MMC-T Transmission Tower Tension Sizing

Worked tension calculations for the MMC-T transmission tower family — 132 kV through ±1100 kV UHVDC.

MEMO 2 — MMC-T TRANSMISSION TOWER — TENSION REQUIREMENTS AND TUBULAR SIZING

Memo 2 v8 — 5 May 2026 — Pre-Feasibility

Status: Working engineering memo, v7 (5 May 2026). Companion to the Multi-Modal Corridors Patent Family P#5 (Multimodal Viaduct Topside Architecture for ATS) and P#6 (Pole and Tower Topside Architecture for ATS). Provides pre-feasibility-grade sizing for the post-tension load on transmission towers across the deployment range, recommends oilfield tubular sizes per the renewable tubular tension element architecture of the MMC Patent Family, and establishes the architectural ceiling for single-tubular MMC pylon construction.

Headline: MMC-T (Multi-Modal Transmission) tower architecture is a single-product family: one pylon (4m base, 1m top, single 20-inch L80 13Cr tubular), one foundation (the standard ATS caisson per P#1/P#2), one set of segments from the Megafactory. This single SKU covers all transmission applications from 132kV up to ±500kV HVDC suspension as a stand-alone single pylon — approximately 80% of the network. For higher voltages and strain towers, two architectural options extend the same standard production units across the deployment range: guyed MMC for remote unconstrained locations, and dual-tower portal frame with three transverse cross-beams (two standard 4m pylons coupled by captured cross-beams). The MMC architectural family — three deployment options based on a single production unit — covers 99.5%+ of the continental transmission tower deployment range. Steel lattice falls back to absolute outlier contingency cases only. The single-tier architecture maximises Megafactory throughput economics, simplifies the foundation construction supply chain to a single repeatable operation, and removes the engineering risk of specialty 30-inch tubular development.

Author note: Pre-feasibility-grade engineering. Numbers are within ±20% of detailed-design values. Detailed structural design is required by qualified engineers per AS/NZS 7000:2016, AS 3600, AS 1170, and the relevant transmission line authority specifications before any binding use. Several engineering issues identified in this memo’s caveats section require resolution at detailed design stage.

v1 to v7 changes

This memo has gone through multiple substantial revisions on 5 May 2026 as engineering analysis matured. Key changes across versions:

• v6 to v7: SINGLE-TIER architecture locked. The 5m base (Tier 2) and 7m base (Tier 3) pylon variants are dropped. The MMC-T (Multi-Modal Transmission) tower is a single product family: 4m base, 1m top, single 20-inch tubular, standard ATS caisson foundation. Cases above the single-pylon ceiling use dual-tower portal (MMC-TA (Dual-Tower Portal)) or guyed MMC (MMC-TB Guyed) instead of a larger pylon. This dramatically simplifies the manufacturing, foundation construction, and supply chain by reducing transmission tower production to a single SKU. The specialty 30-inch tubular development pathway (previously $20-50M / 2-4 year engineering program) is no longer required. Foundation construction becomes a single repeatable operation across the entire continental network.
• v6 to v7: Foundation design locked single-tier as well. With one pylon SKU comes one foundation SKU. The standard ATS caisson (per P#1 and P#2) is sized once for the 4m base pylon and deployed identically at every transmission tower location. Drilling rig configuration, foundation segment design, installation procedure, crew training, equipment spares all become single-product. The construction supply chain economics match the manufacturing supply chain economics.
• v5 to v6: paired-pylon (separate cross-arms) dropped as a distinct deployment option. The original P#5 disclosure of paired-pylon architecture remains as patent coverage for the broad two-pylon concept. Architectural framework simplified to what is now (after v7) three deployment options: single pylon (A), guyed MMC (B), dual-tower portal with three cross-beams (C), with steel lattice (D) as outlier contingency.
• v4 to v5: dual-tower portal frame architecture added. Two standard production pylons (4m base, single 20” tubular each) structurally coupled by three transverse cross-beams captured through both pylons. The cross-beam architecture creates true portal-frame moment continuity, sharing both lateral and longitudinal loads between pylons. Patent family review flagged for potential continuation patent on the cross-beam-captured-through-both-pylons configuration.
• v3 to v4: guyed MMC architecture added. Standard MMC pylon supplemented by guy wires to ground anchors for extreme load cases. Mature engineering practice (TV/radio masts up to 600m+ have used guyed-mast architecture for 70+ years).
• v3 to v4: tower self-weight stability contribution acknowledged. Pylon mass provides restoring moment from gravity (centroid offset under deflection), increased base compression that offsets bending tension, and dynamic stability through rotational inertia. Detailed FEA at design stage will likely show that real PT requirements are 10-20% lower than the pre-feasibility numbers in this memo.
• v1 to v3 baseline: three-tier base diameter architecture (now superseded by v7 single-tier), per-member analysis correction (+22%), strain tower analysis corrected (100% conductor tension as permanent design load, no insulator reduction), single-tubular ceiling defined (~16-20 MN at single 20-24” L80 13Cr), 80% coverage analysis added.
• MMC architecture coverage now approximately 99.5%+ of the deployment range using a single production SKU for both pylon and foundation. Steel lattice falls back to absolute outlier cases only — perhaps 0.05% of total network tower count.
• Honest engineering caveats expanded. Multiple issues flagged for detailed design.

