9.1 Structural Selection & Spanning Rules of Thumb

ARE 5.0 Project Planning & Design PPD

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1 Chapter 1: Exam Overview & Schematic Design Process

1.1 ARE 5.0 PPD Format, Scoring & Case Study Navigation 1.2 The Schematic Design Phase & Multi-Discipline Workflow 1.3 Regulatory Framework: Navigating Codes, Standards & Jurisdictions

2 Chapter 2: Site Analysis, Topography & Soil Conditions

2.1 Site Topography, Grading, Slopes & Cut/Fill Calculations 2.2 Geotechnical Reports, Soil Classification & Foundation Selection 2.3 Site Hydrology, Drainage & Stormwater Management Neighborhood Context, Historic Settings & Site Integration

3 Chapter 3: Climate, Microclimate & Solar Design

3.1 Solar Geometry, Sun Angles & Sun Path Diagrams 3.2 Microclimate Analysis, Prevailing Winds & Passive Ventilation 3.3 Passive Solar Heating & Thermal Mass Strategies

4 Chapter 4: Sustainable Principles & Energy Standards

4.1 Daylighting Metrics: Spatial Daylight Autonomy (sDA) & Annual Sun Exposure (ASE) 4.2 Building Energy Standards: ASHRAE 90.1, IECC & Energy Use Intensity (EUI) 4.3 High-Performance Frameworks: LEED v4.1, Net Zero & Embodied Carbon

5 Chapter 5: Zoning Regulations & Land Use Controls

5.1 Zoning Envelopes: Setbacks, Floor Area Ratio (FAR) & Lot Coverage 5.2 Parking Requirements, Access & Site Circulation Planning 5.3 Environmental Regulations: Wetlands, Flood Zones & Brownfields

6 Chapter 6: Building Code Fundamentals: Occupancy & Construction Types

6.1 IBC Occupancy Classifications & Group Characteristics 6.2 Mixed Occupancy Strategies: Nonseparated, Separated & Accessory 6.3 IBC Construction Types I Through V & Fire-Resistance Ratings 6.4 Allowable Building Height & Area Calculations (IBC Tables 504 & 506)

7 Chapter 7: Means of Egress & Fire Protection

7.1 Occupant Load Determination & Egress Sizing (IBC Chapter 10) 7.2 Egress Components: Common Path, Travel Distance & Exit Separation 7.3 Passive Fire Protection: Fire Walls, Barriers & Smoke Partitions 7.4 Active Fire Suppression: Sprinklers, Standpipes & Fire Alarms

8 Chapter 8: Accessibility & Universal Design

8.1 Accessible Routes, Curb Ramps & Parking Layouts (ADA / ICC A117.1) 8.2 Interior Accessibility: Doors, Maneuvering Clearances & Corridors 8.3 Accessible Plumbing Facilities: Restrooms, Grab Bars & Fixture Layouts

9 Chapter 9: Structural Systems: Wood, Steel, Concrete & Masonry

9.1 Structural Selection & Spanning Rules of Thumb 9.2 Steel Framing Systems: Beams, Girders & Open-Web Joists 9.3 Concrete Framing Systems: One-Way, Two-Way & Post-Tensioned Slabs 9.4 Wood Systems: Light-Frame, Heavy Timber & Mass Timber (CLT)

10 Chapter 10: Lateral Force Resisting Systems & Foundations

10.1 Lateral Load Paths: Wind vs. Seismic Forces 10.2 Shear Walls, Braced Frames & Moment Frames 10.3 Shallow vs. Deep Foundation Systems: Footings, Mats, Piles & Caissons

11 Chapter 11: Building Envelope: Physics, Assemblies & Moisture Control

11.1 Envelope Thermal Performance: U-Values, R-Values & Thermal Bridging 11.2 Moisture Management: Vapor Retarders, Air Barriers & Water Resistive Barriers 11.3 High-Performance Wall & Roof Assemblies: Rainscreens & Inverted Roofs 11.4 Fenestration & Glazing: U-Factor, SHGC, Condensation Resistance & VT Interior Materials, Durability & Program Performance

