×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Mass eccentricity

Mass eccentricity

Mass eccentricity

(OP)
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???

RE: Mass eccentricity

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-des...

RE: Mass eccentricity

(OP)
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

RE: Mass eccentricity

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.

RE: Mass eccentricity

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.

RE: Mass eccentricity

(OP)
@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!

RE: Mass eccentricity

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.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources