Analyze and compare two trusses, arched configurations of the Pratt Truss and the Howe Truss in Figures P4.56 (a) and (b), respectively. The trusses have the same depth, length, panel spacing, loading, and supports. All joints are pinned. For each truss, do the following.

Question

a) Compute all bar forces and indicate tension or compression for each force.

b) Determine the required cross-sectional areas for each bar, given an allowable tensile

stress of 45 ksi,

and an allowable compressive stress of 24 ksi. Note that the allowable compressive stress is

lower due to buckling.

c) Tabulate your results showing bar forces, cross-sectional areas, and lengths.

d) Calculate the total weight of each truss and determine which truss has a more efficient

configuration. Explain your results.

e) What other conclusions can you draw from the study?

Answer 1

(d) Final relative truss weights for the trusses indicated in the tables above, show that the

Howe Truss has a slightly lower weight by requiring approximately 2% less material than the

Pratt Truss. All members in the Howe Truss are in tension, except for the top chord, which

requires less cross-sectional area. However, the variation of forces between corresponding

members of each truss is relatively small so the additional area required for the compression

members does not significantly change the overall weight.

(e) The Howe Truss has the advantage of fewer compression members. Since the top chord

of the truss carries the largest forces, these members would be the most impactful towards

reducing the overall weight. The designer could also investigate the shape of the arched

truss.