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
- Span requirement — What clear span does the programme demand?
- Building height — Low-rise (1-4), mid-rise (5-12), high-rise (13-40), supertall (40+)?
- Programme type — Residential, office, retail, industrial, mixed-use, cultural?
- Cost envelope — Budget-constrained, mid-market, premium?
- Construction speed — Is programme duration a critical driver?
- Sustainability target — Embodied carbon budget (kgCO2e/m²)?
- Fire rating — 30/60/90/120 minutes required?
- 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
- Span range: Hollowcore plank 6-16m; double-tee 12-24m; precast beams 8-18m
- Height limit: Up to 30+ storeys (with in-situ cores or shear walls for stability)
- Typical element sizes: Hollowcore 150-400mm deep x 1200mm wide; double-tee 400-800mm deep x 2400mm wide; columns 300x300 to 600x600mm
- Cost range: 130-230 $/m² (structure only)
- Construction speed: Very fast — factory quality control, minimal wet trades on site. 3-5 days per floor.
- Fire rating strategy: Inherent — similar to in-situ RC. 150mm hollowcore achieves 60 min; 200mm achieves 90 min; 250mm+ achieves 120 min.
- Embodied carbon: 230-370 kgCO2e/m² — similar to in-situ RC but with less waste
- Best-fit building types: Car parks, retail, warehouses, student housing, prisons, repetitive residential
- Exemplar buildings:
- Welbeck Street Car Park, London (1970) — precast diagrid facade/structure
- De Rotterdam, Rotterdam (2013) — precast concrete frame, 150m, 162,000m² mixed-use
- Habitat 67, Montreal (1967) — prefabricated concrete modules, 354 units
Key architectural implications: Requires early coordination — precast elements are manufactured weeks before erection. Modifications on site are difficult and expensive. Joints and connections require careful architectural detailing. Standardization of grid and element sizes is essential for economy. Excellent surface finish achievable (fair-faced, polished, textured, coloured aggregate).
Section 2: Structural Grid Design
The structural grid is the skeleton upon which all architectural decisions hang. Grid dimensions drive column positions, facade rhythm, parking efficiency, service routes, and spatial quality. Getting the grid right at concept stage prevents costly redesigns later.
Grid Spacing by Building Type
| Building Type | Typical Grid (m) | Notes |
|---|
| Residential (apartment) | 5.0-8.0 x 5.0-8.0 | Party wall crosswall at 5.0-7.5m centres; 6.0m allows 2-bed flat width |
| Residential (hotel) | 3.6-4.2 x 7.5-9.0 | Room width 3.6-4.2m; room depth 7.5-9.0m including bathroom |
| Office (speculative) | 7.5-10.8 x 7.5-10.8 | 9.0m is the UK standard; 10.8m for premium Grade A |
| Office (owner-occupied) | 6.0-9.0 x 6.0-12.0 | More flexibility in grid; can accept transfer structures |
| Retail (shopping centre) | 8.0-12.0 x 8.0-12.0 | Column-free retail units preferred; 8.1m suits standard shopfronts |
| Retail (supermarket) | 10.0-16.0 x 10.0-16.0 | Large clear spans for racking and flexibility |
| Car parking (above-ground) | 8.1 x 5.4 (single bay) | 2 cars + aisle = 5.0m car + 6.0m aisle + 5.0m car = 16.0m double bay |
| Car parking (basement) | 7.5-8.1 x 15.4-16.2 | 16.2m double-bay span is standard for post-tensioned car parks |
| Hospital | 7.2-8.4 x 7.2-8.4 | Suits clinical room modules; 7.2m = 2x3.6m clinical bays |
| School (classroom) | 7.5-9.0 x 7.5-9.0 | 8.1m x 8.1m suits 60m² classroom |
| Warehouse/logistics | 12.0-24.0 x 12.0-24.0 | Long-span portal frames or trusses; 24m clear span is standard |
| Laboratory | 6.6-7.5 x 9.0-10.8 | Module driven by 3.3m bench spacing (2 x 3.3m = 6.6m) |
Column-Free Span Requirements
| Use | Minimum Clear Span |
|---|
| Badminton court | 6.1 x 13.4m |
| Basketball court | 15.2 x 28.6m |
| Swimming pool (25m) | 25.0 x 16.0m minimum |
| Swimming pool (50m) | 50.0 x 25.0m minimum |
| Theatre auditorium | 15-30m depending on seating capacity |
| Concert hall | 25-50m |
| Exhibition hall | 30-100m+ |
| Airport terminal | 30-70m (check-in halls) |
Grid Alignment with Facade Module
The structural grid should coordinate with the facade module to avoid awkward junctions:
- Curtain wall: Typical mullion spacing 1.2m, 1.5m, or 1.8m. A 9.0m structural grid divides into 6x1.5m or 5x1.8m facade bays.
