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Re: why the moment frame must be capable to resist at least 25% of lateral loads؟

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It's a back up frame in case the walls begin to yield/fail. The 25% comes in because, in general, the shearwalls will be very stiff in comparison to the moment frames, so when you consider rigidity, the frames will get very little load and the shearwalls will get all the load.

Rshearwall >>>> Rmoment Frame

So, to ensure that the moment frames can do some work and are stout enough, the magical 25% of the seismic load is in there. So what you do is take your building base shear, divide it by 4, and make your shearwalls inactive in your model and design the frames assuming those shearwalls aren't doing anything.

Usually the shearwalls are at the core/center of the building and the moment frames are at the perimeter. By having stouter Moment frames, it will also reduce torsional irregularity concerns somewhat, although still the shearwalls will be much stiffer and still receive the brunt of the load.

-g

On 5/3/07, hossein mardanlo <hosein.mardanloo(--nospam--at)gmail.com> wrote:
Hi,

In lateral load bearing system, where two kinds of bracing and moment frames
are cmbined, seismic codes require that the moment frame must be capable to
resist at least 25% of lateral loads. My question is what is the theoritical
point behind this requirement?

Thanks,


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-gm