# LDD-01 · Structural — PEMB + Spine Beam

> **Status:** 🟢 LOCKED v2.0 — replaces all prior structural concepts.

## One-line intent

A clean-spanned pre-engineered metal building with a secondary spine beam system that supports the second floor while preserving a column-free ~30' viewing zone between gym and living wing.

## Why this matters

The structural system sets every other LDD's constraints — column placement determines floor plan freedom, spine beam location anchors the gym/living boundary, the rigid frame defines what the IMP envelope and exposed ceiling can do. Locking this first is correct.

## Locked decisions

**Geometry**

- 120' (N–S) × 60' (E–W) rectangular footprint
- Continuous roofline (no stepped geometry)
- Four equal 30' structural bays N–S
- Orientation: North = top, South = bottom, West = gym, East = living

**Program distribution**

- North 30' bay → ILS zone
- Central 60' (two bays) → gym (west) + living wing (east)
- South 30' bay → garage / workshop / service

**Primary structure — PEMB**

- Rigid frames at 0', 30', 60', 90', 120'
- ~60' clear span E–W (no interior columns across width)
- Perimeter columns at all grid intersections (vertical + lateral support)

**Secondary structure — spine system**

- Spine beam runs N–S along gym/living boundary, extends through the central 60' core
- **Two interior columns only**, offset from center, preserving ~30' clear central viewing zone
- **No center column** — final decision

**Second floor support**

- Supported by perimeter PEMB frame + spine beam
- Floor framing spans between these
- Minimal columns, open visual continuity below

**Roof**

- Monopitch, west-high → east-low
- **West eave 26'+ → East eave 24'** (≈ 0.4" per foot over 60' span)
- Continuous roof plane prioritized
- **Drainage strategy: gravity-sheds east toward the stream** (per builder; no engineered detention required because building sits ~6' above the floodplain study line)
- Designed to support exposed ceiling systems, organized MEP routing, future solar integration, IMP roof compatibility

**Site context (Delaware)**

- Site benchmark elevation 252.6
- **Proposed dwelling FFE 252.4 — pier foundation** (not slab-on-grade)
- Floodplain study line at elevation 246 (~6' below FFE)
- FEMA floodway zone (Map 1800, dated Feb 2026) is east-downhill, well off the building footprint
- Existing site features: existing sanitary sewer lateral, existing garage, shed, driveway, gazebo
- Building sits on the elevated west portion of the lot; orientation slightly rotated to lot lines

**Second-floor framing**

- Per builder, **18" floor assembly above** (deck, joists, gypcrete radiant topping, finish floor) — span between PEMB perimeter frames and the spine beam
- Combined with the two offset spine columns, this avoids the cost premium of a fully-clear 60' beam

## Open items / requires engineer review

- Final PEMB frame sizing (depth, flange dimensions) — PEMB manufacturer
- Spine beam sizing — structural engineer once second-floor loads finalized
- Lateral system: PEMB rigid frames take E–W loads; need engineer confirmation that the N–S spine beam + offset columns + diaphragm provide adequate N–S lateral resistance, particularly with the 18' glass garage door breaking the spine wall mid-span
- Snow load / seismic / wind specific to site — engineer once site known
- Foundation: per-column footings vs continuous footings; coordinate with radiant slab and underslab insulation strategy
- Frame-to-slab anchorage detail at perimeter (thermal break)

## Cross-references

- → [LDD-03 spine wall](03-spine-wall.md) — uses the spine beam as primary support and the two offset columns as its only interior structure.
- → [LDD-11 envelope](11-exterior-envelope.md) — IMP wall system attaches to PEMB girts; envelope strategy depends on PEMB hot-rolled vs cold-formed secondary framing.
- → [LDD-12 exposed ceilings](12-exposed-ceilings.md) — exposed PEMB structure is layer 1 of the ceiling hierarchy; every other system defers to it.
- ← [LDD-05 HVAC](05-hvac-system.md) — main trunk position (~12' from spine wall) depends on frame geometry.

## Cost drivers

- **PEMB shell package**: ~$30–45/sqft of footprint for engineered shell (frames, purlins, girts, fasteners). 7,200 sqft × $33 ≈ $240K. Includes erection.
- **Foundation**: continuous perimeter + isolated frame footings. Subgrade work dominates the unknown — and is **excluded from this rollup pending civil engineer**.
- **Spine beam** (steel W-section, ~60' span with two intermediate columns): $25–40K including erection.
- **Roof insulation + membrane** (separate from IMP walls): $8–12/sqft × 7,200 ≈ $65–85K depending on system.

The PEMB strategy is one of the strongest cost-control moves in the design. A site-built wood-frame equivalent at this footprint and column-free clear span would run 1.4–1.8× more.

## Air-gap concerns

1. **Spine wall structural rhythm — clarified per builder.** The spine wall is divided by the two offset columns into **three structural spans** (north end ~17' · central ~26' · south end ~17'). The 18'×10' glass garage door sits within the central span. The "Signature Move" per Peter is that this avoids a true 60' clear span and lets the project use standard steel gauges for the 18" floor assembly above. Diaphragm continuity at the door opening is still worth confirming with the structural engineer, but it's not the alarm bell my earlier framing suggested.
2. **Roof slope is shallower than I noted.** Updated: 0.4"/ft (26' W → 24' E over 60'), not ½"/ft. This is at the lower end of typical PEMB minimums; confirm with the IMP roof manufacturer for warranty and snow-load behavior. Delaware snow loads are modest — the shallow pitch is reasonable but not generous.
3. **Pier foundation changes the slab discussion.** The original LDD-02 radiant strategy is written for slab-on-grade. With pier foundation, the first floor is either (a) a structural slab on piles + grade beams or (b) a structural wood subfloor on piers with radiant overlay. **These produce very different radiant assemblies, slab-edge insulation strategies, and underslab insulation requirements.** Reconcile LDD-02 with the actual foundation type before bidding.
4. **Second-floor framing — 18" assembly is now confirmed.** This is the biggest single cost-and-acoustic variable for upstairs and is closely coupled to [LDD-02 radiant §11A gypcrete](02-radiant-slab.md#open-items--requires-engineer-review). Verify that 18" includes the gypcrete topping and the finish floor.

## Diagram

![Building footprint with bay structure](../../diagrams/01-building-footprint.svg)

## Status

🟢 **Green — ready as written.** Clear hierarchy, no center column commitment is excellent. Engineer review needed for lateral, but no fundamental rethink expected.