- Precast cladding: Panel widths typically 1.5-3.6m. Grid must suit panel module.
- Masonry facade: Brick module 225mm (UK) or 200mm (modular). Grid dimensions should be multiples of brick module.
- Unitised curtain wall: Panel widths 1.2-1.8m. Structural grid should be a whole multiple of panel width.
Podium-Tower Transition
Where a tower sits on a podium (common in mixed-use: retail/parking podium + residential/office tower), the grids rarely align:
- Transfer structure: Use transfer beams (1.0-2.5m deep) or transfer slabs (600-1200mm) to redirect loads. Expensive and carbon-intensive.
- Offset columns: Step columns inward using inclined members. Requires careful detailing.
- Aligned grids: Best solution — design tower grid to work with podium grid. Typical approach: 8.1m podium grid (parking) with tower columns at every second or third podium column.
- Mega-columns: Carry tower loads directly through podium on large-section columns or walls, independent of podium grid.
Section 3: Lateral Stability Systems
Every building must resist lateral loads (wind, seismic, notional horizontal loads). The lateral system profoundly affects architectural planning — it determines where solid walls must go, where bracing appears, and what facade expression is possible.
Shear Walls (Concrete or Masonry)
- Principle: Planar walls acting as vertical cantilevers fixed at foundation
- Minimum thickness: 150mm (concrete, up to 10 storeys); 200-300mm (taller buildings); 215mm (masonry)
- Planning impact: Walls must be continuous from roof to foundation. Cannot be removed or relocated. Must be arranged in two orthogonal directions.
- Height suitability: Up to 30 storeys (concrete); up to 10 storeys (masonry)
- Advantages: Excellent fire compartmentation, acoustic separation, no visible bracing
- Location strategy: Around cores (lifts, stairs), party walls, gable ends, flanking corridor walls
- Minimum shear wall provision: Approximately 3-5% of floor area in plan for buildings up to 20 storeys
Braced Frames
- Types: X-bracing, V-bracing (chevron), inverted-V, K-bracing (avoid — progressive collapse risk), eccentric bracing
- Member sizes: Typically 150x150mm to 300x300mm hollow sections (steel); or 200x200mm to 400x400mm timber
- Planning impact: Bracing panels restrict movement through the braced bay. Must be resolved at concept stage.
- Height suitability: Up to 20 storeys (concentric bracing); up to 40 storeys (eccentric bracing with ductile links)
- Advantages: Efficient, lightweight, expressed bracing can be architecturally dramatic
- Disadvantages: Obstructs openings in braced bays; requires foundation fixity
Moment Frames
- Principle: Rigid connections between beams and columns resist lateral loads through bending
- Member sizes: Columns 50-100% larger than gravity-only design; beams deeper at connections
- Planning impact: No bracing or shear walls required — maximum spatial freedom
- Height suitability: Up to 25 storeys (steel); less efficient above this
- Advantages: Complete flexibility in planning; transparent facades possible
- Disadvantages: Most expensive lateral system; significant steel tonnage premium (30-50% more than braced frame); larger member sizes; more complex connections
- Drift control: Governing design criterion is usually storey drift (H/500 typical) rather than strength
Core Structures
- Principle: Concrete core (containing lifts, stairs, risers) acts as primary lateral element
- Planning impact: Core is the anchor of the plan. Core position affects structural efficiency, lettable area, and egress distance.
- Central core: Most structurally efficient for lateral loads; standard in commercial office towers. Maximises perimeter usable space.
- Side/end core: Common in residential towers; allows clear-span floor plates. Less efficient for lateral resistance — supplementary walls may be needed.
- Dual core: Used in deep-plan buildings; two cores linked by floor diaphragm.
