location of shear wall in multistory building 3

[zeeshanahmed]

In a 12 Story building with a very irregular grid rectangular in the middle then circular at one end. What are the factors that should be kept in mind when placing shear walls at a certain location in this or any other building?

is there any reference/code stating or justifying the location of a shearwalls in a building.

where should a shearwall be placed in any building?

 

[csd72]

To avoid unnecessary torsion, the center of stiffness of the shear walls should be at the center of the applied load (wind/seismic).

 

[JAE]

Yes, I'd just look at trying to make the layout of shearwalls as balanced and uniform as possible, looking at both orthogonal directions and trying to keep the center of rigidity and the center of mass as close together as possible.

 

[csd72]

JAE,

that is assuming it is a seismic area, if not the ceneter of wind load is more relevant.

 

[JAE]

CSD72, true.

At least in the US, everywhere is a seismic area, though..some more severe than others....but your point is a good one.

 

[zeeshanahmed]

JAE and CSD72 , you both are senior engineers and structural engineers, i am just a civil engineer and not much experienced with structures, its not that i am designing anything but i want my concepts more clear about structures, so please dont mind me asking ABC questions of structural engineering in this respect.

1) what is "center of loading" ? how do you find a center of loading in a building?

2) i know about center of gravity or mass, but what is center of rigidity ? and how do you find that out in a building ?

 

[JAE]

I think what csd72 is stating for "center of loading" is that for each story, either seismic or wind loading is applied to the structure and there is a resultant and effective center point where the load acts as an equivalent point load.

For wind:
Say you have a 40 ft. wide building. The wind would typically be centered at 20 ft. from either end and in the center of the wall surface vertically.

For seismic:
For a uniform building shape, each floor's center of mass is located in the center of each floor..assuming all the floor framing is uniform and all the walls are uniform.

If they are not uniformly laid out, then you have to calculate where the center of mass at each floor is:

[Σ](weight x distance)
------------------------------------ = dist. to c.m.
[Σ]weight

 

[zeeshanahmed]

thanks !

what is center of rigidity ?

 

[JAE]

You might want to look this one up or have someone show you - essentially it is the geometric point, in each direction, that represents a location where a load, applied laterally to the diaphragm, will not induce torsion in the floor, only lateral translation.

You have to determine the rigidity (kips/inch) of each brace or shearwall line and then find their weighted center, again - in each direction.

 

[jike]

As was stated previously, try to align the maximum loading condition such that the center of applied loads align with the center of rigidity of the resisting elements. Keep in mind that you will have some partial loading conditions that will be eccentric to the center of rigidity.

 

[csd72]

The center of rigidity is only really important when you have more than two rows of shear walls/bracing or are in a high seismic region.

If you only have two rows then the horizontal loads are distributed according to simple statics (somewhat akin to a point load on a beam).

 

[tngolfer]

If your shear wall are located around the perimeter, you may need extra steel adjacent to the walls to limit the crack widths in the slab as the slab tries to shrink when it cures. The shear walls will try to restrain the shrinkage. Aalami and Barth wrote a good paper, "Cracking in Prestressed Concrete Structures" that talks about this.

 

[csd72]

Put your shear walls half way along each side and this is not so bad.

 

[hokie66]

I think what tngolfer is referring to applies primarily to basement walls. He is correct, but that is a different topic from the OP.

 

[zeeshanahmed]

Does IBC2006 addresses this issue ?

 

[tngolfer]

Hokie, it applies to any concrete framed slab but is generally worse the closer you get to any point of high restraint (the ground for example). Granted, the restaint is much worse if the shear wall is part of an elevator/stair box-shape or in it's strong direction though but it will still try to restain the shrinkage.

I have uploaded the article if you are interested. (Sorry for the picture quality - scan of a copy.)

http://files.engineering.com/getfil...acking_in_Prestressed_Concrete_Structures.pdf

 

[hokie66]

tngolfer,

Thanks for that. Restraint is always an important issue. I have not read this particular article, but will do so when I get a chance. Looks like one a lot of people on this site could use, as I have the impression that some think that concrete just cracks for no apparent reason. By the way, and you probably know this, but the reinforcement doesn't "restrain the shrinkage" or prevent cracking, it controls the cracks to an acceptable width.

 

[hokie66]

tngolfer,

I did read the article, and it is an excellent, largely non-technical explanation of restraint cracking with a lot of examples. It correctly discusses the problem as being worse in unbonded post-tensioned slabs than in conventionally reinforced slabs. What is not said is that bonded post-tensioned slabs work better than unbonded slabs, but even so, deformed bars are required in many cases for effective crack control.

 

[OzEng80]

zeeshanahmed,

There is advantages in locating your shear walls (where possible) to correlate with your gravity load structure. By that I mean selecting walls with sufficient compression in them due to dead load / self weight so that overturning moments induced by lateral loading do not result in tension (or very little) across the section. I have found large compression is easier (and more efficient) to handle than large tension.

 

[ryansteel]

Whooa! placing shearwalls are for really experienced structural people. also to consider ARCHITECTS restrictions.

Where the objective is that center of load should
coincide with the center of rigidity. which is very tricky to do and takes a lot of experienced Engineering Judgement.
we know we cannot in reallity made this perfect,
dont hesitate to do "trial and error" using a good 3d analysis software.

I learned a lot in this thread (thanks to all).

one question....i know it is logical to loose the 5% eccentricity as required by code when your structure is irregular. has anyone tried? coz I havn't.