Building fundamental modes in ETABS 3

[Robert-32]

By experience and to verify my model of 10 story building in Seismic zone 2B, what should mode 1, 2,3 look like? The building foot print is rectangular about 35*20m, with distributed sheer walls. How long should the period be?

 

[slickdeals]

Traditionally, translation, translation, torsion would your first 3 modes but this is not always the case as it would depend on your building. Your period should be in the range of 0.7 - 1s.

 

[Robert-32]

Thank you slickdeals, actually the period appeared to be either 0.95 when I use semi rigid diaphragms in Etabs or 0.75 when I use rigid diaphragms. Witch one do you think is the correct period? My slab is one way ribbed slab with hidden beams.

 

[SCper]

- The ASCE7-05 12.8.2.1 gives you and Approximation to the fundamental period
- You should use rigid diaphragm

 

[Robert-32]

I agree on using rigid diaphragm gives a value of 0.75 and this is more realistic than 0.95 since Ta is calculated at 0.614, but In one of the posts somebody said that from the moment they included the semi rigid option in Etabs he used this option all the time. What do you think?

 

[structSU10]

The use of semi-rigid should be based on how your diaphragm is laid out. If there are a lot of opening in the diaphragm that are large and may affect its stiffness, semi-rigid may be required, or at least more appropriate.

Also note, semi-rigid is more 'realistic' but thats only if the diaphragm properties are defined properly for the program to use.

If you are not 100% sure, you can run it both ways and use the worst case for your design.