Starting from scratch in designing a steel joist... 1

[Hemi79]

If I were to design a steel joist. And by this I mean structurally design the joist, not lay them out for a building, but actually designing the top chord, web and bottom chord. What would be the code checks to follow from the AISC code book?

For a K series joist I want to use the following:

Top Chord: Double Angles (Equal lengths)36 Ksi
Web: Single Angle (Uncrimped-First analysis, Crimped-Second Analysis)36 Ksi
Bottom Chord: Double Angles (Equal lengths) 36 KSI
Rods at each ends from bottom chord to top chord.

The Vulcraft Manual states the bottom chord to be checked in axial tension only, web in compression, and top chord in axial compression ONLY if panels are less than 24" apart.

I've used Chapters D,E,and F.
D2
E3 & E4
F9

When I asked my boss about this he said we would never design our own joists for a client, we would just assign proper loads for someone like Vulcraft to meet our specifications. The reason I want to learn to do this is that this summer, for my masters program, I will be taking Advanced Strength of Materials and Advanced Structural Analysis. For both these classes we must do a project. I would like to make this one of my projects. However, with my checks on my progress so far I certainly feel their are many other checks I am missing. Any guidance is appreciated.

Thanks,

 

[NITTANYRAY]

K Series Joists are made with 50 Ksi Steel.

 

[TDIengineer]

I would suggest contacting a manufacturer's representative. Often their engineer's are very helpful. If you explain you are a student, they might help you with some design guides they use or sample calcs they run.

Just a thought.

 

[JedClampett]

Don't they design steel joist to the SJI Specifications, not AISC?

 

[Hemi79]

I don't have their manual (SJI). But the excerpts I've caught online don't show design process. I will start calling manufacturers tomorrow. Also, I did read on the Vulcraft manual that they use both 36KSI and 50KSI for their designs. I was reviewing and using 36 KSI because the AISC manual shows it as the preferred yield stress for angles.

 

[SteelPE]

What is the purpose of the exercise? You would design the joist just like you design a truss, only using smaller members.

 

[Once20036]

As Jed said they use the SJI manual.
I talked to one of the Vulcraft Engineers recently who said it would be very difficult to replicate their their numbers without SJI because of a number of issues, including:

To minimize member sizes, they don't use K=1.0 for compression analysis. The actual K they use is based on the size, shape, and welds for that member and will often be less than 1.
Their calculations include eccentricity in the joints, ie - what if the truss isnt perfectly coplanar? I believe they assume 1" eccentricity.
Their calculations include eccentricity at the joints, ie - the web members may not meet at a theoretical perfect point due to manufacturing tolerances.

If you're trying to replicate the design for an existing joist, or existing calculations - good luck.
If you're doing it as an academic excercise, maybe these points can be ignored and you can proceed with the design via AISC.

 

[dik]

Done lots of these way back when... for preliminary, determine moment at mid span (for UDL) using wl^2/8 and using a depth = 0.9*actual depth... determine chord forces and double angle size required... Iused to use compression force on chord panel point distance*.9 due to 'fixity'. Web members are designed based on shear at supports... welding follows... remember to design the connection at the chord panel poins for the horizontal difference in the forces in the web members... It's a place to start and will give you a pretty good approx for what will follow.

Dik

 

[jike]

The extension of the top chord at each end for bearing will cause bending to be induced into the top chord. This may require a change in panel point spacing toward the end to control the stresses in the top chord.

 

[a2mfk]

Like SteelPE said, there is nothing magical about joists (trusses). I have designed them "from scratch" for a steel fabricator client before and hardly needed anything from SJI other than some bracing guidelines.

If you do this for your project, one trick is roof joists in uplift have bottom chord compression so you have to have permanent continuous lateral bracing of the bottom chord.

 

[oldestguy]

For a master's Degree project I'd not use the usual sections. I'd try to come up with something different that may be more economical, perhaps tubing, Maybe aluminum, who knows. You might even develop something useful and of value. How about prefabed sections of roof along with the joists? Then in the process get a manufacturer to try it.

 

[wannabeSE]

The Steel Joist Institute's Catalog of Standard Specifications And Load and Weight Tables for Steel Joists And Joist Girders can be downloaded for free after registering on there website. See

http://steeljoist.org/free-download-form.

I think the same specification is printed in Vulcraft's catalog.