Bond Stress in Composite Columns

[EngJoe]

I'm designing a composite column. It's a 30" diameter column with a W 14x102 encased. I'm trying to calculate the bond stress between the steel section and the concrete.

How do you calculate the bond stress for a tension and a moment force applied on the column?

Any help is greatly appreciated.

 

[ishvaaag]

Whilst surmised adhesion is standing you can do that with FEM. The nodes will need then be modeled as having no relative displacement at interfaces. Also, the way you are applying the load will have a say; and the ways allowed by the FEM program in representing the action can also limit the accuracy of the answer, particularly near the application points. Solving the model will give the compatible deformations at every point, and the internal stresses in both the steel and concrete will be extant.

For the bond stresses themselves you will have to look at the forces exerted by steel on concrete or viceversa at the contact nodes.

It is a 2 materials model, in the end, like you would model a pile embedded in homogeneous soil. The art resides in knowing the FEM program and setting well the proper engineering restrictions standing in the model.

 

[ishvaaag]

You may also study it without as much complication in Timoshenko beam theory. Reduce one of the materials through n coefficient of equivalence to the other material, then resolve the sectional state under Tension, Moment. Disengage each part and state what forces are standing in the steel and concrete parts for each section. Through an appropriate exceel sheet or mathcad worksheet (to be made) you can resolve either numerically or symbolically the values at some particular points of interests, or state a generical formulation for points of interest.

For a less complete but equally effective solution you disengage particular parts of the concrete from the tube insert and state the forces at the ends of the chunks and also the shear along the axis of the column for the cut chunk within the concrete; then the shear at the interface with the steel needs to make for any difference of forces standing.

 

[ishvaaag]

Think of the last method as we would do for compression heads in composite beams: take a chunk of compression head, compression to the left less compression to the right needs equal to total shear at the interface with the steel.