Sequential Construction

[azureblue83]

The attached picture is from ETABS video regarding sequential construction modeling: [URL unfurl="true"]https://www.youtube.com/watch?v=oi2rdht_pZY[/url]

The left and right images are identical (same structural members and loads), except that the left image is considering the entire structural as a whole; while the right image is taking sequential construction into account, i.e. adding load one story at a time. The analysis shows that the deflection and internal forces for the transfer beam (1st fl beam) from sequential load is much higher.

I'm having trouble wrapping my head around why the two are so different. At the end of the day, when all the stories are built, shouldn't the right image be the same as the left? Can someone provide an elaborated explanation?

Thanks!

 

[Agent666]

It's due to the fact that the way the left one is modeled it assumes all the structure is present before the load is magically added instantly. So in the left example the load is shared more or less equally over all levels due to the presence of the transfer column mid span. If you propped under the transfer column until the entire building was complete and removed the prop you would get something similar to the left case.

Whereas the right one it attempts to more closely model what occurs in reality, whereby each floor is built sequentially, so the load comes into the system during/after each level is complete, so the lower beams carry more of the self weight of the structure. The weight of each floor that is added is shared proportionally between the number of completed floors below (via the transfer column at mid span). Hence why the lower levels have more load than the upper levels.