Comments

• Victor

  November 2016

  Not directly but two ways around it:

  
  

  if you have base excitation of a rigid base, then you can use a fixed base with gravity load on the whole structure as acceleration. You can copy and paste tables of time-acceleration data into that.

  
  

  You could also use the large mass method where you convert the accelerations to forces apply them to a large mass (eg Loads & Constraints -> mass) which then accelerates correctly and transmits that acceleration to the less massive structure. This allows you to have different accelerations on different parts of the structure.

  
  
• Stefanos

  November 2016 edited November 2016

  Thanks Victor, great!

  
  

  Please let me ask you one more thing.

  
  

  Let's say we want to evaluate the envelope of internal forces N,V,M of a column member under time- acceleration excitation in two horizontal directions (for example in X and Z). After putting masses in selected nodes (representing dead and a portion of live loads) is it necessary to use a CONSTANT gravity load in direction of gravity (for example 9.81m/s2 in -Y), along with the two time-dependent gravity loads, for taking into account the self-weight of members, or this is taken automatically in dynamic analysis?
• Victor

  November 2016

  Self weight isn't automatic so yes, you have to specify -9.81m/s^2 gravity.

  
  

  
  
• Stefanos

  November 2016 edited November 2016

  I solved some simple structures with this way and I found some problems in evaluating the envelope of internal forces (or stresses). For example, when I am applying a constant gravity load in the direction of gravity (for example -10m/sec^2 in Y) the forces resulting from the accelerated masses have a variation from zero to double, so it's difficult to evaluate properly the internal forces or stresses. 

  
  

  Bellow is the simplest example of a (zero density) member under a 3 t mass and -10 m/s^2 acceleration (Y), while the horizontal time-dependent accelerations have been excluded (either are present or not the axial force is also doubled) . The correct axial force is 30 KN, but the analysis gives results over time from 0 to 60 KN (from zero to double). Thus, even if this is normal, for more complicated structures and load conditions will be really difficult the proper evaluation of the desired internal forces or stresses. 

  Is this correct or I understand something wrong;

  test.jpg 135.5K

  test.liml 1.4M
• Victor

  November 2016

  Sorry about the late reply. The oscillation is because the initial condition is the undeformed state, so it falls down under gravity, then bounces back and oscillates. Mecway doesn't currently allow non-zero initial conditions so you might have to use damping, or a vertical spring-damper element to eliminate that. Or ramp up the gravity initially.

  
  

  Here's an example with a damper. For linear analysis, it only acts in the vertical direction, so it won't affect horizontal accelerations.

  test with damper.liml 2.6K
• Stefanos

  November 2016

  I see, thanks.

  
  

  Could you please tell me how you calculated the proper value of the damper coefficient in this specific example?
• Victor

  November 2016

  I used trial and error until it damped the oscillation for most of the time steps. There isn't really a proper value since it doesn't represent a real damping effect. It's only valid if none of the time-dependent loading (ie. accelerations) have any components in the vertical direction.

  
  
• Stefanos

  November 2016

  OK, thank you Victor