12 Chapter 12: Mechanical & HVAC Systems

12.1 HVAC System Types: All-Air, Air-Water, All-Water & VRF Systems 12.2 Central Plant Equipment: Chillers, Boilers, Cooling Towers & Geothermal 12.3 Ductwork Distribution, Air Handling & Plenum Space Requirements 12.4 Indoor Air Quality, Ventilation Rates & ASHRAE 62.1 Baselines

13 Chapter 13: Plumbing, Electrical & Life Safety Systems

13.1 Domestic Water Supply & Pressure Boosting Systems 13.2 Sanitary Drainage, Venting & Fixture Count Calculations 13.3 Electrical Service, Transformers, Switchgear & Backup Generators 13.4 Lighting Design, Controls & Lighting Power Density (LPD) Limits

14 Chapter 14: Specialized Building Systems & Vertical Circulation

14.1 Architectural Acoustics: STC, NRC, RT60 & Noise Criteria (NC) 14.2 Elevator Systems: Traction vs. Hydraulic, Core Layout & Sizing 14.3 Building Telecommunications, Security & Building Automation Systems (BAS)

15 Chapter 15: Systems Integration & Coordination

15.1 Floor-to-Floor Height & Plenum Coordination (Structure + MEP + Architecture) 15.2 Structural Grid Planning & Spatial Adjacencies 15.3 Building Massing, Orientation & Envelope Integration 15.4 Case Study Strategy: Synthesizing Drawings, Code Sheets & Schedules

16 Chapter 16: Project Costs, Budgeting & Life-Cycle Analysis

16.1 Schematic Estimating Techniques: Area, Volume & Parametric Costs 16.2 Project Budgets: Contingencies, Overhead, Profit & Escalation 16.3 Life-Cycle Cost Analysis (LCCA) & Value Engineering

9.1 Structural Selection & Spanning Rules of Thumb

Key Takeaways Explain This Topic AI

Fundamental structural load mechanisms govern member sizing: axial tension, axial compression, transverse shear, flexural bending, and torsion.

ASCE 7-16 establishes mandatory design live loads: 50 psf for general offices (plus a 15 psf movable partition allowance), 40 psf for classrooms, 100 psf for assembly spaces, and 100 psf for egress corridors and balconies.

Empirical depth-to-span ratios allow rapid schematic member sizing: steel wide-flange beams require L/20 to L/24, steel girders require L/15, open-web steel joists require L/24, concrete one-way slabs require L/24 to L/28, two-way flat plates require L/30 to L/33, and glulam timber beams require L/16 to L/20.

Modular structural grid planning coordinates subterranean parking layouts (typically 27 to 30 feet for 3-car bays) with vertical core shafts and upper-floor partition layouts.

Structural system selection involves balancing structural depth, vertical plenum space, embodied carbon, construction schedule speed, fireproofing requirements, and lateral force resistance.

Last updated: October 2026

9.1 Structural Selection & Spanning Rules of Thumb

In architectural schematic design, structural system selection is a defining early decision. Structural framing establishes the building dimensional grid, column locations, vertical core configuration, floor-to-floor height, mechanical distribution plenums, and exterior fenestration rhythm. On the ARE 5.0 Project Planning & Design (PPD) division, candidates are evaluated on their ability to match structural systems to specific building programs, spans, load intensities, fire-resistance requirements, and spatial clearances.

Selecting an optimal structural system requires understanding how fundamental forces move through materials, how design loads are categorized by national standards, and how empirical depth-to-span ratios ( L / d L/d L / d ) allow rapid estimation of member profiles before detailed engineering begins.

Fundamental Structural Load Mechanisms

All structural framing assemblies resist gravity and lateral forces through five fundamental internal stress mechanisms:

Axial Tension : Forces acting along the longitudinal axis that pull a member apart, elongating it. Tensile stress is uniformly distributed across the net cross-sectional area ( f t = P / A f_t = P/A f t ​ = P / A ). Steel cables, tension tie rods, and the bottom chords of simply supported trusses operate purely in tension. Materials with high tensile strength (structural steel, reinforcing rebar) excel in tension.

