Civil Engineering PE Structural Analysis of Trusses 2 — Questions and Answers
Question 1: A Pratt truss has vertical members under compression and diagonal members under tension when subjected to gravity loads. Which statement correctly describes the force distribution in a Pratt truss?
- Diagonals are in tension and verticals are in compression (Correct answer)
- Diagonals are in compression and verticals are in tension
- Both diagonals and verticals are in compression
- Both diagonals and verticals are in tension
Correct answer: Diagonals are in tension and verticals are in compression
In a Pratt truss under gravity loading, diagonal members carry tension forces while vertical members carry compression forces.
Question 2: Using the method of sections, a cut through a truss exposes three unknown member forces. To solve for all three unknowns, you need to apply which set of equilibrium equations?
- ΣFx = 0, ΣFy = 0, and ΣM = 0 about a strategic point (Correct answer)
- ΣFx = 0 and ΣFy = 0 only
- ΣM = 0 about three different points
- ΣFx = 0 and ΣM = 0 about one point
Correct answer: ΣFx = 0, ΣFy = 0, and ΣM = 0 about a strategic point
Three equilibrium equations (ΣFx=0, ΣFy=0, ΣM=0) are needed to solve for three unknowns when using the method of sections.
Question 3: A truss panel point has two members meeting at 45° and 90° to the horizontal, with no external load applied. The member at 90° (vertical) carries zero force. What is the force in the 45° member?
- Zero (Correct answer)
- Equal to the reaction force
- Equal to the applied load divided by cos(45°)
- Cannot be determined without additional information
Correct answer: Zero
By the zero-force member rule, if two members meet at an unloaded joint and one is collinear with no load, the other carries zero force as well.
Question 4: For a simply supported Pratt truss with span L and panel width d, how many panels does the truss have if the top chord members are all horizontal and there are 8 vertical members?
- 8 (Correct answer)
- 7
- 9
- 6
Correct answer: 8
A Pratt truss with 8 vertical interior members has 8 panels, since each panel contains one vertical member.
Question 5: A K-truss has members arranged in a K-pattern. Compared to a Pratt truss of the same span and load, the K-truss is typically used to:
- Reduce the unsupported length of compression chord members (Correct answer)
- Increase the number of panel points for load application
- Eliminate diagonal members to reduce weight
- Allow for variable depth along the span
Correct answer: Reduce the unsupported length of compression chord members
The K-truss subdivides the panels so that vertical members have intermediate bracing points, reducing the effective buckling length of compression chords.
Question 6: In a Warren truss without vertical members, all interior members are diagonals. Under a midspan point load, the diagonal immediately to the left of midspan is in what state?
- Tension
- Compression (Correct answer)
- Zero force
- Bending
Correct answer: Compression
In a Warren truss under a midspan load, diagonals on the loaded side carry compression while those on the far side carry tension.
Question 7: A truss is said to be 'determinate' when m + r = 2j. For a truss with 15 members and 3 reactions, how many joints must it have to be statically determinate?
- 9 (Correct answer)
- 12
- 6
- 18
Correct answer: 9
Using m + r = 2j: 15 + 3 = 18 = 2j, so j = 9 joints.
Civil Engineering PE Structural Analysis of Trusses 2
A Pratt truss has vertical members under compression and diagonal members under tension when subjected to gravity loads.
Which statement correctly describes the force distribution in a Pratt truss?