Difference Between a Girder and Truss Bridge

What is Girder?

Girder beam is a large and deep type of beam that is used in construction. It is typically capable of longer spans and taking greater loads than a normal beam due to its structure and material used, and is often used as a main horizontal structural support for smaller beams, such as in bridge construction.

Difference between a girder and truss bridge:

In engineering terms the difference between truss bridge and girder bridge lies as:

• A Truss is a structural members whose components are only ever in tension or compression but not bending.
• A Girder is something that carries loads by bending.

Steel Bridge Manufacturer

Steel is one of the most reliable and sturdy construction material used for building  long span bridges. The properties such as strength, ductility, toughness and endurance offered by steel makes it as the perfect material used for constructing modern bridges that can withstand diverse weather conditions. Ranging from small foot flyovers, high-speed highways, mountainous connecting bridges , there is a wide scope of opportunities for a steel bridges manufacturer in India. Below are some of the reasons why steel are preferred as the best material to be used for the construction of  bridges in India.

Why steel for bridges?

Fast Construction: Using steel considerably reduces the span of time required needed to build a bride. Unlike concrete which demands lengthy time to set up slabs and during formation, steel speeds up the process due t its easy manufacture and wieldable nature.

Minimal Disturbance: Constructions of steel bridges proves highly advantageous for the general public, especially in cities and towns. This is because steel does not produce the usual dirt and dust that is an integral part of building the usual concrete bridges. Moreover, the rapid pace of construction ensures minimal disturbance to everyday vehicular movement and traffic goers.

Eco Friendly: They are a completely eco-friendly as do not cause any significant damage to the local surroundings or lead to long term environmental ramifications. Most importantly they cause minimal pollution even during the construction phase neither dust nor any harmful chemicals into the atmosphere during or after they are built.

Steel Girder Bridge

Girder bridges are the simplest bridge type in structure and consist of steel beams shaped to an I-section or box section, called a plate girder bridge or a box girder bridge, respectively. Girder bridges are comprised of deck slabs, on which vehicles and people pass, and of main girders supporting the deck slabs. Deck slabs include RC deck slabs, steel deck slabs, composite deck slabs, and PC deck slabs. Bridges where the deck slabs and the main girders work together to resist loads are called composite girder bridges, and bridges designed to resist loads with the main girders only are called non-composite girder bridges.

The superstructure of the girder bridge mainly consists of the following components:

Deck:

The deck refers to the surface of the girder bridge which is used as roadway or walkway surface.

Mostly, the deck may be made up of steel grids, wood planks, or reinforced concrete slabs.

The deck mostly includes the road lanes, medians, sidewalks, parapets or railings, and miscellaneous items like drainage and lighting.

Supporting Structures:

The superstructure of the girder bridge contains supporting structures of steel or concrete that support the deck. These structures mainly include the girders, diaphragms or cross-braces, and sometimes even the truss or arch system.

Girders and bracing are the most commonly adopted supporting structures.

In such a system of girders and bracing, the girders act as the primary load support whereas the bracing prevents the beam from toppling.

Bearing Pad:

The bearing pad permits the superstructure to move slightly independently over the substructure.

The objective of allowing such movement of the superstructure is to prevent the development of unnecessary stress and failure of the structure by the structural expansion and contraction due to temperature variations.

Plate Girder Bridge

A plate girder bridge is fabricated by welding plates together to create the desired shape. Usually, the large steel plates of desired thickness are cut into the flanges and webs from the plates in the desired length and shape.

The plate girder offers great flexibility in terms of height and shape. The span of the plate girder bridge can range from 10 meters and more than 100 meters (33 feet to more than 330 feet).

Plate girders became popular in the late 1800’s, when they were used in construction of railroad bridges. The plates were joined together using angles and rivets to obtain plate girders of desired size. By 1950’s welded plate girders replaced riveted and bolted plate girders in developed world due to their better quality, aesthetics and economy.

The different types of plate girder bridges:

Deck type plate girder bridge:

Wood, steel or reinforced concrete bridge deck is supported on top of two or more plate girder and act compositely. For the railroad bridge, the railroad will be fixed onto the girder to form the bridge deck and the deck will support ballast on which the track is placed. Bracing is added to the structure to prevent the girders from buckle.

Half through plate girder bridge:

 This is also called as ponny truss. The deck is supported between two plate girders, usually on top of the bottom flange. The vertical stiffeners are used to prevent the girder from buckle instead of cross bracing. Usually used on railroads and the construction depth (distance between the underside of the vehicle, and the underside of the bridge) is less. This is to allows obstacles to be cleared with less change in height.

Multi-span plate girder bridge:

 Piers act as the intermediate abutments between the end abutments of bridge. Separate plate girder bridge span between each pair of abutments in order to allow for the expansion joints between the spans. Concrete will be used for low piers and steel trestle work will be used for the high bridge.

Welded Girder Bridge

Beams of long span subjected to heavy loads are encountered in buildings and bridges. Due to buckling of the web, ready-made I sections are not suitable since their depth is limited. In this situation, girders are to be fabricated using flange and web plates as per requirement. Usually, the flange and web plates are connected by welding. Girders fabricated in this manner are known as welded plate girders and their depth to web thickness ratio (d/t_w) may be up to 400.

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