Axial Compression : Forces acting along the longitudinal axis that crush or shorten a member ( f c = P / A f_c = P/A f c ​ = P / A ). Compression members (columns, bearing walls, compressive arches, and truss top chords) are limited not only by material crushing strength but also by geometric instability—specifically buckling governed by member slenderness.

Transverse Shear : Internal sliding forces acting parallel to a cross-sectional plane, where adjacent material planes slide past one another. In flexural members, vertical shear stresses peak at the neutral axis and are highest immediately adjacent to column or beam supports.

Flexural Bending : A coupled internal stress state induced by transverse loads perpendicular to a member longitudinal axis. In a simply supported beam subject to gravity loads, flexural bending creates compressive stress in the top fibers and tensile stress in the bottom fibers , separated by a neutral axis where normal stresses are zero. Bending moment capacity is directly proportional to member depth ( M = F ⋅ d M = F \cdot d M = F ⋅ d ), making structural depth the single most effective parameter for controlling bending stresses and elastic deflection.

Torsion : Twisting moments acting about a member longitudinal axis, induced by eccentric loading or curved geometries. Closed hollow structural sections (HSS tubes and pipes) exhibit vastly superior torsional resistance compared to open cross-sections (wide-flange I-beams or channels).

Design Loads: ASCE 7-16 Classifications & Baselines

Building structures must be engineered to resist combined gravity, environmental, and lateral loads in accordance with ASCE 7-16 (Minimum Design Loads and Associated Criteria for Buildings and Other Structures) , as referenced by IBC 2021:

1. Dead Loads ( D D D )

Dead loads represent permanent, stationary gravity loads comprising the weight of all building materials incorporated into the construction:

Structural Self-Weight : Reinforced concrete slabs, steel columns and framing, timber trusses, foundation elements.

Superimposed Architectural Finishes : Floor toppings, ceramic tiles, suspended acoustic ceilings, rigid insulation, roofing membranes, and exterior cladding.

Permanent MEP Services : Mechanical ductwork, chillers, boilers, piping, electrical conduits, and fire sprinkler piping.

Movable partitions : Apply the adopted ASCE 7 allowance where partitions are subject to relocation and the provision applies. A 15-psf allowance is used in the office exercise; the code includes applicability conditions and exceptions, so it is not added to every conceivable floor load without review.

2. Live Loads ( L L L )

Live loads encompass transient, movable gravity loads produced by the use and occupancy of the building. These loads fluctuate over time and include human occupants, furniture, movable equipment, and vehicular traffic. Unlike dead loads, live loads cannot be determined with exact certainty and are specified as minimum uniform surface pressures (psf) or concentrated loads (lbs).

3. Environmental & Lateral Loads

Snow Loads ( S S S ) : Calculated from ground snow load maps ( p g p_g p g ​ ), exposure factor ( C e C_e C e ​ ), thermal factor ( C t C_t C t ​ ), and importance factor ( I s I_s I s ​ ). Snow drifts accumulate along parapets and roof elevation steps.

Wind Loads ( W W W ) : Dynamic pressures acting horizontally and upward (uplift) on the building envelope, proportional to the square of basic wind velocity ( q z ∝ V 2 q_z \propto V^2 q z ​ ∝ V 2 ), topographic exposure category (B, C, or D), and building aerodynamic geometry.

Seismic Loads ( E E E ) : Inertial ground forces induced by earthquake ground accelerations ( F = m ⋅ a F = m \cdot a F = m ⋅ a ). Seismic base shear ( V = C s W V = C_s W V = C s ​ W ) is directly proportional to the total effective seismic weight ( W W W ) of the building, meaning heavier structures (concrete, masonry) experience substantially higher earthquake forces than lightweight structures (wood, steel).

