Architecture: Building Design & Structural Systems

# Architecture: Building Design & Structural Systems

## Structural Fundamentals

### Load Path Concept
Every building must transfer lds from point of application to the ground through a continuous ld path. Gravity lds (DL + LL) flow: roof/flr surface → slabs → bms → clms → fdns → soil. Lateral lds (WL, EQ) flow: building surface → diaphragms (flrs/roof acting as horizontal plates) → shear walls or braced frames → fdns → soil.

Breaking ANY link in the ld path causes failure. The 1981 Hyatt Regency walkway collapse killed 114 people because a connection detail change broke the ld path — the rod-to-beam connection carried double the intended ld. Every connection must be dsgn for the actual forces passing through it.

### Load Types
**DL**: Weight of the building itself — str framing, slabs, walls, roofing, mechanical equipment, finishes. Constant and predictable. Typical residential flr DL: 10-15 psf. Commercial with conc slab: 50-80 psf.

**LL**: Occupancy and use — people, furniture, moveable equipment. Code-specified minimums: residential 40 psf, office 50 psf, assembly/auditorium 100 psf, library stacks 150 psf, heavy storage 250 psf. LL can be reduced on large tributary areas and on clms supporting multiple flrs (probability that all flrs are fully loaded simultaneously is low).

**WL**: Increases with height (logarithmic velocity profile), varies by geographic region, and creates both positive pressure (windward) and negative pressure/suction (leeward, roof). Hurricane zones require dsgn for 150+ mph winds. Suction forces on roofs and cladding connections often govern dsgn — buildings don't blow over, they blow apart.

**EQ**: Seismic force = mass × acceleration. Heavier buildings attract more EQ force. Ductile materials (stl) absorb energy through deformation. Brittle materials (unreinforced msnr) fail suddenly. Seismic dsgn philosophy: resist minor EQs without damage, resist moderate EQs with repairable damage, resist major EQs without collapse (life safety).

### Structural Systems

**Post-and-beam (frame)**: clms and bms connected rigidly (moment frames) or with bracing. Moment frames resist lateral lds through bending of members and rigid connections — expensive but allow open floor plans. Braced frames use diagonal members to create triangulated (inherently stable) geometry — more efficient but diagonals interrupt space.

**Load-bearing wall**: Walls carry both gravity and lateral lds. Efficient for residential and low-rise (typically ≤6 stories). Limits interior flexibility — you can't remove a ld-bearing wall without providing an alternative ld path (bm + clms).

**Shear wall**: Solid walls (conc or reinforced msnr) that resist lateral lds through in-plane shear. The most common lateral system for mid-rise buildings. Core walls around elevator/stair shafts do double duty — enclose vertical circulation AND resist lateral forces.

**Tube structures**: For tall buildings (40+ stories). The exterior fac becomes the str system — closely spaced perimeter clms connected by deep bms form a rigid "tube" that resists WL and EQ. Fazlur Khan pioneered this at Sears Tower (bundled tube) and John Hancock Center (diagrid tube). Modern supertalls use outrigger systems connecting a central core to perimeter clms.

## Materials

### Concrete
Compressive strength: 3,000-10,000+ psi (standard to high-performance). Tensile strength: only ~10% of compressive. This weakness in tension is why conc is almost always reinforced with stl rebar — stl handles tension, conc handles compression.

Mix dsgn variables: water-cement ratio (lower = stronger but harder to place), aggregate size and gradation, admixtures (superplasticizers for workability, air-entrainment for freeze-thaw resistance, accelerators/retarders for set time). Standard 28-day cure achieves ~90% of ultimate strength. Premature loading or inadequate curing is a primary failure cause.

Prestressed conc: stl tendons are tensioned BEFORE or AFTER conc placement, putting the conc into compression. Under service lds, the pre-compression must be overcome before the conc experiences tension — effectively eliminating cracking. Enables longer spans with shallower members: 60-100 ft spans common for parking garages and bridges.

### Steel
High strength-to-weight ratio: stl weighs ~490 lb/ft³ vs conc at ~150 lb/ft³, but stl strength is 5-10x higher, resulting in much lighter str for the same capacity. Ductile — deforms significantly before failure, providing warning and energy absorption (critical for EQ resistance).

Wide-flange (I-shaped) sections are the workhorse: flanges resist bending, web resists shear. HSS (hollow structural sections — tubes and pipes) excel in clm applications and architecturally exposed structure. Connection dsgn often governs: bolted vs welded, each with specific strengths. Post-Northridge EQ (1994), welded moment connections were redesigned to ensure ductile behavior.

Fire is stl's weakness — it loses 50% of strength at 1,100°F. Fireproofing (spray-applied, intumescent paint, or encasement) is required by code for all str stl.

### Mass Timber (CLT)
CLT panels are layers of lumber glued in alternating directions, creating panels with str properties comparable to conc in many applications. Carbon-negative material (trees sequester CO2 during growth). 5x lighter than conc for equivalent str capacity, enabling lighter fdns.