- Height suitability: Up to 40-50 storeys (core alone); taller with outriggers
- Core dimensions: Typically 15-25% of typical floor area for buildings above 20 storeys
Outrigger Systems
- Principle: Deep beams or trusses connect core to perimeter columns, engaging the full building width in lateral resistance. Dramatically reduces core bending moments and roof deflection.
- Outrigger depth: Typically 1-2 storeys (mechanical plant floors are ideal locations)
- Height suitability: 40-80 storeys
- Planning impact: Outrigger floors cannot have normal occupancy — dedicate to mechanical plant
- Efficiency: Single outrigger at 2/3 height reduces drift by ~50%; dual outriggers (1/3 and 2/3) reduce by ~65%
- Exemplar: Taipei 101, One World Trade Center, Shanghai Tower
Tube Structures
- Framed tube: Closely spaced perimeter columns (2-4m centres) with deep spandrel beams form a tube. 40-80 storeys.
- Bundled tube: Multiple tubes grouped together (Sears/Willis Tower). 80-110 storeys.
- Braced tube: Diagonal bracing on building perimeter (John Hancock Center, 30 St Mary Axe). 40-100 storeys.
- Diagrid: Triangulated perimeter structure — no vertical columns on facade. Very efficient. 30-70 storeys.
- Planning impact: Perimeter columns/diagonals are architecturally expressive but restrict views and facade openings. Ground-floor column transfer typically needed for entrances.
Mega-Frame / Belt Truss
- Principle: Mega-columns at corners connected by belt trusses at intervals. Interior structure is lightweight gravity-only framing.
- Height suitability: 60-120+ storeys
- Exemplar: HSBC Building Hong Kong (mega-frame), Leadenhall Building London (mega-frame)
- Planning impact: Mega-columns are very large (1.5-3.0m dimension) — integrate into planning from day one
Section 4: Foundation Types
Foundation selection depends on soil conditions, structural loads, settlement tolerance, water table, and site access constraints. Architects must understand foundation types to assess basement feasibility, coordinate below-ground structure, and understand programme implications.
Strip Foundations
- Description: Continuous concrete strip beneath load-bearing walls
- Typical dimensions: 450-900mm wide x 200-450mm deep (unreinforced); wider/deeper with reinforcement
- Suitable for: Low-rise load-bearing masonry, light framed structures
- Soil requirement: Firm ground at shallow depth (typically <1.5m below ground level)
- Bearing capacity utilised: 50-200 kPa
Pad Foundations
- Description: Individual concrete bases beneath each column
- Typical dimensions: 1.0x1.0m to 3.5x3.5m plan; 500-1500mm deep
- Suitable for: Framed structures (steel, concrete, timber) on competent ground
- Soil requirement: Firm to stiff soil at shallow depth
- Bearing capacity utilised: 100-600 kPa
Raft / Mat Foundations
- Description: Continuous reinforced concrete slab beneath entire building footprint
- Typical thickness: 300-600mm (light buildings); 600-1500mm (heavy buildings/towers)
- Suitable for: Variable or weak soils, high water table, buildings requiring basement waterproofing
- Advantages: Spreads load over large area; acts as basement floor slab; reduces differential settlement
- Soil requirement: Can work on relatively weak soils (50-150 kPa)
Piled Foundations
When surface soils cannot support the building loads, piles transfer loads to deeper competent strata.
- Bored piles (CFA — Continuous Flight Auger): 300-1200mm diameter, depths to 30m. Low vibration, low noise. Suit urban sites. Typical capacity: 500-5000 kN per pile.
- Bored piles (rotary): 600-2400mm diameter, depths to 60m+. For heavy loads. Capacity: 2000-30000 kN per pile.
- Driven piles (precast concrete): 250-450mm square, depths to 30m. High vibration — restricted on urban sites. Capacity: 500-3000 kN per pile.
- Driven piles (steel H-section): 200-350mm, depths to 40m. Can be driven through obstructions. Capacity: 500-4000 kN per pile.
- Screw piles: 200-800mm diameter, quick installation, removable. For temporary structures or light permanent loads. Capacity: 100-1500 kN.
