Cantilever Flexible Diaphragm

[lutein]

I am working on simple rectangular building with metal roof deck, i.e. flexible diaphragm. Have you ever designed a flexible diaphragm building with only shear wall at 1 side? i.e. the building has a L shape shear wall, and that's all for the whole structure. In this case, I have assumed the diaphragm is cantilevering out from the wall, is this a valid assumption for a flexible metal roof diaphragm?
Please help and advise.
Thank you very much.

 

[FFIELD]

No, you can not design a building with only shear walls on two sides. Depending on you local code and what type of lateral loads you have you need at least 3 sides with shear walls. Lateral loads can be taken in rotation in this type of system. You would be better of with shear walls on four sides. In earthquake country you cannot use 3 sides of shear walls with a flexable diaphram. In in any case you cannot cantilever a diaphram from a shear wall!

 

[lutein]

FFIELD,
Thanks for your help and reply. Well, the lateral load is only minimum wind load. It is a 1 story ractangular building.
I do not have the the option of putting 4 sides of shear wall due to arhitecture features. and I don't want to put up a moment frame to resist the minicure wind load either due to the cost. So, I am kind of debating in between myself. I will definitely use 3 sides of shr wall as recommended. Therefore, there will be a U shape shr wall. Shear walls shall be designed for torsion. We could look at this as a plate welded at 3 sides at 1 end, and calculate the reaction of the eccentric weld (the shr wall reaction), can we not?
Again, thanks for your advice.

 

[ARIFSIDDIQUI]

as long as the system has to bear only wind load further sice the building is just 1 story the magnitude of the wind loading is very low which in general should not afffect the system as u planned and it must be ok if designed properly but definitly it will not act as desired under seismic situations.

u must consider the diagnoal wind case also may be that will be critical since one side shear wall is missing.

ARIF ALI SIDDIQUI
STRUCTURAL DESIGN ENGINEER
IMCC 8 JANTAR MANTAR MARG
NEW DELHI INDIA

 

[shemp]

FFIELD,
Where in a code (say, IBC) does it say you can't construct a building with a flexible diaphragm with only two shear walls, one in each direction? It seems to me as long as you account for the rotation of the diaphragm and the force that is applied to the perpendicular wall, then the building will be in equilibrium.

Why can't a flexible diaphragm cantilever from a shear wall? A shear wall itself is nothing more than a cantilevered diaphragm, no?
Thanks for your response,
Shemp

 

[Taro]

shemp,

IBC 1604.4: "Any system or method of construction to be used shall be based on a rational analysis in accordance with well-established principles of mechanics."

If you have only 2 walls in an L shape, the system will not satisfy equilibrium. Try summing moments about the heel of the L. The forces in both walls are coincident with the point of summation. Therefore you cannot resolve the diaphragm rotation.

 

[shemp]

Thanks Taro, I spoke without thinking. You're right, having only two shear walls would cause the walls to twist.
But as far as having a cantilevered flexible diaphragm, as long as you have collectors, it can be done, right?

 

[JAE]

Taro - I agree with you - just another thought on this - what if the walls were semi-rigid in the direction perpendicular to their face? Like a wall cantilievering up out of the ground from a moment footing. then, you would have the ability to resolve the forces down through to the ground... a complete load path.

 

[Taro]

JAE,
Not a very efficient system, but it could work if the loads are very low (say for a small bus shelter or shed).

Shemp,
Yes, it is quite common to have walls/frames on 3 sides and cantilever flexible diapgragms. Examples include strip-malls with glass storefronts along one entire wall or apartment buildings with shearwalls along the interior corridor but not at the exterior walls. Diaphragm spans and/or aspect ratios are often limited by building codes for these systems.

 

[lutein]

Thank you all for your kind reply.
I have also investigated this by using FEM. Metal deck and shear wall were modeled as shell elements and aiaphragm shear is obtained and checked from the FEM results.

This brought up another question, I am looking for a good reference book that has guidelines to analyze for buildings with odd shape, or non-typical buildings, like a guideline for lateral analysis of building structure, could you guys give me some names please?