Structural spans, member-depth ratios and preliminary capacities in this section are design-screening assumptions, not completed structural designs. Check loads, serviceability, fire rating, vibration, connections, bracing, durability and constructability with the engineer. IBC 2021 references ASCE 7-16; an exam-provided reference or a locally adopted edition may differ. Never transfer a numerical limit between editions or structural systems without checking its scope.

Loading diagram... Structural System Selection Tree by Span and Program Demands

Structural Grid Planning & Modular Coordination

A structural grid is the foundational organizational framework of a building. It coordinates vertical load-bearing columns and walls with interior spatial layouts, window mullions, and subterranean foundation grids.

Grid Principles & Parking Coordination

In commercial and mixed-use structures, the structural grid is frequently dictated by subterranean parking geometry . A standard 90-degree parking bay requires:

Standard vehicle stall width: 8.5 to 9.0 feet 8.5\text{ to }9.0\text{ feet} 8.5 to 9.0 feet .

Minimum stall length: 18.0 feet 18.0\text{ feet} 18.0 feet .

Two-way drive aisle width: 24.0 feet 24.0\text{ feet} 24.0 feet .

The 3-Car Bay Standard : To accommodate three parked vehicles comfortably between column faces without wasting space or creating door-strike obstructions, structural columns are spaced at 27 to 30 feet 27\text{ to }30\text{ feet} 27 to 30 feet on center ( 3 × 9 ft = 27 ft 3 \times 9\text{ ft} = 27\text{ ft} 3 × 9 ft = 27 ft , plus column thickness).

Consequently, a 30 ft × 30 ft 30\text{ ft} \times 30\text{ ft} 30 ft × 30 ft or 30 ft × 40 ft 30\text{ ft} \times 40\text{ ft} 30 ft × 40 ft grid has emerged as the universal standard for commercial speculative offices, aligning subterranean vehicular stalls with open-plan workstation modules on upper floors.

Alignment of Vertical Elements

Discontinuous structural columns that stop or shift at lower levels create severe engineering penalties. Transferring vertical loads horizontally requires deep, heavily reinforced transfer girders ( L / 8 L/8 L /8 to L / 10 L/10 L /10 ) or massive post-tensioned transfer slabs. These members consume vertical plenum space, add massive dead weight, and increase foundation costs exponentially. Architects must align vertical cores (elevator banks, fire exit stair towers, primary MEP risers) continuously from the foundation through the roofline.

Empirical Depth-to-Span Rules of Thumb

During schematic design, architects must quickly allocate ceiling plenum depths to accommodate structure, mechanical supply/return ducts, recessed lighting, and fire sprinkler piping. Structural engineers use empirical depth-to-span ratios ( L / d L/d L / d ) to size members preliminarily before performing load takeoffs:

Estimated Depth ( d ) = Clear Span ( L ) Ratio Constant \text{Estimated Depth } (d) = \frac{\text{Clear Span } (L)}{\text{Ratio Constant }} Estimated Depth ( d ) = Ratio Constant Clear Span ( L ) ​

Structural System & Material Depth-to-Span Ratio ( d = L / X d = L / X d = L / X ) Typical Practical Span Range Estimated Depth for a 30 ft Span

Steel Wide-Flange Girder (Primary) L / 15 L/15 L /15 20 to 45 ft 20\text{ to }45\text{ ft} 20 to 45 ft ( 30 × 12 ) / 15 = 24 inches (30 \times 12) / 15 = 24\text{ inches} ( 30 × 12 ) /15 = 24 inches ( W 24 W24 W 24 )

Steel Wide-Flange Beam (Secondary) L / 20 to L / 24 L/20\text{ to }L/24 L /20 to L /24 20 to 40 ft 20\text{ to }40\text{ ft} 20 to 40 ft ( 30 × 12 ) / 20 = 18 inches (30 \times 12) / 20 = 18\text{ inches} ( 30 × 12 ) /20 = 18 inches ( W 18 W18 W 18 )

