Braced Frame - K factors/Unbraced Lengths

[slickdeals]

Folks,
I have a few questions pertaining to unbraced lengths and effective length factors that "CAN" be used in braced frame design.

I am currently peer reviewing documents for a building design where SCBF's have been used. The following were things I noted. I would appreciate your thoughts on some of the assumptions.

1. An unbraced length equal to that between the ends of gusset plates was used. Is this common? I know using workpoint to workpoint dimensions are conservative, but the use of end-end of gusset plate seems to be unconservative. Your thoughts?

2. The use of K=0.8 for the brace design. They have some substantial gusset plates which will prevent a theoretical pin-pin (K=1.0) from happening. They are not using a K=0.65 for fixed-fixed either. Are there means to determine what effective length factor can be used based on a fixity at a connection? How would one calculate the fixity of such connections?

Thanks

 

[sandman21]

1. It is actually the other way round, the shorter the brace the higher the compression capacity that must be used to design the gusset. I take it from hinge to hinge.
2. What type of connection are they using? Fixed in and out of plane, fixed in-plane pinned out, or pinned in-plane and fixed out-of-plane? The 2006 IBC Structural/Seismic Design manual provides a commentary on the K to be used, they use 1 in there design and AISC seismic manual uses 1 as well. With some references to how they have obtained the recommend values.

 

[slickdeals]

1. The building is in SDC D. I agree with you that the shorter the brace, the higher its compression capacity. I am checking their design right now and their design does not work for the first brace.

They had used in their design a model that had a story height from slab-on-grade to the second floor neglecting the distance to the top of footing (which adds about 4' of diagonal length). They are then using a reduced unbraced length which I think is to account for that mis-step. I must mention that this is a fabricator's engineer who is designing the building. Everything is designed to the gnat's ass. Where do you assume the hinge in the gusset plate? Do you assume it at 2t from the ends of the member?

2. The K factor of 0.8 is used in-plane and out-of-plane. Out-of-plane bendind is in the strong axis of the wide-flange. As to the type of connection, it comprises of flange plated connection between the column and the brace.

 

[slickdeals]

Do you have a page number in the seismic manual that talks about the K factor?

 

[JAE]

Do you assume it at 2t from the ends of the member?

Yes - that would be the hinge point for rotation where the gusset is bent across its own weak axis.

 

[slickdeals]

JAE:
Would that apply also to a flange plated connection i.e. the brace member flanges are connected to the column flanges by 2 gusset plates. This is quite a stiff connection out-of-plane, right?

 

[sandman21]

I will get you the page number tomorrow unless someone beats me to it. If I remember right I believe the k is .7, but I don't have it in-front of me. I measure the brace from hinge to hinge, so if the hinge is above the SOG then so be it, I have had some very tall gussets, I had one frame that had 6" between gusset, lol.

 

[sandman21]

Volume 3 pg 57-60, the section is about cross-brace and splice but it has some references on how they have determined the K, might give you an idea of how to calc. it. All else fails have the engineer prove the K.

 

[JAE]

slickdeals - yes - I think you are correct on that kind of gusset arrangement. I was thinking only of the single gusset with the intended hinge just off the end of the brace.