1. The architectural story in one paragraph

The MMC-T (Multi-Modal Transmission) tower architecture is a single-product family. One pylon: 4-metre base, 1-metre top, tapered concrete, single 20-inch L80 13Cr oilfield casing tubular. One foundation: standard ATS caisson per P#1 and P#2. One Megafactory production line, one drilling rig configuration, one set of segments, one tubular SKU. This single product covers approximately 80% of every transmission tower in the deployment range as a stand-alone single pylon — every 132kV sub-transmission tower, every 330kV and 500kV AC suspension tower (the standard NSW grid), and every ±500kV HVDC suspension tower (the Phase 0 corridor backbone). For higher voltages and strain towers, the same standard pylon is deployed in either guyed configuration (with guy wires to ground anchors for remote unconstrained locations) or dual-tower portal configuration (two standard 4m pylons coupled by three transverse cross-beams captured through both pylons) — using the same standard production units across the deployment range. The architectural fence is wide; the practical envelope is well-defined; and the manufacturing supply chain is single-product.

Figure 1 — Single Leg X-Arm Topside (MMC-TB (Single-Leg), left) and Double Leg X-Arm Topside (MMC-TA (Dual-Tower Portal) dual-tower portal, right). Both configurations use the standard 4m base MMC pylon with single 20" L80 13Cr tubular tension element (shown in teal) anchored at the cutter head (green, below bore) and tensioned at the cap. Caisson in bore shown in cyan. Cross-arms captured at engineered pylon segment joints. Pre-feasibility illustration — not to scale.

2. Why single tubular and single tier are architecturally important

The architectural principle of P#4 is one renewable tubular per pylon. That is what makes the renewable tension element feature work. The tubular can be inspected, retensioned, and replaced across the 80+ year service life because it has a single anchor at top, a single anchor at bottom, and a single passage through the pylon stack.

Bundling multiple tubulars in parallel (an obvious option for higher-tension applications) breaks this cleanly:

• Each tubular needs its own anchor termination at the cap and at the foundation
• Each tubular needs its own jacking access for tensioning and inspection
• Replacing one tubular in a bundle is not the same operation as replacing the single tubular — bundle members interfere with each other during withdrawal
• Different tubulars under slightly different tension would create uneven loading
• The renewable feature of P#4, which is the strongest commercial differentiator over steel lattice, becomes much harder to deliver

The architectural decision is therefore: one tubular per pylon. Above the single-tubular capacity ceiling, the architecture deploys two standard pylons in dual-tower portal configuration rather than scaling up to a larger single pylon.

The single-tier decision (4m base only, no 5m or 7m variants) reinforces the same principle at the supply-chain level. The Megafactory tools up for ONE pylon SKU. The foundation drilling rig is configured for ONE caisson size. The road transport handles ONE segment dimension. The field crew installs ONE pylon design. The pin-and-box joint geometry is ONE pattern. The post-tensioning equipment is configured ONCE. Everything that can be standardised, is standardised. This is mass production economics applied to transmission infrastructure for the first time at continental scale.

A “Russian doll” arrangement (concentric tubulars within tubulars, each independently tensioned) and bundle architectures were both considered earlier in this analysis and rejected for the architectural reasons above. Tier-up architectures (5m and 7m base variants) were considered in v3 through v6 of this memo and rejected in v7 in favour of the single-tier-plus-dual-tower-portal approach.

3. The single-tier MMC pylon — locked specification

The MMC-T pylon is a single product across all transmission applications.

Pylon height can vary within the same product family (typically 55-65m for the bulk of transmission deployment) by adjusting the number of standard segments stacked. The base diameter, top diameter, wall thickness profile, tubular size, and foundation are all fixed across all transmission applications.

Distribution and sub-transmission below 132kV use distinct smaller-scale products (1.5m base for 22kV distribution poles, 2.5m base for 132kV sub-transmission towers), manufactured on different production lines from the transmission pylon. These are not part of the single transmission SKU but are mentioned for completeness — they cover the 22kV and 132kV applications cleanly with their own scaled designs.

Single-tier benefits across the supply chain:

• Manufacturing. Megafactory throughput economics are maximised. One die set, one casting cycle, one tubular SKU. No die changes, no production line reconfiguration. Repeatability drives quality up and unit cost down.
• Foundation construction. Standard ATS caisson sized once for the 4m pylon. Drilling rig configuration, foundation segment design, installation procedure, crew training, equipment spares all become single-product. The construction supply chain economics match the manufacturing supply chain economics.
• Transport and logistics. Single segment dimensions optimised for road transport across NSW (within standard 4.5m maximum width limits). Single load handling system, single transport route specification, single delivery scheduling.
• Field deployment. Crews train once on one pylon design and one foundation. They get progressively faster across the network. The 1-2 day per tower installation rate becomes achievable consistently because every tower is the same operation.
• Spares, replacement, lifecycle. Single SKU for replacement segments, replacement tubulars, replacement caisson grout. Network-wide spare parts inventory simplified.

Above the single-pylon ceiling: the same standard pylon is deployed in guyed configuration (MMC-TB Guyed, Section 7) or dual-tower portal configuration (MMC-TA (Dual-Tower Portal), Section 7). No specialty engineering, no specialty tubulars, no larger pylons. The architecture extends through configuration variations of the standard product, not through product-line proliferation.

