I joists parallel to the exterior wall 5

[DoubleStud]

Folks, help me understand what exactly the blocking and the nailing are resisting. Thanks!

 

[XR250]

Out-of-plane wind on the lower wall

 

[Celt83]

As detailed nothing because the joist has no fastener information at the exterior wall.

 

[DoubleStud]

XR250, that makes a lot of sense. For some reason someone told me a while back it was half of upper wall height and half of lower wall height. It is just half of lower wall height out of plane wind, correct?

 

[TheDW]

That is the total trib height to the diaphragm at that level which is correct.

The upper wall is directly attached to the diaphragm.

The lower wall would have no way to transfer wind load to the diaphragm without something like the blocking you're showing.

 

[dauwerda]

Don't worry, none of the architects around here know what it's for either, so they don't put it in. It's not required in the IRC, so many of the houses built (at least in this area) don't get it. Thankfully my builder had no issue putting it in per my request.

 

[XR250]

Usually, the combination of sheetrock and the wall plates spanning between perp.interior walls work fine to resist the loads without the need for blocking.

 

[DoubleStud]

That is the total trib height to the diaphragm at that level which is correct.

This does not match with what XR250 said. upper floor forces will go to the shear walls? I think this blocking is only resisting wind from out of plane wind from lower wall? (half of wall height below).

 

[phamENG]

The fastening of the blocking to the wall below takes out of plane from the lower wall and delivers it to the diaphragm through the blocking. The upper wall is fastened directly to the diaphragm, so those out of plane loads are delivered directly. But the presence of the blocking doesn't hurt - it's probably a slightly stiffer load path and does the real work when its installed.

There's also an argument to be made that it helps to support gravity loads from above. A lot of people don't like the idea of the wall above being supported on nothing but 1.5" of parallel joist between walls, especially if it's a bearing wall for the roof or another floor above.

 

[jerseyshore]

The blocking is specifically for the lower wall since it has a different attachment than the upper wall. As TheDaywalker said, the upper wall is directly attached to the floor diaphragm so the blocking is not needed to support the out of plane wind forces on that section of wall.

The total trib is the upper half of the lower wall + the lower half of the upper wall, however, the flow of forces is different.

This type of perpendicular blocking is rarely done for wood walls. Maybe for tall walls. It's most important at the top of a basement wall.

 

[TheDW]

This type of perpendicular blocking is rarely done for wood walls

We put it on all of our plans (in a hurricane zone) but it is missed pretty often.

 

[TheDW]

Upper and lower forces both go to shear walls - the blocking is just providing a load path for forces from the bottom

 

[DoubleStud]

It's most important at the top of a basement wall.

To help brace top of wall? This is a topic that me and my coworkers talk about all of the time. How much load per ft can we expect on a typical floor framing to be able to resist top of basement wall? Do you design the basement wall as pinned at the floor framing? We tend to just put blocking every 2 ft with about 5-8d nails. So that is only about 200 lb per ft of bracing.

 

[jerseyshore]

For typical residential construction with a basement I don't think anyone really checks the floor framing resistance at the top of the wall (where the framing runs perpendicular). Maybe for really tall basements that's more important, but failures in that regard are very uncommon.

This perpendicular blocking that supports the top of those basement walls where the floor framing runs parallel is definitely more important because those failures are more common. I usually like to see at least 2-3 bays of perpendicular blocking for residential basements.

Then you can feel confident that all of your walls are true pinned-pinned walls braced at the top and not cantilevers.

But back to the original topic, it's still very useful to have this blocking at the wood/wood levels, but less chance of a failure so it is rarely installed even when specified as TDW mentioned.

 

[dhengr]

DoubleStud:
This type of blocking detailing is really important where you will have a ext. deck, with ledger, etc., which has to be tied back into the int. floor diaphragm, for the lateral support of the deck. As Jerseyshore suggests, this blocking system should go back several joist bays into the bldg. to get the concentrated reactions form the deck lateral loading up into the diaphragm. And, nailing btwn. the blocking and the floor diaphragm is all important in doing this.

 

[ARPASEVAN]

I design those blockings to take the out of plane wind and seismic loads on the walls. (Spreadsheet not updated to ASCE 7-16)

 

[Celt83]

But back to the original topic, it's still very useful to have this blocking at the wood/wood levels, but less chance of a failure so it is rarely installed even when specified as TDW mentioned.

This blocking is critical in any location where the floor framing is parallel to the wall. If the blocking, blocking fastening to the floor sheathing, and just as import the fastening of the lower wall top plates to the blocking are not present then there is no load path for the wind pressure on the lower wall panel into the floor diaphragm which makes the system unstable.

 

[XR250]

This blocking is critical in any location where the floor framing is parallel to the wall. If the blocking, blocking fastening to the floor sheathing, and just as import the fastening of the lower wall top plates to the blocking are not present then there is no load path for the wind pressure on the lower wall panel into the floor diaphragm which makes the system unstable.

The sheetrock typically takes care of this. Similar to how a gable end is braced.

 

[Celt83]

XR250:
I disagree the wall gyp typically stops at the wall top plate, the ceiling gyp if connected through a nail block used for this purpose would be loading the bottoms of the I-Joist placing them in torsion which is unreliable.

 

[jayrod12]

From a mechanics standpoint, Celt is right. The blocking "has" to be there.

However, my actual experience on site is more like the others, it's rarely installed. And since the NBCC Part 9 provisions (our IRC) don't mandate it, it's often left omitted. I still show it on any project I have. And when the I-joists exceed the 12" deep stage I hold fast on requiring it. However most of my buildings using I-joists that deep are governed by the commercial code, in which case I can withhold certification if they don't put it in. I don't have that same ace up my sleeve for residential where I practice.