Capped Crane Runway Beam - Class of Built-Up Section / Compactness

[Serhiy2]

Good day,

I'm designing a built-up crane runway beam built of the w-section and a c-channel welded onto the top flange. When checking the built-up member top flange elements for their class/compactness, I'm not sure what criteria to use for the part of the c-channel web located between the edge of the top flange of the w-section and the edge of the c-channel. I think it is a similar case to where the compression flange is supported on two edges with welds/fasteners but just wanted to see if there are other opinions. My assumed scenario would require it to be checked for the following criteria: CSA S16:19 - bel/t<=420/sqrt(Fy); bel/t<=525/sqrt(Fy); bel/t<=670/sqrt(Fy); or AISC - b/t<=1.12xsqrt(E/Fy); b/t<=1.40xsqrt(E/Fy).

I attached the sketch showing which part of the c-channel I'm referring to.

Thanks for you input.

 

[canwesteng]

I would treat it as class 3 since it has a channel - even if you were to go to class 1/2, the requirement that it not yield in service will limit you to the class 3 requirement for service loads anyway. Generally better to just size up the member and use a standard W section, since you save on fabrication and use the full capacity, at the cost of a bit more torsional stress.

 

[Serhiy2]

Thanks Canwest. My concern was that I might need to treat it as class 4.

 

[CDLD]

I wouldn't have any issue, treating this as compact (class 1) if the appropriate checks are made.

As canwest suggests, for CISC you will need to make sure the shape does not yield under service load, which is usually not very critical in my experience.

If torsion is involved, which I assume it is, for AISC you will also need to make sure the factored stress is less than phi*Fy.

The compactness check you have outlined is the correct assumption (case 18 ASIC).
See screenshots below, second photo is from a Steel Interchange article in 2009 (case #s refer to old code I believe).

AISC does a good job explaining the web compactness limits for singly symmetric shapes, for CISC you need to refer to the bridge code -see below.

 

[CDLD]

With a cap channel on top, and for simple span bending it would be conservative to check the web compactness for just the w-shape.
The cap channel lifts the neutral axis up, therefore reducing the compression on the web and improving the stability of web.

 

[dik]

There was an earlier post on section class:

thread507-511413

Although not strictly speaking due to the eccentricity of the reinforcing, I considered it as Class 2 because the beam was braced by the floor and other beams and the eccentricity was relatively small. I also checked that b/t ratios were Class 2. This was necessary because there was a considerable drop in strength if Class 3. Class 1 and 2 sections in Canada can use the Plastic Section Modulus in lieu of the Elastic Section Modulus.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[Serhiy2]

CDLD, thanks a lot - the screenshot from the magazine is exactly what information I was looking for.