In the Load Combination spreadsheet you can turn on P-Delta analysis by entering Y into the P-Delta column. You can quickly turn on P-Delta for an entire column by highlighting the cells in that column and using the Fill Block command. (For more information on the Fill Block command, check out How to Use Spreadsheets More Effectively in RISA.)
The options for P-Delta analysis include Yes for all members or Compression only. The tension members have a reduced flexural demand because the secondary shears act to straighten these members. In the picture below, you can see how the tension member is aided by the direction of the secondary shear application from P-Delta.
You might not want to have P-Delta turned on in your model at all times. It’s a common practice to design all the gravity members first, and then consider the lateral stability of the model. In this case, you want to get the results of your model even if the code requires you to check for P-Delta (AISC, CSA, etc). To turn off the P-Delta requirement for these codes, go to Tools > Application Settings and select the Solution and Results tab.
You can check Don’t require P-Delta analysis for LRFD, CISC, etc.? and you will get a warning that P-Delta is turned off but all the members will be designed. Once you’ve set your preliminary design, you can go ahead and turn P-Delta on as mentioned above.