A structural engineer's guide to sliding door threshold assemblies

to Specifying Sliding Door Threshold Assemblies

Load path analysis, component capacities, and specification review criteria for engineered threshold assemblies in premium sliding door installations.

Why This Deserves a Structural Review

Threshold spacer assemblies rarely appear on structural drawings. They sit below the door line, invisible in plan and section. Load magnitudes are modest — 100 to 200 pounds per anchor point under dead load. Failure modes are gradual rather than sudden. For most projects, the structural engineer sees "premium sliding door per manufacturer's specifications" in the architectural drawings and moves on.

This is a mistake.

Premium sliding door thresholds represent one of the most complex structural interfaces in modern residential construction. Dead load from the door panels, wind load transferred through the frame, seismic accelerations across a rigid mass, thermal expansion of the aluminum threshold, freeze-thaw cycling in the substrate, alkali attack from concrete pore water, long-term creep in the spacer material, and adhesive anchor performance under sustained load — all converge at a single component that must maintain millimeter-precision elevation for 50+ years.

When threshold assemblies fail — and they do, at rates far higher than the industry openly acknowledges — the failures trace back to structural review shortcuts. Undocumented anchor specifications. Assumed rather than verified material capacities. Missing load combinations. Absent creep analysis. Inadequate coordination between structural, thermal, and durability requirements.

This article presents the structural analysis framework for specifying engineered threshold assemblies, with specific reference to KORSYS as basis-of-design. The intent is to give structural engineers reviewing project specifications the analytical framework, component capacities, and specification review criteria needed to approve threshold assemblies with confidence — or to reject inadequate specifications early enough in the design process to require correction.

The Assembly Under Review

KORSYS is an engineered threshold assembly consisting of four load-bearing components in series between the aluminum threshold and the concrete substrate:

• Pultruded GFRP composite block (vinyl ester matrix, ECR-glass fiber, 65-70% fiber volume fraction, 50 × 30 × 230 mm dimensions with 3 mm walls)

• Stainless steel threaded rod (1/2"-13 UNC, ASTM F593 Group 1, 304 SS or 316 SS for coastal exposure)

• Chemical anchor adhesive (Hilti HY-200, Simpson SET-XP, or equivalent with ICC-ES evaluation)

• Concrete substrate (project-specific mix design, minimum 2,500 psi at time of anchor installation)

Each block is supported by two threaded rods on 170 mm center-to-center spacing. The rods pass through the block cavity via Ø14 mm holes in the bottom face; nut and washer hardware clamps the block between upper and lower positions. A second nut+washer assembly above the block provides post-installation elevation adjustment. Post-installation foam infill (polyurethane in the block interior, EPS below the block) provides cavity elimination and thermal break.

This document focuses on the structural design of the load-bearing components. Thermal, moisture, and durability considerations are addressed briefly in the durability section but are treated comprehensively in separate WINDO technical documents.

Design Loads for Threshold Applications

Dead Load

The primary structural load on a threshold spacer is the weight of the door assembly itself, distributed across the threshold support points. For premium sliding doors:

Verify actual door weight from manufacturer's technical documentation for each project. Weights above are typical ranges; specific configurations vary based on glass thickness, frame profile, and hardware.

Wind Load

Wind loads on the door are transferred through the aluminum frame to the threshold. For structural analysis of the threshold spacer, wind load contributions include:

**Horizontal wind (positive pressure): **transferred through the frame as horizontal shear at the threshold interface. Distributed across threshold support points based on frame stiffness. For a 16 ft door in a 90 mph exposure category B environment, design wind pressure is approximately 15-20 psf. Horizontal force per block: typically 50-100 lbf.

**Vertical wind (uplift): **less common but must be considered for exposed installations. Wind uplift on the door panels creates a small uplift component transferred through the frame. Per block uplift: typically 10-30 lbf for standard configurations, but verify per project.

**Combined wind + dead: **the governing load case for most threshold installations is dead + wind horizontal, per ASCE 7 load combinations. Wind uplift rarely governs unless the door is on the leeward face of a tall building.

Seismic Load

Seismic acceleration of the door mass produces horizontal and vertical loads at the threshold. For non-structural components per ASCE 7-22 Chapter 13:

Fp = 0.4 SDS Wp Ip (ap/Rp) (1 + 2z/h)

For a residential sliding door in Seismic Design Category D:

• SDS = 1.0g (typical high seismic zone)

• ap = 1.0 (rigid component)

• Rp = 2.5 (typical component response modification factor)

• Ip = 1.0 (standard occupancy)

• z/h = 0 (ground floor typical for sliding doors)

For a 1,600 lb door: Fp = 0.4 × 1.0 × 1,600 × 1.0 × (1.0/2.5) × 1.0 = 256 lbf total horizontal seismic force. Distributed across 12 threshold anchor points: 21 lbf per block. Generally small compared to wind loads in high-wind zones, but must be considered for load combinations.

Thermal Load

The GFRP block, threaded rod, and aluminum threshold have different coefficients of thermal expansion:

• Aluminum (threshold): 23 × 10⁻⁶ /°C

• 304 Stainless Steel (rod): 17 × 10⁻⁶ /°C

• GFRP (KORSYS block, longitudinal): ~8 × 10⁻⁶ /°C

• Concrete: 10-14 × 10⁻⁶ /°C

Differential thermal expansion between the aluminum threshold and the concrete substrate is real but manageable at typical threshold dimensions. For a 4.87 m (16 ft) aluminum threshold across a 50°C temperature range, expansion is approximately 5.6 mm — but this expansion occurs along the length of the threshold, not through the KORSYS block interface. The KORSYS block sees thermal expansion primarily in the vertical direction (30 mm × 8 × 10⁻⁶ × 50°C = 0.012 mm) which is negligible.

Thermal load in the KORSYS block is not a governing design consideration.

Load Path Analysis

Force transfer through the KORSYS assembly follows a specific sequence. For downward compressive load (the primary case):

• Aluminum threshold applies compressive load to top of KORSYS block via 1.5" OD fender washer bearing on top face

• Load transfers through the four vertical walls of the GFRP block (2 long walls of 3 × 230 mm = 1,380 mm² effective load-bearing area)

• Load transfers from bottom face of block to lower fender washer

• Lower washer bears on lower hex nut

• Lower nut is threaded to the stainless steel rod (compressive load in rod along embedded length)

• Rod transfers compressive load to concrete via bond shear along embedded length (chemical anchor)

• Concrete substrate reacts in compression at the rod tip

For uplift loads (wind uplift on door):

• Aluminum threshold pulls upward on upper fender washer

• Upper washer pulls upward on upper leveling nut

• Leveling nut pulls upward on rod (tensile load in rod)

• Chemical anchor bond resists rod pullout via shear along bond interface

• Concrete substrate reacts in tension along bond area

For horizontal loads (wind horizontal, seismic):

• Aluminum threshold applies horizontal shear at bearing interface with top of KORSYS block

• Block resists shear via combination of friction and shear in rod hardware

• Shear transfers through rod to concrete via chemical anchor bond shear

The load path is critical for identifying failure modes. Each interface must be evaluated: bearing at top of block, compression in block walls, bearing at bottom of block, tension/compression in rod, shear in rod, bond capacity in chemical anchor, breakout capacity in concrete.

Threshold Deflection Analysis

Beyond component capacity analysis, structural review must consider serviceability — specifically the deflection of the aluminum threshold as a continuous beam spanning between KORSYS support points. Threshold deflection between supports controls door operability, gasket seating, and long-term threshold aesthetics. This section provides the beam deflection framework for verifying spacing adequacy.

The Threshold as a Continuous Beam

The aluminum threshold acts as a continuous beam supported at each KORSYS block position. Between supports, the threshold deflects under three load categories:

• Self-weight of threshold profile (uniform distributed load)

• Concentrated hardware loads (wheel positions, interlock guides, panel stops)

• Occasional live loads (foot traffic when door is open, maintenance)

Aluminum threshold profiles vary significantly in stiffness. A typical single-track profile has moment of inertia approximately 500,000 mm⁴. A heavy multi-track profile may exceed 2,000,000 mm⁴. Verify moment of inertia from the specific door manufacturer's technical documentation.

Aluminum modulus of elasticity: E = 69,000 MPa

Threshold self-weight (linear): w ≈ 0.05-0.08 N/mm (depending on profile)

Deflection Formulas

For continuous multi-span beams (KORSYS blocks providing continuous support at both ends of each interior span), the appropriate deflection formulas at midspan are:

Distributed load: δ = wL⁴ / (384 EI)

Point load at center: δ = PL³ / (192 EI)

For end spans (nearest to jambs), simply-supported boundary conditions apply. Simply-supported deflection is approximately 4× greater than fixed-fixed deflection for the same span and load. This makes the end spans the critical case for deflection control.

Simply supported, distributed load: δ = 5wL⁴ / (384 EI)

Simply supported, point load at center: δ = PL³ / (48 EI)

Calculated Deflection at Various Spacings

Deflection calculations for a typical threshold profile (I = 1,000,000 mm⁴), with 150 lbf concentrated hardware load at midspan and threshold self-weight of 0.05 N/mm:

Under ideal calculation assumptions, all spacings from 12 to 24 inches produce deflection well within the L/720 architectural limit for precision applications. Under ideal conditions, 24-inch spacing is structurally adequate.

Real-World Deflection Amplification

The ideal beam calculation assumes uniform threshold stiffness, static loads, elastic behavior, and perfect support conditions. Real installations depart from these assumptions in several ways that amplify actual deflection:

**Variable stiffness along threshold length. **Aluminum threshold profiles have variable moment of inertia along their length. At track cutouts, drainage channels, and thermal break locations, local I values may be 40-60% of the nominal profile I. If KORSYS support positions align with lower-stiffness zones, effective deflection is 2-3× the ideal calculation.

**Dynamic loading. **Static calculations understate real loads. Door slam impact, gusty wind loads, and dynamic foot loading (someone stepping suddenly onto the threshold) create dynamic amplification factors of 1.5-2.0. Realized peak deflection is meaningfully higher than static prediction.

**End-span behavior. **End spans have simply-supported behavior with 4× the deflection of equivalent fixed-end interior spans. Where the threshold cantilevers slightly beyond the last KORSYS block near the jamb, deflection increases further.

**Long-term creep. **Aluminum exhibits primary creep at sustained loads even at ambient temperature. Over 50-year service life, cumulative creep can add 20-40% to initial elastic deflection at sustained load positions.

**Cumulative assembly tolerance. **Small variations in threshold manufacturing (tolerance on straightness), installation (variations in KORSYS elevation), and thermal cycling produce additional deviation from ideal flat threshold. Tighter support spacing constrains these cumulative effects.

Applying these amplification factors, real-world midspan deflection at 24-inch spacing may be 5-10× the ideal calculation — approaching 0.06 to 0.12 mm at critical end-span locations. Still small in absolute terms, but no longer trivial for zero-threshold applications where 1 mm total tolerance is the design target.

The engineering conclusion: 24-inch spacing is defensible for ideal conditions but leaves limited margin for real-world load variations. 12-16 inch default spacing provides 4-8× additional margin against realized loading conditions while maintaining reasonable installation efficiency.

Recommended Spacing by Application

Additional required placement rules for all configurations:

• Block within 3 inches of each jamb (end-span support)

• Block at each interlock position (concentrated hardware load)

• Block at each wheel/roller stopping position (localized load)

• Maximum uninterrupted span without support: 18 inches

Component Capacities

GFRP Composite Block

KORSYS block mechanical properties are documented in CMA/CNAS certified test report LH250612050101E (Guangzhou Liangheng Testing Laboratory). Key values:

For the threshold spacer application, transverse compressive strength (perpendicular to fiber alignment) governs, since threshold load acts perpendicular to the pultrusion axis of the fibers.

Calculating Block Ultimate Capacity

Load-bearing wall area under vertical compression:

A = 2 × t × L = 2 × 3 mm × 230 mm = 1,380 mm²

Where t = wall thickness (3 mm, two long walls in load path) and L = block length (230 mm).

Ultimate compressive capacity:

Pult = A × σtc = 1,380 mm² × 72.9 N/mm² = 100,602 N = 22,617 lbf per block

Design Allowable Capacity

For composite materials in structural applications, safety factors are typically applied per the following framework:

The composite industry does not use a single universally-adopted safety factor. ACI 440.1R applies environmental reduction factors (CE) and strength reduction factors (φ) rather than a lumped safety factor. For threshold spacer applications, a factor of 4 provides consistency with structural design practice and covers uncertainty in installation execution, long-term creep, and load path variation.

Bearing Capacity Under Washer

The 1.5" OD fender washer bears on approximately 1,140 mm² of the top face. Bearing capacity:

Pbearing = 1,140 mm² × 72.9 N/mm² = 83,106 N = 18,700 lbf per washer

With two washers per block (one over each rod), total bearing capacity is 37,400 lbf. Bearing capacity substantially exceeds design allowable in the load path, so bearing does not govern.

Stainless Steel Threaded Rod

The KORSYS assembly uses 1/2"-13 UNC threaded rod in 304 stainless steel per ASTM F593 Group 1 (or 316 SS per ASTM F593 Group 2 for coastal exposure).

Material Properties

Tension Capacity

Ultimate tensile capacity per rod:

Pult = 75,000 psi × 0.1419 in² = 10,643 lbf per rod

Yield load per rod:

Py = 30,000 psi × 0.1419 in² = 4,257 lbf per rod

Design allowable tensile capacity (using SF = 4 on ultimate, per composite industry convention):

Pallow = 10,643 / 4 = 2,661 lbf per rod

Design allowable per block position (2 rods per block):

Pallow,block = 2 × 2,661 = 5,322 lbf

Shear Capacity

Shear strength of austenitic stainless steel is typically 0.6 to 0.75 × tensile strength. Conservative shear ultimate:

Vult = 0.6 × 10,643 = 6,386 lbf per rod

Design allowable shear (SF = 4):

Vallow = 6,386 / 4 = 1,596 lbf per rod

Per block position (2 rods):

Vallow,block = 3,192 lbf

Chemical Anchor

Chemical anchor capacity depends on the specific product, embedment depth, concrete strength, edge distance, and spacing. Basis-of-design uses Hilti HY-200 with 4 inch (100 mm) embedment in 2,500 psi normal weight concrete.

Reference: Hilti HIT-HY 200-R Product Technical Information, ICC-ES ESR-3187.

Tension Capacity (Bond Failure)

For 1/2" rod at 4 inch embedment in 2,500 psi uncracked concrete, characteristic bond strength τk,uncr = approximately 1,400 psi per Hilti HY-200 published data. Bond area:

Abond = π × d × hef = π × 0.5" × 4" = 6.28 in²

Characteristic bond capacity:

Nb = τk × Abond = 1,400 × 6.28 = 8,792 lbf

Applying ACI 318-19 Chapter 17 factors (with φ = 0.65 for anchor pryout, monolithic concrete):

φNb = 0.65 × 8,792 = 5,715 lbf per rod (uncracked concrete)

For cracked concrete conditions, capacity reduces to approximately 60-70% of uncracked values. Design allowable in cracked concrete: approximately 3,400-4,000 lbf per rod.

Tension Capacity (Concrete Breakout)

For anchors well away from edges (edge distance > 3hef = 12 inches), concrete breakout is calculated per ACI 318-19 Section 17.6.2:

Nb = kc × λa × √f'c × hef^1.5

Nb = 24 × 1.0 × √2,500 × 4^1.5 = 24 × 50 × 8 = 9,600 lbf

Applying φ = 0.65 for anchor breakout in tension: φNb = 6,240 lbf per rod. Concrete breakout typically does not govern unless edge distance or spacing is inadequate.

Shear Capacity

For 1/2" rod anchor at 4 inch embedment, well away from edges, ACI 318-19 Section 17.7 gives shear capacity governed by steel failure or concrete breakout. For 304 SS anchor, steel shear capacity typically governs at approximately:

Vs = 0.6 × futa × Ase = 0.6 × 75,000 × 0.1419 = 6,386 lbf per rod

φVs = 0.65 × 6,386 = 4,151 lbf per rod

Concrete Substrate

Concrete substrate capacity is generally not the limiting factor for KORSYS applications when specified minimums are met (2,500 psi, 7-day cure, adequate edge distance). Concrete substrate becomes a design consideration in:

• Anchor pull-out (already addressed under chemical anchor capacity)

• Substrate crushing under bearing (not applicable to KORSYS since load transfers through anchor to bond interface, not to concrete surface)

• Punching shear (not applicable — KORSYS does not create concentrated point loads on concrete surface)

For projects using concrete strengths below 2,500 psi or with poor concrete quality, consult with concrete subcontractor and consider higher-embedment anchors or supplementary reinforcement.

Governing Component and System Capacity

Comparing design allowable capacities per block position (2 rods per block):

The threaded rod governs system design allowable in tension at ~5,322 lbf per block position. The threaded rod also governs shear at ~3,192 lbf per block position. GFRP block compressive capacity (5,654 lbf) is very closely matched to the rod tensile capacity (5,322 lbf), producing a balanced structural design where no single component is significantly under-utilized.

For design purposes, use rod-limited design allowable values. GFRP block capacity provides small additional margin against overload conditions.

Combined Loading Analysis

For threshold assemblies subject to combined tension and shear (from wind or seismic combined with dead load), ACI 318-19 Section 17.8 provides the interaction equation:

(Nu / φNn)^(5/3) + (Vu / φVn)^(5/3) ≤ 1.0

Or the simplified linear interaction:

(Nu / φNn) + (Vu / φVn) ≤ 1.2

Worked Example: Combined Loading Check

Consider a 4-panel, 16 ft door installation in a Wind Exposure Category B, 90 mph design wind environment, Seismic Design Category C:

• Dead load per block: 133 lbf (compression)

• Wind horizontal per block: 75 lbf (shear)

• Wind vertical per block: 15 lbf (net tension after subtracting dead load: not applicable — dead load exceeds wind vertical)

• Seismic horizontal per block: 21 lbf (shear, but occurs at different time than wind — use governing case)

Governing load case: Dead + Wind (per ASCE 7 load combinations)

• Vertical: 133 lbf (compression — carried by GFRP block directly)

• Horizontal shear: 75 lbf per block

Demand-to-capacity ratios:

• Compression: 133 / 5,654 = 0.024 (2.4% of GFRP block design allowable)

• Shear: 75 / 3,192 = 0.023 (2.3% of rod design allowable in shear)

Combined check (simplified linear):

0.024 + 0.023 = 0.047 << 1.2 ✓

Utilization is 4.7% of allowable. The assembly is enormously over-designed for typical service loads, with substantial margin for wind gusts, seismic events, and construction anomalies.

Wind Load Considerations

Wind load transfer from the door frame to the threshold varies with door configuration. Key considerations:

**Frame stiffness. **Rigid aluminum frames distribute wind load evenly across threshold support points. Flexible frames concentrate load at end supports. Verify manufacturer's frame stiffness rating and use their recommended distribution assumption.

**Corner effects. **Wind pressures near building corners are 30-50% higher than field pressures per ASCE 7-22. Threshold spacers within 10% of building height from corners should use elevated design pressures.

**Overhang effects. **Doors on elevated terraces, cantilevered sections, or above overhangs may see wind uplift on the deck adjacent to the threshold. This can locally increase wind uplift on the door assembly.

**Debris impact requirements. **For hurricane-prone regions (HVHZ per ASCE 7), impact resistance requirements may control specification. KORSYS assembly structural capacity is unaffected by debris impact requirements, but door selection and frame anchorage details should be coordinated with the door manufacturer.

Seismic Considerations

For most residential sliding door installations, seismic loads on the threshold spacer are modest compared to wind loads. However, several considerations are worth explicit review:

**Ductility. **The KORSYS assembly is not ductile. GFRP composite failure is brittle (rapid crack propagation once yield exceeded). Rod yielding is more gradual but not classical steel ductility. Design should stay well within elastic range under all seismic load combinations — the 38× safety margin over typical service loads provides adequate reserve for this.

**Displacement compatibility. **The threshold assembly does not participate in the building's seismic resisting system. However, it must accommodate story drift without failure. For a residential building with 0.02 drift ratio, threshold displacement is typically well below the assembly's tolerance. Verify per specific project drift analysis.