Fire performance is counterintuitive: large timber members char on the surface, creating an insulating layer that protects the core. CLT systems routinely achieve 2-hour fire ratings. Tall timber buildings now reach 18+ stories (Mjøstårnet, Norway — 280 ft).

## Building Envelope (env)

### Thermal Performance
R-val measures resistance to heat flow (higher = better insulation). U-val = 1/R-val, measures rate of heat transfer (lower = better). Total wall R-val must account for thermal bridging — stl studs conduct heat 400x faster than insulation, reducing effective R-val by 20-50% in stl-framed walls. Continuous exterior insulation eliminates thermal bridging.

Code minimums (climate dependent): walls R-13 to R-25, roofs R-25 to R-49, windows U-0.30 to U-0.65. Passive House standard demands R-40+ walls, R-60+ roofs, triple-glazed windows U-0.14, and airtightness ≤0.6 ACH50.

### Moisture Management
Water infiltration causes more building failures than any other single factor. The four forces driving water through the env: gravity, kinetic energy (wind-driven rain), surface tension (water crawling along surfaces), and air pressure differential (higher pressure outside pushes water through any opening).

Rainscreen principle: the outer cladding deflects most water, but assumes some will penetrate. A drained and ventilated cavity behind the cladding allows penetrating water to drain out and dry. The weather-resistant barrier (WRB) behind the cavity is the true waterproofing layer. This redundant approach is far more reliable than single-barrier "face-sealed" systems.

### Fenestration (Windows)
Windows are the weakest thermal link — even high-performance glass has R-val of 5-8 vs R-20+ walls. Solar Heat Gain Coefficient (SHGC) measures how much solar energy passes through: high SHGC (0.5+) desirable in heating-dominated climates (free heat), low SHGC (<0.25) desirable in cooling-dominated climates. Low-E coatings selectively block infrared while transmitting visible light.

Dsgn strategy: optimize window-to-wall ratio (typically 30-40% balances daylighting vs thermal performance), orient larger windows south (northern hemisphere) for passive solar gain with overhangs for summer shading, minimize east/west glass (low sun angle makes shading difficult).

## Design Principles

### Proportion & Scale
Classical proportioning systems (Golden Ratio 1:1.618, Renaissance orders) establish visual harmony through mathematical relationships. Le Corbusier's Modulor system based proportions on human body dimensions. Modern practice uses regulating lines — geometric frameworks that align building elements (window openings, floor lines, structural grid) to create coherent compositions.

Scale distinguishes architecture from mere building: how the structure relates to human body size. Monumental scale (cathedrals, civic buildings) deliberately exceeds human proportion to create awe. Intimate scale (residential, boutique retail) keeps elements within human reach to create comfort.

### Circulation & Spatial Organization
Circulation is the connective tissue of architecture. Corridors, stairs, elevators, ramps determine how people experience spaces and in what sequence. Le Corbusier's "architectural promenade" — the designed sequence of spatial experiences as one moves through a building — remains fundamental.

Spatial organization strategies: axial (formal, hierarchical — courthouses, temples), radial (central space with radiating wings — airports, hospitals), grid (flexible, expandable — offices, museums), clustered (informal groupings around shared spaces — campus buildings), linear (movement-driven — train stations, piers).

### Daylighting
Well-daylit buildings reduce lighting energy 40-60% and improve occupant satisfaction and productivity. Key metrics: Spatial Daylight Autonomy (sDA) — percentage of floor area receiving adequate daylight for at least 50% of occupied hours. Target: sDA ≥ 55% for good, ≥ 75% for excellent.

Dsgn strategies: shallow floor plates (≤60 ft for bilateral daylighting), light shelves (reflect daylight deep into flr plate), clerestory windows (high windows bounce light off ceiling), atria (bring light into deep buildings), tubular daylighting devices (mirror-lined tubes for interior rooms). Glare control through orientation, external shading, and interior blinds is equally important — uncontrolled direct sun causes discomfort that leads occupants to close blinds permanently, defeating the purpose.

## Sustainability in Architecture

### Embodied vs Operational Carbon
Operational carbon: energy consumed during building use (HVAC, lighting, equipment) — historically the focus of green building. As grids decarbonize and buildings become more efficient, embodied carbon (emissions from material production, transport, construction, and demolition) becomes dominant — 50-80% of lifecycle carbon for high-performance new buildings.

Reducing embodied carbon: specify low-carbon conc (supplementary cementitious materials like fly ash or slag replace 30-50% of portland cement), use mass timber where code allows, reuse existing structures instead of demolishing, dsgn for disassembly (bolted stl connections vs welded, mechanical fasteners vs adhesives).

### Passive Design Strategies
Before adding mechanical systems, optimize the building form itself: orientation (long axis east-west maximizes south fac for solar control), thermal mass (conc or msnr stores heat, moderating temperature swings), natural ventilation (cross-ventilation requires openings on opposite facs, stack ventilation uses height to drive airflow), earth sheltering (soil provides natural insulation and temperature stability).

The best mechanical system is the one you don't need. Passive strategies in the right climate can eliminate or dramatically reduce HVAC — the largest energy consumer in most buildings.