# Strutting between floor joists



## tony.g (Apr 15, 2012)

This is not a specific problem, more a request for opinions, so hope its the right forum.
There is a heated debate at the moment on another (non-US) forum about the merits of herring-bone strutting v. solid blocking between floor joists (solid timber joists, not engineered joists). There are also differences on whether herringbone should be in timber or metal. As US domestic construction is essentially timber frame, do any framers/carpenters/DIYers out there have any views on this, and on any benefits/disadvantages of strutting?
Thanks.


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## Daniel Holzman (Mar 10, 2009)

Personally I like the Deodata version of Superstrut as featured on Grand Theft Auto. There are those who prefer the Kashmere Stage Band version. Of course, you could get the Pepe release, or the John Tropea cut. I can certainly understand why this topic would generate heated debate.


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## cortell (Nov 27, 2010)

I often find that when there's a heated debate about something, it tends to be a religious war with no clear winner; it mostly comes down to preference. 

The only thing I would add to such a debate is the surprising position that blocking should be avoided altogether--an opinion I came across last year in a Fine Homebuilding article titled _Framing and Sheathing Floors_ (Rick Arnold and Mike Gruterin). You can pretty much find the entire justification by searching for "just say no to bridging" (include the double-quotes) in http://google.com/books


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## Gary in WA (Mar 11, 2009)

Our term (around here) now is "bridging", years ago, used to be "strut'. The solid blocking doesn't transfer the lateral force as well as the 1x4 bridging which; allows the thrust to act parallel to the axis of the strut: pp. 749, click on the words "clear search" on top right corner of: http://books.google.com/books?id=DW...result&ct=result&resnum=4#v=onepage&q&f=false if it doesn't come up right away.

http://publicecodes.citation.com/icod/irc/2009/icod_irc_2009_5_sec002_par026.htm


Gary


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## cortell (Nov 27, 2010)

GBR in WA said:


> Our term (around here) now is "bridging", years ago, used to be "strut'. The solid blocking doesn't transfer the lateral force as well as the 1x4 bridging which; allows the thrust to act parallel to the axis of the strut


While that makes sense to me from a physics point of view, I'm doubtful it's ever been proven to make a difference in practice. If solid blocking is continuous (every joist cavity is blocked along the same line, or a slightly staggered pattern to accommodate end nailing), I suspect that method is as effective in transferring loads as cross bridging in unexceptional situations.


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## woody4249 (May 4, 2012)

From my recollection, granted this was a number of years ago......."Herringbone strutting" was favored over blocking as it allowed airflow between joists. As an apprentice, I sometimes wondered if it was also a test of my ability to cut bevels by hand and hammer the buggers into place balanced on a skinny step ladder.
Once you've done this to a 2000 sq ft floor...........you get pretty good at it.


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## framer52 (Jul 17, 2009)

As a framer that has had a hand in several house builds, I realize there are different views on whether these do any good at all.

I prefer to instal the cross bracing and detest the metal bridging as it requires hand nailing.

I became adept over the years at installing without using a step ladder....+


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## tony.g (Apr 15, 2012)

Many thanks to all for the replies on this and also the references.
My own thoughts were that herringbone strutting (sorry, 'bridging') is more effective at stopping the joists twisting. 
Attached is a sketch showing my reasoning; as the joist shrinks when the heating is on, it will reduce in depth more than in thickness, shown by the dotted line on the sketch (exaggerated). The effect would be that the angle between the strut and joist will increase slightly, resulting in the strut being forced into a smaller gap, thus tightening up against the joist. 
Oh, and a special thanks to Mr D.H., whose erudition shows that his 30-plus years in civil engineering have stood him in good stead when it comes to strutting! Moisseiff himself would have been impressed!! (though possibly not after his last job).


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## cortell (Nov 27, 2010)

tony.g said:


> My own thoughts were that herringbone strutting (sorry, 'bridging') is more effective at stopping the joists twisting.


