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16-Civ-A1 Elementary Structural Analysis · December 2015

Question 6 of 8: Three-hinged gable frame

Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)

Notes on this paper

Reference texts: R.C. Hibbeler, Structural Analysis (10th ed., Pearson) — determinacy, method of joints/sections, virtual-work deflections, moment distribution / slope–deflection, influence lines; A. Kassimali, Structural Analysis (6th ed., Cengage) — internal hinges, compound (Gerber) beams and three-hinged frames. Sign convention: sagging bending moment positive, upward reactions positive, member tension positive (T) / compression negative (C).

The paper requires Q1–Q4 in full plus two of Q5/Q6/Q7/Q8. For completeness all eight questions are worked here.

Question 6: Three-hinged gable frame (20 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

Check (reconstructed figure). Adopted geometry: pin at A$(0,0)$, crown hinge C$(8,6)$, pin at E$(18,1)$ (right base 1 m above the left); an $80$ kN vertical load at the crown; a wind/pressure load on the right rafter taken as $24$ kN/m over its 5 m rise, i.e. a $120$ kN horizontal resultant ($\leftarrow$). The three-hinge statics is exact for this model.

Given. Determinate three-hinged frame ($r=4$, one crown condition).

80 kN 24 kN/m ACE
6 (adopted): three-hinged gable; 80 kN at crown, horizontal wind on the right rafter.

Approach. Four reaction components from three global equations plus the crown condition $M_C=0$ (moment of the forces on one side of C vanishes).

  1. Crown condition (right side). Taking moments about C of the E-reaction and the wind gives one equation relating $E_x,E_y$.
  2. Global equilibrium. Solving with $\sum F_x,\sum F_y,\sum M_A$ yields $A=(A_x,A_y)=(\boxed{88,\,66})$ kN and $E=(E_x,E_y)=(\boxed{32,\,14})$ kN; resultants $|A|=110$ kN, $|E|=34.9$ kN.
  3. Left rafter is a two-force member. The support reaction at A acts along A–C (direction $0.8,0.6$), so the left rafter carries pure axial force — $\boxed{M=0}$ along its whole length.
  4. Right rafter. Moment is zero at the crown hinge and at the pin E; the wind produces a mid-rafter peak $M_{\max}\approx\boxed{+75\text{ kN}\cdot\text{m}}$ (at $x\approx13$ m). This is the governing bending ordinate of the frame.
Quantity (adopted model)Value
Reaction at A$A_x=88$ kN →, $A_y=66$ kN ↑ (|A|=110)
Reaction at E$E_x=32$ kN ←, $E_y=14$ kN ↑ (|E|=34.9)
Left rafter moment0 (two-force member)
Right rafter $M_{\max}$≈ +75 kN·m at mid-length