Typical Bearing Pressures by Soil Type
| Soil/Rock Type | Presumed Bearing Capacity (kPa) |
|---|
| Strong igneous/metamorphic rock | 10,000+ |
| Strong limestone/sandstone | 2,000-10,000 |
| Weak rock (chalk, mudstone) | 500-2,000 |
| Dense gravel / dense sand | 300-600 |
| Medium-dense sand | 100-300 |
| Loose sand | 50-100 (not recommended) |
| Stiff clay (undrained shear strength >150 kPa) | 150-300 |
| Firm clay (Su 75-150 kPa) | 75-150 |
| Soft clay (Su <75 kPa) | 50-100 (settlement-critical) |
| Very soft clay / peat | Not suitable for shallow foundations |
Groundwater and Basement Design
- Water table above basement slab: Requires tanked construction (Type A waterproofing — membrane), structurally integral waterproof concrete (Type B — BS 8102), or drained cavity (Type C).
- Uplift resistance: In high water table conditions, the foundation must resist hydrostatic uplift. A 3m-deep basement below water table generates 30 kPa uplift pressure.
- Typical basement cost: 800-1500 $/m² (single level); 1000-2000 $/m² (multi-level) — substantially more than superstructure.
Section 5: Span-to-Depth Ratios
These rules of thumb allow architects to estimate structural depths at concept stage, determining floor-to-floor heights before detailed engineering.
Concrete Elements
| Element | Span/Depth Ratio | Example: 8m span |
|---|
| One-way RC slab (simply supported) | L/28 | 286mm |
| One-way RC slab (continuous) | L/32 | 250mm |
| Two-way RC slab (simply supported) | L/33 | 242mm |
| Two-way RC slab (continuous) | L/40 | 200mm |
| Post-tensioned flat slab | L/40 to L/48 | 167-200mm |
| RC beam (simply supported) | L/12 | 667mm |
| RC beam (continuous) | L/18 | 444mm |
| Post-tensioned beam | L/18 to L/24 | 333-444mm |
| RC column (height/least dimension) | H/15 to H/8 | — |
Steel Elements
| Element | Span/Depth Ratio | Example: 12m span |
|---|
| Steel beam (UB, simply supported) | L/20 | 600mm |
| Steel beam (continuous) | L/24 | 500mm |
| Castellated beam | L/18 to L/22 | 545-667mm |
| Cellular beam | L/20 to L/25 | 480-600mm |
| Steel truss (parallel chord) | L/10 to L/15 | 800-1200mm |
| Steel truss (pitched) | L/8 to L/12 | 1000-1500mm |
| Plate girder | L/12 to L/15 | 800-1000mm |
Timber Elements
| Element | Span/Depth Ratio | Example: 7m span |
|---|
| Glulam beam (simply supported) | L/15 to L/20 | 350-467mm |
| Glulam beam (continuous) | L/18 to L/24 | 292-389mm |
| CLT floor slab | L/25 to L/30 | 233-280mm |
| LVL beam | L/16 to L/22 | 318-438mm |
| Timber I-joist | L/15 to L/20 | 350-467mm |
| Timber truss (pitched roof) | L/8 to L/12 | 583-875mm |
Precast Concrete Elements
| Element | Span/Depth Ratio | Example: 10m span |
|---|
| Hollowcore plank (150mm) | L/30 to L/35 | 286-333mm |
| Hollowcore plank (250-400mm) | L/35 to L/40 | 250-286mm |
| Precast double-tee | L/20 to L/25 | 400-500mm |
| Precast beam (rectangular) | L/12 to L/16 | 625-833mm |
| Precast beam (L-shaped/inverted-T) | L/14 to L/18 | 556-714mm |
Applying Span-to-Depth Ratios
Procedure for estimating floor-to-floor height:
- Determine structural span from grid
- Select element type (slab, beam, etc.)
