A Draw a truss scaled as large as possible (1) and compute the reactions as for beams (by moment method for asymmetrical trusses).
B Letter the spaces between loads, reactions, and truss bars. Name bars by adjacent letters: bar BH between B and H, etc.
C Draw a force polygon for external loads and reactions in a force scale, such as 1”=10 pounds (2). Use a large scale for accuracy. A closed polygon with head-to-tail arrows implies equilibrium. Offset the reactions to the right for clarity.
D Draw polygons for each joint to find forces in connected bars. Closed polygons
with head-to-tail arrows are in equilibrium. Start with left joint ABHG. Draw a
vector parallel to bar BH through B in the polygon. H is along BH. Draw a vector
parallel to bar HG through G to find H at intersection BH-HG.
E Measure the bar forces as vector length in the polygon.
F Find bar tension and compression. Start with direction of load AB and follow polygon ABHGA with head-to-tail arrows. Transpose arrows to respective bars in the truss next to the joint. Arrows pushing toward the joint are in compression; arrows pulling away are in tension. Since the arrows reverse for adjacent joints, draw them only on the truss but not on the polygon.
G Draw equilibrium arrows on opposite bar ends; then proceed to the next joint with two unknown bar forces or less (3). Draw polygons for all joints (4), starting with known loads or bars (for symmetrical trusses half analysis is needed).
1 Truss diagram
2 Force polygon for loads, reactions, and the first joint polygon
3 Truss with completed tension and compression arrows
4 Completed force polygon for left half of truss
5 Tabulated bar forces (- implies compression)
|STRUCTURES - TRUSS ANALYSIS|