Mass for calculate frequency of building 1

[SPR Baker]

Dear All
what mass combination (Masssource) for calculate frequency of building?
Thank you

 

[Blackstar123]

Dead + Some portion of the live load?

 

[SPR Baker]

Blackstar123: which standard referent ?Blackstar123

 

[r13]

Structure weight considered fixed/permanent in place. Mess is something tangible, not the effect of forces.

 

[HTURKAK]

what mass combination (Masssource) for calculate frequency of building?'

I understand Effective Seismic Weight which ASCE 7-16 defines at 12.7.2 =

' Effective Seismic Weight.W, of a structure shall include the dead load, as defined in Section
3.1, above the base and other loads above the base as listed
below...'

ASCE 7-16 C 12.7.2

'Effective Seismic Weight.... When a building vibrates during an earthquake, only that
portion of the mass or weight that is physically tied to the structure needs to be considered as effective. Hence, live loads (e.g., loose furniture, loose equipment, and human occupants)
need not be included. However, certain types of live loads, such as storage loads, may develop inertial forces, particularly where they are densely packed.
Also considered as contributing to effective seismic weight are
the following:
1. All permanent equipment (e.g., air conditioners, elevator
equipment, and mechanical systems);
2. Partitions to be erected or rearranged as specified in
Section 4.3.2 (greater of actual partition weight and...'

 

[JoshPlumSE]

One thing to point out, sometimes we can be really conservative with our dead load. Like assuming a higher dead load to give the future tenants the ability to place equipment anywhere, or use any type of heavy flooring, etc.

However, this is not always a good idea with seismic loads. It's often better to use a more realistic value for dead load when calculating seismic mass and seismic forces.

 

[Blackstar123]

HTURKAK answered the question about reference.

JoshPlumSE, I don't understand. If we're designing for higher dead load and indirectly higher seismic force, then how could be this not a good idea for seismic design? Or did you meant that it will not be a good idea economically?

 

[r13]

.. that it will not be a good idea ..

That will be in line with my thought - add uncertainty on top of another uncertainty.

 

[JoshPlumSE]

Blackstar -

For dead load, we may intentionally over design our gravity system. Just to provide extra flexibility to the owner.... to locate their heavy equipment anywhere in the building. The owner may gladly deal with some extra cost for that extra flexibility.

But, if we assume the heavy loads exist over the entire building when we know for sure that they're only in a small portion of it, then we being over conservative there as well. And, the owner may not wish to have that extra conservatism since he / she doesn't get benefit from it.

On another note:
If you want to get the most "accurate" natural frequency, this requires you model the mass and stiffness as they actually exist. So, if you need to avoid certain frequencies (because of dynamic equipment or such), then being over conservative with your assumed mass is not desirable.