Time History Analysis (UBc/ASCE-7)

[isles19]

I am interested in performing a time history analysis. Both UBC 1997 and ASCE-7 02 state that for a three-dimensional analysis "ground motions shall consist of pairs of appropriate ground motion acceleration components..." I am unclear what they mean when they say "pairs". Does this mean one ground motion applied in two orthogonal directions or one analysis consists of two ground motions applied in orthogonal directions or something entirely different.

Any information would be helpful.

 

[Taro]

It means that the instrumentation that recorded the earthquake accelerations captured the ground motions in both directions simultaneously. The two components of the earthquake record are applied simultaneously to the computer model. The orientation of the applied loads is then varied to determine the worst-case direction for design.