Composite Steel Beam L / 22 to L / 26 L/22\text{ to }L/26 L /22 to L /26 25 to 50 ft 25\text{ to }50\text{ ft} 25 to 50 ft ( 30 × 12 ) / 24 = 15 inches (30 \times 12) / 24 = 15\text{ inches} ( 30 × 12 ) /24 = 15 inches ( W 16 W16 W 16 )

Open-Web Steel Joist (K / LH) L / 24 L/24 L /24 20 to 60 + ft 20\text{ to }60+\text{ ft} 20 to 60 + ft ( 30 × 12 ) / 24 = 15 inches (30 \times 12) / 24 = 15\text{ inches} ( 30 × 12 ) /24 = 15 inches ( 16 K 16K 16 K )

Concrete One-Way Solid Slab L / 24 to L / 28 L/24\text{ to }L/28 L /24 to L /28 10 to 20 ft 10\text{ to }20\text{ ft} 10 to 20 ft Exceeds typical solid slab span ( 13 – 15 in 13\text{–}15\text{ in} 13 – 15 in )

Concrete One-Way Joist (Pan-Joist) L / 18 to L / 22 L/18\text{ to }L/22 L /18 to L /22 20 to 35 ft 20\text{ to }35\text{ ft} 20 to 35 ft ( 30 × 12 ) / 20 = 18 inches (30 \times 12) / 20 = 18\text{ inches} ( 30 × 12 ) /20 = 18 inches (incl. slab)

Concrete Two-Way Flat Plate L / 30 to L / 33 L/30\text{ to }L/33 L /30 to L /33 15 to 25 ft 15\text{ to }25\text{ ft} 15 to 25 ft ( 30 × 12 ) / 32 = 11.25 inches (30 \times 12) / 32 = 11.25\text{ inches} ( 30 × 12 ) /32 = 11.25 inches (borderline)

Concrete Two-Way Waffle Slab L / 24 to L / 28 L/24\text{ to }L/28 L /24 to L /28 30 to 45 + ft 30\text{ to }45+\text{ ft} 30 to 45 + ft ( 30 × 12 ) / 26 = 14 inches (30 \times 12) / 26 = 14\text{ inches} ( 30 × 12 ) /26 = 14 inches (incl. topping)

Glued-Laminated Timber (Glulam) Beam L / 16 to L / 20 L/16\text{ to }L/20 L /16 to L /20 20 to 60 ft 20\text{ to }60\text{ ft} 20 to 60 ft ( 30 × 12 ) / 18 = 20 inches (30 \times 12) / 18 = 20\text{ inches} ( 30 × 12 ) /18 = 20 inches

Wood Light-Frame Solid Sawn Joists L / 16 L/16 L /16 10 to 22 ft 10\text{ to }22\text{ ft} 10 to 22 ft Exceeds dimension lumber limits

Wood Engineered I-Joists (TJI) L / 18 to L / 20 L/18\text{ to }L/20 L /18 to L /20 16 to 30 ft 16\text{ to }30\text{ ft} 16 to 30 ft ( 30 × 12 ) / 18 = 20 inches (30 \times 12) / 18 = 20\text{ inches} ( 30 × 12 ) /18 = 20 inches

Precast Concrete Hollow-Core Planks L / 35 to L / 40 L/35\text{ to }L/40 L /35 to L /40 20 to 40 ft 20\text{ to }40\text{ ft} 20 to 40 ft ( 30 × 12 ) / 36 = 10 inches (30 \times 12) / 36 = 10\text{ inches} ( 30 × 12 ) /36 = 10 inches

Precast Concrete Double Tees L / 24 to L / 28 L/24\text{ to }L/28 L /24 to L /28 30 to 65 + ft 30\text{ to }65+\text{ ft} 30 to 65 + ft ( 30 × 12 ) / 24 = 15 inches (30 \times 12) / 24 = 15\text{ inches} ( 30 × 12 ) /24 = 15 inches