Assuming that progression is true, how does it lead to the conclusion that it is _more effective_ than solid blocking? I wouldn't expect shrinkage along the length of the block, or the width of a joist to amount to anything that would allow any perceivable twisting of the joists. Plus solid blocking provides resistance along the entire depth of the joist, preventing it from potentially cupping, which cross bridging does not.

I'm not saying one form of bridging is better than another. I believe there are pros and cons to each.


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## tony.g (Apr 15, 2012)

Interesting point about the joist cupping - I hadn't thought of that.

Attached is a diagram from an old book of mine (1916) on building construction which suggests putting a 3/4"rod through the centreline of all the joists, with threaded ends for tightening it up. The book doesn't say how to get the thing in, though!


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## woody4249 (May 4, 2012)

tony.g said:


> Interesting point about the joist cupping - I hadn't thought of that.
> 
> Attached is a diagram from an old book of mine (1916) on building construction which suggests putting a 3/4"rod through the centreline of all the joists, with threaded ends for tightening it up. The book doesn't say how to get the thing in, though!


The only time I have seen this done with a rod is to tie the outsde walls together to prevent further bowing of an unstable wall. We see them here in the U.SA. as a cast iron "star". 
In England I remember they were in the shape of a slow "S:"
From my experience of installing second floor joists back as an apprentice, they were always done when the exterior brickwork reached second floor height. The joists ends were placed on top of the inner wall and then bricked into place.
It would have been a young fool like me who perched himself on a step ladder with a brace and bit in his hands to bore the 3/4" holes. :wink:

Mike


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## tony.g (Apr 15, 2012)

Here are simple cross braces put in years ago to bind opposing walls. The one on the left, and another just behind the viewer, are higher than the 2 in the foreground. The rods for those run an inch above the floor, so they built a new floor above it using 3x2 on top of the old floor. Reduced the headroom a bit but must have saved getting the auger out!


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## Gary in WA (Mar 11, 2009)

cortell got me thinking....... I hadn't done enough research on that subject- in depth, yet. 

Did a lot more: *less *deflection with solid blocking (15%) than with cross-bridging (9%); http://www.ewpa.com/Archive/2004/jun/Paper_278.pdf

10 times less deflection with a scab (45% reduction) at the bearing than mid-bridging (4% reduction); http://www.awc.org/pdf/TR05.pdf

Gary


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## cortell (Nov 27, 2010)

GBR in WA said:


> cortell got me thinking....... I hadn't done enough research on that subject- in depth, yet.
> 
> Did a lot more: *less *deflection with solid blocking (15%) than with cross-bridging (9%); http://www.ewpa.com/Archive/2004/jun/Paper_278.pdf
> 
> ...


Thanks, Gary. Great stuff. Surprising that the first study used such a small deflection range in the test. With a max deflection of 1.3 mm, I'd be concerned with the margin of error in the tool they used to measure the deflection. Not sure what the precision of a dial detector is, but I assume they took that into consideration.

Also surprising is the effect of the 1x4 strapping along the bottom of the joists. I wouldn't expect that to do much in transferring load to other joists. Perhaps I'm misunderstanding that part.


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## loneframer (Mar 27, 2009)

Here's a few pics I took while goofing around with an X-bridging model. In my model, the field joists are supported only by the bridging itself, with no end bearing. The bridging happened to be some wet firring strips I had laying outside my garage. They were stapled in with 7/16 crown staples. The failure was due to the staple legs being pulled through the wet wood when a point load was applied to the model. I typically use 1x4 pine for bridging, which certainly would've produced better results.


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## loneframer (Mar 27, 2009)

the staples


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## cortell (Nov 27, 2010)

loneframer said:


> Here's a few pics I took while goofing around with an X-bridging model.


I wonder if this wasn't really a test of the OSB panel's strength perpendicular to the plane. :wink:


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## Gary in WA (Mar 11, 2009)

cortell, the strapping was 1x8x8' nailed to both, butted-over-the-center-beam, joists.

Gary


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## loneframer (Mar 27, 2009)

cortell said:


> I wonder if this wasn't really a test of the OSB panel's strength perpendicular to the plane. :wink:


OSB didn't lend much strength to the assembly.:laughing:

I built this model in about an hour out of scraps laying around my house to illustrate the concept of load sharing.

