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Hello everyone, I have a question from steel structural designers. Does purlin help in reducing effective span of main rafter? and similarly does girt help in reducing effective height of column, does it give any lateral stability to column?

You should be using SAFE for this. If you want to do it in ETABS, the best way would be draw piles under the mat foundation or use springs. You can ask Geotechnical engineer to provide your formula for pile settlement or if you don't have that option, assume a 25mm settlement of piles (this analysis might not be very accurate) and divide the load by 25mm to get spring constant and model those springs at pile locations to analyze the raft. You can then design piles manually considering geotechnical and structural aspects. For more information see: Thanks.