Beams are the horizontal load carriers of almost every structure. They span openings, collect loads from slabs and walls, and hand them down to columns and foundations. A beam can be classified by its material, its support conditions, its cross-sectional shape, and the job it performs inside the building. The right choice depends on span, load intensity, deflection limits, and cost. On short spans with light loads, the difference between steel I beams and H beams rarely changes the outcome; on long spans and heavy loads, section shape and material dominate the decision.
Beams grouped by construction material
The material sets the beam’s strength, stiffness, durability, and fire resistance, and it fixes practical limits on span and depth.
Concrete beams
Reinforced concrete beams are cast in place or precast, with the reinforcement carrying tension while the concrete carries compression. Rectangular sections are the most common because they are simple to form and the steel ratio stays economical. Circular beams appear in tanks, silos, and curved walls, while flanged sections use the slab above. At foundation level, beams tie the columns together and limit differential settlement; the difference between plinth beams and tie beams comes down to position and the loads they actually carry.
Rectangular and flanged sections
Rectangular beams are cheap to form, which keeps labor costs down on small projects. Flanged beams are cast monolithically with the slab, so the slab acts as a compression flange and the member carries more load.
Steel beams
Steel sections give high strength per unit of weight and predictable material properties. Universal beams have higher stiffness in the direction of bending and dominate floor framing. Plate girders are built up from web and flange plates when loads are heavy and spans are long. Other steel sections used as beams include:
- C channels, for light loads and simple connections
- Rectangular hollow sections, favored for torsional resistance and a clean appearance
- Circular hollow sections and solid pipes, for curved layouts and architectural effects
Timber beams
Timber stays popular in residential work, and engineered products push it into longer spans. Treated timber is used wherever moisture or insect attack threatens durability. Solid sawn beams are limited by natural defects, while glulam and laminated veneer lumber grade out more predictably.
Composite beams
Composite beams combine steel and concrete so each material works where it is strongest. Shear studs welded to the top flange are embedded in the concrete slab, and the two parts then act as a single section. Composite action raises stiffness and capacity without adding depth, which is why composite floors dominate high-rise steel construction.
Beams grouped by support condition
The supports decide how a beam bends, where the maximum moment sits, and how loads redistribute when a support settles. Once materials, cross-sections, and functions are combined, catalogs list more than 22 types of beams, but the support condition is the classification that drives the analysis.
| Support arrangement | Bending moment | Shear force | Typical use |
|---|---|---|---|
| Simply supported | Maximum positive moment near mid-span | Peaks at the supports | Floor beams and short bridge spans |
| Continuous | Negative moment over interior supports, positive at mid-spans | Lower peaks than a series of simple spans | Multi-span decks and building frames |
| Cantilever | Maximum moment at the fixed end | Maximum shear at the fixed end | Balconies, canopies, and projecting slabs |
| Fixed | Hogging at both ends, reduced sagging at mid-span | Shared between both ends | Heavy industrial frames and bunkers |
| Overhanging | Negative moment over the support, reduced positive moment in the span | Peaks near the support | Porch projections and bridge end spans |
Simply supported beams
A simply supported beam rests on two supports that allow rotation, so the ends are free to rotate. The bending moment peaks at mid-span, or near it, depending on the load arrangement, while the shear force peaks at the supports. Simple supports are forgiving: a small settlement of one support barely changes the internal forces.
Continuous beams
When a beam runs over two or more supports it becomes continuous. Interior supports develop negative moments, so top reinforcement matters as much as bottom steel. The analysis must account for column stiffness; assuming knife-edge supports ignores real column restraint and changes the moment diagram.
Cantilever, fixed, and propped beams
A cantilever is fixed at one end and free at the other, with the maximum moment at the fixed end. A fixed beam is restrained against rotation at both ends, which lowers the mid-span moment versus a simply supported beam. A propped cantilever has one fixed end and one simple support, common for stair flights and retaining wall stems.
Overhanging beams
An overhanging beam extends beyond one support. The overhang creates a negative moment over the support, which reduces the positive moment in the main span and makes the section more efficient.
Beams grouped by cross-section and shape
The cross-section sets the moment of inertia, and with it stiffness and deflection. Shape selection puts material where bending stress is highest, at the top and bottom fibers, and keeps it thin near the neutral axis.
