Comprehensive structural systems knowledge for architects covering structural typology selection, grid design and spacing, lateral stability systems, foundation types, span-to-depth ratio tables, load path narratives, and architect-engineer coordination strategies. Provides decision frameworks for selecting structural systems based on span, height, program, cost, construction speed, and sustainability targets.
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?
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.
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.
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
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.
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.
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
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
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
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.
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.
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
Structural grid agreed and coordinated with parking, facade, and planning module