Multi-Criteria Framing Comparison Matrix

Structural System Typical Spans Structural Depth Profile Inherent Fire Resistance Embodied Carbon Profile Programmatic & Economic Fit

Structural Steel with Composite Deck 25 – 45 ft 25\text{–}45\text{ ft} 25 – 45 ft Moderate to deep ( 18 – 28 in 18\text{–}28\text{ in} 18 – 28 in ); requires plenum Noncombustible; requires applied SFRM or intumescent paint High (fossil-fueled smelting); high recycled content Commercial offices, medical facilities, academic buildings

Open-Web Steel Joists & Joist Girders 30 – 100 + ft 30\text{–}100+\text{ ft} 30 – 100 + ft Deep ( 24 – 60 in 24\text{–}60\text{ in} 24 – 60 in ); open webs allow duct routing Poor; highly vulnerable to heat collapse; requires encasement or sprinklers Moderate; very efficient strength-to-weight ratio Warehouses, big-box retail, gymnasiums, single-story roofs

Cast-in-Place Two-Way Flat Plate 15 – 25 ft 15\text{–}25\text{ ft} 15 – 25 ft Shallowest ( 7 – 10 in 7\text{–}10\text{ in} 7 – 10 in ); completely flat underside Excellent; inherently 2 to 4-hour rated without coatings High (cement calcination emissions); long thermal mass Multi-family residential, hotels, dormitories with tight height limits

Cast-in-Place Two-Way Waffle Slab 30 – 50 ft 30\text{–}50\text{ ft} 30 – 50 ft Moderate ( 14 – 24 in 14\text{–}24\text{ in} 14 – 24 in ); coffered ceiling profile Excellent; robust concrete cover over ribs and heads High; reduced volume relative to solid slab of equal depth Public libraries, institutional atriums, heavily loaded laboratories

Precast Hollow-Core Concrete Planks 20 – 40 ft 20\text{–}40\text{ ft} 20 – 40 ft Shallow ( 8 – 12 in 8\text{–}12\text{ in} 8 – 12 in ); flat soffit underside High; 2 to 3-hour fire rating; factory quality control Moderate to high; optimized factory curing and hollow voids Multi-family housing, motels, prisons, parking garage decks

Mass Timber (CLT & Glulam) 15 – 30 ft 15\text{–}30\text{ ft} 15 – 30 ft (CLT) / 20 – 60 ft 20\text{–}60\text{ ft} 20 – 60 ft (Glulam) Moderate to deep; exposed architectural timber Predictable charring rate (~1.5 in/hr); meets Type IV-A/B/C Lowest; net carbon sequestering material when sustainably harvested Sustainable office flagships, university academic centers, civic pavilions

Light-Frame Wood Construction 12 – 22 ft 12\text{–}22\text{ ft} 12 – 22 ft Shallow to moderate ( 10 – 16 in 10\text{–}16\text{ in} 10 – 16 in ); concealed spaces Combustible; requires Type X gypsum protection (Type V/III) Low; fast renewable framing with minimal processing energy Low-rise multi-family (IBC Type V-A, Type III-A podiums)

• Worked Scenario: System Selection for a Height-Restricted Building
• Scenario

An architectural project involves a new 6-story residential building located in an urban core with a strict municipal zoning height ceiling of 65 feet 65\text{ feet} 65 feet above grade . The programmatic layout requires a standard residential floor-to-floor height that provides a minimum clear finished ceiling height of 9 feet 0 inches 9\text{ feet } 0\text{ inches} 9 feet 0 inches in all dwelling units. The structural grid is established at 22 ft × 22 ft 22\text{ ft} \times 22\text{ ft} 22 ft × 22 ft . The design team must choose between a structural steel frame with composite metal deck versus a two-way cast-in-place concrete flat plate .

