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Re: EQ resistant Design

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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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