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lylsutherland 10-29-2012 11:58 AM

Mid span bridging
As I read through the different threads regarding mid span blocking/bridging,I find many different views on what part blocking/bridging plays in a floor system ,so I thought I should put my 2 cents in.

Full depth bridging has traditionally been used for long-span dimensional lumber as a load sharing device, because vibration in a floor has been a problem for at least four decades.Floor vibration is not a structural problem with any type of joist you use in a floor.As we use lighter Engineered I-Joists in light residential construction, bounce and vibration is more prone to be a problem.

Bridging/Blocking plays a very significant part of a floor system,just like sub floor thickness and adhesives, strapping or direct applied drywall, depth and O.C joist spacing or the series of I-Joist used in a floor.

I-Joist manufacturers don't recommend any form of mid span blocking,but they do say it can help with floor performance. They tell you to use a deeper joist, closer o.c. spacing or a more expensive joist to get a better performing floor which in turn increases the L/over and decreases the deflection to help limit the bounce and vibrations in floors which occur in longer spans.They started at L/360 then L/480 and are now recommending using up to L/960 to help address bounce and vibration in their floors and by doing so, you add more mass and cost to the floor. Deeper I-Joists,deeper flush beams,deeper hangers,deeper rim board,more siding, more money for your floor system.

As we change the parameters of a floor you achieve longer spans .I.E. glued floor versus unglued floor,strapping versus no strapping. All floors are limited to a L/360 deflection, which in long spans could deflect up to 1/2" or more and give you a trampoline effect in the floor or vibrate when you walk across the floor.The floor can deflect an inch or more and it don't have any problems until somebody walks on the floor

Historically, designers have used Uniform Live Loads Deflection to predict floor performance. Designers and builders tell us that the performance results using uniform load criteria are not consistent. Uniform live loads never occur in floor systems so the concept of using non-existent conditions to measure the acceptability of floors is neither practical nor accurate.The acceptability of floors is determined by more factors than deflection. Velocity of travel,acceleration,duration of vibration and frequency of movement all contribute to the dynamics of a floor's performance.

The proper measurement of performance requires a good tool and proper calibration. For floor's, the best performance measurement system combines the point load response of a floor to footsteps with a floor performance rating system. Uniform live load is not the service condition that causes floor performance problems. The acceptability of a floor is determined by its response to a person walking on it.These impact loads create the annoying bounce and vibration problems in floor systems.

A Load Sharing Connector improves floor performance by structurally connecting the floor joists together which forces the floor to act as an integrated system,thereby gaining stiffness and rigidity. Load sharing design offers new alternatives in floor design that can address the common problems of soft,bouncy floors and by sharing impact loads you reduce the annoying effect of bounce and vibration in your floor. With a systems approach,you transfer impact point loads such as those generated by foot traffic to as many as eleven joists.

Bridging/blocking requires an effective means of connection and must be installed prior to any loads imposed on the floor. With this connection, the proprietary bridging is faced nailed through the wide plywood uprights to the joist flange so when the sub floor is put down,the bridging is put in a tension mode,which starts the load sharing capabilities. When the bridging is placed in a row,it's like having a flush beam within the floor. Testing has been done with nailing the flanges of an I-Joist to a effective proprietary bridging product using 2-7d common nails on the top flange and 3-7d nails on the bottom flange.

tony.g 10-29-2012 05:59 PM


Originally Posted by lylsutherland (Post 1040292)

the concept of using non-existent conditions to measure the acceptability of floors is neither practical nor accurate.

The concept of using deflection limits to address floor bounce may not be accurate, but it is certainly practical. How else could you possibly account for all the static and dynamic load-conditions which may arise?
It would be easy for regulators to reduce deflection limits to, say, 1/960, but at what cost? A balance has to be struck between performance and cost.
I agree that a deflection limit by itself does not reduce bounce; our code also sets an absolute limit of deflection of 14mm (5/8" approx) in domestic floors regardless of span.
I doubt anyone on this forum would disagree with you on bridging between joists - you are probably pushing at an open door.

Gary in WA 10-29-2012 11:10 PM

Nice write-up, and welcome to the forums!

I've sited these many times here and you may have run across them in your reading;


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