Time-history analysis FAQ

What value of scale factor should I use to define a time-history load case?

Answer: The two options available are described as follows:

• Load pattern. When load type is set to load pattern, the values of the time-history function are multiplied by the scale factor to produce unitless terms. Here, the record should describe the variation in loading over time, as applied in the load pattern.

• Accel. When load type is set to Accel, the scale factor should be specified such that it multiplies with the values of the time-history function to produce the acceleration desired in the current units. If the values of the time-history function are given in the current units, the scale factor should be set to 1.0. Otherwise, the scale factor should reflect the conversion from the units of the time-history function to the current units.
  
  For example, if time-history data is in cm/sec/sec, and current units are ft/sec/sec, then a scale factor of 1ft / 30.48cm = 0.0328 should be used.

What forces are included in reactions from time-history analysis?

Extended Question: Are the reaction forces which are reported for direct-integration time-history analysis as follows:

• Elastic force = K*u
• Damping force = C*du/dt
• Inertia force = M*d²u/dt²

Answer: Reactions include only the effects of elastic, and possibly inelastic, forces, unless links are connected to joint locations, in which case the damping forces within link objects are included during computation of joint reactions.

What value should be specified for the output time-step size during time-history analysis?

Answer: Output time-step size should be small enough to provide sufficient resolution for analysis. The required resolution is dependent upon the characteristics of applied loading and such structural properties as the natural periods. The sufficiency of resolution may be determined by decreasing the time-step size until it is small enough to not affect results.

Can I specify different time-step sizes for different phases of the time-history analysis?

Answer: Time-step size is fixed for a given time-history analysis. However, a series of nonlinear time-history analyses may be defined with variable time-step sizes, then these analyses may be chained together.

How are time-history results displayed at each time step?

Answer: Time-history results may be displayed at each time step by selecting the appropriate response quantity through Display > Show Tables, then by selecting Modify/Show Options, as shown in Figure 1:

Direct History Results should then be changed from Envelopes to Step-by-Step, as shown in Figure 2: