We just completed the construction of our new house (did all work
ourselves).

All floors are constructed with 2x10 joists at 16" OC, with 3/4" OSB
subfloor, and solid blocking every 8' of the span.

The floors feel quite solid throughout the house, except in our living room
where the floor is a bit bouncy. The span there is about 15' from sill to
sill, just about the max for 2x10 Doug Fir joists.

We have the same 15' span in our master bedroom and the floor there is
quite solid, so I suspect the problem is due to our large entertainment
center (a few hundred pounds I'm sure) that sits about mid-span in the
living room.

Since the house is single story over a crawlspace, I decided to add a beam
mid-span to stiffen up the living room floor under the entertainment
center.

I used two 2x8 x 10' boards screwed together for the built-up beam. I
supported the beam on three posts, spaced about 4' apart, which rest on top
of concrete deck piers. I would have preferred a poured footing, but
getting the materials back to where I need them in the crawlspace was
rather difficult. The deck blocks seemed like the easiest approach, and I
spaced the piers four feet apart to minimize the load on each pier.

The floor had sagged about 1/2" where we have the entertainment center, so
I used an automotive floor jack to jack the beam up level and tight against
the floor joists.

Everything went rather smoothly, and should have effectively reduced the
span to about 8' on each side of the beam.

Unfortunately, when I climbed out from under the house and came in
expecting the bounce to be gone, I discovered the floor still bounces...
Not as bad, but it is still significant.

So, now I'm wondering what went wrong and what I do now.

The beam should have made the floor quite solid, and yet it still bounces?
The ground in the crawlspace is quite firm, so I don't think there is any
"give" to the piers.

This weekend I'll probably have my wife bounce the floor from above while I
climb underneath to see if I can discover anything. But, I'm interested in
hearing comments from others with similar experiences.

Thanks,

Anthony

On 21-Oct-2004, HerHusband wrote:

The floors feel quite solid throughout the house, except in our living room
where the floor is a bit bouncy. The span there is about 15' from sill to
sill, just about the max for 2x10 Doug Fir joists.

I'd not use any joist near its max span limits. Be conservative and
switch to the next bigger. Undersized joists are a surefire way to
get excess deflections and vibrations. Not undersized according to
code, but undersized compared to that nice, solid feeling folks expect
in a house.

We have the same 15' span in our master bedroom and the floor there is
quite solid, so I suspect the problem is due to our large entertainment
center (a few hundred pounds I'm sure) that sits about mid-span in the
living room.

There can be a couple of reasons why these are different. Distribution
of weight can be one, since the heavy entertainment center in the middle
of the room could result in a different, lower, resonant frequency and
that can be more noticible.

The beam should have made the floor quite solid, and yet it still bounces?
The ground in the crawlspace is quite firm, so I don't think there is any
"give" to the piers.

If the new support beam doesn't give, you would expect a big change - the
reduced span would have a much higher resonant frequency (about 4 times
higher due to halving the span). This makes me suspect that the support
is actually not that solid. Even if the initial installation is solid, the
concrete piers will settle under load. Some settlement will happen quickly
and some will occur over a longer period of time. This is dependent on the
soil characteristics.

Let us know what happens when you watch from underneath.

Mike

HerHusband wrote:

We just completed the construction of our new house (did all work
ourselves).

All floors are constructed with 2x10 joists at 16" OC, with 3/4" OSB
subfloor, and solid blocking every 8' of the span.

The floors feel quite solid throughout the house, except in our living room
where the floor is a bit bouncy. The span there is about 15' from sill to
sill, just about the max for 2x10 Doug Fir joists.

We have the same 15' span in our master bedroom and the floor there is
quite solid, so I suspect the problem is due to our large entertainment
center (a few hundred pounds I'm sure) that sits about mid-span in the
living room.

Since the house is single story over a crawlspace, I decided to add a beam
mid-span to stiffen up the living room floor under the entertainment
center.

I used two 2x8 x 10' boards screwed together for the built-up beam. I
supported the beam on three posts, spaced about 4' apart, which rest on top
of concrete deck piers.


I would have preferred a poured footing, but
getting the materials back to where I need them in the crawlspace was
rather difficult. The deck blocks seemed like the easiest approach, and I
spaced the piers four feet apart to minimize the load on each pier.

