AISC Steel Construction Manual - J10 - Flanges and webs with concentrated forces - stiffeners 3

[Deener]

Hey guys,
I'm designing a simple frame. Each connection is designed as a full moment connection. I have checked all of the cases for which stiffeners may be required and it appears they are not (J10.1 to J10.9). Deformations in the frame are to be kept to a minimum. I would imagine that adding stiffeners to the columns and beams would reduce deflection. The issue with that manifests when designing the welds. AISC simplifies this process by stating the welds in the stiffeners should be sized to resist a force which is based on the difference between the required strength and the available strength of the unstiffened member. If no stiffeners are required, this is of course negative as the required strength is less than the available strength. Has anyone come across this circumstance before? I see a couple solutions.
1) Assume stiffeners don't contribute much to the stiffness of the connection if the specification dictates they are not required so don't use them. (I don't like to assume)
2) Attempt to calculate the deflection of the flanges and see how this effects rotation at the connection. I would likely model the flanges as a cantilevered beam with an end load to be conservative.
3) Simply specify the minimum weld size based on the thickness of the stiffener plates.
Thanks in advance for your input.

 

[JAE]

I'm not sure why stiffeners would increase overall frame stiffness all that much.
It might reduce shear panel deformations a small amount.

I'd do your number 3)

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[CCox]

I don't think stiffeners will increase overall frame stiffness much either. Is the deflection you are talking about rotational at the joint or overall drift? If it is overall frame drift you are trying to control, you will be best served by investigating the size and stiffness of the moment frame members and end fixities (especially fixing your column base connections), not the stiffeners in the connection. Stiffeners are used for addressing local failure modes (plate buckling and yielding, concentrated loads, etc).

As an added note, take stiffness of column that is pin-fix: k=3EI/L^3. fix-fix: k=12EI/L^3. Fixing the column bases makes it 4 times stiffer. Tradeoff is you now have a foundation and anchor bolts that need to be designed for a moment.

I would use 1 above, and it isn't bad to assume if your assumption is correct.

 

[Deener]

Thank you both for the responses. You both make a good point about the stiffeners increasing stiffness. It is rotation about the joint that I'm concerned about. I think I have a bit of a special case here (which I should have mentioned before) where the connections are bolted (installers don't like field welds and the crane capacity limits welded assemblies). This means that the moment in the connection is transferred through bolts which are located near the edges of the flange meaning we will get some flange bending. The flanges are thick enough to not require any stiffeners so I'm thinking this isn't an issue. Beams are W30x292. There's a very large dead load on the frame so the overturning moment at the anchor joints should not exceed dead load plus the pretension in the anchors (assuming loss of pretension isn't terrible if re-torqueing occurs relatively frequently.

 

[T_Bat]

I'd say if calcs show flange bending is OK then you probably won't get much additional connection rotation based on the column flanges bending so I also agree with #1. As JAE hints at - most additional rotation is typically due to panel zone deformation. If you model these members with beam centerline to column centerline then you probably don't have much to worry about there (someone can correct that if I'm wrong).

If you want stiffeners to feel better then #3 works too. I would pay attention to your endplate thickness.