- Calculate structural depth using L/ratio
- Add ceiling void for services: 150-400mm (residential), 400-800mm (office), 600-1200mm (hospital)
- Add floor finishes: 50-150mm (raised floor) or 20-50mm (screed/carpet)
- Add ceiling finish: 15-25mm
- Sum = floor construction zone
- Add clear room height: 2400-2700mm (residential), 2700-3000mm (office), 2700-3600mm (retail)
- Total = floor-to-floor height
Example 1 — 9m span office with composite steel beam:
- Structural depth: 9000/20 = 450mm beam (with 150mm composite deck = 600mm total)
- Ceiling void: 200mm (services through beam web openings, additional below beam)
- Raised floor: 150mm
- Ceiling tile: 15mm
- Clear height: 2750mm
- Floor-to-floor: 600 + 200 + 150 + 15 + 2750 = 3715mm → round to 3750mm or 4000mm
Example 2 — 6m span residential with RC flat slab:
- Structural depth: 6000/28 = 214mm → use 225mm flat slab
- Ceiling void: 200mm (pipework, sprinkler, cabling)
- Floor finish (screed + carpet): 75mm
- Ceiling finish: 15mm
- Clear height: 2500mm
- Floor-to-floor: 225 + 200 + 75 + 15 + 2500 = 3015mm → round to 3100mm
Example 3 — 7.5m span hospital with RC beam-slab:
- Structural depth: 200mm slab + 600mm beam = 600mm (beam governs zone)
- Ceiling void: 800mm (extensive services — HVAC, medical gases, data, sprinkler)
- Floor finish (vinyl on screed): 75mm
- Ceiling tile: 15mm
- Clear height: 2700mm
- Floor-to-floor: 600 + 800 + 75 + 15 + 2700 = 4190mm → round to 4200mm or 4500mm
Example 4 — 16.2m span car park with PT flat slab:
- Structural depth: 16200/40 = 405mm → use 400mm PT slab
- Services below slab: 150mm (sprinklers, lighting, drainage only)
- No floor finish; no ceiling finish
- Clear height: 2400mm minimum (2100mm at obstructions per BS 8110)
- Floor-to-floor: 400 + 150 + 2400 = 2950mm → round to 3000mm
Structural System Comparison Matrix
| Criterion | Masonry | RC Frame | PT Concrete | Steel Frame | Composite | Mass Timber | Precast |
|---|
| Max economic span | 7m | 12m | 16m | 20m | 18m | 15m (glulam) | 16m (HC) |
| Max height | 6 storeys | Unlimited | Unlimited | Unlimited | Unlimited | 18 storeys | 30 storeys |
| Floor-to-floor (office, 9m) | N/A | 3.6-4.0m | 3.4-3.8m | 3.7-4.0m | 3.5-3.8m | 3.5-3.9m | 3.5-3.8m |
| Embodied carbon | Low-Med | High | Medium | Medium | Med-High | Very Low | Medium |
| Construction speed | Slow | Medium | Med-Fast | Very Fast | Very Fast | Very Fast | Very Fast |
| Planning flexibility | Low | High | High | Very High | Very High | Medium | Low |
| Acoustic performance | Good | Good | Good | Poor (needs treatment) | Fair | Poor (needs treatment) | Good |
| Fire resistance | Inherent | Inherent | Inherent | Requires protection | Partial | Charring + encapsulation | Inherent |
| Wet trades on site | High | High | High | Low | Medium | Very Low | Low |
| Quality control | Variable | Variable | Good | Good | Good | Excellent (factory) | Excellent (factory) |
Vibration Considerations
Lightweight floor structures (steel, timber) are susceptible to human-induced vibration (footfall):
- Concrete slabs (>250mm): Natural frequency typically >8 Hz — acceptable for all uses
- Composite steel deck (130-170mm): Check required for spans >7.5m. Natural frequency target: >4 Hz (office), >8 Hz (hospital operating theatre)
- CLT floors: Susceptible above 5m span. Add concrete topping (50-80mm) to increase mass and damping. Target: >8 Hz for residential
- Post-tensioned slabs: Generally satisfactory due to high stiffness-to-weight ratio
- Long-span steel beams (>12m): Frequently require vibration analysis. Remedies: increase beam stiffness, add concrete topping mass, tune dampers (TMDs)
Vibration-sensitive uses requiring special attention:
- Hospital operating theatres and imaging suites (VC-A to VC-D criteria)
- Laboratory precision equipment (VC-A to VC-E)
- Recording studios and concert halls
- High-rise residential above 20 storeys (wind-induced sway)
Section 6: Load Path Narrative
Gravity Load Path
Loads flow downward through the structure in a continuous chain:
- Applied loads (people, furniture, equipment, snow) act on floor finishes
- Floor finishes transfer load to floor slab/deck (one-way or two-way spanning)
- Slab transfers load to beams (if present) or directly to columns/walls (flat slab)
- Beams transfer load to columns or load-bearing walls
- Columns/walls transfer load through each storey to foundations
- Foundations transfer load to ground
Critical rule: Every element in the chain must have adequate strength, and the path must be continuous. A column on the 5th floor must have a column below it on the 4th floor — or a transfer structure.
