time history - gravity load

[chiefchester]

Hi all,

I want to preload my structure with a gravity load before applying a lateral earthquake load. I'm using a ramp function and applying the dead static load case.

When I look at the time history results, I do not see any effect of vertical load (but I do see horizontal if I put a lateral load in the gravity case). Can someone tell me what I'm doing wrong or what I'm not doing?

I am working with ETABS 9.5 NL.

Thanks so much for your help.

 

[chiefchester]

I have figure out the solution to my problem. I needed to use Ritz Vectors as opposed to Eigenvectors. Can anyone easily and clearly explain why? I understand that complex models may need this distinction but my model was very simple (one beam simple). If I figure it out, I'll let you know.

 

[ge0san]

In my experience and understanding, to be able to get results from a vertical dynamic analysis, you need to allow for vertical translation degree of freedom to be taken into account. To do that, in the "Define" > "Mass source" dialogue, you must uncheck the "Include lateral mass only" option. This way, you get vibrating mass (as percent of total mass) on vertical direction. Then, the mass is multiplied by the corresponding value of acceleration (from time hystory or spectrum case) to yield a force, as action. Then you can get results different from zero, which can be read the standard way.
You may find an advantage in using Ritz vector analysis, as by using this option you can reduce the analysis time, investigating only for OZ acceleration. Also, many codes usually require a minimum of 90% of vibrating mass to be taken into account for the degree of freedom in discussion. In modal analysis, from the number of modes requested, a vast majority of them will be associated with lateral vibration for structures not sensitive to vertical vibration (not having second order supports, long cantilevers, etc).

 

[ge0san]

I'm sorry, I misunderstood you question.
In "Define" > "Time hystory cases" add a history case for Dead load only (or other initial static gravitational loads you need). Then define a second history case for the lateral action, but at "Start from previous history" drop-down menu select your gravity Dead load history case previously defined.
This should work.