Expert: Bruce E. Johnson - 8/23/2008

Question
Question about partial casting of footing.
Sir ,

I want to know about old & new casting bond in isolated footing. A 6 storied apartment building designed (structural) by me. One of the footing size 13’ * 13’-9”*23” & depth of footing 8’ from existing plinth level. Bottom  reinforcement bar 16 mm dia@5” c/c both way (no top reinforcement in footing ).column size =12”*20” below plinth(10”*18” above plinth level). Rebar =16-20mm bar. Ultimate column loading=450 kip. Bearing capacity of soil is 2.75 ksf .(Accidentally my site engineer cast 14” tapered part at a time (surface was rough) after three days later rest of 9” cast .Rest casting was done by cleaning surface & grouted cement . Do u think the bond for shear is o.k. Do u think additional rebar was required for joint between old & new bonding. Please suggest me for the interior footing casting .


Answer
Hi Ahsanul, unfortunately I don't like the idea of pouring a footing in multiple pours unless there is continuous reinforcement tying it together and even then it is a questionable procedure.  Especially with the weight of a six story building sitting on it.  Over time the nine inch layer will begin to break down and eventually fail.  At this point you can only compensate by making sure that any column connections such as bolts or reinforcing bars are long enough to reach the lower section of the footing and epoxy set them.  The concentrated load of your columns is going to put a great stress on the dual layered footing.  If the top layer breaks down you will be stuck trying to keep the main columns from sinking over time and potentially punching through the footings.  Only you can make the determination as to whether the connection between the two footing layers is adequate.  Without seeing the actual structure I can't say for sure whether you need to tear out the footings and repour them or can use them as poured.  I hope this information helps feel free to write again regarding this or other matters, sincerely bruce e johnson..bejohnsonconsulting.com