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Re: DOG BONE CONNECTION - HELP

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At 11:56 AM 5/22/98 EDT, you wrote:
>FEMA gives you a design procedure to determine the increased forces at the
>connection assuming a plastic hinge is formed. The paper 'Ultimate Strength
>Considerations for Seismic Design of the Reduced Beam Section, Engineering
>Journal/ First Quarter/1997' gives you the guidelines to determine the
>properties of the reduced section. I understand how to do the moment
>connection, but how can you just reduce a beam section and assume it is
>adequate for dead+live+seismic or any load combination that I might have? If
>my beam properties are reduced, how can it still be adequate? Is there a check
>that I need to perform at the reduced section to find out if the beam is
>adequate for my load combinations. Do you know about a design example to
>follow?
>Thank you


I hope my humble explaination below helps understaning a genereal concept in
this regard.

When you are dealing with sesimic forces, according to most of the design
codes in the world, we do not design structural members to remain elastic
during sever earthquakes. we do expect structural members to develop
inelastisity. The actual seismic forces induced on structural connections of
a moment frame will be in fact ultimate loads devided by response
modification factors which are somehow represetative of the structures
ductility in a global sense, which in turn, are related to ductilities
provided by yielding component of structures in a local sense. In theory if
you had a infinitely ductile structural components, letting aside the
instability due to p-delta effect, you could virtually chose any design load
for your sesimc loads. The smaller the design load, the smaler your
components capacity, at the cost of higher ductility demand. The reality is
strtuctural components have limited ductility capaity to sustain many large
inelastic cycles experinced ducting sever earthquakes. So if you cannot rely
much on the ductility capacity of compents,( e.g fully welded connections
suffered damage during Northridge EQ) then you want to move the plastic
hinge formation to another location which has a lot more ductility even if
it has 10 or 20 percent less capacity. So what happens then is that you have
protected the vulnurable connection by providing a ductile fuse (of less
capacity) of good ductility. Thus, even if your structure may have some
reduced strength but instead it will absorbe sesimic energy induced by
deforming more at ductile locations. Mind you most of the time in design we
have ignored the overstrength in members for, so actually they posses higher
strength than what we think. 

In dog bone type of detail, we allow plastic moment to develop in the narrow
part of beam, since we know at this location much more ductility is gained
compared to beam end connected by brittle welds. As we allow the member at
this location to develop hinging, we have to think that it strain hardens
too. Thus increased strength of it should be taken to account and yet it
should be kept less than our fully welded end connection capacity to protect
the connection from brittle failure at ultimate stage of loading and when
dog bone area is well into post yield range.

To summerize, 1) actual strength of the dog bone is not significantly less
than end connection  2) even if we have a reduced strength, providing that
dog bone area had a great ductility capacity, we only get larger
deformation, however this provides us a reliable energy dissipating system
which protects brittle welded conection next to it.


I hope my lengthy explaination is of any help to you.

Majid Sarraf
Ph.D Candidate/ Lecturer
Dept. of Civil Eng.
Univ. of Ottawa
Ottawa, Ont. 
Canada K1N 6N5
  


  



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