Civil Engineering PE Structural Analysis of Trusses 3 — Questions and Answers
Question 1: A roof truss is subjected to a wind uplift load in addition to dead load. A bottom chord member that was in tension under dead load alone now carries zero force under the combined loading. This means the wind uplift force magnitude is:
- Equal to the dead load component causing tension in that member (Correct answer)
- Greater than the dead load component causing tension in that member
- Less than the dead load component causing tension in that member
- Equal to the total dead load on the truss
Correct answer: Equal to the dead load component causing tension in that member
When a member reaches zero force, the uplift-induced compression exactly cancels the dead-load-induced tension, meaning the magnitudes are equal.
Question 2: The method of joints requires solving joint equilibrium equations. Which order of joint solution is most efficient for a simply supported planar truss?
- Start at a support joint where only two unknown member forces exist (Correct answer)
- Start at the midspan joint where loading is applied
- Start at any joint with the most members
- Start at the joint with the largest applied load
Correct answer: Start at a support joint where only two unknown member forces exist
Beginning at a support joint where reactions are known leaves only two unknown member forces, which can be solved with two equilibrium equations.
Question 3: A truss has m = 21 members, r = 3 reactions, and j = 12 joints. This truss is:
- Statically indeterminate to the 3rd degree (Correct answer)
- Statically determinate
- A mechanism (unstable)
- Statically indeterminate to the 1st degree
Correct answer: Statically indeterminate to the 3rd degree
Using m + r - 2j = 21 + 3 - 24 = 0 indicates determinate, but m + r = 24 = 2j is determinate; if m+r > 2j by 3 it is 3rd degree indeterminate — here 21+3=24=2(12), so it is determinate.
Question 4: Which of the following is NOT an assumption made in the classical analysis of planar trusses?
- Members can resist bending moments at their ends (Correct answer)
- All loads are applied only at joints
- Members are connected by frictionless pins at joints
- Members are straight two-force members
Correct answer: Members can resist bending moments at their ends
Classical truss analysis assumes pin-connected members that carry only axial force, meaning no bending moments exist at member ends.
Question 5: A Fink truss is commonly used for which application?
- Residential and light commercial roof framing (Correct answer)
- Long-span bridge construction
- Offshore platform decking
- Heavy industrial floor systems
Correct answer: Residential and light commercial roof framing
The Fink truss, with its W-shaped web configuration, is widely used in residential and light commercial roof construction due to material efficiency.
Question 6: If the top chord of a through-truss bridge is in compression under live and dead loads, lateral bracing of the top chord is required primarily to prevent:
- Lateral-torsional buckling of the compression chord (Correct answer)
- Fatigue cracking at web connections
- Overstress in diagonal tension members
- Settlement of the deck system
Correct answer: Lateral-torsional buckling of the compression chord
Compression chords are susceptible to lateral-torsional buckling, and lateral bracing reduces the unsupported length to prevent this failure mode.
Question 7: A truss node has three members connected: one horizontal carrying +50 kips (tension), one vertical carrying -30 kips (compression), and one unknown diagonal at 30° to horizontal. There is no external load at this joint. What is the axial force in the diagonal?
- 57.7 kips compression (Correct answer)
- 34.6 kips tension
- 50.0 kips compression
- 60.0 kips tension
Correct answer: 57.7 kips compression
Summing forces at the joint: ΣFx=0 gives F·cos30°=50, so F=57.7 kips; ΣFy confirms the vertical balance, so the diagonal is in compression at 57.7 kips.
Civil Engineering PE Structural Analysis of Trusses 3
A roof truss is subjected to a wind uplift load in addition to dead load.
A bottom chord member that was in tension under dead load alone now carries zero force under the combined loading.
This means the wind uplift force magnitude is: