Wood floor joist bridging-This Old House 1

[vmirat]

I just watched an episode of "Ask This Old House" which dealt with cross bridging between wood floor joists of a house. Tom Silva explained that the cross bridging transferred the load on the joist to the other joists, thus strengthening the floor.

This is contrary to my understanding of the purpose of bridging. I was always under the impression that bridging was intended to reduce the unsupported length of the beam based on lateral-torsional buckling, not load distribution. During construction, the cross bridging was to support both the top and bottom edge of the floor joist initially until the floor sheathing is installed, and then later on to prevent warping due to drying.

I can't find anything in the NDS or the AITC documents which say that bridging is used to distribute load to adjacent members. However, I have found other posts in Eng-Tips and other web sites which reiterate Tommy's explaination.

I found one web site [

http://www.openjoist.com/tech/bridging.htm

] by Universal Forest Products discussing the value of adding strongbacks to wooden open joists to reduce vibration of the bottom edge. This web site specifically states, "Strongbacking does not, however, contribute to or enhance the strength or structural integrity of the system." This seems to support my position.

 

[jdgengineer]

Intuitatively I would think it would provide some additional stiffness of the floor system due to load sharing. However, the amount of load sharing would be questionable. Usually the bridging is only installed with a couple of nails which would limit the amount of load that could transfer through.

An excerpt from the Residential Structural Design Guide (US Department of Housing and Urban Development) "Bridging or cross-braces were formerly thought to provide both necessary lateral-torsional bracing of dimension lumber floor joists and stiffer floor systems. However, full-scale testing of 10 different floor systems as well as additional testing in completed homes has conclusively demonstrated that bridging or cross- bracing provides negligible benefit to either the load-carrying capacity or stiffness of typical residential floors with dimension lumber framing (sizes of 2x6 through 2x12) and wood structural panel subflooring (NAHB, 1961). These same findings are not proven to apply to other types of floor joists (i.e., I-joists, steel joists, etc.) or for dimension lumber joists greater than 12 inches in depth. According to the study, bridging may be considered necessary for 2x10 and 2x12 dimension lumber joists with clear spans exceeding about 16 feet and 18 feet, respectively (based on a 50 psf total design load and L/360 deflection limit). To the contrary, the beam stability provisions of NDS•4.4.1 conservatively require bridging to be spaced at intervals not exceeding 8 feet along the span of 2x10 and 2x12 joists." This statement essentially says that the cross bridging would have little impact...

However, typical open web truss manufacturers do require the installation of bridging specifically for the purpose of load sharing. Here is an excerpt from RedBuilts Specifiers Guide "Bridging is used to make each truss act with those next to it (load sharing) and minimize or equalize deflections from non-uniform loads. Bridging should not be confused with bracing, which has an entirely different purpose.
Roof Systems do not require bridging because differential deflections, vibrations, etc. are typically not a problem with roof systems. However, we do require bridging for load sharing with Red-LTM, Red-LTTM and Red-WTM trusses because of their single-member chords and their common use in relatively long spans with wide on-center spacing.
Floor Systems perform better under typical loads—particularly with regard to deflection and vibration—if they have an effective bridging system."

I would assume that Redbuilt did testing that showed this bridging was beneficial for load sharing or else they wouldn't require it.

In answer to your question... I've never counted it as load sharing. It seems that based on past testing it may be beneficial in some situations (deep trusses) and not in others (2x floor systems). I suspect the plywood in typical floor construction already accomplishes the minimal amount of load sharing that would occur between the bridging members.

 

[ztengguy]

I dont think Tom Silva is a structural engineer. Now, it still doesnt mean there isnt some additional load sharing capabilities of the floor by transferring the load. Wood is very forgiving and easily shares load.

 

[SteelPE]

Doesn't the code recognize load sharing elements through the value Cr? I always took this as bridging or sheathing. What about the little pieces of metal X bracing that people have as bridging in their houses? I don't think it really takes much to transfer load from one member to another 16" away.

 

[jdgengineer]

The code does recognize load sharing through the value of Cr. The load sharing element is typically considered to be the floor diaphragm. However, some people add additional bridging to try and stiffen the floor and provide additional load sharing (or to brace the joists per NDS 4.4.1). Per the testing I quoted above, in typical floor construction this does not have a significant impact.

NDS Section 4.3.9 defines a load distributing element as "any adequate system that is designed or has been proven by experience to transmit the design load to adjacent members, spaced as described above, without displaying structural weakness or unacceptable deflection. Subflooring, flooring, sheathing, or other covering elements and nail gluing or tongue and groove joints, and through nailing generally meet these criteria".

Bridging could potential be the load sharing element in the absence of sub floor, but I can't think of an installation where this would every occur (perhaps a large trellis or something)

 

[Triangled]

I understand the same as jdgengineer. Additional uses of bridging are the stability of the compression flange which may be needed a) during erection prior to the installation of the diaphragm, b) a continuous or cantilevered system which sees compression on the bottom face, and c) uplift conditions which create compression on the bottom face. But these normally occur on larger members.

 

[focuseng]

My understanding is that solid wood members are dimensionally unstable and subject to warping and twisting due to temperature and moisture conditions. Nothing to do with structural torsional bracing or load sharing but because wood is "organic". Engineered lumber (LVL and I-joist) does not require the bracing.

Bracing does assist some with vibration control. Load sharing I am very doubtful.

______________
MAP

 

[JAE]

Keep in mind that wood X-bridging, traditionally in the US anyway, was never fully nailed until later in construction - It has historically been the practice of carpenters to nail up the tops of the 1x "X" bridging members but they will only nail the bottoms after much of the dead load is in place.

This may not be what is done today - but in years past that was the practice.

 

[Sawbux]

IBC code requires bridging at 8 ft o.c for 2x12 or deeper joists, 1:6 width to depth ratio. This is for lateral stability. Bridging of roof decks is also needed for wind uplift conditions where stress reversal can occur, putting bottom of joists into compression - lateral stability concern.

The most common justification of bridging in floor systems is to load share between joists - especially under point loading such as foot traffic. The floor decking will distribute some of the load, but is not as stiff as bridging. Bridging takes a lot of the bounce out of the floor under foot traffic.

Note also that wood is a natural material, with a lot of variability in E. Bridging distributes the load to the stiffer joists. It also will help even the floor out and restrains the wood joists as they twist and sag as they dry out from the installed moisture contenet of about 19% to the equilibrium value.

 

[msquared48]

I aqree with Sawbux and JAE.

Mike McCann
MMC Engineering

 

[focuseng]

From 2005 NDS commentary C.4.4.1.2:
Cross bridging addresses 1) redistribution of Concentrated loads on Long span members and 2) stabilize local eccentricity due to cupping or twisting of deep members.

The 1993 commentary has a much more long winded background.

______________
MAP

 

[XR250]

I see alot of the old X-Bridging not even nailed at the bottom. I guess they forget to go back and do that.
Some carpenters tell me the bridging makes the floor squeak. Can't say I have ever experienced that, however.

 

[vmirat]

I was reading through the Simpson Strong-Tie web site and looked up their ICC-ES report (ESR-2608) about their metal joist bridging which is either tension-only type or compression type (V-grooved). The report states, "Simpson Strong-Tie bridging components described in this report are used to provide lateral support for wood joists (sawn lumber and prefabricated wood I-joists) in accordance with Section 2308.8.5 of the IBC and Section R502.7.1 of the IRC." There is no mention of load distribution. It seems that the tension-only bridging, which is nailed on the top edge and bottom edge of each joist, would provide the best form of load transfer, if there was any, as opposed to the 1x4's nailed to the sides of joists. However, Simpson does not count this benefit. You would think they would as a selling point.

My feeling is that the old wood cross bridging was an empirical design feature from long ago, kind of like empirical masonry design. As a side note, Tom Silva also mentioned solid blocking as a better way of bracing the joists because then the mechanical contractor can cut holes in it wherever they want as opposed to cross bracing that could get in the way. I can see solid blocking providing more load transfer than the 1x4's. I think the most benefit of bracing/blocking is in the form of vibration dampening. Whatever load transfer there may be is ignored by structural engineers anyway.

 

[msquared48]

Simpson does not make claims to anything that has not been lab tested, so, no mention merely means that it was not tested for that, not that there is no benefit for load transfer.

As for the solid blocking, I have always considered it to be less effective at lateral load transfer than the X bracing.

Mike McCann
MMC Engineering