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RE: HSS and FEMA 350

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Charlie, while I have your attention:

My latest foray into structural engineering has required me to design
eccentrically braced frames per the AISC Seismic Provisions.  The building was
a single-story fire station, Sesimic Zone D.  One of six braced frames was
required to be an eccentrically braced due to architectural constraints, so I
went ahead to make all the rest eccentric to reduce the R-value (not
insulation).  Since I hadn't run thru the new code yet, it took several
iterations to discove what I needed to do to get a code-compliant design. 
Hopefully I got it right.  Here's my final design procedure (after several
false starts):

0. Assumptions.  Although other approaches might work, certain constraints
limited my design approach.  First, I needed to keep the brace size to a
minimum.  Also, I needed to keep the beam flange size small.  Columns and
braces were both HSS.  Brace connections are modeled as pins.  In my case, all
frames were inverted v-braces.  [Link to column connections seem daunting.]

1. Select initial sizes.  The sizes should not be conservatively large, as this
will cause beams, braces, and columns to be unnecessarily large to create a
plastic hinge in the beam.  Also, I had to dog-bone the beam to form the link,
otherwise, I couldn't get the process to work.  More about that later.

2. Run analysis.  Include all code-required load combinations.

3. Check link rotation angle.  I used a method recommended in the military's TI
809-4.  Link rotation angle appears to be the most critical piece of
information for eccentric braced frame design, except possibly eVp/Mp of the
link.  If the link rotation angle is too large for the ratio eVp/Mp, increase
frame (read:link) stiffness and re-run analysis.  Eventually, the rotation
angle meets code requirements, and design continues.

4. Check the link.  The link needs to be initially selected as a dog-boned
section of the W-section beam, otherwise, the forces the link generates will
exceed the code-defined capacity of the rest of the beam, and the plastic hinge
cannot form.

5. Check the brace.  Forces for the brace must be calculated by multiplying the
brace force by the ratio of the the link plastic hinge forces over the forces
generated in the analysis.  Which forces?  I chose Brace Earthquake Load x Link
Capacity / Link Earthquake Load.  If somebody has another opinion, I'd like to
hear it.  If the brace was too small, I picked a larger one, and re-ran the
analysis.  Analysis forces don't usually change much based on the brace size.

6. Check beam.  This is where things can get sticky.  I went over and over
again trying to get the frame to work w/o dog-boning the beam for the link, w/o
success.  After dog-boning the beam, things worked better, except that axial
load sometimes becomes a problem if most of the frame force comes from only one
side of the frame.  In such a case, the link axial load tends to be half that
of the beam load, so that if the link beam is nearly at capacity, the beam is
likely to be over capacity.  In such a case, the beam (and link) must be
re-selected, based on the forces available, and the analysis must be re-run. 
With a new link size, link rotation angles, forces, and forces required to form
the plastic hinge all change, so member checks must start over.  I got lucky,
and only had to do this once.

7. Check column.  This part is easy.

8. Design brace connections.  The brace to column connection is
straightforward.  I welded the gusset to the base plat and column face, and the
tube brace to the gusset.  The brace to beam/link connection is a little more
difficult.  Where the gusset connects to the beam, it cannot infringe on the
length of the link, so odds are the gusset weld to the beam will have an
eccentric load at an angle.  I've used the instaneous center of rotation tables
in the AISC Manual to handle that.

There are some other design checks, but the rest are fairly straight-forward. 
Am I on the right track?

-Keith Fix
-Little Rock, AR


--- "Carter, Charlie" <carter(--nospam--at)aisc.org> wrote:
> >I presume this discussion is limited
> >to moment-resisting frames?  For example,
> >are eccentrically-braced, non-moment-resisting
> >frames impacted?
> 
> Yes, FEMA 350 is limited in scope to moment frames, so CBF, EBF, etc. are
> not affected. One caveat: if you put the EBF link directly to the column, as
> opposed to internal between braces, that moment connection to the column may
> be affected.
> 
> Charlie
> 
> 
> 
> 
> 
> 
> 


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