x-bracing intersection connection

[senthil83]

hi

we have been using x bracing in our support structure,

refer the attached sketch

where the support structure was designed as SCBF(special concentric braced frames)
since there is no length at x-bracing intersection for making gusset conn, we have
made direct pipe to pipe welding at intersection.

The pipe to pipe connection has been designed as per sec.K of AISC 360-10 for
the actual tension/compression brace force coming from staad analysis.

But the client asking to consider higher forces (1.6fy.Ag) as per aisc 341-05 for designing this conn.

we have already designed corner gusset connection as per aisc 341-05 for higher forces 1.6.fy.ag.
since there is no gusset / reduction in gross area I think we can design the intersection only for actual
staad analysis forces.

please clarify.

thanks
SENTHILKUMAR.B

 

[KootK]

Frankly, I don't think that connection is suitable for 1.0 FyAG yet alone 1.6 FyAg. You just can't push or pull force like that across a hollow tube.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[JAE]

Agree with KootK - not a good connection.

And in answer to your question - In my view you do need to use the amplified load on all brace connections, whether they occur at the ends or at the intersections.

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[Shu Jiang PEng SE]

The SCBF structure is expected to experience nonlinear behavior under seismic loads, the required (expected) force of the brace and its connection supersede the derived force using elastic FEA analysis. It is not appropriate using the derived brace force for design.

Also you can design two story "X" brace to avoid the connection of the two pipes.

 

[jayrod12]

Also you can design two story "X" brace to avoid the connection of the two pipes.

Not sure I follow how a two storey compared to a 1 avoids the fact that the braces cross each other.

I agree with the others however that the proposed connection likely won't be capable of transferring even the analyzed bracing force, much less the amplified force. Why can't you just introduce a gusset?

 

[Shu Jiang PEng SE]

An example of pipe bracing spans two storys

 

[sandman21]

@jayrod12 The beam would be the common intersection point and gusset plates would be provided, eliminating the crossing of the braces. Beam would then need to be design for additional horizontal and vertical loading.

I understand we do not have a complete set of drawings but, it appears that you have a slotted pipe with no cover plate, the material removed is required to be replaced. Additionally its not clear where the yield line, 2t, mechanism is located. If you are trying to have the plate in-plane buckling of the plate, there are limitation on the distance from the edge of the plate to the end of the pipe also the plate is typical perpendicular to the gusset plate.

 

[jayrod12]

If you're going to the effort to put an additional beam in, why not just put a gusset plate. That seems like a giant waste of steel in a situation like the one shown in the OP's diagram.

 

[senthil83]

thanks for your replies,

this x-bracing is already in site, we want use with minimum repair,
what would be the good solution?
introducing the gusset at center, but the there is no enough length

 

[canwesteng]

Remove and replace with the gusset plates resused would be my guess for the cheapest option, vs welding in some goofy gusset or reinforcement.