Chapter 14 : Analysis of Perfect Frames (Graphical Method) 291
14.4.FORCE TABLE
After drawing the vector diagram, the next step is to measure the various sides of the vector
diagram and tabulate the forces in the members of the frame. For the preparation of the table, we
require :
- Magnitude of forces, and 2. Nature of forces.
14.5. MAGNITUDE OF FORCE
Measure all the sides of the vector diagram, whose lengths will give the forces in the corre-
sponding members of the frame to the scale e.g., the length ps of the vector diagram will give the
force in the member PS of the frame to the scale. Similarly, the length sr will give the force in the
member SR to the scale and so on as shown in Fig. 14.2. (b).
If any two points in the vector diagram coincide in the each other, then force in the member
represented by the two letters will be zero.
Fig. 14.2.14.6.NATURE OF FORCE
The nature of forces in the various members of a frame is determined by the following steps:- In the space diagram, go round a joint in a clockwise direction and note the order of the
two letters by which the members are named e.g., in Fig. 14.2 (a) the members at joint (A)
are RP, PS and SR. Similarly, the members at joint (B) are QR, RS and SQ. And the
members at joint (C) are PQ, QS and SP. - Now consider a joint of the space diagram and note the order of the letters of all the
members (as stated above). Move on the vector diagram in the order of the letters noted
on the space diagram. - Make the arrows on the members of the space diagram, near the joint, under consider-
ation, which should show the direction of movement on the vector diagram. Put another
arrow in the opposite direction on the other end of the member, so as to indicate the
equilibrium of the method under the action of the internal stress. - Similarly, go round all the joints and put arrows.
- Since these arrows indicates the direction of the internal forces only, thus the direction of
the actual force in the member will be in opposite direction of the arrows, e.g., a member
with arrows pointing outwards i.e., towards the joints [as member PS and SQ of Fig. 14.2
(a)] will be in compression; whereas a member with arrow pointing inwards i.e., away from
the joints [as member SR in Fig. 14.2 (b)] will be in tension.