Simply Supported Beam (What it means in buildings)
A simple, practical explanation for site engineers: supports, behavior, diagrams, and RC reality.
Structure Basics
Support Pin + Roller
Design Simple Model
1) Definition (Simple words)
A simply supported beam is a beam supported at two ends that can rotate at the supports,
and the supports prevent vertical movement.
- Usually modeled as: Pin at one end + Roller at the other end.
- Why roller? It allows small horizontal movement (temperature, shrinkage), reducing unwanted restraint.
- In a pure simply supported model: no fixing moment at the supports.
Pin (hinge) Roller
▲ ▲
│ │
└───┬───────────────────────────┬───┘
│ Beam │
└───────────────────────────┘
Key idea:
- Both supports stop vertical movement
- Beam can rotate at both ends
- One end can slide horizontally (roller)
2) Support types
| Support | Stops movement | Allows | Moment resistance |
|---|---|---|---|
| Pin (Hinge) | Vertical + Horizontal | Rotation | ≈ 0 |
| Roller | Vertical only | Rotation + Horizontal sliding | ≈ 0 |
Note: This is the ideal model used for analysis. Real connections can behave “partially restrained.”
3) Structural behavior (bending moment & shear)
- Moment at supports ≈ 0 (for ideal simply supported).
- Maximum positive moment usually near mid-span under uniform load.
- Shear is typically highest near supports.
Typical moment shape for UDL (concept)
M
│ ▲ Mmax (mid)
│ / \
│ / \
│_____/___________\_____ → x
0 mid 0
Quick memory:
Simply supported → 0 moment at ends,
max moment in the middle.
4) Real construction examples
In the field, we call it “simply supported” when the beam is basically resting on supports:
- Steel beam sitting on a beam seat / corbel.
- Precast beam placed on bearing pads.
- RC beam bearing on wall or small support with no strong moment detailing.
Practical check: If there is no strong moment connection detailing (no “fixity”),
engineers often idealize it as simply supported.
5) Simply supported vs Fixed (quick compare)
| Item | Simply Supported | Fixed |
|---|---|---|
| Rotation at support | Allowed | Restrained |
| Moment at support | ≈ 0 | Non-zero (hogging) |
| Typical crack risk at support (RC) | Lower | Higher if detailing is poor |
| Analysis | Simple | More complex |
| Common usage | Basic beams, precast | Rigid frames, continuous systems |
6) Important note for RC buildings
In reinforced concrete, beams and slabs are often cast monolithically.
Even if we analyze as “simply supported,” the real behavior can be partly continuous.
- If top bars are continuous over supports and anchored well → more continuity (negative moment can develop).
- If detailing is weak or joint is like bearing only → closer to simply supported behavior.
- This is why rebar detailing + anchorage decides whether a connection acts “simple” or “fixed-like.”
RC reality (concept)
- More top reinforcement continuity → more "frame action"
- Less continuity / bearing connection → more "simple support"
If you want, I can make a second Smart-Book post: Simply Supported vs Continuous Beam + typical RC detailing notes (top bars, anchorage, development length).
7) Quick Q&A for site engineers
- Q: Why do we use a roller support in theory?
A: To allow thermal/shrinkage movement and avoid extra restraint forces. - Q: Where is the maximum bending moment for UDL?
A: Near mid-span (for simply supported). - Q: Can an RC beam be truly simply supported?
A: Yes, if the connection is detailed as bearing with little moment transfer (e.g., precast/bearing seats).
Tags:
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