Plate girders

Portal frames may be single-bay, multi-bay (Figure 6.4), propped, tied, or mansard with pinned or fixed foot. They may be composed of universal beams or tapered plate girders. 

Tempered steel is used for the construction of all these pressure reactors, alloyed with chromium and molybdenum to make them resistant to hydrogen under pressure, so that no decarburization, embrittlement and grain boundary splitting occurs. These pressure reactors are more and more being constructed out of several layers. In this way the total reactor wall is built up of a multitude of individual layers, which are wound round a central tube. The advantage of such a multiple layer construction, as against pressure chambers with a plate-girder construction, is that the central tube is subject to chemical demands and the adjacent layers... 

The foundation soil at the site of the plate girder bridge showed signs of liquefaction, as indicated bythe sand boils seen in Figure 6. The plate girder bridge introdueed earlier, suffered some rotation... 

The lateral bending stifihess ofthe plate girder bridge is quite small in comparison to the axial stiffness (1 x EA = 3.255 GNm ) or the Euler buckling load of each span. So when the piers try to rotate about their cross-sectional axis, the superstructure i.e. bridge deck offers least resistance. 

Figure 16. Approximate sections of the two bridges (a) Cross-section of the plate girder bridge (b) Cross-section of the arch bridge (c) Sectional view of the plate girder bridge (d) Sectional view of the arch bridge...

These calculations of bending stiffness are rather simple, but sufficient to illustrate that the super structural stiffness of the bridge influences the failure mechanism suffered by the foundation. A corollary to this statement, that may have more significant impact on liquefaction behaviour, is that once the foundations suffer liquefaction, the superstmcture will try to fail in a mode that offers least resistance. Forthe plate girder bridge this may... 

Protective scheme Average thickness, Small assorted sheet metal items Structural steelwork Heavier structural steelwork Plate girder steelwork Box girder steelwork (outside only)... 

Seismic Analysis of Steel and Composite Bridges Numerical Modeling, Fig. 8 Grillage model of a plate girder... 

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