Stiffness of a buckled knee brace

[CrabbyT]

I'm working on a project where a girder for an existing equipment platform is bolstered by a knee brace. The knee brace was supposed to be a 2L 2.5×2.5×0.25 angle. They only installed it as a single angle.

Under full design load, the angle fails but the girder has a stress ratio around 0.5.
If I delete the angle, the girder still passes, but the stress ratio is around 0.97.

In either case, it's not an issue because the girder passes strength checks, but it raised some questions.

How much strength does an overstressed member contribute to the system?
Is it possible (or permissible by code) to consider strength contributions from overstressed steel?

In reality, my belief is that if the knee brace failed in compression, it might buckle but it would still contribute strength. Or maybe the forces would redistribute once the angle reached its Euler buckling limit. I appreciate that RISA 3D includes Euler buckling as an axial member condition; this feature isn't in Bentley RAM. Maybe I could model the knee brace as a spring. I dunno. What do you all think?

 

[phamENG]

In tension or bending, yes. It can still provide its plastic strength until ultimate failure/rupture. Buckling? Not so much. Buckling is a stability failure. Once the member buckles, continued loading will cause p-delta effects that will cause the member to fail in flexure. Essentially, a buckled member's stiffness goes to zero. Is there some residual strength? Sure, but I wouldn't depend upon it. The load is going to transfer to other, stiffer elements in the frame or the frame will collapse.

 

[WARose]

Is it possible (or permissible by code) to consider strength contributions from overstressed steel?

In day to day elastic design, I'm not sure the code considers post-buckling strength anywhere.

That being said, if this member can have any post-buckling strength and stiffness and not be destroyed (by high stresses) or have excessive displacements.....it may be worth looking at. But I have a hard time picturing a L2.5 having much impact. Better just to fix it and move on. (Since that was the original call out anyway.)

 

[bones206]

Another thing to consider is the lateral system. The knee brace may be a necessary lateral force resisting member in for platform. So if it fails you could also have a lateral stability problem.