Understanding Girders with Multiple Webs: The Box Girder

This question asks us to identify the type of girder that features two or more than two webs. Girders are fundamental structural elements, typically horizontal, used to support loads and transfer them to supports like columns or walls. Their cross-sectional shape significantly impacts their strength and how they are classified.

Different Types of Girders and Their Web Structures

Let's look at the options provided and understand their typical structure concerning the number of webs:

• Plate Girder: A plate girder is usually fabricated by welding together steel plates to form an I-beam or C-beam section. It typically consists of a single vertical web plate connected to horizontal flange plates at the top and bottom. Therefore, a standard plate girder has only one web.
• Box Girder: A box girder is characterized by its closed, box-like cross-section. This shape is typically formed by two vertical web plates connected by top and bottom flange plates. It can also be multi-cell, meaning it could have internal webs resulting in more than two webs in total. The defining feature relevant to the question is that it has two or more webs forming a hollow section.
• Gantry Girder: A gantry girder is a specific type of girder designed to support the rails on which overhead cranes travel. While gantry girders need to be very strong and stiff, their cross-section can vary. They can be made from rolled steel sections, built-up plate girders, or even box sections. The term "gantry girder" describes its function, not necessarily the number of webs it must have, although built-up gantry girders often have a single web with additional stiffening or a cap channel.
• Balanced Girder: This term usually relates to the structural behavior or support conditions of a girder, often referring to a continuous beam or a cantilever beam where moments are 'balanced'. It does not define the cross-sectional shape based on the number of webs.

Identifying the Correct Girder Type

Based on the structural definitions, the type of girder specifically known for having two or more than two webs is the box girder. Its multi-web construction provides excellent torsional stiffness and strength, which is advantageous in many applications like bridges and heavy industrial buildings.

Therefore, the girders having two or more than two webs are called box girders.

Conclusion

Reviewing the properties of each type, the box girder is the only option explicitly defined by having two or more webs forming a box or multi-cell section. This structural feature gives box girders specific advantages in load-bearing and torsional resistance.

Structural Girder Classification Revision

Understanding the different ways girders are classified is important in structural engineering. They can be classified by:

• Material (Steel, Concrete, Timber)
• Cross-section shape (I-beam, Channel, Box, T-beam) - This is relevant to the number of webs.
• Support conditions (Simply supported, Cantilever, Continuous)
• Function (Bridge girder, Floor girder, Gantry girder)

Additional Information on Box Girders

Box girders are particularly useful in situations where high torsional strength is required, such as curved bridges or bridges with eccentric loading. The closed section also provides a greater resistance to buckling compared to open sections like I-beams or plate girders with similar material usage. They can be made from steel, concrete, or composite materials.