EXPERIMENT NO.6
OBJECTIVE:
To experimentally measure the forces in a loaded frame and
then compare them with values calculated from theory.
APPARATUS:
Compressor Rod, Hanger, Weights, Spring Gauge, Frame, Chain attach to spring Gauge.
Figure 6.1: Roof Truss Apparatus
THEORY:
This apparatus is used to
measure the deformation in elements of roof truss. Whenever a load is applied
to a roof truss, its members undergo a strain. This unit consists of a base
structure which provides the support locations for the roof truss. Two metallic
elements and one adjustable element with spring balance and chain form a roof
truss in a triangular shape.
Load is applied from the top
of the roof truss structure via a hanger. Each member has a force gauge (two
consisting force gauge and one comprises of spring balance) connected to it
which shows the forces produced due to applied load in each member.
This apparatus represents a
simple roof truss bar structure. In the single plane system, the bars are only
subjected to compression and tension. Loads are applied only to the nodes.
The unit comprises three members that are joined together
using pin joint such that the joints are free to move. A longitudinally
adjustable chain permits the structure to be constructed with different angles.
Two of the pin joints also form the supports (fixed and free with bearings) and
are clamped to the sturdy aluminium section base frame. The external load is
applied to the upper pin joint by means of weights. The bar forces occurring
are measured directly on force gauge in the middle of the bar. The method of
joints enables the bar forces to be determined by formulating a system of
equation.
PROCEDURE:
1. Place
the 10N to the load hanger (W) and record the loaded reading in
gauges.
2. Increase
the load gradually each time, by adding different loads to the load hanger and
record the readings.
3. 1st
division of the tie member equals to ½
kg and each division of the compression member equals to 1 kg.
4. Use
a steel tape to measure the joint centers.
5. Use
the cosine rule to find an angle in the frame;
a2=b2 + c2 -2bcCosA
6. After
determining the first angle it may be easier to use the sine rule;
7. Calculate
both the experimental and theoretical values of the forces in the members and
tabulate the result.
OBSERVATIONS:
CALCULATIONS:
Use the data of frame lengths and angles together with the
equations in the theoretical section to determine the forces in the members
under the applied load, W.
CONCLUSION:
Discuss the impact of applied load on the frame
members.
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