ft spans...I would not want go any further than that...sounds way overspanned to me.

Which code calls out 8' ceilings in a brownstown?... Just curious.

Just because the joist run wall to wall doesn't mean you didn't take out a bearing wall.

Why doesn't a steel beam supported with post at ends leave an open floor plan?

I'd probably sister each joist with a 12" lvl, glued with pl premium and screwed at 12"oc staggered using T30 5/16" x 4" GRK- RRS...obviously having to drop the ceiling 4" though.

Level as you go and block the chit out of it when your done...two rows min if not three.


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## GettingBy (Aug 19, 2010)

hdavis said:


> Sounds like a resonant frequency issue (structural should have been spotted right away).


Good call.

This cause can be verified by the easy and free test of varying HO walking speed from very slow to very fast.

Stiffening lowers the resonant frequency because it more closely couples the building to the earth, which has a very low mechanical resonant frequency and absorbs vibrations.

You could also reduce the floor vibrations with energy absorbing material (caulk?). This last one is how crankshaft harmonic balancers work to damp out crankshaft ringing.


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## hdavis (Feb 14, 2012)

superseal said:


> My own house is 3 x 8's 16oc and 12ft spans...I would not want go any further than that...sounds way overspanned to me.


I meant to comment on this. I'd expect 2X12s for 18', but if an archi stamps it with sistering every other, I probably won't argue too much.


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## PerfectCntng (Jan 13, 2015)

Thank you all for the great advice. We ended up providing one row of full blocking and expect to install one more. This was installed 1 feet off center of room and the other row will be 1 feet off center the other way. With the one row of full blocking, the HO feels a big difference already. I'm expecting with another row plus 5/8 Sheetrock underneath, things will be sufficient. Thanks again.


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## GettingBy (Aug 19, 2010)

PerfectCntng said:


> With the one row of full blocking, the HO feels a big difference already.


So it was a resonance effect.


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## carp.addict (Dec 26, 2015)

superseal said:


> My own house is 3 x 8's 16oc and 12ft spans...I would not want go any further than that...sounds way overspanned to me.
> 
> Which code calls out 8' ceilings in a brownstown?... Just curious.
> 
> ...


I agree, no span table I've ever seen would say 2x8 @18' clear span without reinforcement. Not even the old true 2by's before we switched to nominal lumber. 2x10 sisters with three rows of blocking would probably be a safe bet. PL on top of blocks to avoid squeaks.


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## AustinDB (Sep 11, 2006)

I was thinking the same but wondered if there could be a quick test to help out. Could placing a heavy object near the middle of the span help to identify the problem? Seems it would either increase or decrease the frequency. 

JLC (or possibly another trade magazine) recently had an article describing the resonant frequency most noticeable to a person walking across a floor structure, seems it was in the 12-14hz range (?)


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