---
title: "Plate girder elements; web and flange splices"
subject: "Design of Steel and Timber Structure"
chapter: 7
key: "semester_6/design_of_steel_and_timber_structure:plate-girder-elements-splices"
source: https://www.bctengineer.com/q/semester_6/design_of_steel_and_timber_structure/plate-girder-elements-splices
site: BCT Engineering
---

# Plate girder elements; web and flange splices

Design of Steel and Timber Structure · Chapter 7: Flexure Members

- Asked in: 2071 Bhadra, 6 marks
- Video: [Plate Girder components of plate girder web flange stiffener 3d presentation](https://www.youtube.com/watch?v=wlJ33KnBx9I) — Civil Topicwise
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/design_of_steel_and_timber_structure/plate-girder-elements-splices
- Whole chapter: https://www.bctengineer.com/md/semester_6/design_of_steel_and_timber_structure/chapter-7

## Question

Explain about elements of plate girders, web and flange splices of plates.

## Short answer

Plate girder elements: web plate (shear), flange plates (moment), stiffeners, flange angles (older), welds/bolts, splices, bracing. Flange splices use full-penetration butt welds or bolted cover plates designed for flange force. Web splices use plates both sides designed for shear and web moment; splices staggered; HSFG bolts preferred.

## Answer

#### Elements of a plate girder
```text
   +-------------------------------+  <- 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.
```text
      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.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. In a plate girder, what are the main roles of the web and the flange plates?
   A. Web resists bending as a couple; flanges resist shear
   B. Both resist bending equally
   C. Web resists shear; flanges resist bending moment as a couple
   D. Web only stiffens the flanges; flanges resist shear and moment
   Answer: C. The deep thin web carries shear while the flanges carry the bending moment as a force couple. (Key point: Functions of web and flanges)

2. For what is a web splice designed?
   A. Shear V at the section and the part of the moment carried by the web, Mw = M Iw / I
   B. Axial tension only
   C. Only the flange force M/d
   D. Only the self-weight of the girder
   Answer: A. The web splice must carry the shear and the web's share of the moment; bolts take the vector sum of the shear and moment forces. (Key point: Web splice design)

3. Which practice is correct for splices in a plate girder?
   A. Place the flange splice at the section of maximum moment
   B. Place it away from maximum moment, stagger web and flange splices, and prefer HSFG bolts
   C. Use thinner cover plates than the flange area
   D. Use ordinary bolts since slip is unimportant
   Answer: B. Splices are kept away from maximum moment and staggered to avoid a weak section; HSFG bolts give no slip and good fatigue behaviour, and cover plates should be at least equal to the flange area. (Key point: Flange splice location and fasteners)

## Related in Design of Steel and Timber Structure

- [Size of web and flange plate in plate girder](https://www.bctengineer.com/question.md?key=semester_6%2Fdesign_of_steel_and_timber_structure%3Aplate-girder-web-flange-size) — Design of Steel and Timber Structure, chapter 7, asked 2×
- [UDL capacity of MB 300 with flange plates](https://www.bctengineer.com/question.md?key=semester_6%2Fdesign_of_steel_and_timber_structure%3Abuiltup-beam-mb300-plates) — Design of Steel and Timber Structure, chapter 7
- [Beam with flange plates, 30 kN/m + 16 kN](https://www.bctengineer.com/question.md?key=semester_6%2Fdesign_of_steel_and_timber_structure%3Abeam-additional-plates-30knm-16kn) — Design of Steel and Timber Structure, chapter 7
- [Components of welded plate girder and selection](https://www.bctengineer.com/question.md?key=semester_6%2Fdesign_of_steel_and_timber_structure%3Aplate-girder-components-selection) — Design of Steel and Timber Structure, chapter 7
- [What is plate girder? Preliminary design](https://www.bctengineer.com/question.md?key=semester_6%2Fdesign_of_steel_and_timber_structure%3Aplate-girder-preliminary-design) — Design of Steel and Timber Structure, chapter 7
- [Stiffeners in plate girder: types and function](https://www.bctengineer.com/question.md?key=semester_6%2Fdesign_of_steel_and_timber_structure%3Aplate-girder-stiffeners) — Design of Steel and Timber Structure, chapter 7

## Related in other subjects

- [Simply supported I-beam: stress at flange-web junction](https://www.bctengineer.com/question.md?key=semester_3%2Fstrength_of_materials%3Ai-beam-flange-web-junction-stress) — Strength of Materials, chapter 7
- [Symmetrical I-beam: moment, flange and web share](https://www.bctengineer.com/question.md?key=semester_3%2Fstrength_of_materials%3Asymmetrical-i-beam-max-moment-flange) — Strength of Materials, chapter 7
- [Principal stresses at web-flange junction](https://www.bctengineer.com/question.md?key=semester_4%2Fstrength_of_materials_mechanical%3Aprincipal-stresses-web-flange-junction) — Strength of Materials, chapter 9
- [Plate stiffness matrix with two elements](https://www.bctengineer.com/question.md?key=semester_8%2Fcomputational_techniques_in_civil_engineering%3Astiffness-two-element-plate) — Computational Techniques in Civil Engineering, chapter 4