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Old 05-06-2012, 03:36 PM   #1
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Strutting between floor joists


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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Old 05-06-2012, 04:18 PM   #2
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Strutting between floor joists


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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Old 05-06-2012, 05:13 PM   #3
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Strutting between floor joists


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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Old 05-06-2012, 05:27 PM   #4
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Strutting between floor joists


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=DWs...page&q&f=false if it doesn't come up right away.

http://publicecodes.citation.com/ico...002_par026.htm


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Old 05-06-2012, 05:53 PM   #5
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Strutting between floor joists


Quote:
Originally Posted by GBR in WA View Post
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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Old 05-07-2012, 09:57 AM   #6
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Strutting between floor joists


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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Old 05-07-2012, 10:14 AM   #7
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Strutting between floor joists


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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Old 05-08-2012, 04:59 PM   #8
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Strutting between floor joists


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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Old 05-08-2012, 05:17 PM   #9
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Strutting between floor joists


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Originally Posted by tony.g View Post
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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Old 05-09-2012, 01:48 PM   #10
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Strutting between floor joists


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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Old 05-09-2012, 02:13 PM   #11
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Strutting between floor joists


Quote:
Originally Posted by tony.g View Post
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.

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Old 05-09-2012, 02:52 PM   #12
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Strutting between floor joists


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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Old 05-10-2012, 12:03 AM   #13
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Strutting between floor joists


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

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Old 05-10-2012, 09:03 AM   #14
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Strutting between floor joists


Quote:
Originally Posted by GBR in WA View Post
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
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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Old 05-10-2012, 06:55 PM   #15
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Strutting between floor joists


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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Strutting between floor joists-003.jpg   Strutting between floor joists-006.jpg   Strutting between floor joists-007.jpg   Strutting between floor joists-009.jpg   Strutting between floor joists-011.jpg  

Strutting between floor joists-012.jpg  

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