Effectibve mass in mass irregularity 3

[NewbieInSE]

Dear Engineers,
I would like to know, as I'm confused, what is meant by "Effective mass" in mass irregularity when the codes say "Mass irregularity shall be considered to exist where the effective mass of any storey is more than 150 percent of the effective mass of an adjacent storey. A roof which is lighter than the floor below need not be considered.  

Thanks.

 

[Ganesh Persaud]

This is used for the purpose of estimating seismic loads, ASCE requires calculating the effective seismic weight (effective mass) which includes dead load, partitions and permanent equipment, plus 25% of the floor live load in areas used for storage. This contribution of live load as inertia seems to correlate with the low likelihood that live load objects be present at the time of occurrence of the design earthquake. After computing, it’s when one of the story mass in a multistory structure is larger than the above story mass by 150%. When this happens you will have vertical structural weight irregularity.

 

[r13]

I think "effective mass" is just the "seismic mass (weight)" of a story, that is to be included in the seismic design.

 

[NewbieInSE]

Couldn't code explicitly use "Seismic Weight" in the definition of Mass Irregularity, or may be effective mass is the sum of all dead loads plus all live loads.

seismic weight (effective mass) which includes dead load, partitions, and permanent equipment, plus 25% of the floor live load in areas used for storage.

Do you know of any source where effective mass is meant to be seismic weight or any book reference where I could confirm this?
Thanks, Ganesh Preaud and retired13.

 

[Ganesh Persaud]

The effective mass in this case would be, for example, building story effective weight lumped at the center of gravity of the building story. If the effective mass during seismic event is away from its center of gravity, you’ll have the irregularities.

You could also search within asce7-10 “mass”, “weight” And “effective mass”

See section 15.4.3 and then read section 15.7.9.3.2 In asce7-10 and you’ll see the correlation.

Look at this example from asce also.

 

[NewbieInSE]

Thanks Ganesh Persaud
It looks convincing.

 

[JoshPlumSE]

The purpose of this "mass irregularity check" is to compare the relative weight / mass of one story to the ones above and below.

Why is that important? If you've got large differences in the weight / mass of adjacent stories, then the Equivalent Lateral Force method of seismic force distribution isn't going to be very accurate. That's because the ELF method is based on the assumption of a single, dominant mode with even distribution of mass. So, for structures with mass irregularities, the code may require you to use a different method (response spectra analysis) in order to better capture the dynamic forces on the structure.

 

[NewbieInSE]

Why is that important? If you've got large differences in the weight / mass of adjacent stories, then the Equivalent Lateral Force method of seismic force distribution isn't going to be very accurate. That's because the ELF method is based on the assumption of a single, dominant mode with even distribution of mass. So, for structures with mass irregularities, the code may require you to use a different method (response spectra analysis) in order to better capture the dynamic forces on the structure.

Thanks JoshPlumSE for your insightful comment.