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Re: BS5950-Pt1 Sect 4.4.4.2a Thin web Plate Girders

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Pat Torey wrote:

> I have problems in getting a clear interpretation of what the Standard's
> intent is in relation to Sect 4.4.4.2a Sections with thin or slender webs.
>
> In my example I have a very thin webbed (d/t >63e), stocky (plastic) flanged
> plate girder with both moment and axial combined loading.
>
> The issues I have with Section 4.4.4.2a are:
>
> This section states that the flanges only are to be designed to resist the
> moment and AXIAL loads (ie web excluded), BUT this section is headed Moment
> capacity and there is no provision or inference in Section 4.7 Compression
> members that the web should be excluded from resisting compressive axial
> forces.
>

I think you are to use the flanges to calculate the Plasitic modulus Sx from which can
calculate the Moment capacity of the section and use py for flange from table 6.  Where
Sx=Af x hs.

>
> If you include the web in the section to resist the axial load, py becomes
> extremely small (calc. in Sect 3.6.3), especially at inflexion points where
> there are no moments.This small py is used for the whole section and thus
> fails even though the flanges have significant resistance to the applied
> loads.

You should not use the web to calculate pc, use flange only.  Even if  do use the web,
then separate pc's must be calculated for web and flange which are multiplied by their
respective areas, added together to get the compressive resistance of the section.  I
think neglecting the web is a more simple and conservative approach.

>
> My interpretation of this is that the girder is designed for ALL axial and
> moment loadings being resisted by the flanges only, with the section
> properties based on the section with the web excluded and py based only on
> the flanges. These properties are then used throughout the Standard
> including Section 4.7 Axial Compression. The web is then designed seperately
> for shear only resistance in accordance with Sec 4.4.5).

I agree with the above.  Remember to reduce py from table 6 by 20 N/mm2 when computing
compressive strength.  You would also need to look at the combined effects of bending and
compression, and check for web for buckling.

Nigel.