**Anchor performance under seismic loading. **Chemical anchors have documented cyclic loading capacity per ICC-ES ESR reports. For SDC C and higher, verify the specified chemical anchor product is certified for cracked concrete and seismic loading. Hilti HY-200, Simpson SET-XP, and other basis-of-design products all meet this requirement.

**Non-structural component classification. **Threshold assemblies are non-structural components per ASCE 7-22 Chapter 13. Force calculation follows the standard Fp equation. Importance factor Ip depends on building occupancy (typically 1.0 for standard residential).

Long-Term Durability and Creep

For permanent embedded structural components, long-term degradation must be evaluated even when short-term capacities are adequate.

GFRP Creep

Pultruded GFRP composites exhibit some creep under sustained load. Long-term creep is typically characterized by the creep-rupture stress ratio: the fraction of ultimate strength that can be sustained indefinitely without failure.

For vinyl ester + ECR-glass composites at 50 years, published data indicates creep-rupture stress ratio of approximately 30-40%. Applying safety factor of 6 (per composite industry convention for long-term applications) yields design allowable of ~17% of ultimate — well above the actual service load of <3% of ultimate. Creep is not a limiting factor at design service loads.

Alkali Attack

Concrete pore water at pH 12.5+ attacks glass fibers over time. The specific vulnerability depends on glass chemistry:

• E-glass: boron content 5-10%, significant leaching under alkaline exposure, service life ~30 years for permanent concrete embedment

• ECR-glass: boron-free, engineered for alkaline environments, service life 50+ years for permanent concrete embedment

• AR-glass: zirconium-enhanced, similar performance to ECR-glass, primarily used in cementitious composites

KORSYS specifies ECR-glass per ACI 440 recommendations. Alkali resistance validated at 88% strength retention after 60 days accelerated aging (CMA/CNAS certified). Long-term strength retention projected at >75% after 50 years based on published accelerated aging data.

Stainless Steel Corrosion

304 stainless steel provides excellent corrosion resistance in concrete embedded applications. 316 stainless steel is recommended for coastal or chloride-exposed environments where chloride concentrations exceed 5,000 ppm at the anchor location. Verify chloride exposure classification per ACI 318-19 Chapter 19.

Chemical Anchor Long-Term Performance

Adhesive anchors are subject to sustained load capacity reduction per ICC-ES AC308. Sustained load factor αN,seis or αN,su applies to sustained load conditions. For Hilti HY-200: sustained load reduction factor αN,su = 0.63 (uncracked concrete). Design capacity for permanent loads should account for this factor.

Specification Language for Structural Engineers

The following specification requirements protect the structural design intent throughout construction:

Material Specification

"Threshold thermal break block shall be pultruded Glass Fiber Reinforced Polymer composite with vinyl ester resin matrix and ECR-glass fiber reinforcement, conforming to ACI 440.1R recommendations for FRP in concrete embedment. Minimum longitudinal compressive strength 75 MPa per ASTM D695 or GB/T 1448. Minimum transverse compressive strength 70 MPa. Minimum alkali resistance 85% bending strength retention after 60 days Ca(OH)₂ exposure per GB/T 1448 or equivalent."

Hardware Specification

"Threaded rod anchor: 1/2"-13 UNC, Type 304 stainless steel per ASTM F593 Group 1 (Type 316 SS per ASTM F593 Group 2 for coastal exposure per project drawings). Hex nuts and washers: matching stainless grade per ASTM F594 and stainless washer specifications. Minimum yield strength 30,000 psi per ASTM F593. All hardware in the buried environment shall be stainless steel; galvanized substitutions are not permitted."

Chemical Anchor Specification

"Chemical anchor adhesive: two-component polymer adhesive with current ICC-ES Evaluation Report for post-installed adhesive anchors in cracked and uncracked concrete per ICC-ES AC308. Basis of design: Hilti HIT-HY 200-R (ESR-3187), Simpson SET-XP (ESR-2508), or approved equal. Minimum embedment: 4 inches (100 mm). Minimum concrete strength at time of anchor installation: 2,500 psi (17 MPa). Minimum edge distance: 3 inches (75 mm). Installation per manufacturer's technical instructions."

