New composite column for high-rises

STRUCTURES

The Canam Manac Group of Quebec has developed a new composite column intended for high-rise construction that is expected to reduce the cost of multi-storey steel columns in the order of 50 per cent.

The column consists of welded light H-steel shapes, with the space between the flanges filled with concrete. The concept was developed by Richard Vincent, P.Eng. It has received an international patent and has a patent pending in the United States.

Various components of the design are being tested at universities in Canada and the U.S. Professor Robert Tremblay, ing. of cole Polytechnique is leading the research team, assisted by Professors Peter Birkemoe, P.Eng., (University of Toronto), James Ricles P.E. (Lehigh) and Bruno Massicotte, ing. (Ecole Polytechnique).

The square H-shape columns are built up from three relatively thin steel plates. The flange widths vary typically from 40 to 70 times their thickness, whereas conventional high rise built-up columns usually have flange widths less than 10 times their thickness. The thin flange tips are prevented from buckling under compressive loads by a series of equally spaced round tie bars. The distance between tie bars can vary from one-half to one times the column depth. The tie bars are shop welded to the inside flanges of the column and look like ladder rungs when viewed from the side.

After the steel column is erected at the site, the voids created between the flanges and web are filled with concrete. The tie bars now function as stirrups helping to confine the concrete and increase its compressive resistance. The column is filled from the floor above at the same time as that slab is poured, thereby minimizing field labour costs.

Floor beams that frame into column flanges are connected using standard shear connections, while beams framing into the column web are framed into a special connection plate. This connection plate is welded to the column flange tips and the length of the plate is extended beyond the underside of the beam.

The connection plate becomes a permanent form at the floor juncture thus allowing the concrete to be poured from the floor above. Field-installed plywood forms close the fourth side of the two rectangular concrete areas created on each side of the column. The tie bars play a third important function as shear connectors distributing the shear between the concrete blocks and the steel flanges.

An experimental test program is now completed and finite element models have been constructed and calibrated to match the test data. Final design equations are being formulated and results from these equations are being compared with the model.

Test results

The full scale test results have indicated that both the steel and concrete components of the column can attain their respective ultimate capacities resulting in very economical columns. Another advantage is that high-rise buildings can now be constructed using the speed of steel construction combined with the economies of concrete columns. Furthermore, it is anticipated that the concrete area within the steel H shape will act as a heat sink and protect the steel portion during fires. It would thus eliminate the need to fireproof the steel portion of the column, resulting in additional economies.

In order to assess the effectiveness and economy of the new column system, some existing high-rise buildings have been redesigned using this column system and compared to the as-built structures. The new column system reduces steel column costs by approximately half, taking into account costs for the raw steel, manufacturing, transportation and erection. Additional savings on crane rental may be possible since columns are usually the heaviest components of a high-rise structure and their weight dictates the size of the crane used for the duration of the project.

Steps have been taken to incorporate this column design into the next version of the Canadian Standards Association steel design standard S16-2000, “Limit States Design of Steel Structures.”

By Richard Vincent, P.Eng. of Canam Manac Group, Montreal.