repair of foundation 3

[swelm]

hi

i have isolated footing with depth 1.2 meter casted already in the site but we need to add new column is very near to the edge of it has heavy load
we need to change this footing to be combind footing to be suitable with the new sutiation.

the steps of repair as below:

1- rough the surface of old footing from all side.

2- we drill in old footing for the bottom bar for 30 cm deep then we fill the hole with epoxy material.

3-we put the steel bar for the new footing in that hole.

4-we cast the new footing with 20 cm concrete above the old footing with new steel layer at the top the total depth became 1.4 meter for the combind footing.


what do you think of that concept of repair?

is that right way or not?

 

[emanc]

Swelm-
Your procedure is ok, though I suggest you also apply concrete bonding agent to old footing surface right before pouring new concrete. This is to bind old concrete with new concrete and act as one.

 

[hokie66]

If the structure above is not already in place, and by needing to add another column I presume it is not, I would bend the column bars, hook up with a big mobile crane, and jerk the footing out.

By the way, swelm, you never did report back on how you got 23 kg/cm^2 bearing pressure on the core of a 58 metre tall building. Unless you give feedback when requested, you will lose enthusiasm for your questions.

 

[swelm]

but if we did not put material for bonding between old and new concrete

is that critical?

for eng'r hokie66 your way is very hard to implement in site

and for the raft stress you will find my answer in thread


thanks for your action

 

[GoddyMgwasa]

your procedure is ok. I think you have taken all neccessary precaution. The footing shall carry all loads perfectly. Although bonding the concrete is not that much critical(bottom bars embedded 30cm into concrete!!!) but if bonding agents are available just use them.

 

[KCRatnayake]

1.2m thick footing ...... I presume heavy loads. What is the distance between columns? If distance is fairly high, then hogging moment at the mid span of the footing can be high. Then bonding between old / new concrete interface is crucial as this interface lies in the tensile region of the concrete. I would calculate the roughness of the concrete to check whether this interface can widstand the applied shear if this is a very critical application. You may have to use shear connectors as well.

Clefcon

 

[emanc]

for strengthening of small footing roughening, of old concrete is enough but for large foundation we always use bonding agent.

 

[hokie66]

IMHO bonding agents just give you a false sense of security, and if incorrectly used they can even act as a bond breaker. What is required for bonding is a clean, slightly roughened, and moist surface. But as Clefcon pointed out, bonding alone may not be enough. Shear connectors could take the form of drilled in dowels or routed out keys. I still think pulling out the footing and starting over is the best approach, provided you have the appropriate craneage. Have done it in the past with footings in the wrong place and columns already cast. Was a simple thing with a 65 tonne mobile crane, but don't know how big your footing is.

 

[zeeshanahmed]

my question on this issue would be that if the footing is designed primarily for the Axial load on the column then most probably AXIAL would be the design governing factor also, right ? so the footing is spread footing as to distribute the axial load over a area of soil right ? now bonding the steel of new footing with existing footing will only transfer (or share) the moment induced in the new footing to old one ! right ? what about axial load distribution ? i mean if you have performed an analysis and the area of footing required under axial of both the columns is say satisfied by the total area of the combined footing can i dont think you can be sure that is actually acting as combined footing!

 

[hokie66]

Zee,

Not sure I understand what you are getting at, but swelm's problem is that he has a column load (axial load) close to the edge of a footing, and to carry the load he needs to enlarge the existing footing, thus creating a footing which carries two columns (two axial loads). In so doing, he has to deal with the shear and bending which this introduces. This would be simple if the footing were monolithic, but he is trying to tie two big lumps of concrete together. Thus his question and the various opinions he has received in answer.

 

[zeeshanahmed]

what iam suggesting or asking is making a steel bar connection between two footings will only transfer (share) local bending moment between the two footings, but "two big lumps of concrete might come together"