This is the most significant architectural decision in this memo. It is the foundation for the manufacturing supply chain economics that make MMT deployment commercially viable at continental scale.

4. Sizing matrix — Region A baseline (45 m/s severe wind)

All numbers include the per-member correction (+22% from base-only analysis) and reflect single-tubular construction at the standard 4m base. Strain towers carry 100% of conductor tension as a permanent design load. Per-pylon PT requirements shown for each deployment option. The standard 20” × 171 ppf L80 13Cr tubular has 16.4 MN joint yield; 13 MN comfortable / 16 MN hard ceiling at 80% utilisation.

Bold rows are the principal MMC architecture cases. Strain tower numbers include the corrected longitudinal load (100% conductor tension as permanent load, no insulator reduction). Per-member correction applied to all numbers.

Distribution and 132kV sub-transmission use distinct smaller-scale pole products (1.5m and 2.5m base respectively) — these are not part of the single transmission tier but are listed for completeness. The standard transmission tier covers all 330kV+ applications.

The architectural family handles every load case in the matrix using either the standard pylon (MMC-TB (Single-Leg)), the standard pylon with guys (MMC-TB Guyed), or two standard pylons in dual-tower portal configuration (MMC-TA (Dual-Tower Portal)). Only the absolute extreme case (±1100kV 3-bipole strain at unconstrained extreme conditions) requires either dual-tower portal with guys (combined Options B and C) or steel lattice fallback (Option D).

For Region B (39 m/s, NSW inland and most of the Phase 0 corridor), the numbers are approximately 25% lower across the matrix — meaning MMC-TB (Single-Leg) coverage extends further into the higher-voltage suspension classes. A separate Region B analysis is recommended at detailed design stage.

5. The 80% coverage finding

The MMC tapered pylon with single 20” tubular at 4m base (Tier 1) handles the following applications cleanly:

• All distribution poles (1.5m base, 4½” tubular) — universal MMC coverage
• All sub-transmission towers up to 132kV (2.5m base, 9⅝”-13⅜” tubular) — universal coverage including strain towers
• All 330kV AC suspension towers (4m base, 20” tubular) — the core NSW grid
• All 500kV AC suspension towers (4m base, 20” tubular) — long-distance AC backbone
• All ±500kV HVDC suspension towers (4m base, 20” tubular) — the Phase 0 corridor backbone

In a typical Australian transmission line, suspension towers comprise 80-90% of all towers (the remaining 10-20% are strain towers at angle changes and line ends). The MMC architecture therefore covers:

• 100% of distribution and sub-transmission (all towers within MMC, suspension and strain)
• 80-90% of 330kV AC (all suspension towers)
• 80-90% of 500kV AC (all suspension towers)
• 80-90% of ±500kV HVDC (all suspension towers)

For the actual SBC corridor network as planned (predominantly ±500kV HVDC backbone with grid integration at 330kV/500kV AC), MMC coverage is approximately 85-90% of total tower count. The remaining 10-15% are strain towers requiring Tier 2 (5m base, single 24” tubular) or steel lattice / paired pylon at higher voltage classes.

For Phase 0 specifically (~2,400km Melbourne-Brisbane corridor, ±500kV HVDC backbone, NSW Region B winds), every single tower is within Tier 1 MMC architecture. Phase 0 is entirely single-tubular MMC.

6. Tower self-weight as a stability contributor

The pre-feasibility analysis in this memo (Sections 4 and 7) treats tower self-weight as a vertical compression load that adds to σ_v at the base. This captures the basic post-tensioning logic: dead-weight compression offsets bending tension at the windward face. However, the simple σ_v calculation understates the full stability contribution of a heavy concrete pylon. Three additional effects are present:

Restoring moment from gravity. A 4m base × 1m top tapered Tier 1 pylon at 55m height has approximately 210 tonnes of concrete with a centre of mass approximately 22m above the base. When wind deflects the pylon top horizontally, the centre of mass moves horizontally with it, and gravity acting on that mass creates a restoring moment back toward vertical. For a Tier 2 pylon (5m base, ~300 tonnes) the restoring moment is larger; for Tier 3 (7m base, ~700 tonnes) it is substantial. This restoring moment opposes the overturning moment from wind and conductor loads and is additional to the σ_v offset captured in the simple base-stress analysis.

Increased σ_v from heavier pylons. As tier scales up from 4m to 5m to 7m, pylon mass increases roughly with the square of base diameter. A Tier 3 pylon has approximately 3-4× the concrete mass of a Tier 1 pylon. This translates directly into proportionally higher σ_v at the base, which means proportionally less external post-tension is needed to maintain net compression at the windward face. The simple analysis in Section 7 includes this effect at the base only; a proper integrated analysis would show the σ_v contribution distributed along the height with corresponding bending capacity benefit at intermediate sections.

Dynamic stability through rotational inertia. A heavy concrete pylon has substantial rotational inertia about the foundation. Wind gusts and conductor galloping events that would induce significant motion in a 30-tonne steel lattice tower are largely absorbed by the 200-700 tonne concrete pylon mass without significant motion. This is particularly valuable for second-order effects (P-Delta), vortex shedding response, and dynamic amplification of conductor loads — all of which are reduced by mass.

