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

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The 25% is actually at the discretion of the code writers.  Deformations on the frame through the combined action with walls, or braced frames should also be analyzed, and torsional effects need to be included as well.
 
The 25% stems largely from the notions that almost across the board researchers and engineers agree that it is very difficult to predict numerically the overstrength for lateral systems.  (Unlike building height limitations, about which there is very little agreement)   Since  we assign numbers to the design overstrength, material overstrength, and system overstrength, codewriters thought that it is best to have a secondary system with greater redundancy and ductility to support service loads after strong motion.  25% appears to be a reasonable estimate (or compromise) for this system.   Design provisions now allow to use an interactive analysis, for a long  time SEAOC  required that shear walls or braced frames be designed to resist the total lateral force independently of the moment frame.

Since actual code language is now more deeply embedded in the force deformation curves there is an emphasis on redundancy, drift and overstregnth.   Controlling drift seems to be a good way of tracking inelastic strain.  The 25% also helps with this.  Ulitmately, the trick is to do an analysis which addresses redundancy, drift, and overstrength as well as including real forces that the structure will see.  Since this can be a tall order for many structures, the 25% combined with the other items seems to do an adequate job of addressing this.
 
As people have previously, and rightly pointed out, the purpose of the frame is to provide a secondary defense with higher degrees of redundancy and ductility, and 25% provides a force level that makes this work most pragmatically.
 
It would be very interesting to have a collection over time of the actual discussions that have gone on, particularly through SEAOC and ATC during the development of these items from the 50's forward, as this is where this item was probably most rigorously addressed.
----- Original Message -----
From: "hossein mardanlo"
To: seaint(--nospam--at)seaint.org
Subject: Re: why the moment frame must be capable to resist at least 25% of lateral loads?
Date: Sat, 5 May 2007 07:45:01 -0700

Thanks for answers. My second question is why 25%? Is there any logic behind it or it is base on experimental observations?
 
----- Original Message -----
From: Gerard Madden, SE
To: seaint(--nospam--at)seaint.org
Sent: Thursday, May 03, 2007 10:39 AM
Subject: Re: why the moment frame must be capable to resist at least 25% of lateral loads¿

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


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