What is the purpose of cold-formed steel floor joist bridging? 1

[cliff234]

When framing floors with cold-formed steel joists, we specify that bridging be installed at 8' on center. I know that bridging is important to keep the joists stable during construction, but what is the purpose of the bridging (specifically the bridging bracing the bottom flanges of the floor joists) after the floor sheathing is in place? I can't find anything in the AISI cold-formed steel publications that clearly states the required spacing of floor joist bridging. Joist catalogs usually have footnotes in the span tables recommending bridging at 7' or 8' maximum spacing.

 

[AELLC]

If they was no ceiling fastened to the bottom flanges of the joists, and the joists were loaded to the max, perhaps the joists would fail in lateral-torsional mode because they aren't symmetric about the Y-Y axis.

 

[hokie66]

Some types of bridging assist in load sharing, but that is usually not considered in the design. I think it is primarily for construction purposes...stability and straightness during installation.

 

[XR250]

The joists like to twist as they are not symmetric. Much better to use block and strap than bridging. It is also even better to flip every other joist when using block and strap as the rotation will oppose one another (and the block is easier to instal)
I was on a job that had really long joist spans (approx 25') They all collapsed as they were putting the sheathing on. The would not have happened if block and strap and every other joist was flipped.

 

[cliff234]

I have calls in to two joist manufacturers to ask them this question. Still waiting for them to respond.

What if I have a 9' span and the recommended spacing of bridging is 8'? Do I need a row of bridging or can I just reduce the usable flexural strength - and how do I compute the reduced flexural strength for the 9' span without bridging?

 

[JLNJ]

When I was trying to do something similar to cliff234 I had a had time finding any useful data.

I had a grid interior to a building with a relatively long span. No sheeting, very light loads.

I ended up specifying some conservative bridging, but it was probably overkill.

 

[JLNJ]

make that "...had a hard time..."

 

[paddingtongreen]

@ EE, It won't be April First for nearly four months!

Michael.
"Science adjusts its views based on what's observed. Faith is the denial of observation so that belief can be preserved." ~ Tim Minchin

 

[rowingengineer]

restraint to the bottom flange joist for torsion or twist will help with vibrations.

http://www.nceng.com.au/

"Programming today is a race between software engineers striving to build bigger and better idiot-proof programs, and the Universe trying to produce bigger and better idiots. So far, the Universe is winning."

 

[XR250]

Download AISIwin 8.0 - free