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Re: Concrete T beams/ ribbed slabs

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Andrew, Peter,

        Yield line theory is quite good; but in this case it might get a little
tricky.  You will have to calculate the plastic moments based on which specific
individual bars are intercepted rather than on the length of the yield line
chosen as would normally be the case.  It's possible that for some trial yield
line patterns some yield lines would not intercept any rebar, hence, no yield
moment, while the symmetrically positioned yield line might intercept two or
three rebars, hence, quite a large plastic moment.

        Another thing: in my student days (1960s) we used 40,000 p.s.i. as a
yield strength for reinforcing steel.  I would strongly advise against using
60,000 p.s.i. as indicated earlier unless there is good back up for this
choice.  I would even suggest 30,000 for 1930s material.

        3,000 p.s.i. is reasonable for concrete but you could use a Schmidt
Hammer and take some actual measurements.  If you do use a hammer keep in mind
that surface carbonation tends to harden the surface so you should use a lower
value than indicated by hammer measurements.

        Good luck.

Regards,

H. Daryl Richardson

Peter James wrote:

> Andrew:
>
> I have analysed tee beam flanges for point loads in the past, and I have
> found yield line theory a useful approach. I can't bring to mind a
> textbook - I used my university notes.
>
> The left side is the open edge, the right side is the face of the web. The
> failure lines will be shaped like this, where the load is applied at the X:
>
>   ---|
>     /|
>    / |
>   /  |
>  X   |
>   \  |
>    \ |
>     \|
>   ---|
>
> If the moment capacities of the flange are the same in the x & y directions,
> the diagonals will be at 45 deg - otherwise the angles will reflect the
> ratio of Mrx & Mry.
>
> Peter James
>
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