Building Shell vs. Tenant Improvement Scope: Core & Shell Boundaries, Code Interfaces, and TI Checklist

Ambiguous Lease Work Letters (Exhibit B) between a landlord’s base building shell and a tenant’s interior fit-out cause six-figure change orders, stalled building permits, and costly rework before construction even begins. Commercial architecture divides multi-tenant building delivery into two separate design, permitting, and financial contracts: the Base Building (Core and Shell), engineered for a 50-to-75-year structural and enclosure service life and recovered through base shell rent, and Tenant Improvements (TI), engineered for a 5-to-10-year lease cycle and funded through a negotiated Tenant Improvement Allowance (TIA) per rentable square foot (RSF) plus direct tenant capital.

Within the commercial architectural design process, disputes rarely occur over the exterior curtain wall glass or the tenant’s workstation carpet. Cost overruns concentrate at four physical and regulatory hand-off zones:

1. Shell Delivery Condition: Cold Dark Shell vs. Warm Lit Shell (GSA Public Buildings Service Pricing Desk Guide standard).
2. Fire & Life-Safety Interfaces (IBC Chapters 7, 9, and 10): Slab-to-slab 1-hour corridor and demising fire partitions (IBC Section 708 and Table 1020.2) vs. ceiling-terminated suite partitions, plus NFPA 13 upright-to-pendent sprinkler conversion.
3. MEP Work-Letter Capacities (ASHRAE 62.1 & 90.1): Base-building outdoor air shaft caps (0.12–0.15 CFM/ft²), 277/480V lighting vs. 120/208V plug-load panels (3.0–5.0 W/ft²), and horizontal plumbing drain slope limits (1/4 in. per ft) to remote wet columns.
4. Structural Slab Live Loads (IBC Table 1607.1 & Section 1607.5): The 50 psf + 15 psf (65 psf total) office slab baseline vs. 80 psf corridors, 100 psf assembly/server rooms, and 150 psf high-density storage stacks.

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Building Shell Delivery Tiers: Cold Dark Shell vs. Warm Lit Shell vs. Turnkey TI

In commercial real estate and federal leasing governed by the GSA Public Buildings Service (PBS) Pricing Desk Guide and Whole Building Design Guide (WBDG) Office Building standards, the term building shell does not describe a single universal physical state. Before an architect prepares a test fit or estimates construction budgets across the stages of architectural design, the project team must classify the landlord’s Work Letter delivery into one of three baseline tiers.

1. Cold Dark Shell (Raw Speculative or Retail/Medical Shell)

A Cold Dark Shell provides the permanent structural skeleton and weather-tight exterior enclosure (CSI Divisions 03, 04, 05, 07, and 08) alongside core vertical circulation, leaving the rentable tenant floor plate completely raw:

• Structure & Slab: Cast-in-place post-tensioned concrete or composite steel deck on structural bays typically spaced at 30 ft × 30 ft to 30 ft × 45 ft, with 12 ft 6 in. to 14 ft 0 in. slab-to-slab floor heights. In ground-floor retail, restaurant, or ambulatory medical shells, the landlord frequently leaves a 4 ft to 6 ft wide slab leave-out strip (filled with compacted gravel) along the rear wet wall so the TI plumber can trench underground sanitary waste and grease-interceptor lines without saw-cutting cured concrete.
• Exterior Envelope: The base building contractor completes all unitized or stick-built curtain walls, punched windows, roof membranes, and perimeter fire containment safing (ASTM E2307) at the floor-slab edge. As detailed in our technical guide to building envelope commissioning and air barriers, verifying 75 Pa air-barrier continuity (<= 0.04 cfm/ft² assembly leakage under ASTM E2357 and <= 0.40 cfm/ft² whole-building leakage under ASTM E779) and ASTM E1105 chamber water resistance belongs 100% to the Base Building scope prior to shell handover.
• Capped MEP & Upright Sprinklers: Inside the tenant suite, there is no suspended ceiling, no interior lighting, no branch ductwork, and no drywall on the perimeter columns. Chilled water, condenser water, or primary supply air ducts terminate at valved/capped stubs at the core shaft wall; electrical service terminates at a spare breaker or lug in the base-building floor electrical closet; and fire protection consists strictly of NFPA 13 upright brass sprinklers turned upward 1 in. to 12 in. below the structural deck.

