Reaction-Seismic analysis-Larger vertical forces without vertical acceleration

[Anonymous]

Hello everybody,

I have modelled a slender, rectangular based steel tower with the height of more than 20 m. I made the modal analysis and it gives 11 modes, which look correct.

After I made a seismic analysis according to IBC 2012. I attached a picture about the resulted reactions for the 4 supports in case of 1*X+1*Y.

My question is how it is possible, that without vertical acceleration(!), how did the vertical forces increase?  Initially, from the self-weight it was 4*~17.8kN=71.20kN, now at this case when the 2 horizontal excitation happened it is 4*~(58.6-58.9)kN=235kN...

Thanks in advance, and I appreciate an early reply if possible.

[Anonymous]

In addition I thought it because the moment, but I am not sure about that regarding the results' table.

[Rafal.Gaweda · Accepted solution]

It seems you missed setting of combination sign \ main mode.

See slide 28 and 41 in webinar 7

https://www.youtube.com/watch?list=PLY-ggSrSwbZqow_60fiqJwS69mg1nQMzk&v=3G5NwyDSzxs

without signing - look at sum of reactions \ sum of forces in Reactions table

[Anonymous]

Thank you for the answer Rafal!

I did now these steps and I got the results. I did it now with another tower as well, which is triangle based (also a slender tower). But now another question just arised for me. I guess it is a basic static. 

The reaction from self-weight is ~30kN and I git now the attached results.. How the static equilibrium appears? If I have larger vertical forces (because of moments) we still need the balance and I don't see that with such moments.

I can send you the model if you can attach an email address or here.

Thank you very much!

[Anonymous]

Dear Rafal, I guess I found the balance that I am looking for. 

However, some explanation would be welcomed according to this phenomena: why did the vertical forces increased ? Why do we need this signed mode calculation?

[Rafal.Gaweda · Accepted solution]

Set  modal case correctly:

or

to get

[Anonymous]

That was again really helpful, thank you. I appreciate your time and effort! I got the results, that you have also got.

What I did before, I used the percent of mass participation limit with 90% of the mass. Why this type of calculation gave wrong results? And why we had the problem without the signed modes?