Components of welded plate girder and selection

A plate girder is a built-up beam made of plates welded together (bolted or riveted in older work), used for large spans or heavy loads where a rolled section is not enough.

Components of a welded plate girder

Elevation:

   +-----------------------------------------------+  top flange plate
   |B|      |S|      |S|      |S|      |S|      |B|
   |B|      |S|      |S|      |S|      |S|      |B|  web plate
   |B|      |S|      |S|      |S|      |S|      |B|
   +-----------------------------------------------+  bottom flange plate
    ^                                             ^
  support                                      support
   B = bearing stiffener   S = intermediate transverse stiffener

Cross-section:

     |<---- b_f ---->|
     +===============+   top flange (t_f)
            ||  <- fillet welds (flange to web)
            ||
            ||   web (t_w), depth d
            ||  <- stiffener plates welded on one or both sides
            ||
     +===============+   bottom flange (t_f)

1. Flange plates (top and bottom): resist the bending moment as a couple, $M \approx F\times d$.
2. Web plate: resists the shear; its depth sets the lever arm.
3. Bearing (load-carrying) stiffeners at the supports and under concentrated loads: prevent web crushing and buckling.
4. Intermediate transverse stiffeners: raise the shear buckling strength of the web (and allow tension field action).
5. Longitudinal stiffener (for very deep webs): placed near the compression flange; controls web buckling in bending.
6. Flange-to-web welds (fillet welds): carry the horizontal shear flow $VA\bar y/I$.
7. Splices in the web and flanges (for transport lengths), and end panels / bracing for lateral stability.

Selecting the section

• Depth $d \approx L/10$ to $L/12$ (or use $d = (Mk/f)^{1/3}$ for minimum weight, with $k = d/t_w$).
• Web thickness $t_w = d/k$, within the $d/t_w$ limits of IS 800:2007 cl. 8.6.1.
• Flange area $A_f \approx M\gamma_{m0}/(f_yd)$; width $b_f \approx d/3$ to $d/5$; thickness $t_f = A_f/b_f$; outstand $b/t_f \le 13.6\varepsilon$ (welded, semi-compact) or $8.4\varepsilon$ for a plastic section.

Brief design procedure

1. Find the factored $M$ and $V$ (add an assumed self-weight).
2. Choose $d$, $t_w$, $b_f$ and $t_f$ as above.
3. Compute $I_{zz}$ and $Z_{zz}$; check $M_d$ (including LTB if the compression flange is not restrained) against $M$.
4. Check the shear $V_d$ and web buckling; provide intermediate stiffeners as needed for the web slenderness (spacing $c$), and design the bearing stiffeners.
5. Check the deflection (Table 6: span/300 in general, span/360 where finishes are brittle or liable to crack).
6. Design the flange-to-web welds for the shear flow $q = VA\bar y/I$ and the splices; curtail the flange plates where the moment falls.