2. Warm Lit Shell (GSA PBS Baseline & Class A Multi-Tenant Office)

The GSA PBS Pricing Desk Guide establishes the industry benchmark for a Warm Lit Shell (often called a Warm Shell in private Class A office Work Letters). It bridges the gap between raw shell construction and tenant customization by installing generic, open-plan interior systems funded under Shell Rent:

• Perimeter & Core Drywall: Exterior curtain wall knee walls, structural column enclosures, and core walls receive metal stud furring, thermal/acoustic insulation, and taped, floated, and prime-painted 5/8 in. gypsum wallboard (GWB).
• Suspended Ceiling Grid & Open-Plan Lighting: A complete 2 ft × 2 ft or 2 ft × 4 ft suspended acoustical ceiling tile (ACT) grid (typically achieving NRC 0.70–0.85 and CAC >= 35) is hung at 9 ft 0 in. to 10 ft 0 in. above finished floor (AFF), Leaving an 18 in. to 24 in. clear plenum below structural steel beams. Parabolic or volumetric 277V LED troffers are wired across the grid to meet ASHRAE Standard 90.1 office Lighting Power Density (LPD) limits (0.65 to 0.79 W/ft²).
• Open-Plan HVAC & Pendent Sprinkler Grid: Primary Variable Air Volume (VAV) terminal boxes (1 box per 1,000 to 1,500 ft²), medium-pressure supply ductwork, open-office supply/return diffusers, and semi-recessed or pendent NFPA 13 sprinkler heads are installed on a uniform open-floor grid.

The Warm Lit Shell Demolition Trap: A Warm Lit Shell only saves Tenant Improvement capital if the tenant occupies an open-plan workstation layout that reuses the landlord’s ceiling grid, troffer layout, and VAV zoning as-is. If a law firm, medical clinic, or executive headquarters signs a Warm Lit Shell lease and builds out 40% to 70% closed perimeter offices and conference rooms, the TI contractor must spend $12 to $28 per USF out of the Tenant Improvement Allowance to demolish or re-frame the ceiling grid, relocate light fixtures, add zone VAV reheat boxes, and re-pipe sprinkler drops to center within the new room dimensions. For partition-heavy programs, negotiating a Cold Dark Shell plus a higher cash TIA credit prevents paying twice for ceiling and MEP distribution.

3. BOMA Z65.1 Rentable vs. Usable Area (RSF vs. USF) Financial Math

Landlords quote Tenant Improvement Allowances in dollars per Rentable Square Foot ($/RSF), whereas general contractors purchase drywall, ceiling tile, ductwork, and carpet across the actual net footprint inside the suite walls—the Usable Square Foot (USF). Under ANSI/BOMA Z65.1 (Office Buildings: Standard Methods of Measurement), a floor plate’s R/U Ratio (Load Factor) allocates a pro-rata share of base-building common areas (ground lobby, multi-tenant floor corridors, core restrooms, mechanical/electrical rooms, and janitor closets) to each tenant:

• Formula: Effective TIA per Usable Sq Ft ($/USF) = Quoted TIA ($/RSF) × (1 + Building Load Factor) — ONLY if the TIA is paid on RSF and spent exclusively inside the USF boundary. However, when comparing a Cold Shell budget against a Turnkey quote where the landlord deducts multi-tenant corridor construction or architectural fees from the allowance, the net purchasing power inside the suite drops sharply:
• Worked Financial Audit: Suppose a tenant leases 15,000 RSF on a multi-tenant floor with an 18% BOMA Load Factor (R/U Ratio = 1.18), yielding 15,000 ÷ 1.18 = 12,712 USF of actual interior build-out area. If the lease provides a $90.00/RSF TIA ($1,350,000 gross allowance), that equals $106.20/USF gross. Once soft costs (8% architectural/MEP engineering fees = $108,000), landlord construction supervision fees (3% = $40,500), and mandatory base-building fire-alarm/BMS tie-in vendors ($35,000) are deducted, only $1,166,500 ($91.76/USF) remains for hard construction trades.

