Skip to main content

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

MBC=15×1=15M_{BC}=15\times1=15 kNm (hogging), a known fixed moment on joint B.

Step 2: Distribution factors at B (k=EI/Lk=EI/L)

  • Joint B: relative stiffness Σk\Sigma k = 0.6500; BA: k=EI/Lk=EI/L = 0.4000, DF = 0.6154; BC: overhang, DF = 0 (its moment is a known fixed value); BD: k=EI/Lk=EI/L = 0.2500, DF = 0.3846

Step 3: Fixed-end moments (clockwise +)

  • AB: ∓wL212=∓50×5212=∓104.17\mp\dfrac{wL^2}{12}=\mp\dfrac{50\times5^2}{12}=\mp104.17 kNm
  • BC (overhang): −15-15 kNm at B
  • BD: 0

Step 4: Distribution

Unbalanced moment at B: +104.17−15=89.17+104.17-15=89.17 kNm. The balancing moment is −89.17-89.17 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.

ABBABCBDDB
DF-0.6150.0000.385-
FEM-104.17104.17-15.000.000.00
Balance0.00-54.870.00-34.290.00
Carry-over-27.440.000.000.00-17.15
Final M-131.6049.29-15.00-34.29-17.15

Step 5: Results and BMD (kNm)

Joint B: 49.29+(−15.00)+(−34.29)=0.0049.29+(-15.00)+(-34.29)=0.00 ✓

MemberBM at first endBM at second endMax within member
AB-131.60-49.2968.51 at 2.83 m
BC-15.000.00-
BD-34.2917.15-

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: MAB=−131.60M_{AB}=-131.60, MBA=49.29M_{BA}=49.29, MBD=−34.29M_{BD}=-34.29, MDB=−17.15M_{DB}=-17.15 kNm, MBC=−15.00M_{BC}=-15.00 kNm (overhang, clockwise end moments +).

✓ Answer checked by Claude Opus 5.5 (high effort)

All Theory of Structures II chapter 3 questions →