Building Materials, Third Edition

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™ III

Deflections of the neutral plane at the centre of the length
are measured with respect to the points in the neutral
plane above the supports. Deflections of the neutral axis
are measured at the centre of the beam.
Deflection is measured at suitable load intervals such
that about 8–10 readings are available up to limit of
proportionality. Beyond the limit of proportionality up to
maximum load or beyond maximum load the test is
continued until a deflection of 150 mm for 750 mm and
60 mm for 300 mm specimens is reached or the specimen
fails to support 1000 N for 50 × 50 × 750 mm or 200 N for
20 × 20 × 300 mm specimens, whichever is earlier.
The failure of the specimen is recorded according to its appearance and development as
indicated in Fig. 4.10. The readings of deflections and the load recorded and a load-deflection
curve is drawn. The various characteristics are determined by the following formulae and from
the load-deflection curve. The area is measured by a calibrated planimeter.

p



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IH p—

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S. No. Characteristic Unit Formula

1. Fibre stress at limit of N/mm^22

3

2

Pl

bh

proportionality

2. Equivalent fibre stress at maximum N/mm^2

2

3

2

Pl

bh

load (modulus of rupture)

3. Modulus of elasticity N/mm^29

3

4 3

Pl

bh

4. Horizontal shear stress on neutral N/mm^2

3

4

P

bh

plane at limit of proportionality

5. Horizontal shear stress N/mm^2

3

4

P

bh

at maximum load

6. Work to limit of proportionality N mm/mm^3

CA

lbh

(elastic resilience)

p



2R

W ƒ—

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