Takeaway: Always convert the Lease Work Letter’s $/RSF allowance into net hard-cost $/USF after deducting soft costs and landlord coordination fees, and refuse Warm Lit Shell ceiling/lighting installations if your space plan requires extensive closed rooms.

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Fire Partitions, Public Corridors, and Life-Safety Hand-Offs (IBC Chapters 7, 9, and 10)

The sharpest code boundary between a building shell and a tenant improvement occurs at the perimeter of the tenant suite—where public exit access corridors, inter-tenant demising walls, and central life-safety systems intersect the tenant’s interior floor plans and space planning.

1. Why Multi-Tenant Corridors and Demising Walls Use 1-Hour Fire Partitions (IBC Table 1020.2 & Section 708)

Under International Building Code (IBC) Section 1020.2 and Table 1020.2 (Corridor Fire-Resistance Rating), the baseline fire-resistance rating for a corridor in a Group B (Business) or Group M (Mercantile) occupancy serving an occupant load greater than 30 is 1 hour in unsprinklered buildings, but drops to 0 hours (non-rated) when the building is equipped throughout with an automatic sprinkler system installed in accordance with NFPA 13.

Despite this 0-hour code allowance for sprinklered Group B offices, institutional landlords and municipal high-rise standards almost universally require multi-tenant public exit access corridors and inter-tenant demising walls to be constructed as 1-Hour Fire Partitions under IBC Section 708:

1. Future Leasing & Mixed Occupancy Flexibility: A floor plate initially leased to Group B office tenants may later accommodate outpatient clinics, diagnostic labs, high-value storage (Group S-1), or dense assembly training centers (Group A-3) that require rated separation or corridor protection without rebuilding occupied public hallways.
2. Slab-to-Slab Continuity (IBC Section 708.4): Unlike intra-suite office partitions that stop at the 9 ft 6 in. AFF acoustic ceiling grid, a Section 708 Fire Partition must extend continuously from the top of the concrete floor slab below to the underside of the structural floor/roof slab or deck above (12 ft 6 in. to 14 ft 0 in. full height).
3. Head-of-Wall Dynamic Deflection & Penetrations (UL 2079, IBC Sections 714, 715, & 717): Because structural floor slabs deflect under live load (L/360), a slab-to-slab 1-hour wall requires a UL 2079 fire-rated slotted deflection track packed with mineral wool and elastomeric fire caulk (IBC Section 715). Every conduit or cable tray crossing the wall above the ceiling requires a tested IBC Section 714 (ASTM E814 / UL 1479) firestop system; every supply or return air duct crossing a rated corridor or demising partition requires a motorized combination fire/smoke damper (IBC Section 717, UL 555 / UL 555S) tied into the fire alarm system; and every suite entry door opening onto a 1-hour corridor requires a 20-minute fire-rated door and frame assembly (IBC Table 716.1(2)) with self-closing, positive-latching hardware and smoke gasketing.

Because of slab-to-slab framing around overhead beams and ductwork, UL 2079 head joints, firestopping, and motorized duct dampers, a 1-hour demising or corridor partition costs 2.2× to 3.0× more per linear foot than a non-rated intra-suite office partition that terminates at the underside of the suspended ACT grid. In the Work Letter, tenants should require the landlord to pay 100% of public corridor walls and at least 50% of inter-tenant demising walls as a Base Building cost.

