Cross Bridging

[marklo154]

Hello everyone, I just wanted everyones thoughts on this.

When installing 2 rows of cross bridging in one span where do you put the bridging, I was once told by a engineering that placing it closest to center span is the correct method.

Any thoughts on this appreciated,.

[dgbldr]

Re: Cross Bridging

You need to state your issue a little better. What kind of bridging? Are we talking about cross bridging joists so they don't roll? If so, the bridging should divide the span into roughly equal sub-spans. So with 2 rows of bridging you would divide the span into thirds.

Depending on the span, you might need more or less than 2 rows of bridging.

[marklo154]

Re: Cross Bridging

Yes cross bridging floor joist, We have typically placed them as you said 1/3 of the span when two rows is need . Not sure the Engineers reasoning behind putting them closest to center span. I was hoping someone here had some insite on this. The local building inspector doesn`t seem to have a problem with where we put it as long as it not more then 6'8" apart.

[Lavrans]

Re: Cross Bridging

I wonder if the thinking is that putting them closer to center, rather than an equal spread, creates more of a center beam effect? I don't know why the Architect would make that statement, so just trying to figure out the reasoning behind it. Anyway, if the blocking is tight and unifies the joists it is acting somewhat like the parts of a truss, so there might be some benefit to having 2 rows slightly closer to each other rather than spaced an equal amount from the sides... But that would make a lot of assumptions about the quality and methods of the install, wouldn't it? Even if the theory is valid.

[mike maines]

Re: Cross Bridging

Marklo, what's going on above the floor? Any point loads or bearing walls?

[jimAKAblue]

Re: Cross Bridging

I agree with the engineer's suggestion because the bridging's job is to prevent the bottom of the joist from rolling. The center is where you'd expect to find the most tendency to roll.

[parkwest]

Re: Cross Bridging

Wouldn't this be specc'ed on the drawings? When in doubt send an RFI to the builder and he can check with the designer and/or engineer to clarify.

The last thing I like to see on my jobsites is a framing contractor doing engineering onsite.

[dgbldr]

Re: Cross Bridging

I don't build new houses, so I don't know if it's customary to specify location of bridging.

I CAN say, however, that I've never seen joists roll because bridging was 2 ft too far this way or that way. Not once. It's not that critical.

[jimAKAblue]

Re: Cross Bridging

I don't build new houses, so I don't know if it's customary to specify location of bridging.

I CAN say, however, that I've never seen joists roll because bridging was 2 ft too far this way or that way. Not once. It's not that critical.

Bridging isn't even required, except on 2 x 12s. I wouldn't put it in my house: it's a squeak hazard.

[Joe Carola]

Re: Cross Bridging

You need to state your issue a little better. What kind of bridging? Are we talking about cross bridging joists so they don't roll? If so, the bridging should divide the span into roughly equal sub-spans. So with 2 rows of bridging you would divide the span into thirds.

Depending on the span, you might need more or less than 2 rows of bridging.

What other kind of cross bridging is there?

[dave_k]

Re: Cross Bridging

To my knowledge bridging is for load sharing between joists. It stiffens the floor assembly by transferring part of the load to the joists on either side of it and in doing so takes the bounce out of floors (reduces deflection). It's movement (deflection) that works nails and building elements loose and causes squeaks to occur.

Cross bridging and solid bridging do the same work, cross bridging does it with less material.

In the building code I follow there is a prescribed maximum span between rows of bridging. All that matters is that you are within that span.

If an engineer has designed a floor and is taking responsibility for it he is the one who dictates where and when bridging is used. If he feels they are more useful in the center of the span and is willing to sign off on it who's going to argue?

[Lavrans]

Re: Cross Bridging

If he feels they are more useful in the center of the span and is willing to sign off on it who's going to argue?

The OP was asking what the reasoning was, not arguing to do it differently.
I agree with your assessment, but I don't have the data to prove it...

[dave_k]

Re: Cross Bridging

The OP was asking what the reasoning was, not arguing to do it differently.
I agree with your assessment, but I don't have the data to prove it...

I didn't mean to imply the OP was trying to argue. It was just a turn of phrase.

What I was trying to say was that the engineer uses his knowledge and calculates all the loads then takes responsibility of the design. If he tells you where to put the bridging it's his work and his stamp on the drawings.

I don't have data to prove what i said either. The engineer that taught me structural design in college told me the purpose of bridging and I believe him ;-).

It's also in a book I read called "Building better floor systems" published by the TARION new home warrantee people which is the agency that governs the mandatory new home warranties here in Ontario.

[Lavrans]

Re: Cross Bridging

It would be interesting to see a real world measurement of how cross bridging acts.

[Dick Hackbarth]

Re: Cross Bridging

Generally speaking, the reason for x-bridging is to distribute loads and deflections btwn. adjacent joists in a more efficient way than by the spanning sub-flooring alone would do. The x-bridging causes a diagonal strut or trussing action. It makes the floor system act stiffer and stronger than the calcs. or tables for a single jst. would indicate the case to be. If one jst. is loaded by a point load or a dynamic load (kids jumping, a heavy person, heavy walking), that load and deflection will be partially distributed to the two adjacent jsts. through two of the pieces of bridging. The top connected two pieces of bridging acting in compression, with the nails/screws acting in shear and compression the way we want them to, are the effective bridging pieces. The other two pieces, acting in tension from the lower edge of the loaded joist, are much less effective in transferring any load because they tend to make the nails act in tension or pull-out. A screw and some adhesive in each end of the x-bridging pieces will help settle down any squeaking. And, you can general feel the movement and loose nails with your hand when you load the offending joist. You can do essentially the same thing with steel strapping, x-bracing in each jst. space; nailed to the bottom of one jst., stretched over the top edge of the next jst. and nailed, then stretched under the bottom of the third jst., etc. The strapping must be stretched tight, and now the effective bridging elements are the ones acting in tension. In fact, the ones acting in compression will tend to buckle.

I suspect the engineer was saying (telling you) that the most effective place for the x-bridging is at the max. deflection location on the jst., or the center, and I agree with that thinking. With two rows of x-bridging I would put them at the 3rd points on the jst. span, or with the slightly shorter spacing at the center of the jst. span. The IRC talks in terms of bridging being req’rd. on 2x12's or deeper jsts. and spaced no more than 8' o/c. On these deeper jsts. the bridging does play a stability function; it’s a lateral torsional stability issue on a deep fairly narrow member which are highly stressed. Of course, the jsts. should be end nailed at rim jsts. and blocked over interior supports, to keep them from rolling, that is, to roll over in a parallelograming fashion, just laying over in unison. The 2x blocking is effective in this situation because it can be nailed to a sill pl. and through the sub-flooring, and it can not parallelogram in this jst. space. This type of blocking is less effective out in the span because at the loaded jst., the jst. deflection makes the lower blocking nails work in pull-out in the end grain of the blocking, as the blocking tries to rotate in the jst. space.