Blodgett Design of Welded Structures 6.4- How to stiffen a panel 1

[Doodler3D]

Hi!

Elementary question, but I was looking at some clarity regarding the width of a full panel versus distance between stiffeners with respect to the following statement mentioned in chapter 6.4 of Omer Blodgett's Design of Welded Structures. I've been using the spacing between stiffeners as the parameter in question.

Thank you

 

[centondollar]

Design of stiffened panels is not something usually done with a cook-book cookie-cutter formula. I am sure that some solutions for simple geometries exist, but they are not based on beam theory, unlike the extremely simple example provided in the book by Blodgett.

Usually, an effective flange width ("left" and "right" from stiffener) must be used due to the effect known as shear lag: the normal stress in the section is non-uniform (parabolic) and not linear as beam theory would suggest. Shear lag is exacerbated as the flanges get wider and stiffener spacing become smaller.

The picture you posted presents a solution that reflects reality approximately only if your "panel" is actually more like a beam (much larger length than width or depth) and if the stiffeners are relatively closely spaced. For more accurate calculations where the load is large and idealizations errors (beam vs. plate theory) become relevant, you should apply plate theory; an orthotropic Kirchoff plate model (with inertia of stiffeners in one or two directions taken from the literature) comes to mind as the simplest solution for a plate with two simply-supported edges and two free edges.

 

[Doodler3D]

Actually, I was reviewing two previous and similar stiffened panel designs where a former engineer used 'b' as single spacing width as well as the whole panel, while citing Blodgett as a reference for both calcs. Sometimes even the simplest of questions throws us into a rabbit hole.

Thank you!

 

[Guest090822]

The length of a panel for a an I-beam is typically taken as 1.0 to 1.5 times the clear height between the flanges according to AASHTO.

There comes a point where a "stiffener" is ineffective and is more of a "connection plate" once the spacing is too far apart.

I also suggest a more recent publication. A lot of steel theory has changed since the 1960s. Blodgett is a great reference, but somewhat outdated.

 

[Doodler3D]

@TheRick109, Could you suggest a good, modern handbook for non-code related structural calcs for machinery? I have Blodgett, Galambos, Hibbeler, Timoshenko and Roarks.

 

[Guest090822]

For machinery I don’t know of a reference that would be useful. If you are just doing beam design then “Steel Structures design and behavior” by Salmon & Johnson is good.

It wasn’t clear in your original post what exactly you are trying to accomplish.

 

[BridgeSmith]

The length of a panel for a an I-beam is typically taken as 1.0 to 1.5 times the clear height between the flanges according to AASHTO.
There comes a point where a "stiffener" is ineffective and is more of a "connection plate" once the spacing is too far apart.

AASHTO considers an end panel (adjacent to the free end of girder) stiffened, if the stiffeners are spaced 1.5 times the web (panel) depth or less. For interior panels, it's 3 times the depth. Obviously, shear capacity increases as the spacing of the stiffeners decreases, but beyond those limits, the web is considered unstiffened.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[centondollar]

FYI BridgeSmith: as far as I understood the original post, the topic of the thread was stiffening of a single plate longitudinally, creating what looks like "many T-beams next to each other" spanning in one direction. These sorts of plates are used a lot in shipbuilding (plate fields with bulb-type stiffening fins to reduce normal stress and increase buckling capacity) and aerospace engineering. It seems that TheRick may also have misunderstood the question, since he mentioned I-beams.

Vertical stiffeners for an I-beam, used to prevent shear buckling and compression buckling of the web, is a different matter altogether.