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Re: EQ resistant Design

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Vijay,

You need to look at each direction (in plan) as a separate lateral system.
A braced frame system that resists a "north-south" seismic load will not
"help" a moment frame that resists the "east-west" seismic load (except
for possible torsional effects).  If this is your case (i.e. in one
direction it is a braced frame and in the other direction it is a moment
frame), then you DON'T have a dual system in the eyes of the code.  You
have a braced frame system that resists the load in one direction and a
moment frame system in the other direction.  This would mean that you
would analyze the brace frame system for 100% of the seismic load (since
it is taking ALL of the seismic load in one direction) and likewise you
would analyze the moment frame for 100% of the seismic load.

If you had a dual system, this would mean that in one direction (either
"east-west" or "north-south" or both for that matter) you would have both
braced frames and moment frames resisting the seismic load.  If this is
the case, you would need to analyze the relative stiffness of the two
systems involved (i.e. brace system and moment frame system) to determine
how much of the seismic force "goes" to each system.  After doing this,
you would likely findout that most would likely go to the braced frames
since braced frames tend to be stiffer than moment frames.  The intent of
the 25% provisions in then to ensure that the moment frames in the dual
system get designed for at least 25% of the load.  For example, let's say
that you run the relative stiffnesses and based upon that analysis you
determine that 83% will "go" to the brace frame system and 17% will "go"
to the moment frame system.  Using this example, the code would have you
design the braced frame system for 83% of the seismic load and the moment
frame for 25% of the seismic load (since 17% is below the minimum of 25%).
Again, though keep in mind that this is for the case of having both braced
frames (or shearwalls) AND moment frames in one direction of the building.

HTH,

Scott
Ypsilanti, MI

On Tue, 19 Mar 2002, Vijay K. Patil wrote:

> Dear Scott,
>
> thanks for the comprehensive reply.
>
> But does that mean I must analyze the Braced frames and Moment resisting
> frames seperately.
>
> i.e analyse the braced frames for 100% of the lateral force.
>
> and analyse the moment resisting frames seperately eliminating the braced
> frames for 25% of the lateral force.
>
> Actually doing this is also equally tricky esp. in steel structures as the a
> particular column might be participating as a braced frame element in  X
> direction of EQ force while may act as a  moment resisting frame member while
> analyzing in Y-direction of EQ force.
>
> another option is to analyze the moment resisting frames in which we must
> assume all elements considered in the braced frames as non existant.
>
> I hope I have been able to ask my doubt clearly.
>
> Vijay Patil
>
>
>
>
>
> Scott Maxwell wrote:
>
> > Vijay,
> >
> > Most (if not all) of the model building codes in the United States have a
> > similar provision for dual systems (i.e. a lateral system that comprises
> > of both shear walls and moment frames).  The provision requires that you
> > design the moment frames for a minimum of 25% of the seismic lateral force
> > even if the distribution of forces based on the stiffness of elements
> > indicates that the moment frames will see less than 25% of the lateral
> > force.
> >
> > As you may be aware, the lateral force resisting elements will "attract"
> > an amount of lateral force that is proportional to how stiff those
> > elements are.  The stiffer the frame or shear wall, the more lateral force
> > that frame or shear wall will attract.
> >
> > Since shear walls are by and large much stiffer than moment frames, they
> > will tend to attract almost all of the lateral force.  As a result, in a
> > dual system (having both shear walls and moment frames) the shear walls
> > would be sized based upon almost all of the lateral load while the moment
> > frames would end up being designed with next to no load on them.  Thus,
> > the provision of designing the moemnt frames of at least 25% of the
> > lateral force.  This provision ensures that the moment frames are design
> > with at least some nominal lateral force on them (otherwise, they might be
> > designed with a force on them that could end up making them not much
> > different that a non-moment frame).  Consider this provision a little bit
> > of redundancy.  After all, the point of using the dual system is to
> > provide some redundancy.  If the shear walls were to "fail", then the
> > moment frames would end up seeing a much larger lateral force.  The 25%
> > minimum is to ensure that those moment frames are at least capable of
> > sustaining some moderate seismic loading rather than "failing" themselves
> > right away.
> >
> > At least that is how I understand it...
> >
> > HTH,
> >
> > Scott
> > Ypsilanti, MI
> >
> > On Fri, 15 Mar 2002, Vijay K. Patil wrote:
> >
> > > Dear Friends,
> > >
> > > In the Indian Standard code of practice for EQ resistant Design of
> > > buildings a factor called performance factor K is considered to
> > > calculate the Lateral forces acting on the buildings. This performance
> > > factor essentially depends on the ductility or rigidity of the
> > > structure.
> > >
> > > 1) For a Moment resisting frame  with ductile detailing performance
> > > factor K=1.0 ( Less
> > >     Lateral force)
> > >
> > > 2) For Frames having shear walls / bracings the performance factor K=1.3
> > > (30% more
> > >     lateral force as compared to above) provided the Moment resisting
> > > frames are
> > >     designed to carry 25% of the lateral force. Again ductile detailing
> > > for these elements
> > >     is required.
> > >
> > > 3) For Moment resisting frames without ductile detailing performance
> > > factor K=1.6(60%
> > >     more than the 1st case).
> > >
> > > To me it is not clear for item no 2. should the Lateral analysis of the
> > > frames and the shear wall/bracings  be carried out seperately to achieve
> > > the 25% force to be carried by the Moment resisting frames and rest by
> > > the Shear walls/bracings.
> > >
> > > What is the provisions in your country codes??
> > >
> > > Is UBC available on the net?
> > >
> > >
> > > Thank
> > > Vijay Patil
> > >
> > >
> > >
> > >
> > >
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