Floor Bounce Joist Sistering - Building & Construction - Page 2 - DIY Chatroom Home Improvement Forum

 DIY Chatroom Home Improvement Forum Floor bounce joist sistering
 Register Blogs Articles Rewards Search Today's Posts Mark Forums Read

01-04-2009, 01:03 AM   #16
Newbie

Join Date: Jan 2009
Posts: 12
Rewards Points: 10

Nester,

Here is a where I learned how to calculate the Inertia: http://darkwing.uoregon.edu/~struct..._lecture28.html
http://darkwing.uoregon.edu/~struct...xample28-4.html.

You will then have to learn how to calculate the location of the centroid. This link will explain that:
http://physics.uwstout.edu/StatStr/...ams/bdsn51a.htm

And one more good source I found for deflection is http://books.google.com/books?id=yZo...um=6&ct=result

With this theory I came up with the following:
Moment of Inertia (I) : Support
99 : 2x10
198 : Two 2x10's
159 : 2x10 with two 2x4's
153 : 2x10 with one 2x8
167 : 2x10 with two 2x6's
210 : 2x10 with one T 2x4
250 : 2x10 with one T 2x6

So you can see that using a 2x4 T will give you a 2.1 benefit and a 2x6 will give you a 2.5 benefit if you start with a 2x10.

Let me show you how I did the T with a 2x4.

First I = bh^3 / 12.
2x10, b = 1.5, h = 9.25. A = bxh = 13.88
2x4: b = 3.5, h = 1.5. A = bh = 5.25
Assuming the bottom of the 2x4 on an x,y plot is y = 0, then the centroid of the 2x4 is y = 0.75". For the 2x10, its center is 4.625, but its sitting on the 2x4 so its centroid distance is 4.625+1.5 = 6.125. Now to combined centroid you will use Yct = (A1*y1 + A2*y2)/(A1 + A2) = (13.88*6.125 + 5.25*.75) / (13.88 + 5.25) = 4.65. Now we need to determine the distance from each center to the combined centorid. The 2x4 is 4.65 - .75 = 3.90 and for the 2x10 its 6.125 - 4.65 = 1.475.

Now we calculate I = Ia + Aad^2 + Ib + Abd^2
I = 1/12(1.5)(9.25^3) + 13.88(1.475^2) + 1/12(3.5)(1.5^3) + 5.25(3.9^2) = 210

So you can see that the moment of inertia for a 2x4 added to a 2x10 takes it from 99 up to 210, or a 2.1 multiplier.

Deflection is proportional to 1/I. So if you double I, you reduce deflection by half.

Here is a link to my major problem that I have not yet solved on another forum: http://www.johnbridge.com/vbulletin/...445#post744445

Regards

Steve

 01-04-2009, 04:37 PM #18 Newbie   Join Date: Jan 2009 Posts: 12 Rewards Points: 10 Try this link: http://darkwing.uoregon.edu/~struct/...lecture28.html the address after the http:// is darkwing.uoregon.edu/~struct/courseware/461/461_lectures/461_lecture28/461_lecture28.html This would be such an easy test to setup for someone who is in the business. Take two 2x10's, seperate by 16", attach some blocking to prevent joists from rotating, add 300 lb load to the middle of it. Measure deflection. Glue and Screw two 2x4's and measure again. If this test shows that D is not reduced by around a factor of 2, then post the results and edjucate us math type people. I don't have the room for a T beam config, one thing I am considering is using a steel plate rather then a wood beem fixed to the bottom of the joist. I have been told that adding a 3/16" x 1.5" steel plate would take my "I" from 99 to 216, or a 2.1 multiplier. I need to find the math to prove this so I can show it to my town inspector. Any ideas on the math? Last edited by ijs12fly; 01-04-2009 at 09:10 PM.
01-05-2009, 06:11 AM   #20
BUILDER / REMODELING CONT

Join Date: May 2008
Location: LONG ISLAND N.Y
Posts: 1,543
Rewards Points: 1,000

Quote:
 Originally Posted by Nestor_Kelebay Steve: And yet, I am at a loss to explain why architects and engineering firms don't suggest doing this as a quick and cheap method for strengthening joists.
Most engineers would see it as, You will be Loosing Head Room by adding wood to the bottom of the beam. but you have my attention on gluing and screwing strapping to the bottom of the beam. BOB

Last edited by buletbob; 01-05-2009 at 06:21 AM.

