Structural system selection by material

One-line orientation

Choosing a structural system is a match between required span/height and the material that delivers it economically — the exam tests whether you can pair a program to the right system.

Key points

• Light wood frame — common for single-family and low-rise residential or commercial work. Small members simplify MEP routing; members are often spaced at 16 or 24 inches on center, and I-joists or trusses extend spans. Wood is combustible, but tested assemblies can still achieve a fire-resistance rating. Height and area limits depend on the complete construction type.
• Heavy / mass timber — mid- to high-rise; larger members with better fire resistance. Solid beams/girders 4–12” thick to ~32 ft; glulam 3–75” deep to ~65 ft; CLT 3–12” thick to ~60 ft lengths.
• Load-bearing masonry — primarily wall construction, with floors and roofs carried by other systems. CMU is generally faster and more economical than brick. A cavity wall commonly uses a structural backup, a drained air space, and a nonstructural veneer tied to the backup.
• Cast-in-place (site-cast) concrete — columns, walls, beams, slabs. Needs formwork + curing; post-tensioning adds strength/span. Columns min. ~10” square / 12” round; beams/girders to ~90–100 ft; slabs ~30 ft (one-way) to ~100 ft (two-way post-tensioned).
• Precast concrete — fast-track, pre-stressed/pre-cured; shallower spanning members; economical on uniform grids. Beams/girders (rect., T, L) to ~75 ft; double-tees to ~90 ft, single-tees to ~110 ft; columns use corbels to support beams.
• Structural steel:
  • Fire behavior: cold-formed steel is noncombustible, but members alone do not create a fire-rated assembly; hot-rolled steel often needs protection.
  • Common column shapes: W and HSS.
  • Preliminary spans: beams/girders 25–40 ft; open-web joists about 90 ft for floors and 140 ft for roofs.

Structural systems — compare members on one span scale

Every row uses the same 0–140 ft scale; gray bands are stated ranges, while labeled dots are single maximum spans.

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Key

• blue band = longest reach
• gray band = stated span range
• dot = stated maximum

Trap: light wood frame and load-bearing masonry are not losing a span race; select them by low-rise scale or wall/height logic.

Confusions / comparison

System	Typical economical span	Height / scale fit	Notes
Light wood frame	Members 16”/24” o.c.; trusses/I-joists for longer spans	Low-rise	Combustible; rated assemblies are possible; height/area depend on construction type
Heavy / mass timber	Glulam ~65 ft · CLT ~60 ft · beams ~32 ft	Mid- to high-rise	Better fire resistance than light frame
Load-bearing masonry	Walls (spanning via other systems)	Brick to ~20 stories	Cavity wall = CMU + air space + veneer
Cast-in-place concrete	Beams ~90–100 ft · slabs 30–100 ft	Any	Formwork + curing; post-tension for span
Precast concrete	Beams ~75 ft · double-tee ~90 ft · single-tee ~110 ft	Fast-track, uniform grids	Pre-cured; shallower members
Structural steel	Beams 25–40 ft · open-web joist 90 ft (floor) / 140 ft (roof)	Any, incl. long-span	Hot-rolled needs fire protection

Related

→ Column buckling & K-values (this topic): why steel HSS/W columns resist buckling · End connections (this topic): how the system’s joints behave · Enclosure control layers (Enclosure): how the chosen structure carries the cladding.