Rigid Diaphragm Analysis

[joemarch]

Can any body direct me to any information on the design of rigid diaphragm torsion using a combination of masonry shear walls and moment frames.

Has anyone ever done this design before?

Thanks for your help.

 

[UcfSE]

Do you have something for a rigid diaphragm with only shear walls? I think the analysis would be similar to having only shear walls except that with a rigid diaphragm the masonry shear walls will attract most of the load because they are so much stiffer than the moment frames. Try modeling the diaphragm as you would normally but with the proper rigidity EI for your frames and shear walls.

 

[joemarch]

UsfSE, I have an old design example from the National Concrete Masonry Association. Otherwise I've used EnerCalc to figure out my shear wall forces. I'm not sure you can accomodate both steel frames and masonry walls. Let me know what you think. I think I need to head to the library.

 

[Lutfi]

I use Enercalc for rigid diaphragm. It gives decent results. Else, you can build a spreadsheet to compute the wall rigidities and have it proportion the shear to each wall based on its rigidity.

Regards,



Lutfi

http://www.cdeco.com

 

[mrengineer]

See Amrhein "Reinforced Masonry Engineering Handbook" for excellent examples of rigid diaphragm analysis with masonry walls, including equations used to calculate lateral deflection of walls. Notice that relative rigidity = inverse of lateral deflection, whether you are dealing with shear wall, or moment frame. Therefore you just need to compute lateral deflection of each shear wall and moment frame for whatever nominal load you choose, say 1000 kips or whatever. You can then use Amrhein procedure, or save yourself some time and use a program such as Enercalc, by coming up with an equivalent wall length/thickness that would result in same lateral deflection for each moment frame.

 

[joemarch]

Lutfi, Say I were to use Enercalc. In order to enter the wall data in for the steel frame, would you convert the steel frames to a concrete transform section and then enter the appropriate hieght, thickness, and length factors.

Thanks for the help.

 

[joemarch]

Thanks guys for all your help. Its well appreciated!!

 

[UcfSE]

I was going to recommend the Amrhein book also. He has some good information on rigid diaphragms. You're right, I don't think Enercalc will let you put in your own moment of inertia. Perhaps you could calculate the stifness of the frame and then find what masonry shear wall has about the same stiffness and use that for your input to Enercalc. You will have EI for the frame, divided by E of masonry and that would be the I for an "equivalent" shear wall. You can solve for the length then knowing the width of masonry you are using. Don't forget to take E into account and not just I. If nothing else that might be a quick sanity check. Does that seem reasonable? The only thing I don't like about Amrhein's book is that I don't remember seeing anything about loading due to rotation of the diaphragm. Walls farther from the center of rigidity take more load than walls closer, due to torsional moment that is. It's been a while though, maybe I forgot.

 

[Lutfi]

Joemarch,

Enercalc is handicapped in this because it does not allow for none homogenous modulus of elasticity. When faced with combo materials, used for shear walls, I resort to spreadsheets and prorate the E based on one of the materials. I normally use Masonry if it is the dominant material and then ratio steel and concrete based on CMU. You also have to compute I for each wall manually (or have excel do it with minor formula).

Regards,
Lutfi


Lutfi

http://www.cdeco.com

 

[haynewp]

Amrhein's book does address rotation as well.

There is a bug in Enercalc's rigid diaphragm program when the walls are not orthogonal. I found it last year and brought it to their attention.

 

[Lutfi]

haynewp,

Do you know if Enercalc took care of the bug? I must say that I have not used Enercalc for non-orthogonal walls.

Regards,


Lutfi

http://www.cdeco.com

 

[haynewp]

They told me the rigid diaphragm program is a very complex module and they would be working on it. That was about 6 months ago, I don't know if a patch has been released or not.