2. Multi-Tenant Egress Thresholds (IBC Chapter 10)

A speculative building shell passes base-building plan review with an unpartitioned open floor plate leading directly to two enclosed core exit stairs. The moment a TI architect subdivides that floor into multiple suites, adds locked reception vestibules, or rings the perimeter with private offices, four IBC Chapter 10 egress rules govern layout approval:

• Two Remote Exit Access Doors (> 49 Occupants per IBC Table 1006.2.1): Under IBC Table 1004.5, Group B business areas are calculated at 150 gross sq ft per occupant (or higher density for concentrated conference/training areas at 15 net sq ft per occupant). Once a suite’s calculated occupant load exceeds 49 occupants (roughly 7,350 gross sq ft of pure office, or much smaller if large boardrooms/training rooms are included), the suite must provide at least two remote exit access doors separated by a distance not less than one-third (1/3) of the maximum overall diagonal dimension of the suite (IBC Section 1007.1.1, Exception 2 for sprinklered buildings; 1/2 diagonal if unsprinklered).
• Common Path of Egress Travel (<= 100 ft per IBC Table 1006.2.1): Measured along the natural path of travel (12 in. clearance around furniture and 90° orthogonal turns) from the most remote desk corner to the point where the occupant has two separate and distinct paths to two exits, the common path cannot exceed 100 ft in a sprinklered Group B suite (75 ft if unsprinklered).
• Maximum Exit Access Travel Distance (<= 300 ft per IBC Table 1017.2): Total walking distance from the most remote point inside the tenant suite, through the suite exit door, and along the public corridor to the door of the pressurized enclosed exit stair (Stair A or Stair B) cannot exceed 300 ft in a sprinklered Group B building (200 ft unsprinklered).
• Corridor Clear Width (>= 44 in.) & Dead-End Limit (<= 50 ft): Public and intra-suite corridors serving an occupant load of 50 or more require a minimum unobstructed width of 44 in. (IBC Table 1020.3; 36 in. for occupant load < 50). Under IBC Section 1020.5 (Exception 3), dead-end corridors in a Group B building equipped throughout with an NFPA 13 sprinkler system cannot exceed 50 ft (20 ft in unsprinklered buildings).

3. NFPA 13 Sprinkler Conversion & NFPA 72 Fire Alarm Tie-Ins

Coordinating fire protection during construction drawings vs. shop drawings review requires resolving how the TI contractor modifies the shell’s active suppression and detection grids:

• Upright Shell Heads vs. Dropped Pendent TI Heads (NFPA 13): In a Cold Dark Shell, NFPA 13 requires upright sprinklers mounted 1 in. to 12 in. below the structural deck. When the TI scope installs a suspended ceiling grid, those high upright heads are obstructed from the room below. The TI fire sprinkler contractor must shut down and drain the floor zone via the core stairwell floor-control assembly (paying the landlord's building engineer a drain-down fee of $350 to $1,000 per shift), plug or convert the upright branch tees, and drop 1 in. rigid black-steel pipe nipples or UL-listed flexible braided stainless-steel sprinkler drops terminating in semi-recessed or concealed pendent heads.
• Light Hazard Spacing & Obstruction Rules: Under NFPA 13 for Light Hazard (Office) occupancies, each pendent head covers a maximum area of 225 ft², with a maximum spacing of 15 ft between heads, a maximum distance of 7 ft 6 in. from any wall, a minimum clearance of 4 in. from any wall, and strict adherence to the Three-Times Rule (keeping deflectors at least 3× the maximum dimension of an obstruction away from pendant light fixtures, acoustic baffles, or structural columns). Every newly partitioned private office, wellness room, or 4 ft × 4 ft phone booth requires its own dedicated sprinkler head and hydraulic shop drawing calculation.
• Fire Alarm Notification Appliances (NFPA 72): The base building provides the main Fire Alarm Control Panel (FACP), primary Signaling Line Circuit (SLC) data loop, and core stairwell/restroom devices. The TI scope must wire all suite wall/ceiling horn-strobes and visual strobes (synchronized to flash at 1 to 2 Hz under NFPA 72 and ADA standards across every conference room, huddle room, wellness room, and open work area). If the tenant’s partitioned layout adds more than 12 to 18 notification appliances, the base floor circuit runs out of 24V DC amperage, requiring the TI budget to purchase a dedicated Notification Appliance Circuit (NAC) booster power panel and pay the landlord’s proprietary fire-alarm vendor for point programming.