Bridging forces all the joists to work as a system, distributing concentrated loads evenly across the floor assembly.


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## cortell (Nov 27, 2010)

loneframer said:


> OSB didn't lend much strength to the assembly.:laughing:


I know, loneframer. It was just my smart-ass way of saying that it wasn't going to take a whole lot for the assembly to come undone with 1x4 furring strips fastened with staples. And once the first set of strips came undone, it became all about what the OSB could hold. 

It's an interesting experiment for sure. I wonder how well it would have performed with 2x3" bridging fastened with 2 8d common nails on each end, as per code. And of course, the same test with solid blocking.


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## cortell (Nov 27, 2010)

GBR in WA said:


> cortell, the strapping was 1x8x8' nailed to both, butted-over-the-center-beam, joists.
> Gary


Thanks. I finally made sense of it. I naively thought strapping was meant to prevent joist rotation, and contributed nothing to load distribution. After more thought, I realized the strapping, because it is fastened to each joist, actually acts to pull down adjacent joists when a load is exerted on any given joist, thus distributing the load. (As a side note, it seems to me the strapping's effectiveness in this respect is mostly limited by the pull out strength of the nails.) Like I said before, I learn something new everyday on this site.


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## tony.g (Apr 15, 2012)

cortell said:


> it seems to me the strapping's effectiveness in this respect is mostly limited by the pull out strength of the nails.)


Yes, loneframer's model seems to show that the failure mode is the tension in the bottom fixings; if one fails, the whole assembly will fail.


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## loneframer (Mar 27, 2009)

Let's take into consideration that the joists would normally have end bearing, so the bridging would only be distributing the load from deflection of the joist under concentrated load. My example was an extreme condition.

Under normal conditions, the bridging could have been installed much tighter.

I've gone back into homes I've framed after the joists have dried and shrunk, to find that the compression on the bridging actually forced a slight S-bend in the bottom of the member.

IMO, staples that are properly placed do a better job on installation because nails tend to split the bridging, rendering it useless for the most part.

I've framed many homes with 1x3 or 1x4 pine bridging, fastened with staples and I promise you, the staples hold incredibly well.


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## cortell (Nov 27, 2010)

loneframer said:


> Let's take into consideration that the joists would normally have end bearing, so the bridging would only be distributing the load from deflection of the joist under concentrated load. My example was an extreme condition.
> 
> Under normal conditions, the bridging could have been installed much tighter.
> 
> ...


I don't mean to question your methods, but I find it hard to believe a 7/16" staple vs a 2-1/2" common nail would perform comparably. But, since I haven't tried it, I can't say for sure. Just saying I'm skeptical.

Now, there's no doubt that having a nail split the wood significantly reduces the effectiveness of the connection.

For these reasons, and others, I generally prefer solid blocking. Now, I might change my mind once utility lines enter the picture.


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## Gary in WA (Mar 11, 2009)

Why is there no bridging/blocking at the joist ends on the bearing wall as per Code? Or do you not have inspections?

Gary


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## cortell (Nov 27, 2010)

(message removed) Never mind :whistling2:


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## kwikfishron (Mar 11, 2010)

GBR in WA said:


> Why is there no bridging/blocking at the joist ends on the bearing wall as per Code? Or do you not have inspections?
> 
> Gary


It's on the punch list Gary. :laughing:


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## Gary in WA (Mar 11, 2009)

Practically impossible to install bridging now, need to use blocking. I figured since they installed bridging at -8' as per Code (IBC) even though it wasn't required when both edges are kept in-line (drywall/decking), yet it is missing at the required support (at joist end); http://publicecodes.citation.com/icod/ibc/2009/icod_ibc_2009_23_sec008_par022.htm

Or under IRC: http://publicecodes.citation.com/icod/irc/2009/icod_irc_2009_5_sec002_par025.htm

Either way, going to be a bear to install unless the solid blocking is sloppy-end gaps.....

Gary


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## loneframer (Mar 27, 2009)

GBR in WA said:


> Why is there no bridging/blocking at the joist ends on the bearing wall as per Code? Or do you not have inspections?
> 
> Gary


Framing houses for almost 30 years in South Jersey area. Never have I been required to install bridging or blocking at interior bearing, with the exception of some I-joist situations. Coastal areas have the most stringent inspections in the state. Framed along the coast for 20 years.


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## loneframer (Mar 27, 2009)

cortell said:


> I don't mean to question your methods, but I find it hard to believe a 7/16" staple vs a 2-1/2" common nail would perform comparably. But, since I haven't tried it, I can't say for sure. Just saying I'm skeptical.
> 
> Now, there's no doubt that having a nail split the wood significantly reduces the effectiveness of the connection.
> 
> For these reasons, and others, I generally prefer solid blocking. Now, I might change my mind once utility lines enter the picture.


A 7/16" crown staple with a 2" adhesive coated leg holds better than any 2.5" common I've ever driven. Refer to the pic. The crown of the staple pulled through the firring strip, not out of the joist.

The builder I framed most of my homes for questioned the integrity of the staples once too....until he had to remove a couple of bridging pieces. 4 staples will hold incredibly well, with zero splitting when fastening within 1" of the end of the bridging.

I always fastened the bottoms as soon as the decking was installed and joists were straightened. As the joists shrink and stabilize, the bridging will tighten up even further

I have no problem with my methods being brought into question. I've defended them at the question of builders and inspectors alike. They often admitted to learning something afterwards.:thumbsup:


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## cortell (Nov 27, 2010)

loneframer said:


> I have no problem with my methods being brought into question. I've defended them at the question of builders and inspectors alike. They often admitted to learning something afterwards.:thumbsup:


And I may have learned something here, too. I take you for your word that those connections are strong. It's apparent you've been doing this for some time, and are passionate about it (hell--I don't know of anyone that experiments with framing in their garage). In this particular case (using staples for cross bridging), I'll avoid it simply because it's outside prescriptive code. But I appreciate you sharing your trick with us.


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## Gary in WA (Mar 11, 2009)

Your State is under the IBC. The joists are required to be laterally supported at each bearing with solid blocking only: http://publicecodes.citation.com/icod/ibc/2006f2/icod_ibc_2006f2_23_sec008_par018.htm
Checking to see if your State omitted that rule.... Ours follows and additionally requires intermediate bearing walls solid blocked.

Gary


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## Gary in WA (Mar 11, 2009)

Nope, NJ follows that IBC 2308.8.2 rule without omissions as far as I see: http://www2.iccsafe.org/states/newjersey/NJ_Building/PDFs/NJ_Bldg_Chapter23.pdf Actually, you are required to block over intermediate walls also.

Interesting...... the blocking is to prevent rotation of the joists (mid-span),* and *over all bearing supports unless other means are used, yet no one required this for that long? Do you have local City/County laws that omit your State Codes?

Gary


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## loneframer (Mar 27, 2009)

GBR in WA said:


> Nope, NJ follows that IBC 2308.8.2 rule without omissions as far as I see: http://www2.iccsafe.org/states/newjersey/NJ_Building/PDFs/NJ_Bldg_Chapter23.pdf Actually, you are required to block over intermediate walls also.
> 
> Interesting...... the blocking is to prevent rotation of the joists (mid-span),* and *over all bearing supports unless other means are used, yet no one required this for that long? Do you have local City/County laws that omit your State Codes?
> 
> Gary


I've been employed or an employer in new construction since 1985 and worked in the remodeling/renovation/repair business since I was a child with my father. Over the years, I have worked in dozens of municipalities.

Not once have I ever seen solid blocking required on an interior bearing wall, unless specified on an I-joist plan, or if a fire break is incorporated in a side by side multi-family building.

As a note, I've never seen a building with issues due to the lack of solid blocking on interior bearing, although, I've seen some half-azzed framing where little care is taken to straighten joists while decking. Those buildings are still standing as well.


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## loneframer (Mar 27, 2009)

.....


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