Diagonal Stiffeners located at knee joints of portal frames

1) Omer Blodgett's book on welded steel construction.

2) AISC's design guide on tapered frame members.

I believe that both references deal with the design of these joints but I don't know that either addresses potential shortcomings.

Did you any chance mean diagonal knee braced portal frames? There has been a lot of seismic testing on these frames recently.

This document from ENZ makes a few comments related to the restraint provided by diagonal stiffeners to the flanges being inadequate.

Potentially the article that's been referred to in other ENZ guidance by Charles Clifton is actually DCB75. This also goes through some of the restraint issues at the knee and recommends against using diagonal stiffeners due to the lack of effective restraint (you'd need a fly brace to the inside flange to provide sufficient restraint. See attachment.

Thanks for the comments. I understand that diagonal stiffeners on their own are problematic, however, my question was relating to the case where we also have rectilinear stiffeners like the snip below that Kootk pointed me towards. I noticed they do make reference to allowable shear stress (which I'm not sure departs from LRFD design?). I suppose I'll just run through these calcs to check if my thickness is sufficient for my case.

Sorry misunderstood the question then!

Check out SCI p398 as well for some additional guidance, because it's based on eurocode it's a bit more like many current NZ rules/approaches than a US code would be given we already have a lot of adopted eurocode type approaches sneaking into the new composite code for example. Any NZ reviewer worth his money is more likely to be familiar with this document as well.

Basically in a NZ context work out the compression capacity of the stiffener using the load bearing stiffener rules in NZS3404 chapter 5, then you can add the horizontal component of this directly to the panel zone shear capacity (referring to the equations in chapter 12) for the total panel zone shear capacity.

P398 link

Thanks, that helps a lot. Was wondering whether you had any idea about what to take alpha_b as for steel plates and for that matter, steel rods? NZS3404 doesn't stipulate.

I believe this is intended to actually be covered by the following clauses for "connection components", i.e. alpha_b=0.5



For comparison, the checking of unstiffened webs for buckling utilises an alpha_b of 0.5 with a form factor of 1.0, as it is a similar plate in compression.

Thanks for that - very helpful

Section 5.6 of AISC Design Guide 13 also covers diagonal web stiffeners.