What is fixed beam? with examples, advantages and disadvantages of a fixed beam over a simply supported beams.

What is fixed beam?

 Fixed: A beam supported on both ends, which are fixed in place.

Fixed Beams

A Beam fixed at both Ends - that is, so fixed that the ends cannot tilt up when the beam is loaded - is shown in Fig. 437.

Such a beam is in the condition of two cantilevers, Af and Bi, carrying a beam fi between them, which is supported at its ends f and i by hanging from the ends f and i of the cantilevers.

From the figure it will be seen that the upper portion of the beam is in tension from A to f and from B to i; the remainder from i to f is in compression.

Fig. 437.

The lower portion of the beam is under compression from A to f and B to i, the central portion if being in tension.

It will be noticed that at the points i and / the nature of the stress in each case changes; i and f are called the points of contra-flexure, and their distances from A and B depend upon the form of section of the beam, and the distribution of the load, etc. Roughly speaking, the points of contraflexure are generally distant about ¼ of the span from the abutments.

A Beam fixed at one End and supported at the other (Fig. 438) is like a combination of a cantilever Af and a supported beam fB; and the portions in tension and compression respectively are shown by the letters ttt and ccc.

A continuous Beam is one that extends without break in itself over two or more spans.

Fig. 439.

If the ends are fixed the compressions and tensions will be as shown by ccc and ttt in Fig. 439, resembling those of two fixed beams.

Fig. 440.

If the ends are supported the stresses will be as shown in Fig. 440, the arms in each span being like those of a beam fixed at one end and supported at the other. (Fig. 438.)

C. " Difference in Strength of a Girder carrying a given Load at its Centre or Uniformly Distributed."

The beam in the image above as its end fixeds which impacts the bending moments seen at the each end.

For a fixed beam, the difference will be seen in the moment diagram. The reaction forces are the same as in the simple beam:

where P is the value of the load force.

The shear diagram will also behave as found in the first case of the simple beam. However, now that the ends are fixed, the moment at the supports will reach differently. The moment at the center and at the ends can be expressed using the same equation:

     Advantages

• For the same loading, the maximum deflection of a fixed beam is less than that of a simply supported beam.
• For the same loading, the fixed beam is subjected to lesser maximum bending moment.
• The slope at both ends of a fixed beam is zero.
• The beam is more stable and stronger.

    Disadvantages

• Large stresses are set up by temperature changes.
• Special care has to be taken in aligning supports accurately at the same level.
• Large stresses are set if a little sinking of one support takes place.
• Frequent fluctuations in loading render the degree of fixity at the ends very uncertain.