Takeaway: Verify before lease signature whether public corridors and demising walls are already built to IBC Section 708 1-hour slab-to-slab standards, check that your suite layout meets the 100 ft common-path and > 49 occupant two-exit rules, and budget for NFPA 13 pendent drops plus NFPA 72 proprietary vendor programming in every room.

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MEP Work-Letter Allocations: ASHRAE 62.1 Outdoor Air, VAV Zoning, and Electrical Density

Even when a building shell boasts modern central chillers and rooftop air handlers, the physical size of vertical shafts and electrical busways fixed during Core and Shell design caps how densely a tenant can program a floor plate. Preparing a quantitative MEP audit when defining how to write an architectural brief prevents discovering mid-construction that the base building cannot ventilate or cool your suite.

1. ASHRAE Standard 62.1 Outdoor Air (CFM) Shaft Bottlenecks

Under the ANSI/ASHRAE Standard 62.1 Ventilation Rate Procedure (VRP), minimum Breathing Zone Outdoor Airflow (V_bz) is governed by both occupant count and floor area:

• Formula: V_bz (CFM) = (R_p × P_z) + (R_a × A_z)
  • Where R_p is the required outdoor airflow rate per person (CFM/person), P_z is the zone population (peak number of occupants), R_a is the required outdoor airflow rate per unit area (CFM/ft²), and A_z is the net occupiable zone floor area (ft²).
  • To find the primary system outdoor air intake (V_ot) required at the base-building air handling unit (AHU) or Dedicated Outdoor Air System (DOAS) shaft, divide the sum of zone breathing airflows by the multi-zone system ventilation efficiency (E_v, typically 0.65 to 0.75 for multi-zone VAV systems): V_ot = ΣV_bz ÷ E_v.

Compare how three common Tenant Improvement space types consume outdoor air under ASHRAE 62.1 Table 6-1 across a 1,000 ft² zone:

1. Standard Commercial Office Baseline (5 people / 1,000 ft²):
   • Table 6-1 specifies R_p = 5 CFM/person and R_a = 0.06 CFM/ft².
   • V_bz = (5 CFM/person × 5 people) + (0.06 CFM/ft² × 1,000 ft²) = 25 + 60 = 85 CFM per 1,000 ft² (0.085 CFM/ft² breathing zone).
   • Dividing by E_v = 0.65 to 0.70, landlords engineer base-building vertical outdoor air shafts for 0.12 to 0.15 CFM/ft² (120 to 150 CFM per 1,000 ft²).
2. Dense Open-Bench Tech Office or Call Center (15 people / 1,000 ft²):
   • V_bz = (5 × 15) + (0.06 × 1,000) = 75 + 60 = 135 CFM per 1,000 ft² (0.135 CFM/ft² breathing zone).
   • At E_v = 0.70, system outdoor air demand rises to 0.193 CFM/ft², exceeding a standard 0.15 CFM/ft² Work-Letter allocation by 29%.
3. Conference, Boardroom, Training, or All-Hands Room (50 people / 1,000 ft²):
   • Table 6-1 default occupant density jumps 10-fold to 50 people per 1,000 ft² (R_p = 5 CFM/person, R_a = 0.06 CFM/ft²).
   • V_bz = (5 × 50) + (0.06 × 1,000) = 250 + 60 = 310 CFM per 1,000 ft² (0.310 CFM/ft²)—3.65× the base office outdoor air requirement.

Engineering Rule-of-Thumb: If conference rooms, training centers, town-hall gathering zones, or medical exam rooms exceed 20% of a tenant's floor area, or if overall seat density is tighter than 125 USF per person, the suite will exceed a standard 0.15 CFM/ft² base-building outdoor air shaft cap. To pass mechanical plan check without cutting new exterior facade louvers for a supplemental DOAS unit, the TI mechanical engineer must specify CO2 Demand-Controlled Ventilation (DCV) dampers (mandatory under ASHRAE 90.1 for spaces > 500 ft² with design occupancy >= 25 people/1,000 ft²) and apply the ASHRAE 62.1 time-averaged multi-zone occupancy diversity credit.

