Engineering Mechanics

Chapter 12 : Support Reactions „„„„„ 237

and vertical component of the total wind load
= 4 sin 60° = 4 × 0.866 = 3.464 kN
∴ Balance vertical reaction at A
= 3.464 – 1.16 = 2.304 kN

and reaction at A, RA=+(2)^22 (2.304) =3.05 kN

Let θ = Angle, which the reaction RA makes with the vertical.

∴ tan 2.0 0.868

2.304

θ= = or θ = 41° Ans.

Graphical Method

Fig. 12.31.

1. First of all draw the space diagram for the roof truss, and name the various forces and
   reactions according to Bow’s notations.


2. Compound (i.e. add) all the forces together and assume them to act through the C.G. of the
   forces i.e. at C.


3. Produce the line of action of the resultant force (compounded together as per item 2) to
   meet the line of action of the roller support (which will be vertical due to support on
   rollers) at O.


4. Join OA, From O cut off OM equal to the total compounded load (i.e. 4 kN) to some
   suitable scale, along the line of action of the resultant load.