[SCJ]

Hello, were in the early stages of a addition renovation and want to firm up the floor in the existing part of home. The floor is relatively level but has a little too much bounce and we would like to firm it up.
The home is on a crawl space ranging from about 3-2.5 feet of working room, built in mid 70's, 2x12 girders and 2x10 joists. My initial plan is to 1/2 all spanned widths and add beams and jacks - but wanted to see if anyone had any experience that could help me avoid any unnecessary jacks/beams. On the drawing my current plan are red lines representing new beams, thick lines are girders and thin a joists. Thanks

 

[joecaption]

What's the spacing on the joist that are there now?
Just going by your drawing looks like there way to far apart.
Looks like West Coast framing (every 4') that would never fly around here.

 

[SCJ]

What's the spacing on the joist that are there now?
Just going by your drawing looks like there way to far apart.
Looks like West Coast framing (every 4') that would never fly around here.

sorry should have clarifies the numbers are pillar to girder lengths the joists look to be on 16in OC but id have to double check, lines were just to show direction of joists.

 

[Daniel Holzman]

I am having a bit of a hard time understanding the drawing. Is the worst case for the joists that you have 2x10 joists 14 foot long span, 16 inches on center?

 

[WhatRnsdownhill]

your plan will work, instead of using expensive jacks, use 4 x 4 pressure treated posts to hold the beams up, just use a bottle jack to snug the new beams to the bottoms of the existing joists and bang in the proper length 4x4s with a sledge hammer and secure in place with nails or brackets...

 

[SCJ]

I am having a bit of a hard time understanding the drawing. Is the worst case for the joists that you have 2x10 joists 14 foot long span, 16 inches on center?

yes the joist are 2x10 16"OC, and yes there is one are with 14' of joist floating.

Ill try and explain the drawing:

• Grey boxes block pillars
• numbers represent distance between pillars
• thicker black lines girders
• thinner black lines direction of joist
• red line possible beam location to 1/2 the spans

 

[SCJ]

Adding pic with what is existing in structure now. Removed red lines which were proposed beam location.

 

[SCJ]

your plan will work, instead of using expensive jacks, use 4 x 4 pressure treated posts to hold the beams up, just use a bottle jack to snug the new beams to the bottoms of the existing joists and bang in the proper length 4x4s with a sledge hammer and secure in place with nails or brackets...

how would you space the 4x4 supports e.g. every 8', 6'...?

 

[ront02769]

Possibly two issues. 1) two bye twelve girders, if that, are WAY inadequate. Do you mean a doubled or tripled up two bye twelve?? 2) too large a span for the joists. While 14' is within the range of a two bye. Ten spaced 16" OC, it is near the upper end of the range. Possibly a simple doubled up two bye six beam just snugged up in the center of the joists with the 14' span would do it. Ron

 

[WhatRnsdownhill]

how would you space the 4x4 supports e.g. every 8', 6'...?

it would depend on what size beams you are going to use underneath the floor joists..for example if you used a double 2x10 I would put the 4 x 4 6ft apart..overkill at a cheap price is fine..what does a 3 ft piece of 4x4 cost under $4.00...

 

[ddawg16]

Personally, I'd double up on the joists. I've done the pad thing and while it does stiffen up things, it also gets in the way when you are moving around down there.

Assuming you can get them into the crawl space....it's not hard to get them up in place.

 

[ddawg16]

I should point out...I have 1x6 diagonal planks for a sub floor. In some places, I put 3/4" T&G plywood down. This did wonders for taking out a lot of the bounce.

 

[WhatRnsdownhill]

material wise it will cost 10x both in labor and materials to double up floor joists, as the floor he is shoring up for bounce is already complete and is his living space, and the crawl space is only 3ft high, so its not an active area...wouldnt you agree?

 

[Nestor_Kelebay]

SCJ:

Here, take a look at this file that gives the formula for deflection of a simply supported beam:

http://de.structural-analyser.com/Modeling_HigherOrder_DE_linear_boundaries/

The top row shows the equations to calculate the deflection of a simply supported beam with a concentrated load in the middle. (The other rows show the formulas for beams with other loading conditions.)

A "simply supported" beam is just one that's supported at it's two ends, very much like a floor joist.

The deflection at mid-span of such a simply supported beam is given by the equation:

d = (P*L^3)/(48EI)
or deflection equals P times L cubed divided by 48 times E times I

Where:
P is the load at the mid-span of the beam
L is the length of the beam

E is something called the Modulus of Elasticity, and is a measure of the strength of the material the beam is made of. E for wood varies from about 50,000 to 150,000 whereas for steel, it's about 30,000,000. For any given beam material, the Modulus of Elasticity is the same, and depends only on what material the beam is made of. So, a wooden floor joist would have the same value for E as a wooden flag pole made of the same wood.

I is something called the Moment of Inertia, and is related to the cross sectional shape of the beam. Every beam with the same cross sectional shape will have the same value for I regardless of what it's made of. So, for example, the difference between using a 2X12 as a floor joists or as a diving board is that the cross sectional shape changes with the orientation of the beam with respect to the applied force. In one case the cross sectional shape of the beam is much taller than it is wide with respect to the applied force, and in the other, the beam is much wider than it is tall with respect to the applied force. It's the difference in the value for I in those two cases that results in the 2X12 deflecting very little when used as a floor joists and it deflecting a lot when used as a diving board.

But, what's important here is that the deflection in the middle of the beam is proportional to the span of the beam CUBED.

By shortening the distance between supports, the amount of deflection in the joists as a result of a load it mid-span will be:
a) (1/2) cubed if you added one new support at mid-span or 1/8 of the amount it was before, or
b) (1/3) cubed if you added two new supports each at 1/3 of the length of the beam, or 1/27th of what it was before, or
c) (1/4) cubed if you added three new supports each at 1/4 of the length of the beam, or 1/64th of what it was before.

Certainly, because of that (Length) cubed term in the deflection formula, shortening the distance between the points where your joists are supported is the most effective way of strengthening your floor.

The problem is, adding supports under your floor that are strong enough to support the joists. How are you going to ensure that the supports you add won't simply sink into the mud under your house as a result of the load put on them, or even just because of their own weight?

 

[ront02769]

All the math not withstanding, we can't be much help if we don't know how the existing beams are made. For example, if the OP actually DOES have single two bye twelves as beams, adding joists will do zip. Step one, determine what existing is. iF double or triple two bye twelve and they are currently well supported, step one figure out where the bounce is. From the pix, it LOOKS as if it is in the side with the 14' span. If so, step buy a half dozen concrete blocks and enough two bye six to make up a double beam. Locate said beam in the middle of the 14' span. Set the blocks flat side down and level and plumb at each end and equally spaced throughout, grab some short adjustable jackposts (which come as short as 16" and put one on each block. Stick a piece of four bye four on top of each and snug up to the bottom of the beam until said bounce is gone. Nail everything in place. Done. Couple of years down the road the bounce is back, give the jackposts another half turn. And remember, the OP is not looking to put a supporting wall over top or install a hot tub; he is simply looking to take the bounce out. Been there, done that, many many times. Ron

 

[SCJ]

Hey guys, first thanks for all the info.
just went over to take some more pics and measure a few things.

1. first pic is bottom of girder. the girders are in fact 2 2x12 nailed together with some 2x2 strips at the bottom
2. general layout of pillars and joists
3. joist tie into girder
4. another joist girder shot

 

[SCJ]

I should point out...I have 1x6 diagonal planks for a sub floor. In some places, I put 3/4" T&G plywood down. This did wonders for taking out a lot of the bounce.

the existing particle board overlay is coming out now and will be replaced with 3/4 T&G - just not sure if its enough, im a big guy and we have three little girls. so I'm trying to get it firm and quiet as best I can.

 

[SCJ]

SCJ:

Here, take a look at this file that gives the formula for deflection of a simply supported beam:

http://de.structural-analyser.com/Modeling_HigherOrder_DE_linear_boundaries/

The top row shows the equations to calculate the deflection of a simply supported beam with a concentrated load in the middle. (The other rows show the formulas for beams with other loading conditions.)

A "simply supported" beam is just one that's supported at it's two ends, very much like a floor joist.

The deflection at mid-span of such a simply supported beam is given by the equation:

d = (P*L^3)/(48EI)
or deflection equals P times L cubed divided by 48 times E times I

Where:
P is the load at the mid-span of the beam
L is the length of the beam

E is something called the Modulus of Elasticity, and is a measure of the strength of the material the beam is made of. E for wood varies from about 50,000 to 150,000 whereas for steel, it's about 30,000,000. For any given beam material, the Modulus of Elasticity is the same, and depends only on what material the beam is made of. So, a wooden floor joist would have the same value for E as a wooden flag pole made of the same wood.

I is something called the Moment of Inertia, and is related to the cross sectional shape of the beam. Every beam with the same cross sectional shape will have the same value for I regardless of what it's made of. So, for example, the difference between using a 2X12 as a floor joists or as a diving board is that the cross sectional shape changes with the orientation of the beam with respect to the applied force. In one case the cross sectional shape of the beam is much taller than it is wide with respect to the applied force, and in the other, the beam is much wider than it is tall with respect to the applied force. It's the difference in the value for I in those two cases that results in the 2X12 deflecting very little when used as a floor joists and it deflecting a lot when used as a diving board.

But, what's important here is that the deflection in the middle of the beam is proportional to the span of the beam CUBED.

By shortening the distance between supports, the amount of deflection in the joists as a result of a load it mid-span will be:
a) (1/2) cubed if you added one new support at mid-span or 1/8 of the amount it was before, or
b) (1/3) cubed if you added two new supports each at 1/3 of the length of the beam, or 1/27th of what it was before, or
c) (1/4) cubed if you added three new supports each at 1/4 of the length of the beam, or 1/64th of what it was before.

Certainly, because of that (Length) cubed term in the deflection formula, shortening the distance between the points where your joists are supported is the most effective way of strengthening your floor.

The problem is, adding supports under your floor that are strong enough to support the joists. How are you going to ensure that the supports you add won't simply sink into the mud under your house as a result of the load put on them, or even just because of their own weight?

I agree with your thought that reducing the span should be the most efficient way. the math is way over my head so I have no real reason behind it just kind of looked good on paper. currently I plan to build beams out of 2 - 2x6 and support with a combination of jacks and 4x4 posts on 4x8x16 concrete blocks on top of gravel - how much gravel will depend on the area of the crawl space some areas will make digging very difficult.

 

[WhatRnsdownhill]

is the floor of the crawl space cement? or just dirt?..

 

[SCJ]

All the math not withstanding, we can't be much help if we don't know how the existing beams are made. For example, if the OP actually DOES have single two bye twelves as beams, adding joists will do zip. Step one, determine what existing is. iF double or triple two bye twelve and they are currently well supported, step one figure out where the bounce is. From the pix, it LOOKS as if it is in the side with the 14' span. If so, step buy a half dozen concrete blocks and enough two bye six to make up a double beam. Locate said beam in the middle of the 14' span. Set the blocks flat side down and level and plumb at each end and equally spaced throughout, grab some short adjustable jackposts (which come as short as 16" and put one on each block. Stick a piece of four bye four on top of each and snug up to the bottom of the beam until said bounce is gone. Nail everything in place. Done. Couple of years down the road the bounce is back, give the jackposts another half turn. And remember, the OP is not looking to put a supporting wall over top or install a hot tub; he is simply looking to take the bounce out. Been there, done that, many many times. Ron

thanks for the reply, just measured and the joist are 2-2x12 nailed together. I think unless I hear some vastly easier way to do supports I will do it exactly as you described. 2-2x6 beams with supports and conc block and a bit of gravel. I thought about just using 4x4 supports but like you said I like the idea of being able to turn the jacks if needed in the future.