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RE: Rigid vs Flexible diaphragm discussion[Subject Prev][Subject Next][Thread Prev][Thread Next]
- To: "'seaint(--nospam--at)seaint.org'" <seaint(--nospam--at)seaint.org>
- Subject: RE: Rigid vs Flexible diaphragm discussion
- From: "Swingle, Mark" <Mark.Swingle(--nospam--at)dgs.ca.gov>
- Date: Wed, 11 Aug 1999 14:49:49 -0700
- Cc: "'mswingle(--nospam--at)earthlink.net'" <mswingle(--nospam--at)earthlink.net>
Martin, Your comments are well taken, and your example is a good illustration for further discussion. I have tried to convey some of the concepts that you outlined, but my explanations are not always so clear. Many others have spoken about this too. I and many others have pointed out that in many situations the lateral system behaves such that the force distribution is somewhere BETWEEN "rigid" and "flexible". This is exactly the situation you described. In your example, the short walls will see LESS force than the tributary load case ("flexible" diaphragm), but MORE force than an infinitely rigid diaphragm. Conversely, the long walls will see MORE force than the tributary, but LESS than the completely rigid case. I previously stated that one must "accurately model the stiffness of the diaphragm" or something to that effect. I never stated the degree of precision that was intended, but this was a poor choice of words. I was only trying to point out that one should not go to the other extreme if one judges or calculates that the diaphragm is not "flexible". I meant that one should at least model it in his mind if not by equations. This is the "considering" that Charles Greenlaw has so eloquently clarified for us all. Several problems exist in the code itself, and several problems exist due to many engineers' MIS-understanding of the code. The ones that misunderstand the code may be your plan checker or an expert witness for the plaintiff (you being the defendant). One problem in the code is the definition of "flexible" diaphragm and its implications as to whether actual and accidental eccentricity must be accounted for (not just "considered"). In your example, the reason that the diaphragm will redistribute forces (from the short walls to the long walls) has more to do with the DIFFERENCE in stiffness between the adjacent shear walls than it does with the deflection of the diaphragm vs the "average story drift" of the shear walls. If one has an infinitely stiff diaphragm, then the concept of torsional eccentricity is well-defined. However, if the diaphragm redistributes forces to a degree less than infinitely stiff (such as in your example), what do we do about torsion? Suppose your calculations (or those by the expert witness 9 years from now) show that the diaphragm is NOT flexible per the code definition. In this case, there are an infinite number of ways that the mass can be "displaced from the calculated center of mass...a distance equal to 5 percent...". In other words, the concept of moving the mass for accidental torsion is not mathematically rigorous when the forces are PARTIALLY determined by tributary loads. It would be easy for an expert witness to show that you did not account for accidental eccentricity according to HIS/HER interpretation of the code language. Some better way needs to be defined IN THE CODE as to how to account for torsion when the diaphragm is BETWEEN "rigid" and "flexible". One other problem in the code is that the formula for shear wall deflection is in the body of the code, although it does not come with CODE LANGUAGE to clarify that it is only valid (?!) for isolated, full-height, cantilevered shear walls. Only one-story commercial warehouses or perhaps some one-story residential garages fit this case. This formula is next to useless for multi-story bldgs and walls with adjacent spandrels that modify the stiffness. This formula is also useless absent load-deflection curves for the various holdown devices. So, if you were to use OTHER researched, tested, or well-reasoned methodologies to come up with shear wall stiffnesses (and thus shear distributions), you could be shown to have violated the language of the code. Or, if you had made an engineering judgement as to how the forces would distribute, as in your example, then you could be shown to have violated the code, because you did not use the deflection formula in the code. These formulas need to be removed from the body of the code (or referenced standards), or they need to be clarified as to their proper application, so that other methodologies would become valid based upon some assumed "rationality" due mechanics or laboratory results. I know some of you are weary of this discussion, but hopefully if you are one of those, you haven't read this far and have long since deleted this message. These are real issues to those that design residential buildings. Mark Swingle, SE Oakland, CA --------------------------------- --------------------------------------- Martin W. Johnson wrote: Most of the comments about this topic have seemed to me to have been missing the real issue of how we are designing diaphragms versus how they in fact behave.......Etc....... Best regards to you all, Martin.
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