Need a book? Engineering books recommendations...

Return to index: [Subject] [Thread] [Date] [Author]

re: joist bracing

[Subject Prev][Subject Next][Thread Prev][Thread Next]
Ed,
A lot of your questions can be answered just by studying Vulcraft's joist manuals. Also,
not to sound jerky or anything because I had to learn all of this too, but these should
be standard, easy questions for your senior level engineer in your office to answer. Most 
offices develop typical bracing details for joists that work 95% of the time.

As far as resolving the bridging/bracing buckling force, the rule of thumb is usually a compressive
force equal to 2% of your max moment, and these are of course accumulate from each joist. This usually means you space your X bracing at a maximum value to control the magnitude of
this force, but if you realize they usually use something as small as L1x1x1/8" for bottom
chord bridging, this force is usually not much to worry about and can be easily detailed. Depends on your wind speed region, type of joist, span, dead load, etc. on how much bridging they need.

You do in theory have some coordination with where your bridging/bracing terminates as far
as at a wall joint, but I have never had that come up as an issue. I think there is the ability
to move the bridging location a few inches in any direction to avoid such a conflict, worse
case they exceed their spacing in the field to avoid conflict and have to throw in another row of bridging. But you can double check that in the shop drawing process. You are the EOR and you have to coordinate this, the joist mfr is not responsible to check where your panel joints are, they are responsible to supply a building element only.

In regards to deflection adjacent to walls, this is something you need to consider during design. 
Lay out the joists where you don't have a large span that would deflect next to an infinitely stiff element in the middle third of the span, say next to a reentrant corner. Example, in an L-shaped building, you may need to cut that span and place a joist girder or beam there to prevent deflection incompatibilities. You can decrease the space of the first joist off the wall so that it is carrying much less load, as well as dictate your deflection limits (use a key note right on the plans). I have heard of long span roof joists having deck fit up issues because of this, but that would be because of the joist weight itself so decreasing the spacing would not help there. Again, the joist mfr is supplying a product, one element of your design, and they will likely do the minimum required by your drawings, specs, code and SJI unless you explicitly tell them otherwise. That is most economical for them but maybe not so good at CYA for you. Get it your special requests right on the drawings and check those shop dwgs!

On the brighter side of things, this is way easier to deal with than wood truss bracing and bridging!

Hope that helps, plus you have already gotten great advice from others...

Andrew Kester, PE




******* ****** ******* ******** ******* ******* ******* ***
*   Read list FAQ at: http://www.seaint.org/list_FAQ.asp
* 
*   This email was sent to you via Structural Engineers 
*   Association of Southern California (SEAOSC) server. To 
*   subscribe (no fee) or UnSubscribe, please go to:
*
*   http://www.seaint.org/sealist1.asp
*
*   Questions to seaint-ad(--nospam--at)seaint.org. Remember, any email you 
*   send to the list is public domain and may be re-posted 
*   without your permission. Make sure you visit our web 
*   site at: http://www.seaint.org 
******* ****** ****** ****** ******* ****** ****** ********