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BS5950-Pt1 Sect 4.4.4.2a Thin web Plate Girders

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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.


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.


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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