The floor had sagged about 1/2" where we have the entertainment center, so
I used an automotive floor jack to jack the beam up level and tight against
the floor joists.

Can you see that the new beam really is "tight" against *all* the
joists? Maybe you'll need to tie each joist to it in case there's enough
variation it the 10" dimension of the joist boards and some of them
aren't really sitting hard on the new beam.

Just a WAG.

Jeff

--
My name is Jeff Wisnia and I approved this message....

(W1BSV + Brass Rat '57 EE)

"As long as there are final exams, there will be prayer in public
schools"

The floor had sagged about 1/2" where we have the entertainment center, so
I used an automotive floor jack to jack the beam up level and tight
against
the floor joists.

Everything went rather smoothly, and should have effectively reduced the
span to about 8' on each side of the beam.

Unfortunately, when I climbed out from under the house and came in
expecting the bounce to be gone, I discovered the floor still bounces...
Not as bad, but it is still significant.

So, now I'm wondering what went wrong and what I do now.

Many years ago had similar problem; our kitchen floor sagged. maybe a half
inch or more due to the weight of the stove and sink counter which sat
longitudinally above approx. three joists. Also a ten foot run of hall
closets over several adjacent joists backed onto the kitchen wall, so there
was a lot of weight up there!
As a solution we jacked up that part of the floor a little bit at a time,
using a small hydraulic car jack on top of a wooden post, sighting across
the kitchen and hall floors from the top of the basement stairs; jacking
gradually until that part of the floor was 'just a little bit too high'!
Then built a load bearing, double header, single floor plate 2x4 stud wall
underneath, in our full height unfinished basement resting on the concrete
basement floor; which spread the weight nicely. Wedging the stud wall firmly
in place we then released the jacking. IIRC after a couple of days and
one/two minor adjustments it settled into place nicely level. And has been
that way for over 30 years. Wedges and/or shims of thin plywood were used
for 'fine' adjustment and 'packing' the gaps to get everything level.
That wall later became one side of a 10 by 13 foot room directly underneath
our kitchen and houses an electronic repair bench. The wall also contains a
storage closet the lower part of which stores unused luggage above which are
hung occasionally used oilskins, sea boots etc. Although added later and not
necessary the sides of the closet also support adjacent joists and buttress
or stiffen the stud wall itself. Come to think of it the secondary breaker
panel for the kitchen end of the house is also in that closet wall above and
the added basement stud wall provided means of later routing down some
wiring for added basement circuits.
Overcoming the problem actually created new possibilities.

Michael,

I'd not use any joist near its max span limits.

Yeah, it was a weak design decision on my part... The majority of the house
has spans that are around 12', well within the capability of the 2x10 @ 16"
joists. Only the living and master bedroom have the 15' span. I approached
the span limits in those two rooms to keep things consistant. In hindsight,
I should have added another support or switched to larger joists in those
two rooms, but that does little good now.

If the new support beam doesn't give, you would expect a big change -
This makes me suspect that the support is actually not that solid.

Yeah, that's what I'm thinking too.

I'm gonna give it a couple of days to settle in a bit, then I'll check
things out better.

Anthony

BB,

What kind of flooring in each room?

Identical floor structure in both rooms.

Carpet
Padding
3/4" T&G "Edgegold" OSB subfloor (nailed every 6").
"Subfloor" construction adhesive between OSB and joists.
2x10 joists @ 16" OC (15' free span between supports).
Solid 2x10 blocking in middle of span (8').

Other than a non-bearing partition wall, the two rooms are basically one.

3/4" OSB doesn't exactly sound "overbuilt" to me.

Nope, it's not overbuilt, but the OSB seems sturdy enough everywhere else
in the house.

Anthony

"HerHusband" wrote in message
...
We just completed the construction of our new house (did all work
ourselves).

All floors are constructed with 2x10 joists at 16" OC, with 3/4" OSB
subfloor, and solid blocking every 8' of the span.

The floors feel quite solid throughout the house, except in our living
room
where the floor is a bit bouncy. The span there is about 15' from sill to
sill, just about the max for 2x10 Doug Fir joists.


