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RE: BS5950-Pt1 Sect Thin web Plate Girders

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Title: RE: BS5950-Pt1 Sect Thin web Plate Girders


Essentially I agree with your interpretetion and this seems a perfectly adequate simple approach to the design of a beam/column.  (I assume that bending dominates.  I might want to think more about it if the section is primarily a column).

My interpretation of the reduction in py given in section 3.6.3 is that this applies to the WEB ONLY.  This is what I understand to be meant by the phrase "that element" stated in the clause.  It doesn't apply to the whole section, so you can use the appropriate (ie not reduced) py for the flanges.

Remember the reduction in py for compression sections fabricated by welding, Clause 4.7.5.

Regards,  Bob

-----Original Message-----
From: Pat Torey [mailto:ozbuild11(--nospam--at)]
Sent: 13 December 2000 01:46
To: seaint(--nospam--at)
Subject: BS5950-Pt1 Sect Thin web Plate Girders

I have problems in getting a clear interpretation of what the Standard's
intent is in relation to Sect 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 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

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

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).

Anyones' comments or clarification on this interpretation would be welcomed.

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