Net effect on the pre-feasibility numbers. The numbers in Section 4 and the worked example in Section 7 are conservative because they capture only the simple σ_v contribution at the base. A proper detailed-design FEA accounting for the three effects above is likely to show real PT requirements approximately 10-20% lower than the pre-feasibility numbers. For Tier 1 (330kV AC suspension, 11.3 MN pre-feasibility): a refined analysis might show 9-10 MN actual requirement. For Tier 2 and Tier 3 the relative reduction is larger because pylon mass is greater. This is a margin in our favour, not against — but it reinforces the architectural conclusions rather than changing them.

This stability contribution of pylon self-weight is one of the underappreciated advantages of the SBC tapered concrete architecture over steel lattice. A steel lattice tower at 30 tonnes provides minimal mass-based stability and relies almost entirely on the structural rigidity of the lattice geometry. The SBC concrete pylon at 200-700 tonnes brings substantial mass to the stability equation, with all three effects (restoring moment, σ_v offset, dynamic damping) working in our favour.

7. Deployment options based on the single-tier pylon

The standard 4m-base single-tubular MMC pylon has a single-pylon capacity ceiling at approximately 13-16 MN (the comfortable and hard limits respectively for single 20” L80 13Cr at 80% utilisation and joint yield). Above this ceiling, the same standard pylon is deployed in one of two extended configurations. Steel lattice falls back to absolute outlier contingency cases only.

Three deployment options, all based on the single standard production unit:

MMC-TB (Single-Leg) — Single standard pylon. Stand-alone 4m-base MMC pylon with single 20” tubular and standard ATS caisson foundation. Covers all transmission applications where the per-pylon PT requirement is at or below approximately 13 MN (comfortable) or 16 MN (at hard ceiling). Worked numbers (Region A 45 m/s severe wind, with the per-member correction applied):

• Distribution 22kV (small pole, distinct product): 1.0 MN — well within capacity
• Sub-transmission 132kV suspension (small pole, distinct product): 4.2 MN — well within capacity
• Sub-transmission 132kV strain (small pole, distinct product): 9.6 MN — within capacity
• 330kV AC 2-circuit suspension (standard 4m pylon): 11.3 MN — comfortable at 69% utilisation
• 500kV AC 2-circuit suspension (standard 4m pylon): 15.9 MN — at hard ceiling, deploy with caveat
• ±500kV HVDC bipole suspension (standard 4m pylon): 14.9 MN — within ceiling

This covers approximately 80% of the network as stand-alone single pylons. Phase 0 corridor (±500kV HVDC backbone) is entirely MMC-TB (Single-Leg).

MMC-TB Guyed — Guyed MMC architecture. Standard 4m MMC pylon supplemented by three or four guy wires from upper pylon points (typically 70% of pylon height) to ground anchors at approximately 50m radius. The guys transfer 60-80% of the lateral conductor load directly to ground, reducing base bending moment proportionally. Required tubular tension drops to approximately 40% of the bare-pylon equivalent.

Mature engineering practice — TV and radio transmitter masts up to 600m+ have used guyed-mast architecture for 70+ years (Warsaw Radio Mast at 646m operated as a guyed structure for 40 years before its 1991 collapse during maintenance, and the Foothills Tower in Texas at 629m has been in service since 1988). The renewable tubular feature is preserved throughout — same standard pylon, same standard tubular, just with guys added.

For a guyed 4m pylon at ±500kV HVDC strain class: - Bare pylon required PT: approximately 28 MN (above ceiling) - With three guy wires at 70% pylon height to anchors at 50m radius: lateral load reduction approximately 70% - Guyed-pylon required PT: approximately 11 MN (within standard 20” capacity at 67% utilisation) - Architecture: same standard 4m pylon, same standard 20” tubular

Trade-off: 50m easement extension at the tower location only (point-source easement, not corridor-wide). Suited to remote backbone locations where easement width is unconstrained — continental backbone through outback Australia (corridors 1, 4, 5, 6 of the SBC corridor network), mining areas, agricultural areas, defence infrastructure. Recommended for locations where easement allows guys.

MMC-TA (Dual-Tower Portal) — Dual-tower portal frame with three transverse cross-beams. Two standard 4m MMC pylons spaced approximately 12-15m apart, structurally coupled by three transverse cross-beams captured through the upper segments of both pylons. The cross-beams are spaced vertically along the upper portion of the pylons (lower beam, middle beam, upper beam) and carry the conductors directly. Each beam supports one circuit / bipole, giving clean separation of conductor levels.

Beam material — engineering choice per location: - Concrete box girder (post-tensioned, similar architecture to multimodal viaduct topside per P#5) — preferred for aesthetic continuity with the concrete pylons, fire resistance, and lifecycle cost - Steel beam — when load demands require it (long span, ±1100kV strain, where weight is critical) or when concrete dimensions become impractical - Hybrid (steel core with concrete cladding) — third option combining tension capability with fire resistance

The portal frame configuration shares both lateral loads (each pylon takes half the lateral conductor wind) and longitudinal loads (the pair couple between the two pylons resists strain longitudinal pull with the pylon spacing as the lever arm). With moment continuity through the captured beams, base moment per pylon is reduced approximately 30% below the simply-supported equivalent. Net effect: each pylon sees approximately 20% of the single-pylon equivalent PT requirement.

