[mske390]

If you cannot sister a joist the entire length of the existing joist will it still provide any benefit? Let's say (for example) you have 2x6 joists spanning 10' sitting on wall framing on each end with an obstruction down the middle so cannot sister another 2x6 the entire length but could probably get on 6' long installed in the middle. Attached this securely throughout the 6' span will this provide any benefit?

 

[Bondo]

Let's say (for example) you have 2x6 joists spanning 10' sitting on wall framing on each end with an obstruction down the middle so cannot sister another 2x6 the entire length but could probably get on 6' long installed in the middle.

Ayuh,.... How ya gonna sister the center when that's where the obstruction is,..??

I doubt it would help much anyways,.....

Just what's the problem yer tryin' to fix anyways,..??

 

[joecaption]

Pictures would be great so we can see what your seeing.
Sounds like you may be better off with some real footers and a doubled up beam down the center of the span.
A sistered joist in two pieces is useless.

 

[mske390]

"Let's say (for example)"...example so not an actual problem seeking opinions if a sistered beam installed in middle not entire length will provide any benefit

 

[jlhaslip]

Fine Home Building magazine has an article which describes a method for using metal straps instead of sistering joists. Apparently it is effective.
Unfortunately, I do not have a link.

 

[Daniel Holzman]

The purpose of sistering a joist is to increase the moment of inertia of the joist at the location of maximum stress, which is usually (not always) in the middle. So sistering a joist only in the middle is a good solution if the problem you are trying to solve is for example excess deflection at the middle of the joist, or a joist which is failing near the middle, but not failing on the ends. This is actually pretty common, you can look at an overspanned joist and often the joist is fine near the walls where it is supported, but not fine in the middle.

The key to making a partial sister work is to make sure the nailing or bolt pattern transfers 100% of the required horizontal shear developed by the load between the two joists. Since hardly any carpenter understands how to compute horizontal shear, the typical solution is to use 10d or 16d nails 6 inches OC, staggered top and bottom.

A full sister provides more strength, but interesting not much more than sistering about half the joist, centered over the middle, for most loading conditions. So a partial sister is OK if you can't do the whole joist, just make sure you use a good nailing pattern.

 

[bob22]

Daniel,
I assume that the original joist is jacked up to a level plane prior to nailing off the sistering joice? Otherwise, I assume it would continue to be in a stressed condition?

 

[Mingledtrash]

Since hardly any carpenter understands how to compute horizontal shear, the typical solution is to use 10d or 16d nails 6 inches OC, staggered top and bottom.

Only a few of us even understand the concept of shear...:surprise::vs_lol:

 

[mske390]

Thanks Daniel, I vaguely remember from my structural courses that it would help in the middle but that was like 20 years ago. I vaguely remember the load profile being the greatest in the middle of the beam when supported on outer ends too and since a floor sits on top of the beam in my example the load would be distributed to the add on joist. Just figured I'd check with others who are current with it.

I appreciate your help.

 

[Daniel Holzman]

It is not necessary to jack up the first joist to get the benefit of sistering. The first joist is stressed because of the load on it, hence takes a deflected shape (all stressed structural elements bend due to the stress). If you jack the first joist back to level before you sister, all sorts of complicated things happen.

First off, the first joist has probably taken a permanent deflection, since wood is an inelastic material which takes a permanent set after it has been exposed to load for a year or more. You can see this in any old house, the floors are out of level, because the joists have permanently deflected due to the load. Even if you remove the load, the joists will NOT spring back to their original shape. If you force them back to their original shape by jacking, you are effectively prestressing the joists in the opposite sense of their deflection, so you may end up in a situation where the top of the joist is no longer in compression, it could be in tension while the jack is in place.

Then when you nail on a sister, you are locking in the reverse stress in the original joist, and you can get a wide range of stress results when you release the jack and reload the joist pair. Very complex.

If you do NOT jack the joist back up but simply nail on the sister, the two joists will share the load. They will share the load approximately evenly as long as the two joists are fastened together sufficiently well that the horizontal shear which develops when the two joists are loaded gets shared. The mechanism for horizontal shear transfer is the nails, bolts, or glue that holds the two together.

If you forget to nail the two joists together, whichever one is higher will carry all the load, until it deflects far enough so the second joist begins to pick up load. This is a bad plan, since the first joist could conceivably crack before the second joist picks up load, then the second joist can crack since the first joist is no longer contributing much strength. This type of failure has happened in industrial and commercial projects where two structural elements were not properly connected, and got loaded before the fastening was complete. Bad news, best avoided.

 

[bob22]

Thanks Daniel for an erudite and clear explanation!

 

[ChuckF.]

Fine Home Building magazine has an article which describes a method for using metal straps instead of sistering joists. Apparently it is effective.
Unfortunately, I do not have a link.

https://m.finehomebuilding.com/PDF/021184090.pdf

This is interesting, -not at all the kind of strap I expected.