Capacity of Existing Steel Columns Encased in Concrete

[nycstructural]

Hello! I have an existing three steel framed structure which we are adding one story to. The beams and columns are encased in concrete. The building was constructed in 1964. From analysis some of the steel columns are overstressed when the vertical addition loads are applied. Can the concrete encasement be considered to be acting compositely with the columns? The encasement is very rebust. For example, 8WF40 encased in 22x22 concrete.

 

[MrHershey]

Would have to verify if limitations of AISC 360-I1.2 and -I2 are met. I would imagine I1.2 is met. I2 may not be.

 

[JAE]

Typically that concrete was added as fire protection. Per MarkH above - need to check the bond condition between the two (concrete and steel) to see if they can work compositely.

Alternatively, you might look at a carbon wrap on the columns - added complexity but it might work.

 

[ishvaaag]

Inspect the current status and the following stages which the column is going to stand, even notionally (i.e. factored loads plus any amplifications from stability).

Regarding to the question, steel at your actual service level will be likely in elastic state -it would be quite anomalous to be otherwise- and your aditament should (or at least could) protect the quasielastic behaviour of the existent steel column for any foreseeable new service level loading ... so in general terms there will be composite action, the specifics as pertaining to the design to be cleared in the process.

 

[ishvaaag]

I see the concrete encasement seems be standing but the general ideas above given may stand. Other thing is that of course a mere encasement without shear connectors will raise a brow with almost any onlooker ... it may work if the shear interfaces are not subjected to high shears but from the normative viewpoint not a very good prospect.