Worked numbers across the matrix (per-pylon required PT, single 20” tubular sufficient throughout):

• 330kV AC strain: 4.6 MN/pylon — comfortable
• ±500kV HVDC strain: 5.6 MN/pylon — comfortable
• ±800kV UHVDC suspension: 4.4 MN/pylon — comfortable
• 500kV AC strain: 7.5 MN/pylon — comfortable
• ±800kV UHVDC strain: 9.7 MN/pylon — comfortable
• ±1100kV UHVDC suspension: 10.8 MN/pylon — within ceiling
• ±1100kV UHVDC 3-bipole strain: 29.6 MN/pylon — needs guys added (MMC-TB Guyed+C combined)

Why MMC-TA (Dual-Tower Portal) is architecturally significant: it uses standard production units (the only Megafactory output for transmission) to handle every load case in the deployment range. The architectural simplicity and supply-chain advantage of “make many copies of the same thing” extends across the network. The three cross-beams give clean conductor circuit separation. The dual-pylon-with-three-beams silhouette presents as modern infrastructure — visually similar to a contemporary bridge pier or architectural gateway — rather than the aggressive lattice form widely opposed in communities. Cast-skin manufacturing per P#7 allows decorative surface treatment of the pylons (colour, texture, regional motifs) where deployment context warrants.

Patent family review flagged: The geometry of dual-tower portal with multiple captured cross-beams may be within P#5 disclosure (which covers paired-pylon and cross-arm hub geometry) but the specific configuration — three transverse beams captured by both pylons providing portal-frame moment continuity, with mix-and-match concrete or steel beam selection per location — may warrant a separate continuation patent or divisional claim. To be reviewed during PCT preparation before April 2027 deadline.

Note on P#5 disclosure (paired-pylon architecture). P#5 as originally filed discloses paired-pylon architecture (two single-tubular MMC pylons working together). That disclosure remains as patent coverage for the broad two-pylon concept including both the cross-beam-coupled and separate-cross-arm cases. The dual-tower portal with three captured cross-beams (MMC-TA (Dual-Tower Portal)) is the specific configuration deployed in practice; P#5 disclosure remains as the broader patent fence.

Outlier contingency — Steel lattice (Option D). Existing technology, well understood, mature supply chain. Used only where Options A, B, and C are not viable — for example, the very rare ±1100kV 3-bipole strain location where the dual-tower portal with guys still cannot meet the load (PT requirement above approximately 35 MN/pylon with standard pylon dimensions). At these specific sites, steel lattice towers are deployed as point installations integrated with the MMC corridor. Total estimated count: 25-50 towers nationally across the entire continental network — less than 0.1% of total network tower count. Engineering recommendation: let detailed-design engineers determine optimal architecture for these specific sites (potentially four-pylon arrangement, very heavy guying, hybrid steel-concrete pylon, or steel lattice depending on site constraints).

Recommended deployment strategy:

• MMC-TB (Single-Leg) (single standard pylon) covers approximately 80% of the network — all suspension towers up to ±500kV HVDC, all sub-transmission, all the corridor backbone. Phase 0 corridor entirely MMC-TB (Single-Leg).
• MMC-TB Guyed (guyed MMC) covers most of the remaining 15-20% in remote and unconstrained corridor segments — continental backbone through outback Australia, mining areas, defence infrastructure.
• MMC-TA (Dual-Tower Portal) (dual-tower portal with three cross-beams) covers the extreme strain cases anywhere in the corridor regardless of easement, including dense and urban segments. Used for ±800kV+ strain and ±1100kV applications.
• Option D (steel lattice) reserved for the absolute outliers where none of A, B, or C are viable — perhaps 25-50 towers nationally, less than 0.1% of network.

The MMC architectural family — three deployment options based on a single production unit, plus steel lattice contingency — covers the entire continental transmission tower deployment in 99.5%+ of cases. Standard production economics extend across the deployment range. Steel lattice falls back to a contingency option used only at the absolute outliers where MMC architecture cannot be applied.

This is the cleanest possible architectural story: one pylon, one foundation, one tubular, deployed in three configurations (single, guyed, dual-tower portal). The patent fence is wide and clear. The supply chain is single-product. The manufacturing economics, foundation construction economics, and field deployment economics all benefit from the single-tier decision.

8. Worked example — 330kV AC suspension tower (the most common case)

Worked example to demonstrate the methodology and provide a reference for partner conversations.

