INFLUENCE OF FLANGE STIFFNESS ON DUCTILITY BEHAVIOUR OF PLATE GIRDER: DETAILS

The girders were designed using tension field theory in accordance with BS5950: Part 1: 1990. When d/t ratio is less than 63e (e = (275/py)05), the girder has to be considered as a beam in which no tension field effect can be considered; when the ratio exceeds the above value the effect of tension field action can be included and designed as plate girder. In normal practice plate girders are designed with a d/t ratio ranging from 120 to 160 and BS5950: Part1 allows up to a maximum value of 250. It is also noted (Evans and Moussef, 1988) that the contribution from the post buckling reserve strength of the web plate increases with increasing d/t ratio. In view of the above factors, Shanmugam and Baskar (2003) considered two different d/t ratios viz. 250 and 150 in order to study the behaviour of plate girders. From a practical consideration of welding and availability of minimum thickness of plates, a 3mm thick plate was chosen for the web for girders with a d/t ratio of 250 (SPG1), and a 5mm thick plate for girders with a d/t ratio 150 (SPG2). The panel aspect ratio of the web was restricted to 1.5 in all girders. SPG3 and SPG4 are provided with a web thickness of 6mm and 8mm, respectively, such that their d/t ratios are 125 and 94. The d/t ratios of all the four girders are more than 63e and therefore, the tension field theory can be applied.

Proportioning the flange dimension is critical in the case of girders subjected to tension field action. The shear carrying capacity is calculated from the three different contributions such as critical buckling strength of web, post buckling reserve strength of web panel and the yielding of flanges. At the beginning, the flange sections are designed by beam theory; but such a calculation provides a minimum flange dimension which leads to the ultimate load in perfect cases and lateral torsional mode of failure in some other cases. Therefore, the SPG1 and SPG2 were designed with larger Mp/M ratio where Mp is the plastic moment capacity of section provided and M is the bending moment due to the load which is calculated from the ultimate shear capacity of the girder. The minimum flange thickness required from the beam theory is shown in Table 1. In view of controlling the experimental behaviour of SPG1 and SPG2, the flange dimensions were taken as 200mm x 20mm and 260mmx20mm, respectively. These dimensions were decided based on a parametric study through FE analyses. The details of the considered girders are shown in Fig. 1 and Table-1.

l. No Girder d/t 

ratio

Thickness of web provided, mm Min. Flange size required, mm Actual flange size of tested girders, mm
bf tf bf tf
1 SPG1 250 3 200 6.71 200 20
2 SPG2 150 5 260 9.99 260 20
3 SPG3 125 6 260 13.35
4 SPG4 94 8 260 21.72

Fig1Influence Of Flange Stiffness_decrypted
Fig. 1 Variation of Shear Strength with Web

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