Rienforcing floor joists......

[Thezapper]

Hello everyone i have a question for you guys. My friend has a 16" x 16" addition that was built in 1989 apparently it was code to span 16 feet with a 2x8 on 12" centres. The above room is a kitchen with ceramic floor + scratch coat on top of 1/2" plywood. theres a kitchen island right in the center of the room so theres quite abit of load on the floor. The problem I'm having is that the floor is so bouncy! Standing under the said floor and have a 240 lb man walk up there while you watch from under is crazy, you see the joists and floor deflect about 1/2". The deflection on theses 2x8 are rediculas, and It has created a bowl effect in the center and has a obvious sag of about 1" in the center of the room.

So the question, how can I stiffen up this structure without demolishing the whole thing and staying within the height of 7" 1/4 inches ? I was thinking of sistering up another 2x8x16 SPF glued and screwed and lots of blocking. But then hearing opinions from other friends one said to get a 4x8 sheet of 3/4 plywood and cut it to 7" 1/4 and glue and screw the plywood to the beam. The plywood will have 2 laminations so it would be the same thickness 1 1/2" thick.

So what would you guys do? What option will be the strongest? Or any other ideas are welcomed. The space under the floor cannot have a wall for example in the middle , or a beam in the middle as head space is a problem and whatever has to be done must stay flush to the 2x8 floor joists. Just trying to help a good friend of mine.

 

[bczygan]

Cut all the joists in the middle and install a 7 1/4" steel beam of adequate strength and weight. The beam will be installed in the same plane as the existing joists and have nailing plates on each side for joist hangers.

If necessary, use 2 beams at the 1/3 points.

Install columns under the ends of the beams, either as pilasters or within the walls.

Alternatively, use steel plates to turn the joists into flitch plate beams. May only need to do a few.

There are lots of possible solutions.
Calculations need to be done to determine the most cost effective solution for the degree of modification desired.

Bill

 

[kbs2244]

What is underneath?
I would think of a beam from wall to wall, crosswise, under the existing joists with a post in the middle.
That would give only an 8 foot span.

 

[DTE]

What is underneath?
I would think of a beam from wall to wall, crosswise, under the existing joists with a post in the middle.
That would give only an 8 foot span.

That is what we done under our living room, took three 2 x 12's and bolted them together for a beam, used some support post that are threaded for adjustment and cut to length . Sat those on a concrete pad and ease the floor up to level and fastened the beam in place with some hurricane straps and toe nail it in. Got rid of all the bounce.

 

[Leaflessshadetree]

What about putting a Post in the basement below the center of the room? 12" c toc doesn't allow much room to work. Sistering more 2x8s and adding braces between teh joists should help.
Whatever you do, jack up the floor to take out the sag first. Be prepared for possible cracked tiles or grout.

 

[Thezapper]

Thanks for the suggestions I'll keep you guys posted on what happened

 

[srmofo]

Cross bracing/blocking is another cheap alternative although not as effective.

I laid .25" OSB and construction adhesive before I did my laminate floors (to level/smooth the old floors) and it took a considerable amount of bounce out of my floors. Before the walls would shake just walking through the house

 

[scotw]

Here is a calculator that the ceramic tile people use to determine approximate floor deflection:

http://www.johnbridge.com/vbulletin/deflecto.pl


Your floor has a deflection of around L/185, depending on the type of wood used in the joists. You need at least L/360 for ceramic tile. You can do this by sistering every joist to get L/371. If possible, reduce the span by having a support wall as others have mentioned.