AISI: Flexural Strength for continuous beam near inner support

[DavidStructural08]

Hi all,

I am looking at a multi-span light gauge floor joist now. My questions are that:

1, for inner support where negative moment happens, what is the flexural stength Equation in AISI if the compression (Bottom) flange is NOT braced at the inner support?

2, How to design the bracing or bridging within this negative moment region?

Thanks a lot

 

[DavidStructural08]

forgot to mention: the top flange is continuously braced by concrete slab on metal deck

 

[apsix]

I know nothing about AISI, but isn't it the case that the bottom flange must be braced at the support by default?
If not, there would be overall structure stability problems.

 

[swearingen]

You must have bottom chord bracing at an over-the-column support for a joist. If you don't have bracing, essentially you have a pin at the top of the column.

I was involved in the fix of a situation where a beam was run over the top of a column and the flange just rolled over under load and the column kicked out a little - the flange to web connection became the pin. Luckily, the structure it was supporting was strong enough to support itself without the support and did not come crashing down. The support was there to limit deflection.


If you "heard" it on the internet, it's guilty until proven innocent. - DCS

http://www.eng-tips.com/supportus.cfm

 

[DavidStructural08]

Thanks apsix and swearinggen.

I understand that we must brace the bot. flange at the column point. But should we provide some bracing to the joist bot. flange as well, between the inflection point and the column?

If so, how to do the design?

 

[kslee1000]

I will provide shallower transverse joists to the main joist, and provide kickers from the bottom chord of the transverse joists to the bottom chord of the main joist. It forms a stable grid system to support the floor, also acts as lateral brace of the columns.