Submittals

"Submit for structural review: (1) certified test reports demonstrating GFRP mechanical properties from accredited laboratory (CMA/CNAS, A2LA, ISO/IEC 17025), (2) material certifications identifying fiber type and resin type per production run, (3) chemical anchor ICC-ES Evaluation Report for the specific product used, (4) project-specific shop drawings showing anchor locations, embedment depths, edge distances, and load paths, (5) manufacturer's calculation of design allowable capacity per block position."

Structural Review Checklist

When reviewing threshold assembly specifications for structural approval, verify each of the following:

• ☐ Material specification identifies both resin (vinyl ester) and fiber (ECR-glass) explicitly

• ☐ Test data submitted from accredited laboratory demonstrating specified mechanical properties

• ☐ Alkali resistance test result available (minimum 60-day Ca(OH)₂ exposure)

• ☐ Chemical anchor product has current ICC-ES Evaluation Report

• ☐ Chemical anchor embedment depth is 4 inches minimum

• ☐ Chemical anchor edge distance is 3 inches minimum, larger if edge breakout governs

• ☐ Anchor spacing is 6 inches minimum, or per manufacturer requirements

• ☐ Threaded rod specification is 304 SS minimum, 316 SS for coastal exposure

• ☐ Concrete strength requirements are compatible with anchor manufacturer specification

• ☐ Combined loading analysis performed for dead + wind and dead + seismic load combinations

• ☐ Demand-to-capacity ratios documented for governing load case

• ☐ Sustained load factors applied for chemical anchor per ICC-ES AC308

• ☐ Project-specific shop drawings show all critical dimensions and coordinates

• ☐ Assembly capacity documented with explicit safety factor per composite industry practice

• ☐ Governing component identified (typically the threaded rod)

• ☐ Failure mode analysis documented (block, rod, anchor, concrete)

• ☐ Long-term creep considerations addressed for permanent applications

Standards and Code References

The following standards form the basis for threshold assembly structural design:

Bottom Line

Threshold spacer assemblies for premium sliding doors are more structurally complex than they appear. The load environment includes dead load, wind, seismic, and thermal contributions. Load paths run through four components in series (block, rod, anchor, concrete), any of which can govern. Long-term durability requires consideration of creep, alkaline attack, and sustained-load anchor behavior.

KORSYS provides a specified, tested, and structurally-analyzed assembly for this application, with design allowable capacity per block position of approximately 5,300 lbf (rod-governed) and typical service loads of 100-150 lbf per block. Utilization at typical service loads is approximately 2-3% of design capacity, providing substantial margin for wind, seismic, and unusual load conditions.

For structural review purposes, KORSYS meets or exceeds industry standards for FRP in concrete (ACI 440), stainless steel hardware (ASTM F593/F594), chemical anchors (ICC-ES AC308), and combined loading analysis (ACI 318-19 Chapter 17). All material properties are validated by CMA/CNAS certified test data available on request.

When threshold assembly specifications include these components, meet these standards, and are documented per the review checklist above, the structural review can proceed with confidence that the assembly will perform reliably for the specified 50+ year service life.

About This Article

This article is published by WINDO, a specialty premium sliding door dealer and manufacturer of the KORSYS engineered threshold assembly system. It is intended as a technical resource for structural engineers reviewing threshold assembly specifications on premium residential and light commercial projects.

Technical data for KORSYS is documented in CMA/CNAS certified test report LH250612050101E, available on request for structural review purposes. Additional testing (ASTM C518 thermal conductivity, ASTM E84 fire performance) is currently in progress and will be reflected in future revisions of KORSYS technical documentation.

For project-specific structural questions, load capacity calculations, or specification review support, contact WINDO engineering at [email protected].

Related resources:

• KORSYS Technical Data Sheet — mechanical, physical, and environmental properties reference

• KORSYS Technical Properties — complete engineering documentation

• KORSYS CSI 3-Part Specification (Section 08 41 26.13) — for project manuals

• KORSYS Installation Manual — installer-facing procedural guide

• Related article: The Threshold Thermal Bridge — Passive House thermal analysis

• Related article: Why GFRP? — materials comparison analysis