2. Electrical Power Density (W/ft²), 24/7 Supplemental Cooling, and Wet Columns

• Lighting (277V) vs. Receptacle (120/208V) Power Allocations: Standard Class A Work Letters separate high-voltage lighting power (277/480V panels sized for 0.65 to 0.79 W/ft² under ASHRAE 90.1 office LPD tables) from step-down 120/208V receptacle and equipment power (typically capped at 3.0 to 5.0 W/ft² connected load). While 4.0 W/ft² easily supports laptop workstations (150–250 W per desk), dense trading desks, medical imaging/sterilization suites, or commercial test kitchens require 12 to 25+ W/ft², forcing the TI budget to fund an oversized K-rated step-down transformer and feeder riser tap.
• After-Hours HVAC & 24/7 Condenser Water Tap Fees: Base-building chilled-water or rooftop VAV systems operate only during defined lease business hours (typically Monday–Friday 7:00 AM to 6:00 PM, billed at $50 to $150 per hour for after-hours overtime calls). Tenant MDF/IDF server closets, UPS battery rooms, and laboratory cold rooms generate 25 to 45+ W/ft² of continuous sensible heat 24/7/365. The TI scope must provide a dedicated split-system DX cooling unit or a water-cooled ceiling/floor CRAC unit tapped into the building’s supplemental condenser water riser. Always audit the Work Letter for hidden landlord charges: many leases levy both a one-time condenser water tap fee ($1,500–$3,500 per ton) and an ongoing annual circulation fee ($400–$900 per ton/year).
• Plumbing Wet Columns & Horizontal Drain Slope (1/4 in. per ft): Outside the finished core restrooms, the base building provides 1 to 3 remote wet columns per floor containing a 3 in. or 4 in. sanitary waste stack, a 2 in. vent stack, and a valved cold water line. Under the International Plumbing Code (IPC Table 704.1) and Uniform Plumbing Code (UPC), horizontal 2 in. sanitary drain lines require a minimum 1/4 in. per foot (2%) downward slope toward the wet column stack. If a tenant locates an executive breakroom sink and dishwasher 65 ft away from the nearest wet column, the drain pipe drops 65 × 0.25 in. = 16.25 in. vertically. Because you cannot drain water uphill into a floor slab, that horizontal drain pipe must run below the concrete slab inside the ceiling plenum of the downstairs neighbor’s suite, requiring core-drilling approval, firestopped floor sleeves, after-hours access into the lower tenant’s occupied space, and plenum-rated cast-iron pipe.

Takeaway: Keep breakrooms, coffee bars, and clinical sinks within 30 to 45 ft of a base-building wet column, negotiate waiving annual condenser-water tap fees for your MDF server closet in the Work Letter, and apply ASHRAE 62.1 CO2 DCV dampers whenever conference rooms cluster on a floor.

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Structural Floor Live Loads, Post-Tensioned Slabs, and Slab Leave-Outs (IBC Chapter 16 / ASCE 7)

When evaluating architect vs. structural engineer responsibilities on a commercial interior fit-out, the tenant’s architect cannot assume that a code-compliant office floor slab can support any interior room layout or storage system. Under IBC Chapter 16 (Table 1607.1) and ASCE/SEI 7 (Minimum Design Loads and Associated Criteria for Buildings), structural floor capacities vary sharply by room occupancy and circulation function:

1. The 50 + 15 psf Office Baseline vs. 80 psf and 150 psf TI Zones

Under IBC Section 1607.5 (Partition Loads), in office buildings where partition locations are subject to change during future Tenant Improvements, the base-building floor slab must be designed for an additional uniformly distributed live load of not less than 15 psf on top of the 50 psf office live load—providing 65 psf total uniform capacity. (Note that under Section 1607.5, this 15 psf partition load is not required where the specified live load is already 80 psf or greater.)

