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Old 04-23-2007, 06:57 PM   #1
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Deck vibration problem


I can sure use someone's help with this.

I have a trex composite deck with a max joist span of 10' 10" with 16" oc spacing. The joists are 2x8's, treated southern pine grade #2. The deck was designed for 60 psf.

About midway down between joists, bridging (wood block) is installed for increased stability. However I still notice vibration when walking on this section of the deck.

I know now that for this joist span, minimum 2x10's should have been used and/or 12" oc spacing.

Question: To further reduce vibration, can sister joists be installed? I don't want to remove the bridging first however. SO...if I cut the sister joist in half and run each half along the original beam up to the bridge, will this work if the sister joists are securely anchored to either side of the bridge?

Will this then provide the same additional support as a single sister joist running the full length of the orginal joist?

Any thoughts on best method of anchoring the sister joist to the bridge?

I'm trying to reduce the springy sensation when walking on the deck as well as add additional joist strength. Is this a good idea?

Thanks.

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Old 04-23-2007, 07:03 PM   #2
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Deck vibration problem


Quote:
Originally Posted by tpmanus View Post

Question: To further reduce vibration, can sister joists be installed?
I don't want to remove the bridging first however. SO...if I cut the sister joist in half and run each half along the original beam up to the bridge, will this work if the sister joists are securely anchored to either side of the bridge?
Will this then provide the same additional support as a single sister joist running the full length of the orginal joist?
Any thoughts on best method of anchoring the sister joist to the bridge?
I'm trying to reduce the springy sensation when walking on the deck as well as add additional joist strength. Is this a good idea?
Thanks.
Hi,
What exactly is the design of the deck like in terms of the shape, and connection points to your house? What is the main carrying beam arrangement?

Assuming that the rest of the deck subframe is constructed properly;
You should rip out the bridging (actually, just get a reciprocating saw with a metal cutting blade and cut the nails to remove the bridging)

You can then sister up full length run - 2x10's, to the existing joists. If you are concerned with the unsightly issue of the extra 2" showing under the rim joists, you could 45 degree cut the lower end corners so they are less 'visibly exposed'.

Then go back and re-install new 2x10" bridging. I would suggest that you add additional bridging at regular intervals in the joist bays. For example, instead of the half way point, you could add 3-4 in each bay - spaced evenly....

Bear in mind that you are also adding alot of extra weight to the deck with the 2x10's...so make sure that you are properly lagged into your home structure and the ledger board, Also confirm you have the proper sized footings, posts, joist hangers, etc......Make sure that all is up to code and will support the extra dead load.

There may be more recommendations from other posters...

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Old 04-23-2007, 07:24 PM   #3
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Deck vibration problem


The deck is essentially a large rectangle, except there is a bay extension on the front. The dimensions are 29' wide by 20' deep. The joists are secured to a ledger board using hangers and extend almost 11' to the beam. The beam is sandwich design 2x8's bolted to either side of the posts. From the beam, additional joists extend out to the second beam.

The joists with the greatest span run from the house (ledger board) to the first beam. This is where the springy sensation is. The deck area between the two sets of beams seems solid.
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Old 04-23-2007, 07:38 PM   #4
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Deck vibration problem


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Originally Posted by tpmanus View Post
The deck is essentially a large rectangle, except there is a bay extension on the front. The dimensions are 29' wide by 20' deep. The joists are secured to a ledger board using hangers and extend almost 11' to the beam. The beam is sandwich design 2x8's bolted to either side of the posts. From the beam, additional joists extend out to the second beam.

The joists with the greatest span run from the house (ledger board) to the first beam. This is where the springy sensation is. The deck area between the two sets of beams seems solid.
I'm worried about the size of that main beam arrangement, and also the dreaded 'beam sandwich' arrangment that puts all the weight load on the bolts....

We need to get RobertCDF in on this.....
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Old 04-23-2007, 07:45 PM   #5
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Deck vibration problem


I sent an Email to RobertCDF about this....
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Old 04-23-2007, 08:04 PM   #6
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Deck vibration problem


Thanks I appreciate the assist. The beam sandwich is secured with two large bolts at each post. The posts are buried approximately 36" deep on solid concrete footings. For the ledger board, I used long lag screws, through brick and into the house rim joist.

