What is Box girder bridges?
Girders are the large section beams above which the slab(deck) rests. They are generally adopted for the bridges. The Box Girder are hollow channel-shaped beams containing two (or more) side webs and two flanges.
The box shape can be either trapezoidal or rectangular. In such girder, one of the flanges remains in compression and the other in tension. The Bridges with such girders are called Box girder bridges.
Types of Box Girder bridge
On basis of the material of construction
- Prestressed concrete Box Girder bridge
- Structural steel Box Girder bridge (generally truss structure along with the metal box)
- Composite steel and Reinforced concrete Box Girder bridge
On basis of shape
- Rectangular Shaped Box Girder bridge
- Trapezoidal shaped Box Girder bridge
- Two or multiple chamber Box Girder bridge
- Cellular Chamber Box Girder bridge
Components of Box Girder bridge
- Deck slab: They are the top flat slab portion of the bridge where the dynamic loads and vehicular movement are allowed. They rest upon the girders usually. Sometimes the decks are also supported with the truss structure, suspension cables, and arch over it.
- Box girders: These are the unique and important features of the Box Girder bridge. The box girders further have the following parts. They are Web, Flange, and Ribs.
The web is the side vertical stem of the box-shaped girders. The flange is the top and bottom parts of box-shaped girders. Ribs are the thin middle vertical present in such box-shaped girders. The ribs are generally term in the cellular type box girder.
- Prestress Concrete section: The flanges of the girders are left with cylindrical longitudinal holes throughout the span. And, thus they are later on reinforced with prestress reinforcements and then grouted or concreted.
- Haunch: They are the triangular structure provided at the corners of the flanges and web connection with the purpose of increasing shear resistance and stability.
- Diaphragm: They are the vertical wall present inside the box girder placed cross-sectionally(transversely) to the span of the girder or bridge. The diaphragm is provided an abutment, piers, and hinge joints to resist load and transfer to the point support.
- Braces and cables: They are also provided in some box girder bridges for more stability
They are generally adopted for flyovers, lightweight rails bridge where the vehicular movements and dynamic loads are very high.
Due to their excellent aerodynamic stability, they are also used for bridges at very high elevation depths from the ground.
The box girder bridge has very high torsional stiffness and can encounter a large amount of torque due to which they are very much suitable for the construction of curved-shaped bridges.
Box girder bridges can be constructed in different ways according to their types. The general methods are cast in situ, fabrication and assembling, precast, and arrangement of smaller units.
The concrete box girder bridge is generally cast in situ and done by use of scaffoldings, supports, falseworks, and frameworks. Such bridges are constructed with construction joint into different segments.
However, it can also be pre-casted and lifted through the cranes and then later on concreted over joints. But this requires high loading capacity cranes.
In the case of the steel box girder, different parts and segments are fabricated and constructed in the workshop. Then, they are assembled and jointed with bolting or welding.
But again, in composite steel and concrete bridges, it is often cast at in situ with the false works and framework along with supports of steel girder and bracings which are fabricated and assembled before.
In modern days, there is a new method of construction of these types of bridges. The technique is popularly known as the Incremental launching method of bridge construction.
In such a method, advanced types of machinery and gantry cranes, structurally capable of providing support and hold loads for a temporary time, are used. The cranes then placed the new segments of prefabricated or precast piece/segments of the bridge on the completed portion of the bridge.
And after the placement, the gantry crane is moved again to place the new segments over the recently placed or jointed segment.
They are built from one end of support and continued till another end of support is met. Thus, they act as cantilever until they meet another end. There is also the provision of holes for the prestress concreting in order to join all the segments.
Specification of Box girder bridges
- Used for the larger spans and wider decks. Span can length from 25m to 250 m. They are mostly preferred for spans greater than 50m.
- The single box girder can have a deck slab of 20m – 30m wide. And further width of the deck slab can be increased using a cellular box girder or multichambered box girder.
- Single box girder is cast in-situ if the bridge has a span of 30m – 250m
- Below the 30m span of the bridge, the decks with void for precast concreting are preferred. And over 50m, a single trapezoidal box bridge is more economical.
- The top and bottom of flange thickness should not be less than 1/30 of the clear span between webs.
- Webs may be inclined or vertical. The inclination of the web shall not exceed 1 – 4 to the normal of the bottom flange.
- Minimum values for web shall be according as:
Web with no longitudinal or vertical post tensioning tendons – 8inch
Web with only longitudinal or vertical post tensioning tendons – 12inch
Web with both longitudinal and vertical post tensioning – 15inch
- Minimum thickness of ribbed web – 7inch
- Minimum flange thickness – 8inch
The above-provided information is some basic general geometrical specifications for the box girder bridge. For the detailed specification, the reader can take reference and help from the AASHTO bridge design specification(2007) and TxDOT Bridge design Manual(2001).
Further for the steel box girder bridge, there is a separate specification that can be found over Preferred Practices for Steel Bridge Design, Fabrication, and Erection(2005)
- Aesthetic beauty: The box-shaped and smooth shapes of the girder gives a sleek appearance to the bridge.
- The box chamber present below the deck can be used as space for utilities with the purpose of easy management of electric wires, sanitary pipelines, etc. without hindering the deck of the bridge.
- Structural advantages: The torsional stiffness of the Box girder is 100 to 1000 times greater than the I-section girder of the bridge. So, they are extremely advantageous where the torsional effect is likely to be occurred high like high speed moving rails, curved section of the bridge.
- They have larger shapes and deck slabs.
- They have greater aerodynamic stability.
- Even though a number of bridged had been constructed using Box girder bridge technology, it is a new concept of design which have very less amount of research and analysis done yet. This had created difficulty and non-uniformity over design, specification, methodology of construction, and installment.
- They are highly expensive methods of construction.
- Very difficult and risky for maintenances.
- If the box girder is provided with the cables bracing then, they might get corrosion inside and gets highly cumbersome to maintain as they lie inside the enclosed space.
- Permanent formworks are needed to be used for the construction of the deck over the girders (if the slabs are to be cast in situ). However, the use of temporary falseworks and formworks are also possible but lead to complexity, difficulty, and risk to the workers.
I hope this article on “Box girder bridges” remains helpful for you.
Happy Learning – Civil Concept
Civil Engineer – Rajan Shrestha