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Braced Frame Connections

Braced Frame Connections

Braced Frame Connections

In the design of a concentrically braced frame (See AISC "Provisions for structural steel buildings") the connection of the brace (including the brace to beam connection has to be designed for load cases (4-1 & 4-2) with an overstrength factor of 2. (table I-4-1). My question is, what does the the connection of the column/brace to the foundation need to be designed for.  If the overstrength factor is for the ductility of the brace and beam does it need to included in the design of the conection to the foundation.  Please point me to any references or commentary you might know of.

RE: Braced Frame Connections

Your brace must have the same connection strength at both ends, EVEN AT THE BASE.  Remember, the ideal connection for a brace is Ag*Fy, but codes allow you to use a value corresponding to the structure's elastic response.  The Canadian code S16.1 used to allow this, but now the requirement is that all brace connections develope the full strength of the brace, at both extremities, of course.

Your brace IS most probably going to yield in an earthquake, and you don't want it failing at the connection.


RE: Braced Frame Connections

We usually design the column/brace connection to the foundation for the higher "connection" strength.  

But note:
 The brace doesn't have to necessarily meet the two overstrength factors.  Actually, you design for the SMALLER of the three conditions:
1.  Tensile strength of the brace
2.  The overstrength load combinations (4-1 and 4-2)
3.  The maximum force that can be dragged into the system.

Usually you will find that the third item above controls.  This force may be due to a steel deck diaphragm shear strength (ultimate...not the allowables shown in the tables).  The idea is that you want "other" more ductile elements to fail BEFORE the brace does.

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