when to design with shear wall?

[migs29]

guys I have question for you....

when it is needed to put shear wall in a building?
what is the basis or criteria to consider that the building need to design with shear wall???

there is an instance that we design concrete two storey residential building. We design the frame as a lateral load resisting frame. We used masonry exterior and interior wall. When we submitted the plans for issuance of building permit, the building official required as to put shear wall. It is my question why we required to put shear wall since we design the structure safe to any anticipated load that may occur and I don't have any idea where the shear wall should be located. The building official doesn't explain why they required us... I want to know what to explain why and when does a particular building need to design with shear wall..


thanks to any input you may have....

 

[shin25]

The best place to start with should be the local building codes and their requirements. Since you do not have shear wall, how the frames are transferring the lateral forces, moment frame action? Are you located in seismic area?

 

[csd72]

migs29,

Deflection is the reason. Under wind load the frame will move but the brick will not (and isnt the frame supposed to be supporting the masonry?).

Shear walls deflect much less.

Check th building code to see if there is an in plane deflection limit for masonry veneer and if your frame meets it.

csd

 

[migs29]

thanks guys for the input but i don't really understand the point... in my own understanding shear wall is different from masonry wall right? what I mean masonry wall is CONCRETE HALLOW BLOCKS filled with mortar with horizontal and vertical reinforcement, that wall can resist 250km/h wind speed acting on the plane... that masonry wall can resist lateral load in fact... my point is we design the building that can withstand to any possible load may occur, we can justify that our design is safe and in conformance with the structural code through structural design computation .. so why need for something more, like putting shear wall which is in our assessment is not necessary... and will make construction expensive... Is it the duty of the building official is to check the stability of the design base on the design data and computation submitted? and not to suggest his opinion?

 

[shin25]

My guess is, in this perticular area, two storey buildings with shear walls are the predominant lateral force resisting system? The building official is not comfortable seeing anything else.

 

[hokie66]

If the reinforced masonry walls are acting as bearing walls, they will also act as shear walls. "Shear wall" is a generic term, and a shear wall can be constructed of concrete, masonry, steel, wood, or whatever. So it sounds as if you already have shear walls, but your analysis doesn't recognize it, as you say you designed the building as a moment resisting frame. Possibly the building official just wants you to designate on the plans the shear resisting elements and how much load they take.

 

[csd72]

As I said above, if your frame deflects more then the brick wall then it is no longer supporting the brick and the brick wall is actually taking some of the shear.

This may be what the building official is concerned about.

OR maybe they have the junior involved in your job and they have never encountered a building without shear walls.

csd

 

[msquared48]

The masonry (CMU) wall will only act as a shear wall if the floor or roof diaphragm is connected to it either directly or thry drag struts. In other words, it must be designated and designed as a shear wall. Some are merely partition or firewalls.

In most CMU buildings, the exterior walls are shear walls. If they are needed inside, additional CMU shear walls may be added to limit the diaphragm stresses and establish fire zones among other issues.

It is within the responsibility of the plans checker to comment and have you justify your design, but not to do the engineering. That is your job.

Mike McCann
McCann Engineering

 

[haynewp]

If the walls are not designed as shear walls, the code you are under may require the walls to be isolated from the structure (depending on your seismic category) so that shear is not induced in them. Or else you have to design them as shear walls and meet some minimum detailing requirements.

You really should call and ask the guy what he is thinking. Are you in the U.S.?

 

[ezoto]

First let us look at the design fundamentals.

Structural design is carried out on the basis of transfering actions/loads on structure back to the ground. On top of the vertical loads (dead and live loads due to gravity) that the building is carrying, it is required to resist horizontal forces (eg. from wind) as well.

For the 2 storey RC building mentioned by migs29, there are a few ways of transfering the horizontal forces back to ground:

1) moment frame - connections between main vertical member (column) and horizontal member (beam) are designed as "fixed", thus stabilizing the structure.

2) shear wall(s) - horizontal actions on building are transfered to the shear wall(s), which is designed as a cantilever beam, by the diaphragm actions (from the slab).

Migs29, from what was said i believe you've adopted the 1st solution. If you've justified in your design that, the columns and beams are able to take the additional moment induced when considering the load case with horizontal actions, i don't see why your design is rejected by the officer.

Probably it is worth clarifying your design with him/her in person. It's actually quite worrying that an officer that couldn't understand something so fundamental is doing the checking... Unless he saw something which wasnt pointed out in your questions.