Time-history analysis FAQ

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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.

What is Format Type and Arrival Time?

Extended Question:

1. In the time-history definition, what is Format Type? Also, which option should be used?
2. In the time-history load case, what is Arrival Time? Does this imply that the segment at the beginning of the time-history record may be neglected?

Answer:

1. Format Type options indicate how data is presented in the time-history function text file.
   
   
   • Free Format is applied when spaces or tabs separate the data.
   • Fixed Format is applied when data is not separated by spaces or tabs. In this case, the number of Characters per Item should be specified in the edit box. Also, the Number of Prefix Characters should be specified under Function File. The software begins reading data after skipping this many spaces.
2. Arrival Time is the time at which a load assignment begins. This value may be positive, negative, or zero. Time-history analysis always begins at time zero for a given load case. Therefore, when a negative arrival time is specified for a given load assignment, any portion of the record which occurs before time zero is ignored. For example, an arrival time of -5sec will neglect the first 5sec of the input function. Arrival time may also be useful when staggering the application of ground motion in the global X and Y directions.

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:

Figure 1 - Choose Tables for Display

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

Figure 2 - Output Options