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Re: EQ resistant Design

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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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