A little advice I discovered by accident while renovating my house:

Use pressure treated joists--they are made from a much harder, denser,
stronger grade of wood than typical doug fir. I did some ceiling joists
recently with a 12' span and walking up there (flat roof), it's as if the
frame were sitting on concrete on the ground.

Short of replacing or adding additional PT joists, I'd recommend a carrying
beam perpendicular to the existing joists, supported by lally columns on
cement footings. Lots of extra work, but ultimately, it solves the problem.

Normally, I do 2x12 joists for floors. And I love PT joists. They are almost
like steel "I" beams in terms of rigidity.


--
Take care,

Mark & Mary Ann Weiss

VIDEO PRODUCTION • FILM SCANNING • DVD MASTERING • AUDIO RESTORATION
Hear my Kurzweil Creations at: http://www.dv-clips.com/theater.htm
Business sites at:
www.dv-clips.com
www.mwcomms.com
www.adventuresinanimemusic.com
-

Subject: Bouncy floor. New beam didn't fix?
From: "Michael Daly"
Date: 10/21/04 2:12 PM Pacific Daylight Time
Message-id:

SNIP

I agree with all Mike's comments, except

If the new support beam doesn't give, you would expect a big change - the
reduced span would have a much higher resonant frequency (about 4 times
higher due to halving the span).

Shouldn't that be the square root of 8 (2.8) not 4?

I've investigated the bouncy floor quite a bit. Often it is the furniture in
the room that gives the feeling of bonciness. A tall unit will rock more than
a dresser, thus giving the impression that the "floor is bouncy".

Depending on how the unit is situated you might have the front legs over one
joist & the back legs over another. This will allow the unit to rock back &
forth.

Also i would suggest just putting piers with jack screws (you can buy or make
yourself from 1 1/2 pipe & 1" bolts) under the set of three or four joists
rather than one long beam. With the jack screws you can tighten them up as
needed.

On 21-Oct-2004, (Bob K 207) wrote:

Shouldn't that be the square root of 8 (2.8) not 4?

I'm working from memory and may be wrong (time to review
my old dynamics stuff). At any rate, the frequency of the
halved span will be higher and by more than double.

Mike

Use pressure treated joists--they are made from a much harder, denser,
stronger grade of wood than typical doug fir.

True often but not always. The really tough PT is yellow pine, heavy and dense.
But I got some PT at my local lumber yard a while back that was some kind of
softwood from Nebraska IIRC. Not at all in the same league as the Southern
yellow pine. HTH

Joe

"Joe Bobst" wrote in message
...
Use pressure treated joists--they are made from a much harder, denser,
stronger grade of wood than typical doug fir.

True often but not always. The really tough PT is yellow pine, heavy and
dense.
But I got some PT at my local lumber yard a while back that was some kind
of
softwood from Nebraska IIRC. Not at all in the same league as the Southern
yellow pine. HTH

Joe


PT around here is all Doug Fir or hemlock
it isnt any harder at all. Often times its much weaker.

In article , rnr_construction wrote:

"Joe Bobst" wrote in message
...
Use pressure treated joists--they are made from a much harder, denser,
stronger grade of wood than typical doug fir.

True often but not always. The really tough PT is yellow pine, heavy and
dense. But I got some PT at my local lumber yard a while back that was
some kind of softwood from Nebraska IIRC. Not at all in the same league
as the Southern yellow pine. HTH

PT around here is all Doug Fir or hemlock
it isnt any harder at all. Often times its much weaker.

Replacing joists is probably going to be an enormous job; what's he going
to do - lift up the whole house and replace all the joists to make them
the same size?

I suggest converting the first floor / basement ceiling into a torsion
box to stiffen it. Toss up a 4x8 foot sheet of plywood under the area
that's bouncing and screw it in with drywall screws on six inch centers.
If this seems to help the problem, take it back down and put glue on every
joist before putting it back again. You're not getting this back off, so
be sure you don't cover up anything important (like a cleanout). You can
extend the plywood to cover the entire basement ceiling.

Look up torsion box on Google - the technique is used in furniture, in
interior doors and in cardboard boxes. It's probably more expensive than
using the correct sized joists in the first place, but it's a possible
fix for the problem. Use thin plywood to avoid having to lift a heavy
weight to the ceiling of the basement. Quarter inch should be more than
enough.