Expert: Bruce E. Johnson - 9/2/2009

Question
QUESTION: I have laid down foundation of my house.concrete footing and columns are laid down. The Engineers say footing is weak because the required length was 6 feet and 5 feet wide. Whereas the existing length is 4.5 feet and its 4 feet wide. So how can I extend this cncrete footing to the required size. I have stoped the work at plinth level and plinth beam has been also raised with columns. Thanks

ANSWER: Hi Ata, it is generally better to have a foundation engineered prior to pouring the concrete.  In your case since it is such a small piece of concrete why not tear it out and repour it?  Have the engineers given you any fixes?  You can drill and epoxy rebar into the old foundation and repour the new around it.   I hope this information helps please feel free to write again regarding this or other matters, sincerely bruce e johnson..bejohnsonconsulting.com

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QUESTION: Many Many thanks for your reply I am highly obliged. My an other question in the same context is that threre are 18 under size footings those need repair as you have advised. Will you support me if I raise new footings of full size(5' x 6')and new columns on it with 8(1/2 inch vertical bars & rings)by breaking plinth beam 2 ft wide and bring it out.Will this action subsidize the load of existing 18 columns(with 6 bars)and their footings(4 1/2 x 4 ft)or not.The covered area of my houe will be 2438 sq ft and you can imagine the load this foundation is going tb bear in future. Regards. Ata

ANSWER: Hi again Ata, if you could do a drawing showing me what is existing and then show me what you want to do with dotted lines I can better answer you.  I have done a lot of structural concrete repairs over the years involving large beams and columns but I am having a hard time picturing what you are trying to do.  I am also having a hard time understanding how the construction has progressed to this point with improper footings.  An engineer should be able to calculate the weight of the structure , the bearing capacity of the soil, the bearing capacity of the concrete and design the foundation accordingly.  Some elements of the structure may have concentrated loads requiring larger footings in specific areas.  I hope this information helps, please respond with appropriate drawings or photos and I can respond further,  sincerely bruce e johnson..

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QUESTION: Thanks again.I have attached the drawings that you have advised it will come to you in 2 pieces.The drawings are prepared by the Engineer but the contractor is constructing the house and he made changes at his own to save his labour charges that he gave to steel fixer on weigt basis. One of my friend has checked this all at this stage as i am totally a layman in this regard.The load bearing capacity of the soil is 1.5 tons.Need your advise how to further strenghten this foundatin.Now the footings are 4 1/2 x 4 ft. colunms are 24"x8" and 5 feet deep after that the plinth beam starts that is 2 ft x 6" and is based over the surface of the plot. The concrete ratio is 1:1:1 1/2. The steel is 1/2" in footings and same size in vertical bars in columns and the rings are of 3/8. In plinth beam 4 bars of 1/2" with rings of 3/8 are used.

Answer
Hi again Ata, The bearing capacity of the soil is good if the information given is correct.  Based on the existing footing size of 4.5x4 you will need a footing every eight feet maximum to support a structure of this size..you haven't given me your column spacing so I don't know how often you have your columns or whether or not there are concentrated loads requiring larger footings.  It seems to me that your plinth beam needs to be the same thickness as your columns 8".  Personally I would make the contractor's work conform to the plans as drawn by the engineer unless the engineer approved the changes in advance.  Apparently the engineer figured the weight of the structure, calculated the bearing requirements and determined the footing size based on these calculations.  At this point trying to add width and length to an existing footing won't cut it.  You may be able to add some intermediate footings to compensated for the reduction is bearing area of the footings.  Say your engineer has called for 20 column footings 6'x 6' and 1.5 feet deep..and your contractor has installed 20 footings 4.5 x 4' x 1.5'..the required footing bearing area is 720 sq. feet whereas the actual footing area constructed by the contractor is 360 sq. ft.  This means that you would need to double the amount of 4.5 x 4 footings to come up with the necessary bearing capacity of your foundation.    Another option would be to widen your plinth beam and create a floating footing at ground level to help support the structure.  See if the engineer will allow the addition of more 4.5 x 4 footings or a larger plinth beam in the form of a continuous footing or forego the plinth beam and pour a monolithic slab with a wide thickened edge beam tied into the columns.   I hope this information helps please feel free to write again regarding this or other matters, sincerely bruce e johnson..bejohnsonconsulting.com