# What building mass should be applied for 2D analyses?

#### Sukhi

I am performing a 2D analysis on a steel frame structure that is 8 storeys high using NZS 1170.5:2004 (New Zealand Standard for Earthquake Actions). How much of the original 3D structure's mass should be added to the 2D frame to make the analysis realistic? How should this mass be distributed?
Thank you.

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#### berkeman

Mentor
I am performing a 2D analysis on a steel frame structure that is 8 storeys high using NZS 1170.5:2004 (New Zealand Standard for Earthquake Actions). How much of the original 3D structure's mass should be added to the 2D frame to make the analysis realistic? How should this mass be distributed?
Thank you.
It looks from your other thread like you are taking a class in modeling for mechanical/civil engineering? Can you say more about the class and the projects?

#### Sukhi

It looks from your other thread like you are taking a class in modeling for mechanical/civil engineering? Can you say more about the class and the projects?
I am doing a research project where I have to apply Numerical Integration Time History Analysis to a steel building. Although in other structural design codes I have found different ways by which 3D mass can be applied to 2D, I can't find the same for the New Zealand Standards. It would be greatly appreciated if you can help me with this.
Thank you.

#### JBA

It appears that your real question should be "How should this be done in accordance with the New Zealand Code?" which is best answered by contacting the New Zealand code authority.

PS If you are actually located in New Zealand, it is a beautiful country and you all have my greatest sympathy for the recent devastating South Island earthquake and its ongoing aftershocks.

#### mle

You have to use enginnering judgement on this. If the 3D structure is regular, that is with identical 2D frames, with equal spacing between them, bracing is evenly distributed (or doesn't exist), the loads are uniform at any level etc, then a first approach would be to assign to the 2D frame, the gravity loads, at either side of the frame (up to the half spacing distance). Doing so, you assume that no framing exists in the perpendicular direction, but normally this is not against safety.

For a more robust method, you have to distribute the uniform gravity loads of the 3D straucture to the nearest beams, and then collect for each beam the mass that is assigned to it. Normally no mass is assigned to column elements and bracing, other than self weight.

Also pay attention which loads contribute to the mass. Normally the dead loads and a part of the live load is included for seismic analysis, but that depends on the seismic code.

"What building mass should be applied for 2D analyses?"

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