01-21-2009, 03:41 AM   #21
Newbie

Join Date: Jan 2009
Posts: 17
Rewards Points: 10

## Sistering/strengthening TGIs

Hi

Any one have any information on how to strengthen TGIs?

Thanks Gary

 01-22-2009, 11:43 AM #22 Member   Join Date: Jul 2007 Posts: 2,045 Rewards Points: 1,910 You can also create a similar effect by glueing and nailing/screwing 1/2" or 3/4" plywood across the entire ceiling. It creates a nice box beam configuration and significantly reduces bounce. I'd suggest adding blocking (use glue to prevent squeeks), and see if that's enough before going further. By the way, if you have no windows down there you are likely violated code egress requirements for finished space.
 05-10-2009, 02:19 PM #23 Newbie   Join Date: May 2009 Posts: 26 Rewards Points: 25 With this theory I came up with the following: Moment of Inertia (I) : Support 99 : 2x10 198 : Two 2x10's 159 : 2x10 with two 2x4's 153 : 2x10 with one 2x8 167 : 2x10 with two 2x6's 210 : 2x10 with one T 2x4 250 : 2x10 with one T 2x6 So you can see that using a 2x4 T will give you a 2.1 benefit and a 2x6 will give you a 2.5 benefit if you start with a 2x10. Regards Steve[/quote] Very interesting Steve, I'm wondering if you'd calculate for my situation in order to install tile. My kitchen = Thank you for using the John Bridge Forums Deflect-O-Lator :-) For joists that are Unknown wood, but in good condition, 7.25 inches tall, 1.5 inches wide, 12 inches on center, and 12 feet long between supports, the deflection calculated is 0.468 inches. This translates to a deflection of L / 307. Since the maximum deflection for tile is L / 360, and for natural stone is L / 720, your floor is rated for Sheet Vinyl or wood. My bath = For joists that are Unknown wood, but in good condition, 7.25 inches tall, 1.5 inches wide, 16 inches on center, and 11 feet long between supports, the deflection calculated is 0.457 inches. This translates to a deflection of L / 289. Since the maximum deflection for tile is L / 360, and for natural stone is L / 720, your floor is rated for Sheet Vinyl or wood. My current flooring is 3/4" thick. Making the "T" is a matter of installing the 2X4 flat against the bottom of my existing joist with glue and screws? Thanks, Tom
05-10-2009, 02:50 PM   #25
Newbie

Join Date: May 2009
Posts: 26
Rewards Points: 25

I'm not so much concerned about the head clearance as it is in the garage side of a walkout basement and there is the additional duct work that falls much lower. The good news to my situation is that the joists run parallel with the length of the upstairs bathroom floor. Being a small bathroom, I should only require sistering a couple of the joists. There are also double joists already under each wall of the bathroom.

 Thread Tools Display Modes Linear Mode

 Posting Rules You may not post new threads You may not post replies You may not post attachments You may not edit your posts BB code is On Smilies are On [IMG] code is On HTML code is OffTrackbacks are Off Pingbacks are Off Refbacks are Off Forum Rules

 Similar Threads Thread Thread Starter Forum Replies Last Post norriselijah@yahoo.com Building & Construction 9 09-24-2008 10:18 PM tommyt Building & Construction 2 08-11-2008 08:38 PM teachtech Building & Construction 8 02-19-2008 10:35 PM curls00 Carpentry 3 07-20-2007 10:56 PM peeks Building & Construction 1 07-06-2005 03:02 PM

Top of Page | View New Posts