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Home > Forums > Structural Engineers > Activities > Structural engineering other technical topics Forum

Purpose & function of joist bridging

thread507-324411
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WWTEng (Structural)
21 Jun 12 13:26
What is the function of joist bridging pre & post erection? The reason I am asking is that on one of the jobs the contractor want to place a duct between 2 existing joists and wants to get rid of all the bridging. In my mind since the joists have bridging on one side, this should not be a problem but I am not sure.
focuseng (Structural)
21 Jun 12 13:38
What kind of joists? Wood, steel bar, etc.??
msquared48 (Structural)
21 Jun 12 13:39
The purpose is not only lateral supportr for the bottom flange, but more so to help the floor system distribute point loads to more than one joist.

If the contractor removes the bridging in one bay, then the joists next to the unbraced bay can only get help from one other joist at the most, rather than a maximum of three. Point being that the floor system will be weaker next to the bay where the bridging is removed.

Mike McCann
MMC Engineering
http://mmcengineering.tripod.com
JAE (Structural)
21 Jun 12 13:51
What we usually require is to allow the ducts, but then add X-bridging on the two opposite sides of the joists.
I'm assuming that the existing bridging are horizontal and not X.
WWTEng (Structural)
21 Jun 12 13:53
focuseng: Steel joists.

M^2: Would exchanging X-briding with straight lines acceptable in this case you think?
msquared48 (Structural)
21 Jun 12 14:29
With steel joists it is OK.

My comments were focused more toward wood joists which are closer together than steel joists. With steel joists ususlly spaced from 5 to 8 feet apart, it is mainly the lateral support of the bottom flange that is of concern here, which the horizontal members under the ducts will satisfy. I would still do what JAE suggested on either side though.

Mike McCann
MMC Engineering
http://mmcengineering.tripod.com
MiketheEngineer (Structural)
21 Jun 12 14:51
I have no problems removing one line of bridging. I think it is over-rated - not really sure it helps that much unless you are spanning say over 12' I would rather see walls above prependicular over the joists -- now that does help.
msquared48 (Structural)
21 Jun 12 15:20
Not the bottom chords Mike.

Mike McCann
MMC Engineering
http://mmcengineering.tripod.com
zlindauer (Structural)
21 Jun 12 18:54
The bridging only works in tension. This is why the x bridging is required in adjacent bays if bridging is removed from a bay.
hawkaz (Structural)
21 Jun 12 20:01
Do you have any uplift? If the bottom chord goes into compression, the bridging will brace it.

If this is new construction, I would have the joist supplier answer the question
woodman88 (Structural)
21 Jun 12 20:26
The artical "New Horizons in Open Web Steel Joists and Joist Girders
Longer Spans, Less Bridging and Better End Anchorage" has some good basic information on steel joists. This is the link to it http://www.structuremag.org/article.aspx?articleID....

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

slickdeals (Structural)
21 Jun 12 21:53
@zlindauer,
Not sure why you that bridging is tension only. Is that a assumption based on the sizes of bridging angles?
WWTEng (Structural)
22 Jun 12 8:52
hawkz: No, there is no uplift. And its existing construction. Joist are 8" deep, spanning 15' @ 20" oc.
MiketheEngineer (Structural)
22 Jun 12 10:47
To me a joist is a 2x10 or 2x12 or LVL or ML... I wasn't thinking "bar" joists. Might re-evaluate that..
slickdeals (Structural)
22 Jun 12 11:10
What kind of bridging did they have for a 8" deep joist? Is this a open web joist with rod webs and angle chords?
zlindauer (Structural)
22 Jun 12 15:00
@slickdeals: The source of the information on the bridging being designed to work in tension only is a Vulcraft engineer who I consulted with when I had a similar issue in one of my buildings. If they designed it to work in tension or compression, they would end up with a larger bridging member, and since the bridging is typically continuous through the framing bay, compression capacity is typically not needed.
paddingtongreen (Structural)
22 Jun 12 17:11
It can only be "tension only" if it is anchored at both ends.
I've been out of this for a while, but doesn't OSHA require erection bridging of the joists as they go in? Would that not require tension and compression capability.

Michael.
Timing has a lot to do with the outcome of a rain dance.

WWTEng (Structural)
22 Jun 12 17:42
slick: They are about 50 year old joists, SJ types. I am not quite sure about bridging, I am asking the contractor to send a picture. But if memory serves me, they have a line of X-brdg down the center.
msquared48 (Structural)
23 Jun 12 18:11
Mike:

Bar joists are commonly used for the floor joist in bars.

Mike McCann
MMC Engineering
http://mmcengineering.tripod.com
charliealphabravo (Structural)
13 Jul 12 11:22
bigcheeks
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