Rectangular and square sections
Rectangular concrete sections are cheap to form and easy to reinforce, and they suit moderate spans where depth is not constrained. Square sections appear in short columns and where the architect wants a uniform grid.
T and L beams
T beams use the slab as a compression flange, so the effective flange width becomes a design input. L beams occur at edges and openings where the slab extends on one side only. The flange adds compression area, which lets the member carry larger moments for the same web size.
I sections and box girders
I sections concentrate steel in the flanges and keep the web thin for a high stiffness-to-weight ratio. Box girders close the section and add torsional rigidity, which is why they are standard on curved bridges and long cantilevered roofs.
Prestressed and post-tensioned beams
Prestressing holds the concrete in permanent compression so that service loads produce little or no tension. Crack widths stay small under working loads and the member recovers after overload. Openings for services must be planned at the design stage; a poorly placed opening concentrates stress and turns minor cracking into structural damage. Review the guidance on cracks in prestressed concrete beams with openings before any penetration is approved.
Beams that work inside building systems
Many beams are named for the position they occupy rather than for their section. These members carry modest loads, but the frame depends on them.
Lintel beams
Lintels span openings in walls and carry the masonry above the opening down to the jambs. They are built from reinforced concrete, steel angles, precast units, or timber, and the depth is usually governed by the clear span and the masonry height above. The types of lintel beams available make it possible to match the member to the opening, the wall material, and the floor loads above.
Plinth and tie beams
Plinth beams run at ground level between column bases and support the wall above, while tie beams connect columns at intermediate levels and shorten their effective length. Both are light members that rarely carry floor loads but improve robustness against differential settlement and drift.
Spandrel and ring beams
Spandrel beams run along the building perimeter at each floor level, catching the outer slab edge and supporting the facade. Ring beams wrap masonry walls at roof and floor levels, tying the wall together and distributing loads evenly. Both are sized more by stiffness than by strength.
How beams carry load: analysis in practice
Every beam, from a garage joist to a bridge girder, gets analyzed the same way: find the reactions, draw the shear force and bending moment diagrams, then check the section against the maximum values.
- Calculate the design loads, including self-weight, superimposed dead load, and live load with combination factors.
- Work out the reactions from the support conditions and the load positions.
- Draw the shear force and bending moment diagrams and record the critical values.
- Select a trial section and check bending, shear, deflection, and any torsion.
- Iterate until the section satisfies every limit state with an acceptable utilization ratio.
Bending moments and shear
The maximum bending moment sets the required section modulus, while the maximum shear sets the web thickness and the stirrup spacing. For a simply supported beam under a uniform load w, the moment is wL^2/8 at mid-span and the shear is wL/2 at each support, a quick sanity check before any software run.
Deflection and stiffness
Deflection limits, not strength, often govern shallow beams. A strong member still feels unsafe if it sags visibly. Elastic deflection depends on the load, the span cubed, and the modulus times the moment of inertia, which is why doubling the depth cuts deflection eightfold.
Support modeling matters
Boundary conditions come from the actual connections. Getting them right starts with a clear beam definition and types of supports; roller supports release horizontal force, pinned supports release moment, and fixed supports release nothing, so modeling the wrong restraint can flip the moment diagram.
Practical construction: setting out and level control
Choosing the right beam type is only half the job. On site the member has to be set out and poured or erected to the right level, and the surveyor’s instruments make that possible. The types of levels used in leveling run from simple spirit levels to automatic and digital instruments for long beam lines.
Setting out beam positions
Beam positions are transferred from the drawings using the center line and offsets. Two control points go at each end, and a string line or laser defines the beam axis. On long beams, checks every few meters catch cumulative errors before the concrete is placed.
Checking levels during construction
The finished floor level is the reference most beam levels are set against. A level set on a known benchmark produces staff readings that convert to reduced levels at each formwork location.
Levels for beam soffits
Beam bottoms are set to the design level plus any camber, because the formwork deflects under the wet concrete. Check the soffit at mid-span and at each support after the props are tightened, then re-check after the concrete is placed.
The surveyor’s job does not end when the concrete hardens. Final soffit levels are compared with design levels and recorded in the as-built documentation. A few millimeters of error is absorbed by the floor build-up; a larger error is expensive to fix. The types of leveling in surveying give the team the tools to catch those errors early, which is cheaper than grinding concrete or recasting a member.