Lateral Load Path
Wind and seismic forces follow a horizontal path:
- Wind pressure acts on facade/cladding
- Facade transfers load to floor slabs (acting as horizontal diaphragms) at each level
- Floor diaphragm transfers load (through in-plane shear) to lateral resisting elements (cores, shear walls, braced frames, moment frames)
- Lateral elements transfer load vertically to foundations
- Foundations transfer overturning moments and base shear to ground
Critical rule: Floor diaphragms must be continuous and connected to lateral elements. Large openings in floor slabs (atria, stairs) weaken the diaphragm — reinforcement and collector beams required around openings.
Common Architectural Decisions That Create Structural Problems
- Removing a column on one floor — creates a transfer beam, adds cost and depth, disrupts floor below
- Large atrium void through multiple floors — interrupts diaphragm action; requires edge beams and supplementary bracing
- Cantilevered floors — require back-span anchoring and heavier structure; deflection critical at cantilever tips
- Setting back upper floors — changes lateral force path; columns must transition, adding transfer structures
- Misaligned cores between podium and tower — creates torsional effects and complex transfer
- Ground-floor open on all sides (pilotis) — soft-storey risk in seismic zones; requires careful lateral design
- Curved or irregular floor plates — complicates diaphragm action; may require deeper slabs or additional beams
- Late changes to penetration locations — particularly damaging in post-tensioned slabs where tendons cannot be cut
- Mixing structural systems vertically — differential stiffness and movement between systems requires careful detailing
- Ignoring construction sequence — propping, temporary stability, and pour sequences affect final stresses
Progressive Collapse and Robustness
Building structures must be designed to avoid disproportionate collapse — where local failure of one element does not cascade into collapse of a large part of the structure. This is a code requirement (Eurocode 1991-1-7, ASCE 7, Approved Document A).
Design strategies for robustness:
- Prescriptive tying: Continuous reinforcement or steel ties connecting all elements horizontally and vertically. Floor slabs tied to beams; beams tied to columns; columns tied to floors above and below.
- Alternative load path method: If one column is removed, the structure must redistribute loads to adjacent elements without collapse. Requires ductile connections and redundancy.
- Key element design: Critical elements (transfer columns, single support members) designed for accidental load of 34 kN/m² (Eurocode) applied to the element and any attached structure.
- Segmentation: Subdivide the structure so that collapse of one segment does not propagate to adjacent segments. Expansion joints or structural separation zones.
Architectural implications: Avoid single-column support for large areas. Ensure at least two load paths for every supported area. Transfer structures are key elements by definition — they must be designed for enhanced robustness.
Movement Joints
Buildings expand and contract with temperature changes. Concrete shrinks as it cures. Differential settlement occurs across large footprints. Movement joints accommodate these movements.
- Expansion joints in RC structures: Every 50-60m (or closer in exposed structures). Joint width: 25-50mm.
- Expansion joints in steel structures: Every 80-100m. Steel has lower thermal mass but higher coefficient of expansion.
- Settlement joints: Where building sections have significantly different loads or foundation conditions (e.g., tower meeting podium).
- Seismic joints: In seismic zones, building sections of different heights or stiffness must be separated by seismic joints wide enough to prevent pounding (typically 50-200mm depending on building height and drift).
- Joint covers: Architectural joint covers required at floors, walls, ceilings, and roof. Fire-rated joint seals at fire compartment boundaries.
Architect's Coordination Responsibilities
- Concept stage: Define grid, floor-to-floor height, lateral system location, basement extent with structural engineer input
- Scheme design: Fix column positions, core geometry, transfer locations. Structural engineer provides preliminary member sizes.
- Detailed design: Coordinate penetrations, fixings, movement joints, cladding connections. Structural engineer produces detailed calculations and drawings.
- Construction stage: Review shop drawings, attend site for critical pours/erections, verify as-built conditions
Early Coordination Checklist