Sistering joists with smaller dimenisional members 3

[superdeepsix]

To all structural engineers:

Can you sister an existing joist using smaller dimensional lumber?

For instance I have a 12 ft span with 2x10s Douglas Fir spaced 16". O.C. I want to add 2x8 Douglas Fir sisters across entire span, to each bearing point(mudsill to center I beam), as I can easily thread 2x8s into the joist bays without removing a ton of electrical and plumbing. Note I can only sister one side of 2x10 as the joists across the floors overlap one another on the center I beam.

If you are able to sister using smaller dimensional lumber, does the member attach at the bottom or top of existing joist? I assume bottom as that part of joist is in Tension and the wood is more suspectible to cracking due to the bending stress applied by the floor above.

And lastly 16d nails, carriage bolts, screws, or a combination of two?

 

[msquared48]

Yes, bottoms flush, 3" (10d) nails or screws. Use 10d otherwise you have to clinch the ends.

Mike McCann
MMC Engineering

 

[superdeepsix]

Thanks for the quick reply.

i have one more question. i noted that I cannot sister opposing side of 2x10 because they overlapped the 2x10s that span the floors. However, it is possible to sister a 2x8 on the opposing side of the 2x10, with one end bearing on the mudsill and the other end joined to the end of a 2x10 by an 8ft 2x8 scab (4 feet overlapping 2x10 and 2x8 sister)? I figued I can secure it with 8 1/2"x4" carriage bolts (4 per scab side) and adhesive. here's a pic.

Thanks in advance

<----- to mudsill [I-BEAM]
[ ]
[ ]
________________________________________________________[______]
2x10 ___________________________________________________[______]________________________________
2x8 sister __________________________________________ [______]________________________________ 2x10
_________________[______]_________________ 2x8 scab
[ ]
[ ]

 

[SteelPE]

First of all, I have no idea what that "sketch" is suppose to be above.

Second, what are you trying to achieve? Are you trying to increase the strength of the floor, are you trying to limit deflection. If you are having a hard time trying to get 2-2x8's in, because your joists rest on top of the center beam of the house and they overlap a few feet on each side, would 1 7-1/4" LVL be sufficient?

 

[mijowe]

If shear works on the existing it is possible to sister the joist and ave it be sort of both ends.

If you go with new 2x10s, or push the 2x8s tight to the underside of the floor and block the ends for bearing you can simply double the joist and would nto need to "sister" them.

 

[superdeepsix]

Sorry about that. I tried to post pic and I failed to go so. I tried the txt trick but when it posted spaces dropped out leaving an unintelligible mess.

 

[superdeepsix]

As for adding joists 8" OC, I thought about that but am concerned doing so because the area in question resides under a finished kitchen with granite countertops. I fear the action of blocking the 2x8s will add stress to the floor above.

 

[superdeepsix]

finally here's the pic

 

[SteelPE]

Again, why are you doing this?

I don't see how blocking would add stress to your floor..... but I have been wrong about things before. If you want to sister the existing joists then why don't you add the new 2x8 to the left of the 2x10 you are sistering? That way you can bear on the steel beam and on the "Mudsill" at the same time.

 

[superdeepsix]

To be honest the 2x10 in question is cracked from the bottom up. An electrician drilled a fairly large hole in the bottom third of 2x10 and when the kitchen was finished the bending stress was enough to crack at the point. I simply want to sandwich the 2x10 and stabilize it.

 

[racookpe1978]

One of those extemporaneous, impromtu electro-structural engineering mods, eh?

Would a pair of 2x4x1/4 angle irons, one leg of each angle iron on each side of the busted 2x10 wood piece, through-bolted under the beam be easier to work with?

You would still have to use a jacking beam pushing up on the assembly before bolting up and drilling, but you'd have to do that in every case.

 

[SteelPE]

Now I understand the problem.

If it was my project I would do everything I could to get bearing on both ends. If that meant using 2-2x8 then I would space them in such a way to get proper bearing and if necessary block them back to the existing joist. I would also investigate using a single LVL on one side of the existing joist. They tend to be a bit stronger than conventional lumber you get at your local yard.

In each instance the member could be pushed tight to the underside of the existing floor. Either nail off the LVL to the existing or block back to the existing for the 2x8. A block of wood could be installed at each end to attain proper bearing for each instance.

 

[superdeepsix]

Here's a more complete picture Maybe a 10 ft scab will hold?

 

[superdeepsix]

sorry, here's a better pic.

 

[ztengguy]

To be honest, maybe a photo instead of a sketch would be better.

 

[madmantrapper]

A photo would be nice but I think I understand what you are after. How are you going the handle the hole where the wire is? Are you going to have to pull the wire and redrill the hole? I have fixed situations like this before with 1/4 inch steel plate along the bottom vertically, 4 inches tall. Bolted through, might depend on how bad the original joist is.

 

[superdeepsix]

I am re routing the electrical so it will not need to re thread into damaged joist.

Since I am able to fully sister one end between bearing points, and sister the other side (albeit with only one end on a bearing point/mud sill), I figured I scab across I beam to gain bearing point access (literally inches away). I understand they may be significant scissoring force where sister and scab overlapp since it is not one solid piece. However glues work great in shear and figured a long enough scab across sister (with plenty of fasteners) would work.

 

[superdeepsix]

Looking at some of the suggestions, sistering a 2x8 LVL (point to point) on one side will help.

As for other side, I may just scab 2/3s of damaged area (8 ft scab).

In addition, I may add new joists on either side of the damaged joist (8" o.c.).

Only issue I have with this method is how do I block the joists so they bear load but do not end up cracking tiles above?

 

[racookpe1978]

See sketch.

 

[superdeepsix]

Does the angle iron require support? If so then I imagine I need to jack the damaged joist 3//8" off the bearing point?