# Influence of a diagonal fold on a web + double fold situation

I need to build a steel frame with a steel web in it, as illustrated below. Flanges of the frame will be welded, as well as the web, along its whole perimeter.

Would adding a diagonal fold to the web allow this structure to whistand a bigger load? Or, for an equivalent loading, would the fold allow me to use a thinner web?

Considering the doubly folded strap of the web material welded on the ends to the frame, is there a critical angle for this double fold, where, at some point it is stronger to have no folds at all, rather than two folds.

Intuitively and I may be wrong about this, concentrating matter close to the diagonal tends to make the web behave as a truss. However shear flow must be disrupted as well with two folds of very obtuse angle. See illustration below.

Is this case can it be assumed that one single fold, whatever the angle of the fold, weakens the structure, but two diagonal folds, whatever the angle, makes it stronger?

if you add a fold you disrupt the flow of shear force which is acting in a rotating pattern all around the web, and make it weaker actually. because you are encouraging it to warp.

However if you just put a doubly folded strap of the web material with strong welds on the ends to the frame and leave the web undisturbed it would act as a truss, and be stronger.

Edit

I attached a sketch of shear flow below. as you see you create two triangles of shear panel with much less shear resistance, usually shear strength of a section is related to its side length , a^3 , so we have here roughly 1/2a^3=1/8a multiplied by two, 1/4a or times less shear strength.

• thank you, does warping behaviour comes from having this rotating pattern not coplanar to the 4 nodes anymore? – qq jkztd Apr 14 '19 at 17:01
• @qqjkztd, no it is because effectively we devide the section into two triangles. I modified my answer to show this. – kamran Apr 14 '19 at 19:04
• edit made, don't know if I should post it as a separate question, I will if needed, don't want to pollute the place with layman questions either. – qq jkztd Apr 15 '19 at 10:33
• @qqjkztd, it's discontinuity in the sheet that disrupts the flow of shear, regardless of it being single fold or double. But for a diagonal brace there is only one logical position and that is from top left corner to bottom right. And you can use bolts and nuts instead of welding for connecting it. – kamran Apr 15 '19 at 15:31

There is potential value to adding stiffener plates that are transverse to the plane of the web. For tall welded plate wide flange girders, it is very common to provide a stiffener element in order to reduce the web plate thickness.

As mentioned, the shear wants to flow through the plate analogous to a truss, this same analogy is used by AISC in Chapter G they call it Tension Field Action.

Whether or not this is the most economical solution depends on your actual configuration, loads, and fabricator capabilities.