13.1 Selecting Structural Systems: Steel, Concrete, Wood & Masonry

Structural System Comparative Selection

Selecting an optimal structural system is one of the architect's most consequential schematic design decisions. The choice dictates building geometry, floor-to-floor heights, column spacing, MEP routing strategies, construction logistics, cost, and passive fire resistance. On the ARE 5.0 Project Planning & Design (PPD) division, candidates must evaluate competing structural systems based on span capabilities, structural depth, material behaviors, and architectural program requirements.

Structural Steel Framing Systems

Structural steel offers the highest strength-to-weight ratio of any common building material, making it the primary choice for long spans, high-rise construction, and irregular architectural massing.

• Wide-Flange Shapes (W-Shapes): Standard hot-rolled wide-flange beams and columns (e.g., W18x35, W24x68) typically span 20 to 50+ feet with span-to-depth ratios of approximately $L/20$ to $L/24$ for standard floor beams and $L/28$ to $L/32$ for composite beams. In composite construction, steel shear studs welded through corrugated steel decking lock the cast-in-place concrete slab to the steel beam, forcing them to act as a unified T-beam that substantially increases stiffness and load capacity while reducing structural depth.
• Open-Web Steel Joists (K, LH, DLH Series): Lightweight, shop-fabricated steel trusses composed of hot-rolled angles and round bars. Standard K-series joists span up to 60 feet; Longspan (LH) joists span up to 96 feet; Deep Longspan (DLH) joists span up to 100+ feet. Open webs provide continuous, unobstructed pathways for mechanical ductwork, electrical conduit, and plumbing piping. Commonly used for roof structures in big-box retail, warehouses, athletic facilities, and gymnasiums. Span-to-depth ratios typically range from $L/18$ to $L/24$.
• Light-Gauge Cold-Formed Steel (CFS): Cold-rolled galvanized sheet steel (C-shaped studs and tracks) used for non-load-bearing interior partitions, exterior curtain wall framing, and low-rise load-bearing structures (typically 1 to 4 stories) spanning 15 to 25 feet. CFS provides non-combustible construction (Type I/II), dimensional stability (will not warp, shrink, or twist like dimensional lumber), and resistance to rot and termites.
• Passive Fireproofing for Steel: Structural steel is non-combustible but loses approximately 50% of its structural yield strength at 1,000°F (538°C) and buckles rapidly near 1,200°F. Building codes require passive fire protection to achieve 1- to 4-hour fire-resistance ratings:
  • Spray-Applied Fire-Resistive Materials (SFRM): Cementitious or mineral fiber wet slurries sprayed directly onto steel members; lowest initial cost, lightweight, but visually rough and typically concealed above suspended lay-in ceilings.
  • Gypsum Board Encasement: Multiple layers of 5/8-inch Type X gypsum board fastened around beams and columns; clean, smooth appearance suitable for enclosed partitions.
  • Concrete Encasement: Pouring solid concrete around steel members; extremely durable and impact-resistant, but adds massive dead weight.
  • Intumescent Coatings: Thin, paint-like coatings applied directly to exposed steel. Under high heat (>390°F), the chemical coating undergoes an endothermic reaction, swelling 50 times its original thickness into an insulating cellular carbonaceous char. Ideal for exposed Architecturally Exposed Structural Steel (AESS) where aesthetic elegance and code-mandated fire ratings must coexist.

Concrete Framing Typologies

Cast-in-place and precast concrete systems provide inherent 1- to 4-hour fire resistance without supplemental thermal insulation, superior acoustic separation (high Sound Transmission Class [STC] ratings exceeding STC 50), and massive thermal capacity that damps interior temperature swings.

Critical Exam Concept — Punching Shear in Flat Plates: In a two-way flat plate, the entire gravity load transfers directly from the slab into the column over a small perimeter area ($d/2$ from the column face). Without deep beams, drop panels, or column capitals, concentrated shear stresses can cause the column to punch vertically through the slab like a hole-punch through paper, precipitating progressive structural collapse. Mitigation strategies include increasing slab thickness, increasing column cross-sectional dimensions, embedding vertical steel shear studs on rail assemblies, or fabricating internal structural steel shearheads.

Wood & Heavy Timber Framing

• Light-Frame Wood Construction: Nominal 2x dimensional lumber (2x4, 2x6, 2x8) and prefabricated engineered wood I-joists. Spans typically range from 15 to 25 feet. Dominates residential and low-rise commercial construction up to 5 stories (Type V or Type III construction, frequently combined with a Type I-A concrete podium in "5-over-1" or "5-over-2" podium configurations). Economical, highly workable, but combustible and subject to moisture rot, insect attack, and long-term shrinkage.
• Heavy Timber (Type IV-HT) & Glulam: Glued-laminated timber (Glulam) consists of individual 1.5-inch dimensional lumber laminations glued with water-resistant adhesives. Glulams span 40 to 80+ feet, engineered into straight beams, heavy girders, and dramatic curved arches.
• Mass Timber & Cross-Laminated Timber (CLT): CLT consists of 3, 5, 7, or 9 layers of kiln-dried dimensional lumber stacked perpendicularly to adjacent layers and glued under massive hydraulic pressure. CLT panels span bi-axially, functioning as rigid two-way structural floor and roof slabs, as well as multi-story load-bearing shear walls. Mass timber offers structural strength comparable to reinforced concrete at approximately 20% of the dead weight.
• Wood Charring Mechanics: Heavy timber and mass timber achieve predictable fire resistance through natural charring. When exposed to standard ASTM E119 fire conditions, wood burns from the outside in at a predictable nominal charring rate of approximately 1.5 inches per hour (0.024 in/min). The resulting outer layer of charcoal has virtually zero oxygen permeability and a thermal conductivity only one-sixth that of virgin wood. This sacrificial char layer insulates the interior unburned wood core, which maintains room temperature, nominal yield strength, and full load-bearing capacity throughout the fire duration.

Reinforced Masonry

Reinforced Concrete Masonry Unit (CMU) construction utilizes hollow modular blocks (typically 8x8x16 inches) laid in mortar. Vertical reinforcing steel bars are placed within the hollow cells, and horizontal ladder/truss wire reinforcement is embedded in mortar bed joints. Solid grout is poured into the reinforced cells, and continuous reinforced horizontal bond beams are cast at floor and roof levels. Reinforced CMU provides superior compressive load-bearing strength, 2- to 4-hour inherent fire resistance, high impact durability, and exceptional resistance to windborne hurricane debris and seismic shear forces.