Vertical Structure Irregularties

[ahmedhegazi]

Hi all
I have structure located in Seismic zone 2A, the main lateral load resisting system is core wall, the main problem I have is that some RC walls are added starting from the 4 the floor up to last floor (12 floor), This will result in weak story irregularity according to UBC and IBC, can I assume that this walls will not be part of the lateral load resisting system and the all the load will transfer to my core wall and ignore this weak story effect?

 

[csd72]

ahmed,

Why do you need these walls? If the core is good enough up to 4th floor then it will be good enough above that.

That said, if deflection is an issue then maybe an out rigger system may be feasible to engage the outer lines of columns.

csd

 

[ahmedhegazi]

csd,
walls are need for construction issue and vertical load , the core can handle the lateral load safely. can i ignore this walls in lateral load analysis?

 

[Lion06]

don't tie it into the diaphragm

 

[csd72]

Walls are slow and expensive to construct. I would think it would be quicker and cheaper to build a frame structure with non-loadbearing infill walls (built at a later stage).

But I have not designed anything over 8 stories, so this is not really my forte.

csd

 

[ahmedhegazi]

Thanks all for your replies, the main reason for using wall is that in case I used columns and beams the column will have big width which will affect the Arch, requirements.

StructuarlEIT,

How can I separate the wall from the diaphragm?
Construction section shall be as follows
1-walls will be casted first with its top level equal to the level of slab soffit

2-Reinforcement will be placed for the slab and negative slab Rft will pass over the wall.

3- Slab will be casted with the reaming part of the wall (the slab wall connection will act as diaphragm)

 

[tdt41xf]

If the wall is needed for construction issues and vertical load only. You can engineer the bearing connection between top of the wall and soffit of slab. That means no ties rebar between slab and wall( flat top finish wall and slab bearing on it). For example you can use the fillet material between the slab and wall. Because the core shear wall already take all lateral load.So the lateral deflection should not be issue. Only the bottom wall you have to cast in-situ with slab below.

 

[csd72]

Can you use blade columns instead of walls thus reducing their stiffness for shear.

csd

 

[ahmedhegazi]

Csd72,
what you mean by blade column?

 

[msquared48]

You could also place an embedded steel plate in the top of the wall, and one incorporated into the structure of the diaphragm, or another steel beam above, with locking pins between the two plates. Incorporate slotted holes to engage the locking pins allowing movement only in the long direction of the wall, but have full bearing. The only shear the wall will see will be the frictional resistance of the steel plates due to the vertical load seen by the wall and even this can be mitigated with a teflon coating on the steel, or some other coating.

Mike McCann
McCann Engineering

 

[ahmedhegazi]

Thanks all for this feebback , but just i mean, as a principal of lateral load analysis can i ignore in my analysis this walls and design the corewall to resist all the force, however the wall will connected to the diaphragm in real.

 

[Lion06]

For strength, sure you can ignore it. But for serviceability issues such as potential cracking of finishes below... NO, you can't ignore it. If you tie it into the diaphragm, they are going to see the shear whether you account for it in design or not.

 

[ahmedhegazi]

StructuralEIT,

According to IBC2003 Table 1616.5.1.2 Item 5

Discontinuity in Capacity—Weak Story
A weak story is one in which the story lateral strength is less than 80 percent of that in the story above.
The story strength is the total strength of seismic-resisting elements sharing the story shear for the
direction under consideration.

in case i will apply to this type of vertical irreg. the design will be completly changed, so iw ould like to ignore this extra wear from shearingin the shear force for lateral loads, but for sevriceablty limits and gravity loadings this walls shall be considered

 

[UcfSE]

If the wall is connected to the diaphragm it will attract load because of it's stiffness, regardless of whether you want it to do so. It will change how your building responds. You should account for that in your design.

 

[csd72]

A blade column is a long narrow column that is much longer than it is wide - often in the range of 10" by 4'-0" or longer. They behave like a wall in the stiff directopn and like a column in the narrow direction.

Several of these would be much more flexible than one long wall and may meet the 80% criteria.

csd

 

[msquared48]

If the wall is not rreinforced to act like a shear wall, having the required rigidity to resist the forces it will see as alluded to by UcfSE, then it will appear wo fail in a major event, However, if you design the core to resist all the loads, there will not be a catastropic failure with loss of life - just a few scared clients when they see what turns out to be a structurally weak redundant element fail. It will help out in the smaller events, but will eventually fail.

I would run both scenarios though if you decide to leave the wall in place, as loads can shift with different layouts. Also, ifleft in place, the center of rigidity will be affected, possibly inducing some unanticipated torsional forces.

Mike McCann
McCann Engineering