amanbh997/skills-architects/structural-systems

Structural Systems for Architects

Section 1: Structural System Selection

The choice of structural system is one of the earliest and most consequential decisions in building design. It determines floor-to-floor height, column spacing, facade expression, construction programme, cost, and embodied carbon. The following decision tree and system catalogue provide architects with the knowledge to make informed selections before engaging structural engineers.

Decision Tree: Key Variables

1. Span requirement — What clear span does the programme demand?
2. Building height — Low-rise (1-4), mid-rise (5-12), high-rise (13-40), supertall (40+)?
3. Programme type — Residential, office, retail, industrial, mixed-use, cultural?
4. Cost envelope — Budget-constrained, mid-market, premium?
5. Construction speed — Is programme duration a critical driver?
6. Sustainability target — Embodied carbon budget (kgCO2e/m²)?
7. Fire rating — 30/60/90/120 minutes required?
8. Site constraints — Access, vibration, noise, adjacent structures?

System Type 1: Load-Bearing Masonry

• Span range: 4-7m (with concrete floor slabs or timber joists)
• Height limit: Up to 6 storeys (unreinforced); up to 10+ storeys (reinforced/confined)
• Typical wall thickness: 215mm (single leaf), 300mm (cavity), 215-327mm (solid at lower storeys)
• Slab depth: 150-200mm RC slab on masonry walls; or 200-250mm precast plank
• Cost range: 80-140 $/m² (structure only)
• Construction speed: 4-6 weeks per floor (traditional); faster with thin-joint masonry
• Fire rating strategy: Inherent fire resistance — 215mm solid block achieves 120 min REI
• Embodied carbon: 150-280 kgCO2e/m² (depending on block type and mortar)
• Best-fit building types: Social housing, low-rise residential, hotels, student accommodation, schools
• Exemplar buildings:
• Monadnock Building, Chicago (1891) — 17 storeys, 1.8m thick walls at base
• Mapleton Crescent, London (2018) — 6-storey CLT with masonry facade
• Park Hill, Sheffield (1961) — load-bearing crosswall construction

Key architectural implications: Wall positions fix the plan. Window openings are limited by structural capacity. Interior load-bearing walls constrain future flexibility. Best for repetitive cellular plans (residential, hotels). Crosswall construction (party walls carry loads) is the most efficient variant.

System Type 2: Reinforced Concrete Frame

• Span range: 6-12m (flat slab); 8-15m (beam-and-slab); 10-18m (waffle slab)
• Height limit: Unlimited (with appropriate lateral system)
• Typical slab depths: Flat slab 250-350mm; beam-slab 150-200mm slab + 450-700mm beam; waffle 325-500mm overall
• Column sizes: 300x300mm (low-rise) to 1200x1200mm (high-rise base)
• Cost range: 120-220 $/m² (structure only)
• Construction speed: 7-14 days per floor (traditional formwork); 5-7 days (table forms)
• Fire rating strategy: Cover to reinforcement — 25mm for 60 min, 35mm for 90 min, 45mm for 120 min (BS EN 1992-1-2)
• Embodied carbon: 250-400 kgCO2e/m² (conventional); 180-280 kgCO2e/m² (with GGBS/PFA cement replacement)
• Best-fit building types: All building types — residential, offices, hospitals, mixed-use, high-rise
• Exemplar buildings:
• Barbican Estate, London (1969-76) — exposed concrete frame, residential towers to 42 storeys
• Marina Bay Sands, Singapore (2010) — post-tensioned flat slabs, 57 storeys
• Bosco Verticale, Milan (2014) — RC frame with cantilevered planting balconies

Key architectural implications: Flat slabs give maximum flexibility — no downstand beams, services route freely. But punching shear at columns requires drop panels or shear heads. Beam-slab systems deeper overall but more efficient for longer spans. Waffle slabs suit exposed soffits in public buildings.

System Type 3: Post-Tensioned Concrete

• Span range: 8-16m (flat slab); 10-20m (banded beams)
• Height limit: Unlimited
• Typical slab depths: 200-300mm (flat slab PT); 180-250mm slab + 400-600mm band beam
• Cost range: 140-240 $/m² (structure only — tendons add 15-25% to RC cost but save concrete)
• Construction speed: 5-7 days per floor (table forms); faster cycle than RC due to thinner slabs and earlier striking
• Fire rating strategy: Same as RC, but with specific tendon cover requirements (40mm for 90 min). Bonded tendons preferred for fire safety; unbonded require supplementary reinforcement.
• Embodied carbon: 200-320 kgCO2e/m² — typically 15-25% lower than equivalent RC flat slab due to reduced concrete volume
• Best-fit building types: Large-span offices, car parks, hospitals, airports, podium structures
• Exemplar buildings:
• One Canada Square, London (1991) — PT flat slabs, 50 storeys
• Apple Park, Cupertino (2017) — PT radial slabs, 12m spans
• Broadgate Exchange House, London (1990) — 78m clear span PT arches supporting 10 storeys

Key architectural implications: Thinner slabs than RC (typically 20-30% thinner) — reduces floor-to-floor height, cumulative savings on cladding and services. Tendons restrict location of penetrations: openings must be coordinated early with structural engineer. Core holes cannot be cut through tendons post-construction.