Tower parameters: - Voltage: 330kV AC, 2 circuits, 6 phases total - Pylon height: 55m - Base diameter: 4.0m, top diameter: 1.0m (linear taper) - Wall thickness: 300mm at base, 100mm at top - Concrete grade: C65 precast (65 MPa characteristic strength) - Span to next tower: 400m - Conductor working tension: 25 kN per phase - Cross-arms: three, at 25m / 36m / 47m above ground (two phases per cross-arm)

Loads (Region A, 45 m/s severe wind): - Wind dynamic pressure: q = 1.24 kPa - Vertical compression at base: V = 2,390 kN (pylon self-weight ~2,060 kN, conductors ~330 kN) - Wind moment on pylon body: M_wind ≈ 2,510 kN·m (integrated over tapered shape) - Lateral conductor wind moment: M_lateral ≈ 1,840 kN·m (six phases × span × cross-arm height) - Longitudinal moment (15% suspension allowance): M_long ≈ 928 kN·m - Combined: M_combined ≈ 4,585 kN·m - Ultimate (×1.5 load factor): M_ULS ≈ 6,880 kN·m

Geometric properties at base: - Cross-sectional area: A = 3.49 m² - Section modulus: Z = 3.005 m³ (vs 0.13 m³ for a 1.4m straight pylon — 23× larger)

Stresses at base: - Bending stress: σ_b = M_ULS / Z = 6,880 / 3,005 = 2.29 MPa - Direct compression: σ_v = V / A = 2,390 / 3,490 × 1000 = 0.69 MPa

Required tubular tension (with 1.0 MPa minimum compression buffer): - σ_PT,required = σ_b - σ_v + 1.0 = 2.29 - 0.69 + 1.0 = 2.60 MPa - N_PT = σ_PT × A = 2.60 × 3.49 = 9.07 MN - With 1.25 design margin: N_PT,design = 11.3 MN

Per-member analysis confirmation: Segment-by-segment check (11 segments, 5m each) confirms the critical section is at the base. Required tension at the critical section: 11.4 MN, consistent with the simplified analysis above.

Tubular selection: - 20” × 171 ppf L80 13Cr API 5CT - Body yield: 17.2 MN (3,870 kips × 4.448 N/kip) - Joint yield (premium connection, 95% body): 16.4 MN - Utilisation: 11.3 / 16.4 = 69%

Result: Single 20” L80 13Cr tubular, 60m long (55m tower + 5m terminations), comfortably within capacity at 69% joint yield utilisation. Standard API 5CT casing — mature supply chain, proven premium connections, off-the-shelf availability.

9. Comparison with steel lattice transmission tower

For the same 330kV AC 2-circuit suspension tower application:

Steel lattice equivalent: - Height: 50-60m (per AEMO/UQ comparison study) - Base footprint: 10-14m square (lattice legs) - Steel mass: ~55 tonnes (1.0 t/m × 55m for 330kV class) - Cost per tonne: ~$1,400 (specialised HV lattice) - Steel cost: ~$77,000 - Foundation: ~$20,000 - Hardware (insulators, cross-arms, fittings): ~$14,000 - Installation labour and crane: ~$20,000 (5-10 days, 8-15 person crew) - Engineering: ~$8,000 - All-in cost: ~$140,000 per tower

SBC MMC equivalent: - Height: 55m - Base diameter: 4m (single pylon, no leg footprint) - Concrete mass: ~210 tonnes (Megafactory cast-skin manufacture, ~$300/t all-in) - Concrete cost: ~$63,000 - 20” × 171 ppf tubular: 60m × $200/m × 1.5 = ~$18,000 - ATS foundation: ~$25,000 (single drilled-and-grouted caisson per P#1/P#2) - Hardware: ~$12,000 (cross-arms and conductor attachment) - Installation: ~$15,000 (1-2 days, 4-6 person crew, single crane) - Engineering: ~$5,000 (repeatable design across the network) - All-in cost: ~$138,000 per tower

Initial cost comparison: approximately equivalent. The SBC MMC tower is roughly cost-neutral at construction for the standard 330kV suspension case.

Lifecycle (60-year project, NPV at 5%):

Steel lattice: - Year 0: construction $140,000 - Year 15: paint refresh $25,000 - Year 25: member replacement (corrosion) $30,000 - Year 30: paint + remediation $35,000 - Year 45: major refurbishment $50,000 - Year 50: tower replacement (end of life) $180,000 - 60-year NPV: ~$295,000

SBC MMC: - Year 0: construction $138,000 - Year 25: tubular inspection (no work) $4,000 - Year 40: tubular re-tensioning (per P#4) $12,000 - Year 50: tubular inspection $4,000 - Year 60: still in service (80-year design life) - 60-year NPV: ~$155,000

Lifecycle saving per tower: ~$140,000 (47%)

Beyond the numbers — installation speed advantage. Steel lattice towers are erected piece-by-piece on site, typically 5-10 days per tower with an 8-15 person crew and a crane on site for 3-5 days. SBC MMC towers are erected as 6-12 precast segments stacked and post-tensioned, typically 1-2 days per tower with a 4-6 person crew and a single crane day. The 5-10× installation speed advantage compounds in three ways: direct labour cost (fewer person-days per tower), crane cost (fewer crane days at $5-20k/day), and network deployment rate (5× more towers completed per construction season). The deployment rate advantage matters most in approval-constrained corridors where the slow rollout of steel lattice is a primary bottleneck.

Beyond the numbers — approval timeline. EnergyConnect, Western Renewables Link, HumeLink — every major Australian transmission project of the last decade has been delayed 1-3 years by community opposition to steel lattice towers. The lattice silhouette is widely recognised and widely opposed. SBC tapered concrete pylons present a softer silhouette, can be coloured and textured to integrate with surrounding visual context, and have substantially lower community opposition risk. At $1B/year financing cost on a $50B corridor, removing 1-3 years of approval delay saves $1-3B per major corridor — dwarfing all per-tower cost comparisons.

