Simply supported describes a beam or structural member that rests on two supports at its ends, free to rotate at those points and free to expand or contract along its length. One end is pinned in place, and the other sits on a roller. This is the most basic and commonly taught support configuration in structural engineering, and it shows up everywhere from footbridges to floor joists.
How Simple Supports Work
A simply supported beam has two distinct types of support, one at each end. The left end typically has a pin support, which holds the beam in place both vertically and horizontally but allows it to rotate freely. The right end has a roller support, which only pushes back against vertical loads. The roller can’t resist any sideways force, which means the beam is free to slide slightly in the horizontal direction at that end.
This freedom to slide is actually the point. When a beam heats up on a sunny day or bears a heavy load, it expands slightly. A roller support lets that expansion happen without building up internal stress that could crack or buckle the beam. The freedom to rotate at both ends means the beam can bend under load without the supports trying to hold the ends rigid, which simplifies both the design and the math behind it.
Because neither support resists rotation, a simply supported beam cannot carry a moment (a twisting force) at its ends. The bending moment at both ends is always zero. All the bending happens between the supports, peaking somewhere in the middle of the span.
How It Differs From Fixed and Cantilever Beams
The key distinction is what happens at the supports. A fixed beam (also called a built-in or encastré beam) has both ends rigidly locked in place. The supports resist vertical forces, horizontal forces, and rotation. This makes the beam stiffer, but it also means any expansion, settling, or movement in the structure can create large internal stresses.
Research comparing the two configurations confirms what engineers expect: simply supported beams deflect significantly more than fixed beams under the same load. In one study, simply supported beams deflected roughly 10 to 100 times more than their fixed counterparts under comparable conditions. That extra flex isn’t necessarily a problem. It just means engineers need to account for it when choosing beam sizes.
A cantilever beam, by contrast, is fixed at only one end and hangs freely at the other, like a diving board. It carries all its load through that single fixed connection, which must resist both vertical force and rotation. Simply supported beams spread the load across two supports, making them more straightforward to design and build.
What Happens Inside the Beam Under Load
When weight pushes down on a simply supported beam, two things develop along its length: shear force and bending moment. Understanding both helps explain why beams are sized the way they are.
Shear force is the internal force that resists vertical sliding between adjacent sections of the beam. At the supports, the shear force equals the support reaction (essentially, how hard the support pushes up). For a beam carrying a single concentrated load at its center, the shear force is constant across each half of the beam, jumping abruptly at the point where the load is applied. For a load spread evenly along the entire length, the shear force decreases linearly from one support to the other, crossing zero at the midpoint.
Bending moment measures how much the beam wants to curve at any given point. On a simply supported beam, the bending moment is always zero at both ends and reaches its maximum value somewhere between the supports. For a centered point load, the maximum bending occurs directly under the load. For an evenly distributed load, it peaks at the exact middle of the span. The point where shear force crosses zero is always where bending moment hits its maximum, a relationship that holds for any loading pattern.
How Engineers Size Simply Supported Beams
Two concerns drive beam sizing: strength (can it carry the load without breaking?) and serviceability (does it bend so much that it looks or feels wrong?). For simply supported beams, serviceability often controls the design because these beams deflect more than fixed ones.
Engineers use the span-to-depth ratio as a quick check. This is simply the beam’s length divided by its depth. European concrete design standards, for example, state that if the calculated deflection under sustained loads exceeds 1/250 of the span, the beam’s appearance and function may be compromised. For a 5-meter beam, that limit is just 20 millimeters of sag.
Design codes provide tables of limiting span-to-depth ratios that depend on the material, the cross-section shape, and how heavily the beam is reinforced. A T-shaped concrete beam, for instance, gets a reduction factor of 0.8 compared to a rectangular one, reflecting its different stiffness characteristics. For spans longer than about 7 meters, an additional correction factor reduces the allowable ratio further. These checks let engineers quickly estimate whether a beam depth is in the right ballpark before running detailed calculations.
Where You’ll See Simply Supported Beams
Simply supported beams are common in situations where simplicity, economy, or the need to accommodate movement matters more than minimizing deflection. Short-span floor joists in residential construction often behave as simply supported members, resting on walls or beams at each end without rigid connections. Precast concrete planks dropped onto bearing walls work the same way. Bridge girders frequently sit on elastomeric bearing pads that act like rollers, letting the bridge deck expand and contract with temperature changes while the supports handle only vertical loads.
In practice, many connections that look pinned or simply supported actually provide some rotational resistance. A steel beam bolted to a column through a thin end plate, for example, transfers a small moment even though it’s designed as a simple connection. Engineers typically ignore this extra stiffness for safety, since assuming simple support produces higher deflection and bending estimates than the beam will actually experience. Designing for the worse case builds in a margin of safety.