Three frequent TI design elements exceed this 65 psf envelope and require retaining the landlord’s Base Building Structural Engineer of Record (SEOR):

1. High-Density Mobile Filing & Stack Rooms (150 psf): Rolling track storage eliminates intermediate aisles, concentrating 150 to 250 psf onto the slab. Unless the Base Building Work Letter pre-designated a reinforced “heavy bay” (100–125 psf) along the core girder line, the TI contractor must install structural steel wide-flange spreader beams (steel dunnage) spanning directly between building columns or bond Carbon-Fiber Reinforced Polymer (CFRP) structural strips to the underside of the slab.
2. Overhead Operable Partitions (Folding Air-Walls): Large divisible conference and training centers suspend steel tracks carrying acoustical operable walls weighing 8 to 14 lbs per sq ft of vertical wall surface (80 to 140 lbs per linear foot). Hanging a 4,500 lb folding wall from a single composite steel floor beam causes localized mid-span deflection (> L/480), jamming the wall panels unless supplemental diagonal steel bracing and beam stiffeners are welded to the base structure.
3. Interconnecting Two-Story Tenant Stairs: Cutting a 12 ft × 24 ft slab opening between two leased floors to install a communicating architectural stair requires framing a perimeter structural steel curb header, verifying diaphragm shear transfer with the SEOR, and enclosing or protecting the opening with a 2-hour vertical shaft, draft curtains with closely spaced NFPA 13 deluge/water-curtain sprinklers (IBC Section 712.1.3.1), or accordion fire shutters.

2. Post-Tensioned (PT) Concrete Slab Coring Restrictions

In cast-in-place post-tensioned (PT) concrete office buildings, high-strength braided steel tendons greased inside plastic sheathing—or bonded inside grouted ducts—are stressed to 27,000 to 33,000 lbs of tension per tendon. Severing a PT tendon while core-drilling a 4 in. poke-thru conference table floor box or plumbing drain releases explosive stored energy and compromises slab structural capacity.

Every TI Work Letter in a PT building enforces a strict coring protocol:

• Mandatory GPR or X-Ray Scanning: Before drilling any hole or anchor deeper than 3/4 in., the TI contractor must scan both faces of the slab using Ground-Penetrating Radar (GPR) or radiographic X-ray to map all PT tendons, mild steel rebar, and embedded electrical conduits on the concrete surface.
• SEOR Written Coring Sign-Off: Field markings must be photographed and approved in writing by the Base Building SEOR, maintaining a minimum 6 in. clear distance from PT tendon anchorages and banded tendon bundles over column lines.

Takeaway: Map all 80 psf corridors, 100 psf assembly/server zones, 150 psf storage rooms, and operable conference walls onto the base-building structural framing plan during Schematic Design, and budget for GPR slab scanning plus SEOR review fees whenever coring floor boxes or plumbing drains.

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Pre-Lease Commercial Tenant Improvement (TI) Work-Letter Audit Checklist

Execute this five-part architectural and engineering checklist before signing the commercial Lease Work Letter (Exhibit B) so scope gaps are shifted into the Landlord’s Shell obligations or compensated through an increased Tenant Improvement Allowance:

Phase 1: Shell Delivery & Financial Work-Letter Audit

• Confirm Exact Shell Tier (Cold Dark vs. Warm Lit vs. Second-Generation): Walk the floor plate with base-building as-built drawings (CSI Divisions 01–28) to verify whether perimeter drywall, suspended ACT ceilings, 277V lighting, and branch VAV ductwork exist—and require the Landlord to demolish unusable second-generation partitions at Landlord’s sole cost prior to delivery.
• Convert $/RSF Allowance to Net Hard-Cost $/USF: Divide the quoted $/RSF TIA by the BOMA Z65.1 load factor (1 + Load Factor converts USF to RSF), then cap soft-cost deductions, eliminate or cap Landlord Construction Supervision / Coordination Fees at <= 1%–2% (or flat fixed fee), and prohibit the Landlord from charging elevator hoist, water, temporary power, or security guard fees during TI construction.
• Identify Mandatory Base-Building Proprietary Vendors: Obtain upfront unit-price rate sheets from the Landlord’s designated single-source subcontractors for Fire Alarm (NFPA 72), Building Management System (BMS / DDC controls), Security/Access Control, Roofing penetrations, and Structural Engineer of Record (SEOR) reviews so proprietary contractors cannot inflate change orders after lease execution.