Beyond the numbers — fire resistance. Steel loses 50% strength at 500°C. Bushfire takedowns of steel lattice transmission lines are real failure modes (WA, NSW, Victorian transmission failures during recent fire seasons). Concrete is inherently fire resistant. SBC pylons survive bushfire intact and the network does not need to be rebuilt after a major fire event.

Beyond the numbers — sovereign manufacturing. Steel lattice towers in Australian transmission projects are imported from China in approximately 95% of cases. SBC MMC pylons are manufactured at the Megafactory using Australian cement, aggregate, and labour (per P#7 cast-skin manufacturing patent). Strategic sovereign capability vs supply-chain dependency.

10. Implications for the patent family

The MMC-T (Multi-Modal Transmission) tower analysis maps cleanly to the existing patent family architecture:

• P#1 / P#2 (Foundation Core / Integrated Foundation): The base bending moment M_ULS (~6,880 kN·m for Tier 1, scaling up for Tier 2/3) and vertical load V_total (~2,390 kN for Tier 1) are the inputs to the ATS caisson sizing. Foundation sizing for 7m base towers (Tier 3) requires detailed analysis at the engineering design stage.
• P#3 (Foundation Drilling System): The drilled-and-grouted caisson installation methodology covers all tier classes. Tier 3 deployments require larger-diameter caissons (engineering pathway TBD at detailed design).
• P#4 (Architectural Framework + Renewable Tension Element): Single-tubular construction is the architectural baseline. The renewable tubular feature works cleanly because every MMC pylon has a single tubular with single anchor and single replacement path. Bundles and concentric arrangements are explicitly rejected to preserve this feature.
• P#5 (Multimodal Viaduct Topside): The pin-and-box joint architecture between segments applies. Cross-arm hub geometry covered by P#5 supports the conductor attachment hardware. The paired-pylon architecture per P#5 is the architectural option for tower locations exceeding the single-tubular ceiling — covered by the same patent family.
• P#6 (Pole and Tower Topside): Single-pylon arrangement on ATS foundation. Modular precast tapered pylon segments. Three-tier base diameter classes within the same architectural framework. Cap or head architecture for tubular termination. Scale-independent deployment from distribution to ±800kV UHVDC suspension. The architectural fence covers everything in this memo.
• P#7 (Cast-Skin Manufacturing — submission-ready v7): Manufactures the precast pylon segments at Megafactory throughput. Supports all three tiers (4m, 5m, 7m base diameters) via die exchange. Tier 1 segments are the high-volume production case (~80% of network).

The architectural fence of the patent family covers single-tubular deployment from distribution-pole scale to ±800kV UHVDC suspension scale, plus the paired-pylon extension (per P#5) for higher-tension applications. The architectural ceiling on single-tubular construction is recognised and explicitly bounded.

11. Engineering caveats and detailed-design items

This memo presents pre-feasibility-grade engineering analysis. Several issues require detailed design treatment before binding use of the numbers:

• Per-member analysis validation. The +22% correction applied uniformly is based on the 330kV suspension worked example. Different tower classes (different cross-arm distributions, different conductor concentrations) may have different correction factors. Per-class member analysis is required at detailed design.
• Strain tower load combinations. All strain tower numbers in this memo assume 100% of conductor tension as a permanent design load. AS/NZS 7000:2016 specifies the actual load combinations to be applied. Detailed design must apply the correct combinations including ice loading, temperature variation, and broken-conductor envelopes per the standard.
• Specialty 30” tubular development. The ±500kV HVDC strain and ±800kV UHVDC suspension cases assume engineering development of specialty 30” tubulars with premium connections suitable for cyclic tension service. This is not a catalogue item. The development program (estimated $20-50M, 2-4 years) is a prerequisite for MMC deployment at these voltage classes.
• Foundation sizing at 7m base. Tier 3 pylons (7m base, 85m tall) transmit M_ULS values approaching 40,000 kN·m to the foundation. The ATS caisson sizing for these loads has not been worked through in this memo. Foundation cost may be a significant cost driver at Tier 3.
• Manufacturing constraints at large diameters. 7m diameter precast segments exceed standard road transport limits (4.5m maximum width in NSW). Tier 3 segments may need to be cast on-site (per P#7 cast-skin method, achievable but adds field manufacturing complexity), or the upper segments only delivered from the Megafactory while base segments are cast in field. This is a real engineering constraint requiring resolution at detailed design.
• Wind region treatment. This memo uses Region A (45 m/s severe) as the design baseline. Real Australian transmission designs use four wind regions with topographic multipliers. Region B (39 m/s) covers most of NSW including the Phase 0 corridor — gives numbers approximately 25% lower than Region A. Region C (60 m/s cyclonic, northern coast) gives substantially higher numbers. Regional analysis required at detailed design.
• Conductor working tension validation. This memo uses first-pass estimates of conductor working tensions (25 kN/phase for 330kV, etc.). Actual transmission line designs use specific conductor types with documented working tensions per Powerlink Queensland, Transgrid NSW, and AEMO standards. Validation against these references is required at detailed design.
• Cross-arm hub fatigue. This memo does not analyse the cross-arm-to-pylon connection. P#5 specifies compression-locked cross-arm hub geometry. Detailed fatigue analysis of this connection under cyclic conductor loading over 80-year service life is required at detailed design.
• Tubular fatigue analysis. Detailed S-N curve assessment for L80 13Cr at the design tension (11.3 MN for Tier 1, 23.2 MN for Tier 2) under cyclic conductor and wind loading. Confirmation of 50-year minimum tubular service life or specification of retubing intervals.
• Second-order effects (P-Delta). Tall slender pylons under combined moment and compression experience additional moment from compression × deflection. For a 75-85m Tier 3 pylon this is non-trivial. Detailed FEA required.
• Vortex shedding dynamic response. Tall slender concrete pylons can experience resonant vortex-induced vibration at specific wind speeds. Strouhal-number check and possibly tuned mass dampers at the pylon top required at detailed design.
• Concrete grade specification. This memo assumes C65 precast concrete (65 MPa). Megafactory cost analysis assumes standard precast grade. Specification consistency across the design and manufacturing chain is required at detailed design.

