Composite column:Fire resistance

[cecil123]

Let us say i have a composite column section consisting of structural steel, concrete and reinforcing steel.
Under fire, as per eurocode, we need to divide the column section into various layers and get the temperature in every layer.

What is the reason that the temmperature in every layer varies so largely?Can anyone explain with a intuitive/physical reasoning??

Plz help!!

 

[DonPhillips]

I am not familiar with the Eurocodes but Chapter 7 of the International Building Code adopted almost entirely across the United States, allows you to "build" an equivalent rating by adding up the prescriptive fire-resistance values of each material as you build your section. If you have a Type I or IIA building, and you need a 2 hour rating on your columns, you can use, perhaps, a combination of drywall and masonry to create that rating to protect the steel - or a combination of drywall and concrete encasement of the steel.

Don Phillips

http://worthingtonengineering.com

 

[csd72]

cecil123,

The fire rating is based on the member heating up over a certain period of time. The outer layers heat up faster than the inner layers therefore creating a temperature gradient between the out side surface and the core.

csd

 

[cecil123]

Is it anyways related to the difference in thermal conductivity between steel and concrete?Because steel has very high thermal conductivity and concrete comparatively low?

 

[csd72]

That is on factor.

The concrete effectively acts as an insulator meaning that the steel closer to the surface heats up faster than that further from the surface.

The concrete will also have a varying temp across the section

 

[concretemasonry]

The properties of the layers of materials on a steel structural material provide reduced temperatures.

Temperature is a factor when it comes to buckling or other rapid failures. - A critical factor if you are considering life safety.

A parallel example is factors used by some insurance companies when quoting rates for interior contents. - A building with a wood truss may have a lower rate than a building with a steel truss because of a the unpredictable collapse. A wood structure loses strength according to loss of material (more or less), so the strength can be estimated (if it isn't burned much it is reasonsbly good), while you cannot determine the steel temperature, so fire fighters must "error" on the side of their life safety and evacuate. - More fire fighting time means less loss.

An explantion from an insurance company.