- 2071 Shrawan · 10 marks
Moment distribution: frame with cantilever 15 kN
Short answer
Using moment distribution: overhang BC contributes a fixed moment of -15 kNm at B. Fixed-end moments for AB are ±104.17 kNm. After distribution with DFs 0.615 (BA) and 0.385 (BD) and carry-over, final moments are M_AB=-131.60, M_BA=49.29, M_BC=-15.00, M_BD=-34.29, M_DB=-17.15 kNm.
Analyse the frame shown in figure below by using the moment distribution method. Draw bending moment diagram. [Figure: beam of 5 m (2I) fixed at the left end with 50 kN/m UDL; 1 m overhang with 15 kN at the end; column (I) of height 4 m below the junction, fixed base.]
Assumptions: the beam AB = 5 m (2I) is fixed at A and carries 50 kN/m; it continues as a 1 m overhang BC (2I) with 15 kN at C; the column BD (I, 4 m) runs down from the junction B to its fixed base D. The fixed support A prevents sway, so only joint B rotates.
Step 1: Overhang BC
kNm (hogging), a known fixed moment on joint B.
Step 2: Distribution factors at B ()
- Joint B: relative stiffness = 0.6500; BA: = 0.4000, DF = 0.6154; BC: overhang, DF = 0 (its moment is a known fixed value); BD: = 0.2500, DF = 0.3846
Step 3: Fixed-end moments (clockwise +)
- AB: kNm
- BC (overhang): kNm at B
- BD: 0
Step 4: Distribution
Unbalanced moment at B: kNm. The balancing moment is kNm, divided 0.615 : 0.385 between BA and BD, and half of each share is carried over to the fixed ends A and D.
| AB | BA | BC | BD | DB | |
|---|---|---|---|---|---|
| DF | - | 0.615 | 0.000 | 0.385 | - |
| FEM | -104.17 | 104.17 | -15.00 | 0.00 | 0.00 |
| Balance | 0.00 | -54.87 | 0.00 | -34.29 | 0.00 |
| Carry-over | -27.44 | 0.00 | 0.00 | 0.00 | -17.15 |
| Final M | -131.60 | 49.29 | -15.00 | -34.29 | -17.15 |
Step 5: Results and BMD (kNm)
Joint B: ✓
| Member | BM at first end | BM at second end | Max within member |
|---|---|---|---|
| AB | -131.60 | -49.29 | 68.51 at 2.83 m |
| BC | -15.00 | 0.00 | - |
| BD | -34.29 | 17.15 | - |
BM sign: sagging positive for the horizontal members; for the column, positive = tension on the right-hand side when travelling from B down to D. The span AB has hogging at both ends and a sagging span moment; the overhang has a straight-line BMD from 0 at C to 15.00 kNm (hogging) at B.
Answer: , , , kNm, kNm (overhang, clockwise end moments +).
✓ Answer checked by Claude Opus 5.5 (high effort)
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