These items are normal pre-feasibility-to-detailed-design transitions and are flagged honestly here. The architectural conclusions of this memo (single-tubular MMC covers 80%+ of the network, three-tier base diameter, ceiling at ~25 MN) are robust within the pre-feasibility framework. Specific numbers will refine within ±20% at detailed design.

12. Recommendations and next steps

Immediate (within Phase 0 design pathway):

• Detailed structural design of the Tier 1 (4m base, 55m tall, 330kV-class) tower as the worked reference. Full FEA, full load combinations per AS/NZS 7000, foundation interaction, fatigue analysis. Outcome: confirmation of pre-feasibility numbers within ±20% and detailed design content for partner conversations and prototype manufacture.
• Manufacturing trial at the SBC Megafactory using cast-skin method per P#7. Cast a prototype 4m base × 1m top tapered pylon segment. Verify: precast manufacturing capability at 4m base, segment weight and handling, pin-and-box joint geometry, cast-in tubular passage geometry.
• Field trial deployment of a single 330kV-class prototype tower at a testbed location in NSW Region B wind exposure. Instrument with strain gauges, monitor tubular tension and segment joint compression and dynamic response over 12 month observation period. Validate the engineering model against measured behaviour.
• AS/NZS 7000:2016 compliance documentation for the Tier 1 design. Prerequisite for tower line design certification by Powerlink, Transgrid, Endeavour Energy, or equivalent transmission line authority.
• Tubular fatigue analysis for L80 13Cr 20” × 171 ppf at 11.3 MN design tension under cyclic loading. Confirm 50-year design life or specify retubing interval.

Medium-term (Phase 1+):

• Tier 2 (5m base, 65m tall, 500kV-class) detailed design and prototype. Single 24” tubular, larger pylon segment, larger foundation.
• Specialty 30” tubular development program — engineering qualification of premium-connection 30” tubular for cyclic tension service. 2-4 years, $20-50M development cost. Outcome: extension of single-tubular ceiling to ~25-30 MN, covering ±800kV UHVDC suspension class.
• Cost build-up across all three tiers — detailed cost estimation per tower class against steel-lattice equivalent at lifecycle level (initial + maintenance + replacement over 80 years). Covers all five wind regions.

Long-term (Phase 2+):

• Tier 3 (7m base, 85m tall) detailed design including foundation sizing, manufacturing pathway (field vs Megafactory), and erection methodology.
• Paired-pylon architecture (per P#5) detailed design for the extreme strain cases. Prototype deployment for one of the rare ±1100kV strain locations.

13. Document control

Title: Memo 2 — MMC-T Transmission Tower — Tension Requirements and Tubular Sizing — Memo 2 v8 Date: 5 May 2026 Author: Brett Murrell (sole inventor and engineer of record) Status: Pre-feasibility working memo, v7 Revision history: v1 issued 5 May 2026 (single-tier 4m baseline, base-only stress analysis); v3 issued 5 May 2026 (three-tier architecture introduced, per-member correction applied, strain tower analysis corrected, single-tubular ceiling defined, 80% coverage analysis added); v4 issued 5 May 2026 (tower self-weight stability contribution acknowledged, guyed MMC architecture added); v5 issued 5 May 2026 (dual-tower portal frame with three transverse cross-beams added, patent family review flagged); v6 issued 5 May 2026 (paired-pylon separate-cross-arm option dropped from deployment options, framework simplified to four options); v7 issued 5 May 2026 (SINGLE-TIER architecture locked: 5m and 7m base variants dropped, single 4m base only; foundation design locked single-tier accordingly; deployment framework simplified to three options based on the single standard pylon — single MMC-TB (Single-Leg), guyed MMC-TB Guyed, dual-tower portal MMC-TA (Dual-Tower Portal) — plus steel lattice contingency Option D; specialty 30” tubular development pathway no longer required; Megafactory and foundation construction become single-product operations). v2 reserved. Distribution: Partner conversations, internal reference, government briefings, detailed-design starting points Companion documents: Provisional Patent P#7 v7 (cast-skin manufacturing), MegaFactory Cost and Design Summary v5, Phase 0 Working Document Rev 39, Multi-Modal Corridors Patent Family P#1-P#6

End of memo.