Web thickness: Web thickness, t w , will determine the exact basis for web design, depending on whether the web is classified with regard to shear buckling as "thick" or "thin" see later. Thin webs will often require stiffening; this may take the form of transverse stiffeners, longitudinal stiffeners or a combination, see Figure 2.
Manual Plated Structures: Stability and Strength (Stability & Strength)
Clearly, depending on the particular loading pattern, and on depth and breadth restrictions, one can expect wide variations within all the above limits which should be regarded as indicative only. Under static loading, ultimate limit states such as strength and stability will normally govern most plate girder design, with serviceability limit states such as deflection or vibration being less critical.
Some absolute limits on plate slenderness are advisable so as to ensure sufficient robustness during erection. A generally accepted method  for designing plate girders which is permitted by Eurocode 3 subject to a moment M ad and a coincident shear V ad is to proportion the flanges to carry all the moment with the web taking all the shear.
This provides a particularly convenient means for obtaining an initial estimate of girder proportions. Thus, at any particular cross-section along a laterally-restrained plate girder, subject to specific values of bending moment and shear force, the flange and web plates can be sized separately.
Plated Structures: Stability and Strength (Stability & Strength)
The required flange plate area may readily be obtained as follows:. An iteration or two may be required depending on an assumed value of t f and its corresponding f y value from Table 3. The cross-sectional moment of resistance may then be checked using:. Unfortunately, economic sizing of the web plate is not quite as straightforward, although where a thick web defined later is acceptable it can be rapidly sized by assuming uniform shear stress t y over its whole area.
Strength and stiffness
Provided that the individual plate elements in a girder are each kept sufficiently stocky, the design may be based on straightforward yield strength considerations. Economic and practical considerations will, however, dictate that not all of these conditions will always be met. In most cases various forms of buckling must be taken into account.
Figure 8 lists the different phenomena. Diagonal buckles, of the type shown in Figure 9 a , resulting from the diagonal compression associated with the web shear will form.
This topic is covered in Lecture 7. If it is desired to reach the girder's full plastic moment resistance a stricter limit will be appropriate. If particularly slender webs are used, the compression flange may not receive enough support to prevent it from buckling vertically rather like an isolated strut buckling about its minor axis. Transverse stiffeners also assist in resisting this form of buckling. Vertical loads may cause buckling of the web in the region directly under the load as for a vertical strut.
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The level of loading that may safely be carried before this happens will depend upon the exact way in which the load is transmitted to the web, the web proportions, and the level of overall bending present. Owing to the post-buckling behaviour see Lecture 8. In plate girder webs a special form of post-buckling termed "tension field action" is possible. Tension field action involves a change in the way in which the girder resists shear loading from the development of uniform shear in the web at low shear loads, to the equivalent truss action, shown in Figure 10, at much higher loads.
In this action the elements equivalent to truss members are: the flanges, which form the chords; the vertical stiffeners which form the struts; and the diagonal tension bands which form the ties. The compressive resistance of the other diagonal of each web panel is virtually eliminated by the shear buckling. The way in which this concept is utilized in design is explained in Lecture 8. The principal functions of the main components found in plate girders may be summarised as follows:.
The design model is valid for single- and double-plane corner gusset plates in steel buildings and bridges, including extended corner gusset plates commonly used for seismic design. Compared to the results of specimens from 15 previously-published research projects, the design model is reasonably accurate. Structures Congress Previous paper. Next paper. Downloaded 71 times. Bo Dowswell , Ph. Journal of Structural Engineering May