LDD-01 · Structural — PEMB + Spine Beam
One-line intent
A clean-spanned PEMB with a secondary spine beam that supports the second floor while preserving a column-free ~30' viewing zone between gym and living wing.Locked decisions
Geometry
- 120' (N–S) × 60' (E–W), four 30' bays, continuous roofline
- North = ILS · Central 60' = gym (W) + living wing (E) · South = 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 every grid intersection
Secondary structure — spine beam
- N–S along gym/living boundary, central 60' core
- Two interior columns only, offset from center
- ~30' clear central viewing zone preserved
- No center column — final decision
Roof
- Monopitch, west-high → east-low
- West eave 26'+ → East eave 24' (~0.4" per foot, per builder)
- Drainage: gravity-sheds east toward stream (no engineered detention required because building is on elevated west side, ~6' above floodplain)
- Designed for exposed ceiling + organized MEP + future solar + IMP roof
Site context (Delaware) — added from builder drawings
- Site benchmark elevation 252.6
- Proposed dwelling FFE 252.4 — pier foundation (not slab-on-grade; cascades into LDD-02)
- Floodplain study line at elevation 246 — building sits ~6' above
- FEMA floodway zone (Map 1800, dated Feb 2026) is east-downhill off the footprint
- Existing: sanitary sewer lateral, garage, shed, driveway, gazebo
Second-floor framing — added from builder rebuttal
- 18" floor assembly above (deck + joists + gypcrete radiant topping + finish)
- Spans between PEMB perimeter frames and the spine beam
- Combined with the two offset spine columns, avoids cost premium of a fully-clear 60' beam
Open items / engineer review
- Final PEMB frame sizing — PEMB manufacturer
- Spine beam sizing — structural engineer once 2F loads finalized
- Lateral system: confirm 18' garage door breaking spine doesn't compromise N–S lateral
- Snow / seismic / wind specific to site
- Foundation: per-column footings vs continuous; coordinate with radiant + insulation
Cost drivers
PEMB shell ~$240K (4 bays × $33/sqft of footprint, includes erection). Spine beam $25–40K. Roof insulation + membrane $65–85K. The PEMB strategy saves 30–60% vs site-built wood frame at this clear span.Air-gap concerns (revised after builder rebuttal)
- Spine wall structural rhythm — clarified. Two offset columns divide the spine wall into three structural spans (north end ~17' · central ~26' · south end ~17'). The 18'×10' garage door sits within the central span. Per Peter, this avoids a true 60' clear span and uses standard steel gauges. Diaphragm continuity at the door opening still worth confirming, but it's not the alarm bell my earlier framing suggested.
- Roof slope is shallower than I noted. Updated: 0.4"/ft (26' W → 24' E over 60'), not ½"/ft. At the lower end of typical PEMB minimums; confirm with IMP roof manufacturer for warranty + Delaware snow loads (modest but non-zero).
- NEW: Pier foundation changes the slab discussion. LDD-02 radiant strategy is written for slab-on-grade. Pier foundation typically supports either (a) structural slab on piles + grade beams (radiant strategy applies) or (b) wood subfloor on piers with radiant overlay (entirely different assembly). Reconcile LDD-02 with actual foundation type before bidding.
- Second-floor 18" assembly is now confirmed. Verify the 18" includes gypcrete topping + finish floor, coupled to LDD-02 §11A gypcrete.
Cross-references
← Inputs from
LDD-05 HVAC trunk position depends on frame geometryDiagram
60' × 120' footprint, four 30' bays, spine beam with two offset columns.