Web crippling in fully supported joists 1

[Structintern3]

I'm designing cfs joists that are fully supported along the entire length of their bottom flange by a slab but my understanding is they may still be subject to web crippling failure. I'm used to only checking web crippling at supports, and when these are fully supported I'm not understanding why they are still subject to web crippling. Could anyone maybe elaborate on why that is please? If you were going to check against web crippling, what tributary load would you place on them/how are you checking them?

 

[driftLimiter]

The concept is the same at a support as it is in your case.

The member is loaded on the top, and the reaction comes out the bottom. Thus the web is right in the path to transfer the load.

The web has some component of axial loading in this condition, and slender elements in compression tend to become unstable geometrically (web crippling).

In your case the load is uniform and the reaction is uniform, so I would consider check a strip of the web region near the end and one near the middle.

 

[Structintern3]

@driftlimiter Thank you for the explanation. I guess the way I'm visualizing it is at a simple beam support we have let's say 3" of support and then our support reaction. So would be our support reaction/3 per inch to check for web crippling. But with the fully supported joists, isn't the end and the middle the same load? Wouldn't the section be infinitely small since it's fully supported?

 

[KootK]

I don't see web crippling for this if it's envisioned, truly, as uniform load on uniform bearing. That case, "crippling" is probably just straight up Euler column buckling on a unit strip width of web.

An interesting question will be, however, whether this is really uniform bearing when one considers flatness tolerances etc. That might be what drives you to consider non-uniform support and, thus, crippling.

 

[driftLimiter]

I agree with Koot in the interior its just buckling of the web, at the end though the boundary conditions of the web are different and we will see more of a crippling, the challenge is what length to consider "the support" near the end. Would start with a few inches and only consider the load over those few inches then work my way out a few iterations and see which way things go.

 

[KootK]

Perhaps you could just assume supports at intervals of, say 8X the depth of the member and design for that.

 

[XR250]

What load, what gage, what depth?

 

[bones206]

It might be similar to designing a load bearing rim track, if you assume intervals as KootK suggested.

 

[SWComposites]

if there are vertical loads on the beam that get transmitted down thru the web, then you need to check web buckling/crippling.

show a detailed sketch of what you have and loading.