Step-by-Step Resolution

Option A: Structural Steel Frame with Composite Deck

Primary Girder Depth: d = L / 15 = ( 22 × 12 ) / 15 = 17.6 inches → d = L/15 = (22 \times 12) / 15 = 17.6\text{ inches} \rightarrow d = L /15 = ( 22 × 12 ) /15 = 17.6 inches → Select W 18 W18 W 18 ( 18 in 18\text{ in} 18 in depth).

Composite Slab & Metal Deck: 4.5 inches 4.5\text{ inches} 4.5 inches topping slab above flutes.

Mechanical / Lighting / Ceiling Plenum Allowance: 14 inches 14\text{ inches} 14 inches (to route 8-inch horizontal supply ducts, recessed LED fixtures, and fire sprinkler piping under structural beams).

Total Floor Assembly Depth: 18 in (girder) + 4.5 in (deck) + 14 in (plenum) = 36.5 inches ≈ 3 ft 0.5 in 18\text{ in (girder)} + 4.5\text{ in (deck)} + 14\text{ in (plenum)} = 36.5\text{ inches} \approx 3\text{ ft } 0.5\text{ in} 18 in (girder) + 4.5 in (deck) + 14 in (plenum) = 36.5 inches ≈ 3 ft 0.5 in .

Floor-to-Floor Height Required: 9 ft 0 in (clear ceiling) + 3 ft 0.5 in (assembly) = 12 ft 0.5 in 9\text{ ft } 0\text{ in (clear ceiling)} + 3\text{ ft } 0.5\text{ in (assembly)} = 12\text{ ft } 0.5\text{ in} 9 ft 0 in (clear ceiling) + 3 ft 0.5 in (assembly) = 12 ft 0.5 in .

Total 6-Story Building Height: 6 × 12.04 ft = 72.25 feet 6 \times 12.04\text{ ft} = 72.25\text{ feet} 6 × 12.04 ft = 72.25 feet .

Zoning Evaluation : 72.25 ft > 65.0 ft 72.25\text{ ft} > 65.0\text{ ft} 72.25 ft > 65.0 ft . The steel framing option violates the municipal zoning height limitation by over 7 feet .

Option B: Two-Way Cast-in-Place Concrete Flat Plate

Slab Thickness: t = L / 32 = ( 22 × 12 ) / 32 = 8.25 inches → t = L/32 = (22 \times 12) / 32 = 8.25\text{ inches} \rightarrow t = L /32 = ( 22 × 12 ) /32 = 8.25 inches → Select 8.5 inch 8.5\text{ inch} 8.5 inch uniform solid slab.

Underside Finish: The concrete slab soffit can be skim-coated and painted directly as the finished ceiling, eliminating a suspended ceiling assembly.

MEP Distribution: Dropped soffits are utilized only in localized entry vestibules and bathroom zones; main living spaces feature wall-sconce lighting with vertical mechanical chases.

Total Floor Assembly Depth: 8.5 inches 8.5\text{ inches} 8.5 inches ( 0.71 ft 0.71\text{ ft} 0.71 ft ).

Floor-to-Floor Height Required: 9 ft 0 in (clear ceiling) + 0 ft 8.5 in (slab) = 9 ft 8.5 in 9\text{ ft } 0\text{ in (clear ceiling)} + 0\text{ ft } 8.5\text{ in (slab)} = 9\text{ ft } 8.5\text{ in} 9 ft 0 in (clear ceiling) + 0 ft 8.5 in (slab) = 9 ft 8.5 in ( 9.71 ft 9.71\text{ ft} 9.71 ft ).

Total 6-Story Building Height: 6 × 9.71 ft = 58.25 feet 6 \times 9.71\text{ ft} = 58.25\text{ feet} 6 × 9.71 ft = 58.25 feet .

Zoning Evaluation : 58.25 ft ≤ 65.0 ft 58.25\text{ ft} \le 65.0\text{ ft} 58.25 ft ≤ 65.0 ft . The two-way flat plate structure fits comfortably within the 65 -foot 65\text{-foot} 65 -foot cap with over 6 feet 6\text{ feet} 6 feet of zoning buffer, confirming it as the correct technical selection.