Mixed steel-concrete earthquake-resistant constructions - Within the Eurocode 8 context

In recent years, mixed steel/concrete construction has enjoyed a boom. This trend can be explained by the combination of the two materials' respective qualities.

Steel framing offers speed of execution, as well as high strength and ductility for a limited footprint.

Concrete, the irreplaceable material for floors, provides additional compressive strength to the special reinforcement provided by the steel sections making up the framework. Concrete also provides geometric flexibility during installation, as well as thermal inertia.

But seeking to use concrete for its structural collaboration in primary composite structures, i.e. those subject to actions other than gravitational ones, poses new problems, which are not addressed by Eurocode 4.

Some of these issues were resolved during the development of Eurocode 8, the seismic standard that will come into force in 2011. These include:

• definition of conditions to ensure the ductility of sections partially or completely encased in concrete;

• the definition of "seismic" reinforcement for slabs, and other conditions at the nodes guaranteeing the development of full mixed plastic moments at the nodes of self-stabilizing portal frames;

• expression of conditions for mixed assemblies ;

• definition of project elements for "mixed systems".

All these new aspects of mixed construction are described and explained, within a text that defines and motivates, with the formulations and symbols of Eurocode 8, the current state of project elements for earthquake-resistant mixed steel/concrete construction.

We also mention the limits of our knowledge, as some problems remain to be studied, such as the cyclic behavior of mixed diagonals, or the conditions for benefiting from a full mixed plastic resisting moment at the foot of a portal column, coupled with a full mixed shear resistance.

Questions relating to exclusive steel structures are covered in the [C 2 559] file.