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Re: Plastic moment capacity of concrete filled steel pipe

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Anand Nene wrote:
> Thank you, John Hubert, for your response.  I have the blue colored edition
> and section I3.2 gives criteria for plastic capacity of beams WITH shear
> connectors.  I am particularly concerned with Steel pipes with infill concrete
> WITHOUT shear connectors.  As I see it, the capacity is essentially that of
> the steel pipe if no connectors are provided.  Could you or someone provide
> more information?
> Thanks.
> Anand Nene, S. E.
> In a message dated 98-03-12 16:23:30 EST, you write:
> << Anand,
>       See section I2-2 of the LRFD Manual of Steel Construction (this is for
>       the edition which is silver colored).  Essentially, a modified yield
>       stress and modulus are calculated.  These are then used to determine
>       the ultimate compression and bending strength, which are then used in
>       an interaction equation.
>       John Hubert
>  ______________________________ Reply Separator
> _________________________________
>  Subject: Re:Plastic moment capacity of concrete filled steel pipes
>  Author:  AnandNene(--nospam--at) at Internet
>  Date:    3/12/98 7:24
>  I would like to know a reliable reference to estimate plastic moment capacity
>  of concrete filled steel pipe and detailing requirements.
>  I am reviewing a sope stabilization design where 8" dia. steel pipes
>  (Fy=50ksi) filled with f'c=5000 psi concrete are proposed.  There is no
>  reinforcing inside the pipe nor any provision for shear transfer.  The
>  designer used capacity of both cocnrete and steel to estimate total plastic
>  moment capacity claiming that for small diameter pipes this is the way things
>  are done - on Caltrans projects.  I do not do work for Caltrans and not too
>  familiar with bridge projects.
>  I would appreciate comments and references on this subject.
>   >>

I don't think you need studs for a concrete filled pipe.

This is different than a composite steel girder with concrete slab on
top. When the girder deflects, there is a very small relative movement
between the girder and the slab. This movement engages the stud. Once
the stud engaged, the composite action develops.

For the case of a pipe filled with concrete, I can't imagine the
relative movement between the concrete and the pipe happening. This is
due to the confinement of the concrete inside the pipe. The composite
action develops without the studs.If you had studs, I don't think they
would be engaged at all.

The same principle applies when steel girders are encased in a concrete
slab. I am aware of old structures designed according to this principle
which are still standing.

Bruno Côté