Mass eccentricity

[jsmatte]

Anyone ever heard of studies made on structures with mass eccentricity, i.e. the center of mass of adjaent floors do not have the same location???

 

[johnbridge231]

Mass eccentricities are the main cause for torsional behaviour of a building, which should be normally avoided using shear walls at the perimeter of the floor. Torsional rotation of diaphragms produces extra shear forces at columns, which results into larger bending moments and consequently not economical design. Moreover, such deformation of buildings (torsional) are not considered energy dissipative, ie their ductility is very low.

Analysis and Design of arbitrary cross sections
Reinforcement design to all major codes
Moment Curvature analysis

http://www.engissol.com/cross-section-analysis-design.html

 

[jsmatte]

I agree, but in the case of structures with setback (stepped structures) the centers of mass of the two parts of the strucutre do not have the same location. it is therefore an important parameter to consider if you are looking at the seismic behaviour of the structure. I am just wondering how important the effect of the mass eccentricity is

 

[johnbridge231]

Then comes engineering judgement...

Analysis and Design of arbitrary cross sections
Reinforcement design to all major codes
Moment Curvature analysis

http://www.engissol.com/cross-section-analysis-design.html

 

[wannabeSE]

jsmatte,
I do not understand the question. Regularly, buildings are analyzed and designed with the mass not at the same location on all floors. The location of the center of mass for each level must be considered in the analysis.

 

[TLHS]

What exactly are you interested in? I'm sure there are studies, but it's a pretty broad question.

It's a condition that would generally disqualify a building for the simplified equivalent static method of seismic analysis under the Canadian Building Code. Seismic response would generally be the big issue for me.

 

[jsmatte]

@TLHS, indeed, this is a condition upon which a structure can be classified as an irregular structure and have the restriction of not being able to be analysed with the ELF method. And the point you are bringing by mentionning the ELF method is very good as it was the answer I am looking for (sorry if it was not clear...).

In the codes, you have certain criteria that define each irregularity. For example, the torsional sensitivity irregularity is defined by the B factor. If the latter is greater than 1.7, then the structure is considered torsionally sensitive and it was shown that it cannot be satisfactorily analysed by the ELF method. This particular definition was developed of stiffness in-plan irregular structure. But what about the in-plan mass eccentric systems? Is there a value ( max eccentricty expressed as a % of the plan dimension of the floor) that was found and that allows to make the difference between a mass eccentric torsionally sensitive and non-sensitive system?

Hope it is clearer now, let me know if you have any questions... Or if you have an answer!

 

[TLHS]

I don't have a copy of the code here to reference, but at least in the Canadian code I believe the B value is determined as the ratio between the greatest deflection to the average deflection (i.e. if the system torques such that the corner on the right deflects out 15mm and the average deflection is 7.5mm then it's torqued counter clockwise with a B factor of 2.

Being defined this way means that it takes into account both eccentricity due to location of load and eccentricity due to location of resistance. The two items can't really be looked at independently anyway, since it's the relation between the two locations that creates the torsion to begin with. Don't forget to include any code required additional assumed eccentricity when you're checking this.