Bar Joist Properties

[BadgerPE]

Hello all,

Does anyone have any literature to find the chord area, chord moment of inertia and the C.O.G. for Vulcraft bar joists? I have emailed them, but have not received word back yet. I need the information to perform a vibration analysis per AISC DG 11 for a bar/dance floor project that I am working on.

 

[XR250]

You could probably back the "I" out of the deflection tables. And then get the chord areas using the parallel axis theorem +/-

 

[JedClampett]

I wouldn't hold my breath. Vulcraft makes thousands of these each year, and they're not known for their record keeping. And they're one of the better ones. A lot of these designs are based on the posted loading and the manufacturers will adjust the members accordingly. They'll tell you a 300/100 joist is good for 100 plf dead load and 300 plf total load and that's about it.
If you absolutely need to know, get a ladder.

 

[cfox142]

A few things. New Millennium has an equation for the approximate moment of inertia for K-series joists as :

I=26.767(W)L^3)(10^-6) where W is the load from the joist table for L/360 deflection.

I've always used the Floorvibe software in RAM structural system for this. Its worth it if you do this by hand a few times. I just checked the report from floorvibe and it calculates the A and I of the joists somehow so there must be an equation somewhere to do this.

I've also used the attached joist chord sizes in the past. I'm pretty sure I found this on eng-tips a while back so use with some extra caution.

 

[cfox142]

Also just saw this from SJI: Technical Digest No. 5 Vibration of Steel Joist – Concrete Floors might be helpful not sure.

https://steeljoist.org/product/td5-vibration-of-steel-joist-concrete-floors/

 

[BUGGAR]

"and it calculates the A and I of the joists somehow so there must be an equation somewhere to do this."
Somehow and somewhere!

 

[BadgerPE]

XR250

The example in DG11 has a CG above the midpoint so I am assuming that the top chord angle are larger than the bottom. Good idea though!

Jed

I got a hold of Vulcraft and they will get me the individual properties for my proposed shapes. It seems strange that they can't just give top and bottom chord sizes and let me figure out the rest.

cfox

Thanks for the equation. It got me to within 5% of the example in DG11 so it is a good starting point.

I NEED to get through this ridiculous vibration calc today so it doesn't follow me to 2016! Happy New Year all!

 

[XR250]

I wonder why the vibration calc requires chord areas? Seems like only the overall moment of inertia would matter.

 

[Ron]

I just checked the report from floorvibe and it calculates the A and I of the joists somehow so there must be an equation somewhere to do this.

.....remember.....In the US, we are usually required by law (depends on your licensing state, so check it) to validate the software we use, whenever possible, by hand calcs or other means (comparison of answers with other software, etc.)