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RE: Rigid diaphragm question

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If the building and lateral system are symmetrical all the way down then you can just apply the story shear at the center of mass at each level.   For the wall design then you add the effects from the wall stacked above to the wall below.
On a pitched roof acting as a flat diaphragm - if it constrains everything on a level to deflect together then it is acting like a diaphragm, how it is designed is up to the engineer...
-----Original Message-----
From: Mark D. Baker [mailto:shake4bake(--nospam--at)]
Sent: Monday, July 19, 2004 1:28 PM
To: seaint(--nospam--at)
Subject: RE: Rigid diaphragm question



My building is a rectangle, center of roof mass is at geometric center of building. Sounds like you are talking about calculating the center of rigidity of second floor walls and applying roof load to floor at that location….yes? If so, one would have to assume this trussed roof, 4:12 pitch was rigid enough to send loads to walls based upon their rigidity. I am not one of those in our profession who believes this type of diaphragm will behave in this manner.




From: Haan, Scott M. [mailto:HaanSM(--nospam--at)]
Sent: Monday, July 19, 2004 2:07 PM
To: seaint(--nospam--at)
Subject: RE: Rigid diaphragm question



I would do the roof first - figure where the center of reactions was for the roof forces and impose the roof shear on the second floor diaphragm at that location. 


The problem with the method portrayed in most text books for rigid diaphragm analysis by hand is that they use buildings are proportionate twisting structures.    If you are doing it by hand there are going to have to be simplifying assumptions no matter what. The problem is that the stiffness at a level along a grid depends on everything that is under it on the grid in a non-proportionate structure.


 Most small residential wood buildings are non-proportionate.  How do you trace the accidental eccentricity from floor to floor for a multistory non-proportionate twisting structure.


Scott Haan

-----Original Message-----
From: Mark D. Baker [mailto:shake4bake(--nospam--at)]
Sent: Monday, July 19, 2004 8:03 AM
To: seaint(--nospam--at)
Subject: Rigid diaphragm question

I have a two story building, roof construction is wood framed, floor construction is concrete, 1st and 2nd story walls are concrete.


I am considering roof diaph. flexible, floor diaph rigid.


When distributing floor lateral loads based on rigid diaph analysis,  would it be appropriate to bring the roof lateral forces directly down to the 1st floor walls added to the floor forces determined from rigid diaph analysis, or should I consider the roof level forces to flow into floor diaph and then be distributed to the 1st floor walls, ie: add roof and floor level forces and distribute the total based on rigid diaph analysis at floor level.


It seems the floor diaph would be more rigid than the 1st floor walls so the forces coming down from roof would first flow into floor before going into 1st floor walls.


Comments please.




Mark D. Baker

Baker Engineering