I-Shape Girders

I‒SHAPE GIRDERS

Definition:

The girders with upper and lower section broadened and the middle section tapered, so that it can withstand heavy loads over it is called as I shape girders. Since the girder look like letter I as shown in Fig.1.16 they are named as I shape girder.

Explanation:

In general, any girder supported at its two ends as on the opposite walls of a room, bends under its own weight and a small depression is produced at the middle portion. This may also be caused when loads are applied to the beams.

Due to the depression produced, the upper parts of the girder above the neutral axis contracts, while the lower parts below the neutral axis expands. (i.e) the stresses have a maximum value at the top and bottom. The stresses progressively decreases as it approaches towards the neutral axis. Therefore the upper and lower surfaces of the girder must be stronger than the intervening part. Thus the girders are made of I shape and are called as I‒shape girders.

Minimisation of the depression:

We know the depression produced in the case of a rectangular section

 y = 4Wl³ / Ybd³

Here, the depression can be minimised by either decreasing the load (W) (or) the length of the girder (l) (or) by increasing the youngs modulus (or) the breadth (b) (or) the thickness of the girder.

Since the length of the beam 'l' is the fixed quantity, it cannot be decreased. Therefore the breadth and thickness may be adjusted by making the girder of large depth and small breadth (since thickness increases by b³). Thus the volume of the girder is increased and hence the depression produced is reduced.

Therefore for stability, the upper part and the lower part is made broader than the centre part and hence forming an I shape called as I shape girders. The depression can also be reduced by properly choosing the materials of high youngs modulus.

APPLICATIONS OF I‒SHAPE GIRDERS

(i) They are used in the construction of bridges over the rivers.

(ii) They are very much useful in the production of iron rails which are employed in railway tracks.

(iii) They are used as supporting beams for the ceilings in the construction of buildings.

(iv) They are used in the construction of iron beams to support the bridges for the heavy vehicles and also in the construction of dams.

Advantages of I shape girders

(i) More stability

(ii) More stronger

(iii) High durability