System Type 4: Steel Frame

• Span range: 6-20m (standard sections); 15-60m+ (trusses, castellated beams)
• Height limit: Unlimited (with appropriate lateral system)
• Typical beam depths: 300-600mm (6-12m span); 500-900mm (12-20m); trusses 1.5-3.0m (30-60m)
• Column sizes: 200x200mm UC to 400x400mm (or circular 219-508mm dia.)
• Cost range: 150-280 $/m² (structure only, including fire protection)
• Construction speed: 3-5 days per floor (steel erection); fastest structural system. 4-6 month programme advantage over RC for high-rise.
• Fire rating strategy: Intumescent paint (thin-film for 30-60 min, thick-film for 90-120 min), board encasement, concrete encasement, water-filled hollow sections, or fire-engineered unprotected steel where justified.
• Embodied carbon: 200-350 kgCO2e/m² (conventional); 120-200 kgCO2e/m² (high recycled content, EAF steel)
• Best-fit building types: Offices (City of London standard), tall buildings, long-span structures, exhibition halls, stadia, fast-track projects
• Exemplar buildings:
• The Shard, London (2012) — steel frame with concrete core, 72 storeys, 310m
• Centre Pompidou, Paris (1977) — exposed steel gerberette trusses, 48m clear spans
• Beijing National Stadium "Bird's Nest" (2008) — steel space frame, 330m span

Key architectural implications: Lightest structural system — advantageous on poor soils. Fastest erection — critical for commercial projects. Fire protection is an additional cost and aesthetic consideration. Exposed steel requires careful detailing for fire engineering or acceptance of fire-engineered solutions. Long-span steel enables column-free spaces impossible with concrete.

System Type 5: Composite Steel-Concrete

• Span range: 9-18m (composite beams with metal deck); 12-20m (composite trusses)
• Height limit: Unlimited
• Typical floor construction: 130-170mm composite metal deck slab on 400-600mm downstand steel beams (or 200-350mm ASB/slim-floor beams)
• Cost range: 140-250 $/m² (structure only)
• Construction speed: Matches steel frame — metal deck acts as permanent formwork, no propping required for spans up to 3.6m
• Fire rating strategy: Mesh-reinforced deck achieves 60 min; additional bar reinforcement for 90-120 min. Steel beams protected as per steel frame.
• Embodied carbon: 220-340 kgCO2e/m²
• Best-fit building types: Commercial offices (UK standard), mixed-use podiums, car parks
• Exemplar buildings:
• 22 Bishopsgate, London (2020) — composite frame, 62 storeys
• One World Trade Center, New York (2014) — composite steel-concrete
• Leadenhall Building "Cheesegrater", London (2014) — mega-frame with composite floors

Key architectural implications: The standard commercial office solution in the UK. 150mm slab + 400-500mm beam gives total structural zone of 550-650mm. Slim-floor variants (ASB, Slimdek) reduce this to 300-400mm but at higher cost. Web openings in beams allow services to pass through the structural zone, reducing overall floor-to-floor height.

System Type 6: Mass Timber

• Span range: CLT floors 5-8m; glulam beams 8-15m; timber trusses 12-30m
• Height limit: Up to 18 storeys (current codes; taller in engineered solutions — Mjosternet, Norway is 18 storeys/85.4m)
• Typical element sizes: CLT panels 100-300mm thick; glulam beams 200x400mm to 300x900mm; LVL beams similar range
• Cost range: 160-300 $/m² (structure only — premium over concrete but falling)
• Construction speed: Very fast — 3-5 days per floor. Prefabricated, dry construction, reduced site labour. 30-50% faster than RC.
• Fire rating strategy: Charring rate method — timber chars at 0.65mm/min (softwood), sacrificial timber layers. 60-90 min easily achievable with oversized sections. Encapsulation for 120 min. Sprinklers typically required above 4 storeys.
• Embodied carbon: 80-180 kgCO2e/m² — significantly lower than concrete or steel. Biogenic carbon sequestration potentially makes it carbon-negative.
• Best-fit building types: Residential (mid-rise), offices, schools, cultural buildings, wellness/healthcare where biophilic design is valued
• Exemplar buildings:
• Mjosternet, Brumunddal, Norway (2019) — 18 storeys, 85.4m, world's tallest timber building
• Dalston Works, London (2017) — 10-storey CLT residential, 121 units
• Sara Kulturhus, Skelleftea, Sweden (2021) — 20 storeys, mass timber hybrid

Key architectural implications: Warm, biophilic aesthetic when exposed. Acoustic separation requires careful detailing (resilient layers, concrete topping on CLT). Moisture management critical during construction. Connection design is the key challenge — steel brackets, self-tapping screws, dowelled connections. Timber moves (shrinkage, creep) — allow for differential movement at connections to other materials.

System Type 7: Hybrid Systems

• Timber-Concrete Composite (TCC): CLT or glulam with concrete topping — improves acoustic performance, adds thermal mass, increases stiffness. Spans 6-10m. Common in residential.
• Steel-Timber: Steel primary frame with CLT floor panels — combines long steel spans with timber's sustainability. Spans 8-16m.
• Concrete Core + Timber Frame: Concrete core for lateral stability, timber for gravity frame — common in tall timber buildings.
• Cost range: 150-280 $/m² depending on combination
• Embodied carbon: 120-250 kgCO2e/m² — intermediate between pure timber and pure concrete
• Exemplar buildings:
• 25 King, Brisbane (2018) — timber-concrete composite, 10 storeys
• Brock Commons, Vancouver (2017) — concrete core + CLT floors + glulam columns, 18 storeys
• 80 Atlantic Avenue, Toronto (2022) — steel-timber hybrid office

System Type 8: Precast Concrete