Plate girder elements; web and flange splices

Elements of a plate girder

   +-------------------------------+  <- flange plate (top)
   |  | |  | |  | |  | |  | |      |
   |  | |  | |  | |  | |  | |      |  web plate with stiffeners
   +-------------------------------+  <- flange plate (bottom)

1. Web plate: a deep, thin plate that resists shear; its thickness is limited by buckling ($d/t_w$ limits, IS 800:2007 cl. 8.6).
2. Flange plates: resist the bending moment as a couple; their thickness may be varied along the span (curtailed).
3. Flange angles (in old bolted or riveted girders): connect the flange plates to the web.
4. Stiffeners: intermediate transverse, bearing (load-carrying) and longitudinal stiffeners.
5. Welds or bolts connecting the flange to the web (horizontal shear flow $q = VA\bar y/I$).
6. Splices where plates are joined, and bracing and bearings.

Splices

Plates are limited in length by rolling and transport, so splices are needed.

Flange splice. A butt weld (full penetration, ground flush) of the flange plate, or a bolted splice with cover plates on both sides. The splice should be placed away from the section of maximum moment. A bolted flange splice is designed for the force the flange carries at the splice section:

$$F_f = \frac{M_f}{d + t_f} \quad (\text{not less than the force from the moment at the splice section})$$

where $M_f$ is the part of the moment taken by the flanges. Many designers design the splice for the full flange strength $A_ff_y/\gamma_{m0}$. The cover plates should have an area at least equal to the flange area, with enough bolts or weld on each side of the joint to carry the force.

Web splice. Made with splice plates on both sides of the web (or a butt weld). It is designed for the shear $V$ at the section and for the part of the moment carried by the web, $M_w = M\,I_w/I$ (with $I_w = t_wd^3/12$). The bolt group takes the combined effect of shear and the web moment:

$$F_V = \frac{V}{n},\qquad F_M = \frac{M_wr_{max}}{\Sigma r^2},\qquad R = \sqrt{F_V^2 + F_M^2}\ (\text{vector sum}) \le \text{bolt value}$$

where $n$ is the number of bolts on one side of the joint and $r$ is the distance of a bolt from the centroid of the bolt group.

      flange cover plate
   ===========  ===========
        | |  splice  | |
        | | plates   | |   web splice (both sides)
   ===========  ===========

The web and flange splices are staggered, or the web splice is kept clear of the flange splice, to avoid a weak section. HSFG bolts are preferred because they do not slip and behave well in fatigue. The two web splice plates together should be at least as thick as the web (each at least half the web thickness), and where practical the splice is designed to resist the full strength of the web.