3 story: Floor Framing Blocking at 4' o.c. for 4' for joists parallel to wall? WHY? 2

[Materofact]

Floor Framing Blocking at 4' o.c. for 4' for joists parallel to wall? WHY?

I see this requirement on a previous plan check for a 3 story wood structure. AND i see it in this 3 story example courtesy of FEMA. But i cant figure out why. the horiz diaphragm doesnt seem to need it due to high loads in the fema example (see the entire example

https://c.ymcdn.com/sites/

http://www.nibs.org/resource/resmgr/BSSC/p751_ch11.pdf

) , so what is it for? what is the code? attached is pic.

 

[jayrod12]

It's a good way to transfer your horizontal load from your walls into the floor diaphragm. If you don't block the first joist space at a minimum, your edge joist will just want to roll over. I'm not partial to relying on drywall to prevent the horizontal movement of the edge joist.

 

[msquared48]

Agree with Jayrod. Normally done with any wood framed structure - particularly with brick or concrete walls.

Under a seismic event, the walls want to separate from the floor structure. Blocking with metal ties prevents the separation from happening.

Mike McCann, PE, SE (WA)

 

[Materofact]

well. ok. in the particular building i am working on there are are bearing walls supporting the parallel floor joist at 15' spacing. This is where blocking (by standard constrution) between the floor joists (at each supported end of the joist)exists that stops the "rolling over" of the joist (and also where metal shear clips are attached from this blocking to the top plate of shear walls below). I do not see anything special about the floor joists that are closer to the exterior wall as far as "rolling over" as opposed to those that are more removed. if this is true, why not block full length at 4 ft centers between all the joists?

thank you for you responses.. but is there a code requirement? a definitive load path requirement? the building is almost 200 feet long, blocking for 3 bays of floor joist at FOUR foot on center for 400 feet of wall. shall i also require it at 4 ft centers for the 2 interior shear walls? - thats 564 pieces of 16" 2x blocking!!!! comon!!! if there isnt a concrete definitive reason that isnt taken care of by other standard framing, i need to figure that out ?? i know about the tension requirements for concrete/masonry attachment, but this is all wood construction.

for the exterior (or interior) shear walls to "separate from the floor" the floor diaphragm nailing/plate nailing, needs to fail, which if your floor diaphgram is not loaded large enough to need blocking, means that your shear walls perpendicular to the outside walls are failing. and besides, just how much purchase you think you can get from 2x blocking attached to the top plates? clip at 500 lbs maybe?

Sorry, gentlemen, i dont get it. and im having a hard time finding how its "typical" or "standard". (could it just be for ceiling drywall attachment? nah)

thanks for any input. Eng-tips rarely fails me!

 

[jayrod12]

It's not the in plane shear, it's the out of plane shear from the wall studs below that create a knuckle without the blocking.

 

[Materofact]

indeed, yes, its an out plane resisting framing.agreed. You are saying its to take care of the BOTTOM of the RIM joist attachment to the top plates below, under wind/seismic loading of the joists below? so, for sake of math, i think what the standard nailing of rim to top plates is... 8d tonails at 6" on center. so lets say with ST increase, 150x2= 300 pounds a foot.blocking at 4 foot with a 500 lb angle clip only gives another 125 lbs a foot, unless its a flush bearing against the rim, but it should be for both out and inward out of plane shear. i just found the requirement again here

https://www.cityofpaloalto.org/civicax/filebank/documents/17662.

but its only for one bay or 2, not 3. would still like to see where it is in the code, it must be there somewhere.

 

[jayrod12]

Would you consider nails from the rim joist to the top plate a pinned or fixed connection? What about the nails/screws from the top of the rim joist to the sheathing/bottom plate above?

Most engineers would consider those pinned. Therefore when the top plate tries to move laterally, the two pins at the top and bottom of the rim joist rotate. The blocking stops that rotation.

 

[Shu Jiang PEng SE]

Hop image below give some help

 

[Materofact]

There it is!! So the rim joist, unless blocked at 4 ft centers, needs to be designed to bend between its fixed “anchored” support ends (ignoring the nailing of rim to plate eh?)So, probably relatively easy to “design” in some curcumstances, but the conservative way is to block it at 4 ft and not worry about it. Bloxking for 3 bays seems little excessive however (such as shown in that fema example in the original post).

 

[jayrod12]

I'd agree 3 bays is excessive. But yes, outside of blocking, you'd need to design the rim, or the top plates to span between sources of lateral resistance. Which although may seem easy, when it comes to wood framing you're best off expecting splices all over the place for wall plates and rim boards. So just install the blocking as it's easy on all fronts, design, construction and review.

 

[Materofact]

The fema example also shows a double rim? Perhaps that could rid the nexessity of this blocking? But then i think they imply the bloxking anyway. I think It also needed when your ext plywood shear does not nail into the rim and u need to transfer shear between floors elsewhere (never been a fan doing that even with the plwood savings (get 9 ft sheets). Or ofxourse if u got too heavy vert loads for one rim). Also makes nailing the sole plate down easier

 

[jayrod12]

I practice north of the 49, so I am not familiar with the FEMA documents or where their requirements/typical details come from. But I assume since they only come in after a disaster strikes, that they'd naturally be more conservative than typical code stipulated values.

 

[Materofact]

https://www.structuremag.org/?p=13553

 

[Materofact]

for the link above, love how they ignored how the strap for the blocking connects INTO the wall plates for 800 lbs TENSION per strap. what, gonna use bolts??? gonna use an L strap? ???? jeeez....