Phase 2: Life-Safety, Corridors & Egress Verification (IBC Ch. 7, 9, 10)

• Verify IBC Section 111.2 Shell Sign-Off Status: Confirm that the Base Building has already passed municipal core/shell inspections so a lag in base-building facade, elevator, or fire-pump sign-off cannot block issuance of the tenant’s final IBC Section 111.1 Certificate of Occupancy.
• Assign 1-Hour Corridor & Demising Wall Costs (IBC Section 708): Require the Landlord to fund 100% of multi-tenant public exit access corridors (framed slab-to-slab with UL 2079 deflection tracks, IBC 717 smoke/fire dampers, and >= 44 in. clear width) and at least 50% of inter-tenant demising partitions.
• Test-Fit Egress Distances & Door Counts: Confirm that the schematic test fit provides 2 remote exit access doors (separated by >= 1/3 diagonal) if occupant load exceeds 49, keeps the common path of travel <= 100 ft (Table 1006.2.1), keeps total travel distance <= 300 ft (Table 1017.2), and eliminates dead-end corridors > 50 ft (Section 1020.5).

Phase 3: Fire Protection (NFPA 13 & NFPA 72) Scope Check

• Audit Sprinkler Head Orientation & Zone Drain Fees: Count existing upright vs. pendent sprinkler heads, verify that every proposed closed room satisfies Light Hazard spacing (<= 225 ft²/head, <= 15 ft spacing, >= 4 in. from walls), and require the Landlord to waive building-engineer sprinkler drain-down and refill fees during TI construction.
• Verify FACP Loop Capacity & NAC Booster Power: Confirm that the base-building Fire Alarm Control Panel has spare addressable points on the floor signaling loop and budget for a 24V DC NAC strobe synchronization booster panel in the TI electrical scope.

Phase 4: MEP Capacity & Wet-Column Verification (ASHRAE 62.1 & 90.1)

• Run the ASHRAE 62.1 Outdoor Air (V_bz) Calculation: Compare your suite’s peak population and conference/training room density (0.31 CFM/ft²) against the Work Letter’s outdoor air shaft allocation (typically 0.12–0.15 CFM/ft²), specifying CO2 Demand-Controlled Ventilation (DCV) dampers where high-density rooms cluster.
• Check VAV Box Density & 24/7 Server Room Cooling: Verify base VAV zone density (>= 1 box per 1,000–1,200 USF, with separate boxes for solar-exposed west/south glass), confirm 3.0–5.0 W/ft² 120/208V panel capacity, and negotiate zero annual tap fees for tapping the base condenser-water loop for 24/7 MDF/IDF cooling.
• Measure Horizontal Distance to Wet Columns (<= 45 ft): Verify that all breakroom sinks, dishwashers, and coffee stations sit close enough to a base-building wet column to maintain a 1/4 in. per ft (2%) drain slope within the floor/plenum zone—or secure Landlord contractual guarantee of ceiling access into the suite below for under-slab plumbing runs.

Phase 5: Structural Slab & Plenum Clearance Audit (IBC Ch. 16)

• Verify Slab Live Loads & Heavy Bays (IBC Table 1607.1): Confirm the 50 + 15 psf (65 psf) office slab rating, locate 150 psf high-density filing or 100 psf UPS/server rooms inside pre-reinforced column/girder bays, and verify whether the slab is post-tensioned (PT) requiring GPR scanning and SEOR coring sign-off.
• Measure True Under-Beam Plenum Clearance: Field-verify the laser dimension from the top of the concrete slab to the bottom of the lowest fireproofed structural beam, main VAV trunk duct, and cast-iron storm leader so 9 ft 6 in. or 10 ft 0 in. finished ceiling targets do not collide with base-building utilities.

Takeaway: Complete all five pre-lease Work-Letter audit phases prior to lease execution so base-building corridor, sprinkler, outdoor-air, and structural slab deficiencies are resolved at the Landlord’s expense rather than deducted from the Tenant Improvement Allowance.