So I think the support is there for additional lumber. I really wanted to avoid pulling out the existing bridging though. This deck is at low elevation...it's roughly equal to a crawl space underneath. I'd like to try and remedy this from underneath rather then pulling up the floor.
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Old 04-23-2007, 08:13 PM   #7
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Deck vibration problem


One more thing I thought was worth mentioning, the posts are spaced about 6' feet apart.
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Old 04-23-2007, 09:59 PM   #8
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Deck vibration problem


Ok.... First of all the center beam is undersized. It carries 1/2 the span from the ledger to it and 1/2 the span between 1st beam and end beam. So therefore it carries 2X as much weight as the end beam does.

I also HATE the sandwich style construction. Its can never be as strong as a beam on top of post.

As far as the center beam goes... It should be a minimum 2 2x12's with posts at 6' O.C. and the piers would probably need to be enlarged. The end piers on the center beam should be min 10" DIA and however deep as required by your local building department. The CENTER piers on the center beam should be MIN 16" DIA.

As far as fixing... this is what I would recomend you do.

Support the deck near the center beam temporarly and tear out the center beam. Install a new one with doubled 2x12's sitting on top of the posts.

Tear out the midspan blocking and double up every 2x8 joist. Or you could add 1 joist in between each joist to make it 8" O.C. but I prefer a double joist as 2 nailed together is stronger than 2 8" apart.

You may still feel a little bit of "flex" in your deck but I would think it would come from the trex spanning 14.5"

If you have any other questions let me know and I will try to answer as best as I can.
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Old 04-24-2007, 08:46 AM   #9
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Deck vibration problem


Thanks Robert. What if I tear up some of my deck floor and add an additional support beam about midspan? This way I could leave the bridging in place, not worry about sistering joists and use the same material as my present beams.
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Old 04-24-2007, 10:02 AM   #10
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Deck vibration problem


Have considered looking at the bracing and stability of the deck? How high and what lateral bracing?

Vibration is not a structural issue, but can be related to stiffness and stability, just as cumulative deflection is.
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Old 04-24-2007, 10:20 AM   #11
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Originally Posted by tpmanus View Post
Thanks Robert. What if I tear up some of my deck floor and add an additional support beam about midspan? This way I could leave the bridging in place, not worry about sistering joists and use the same material as my present beams.

That would work as well... However digging holes and pouring footers around framing can be a REAL pain in the neck. If you do add another beam it would be best if you could fix the other beam so that it at least sits on top of the posts.

If you do go the route of adding another beam you can use a double 2x8 and have yours posts at about 6' then your piers only need to be 12" Dia
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Old 04-24-2007, 10:35 AM   #12
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Deck vibration problem


The deck elevation at midspan is roughly 25"-26". I used 2x8 block bridging between joists all the way across midspan, i.e. same material used for the joists.

Regarding deflection, I noticed some deflection of the rim joist when my 195 lb son stepped to this area.

It's apparent to me that I should have used 2x10's for the joists (and for the beams, at least). I had this deck designed for 60 psf, however as it stands, it's probably only rated at 40 psf if I'm lucky. For this reason, it's probably a given I need to strengthen the 2x8's??

For the springy sensation, is there different or additional bridging needed that will help?
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Old 04-24-2007, 10:44 AM   #13
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Deck vibration problem


Thanks Robert...if I add the additional beam, this should take care of my live load rating issue and vibration issue. Like you said, this will be very tricky working between joists.

Any other recommendations are welcome. Thanks for everyone's help.
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Old 04-24-2007, 03:27 PM   #14
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Deck vibration problem


I like the additional beam approach.Ii cant envision why it would be that hard. and why your would be "working between joists". Wouldn't you put the beam under your joists, mid span, under your springy area?
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Old 04-25-2007, 10:29 AM   #15
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I like the additional beam approach.Ii cant envision why it would be that hard. and why your would be "working between joists". Wouldn't you put the beam under your joists, mid span, under your springy area?

Because the deck is only 25-30" off the ground... And he would have to dig the holes while standing in between joists.

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